Handbook of Lead-Free Solder Technology for Microelectronic Assemblies

Handbook of lead-Free Solder Technology for Microelectronic AssembIies

edited by

Karl J. Puttlitz ISM Corporate Staff Corporate Environmental Affairs ISM Corporation East Fishkill, New York, U.S.A.

Kathleen A. Stalter ISM Technology Group ISM Corporation Hopewell Junction, New York, U.S.A.

a% MARCEL

DEKKER

MARCELDEKKER, INC.

NEWYORK BASEL

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MECHANICAL ENGINEERING A Series of Textbooks and Reference Books

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L. L. Faulkner Columbus Division, Battelle Memorial Institute and Department of Mechanical Engineering The Ohio State University Columbus, Ohio

1. Spring Designer's Handbook, Harold Carlson 2. Computer-AidedGraphics and Design, Daniel L. Ryan 3. Lubrication Fundamentals,J. George Wills 4. Solar Engineering for Domestic Buildings, William A. Himmelman 5. Applied Engineering Mechanics: Statics and Dynamics, G. Boothroydand C. Poli 6. Centrifugal Pump Clinic, lgor J. Karassik 7. Computer-AidedKinetics for Machine Design, Daniel L. Ryan 8. Plastics Products Design Handbook, Part A: Materials and Components; Part 6: Processes and Design for Processes, edited by Edward Miller 9. Turbomachinery: Basic 'Theory and Applications, Earl Logan, Jr. 10. Vibrations of Shells and Plates, Werner Soedel 1 1. Flat and Corrugated Diaphragm Design Handbook, Mario Di Giovanni 12. Practical Stress Analysis in Engineering Design, Alexander Blake 13. An lntroduction to the Design and Behavior of Bolted Joints, John H. Bickford 14. Optimal Engineering Design: Principles and Applications, James N. Siddall 15. Spring Manufacturing Handbook, Harold Carlson 16. hdustrial Noise Control: Fundamentalsand Applications, edited by Lewis H. Bell 1 7. Gears and Their Vibration: A Basic Approach to Understanding Gear Noise, J. Derek Smith 18. Chains for Power Transmission and Material Handling: Design and Applications Handbook, American Chain Association 19. Corrosion and Corrosion Protection Handbook, edited by Philip A. Schweitzer 20. Gear Drive Systems: Design and Application, Peter Lynwander 21 . Controlling In-Plant Airborne Contaminants: Systems Design and Calculations,John D. Constance 22. CAD/CAM Systems Planning and Implementation,Charles S. Knox 23. Probabilistic Engineering Design: Principles and Applications, James N. Siddall 24. Traction Drives: Selection and Application, Frederick W. Heilich 111 and Eugene E. Shube 25. Finite Element Methods: An Introduction, Ronald L. Huston and Chris E. Passerello 26. Mechanical Fastening of Plastics: An Engineering Handbook, Brayton Lincoln, Kenneth J. Gomes, and James F. Braden 27. Lubrication in Practice: Second Edition, edited by W. S. Robertson 28. Principles of Automated Drafting, Daniel L. Ryan 29. Practical Seal Design, edited by Leonard J. Martini 30. Engineering Documentationfor CAD/CAM Applications, Charles S. Knox 31 . Design Dimensioning with Computer Graphics Applications, Jerome C. Lange 32. Mechanism Analysis: Simplified Graphical and Analytical Techniques, Lyndon 0. Barton 33. CAD/CAM Systems: Justification, Implementation, Productivity Measurement, Edward J. Preston, George W. Crawford, and Mark E. Coticchia 34. Steam Plant Calculations Manual, V . Ganapathy 35. Design Assurance for Engineers and Managers, John A. Burgess

36. Heat Transfer Fluids and Systems for Process and Energy Applications, Jasbir Singh 37. Potential Flows: Computer Graphic Solutions, Robert H. Kirchhoff 38. Computer-Aided Graphics and Design: Second Edition, Daniel L. Ryan 39. Electronically Controlled Proportional Valves: Selection and Application, Michael J. Tonyan, edited by Tobi Goldoftas 40. Pressure Gauge Handbook, AMETEK, U.S. Gauge Division, edited by Philip W. Harland 41. Fabric Filtration for Combustion Sources: Fundamentals and Basic Technology, R. P. Donovan 42. Design of Mechanical Joints, Alexander Blake 43. CAD/CAM Dictionary, Edward J. Preston, George W. Crawford, and Mark E. Coticchia 44. Machinery Adhesives for Locking, Retaining, and Sealing, Girard S. Haviland 45. Couplings and Joints: Design, Selection, and Application, Jon R. Mancuso 46. Shaft Alignment Handbook, John Piotrowski 47. BASIC Programs for Steam Plant Engineers: Boilers, Combustion, Fluid Flow, and Heat Transfer,V. Ganapathy 48. Solving Mechanical Design Problems with Computer Graphics, Jerome C. Lange 49. Plastics Gearing: Selection and Application, Clifford E. Adams 50. Clutches and Brakes: Design and Selection, William C. Orthwein 51. Transducers in Mechanical and Electronic Design, Harry L. Trietley 52. Metallurgical Applications of Shock-Wave and High-Strain-Rate Phenomena, edited by Lawrence E. Murr, Karl P. Staudhammer, and Marc A. Meyers 53. Magnesium Products Design, Robert S. Busk 54. How to Integrate CAD/CAM Systems: Management and Technology, William D. Engelke 55. Cam Design and Manufacture: Second Edition; with cam design software for the ISM PC and compatibles, disk included, Preben W. Jensen 56. Solid-state A C Motor Controls: Selection and Application, Sylvester Campbell 57. Fundamentals of Robotics, David D. Ardayfio 58. Belt Selection and Application for Engineers, edited by Wallace D. Erickson 59. Developing Three-DimensionalCAD Software with the ISM PC, C. Stan Wei 60. Organizing Data for ClM Applications, Charles S. Knox, with contributions by Thomas C. Boos, Ross S. Culverhouse, and Paul F. Muchnicki 61. Computer-Aided Simulation in Railway Dynamics, by Rao V. Dukkipati and Joseph R. Amyot 62. Fiber-Reinforced Composites:Materials, Manufacturing, and Design, P. K. Mallick 63. Photoelectric Sensors and Controls: Selection and Application, Scott M. Juds 64. Finite Element Analysis with Personal Computers, Edward R. Champion, Jr., and J. Michael Ensminger 65. Ultrasonics: Fundamentals, Technology, Applications: Second Edition, Revised and Expanded, Dale Ensminger 66. Applied Finite Element Modeling: Practical Problem Solving for Engineers, Jeffrey M. Steele 67. Measurement and lnstrurnentation in Engineering: Principles and Basic Laboratory Experiments, Francis S. Tse and Ivan E. Morse 68. Centrifugal Pump Clinic: Second Edition, Revised and Expanded, lgor J. Karassik 69. Practical Stress Analysis in Engineering Design: Second Edition, Revised and Expanded, Alexander Blake 70. An Introduction to the Design and Behavior of Bolted Joints: Second Edition, Revised and Expanded, John H. Bickford 71. High Vacuum Technology: A Practical Guide, Marsbed H. Hablanian 72. Pressure Sensors: Selection and Application, Duane Tandeske 73. Zinc Handbook: Properties, Processing, and Use in Design, Frank Porter 74. Thermal Fatigue of Metals, Andrzej Weronski and Tadeusz Hejwowski 75. Classical and Modem Mechanisms for Engineers and Inventors, Preben W. Jensen 76. Handbook of Electronic Package Design, edited by Michael Pecht 77. Shock-Wave and High-Strain-Rate Phenomena in Materials, edited by Marc A. Meyers, Lawrence E. Murr, and Karl P. Staudhammer 78. Industrial Refrigeration: Principles, Design and Applications, P. C. Koelet

79. Applied Combustion, Eugene L. Keating 80. Engine Oils and Automotive Lubrication, edited by Wilfried J. Bartz 8 1. Mechanism Analysis: Simplified and Graphical Techniques, Second Edition, Revised and Expanded, Lyndon 0. Barton 82. Fundamental Fluid Mechanics for the Practicing Engineer, James W. Murdock 83. Fiber-Reinforced Composites: Materials, Manufacturing, and Design, Second Edition, Revised and Expanded, P. K. Mallick 84. Numerical Methods for Engineering Applications, Edward R. Champion, Jr. 85. Turbomachinery: Basic Theory and Applications, Second Edition, Revised and Expanded, Earl Logan, Jr. 86. Vibrations of Shells and Plates: Second Edition, Revised and Expanded, Werner Soedel 87. Steam Plant Calculations Manual: Second Edition, Revised and Expanded, V. Ganapathy 88. Industrial Noise Control: Fundamentals and Applications, Second Edition, Revised and Expanded, Lewis H. Bell and Douglas H. Bell 89. Finite Elements: Their Design and Performance, Richard H. MacNeal 90. Mechanical Properties of Polymers and Composites: Second Edition, Revised and Expanded, Lawrence E. Nielsen and Robert F. Landel 91. Mechanical Wear Prediction and Prevention, Raymond G. Bayer 92. Mechanical Power Transmission Components, edited by David W. South and Jon R. Mancuso 93. Handbook of Turbomachinery,edited by Earl Logan, Jr. 94. Engineering Documentation Control Practices and Procedures, Ray E. Monahan 95. Refractory Linings: Thermomechanical Design and Applications, Charles A. Schacht 96. Geometric Dimensioning and Tolerancing: Applications and Techniques for Use in Design, Manufacturing, and Inspection, James D. Meadows 97. An Introduction to the Design and Behavior of Bolted Joints: Third Edition, Revised and Expanded, John H. Bickford 98. Shaft Alignment Handbook: Second Edition, Revised and Expanded, John Piotrowski 99. Computer-Aided Design of Polymer-Matrix Composite Structures, edited by S. V. Hoa 100. Friction Science and Technology, Peter J. Blau 101. Introduction to Plastics and Composites: Mechanical Properties and Engineering Applications, Edward MiIler 102. Practical Fracture Mechanics in Design, Alexander Blake 103. Pump Characteristics and Applications, Michael W. Volk 104. Optical Principles and Technology for Engineers, James E. Stewart 105. Optimizing the Shape of Mechanical Elements and Structures, A. A. Seireg and Jorge Rodriguez 106. Kinematics and Dynamics of Machinery, Vladimir Stejskal and Michael ValaSek 107. Shaft Seals for Dynamic Applications, Les Horve 108. Reliability-Based Mechanical Design, edited by Thomas A. Cruse 109. Mechanical Fastening, Joining, and Assembly, James A. Speck 110. Turbomachinery Fluid Dynamics and Heat Transfer, edited by Chunill Hah I 1I.High-Vacuum Technology: A Practical Guide, Second Edition, Revised and Expanded, Marsbed H. Hablanian 112. Geometric Dimensioning and Tolerancing: Workbook and Answerbook, James D. Meadows I13. Handbook of Materials Selection for Engineering Applications, edited by G. T. Murray 114. Handbook of Thermoplastic Piping System Design, Thomas Sixsmith and Reinhard Hanselka 115. Practical Guide to Finite Elements: A Solid Mechanics Approach, Steven M. Lepi 116. Applied Computational Fluid Dynamics, edited by Vijay K. Garg 117. Fluid Sealing Technology,Heinz K. Muller and Bernard S. Nau 118. Friction and Lubrication in Mechanical Design, A. A. Seireg 119. Influence Functions and Matrices, Yuri A. Melnikov 120. Mechanical Analysis of Electronic Packaging Systems, Stephen A. McKeown

121. Couplings and Joints: Design, Selection, and Application, Second Edition, Revised and Expanded, Jon R. Mancuso 122. Thermodynamics:Processes and Applications, Earl Logan, Jr. 123. Gear Noise and Vibration,J. Derek Smith 124. Practical Fluid Mechanics for Engineering Applications, John J. Bloomer 125. Handbook of Hydraulic fluid Technology,edited by George E. Totten 126. Heat Exchanger Design Handbook,T. Kuppan 127. Designing for Product Sound Quality, Richard H. Lyon 128. Probability Applications in Mechanical Design, Franklin E. Fisher and Joy R. Fisher 129. Nickel Alloys, edited by Ulrich Heubner 130. Rotating Machinery Vibration: Problem Analysis and Troubleshooting, Maurice L. Adams, Jr. 131. Formulas for Dynamic Analysis, Ronald Huston and C. Q. Liu 132. Handbook of Machinery Dynamics, Lynn L. Faulkner and Earl Logan, Jr. 133. Rapid Prototyping Technology: Selection and Application, Ken Cooper 134. Reciprocating Machinery Dynamics: Design and Analysis, Abdulla S. Rangwala 135. Maintenance Excellence: Optimizing Equipment Life-Cycle Decisions, edited by John D. Campbell and Andrew K. S. Jardine 136. Practical Guide to Industrial Boiler Systems, Ralph L. Vandagriff 137. Lubrication Fundamentals: Second Edition, Revised and Expanded, D. M. Pirro and A. A. Wessol 138. Mechanical Life Cycle Handbook: Good Environmental Design and Manufacturing, edited by Mahendra S. Hundal 139. Micromachining of Engineering Materials, edited by Joseph McGeough 140. Control Strategies for Dynamic Systems: Design and Implementation, John H. Lumkes, Jr. 141. Practical Guide to Pressure Vessel Manufacturing, Sunil Pullarcot 142. Nondestructive Evaluation: Theory, Techniques, and Applications, edited by Peter J. Shull 143. Diesel Engine Engineering: Dynamics, Design, and Control, Andrei Makartchouk 144. Handbook of Machine Tool Analysis, loan D. Marinescu, Constantin Ispas, and Dan Boboc 145. Implementing Concurrent Engineering in Small Companies, Susan Carlson Skalak 146. Practical Guide to the Packaging of Electronics: Thermal and Mechanical Design and Analysis, Ali Jamnia 147. Bearing Design in Machinery: Engineering Tribology and Lubrication, Avraham Harnoy 148. Mechanical Reliability Improvement: Probability and Statistics for Experi-mental Testing, R. E. Little 149. Industrial Boilers and Heat Recovery Steam Generators: Design, Applications, and Calculations, V. Ganapathy 150. The CAD Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design, Stephen J. Schoonmaker 15 1. Industrial Noise Control and Acoustics, Randall F. Barron 152. Mechanical Properties of Engineering Materials, Wole Soboyejo 153. Reliability Verification, Testing, and Analysis in Engineering Design, Gary S. Wasserman 154. Fundamental Mechanics of Fluids: Third Edition, I . G. Currie 155. Intermediate Heat Transfer, Kau-Fui Vincent Wong 15 6. H VAC Water Chillers and Cooling Towers: Fundamentals, Application, and Operations, Herbert W. Stanford Ill 157. Gear Noise and Vibration: Second Edition, Revised and Expanded, J. Derek Smith

158. Handbook of Turbomachinery: Second Edition, Revised and Expanded, Earl Logan, Jr., and Ramendra Roy 15 9. Piping and Pipeline Engineering: Design, Construction, Maintenance, Integrity, and Repair, George A. Antaki 160. Turbomachinery: Design and Theory, Rama S. R. Gorla and Aijaz Ahmed Khan 161. Target Costing: Market-Driven Product Design, M. Bradford Clifton, Henry M. B. Bird, Robert E. Albano, and Wesley P. Townsend 162. Fluidized Bed Combustion, Simeon N. 0 ka 163. Theory of Dimensioning: An Introduction to Parameterizing Geometric Models, Vijay Srinivasan 164. Handbook of Mechanical Alloy Design, George E. Totten, Lin Xie, and Kiyoshi Funatani 165. Structural Analysis of Polymeric Composite Materials, Mark E. Tuttle 166. Modeling and Simulation for Material Selection and Mechanical Design, George E. Totten, Lin Xie, and Kiyoshi Funatani 167. Handbook of Pneumatic Conveying Engineering, David Mills, Mark G. Jones, and Vijay K. Agarwal 168. Clutches and Brakes: Design and Selection, Second Edition, William C . Orthwein 169. Fundamentals of Fluid Film Lubrication: Second Edition, Revised and Expanded, Bernard J. Hamrock, Steven R. Schmid, and Bo 0. Jacobson 170. Handbook of Lead-Free Solder Technology for Microelectronic Assemblies, edited by Karl J. Puttlitz and Kathleen A. Stalter

Additional Volumes in Preparation

Mechanical Wear Fundamentals and Testing: Second Edition, Revised and Expanded, Raymond G. Bayer Engineering Design for Wear: Second Edition, Revised and Expanded, Raymond G . Bayer Progressing Cavity Pumps, Do wnhole Pumps, and Mudmotors, Lev Nelik

Mechanical Engineering Sofhyare Spring Design with an IBM PC, Al Dietrich Mechanical Design Failure Analysis: With Failure Analysis System Software for the IBM PC, David G. Ullman

Preface

The inspiration and need for this book arose from our task at IBM of implementing leadfree/reduced products to be in conformity with legislation proposed by the European Union (i.e. the RoHS Directive), and from the growing worldwide concern about the toxic agents utilized in electronics manufacture and products, particularly lead. The lead (Pb)-free implementation issue is of paramount interest to the electronics industry due to the magnitude of the undertaking and the many unanswered questions that must be addressed. It is an area that has been much discussed in trade press publications, at workshops, and it has been the subject of portions or entire international conferences. Given the importance and level of activity worldwide, it is both timely and appropriate to assemble the information surrounding the key aspects of the lead-free issue, particularly as they relate to electronic assemblies, from among the most knowledgeable practitioners and researchers in the world. Virtually every major electronics component or product manufacturer is involved in activities in response to both legislative and market pressures to eliminate toxic materials, particularly lead, from their products. The purpose of this book, therefore, is to serve as a handbook/primer for those who are contemplating or have already started the process of conversion, from standard Sn-Pb solder microelectronic assemblies to Pb-free. The book serves as a single source that covers all key aspects associated with this issue, including health, total environmental impact, legislation, market economics, and the materials, process, and reliability challenges; not just a narrow focus on a particular aspect. Accordingly, the book also serves as a valuable resource for those actively engaged in procuring, manufacturing, and utilizing lead-free products in all regions comprising the world market. The audience for this book includes almost everyone either peripherally or directly involved in bringing microelectronics products to the marketplace, and users as well. This book would certainly be a valuable resource for individuals engaged in producing and marketing low-end products such as cameras, camcorders, cell phones, pagers, games, lapand desktop computers, and other consumer products. Some of these products have been targeted for conversion to lead elimination/reduction and are already available in the marketplace. Typically, these products tend to be high-volume items, destined for disposal in landfills. Although high-end product disposal at end of life is likely to be by recycling, or refurbishing for the secondary market, individuals involved with high-end products will also find the book very useful. While this handbook thoroughly discusses the technological aspects surrounding lead-free implementation, it also provides a detailed understanding of lead-related health issues, tradeoffs and implications in changing over to Pb-free/reduced assemblies; and global regulations, consequences, dispensations, and exemptions with iii

iv

PREFACE

regard to utilizing lead. Since the book contains material that relates to a wide range of electronics products from low- to high-end applications, the book will be useful to a very large audience. The book will serve as a primer for newcomers in the field, as it does not assume prior knowledge of the subjects discussed and provides the necessary background information. It will be a valuable resource tool for those already in the field. Specifically, the book is intended for:


Scientists and engineers engaged in research, development and design, assembly, manufacturing, applications, quality, reliability, equipment engineers and testing
Management, to gain a thorough understanding of the technology challenges, risks, and trade-offs
Marketing, sales, intellectual-property-oriented individuals, and others peripherally involved, who require a working knowledge or a general understanding of the issues
Students, for a thorough understanding of the forces that come into play in shaping business decisions such as ‘‘green’’ manufacturing, especially in electronics; materials technology fundamentals and issues, and potential solutions surrounding Pb-free solders This book is written in a manner that assumes the reader has some knowledge or facility with science or engineering-based principles, but does not assume prior knowledge of the material discussed here. In addition to serving as a handbook, it can be used for instructional purposes, with each chapter being a self-contained unit. That is, those key concepts or principles necessary to understand the material introduced in a chapter are briefly explained in the early sections of a chapter, or at the point of discussion. Also, brief segments may be introduced as restatements or summaries of concepts discussed in some detail in another chapter. This is done as a convenience to the reader, to avoid interruption in seeking information in other chapters. However, this book is well cross-referenced, providing the reader the opportunity for a more thorough explanation and understanding concerning a particular area of interest if desired. The introduction in Chapter 1 provides an overview of why the lead-free matter has become a worldwide issue, some concerns in introducing lead-free substitutes, and perspectives for a successful introduction. The book is divided into three parts, each addressing a major aspect related to the elimination of lead in electronic assemblies: the driving forces; the impact on manufacturing; and reliability-related factors. In Part I, ‘‘Background and Driving Forces,’’ Chapters 2 through 5 provide an understanding of the key issues associated with the worldwide lead-free movement, and the forces that are shaping and driving the movement. For example, Chapter 2 provides a detailed discussion of the toxic effects of lead on humans, how lead enters the body and its effect on various organ systems. Chapter 3 addresses the potential environmental impacts of substituting various materials utilized in microelectronic assemblies. In addition to health and environmental issues, there are also significant legislative drivers, both national and regional, discussed in Chapter 4 that manufacturers and users alike must be aware of because of the constraints and requirements they impose. The marketplace provides a significant driving force for lead-free products (Chapter 5), which is steadily increasing in importance due to worldwide identification with, and sentiment for, the removal of toxic materials in the manufacture of products. This driver may, in fact, ultimately require manufacturers to convert their products to lead-free long before legislation requires it.

PREFACE

v

In Part II, ‘‘Materials and Manufacturing Considerations,’’ Chapters 6 through 18 provide detailed discussions of the changes and impacts associated with various materials utilized, as well as manufacturing practices associated with the fabrication of microelectronic assemblies. Chapter 6 describes the key features of the Pb-Sn system, which the industry unanimously adopted worldwide and successfully utilized for the past 40 years, and serves as a reference or benchmark for comparing replacement systems. Chapter 7 defines the nature and mechanical properties that a solder material must collectively possess to meet the requirements imposed by solder joints in microelectronic assemblies at both the die and package levels. Chapters 8 and 9 describe the melting characteristics, microstructural features, morphology and other characteristics that determine the properties of interest for various binary, ternary and higher-order alloy systems considered as potential candidates to replace the Pb-Sn system. Although Pb-Sn solders possess many characteristics desirable for solder joints, they are not ideal, exhibiting low shear strength and creep resistance, particularly at elevatedtemperature application conditions. Chapter 10 describes several dispersion-strengthening methods for Pb-free solders to help overcome these traditional shortcomings of solder materials. Chapter 11 addresses the fundamentals governing wetting and spreading of liquids on solid surfaces and how they relate to lead-free solders on metallized terminal pads to assure sound connections. The mechanism and role of fluxing agents are also discussed. As a follow-on, Chapter 12 describes the metallurgical compatibility, processing advantages/disadvantages and reliability aspects associated with various types of lead-free board and component finishes in combination with several popular lead-free solder candidates under consideration. Chapter 13 discusses the method of formation, morphology, stability, and properties of intermetallic compounds (IMC) that form at solder/metallized surface interfaces that play an integral part in the quality and reliability of solder joints. Chapter 14 discusses the assembly differences that arise when utilizing lead-free solders as compared to eutectic Sn-Pb, and the impacts of those differences. Of particular interest are situations that necessitate and benefit from inert atmospheres (Chapter 15), which is a significant departure from standard eutectic Sn-Pb practice. Chapter 16 discusses some logistical and other impacts in changing over to a lead-free soldering assembly operation. The manufacturing experiences of various organizations that successfully introduced lead-free technology to their product lines are also reviewed. Numerous studies have been conducted by consortia around the world to determine which solder systems or alloys exhibited the most promise as suitable replacements for eutectic Sn-Pb solder. The major studies and their findings are reviewed in Chapter 17. It is anticipated that non-solder alternatives (Chapter 18) will play an increasing role in providing suitable, cost-effective interconnection solutions for some product segments, particularly in the area of consumer electronics. In Part III, ‘‘Reliability Considerations,’’ Chapters 19 through 22 describe a variety of reliability issues of concern in solders utilized in electronic assemblies, but particularly so for high Sn-based solders typical of lead-free solder alloys. Chapter 19 discusses general reliability concepts in relation to solder alloy composition, phasing, and morphology. Reliability aspects reported in the literature in relation to the use of lead-free alloys for flipchip and various package-level interconnections are also reviewed. Chapter 20 describes the factors associated with thermomigration and electromigration, which are of particular increased interest due to higher electrical power levels in flip-chip solder joints, and the introduction of lead-free solders. Tin-whisker formation and the potential for creating electrical shorts in the field is an issue mostly confined to Sn-rich finishes on components and boards. Chapter 21 describes how and why they form, and addresses the issue from the perspective that most lead-free replacement alloys are Sn-rich, and what the studies in this area have revealed. Finally, Chapter 22 provides a comprehensive explanation of various phe-

vi

PREFACE

nomena associated with Sn-based systems that can have significant reliability implications, among them, soft-error problems due to the presence of radioactive species, catastrophic degradation as a result of an allotrophic phase transformation, and structure-related issues. We are most grateful to our parents for their unending devotion, sacrifices, and love. They instilled the desire to seek the truth and enlightenment to better understand ‘‘how’’ and ‘‘why,’’ and to freely share our talents and knowledge with others in helping to give back what was given to us. We are also thankful to our spouses, Dianne Puttlitz and Mark Stalter, being ever mindful of their understanding and patience over the many months the manuscript was in preparation, and without whose support and encouragement the completion of this book would have been impossible. Finally, we are very appreciative of the effort put forth and information provided by the many contributors to this book who so generously shared their knowledge and expertise, and devoted so much of their precious free time to make this book a reality. Karl J. Puttlitz Kathleen A. Stalter

Contents

Preface

iii

PART I: BACKGROUND AND DRIVING FORCES 1. Overview of Lead-Free Solder Issues Including Selection Karl J. Puttlitz 2. Health and Environmental Effects of Lead and Other Commonly Used Elements in Microelectronics Edwin B. Smith III 3. Environmental Impact of Lead and Alternatives in Electronics Laura J. Turbini and Dennis Bernier 4. Environmental Stewardship with Regional Perspectives and Drives of the Lead-Free Issue Susanna Pelzel, Minna Juuti, and Yukio Sugimoto 5. Market, Product, and Corporate Policy Trends E. Jan Vardaman

1

49 83

115 149

PART II: MATERIALS AND MANUFACTURING CONSIDERATIONS 6. The Metallurgical Aspects, Properties, and Applications of Solders from the Lead–Tin System Paul T. Vianco

167

7. Physical Basis for Mechanical Properties of Solders Morris E. Fine

211

8. Sn-Ag and Sn-Ag-X Solders and Properties Karl J. Puttlitz

239

9. Bi-Sn, Sn-Sb, Sn-Cu, Sn-Zn, and Sn-In Solder-Based Systems and Their Properties Sung K. Kang vii

281

viii

CONTENTS

10. High-Temperature Lead-Free Solders with Dispersoids K. N. Subramanian, Sunglak Choi, and Fu Guo

301

11. Solder Wetting and Spreading Timothy J. Singler, Stephan J. Meschter, and James Spalik

331

12. Lead-Free Finishes for Printed Circuit Boards and Components Rob Schetty

431

13. Formation of Intermetallic Compounds at Pb-Sn/Metal and LeadFree/Metal Interfaces in Solder Joints Eric J. Cotts, Richard Chromik, Peter Borgesen, Robert Kinyanjui, and Anis Zribi

465

14. Electronics Assembly and the Impact of Lead-Free Materials Anthony A. Primavera

495

15. Use of Inert Atmospheres in Lead-Free Soldering Martin Theriault, Jason Uner, and Armin Rahn

569

16. Pb-Free Component Conversion and Some Manufacturing Experiences Kathleen A. Stalter, Tom Baggio, and Kenichiro Suetsugu

591

17. Major International Lead (Pb)-Free Solder Studies Carol A. Handwerker, Frank W. Gayle, Erik E. de Kluizenaar, and Katsuaki Suganuma

665

18. Electrically Conductive Adhesives–A Lead-Free Alternative Daoquiang Lu and C. P. Wong

729

PART III:

RELIABILITY CONSIDERATIONS

19. Reliability Aspects of Lead-Free Solders in Electronic Assemblies Puligandla Viswanadham, Steven O. Dunford, and Jorma Kivilahti

769

20. The Physics and Materials Science of Electromigration and Thermomigration in Solders James R. Lloyd, King-Ning Tu, and Jasvir Jaspal

827

21. The Structure and Kinetics of Tin-Whisker Formation and Growth on High Tin Content Finishes W. J. Choi, George Galyon, King-Ning Tu, and T. Y. Lee

851

22. Degradation Phenomena Michael J. Sullivan and Steve Kilpatrick Index Biographies

915

979 1015

Contributors

Tom Baggio

Panasonic Factory Automation, Tokyo, Japan

Dennis Bernier

Kester Northrop Grummen, Des Plaines, Illinois, U.S.A. Universal Instruments Corporation, Cleveland, Ohio, U.S.A.

Peter Borgesen, Ph.D.

Samsung Electronics, Seoul, South Korea

Sunglak Choi, Ph.D. W. J. Choi, Ph.D.

Intel Corporation, Santa Clara, California, U.S.A.

Richard Chromic, Ph.D. York, U.S.A.

State University of New York at Binghamton, Binghamton, New

Eric J. Cotts, Ph.D. York, U.S.A. Steven O. Dunford

State University of New York at Binghamton, Binghamton, New

Nokia Research Center, Irving, Texas, U.S.A.

Morris E. Fine, Ph.D.

Northwestern University, Evanston, Illinois, U.S.A.

George Galyon, Ph.D. New York, U.S.A.

Systems Development Group, IBM Corporation, Poughkeepsie,

Frank W. Gayle, Sc.D. Maryland, U.S.A.

National Institutes of Standards and Technology, Gaithersburg,

Fu Guo, Ph.D.

Michigan State University, East Lansing, Michigan, U.S.A.

Carol A. Handwerker, Sc.D. burg, Maryland, U.S.A. Jasvir Jaspal Minna Juuti

National Institute of Standards and Technology, Gaithers-

IBM Corporation, Hopewell Junction, New York, U.S.A. IBM Environmental Affairs, Sweden ix

x

CONTRIBUTORS

Sung K. Kang, Ph.D. T.J. Watson Research Center, IBM Corporation, Yorktown Heights, New York, U.S.A. Steve Kilpatrick, Ph.D. Physical Scientist, Sensors and Electron Devices Directorate, Micro-Devices Branch, U.S. Army Research Labs, Adelphi, Maryland, U.S.A. Robert Kinyanjui U.S.A.

State University of New York at Binghamton, Binghamton, New York,

Jorma Kivilahti, Sc.D.

Helsinki University of Technology, Helsinki, Finland

Erik E. de Kluizenaar, Ph.D. T. Y. Lee, Ph.D.

Philips CFT, The Netherlands

Hanbat National University, Daejeon, Korea (South)

James R. Lloyd, Ph.D. T.J. Watson Research Center, IBM Corporation, Yorktown Heights, New York, U.S.A. Daoquiang Lu, Ph.D.

Georgia Institute of Technology, Atlanta, Georgia, U.S.A.

Stephan J. Meschter, Ph.D. U.S.A.

BAE Systems Platform Solutions, Johnson City New York,

Studio Legale Paggi, Venice, Italy

Susanna Pelzel, J.D.

Anthony A. Primavera, Ph.D. U.S.A. Karl J. Puttlitz, Ph.D. New York, U.S.A. Armin Rahn, Ph.D. Rob Schetty

Universal Instruments Corporation, Cleveland, Ohio,

Corporate Environmental Affairs, IBM Corporation, East Fishkill,

Consultant, St. Catherines, Ontario, Canada

Advanced Technology Division, Technic Inc., Plainview, New York, U.S.A.

Timothy J. Singler, Ph.D. New York, U.S.A. Edwin B. Smith III

State University of New York at Binghamton, Binghamton,

Hart InterCivic, Lafayette, Colorado, U.S.A.

James Spalik, Ph.D. IBM Corporation, Endicott, New York, U.S.A. Kathleen A. Stalter U.S.A.

Technology Group, IBM Corporation, East Fishkill, New York,

K. N. Subramanian, Ph.D. Department of Chemical Engineering and Material Science, Michigan State University, East Lansing, Michigan, U.S.A. Kenichiro Suetsugu, Ph.D.

Matsushita Electric Industrial Co., Ltd., Tokyo, Japan

CONTRIBUTORS

xi

Katsuaki Suganuma, D.Eng. Yukio Sugimoto

IBM Corporation, Tokyo, Japan

Michael J. Sullivan, Ph.D. Martin Theriault

Consultant, Red Hook, New York, U.S.A.

Air Liquide America Corporation, Houston, Texas, U.S.A.

King-Ning Tu, Ph.D. U.S.A.

University of California at Los Angeles, Los Angeles, California,

Laura J. Turbini, Ph.D. Jason Uner

Osaka University, Osaka, Japan

University of Toronto, Toronto, Ontario, Canada, U.S.A.

Air Liquide Corporation, Houston, Texas, U.S.A.

E. Jan Vardaman

TechSearch International, Inc. Austin, Texas, U.S.A.

Paul T. Vianco, Ph.D.

Sandia National Laboratories, Albequerque, New Mexico, U.S.A.

Puligandla Viswanadham, Ph.D.

Nokia Mobile Phones, Inc., Irving, Texas, U.S.A.

C. P. Wong, Ph.D. School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, U.S.A. Anis Zribi, Ph.D. U.S.A.

State University of New York at Binghamton, Binghamton, New York,

1 Overview of Lead-Free Solder Issues Including Selection Karl J. Puttlitz IBM Corporation, East Fishkill, New York, U.S.A.

I.

INTRODUCTION

The use of lead (Pb) has been widely accepted in the electronics industry (utilized in a variety of applications for more than five decades), but most notably the use of eutectic tin–lead (Sn–Pb) solders to attach discrete components to printed circuit boards (PCBs). These attachments typically also serve as the electrical interconnection between the attached components and the PCB. Lead-based solders are also used as a coating or finish on metal terminations on PCBs, and on the peripheral leads of components.

A.

Lead Utilization and Consumption

Approximately 5 million tons of Pb are consumed annually on a global basis, of which storage batteries represent the largest single application (about 81%) by far. Lead oxides (for glass, ceramic products, chemicals, etc.) and ammunition together account for about 10%, and solders for electronic assemblies account for only about 0.5%, as noted in Table 1 [1]. Although storage batteries constitute a large percentage of consumption due to an alleged near-100% recycling practice, batteries have not been targeted for a ban because they are believed not to contribute to pollution or contamination, and hence do not pose a problem [2]. The largest use of Pb in electronics is in the manufacture of cathode ray tubes as a component of the glass that protects TV and computer monitor viewers from undue exposure to x-radiation. Only about one tenth of the total (0.05%) is currently used in solder alloys [3]. There are laws in the United States that control the use of lead in gasoline and plumbing solder, and banned its use in consumer paint since 1978 [4].

B.

Efforts to Restrict Lead Use

However, due to an increased awareness of the potential health hazards associated with the toxicity of lead in humans, which can lead to disorders of the nervous and reproductive systems, and also affect neurological and physical development, actions have been taken to eliminate or reduce the use of Pb in a variety of products. Among these are solders used for plumbing, and additives in paint and gasoline since the late 1970s. Banning or restricting the use of Pb did not become an issue until the early 1990s in the United States, but the proposed legislation to ban Pb was never enacted into law due, in large part, to pressures brought about by the electronics

1

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TABLE 1

Lead Consumption by Product

Product Storage batteries Oxides used for glass, ceramic products, pigments, and chemicals Ammunition Sheet lead Cable covering Casting metals Brass and bronze billets and ingots Pipes, traps, and other extruded products Solder (excluding electronic solder) Electronic solder Miscellaneous

Consumption (%) 80.81 4.78 4.69 1.79 1.40 1.13 0.72 0.72 0.70 0.49 2.77

Source: Ref. 1.

industry. However, there has been a growing worldwide movement to eliminate the use of lead in products due mainly to health-related concerns. This will now be mandated through European Union (EU) legislation banning the use of lead in electronics products with some exemptions starting July 1, 2006. Environmental issues, particularly lead elimination, had not received much attention in the electronics industry until the late 1990s when the initial Waste from Electrical and Electronic Equipment (WEEE) Directive was drafted by the EU. It became clear that responsible environmental practices can have a positive impact on business. Failure to embrace prescribed environmental practices would result in a degraded competitive posture and an erosion in market share. Cleaning procedures that were, at one time, an integral part of PCB assembly today have mostly converted to no-clean processes. This shift, like many others, benefits the environment while also contributing to the bottom line [5].

C.

This Handbook

This handbook discusses all the key aspects of the lead-free issue as it relates the implementation of lead-free soldering technology in the electronics industry. Among those areas discussed in detail are the background issues that give rise to the driving forces for lead-free implementation; the factors related to selecting alternative material systems such as flux, printed circuit board, board/component lead finishes, and lead-free solder alloys suitable for both reflow and wave soldering; and also manufacturing and reliability-related considerations.

II.

DRIVING FORCES FOR LEAD-FREE SOLDERS

A.

Health-Related Aspects

Candidate solder materials to replace eutectic Sn–Pb solders must satisfy toxicity regulations and limits set by the government. U.S. occupational exposure limits for selected materials are listed in Table 2 [6]. These are generally the same as for other industrialized countries (e.g., UK limits are the same for inorganic Pb) [6]. Despite the long-term utilization of Pb, it is now well recognized that Pb can have some very serious ill effects on humans. Depending on the aspects, Pb poisoning is sometimes classified as alimentary, neuromotor, or encephalic. Lead poisoning is the result of prolonged exposure to Pb or its compounds.

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION TABLE 2

3

Occupational Exposure Limits of Selected Materials

Material Zinc oxide fume Inorganic tin Copper dust Antimony and compounds Copper fume Organic tin Indium Silver dust and fume Silver, soluble compounds Inorganic lead Bismuth Gallium

U.S. Occupational Safety and Health Administration, permissible exposure lights (mg/m3) 5 2 1 0.5 0.1 0.1 0.1 0.1 0.01 0.05 Not assigned Not assigned

Source: Ref. 6.

1. Contaminated Drinking Water Historians have speculated that Pb poisoning played a role in the collapse of the Roman Empire because of the widespread use of Pb for drinking water pipelines [7]. However, this source of Pb was recognized early on as a source for ingesting Pb, so the use of Pb pipes and lead-containing solders for plumbing of drinking-water lines has been prohibited by federal legislation in the United States (Section 1417, Amendment to the Safe Drinking Water Act, June 19, 1986). a. Leachate from Landfills. Based on similar concerns, the electronics industry is quickly moving ahead with lead-free assemblies and components to avoid potentially Pb-contaminated drinking water. This time, however, the source is alleged to be Pb leached from landfills containing electronic wastes (lead from component finishes, printed circuit board assemblies, etc.) of consumer products such as TVs, PCs, telephones, camcorders, handheld games, refrigerators, microwaves, etc. It is contended that the leachate would find its way into the groundwater system with the potential of contaminating municipal water supplies, as illustrated in Fig. 1. b. Lead Concentration Limits. In Japan, the Pb elution environmental standard in landfills in 0.3 mg/L. In toxic materials detection tests conducted by the Japanese Environmental Agency, it was determined that the amount of lead leaching from the pulverized remains of TV cathode ray tubes, PCBs from PCs, and pachinko machines far exceeded the environmental standard [8]. In the United States, the regulatory limit for Pb in drinking water is 0.015 mg/L (EPA 40 CFR 141). In utilizing the Toxicity Characteristics Leaching Procedure (TCLP) per EPA 40 CFR 261, the concentration limit is 5 mg/L [2]. It has been demonstrated that the concentration of lead leached from solders can be several hundred times higher than the acceptable limit [9].

B.

Legislation

There are several forces driving the movement toward the elimination of Pb. An initial driving force is the fear of legislation, the most important of which is the EU’s proposal (RoHS Directive) to ban lead in all electronics products, including information technology (IT) products such as monitors, printers, and computers. The basis for the legislation is the allegation that Pb could be leached from electronic wastes discarded in landfills, and that the leachate will find its way into the groundwater system, ultimately contaminating the drinking water supplies of municipalities.

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FIG. 1 An illustration depicting the sequence of events that results in the ingestion of lead leached from electronic wastes in landfills. (From Ref. 69. Courtesy of Panasonic Corporation.)

This legislation, which will have a major impact on the electronics industry, is a precautionary measure because there are no known instances of water supplies having been contaminated in this manner. Nevertheless, there is a growing concern about electronic wastes and their Pb content because they are increasing at a rate three times greater than other solid wastes [10].

C.

Market Advantages

Japan has taken aggressive steps in instituting recycling laws to reduce the volume of wastes to be disposed. Because Japan is a small, highly populated country, it does not wish to utilize its precious space for landfills. Accordingly, it requires many products at end-of-life to meet prescribed specifications for disposition, including the removal of lead-bearing parts. To help reduce disassembly costs at end-of-life, Japanese original equipment manufacturers (OEMs) took the approach of introducing lead-free technology into their assemblies, particularly the solders used to reflow attach components to printed circuit boards, and lead-free components and board finishes. This strategy put the Japanese OEMs at the forefront of lead-free implementation. They have made lead-free products available in several niche markets, such as laptop and desktop computers, and some handheld consumer electronic products. Many observers of the lead-free solder issue feel that the market-driven aspect is more important than legal mandates because the market can force a transition to lead-free solders much more quickly as it can provide a substantial marketing advantage in some geographies, particularly for consumer products. Japanese electronics manufacturers have taken the lead in ‘‘green’’ marketing and manufacturing. Consider, for example, a lead-free minidisc player made by Panasonic, bearing a green leaf environmentally safe symbol that was released in October 1998. The product gained a significant increase in market share, moving from 4.7% of the minidisc player market to 15% in a very short time. Much of this success was attributed to the product’s green status. In Japan, products bearing a green sticker are typically not more expensive than competitors’ lead-bearing products—in some instances even less expensive. It appears that there are tradeoffs that can be made to work around cost issues [11]. Although not as pervasive as in Japan, there have been some early lead-free activities in both Europe and the United States as well. For example, the Ford Motor Company launched a large

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

5

green marketing campaign, having declared targets of 2002 for lead-free electronic assemblies, and totally lead-free vehicles by 2004 [12]. Green marketing is rapidly being viewed as a powerful and effective marketing tool. Many respected companies attribute some of their success to having taken a proactive environmental approach. Corporate executives are becoming increasingly aware that benefiting the environment can lead to bottom-line benefits. Among the benefits are a reduction in expenses and the creation of new sources of revenue [13,14].

D.

Mechanical Properties

There are technology-based reasons for eliminating Pb–Sn solders. The trend for continued densification in electronic packages is driving the need for even tighter pitches, which, correspondingly, requires finer-line circuitry. Because this trend results in solder joints with smaller cross sections, it also requires solders whose inherent mechanical properties are better suited to withstand the increasingly severe mechanical demands compared to eutectic Sn–Pb solder. It has been reported that many lead-free candidate solders exhibit significantly better strength and fatigue life properties than eutectic Sn–Pb [6].

E.

International Standards Organization (ISO) 14001 Certification

The international community is becoming ever more environmentally conscious, causing manufacturers and their materials and equipment supplies to develop products and processes that reduce the impact on the environment. The two primary factors are: the growing support for and embracement of the worldwide lead-free movement, and ISO 14001 certification. The certification is a standard for environmental management published by the ISO in 1996. It emphasizes establishing management systems that ensure environmentally sound business practices as opposed to focusing on specific performance targets. ISO 14001, whose attributes and features are listed in Table 3, can also be tailored to individual companies owing to its generic structure. The intent is to integrate the environmental management system (EMS) with companywide business management practices and decisions [15]. The certification supports techniques that are environmentally friendly. Electronics manufacturers are turning to vendors that can provide products that meet ‘‘green’’ standards. For example, the implementation of ISO 14000 prevents volatiles from flux to escape into the atmosphere, a problem aggravated by lead-free solders due to the higher reflow temperatures of furnaces. The reduced flux volume that results in enhanced throughput and efficiency also serves to preserve the internal cleanliness of reflow furnaces by minimizing interruptions for maintenance purposes. Some flux separation and collection systems can remove 95% of flux residues from an oven interior, and extend maintenance for 30–90 days in high-volume operations [16].

TABLE 3

Attributes and Features of Environmental Standard ISO 14001

Standard Sponsor Purpose of standard Emphasis Structure Intent Advantage

Description International Standards Organization Provide an environmental management system Environmentally sound business practices Can be tailored to individual organizations, big or small Integration of environmental management system with company-wide business practices and decisions Benefits the environment, and users are increasingly turning to products that meet ‘‘green’’ standards

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TABLE 4

Some Useful Standards Related to Lead-Free Technology

Standard Association of Connecting Electronics Industries, IPC-J0Std-006 ICP-J-Std-002, 003 MIL-M-38510 JEDEC J-Std-620A

F.

Subject Detailed list of both tin–lead and lead-free solders Board and component lead solderability Plating thickness for good component lead solderability Plastic package moisture sensitivity, heat tolerance

Standards/Practices/Information

Because lead-free technology entered the scene relatively recently, practices are still being established and few standards exist. Some useful standards that do exist are listed in Table 4.

III.

WORLDWIDE ACTIVITIES/PRACTICES

A.

Early Initiatives

Some of the efforts to ban lead were initiated in the U.S. Congress. In 1991, the U.S. Senate proposed the Reid Bill (S.391, Lead Exposure Reduction Act), which proposed restrictions on lead solders, limiting the lead content to 0.1% or less. Subsequently, the bill was modified (S.729, 1993) to propose a ban on all lead-bearing alloys including electronic solders, and a tax of US$1.69 per kilogram on primary lead and US$0.83 per kilogram on secondary lead used in the industry. Although Pb solders were ultimately removed from the bill due to intense lobbying by the U.S. electronics industry, it serves to provide global awareness that Sn–Pb solders may, at some point, be restricted. As a result, many companies worldwide instituted investigations addressing the feasibility of a variety of Pb-free solders [2]. Environmental consciousness was firmly in place in industries, including the electronics industry, which responded to several challenges, most notably the ban on chlorofluorocarbon (CFC) cleaning agents. Prior to 1992, flux residues from soldering operations were removed mainly by industry choice. After the Montreal Protocol and subsequent Amsterdam Regulations that banned manufacturers from using ozone-depleting chemicals effective January 1, 1996, the industry responded by implementing suitable solutions [17].

B.

Europe

In 1994, Denmark, Sweden, Norway, Finland, and Iceland signed a pact to phase out Pb, with Swedish manufacturers under a voluntary ban effective 2000. There are producer responsibility laws in effect in Holland, Switzerland, and Norway. In 1997, the Swedish government identified Pb as an element it intended to eliminate from products over the following 10 years. The Sweden Environmental Quality Objectives direct that new products, including batteries, introduced in Sweden should be mostly Pb-free by 2020 [18]. In 1991, the UK, in collaboration with GEC, BNR-Europe, Multicore Solders, and ITRI, and supported by the Department of Trade and Industry (DTI), undertook a project to make an initial assessment of alternative solders for electronic assemblies. That was followed by a European collaboration in 1994 to address issues associated with the implementation of Pb-free soldering technologies as did several large companies in the UK. 1. WEEE Directive The EU introduced a draft directive (law) called the WEEE, which called for a ban on Pb in all electronics, except automotives. The scope of the ban is far-reaching, banning the selling or

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

7

import of electronics containing lead, normally as part of interconnections that attach components to printed wiring boards (PWBs). But Pb can also be present as part of finishes on cards/ boards, peripheral leads of components, or materials used to fabricate components and peripherals (e.g., monitors). 2. Take Back/Recycling In parallel with initial investigations of lead-free alternatives that commenced in the early 1990s, there were also recycling laws proposed, mostly in Asian countries, that addressed the related issue of an ever-increasing volume of end-of-life electronic wastes to be disposed in landfills. Products must be designed for disassembly to remove parts containing environmentally objectionable materials, including Pb. Various product ‘‘take-back’’ schemes involving electronic wastes have been devised whereby manufacturers of these products are mainly responsible for the collection of wastes from designated locations, businesses, and households when owners decide to discard electrical and electronics products. So-called ‘‘producer responsibility’’ laws for electronics and electrical waste equipment are anticipated to be widespread in Europe. Such laws have already been in effect for some time in Holland and Switzerland, and are also in effect in Norway (1999) and Sweden (2000). Threshold limits for recycling specified materials are typically defined [2].

C.

Asia Pacific

There is a wide disparity in the approach to environmental matters among Asian countries, from virtually no regulations to strict legislation diverting discarded electronic products from landfills. In those cases where regulation is lax, it is most often due to a corresponding lack of infrastructure. For example, in 1995, Thailand had a staff of 43 to oversee 50,000 industrial plants. A reliance on self-regulation of the private sector is not uncommon in these areas where the trend is to adopt environmental standards such as ISO 14000, the environmental impact assessment required by the World Bank and the Asian Development Bank. Although ‘‘command-andcontrol’’ regulations will likely increase in the United States, Europe, Japan, and South Asian countries, other Asian countries will more likely use market-driven regulation and self-regulation because they have experience with these methods and lack necessary resources [19]. 1. Recycling Due to the very significant involvement on the part of the electronics industry, there is a high degree of interest and activity in the Pb-free issue in Japan, Singapore, South Korea, and Taiwan. For example, there has been a recycling law in effect in Taiwan since March 1998 requiring that all electronics products undergo proper waste processing at a product’s end-of-life. Japan enacted the ‘‘Electronics Appliance Recycling’’ law in June 1998 requiring appliance manufacturers by 2001 to collect and recycle their own products. Japan appears to be leaning toward high-technology disassembly coupled with material recovery (i.e., recycling) with many private-operated and government-operated recycling plants coming on-line. An example of these is the highly automated plant that went into operation in April 1998 that has the capacity to service a population center of about 1 million people, equivalent to about 150,000 units (TVs, washing machines, etc.) [19]. So as the EU is focused on a Pb ban, the Japanese Ministry of International Trade and Industry (MITI) is approaching the issue quite differently through socalled ‘‘take-back’’ legislation (i.e., recycling). In the European Directive, the manufacturer or importer is responsible for the cost of recycling [20]. 2. Lead Reduction/Elimination Whereas the official Japanese approach to the ‘‘Pb in landfills’’ issue is recycling, Japanese OEMs, and material and component suppliers, apparently motivated by the benefits of reducing disassembly costs at end-of-life by eliminating lead-containing parts and the specter of significant potential market opportunities, are moving in the Pb reduction/elimination direction. Early in 1998, the Japanese Electronics Industry Development Association (JEIDA) and the Japanese

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Institute of Electronics Packaging presented a Pb-free roadmap. Japanese OEMs announced their own plans to drastically reduce Pb levels in their products by 2002, and some even earlier [21]. The targets of several Japanese OEMs who were among the first to promote and adapt leadfree manufacturing to their products are listed in Table 5.

D.

North America

Since the initial attempts in congress to pass lead-free legislation in relation to electronic products, there has been little activity in the United States until the late 1990s, when it was clear that the Japanese were well positioned to exclude products from Japan that did not meet standards they might require owing to their early lead-free activities. Furthermore, because some Japanese lead-free products were being offered in some niche markets, it demonstrated the viability and manufacturability of lead-free technology, thus justifying the EU-proposed legislation to ban the use of Pb in electronics products. 1. Low Activity Level Early On Significant levels of lead-free activities were carried out over a wide range of consumer and industrial electronic products in the early 1990s. These initiatives were gradually abandoned in many areas due to the lack of legislative pressures to pursue alternatives to lead-containing products and materials, particularly solders for PWB assemblies. A notable exception was the automobile industry, where companies such as Delco, Ford, and Motorola continued to investigate and qualify lead-free products. Some of these activities were motivated by the desire to achieve higher strength and greater fatigue resistance characteristic of most lead-free alloys compared to eutectic Sn–Pb at higher service temperatures. Nortel Networks was one of the few North American companies to utilize lead-free solders in consumer electronics (Meridian phones). Texas Instruments (TI) provided Ni/Pd finishes for its component leads early on, replacing Sn–Pb coatings. TI also replaced the use of high-Pb solder for chip attachments to components with Ag-filled epoxies. These joints, however, are not suitable for high-power applications [19]. 2. Drivers Motivate Activity Aside from these few exceptions, it was both market-driven concerns and legislative concerns that served to revive interest in the lead-free issue among manufacturers in the United States. Although industry associations initially lobbied against the issue, they eventually supported it since the movement gained support worldwide. In fact, National Electronics Manufacturers Initiative (NEMI) initiated a very ambitious and aggressive lead-free effort that sought to select a suitable lead-free alloy, define a process window, produce populated cards/boards, and demonstrate the reliability of the solder joints of a broad variety of components through standard stress testing.

TABLE 5 Products

Several Early Manufacturers to Initiate Lead Reduction/Elimination in a Variety of

Company Hitachi

Action

Year

50% reduction from 1997 levels 1999 Eliminate Pb interconnects April 2001 Toshiba Elimination in mobile phones 2001 Sony All Pb eliminated, except high density electronics 2001 Matsushita (Panasonic) All Pb from products 2001 Been producing 40,000 minidisc players per month Pb-free October 1998 NEC 50% reduction from 1997 levels 2002 NTT No Pb or Cd in newly purchased equipment Now Source: Ref. 21.

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

9

An IPC statement probably reflects the opinion of most U.S. manufacturers, ‘‘. . . lead in electronics is not perceived as a health issue, but government and commercial drivers will push for its adoption anyway. Thus IPC will facilitate activities to enable it to happen’’ [22].

IV.

LEAD-FREE SOLDERS AND SELECTION CONSIDERATIONS

A.

Solder Alloys

1. Brief History The use of solders extends back to about 4000 BC and, as indicated in the timeline of Fig. 2, served as an important factor through the Bronze and Iron Ages up to the current so-called ‘‘Silicon Age.’’ The oldest archaeological evidence relates to gold-based hard solders since gold is impervious to corrosion, so an ample number of objects have been discovered. The earliest of these date back to about 4000 BC when gold-based hard solders were developed by artisans in Mesopotamia [24]. Both the Celts and the Gauls performed high-quality soft soldering using the same solder alloy. The use of Sn–Pb solders in ancient cultures is much more obscure, but by 350 BC, the Greeks sealed bronze-based water pumps and air pumps with Sn–Pb alloys, and the Romans used Sn–Pb solders extensively in the construction of aqueducts [25] and to close the seams of lead water pipes. They used an alloy, ‘‘terrarium,’’ consisting of one part Sn and two parts Pb, still used today for that purpose. There are references to lead–tin soldering going back to biblical days, where its use to solder water pipes is mentioned in the Old Testament [26]. There were few changes to soldering practice during the Iron Age and Middle Age; however, its use was greatly expanded during the Industrial Revolution, such as the use of soft solders for plumbing (i.e., copper tubing). Early in the 20th century, soldering was introduced to the electronics industry as a reliable method of connecting copper wires. More recently, the solder joint took on the dual function of serving both a mechanical and electrical connection, challenging the strength, creep, and fatigue resistance of Sn–Pb solders [23]. In electronics, soldering continues to be the predominant means to attach devices to printed circuit boards or microelectronic chip carrier packages.

FIG. 2 Solder utilization time line. (From Ref. 23.)

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2. Definition and Classification Soldering may be defined as the joining of metals by utilizing another metal, which has a melting point or range below about 450jC (1000jF) and is lower than the metals to be joined. This differs from brazing, in which the added metal generally has a melting range above 450jC (1000jF), but also below the metals to be joined [27]. Fig. 3 illustrates the melting ranges of some typical solder and braze materials. Solders are typically classified as either soft or hard. Some confusion often results from this convention because some hard solders are really braze materials if the melting temperature criterion is applied. Soft solders typically consist of alloys containing lead and tin, but also often contain indium (In), bismuth (Bi), antimony (Sb), or silver (Ag). In practice, most soft alloys melt at temperatures lower than 450jC, usually between 180jC and 300jC. High-tin solders, typical of lead-free solders, tend to be stiffer, harder, and less ductile compared to high-lead solders. Hard solders often contain metals such as Au, Zn, Al, and Si. 3. Traditional Electronics-Based Solders Lead-bearing solders had become the largest group of alloys utilized in electronics products; among these were eutectic (63Sn–37Pb) and near-eutectic (60Sn–40Pb) solders used extensively for PCB assemblies. The melting point or range of these alloys is sufficiently low to maintain the integrity of both components and laminate materials, such as FR-4, typically used for PCBs. There are some specialized applications of several high-Pb alloys (Table 6) utilized as flip-chip solder bumps (e.g., 97Pb–Sn, 95Pb–Sn), and as high-melt balls or columns (90Pb–10Sn) for ball

FIG. 3 Melting ranges for fusible alloys. (From Ref. 27.)

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION TABLE 6

11

Some Solders Utilized for Electronic Assemblies

Application

Solder alloys

Flip-chip solder bumps BGA solder bumps

Soldering to thick film metallizations (e.g., Au–Pd, Au–Pt, Ag–Pd) Soldering to thin films (e.g., Cu, Ni, Au) Low-temperature operations (e.g., attaching temperature-sensitive components, last step in a temperature hierarchy)

Pb–3Sn, Pb–5Sn, Pb–50In, eutectic Sn–Cu Dual-solder bumps: Pb–10Sn (ball) and eutectic Sn–37Pb (ball and card attachments) Single-solder bumps; eutectic Sn–37Pb, eutectic Sn–Pb–2Ag, Sn–Ag–Cu Pb–10Sn, Pb–50In Eutectic Sn–37Pb, Au–20Sn Eutectic Sn–Bi

and column grid array components attached to cards or boards with a low-melt solder (eutectic Sn–Pb) [28]. In all Pb–Sn alloys, Pb is not a metallurgically active species (i.e., does not enter into any reactions with bonding pads or surface metallizations), so that there are no interfacial intermetallic compounds (IMCs) containing Pb, nor any such compounds dispersed within the bulk solder. The presence of lead, however, affects the solid-state growth kinetics of Sn–Cu intermetallic compounds. These factors are all discussed in detail in Chap. 6. The presence of Pb also helps to prevent the white tin (h)-to-gray tin (a) allotropic phase transformation from occurring in high-Sn solders upon cooling below 13jC. The loss of structural integrity that accompanies this transformation during cold-temperature service is referred to as tin pest [29]. The details of this phenomenon are discussed in Chap. 22. There are several non Pb–Sn solders utilized in electronics for specialized applications. One of these, 50Pb–50In, is utilized for hybrid microcircuit applications involving soldering to precious metal (Au–Pd, Au–Pt), thick film, and thin film circuits [30]. One advantage of this solder is that its Au thieving (dissolution rate) is much slower than Sn–Pb solders. a. Role of Lead in Lead–Tin Solders. Lead plays a very important and unusual role in Sn– Pb solder alloys in general, as noted in Table 7, and in solder joints of electronic assemblies in particular. As noted earlier, Pb does not participate in the formation of any intermetallic compounds, neither at the terminal ends of solder joints nor distributed within the bulk solder. Pb is effectively a ‘‘neutral diluent’’ for Sn. It reduces the melt temperature of alloys with Sn, and improves the mechanical characteristics of Sn as a solder joint material (i.e., reduces stiffness, increases plasticity, etc.) without significantly affecting solder chemistry (i.e., interactions between Sn and terminal metallizations, namely Cu and Ni) [31].

TABLE 7

Role of Lead (Pb) in Tin–Lead Solders

Feature Melting point Surface tension Activity Cost

Comments Reduces the melting point of Sn Reduces surface tension of molten Sn–Pb solders, providing their characteristic good wetting properties Does not participate in reactions with circuit metallizations (e.g., Cu, Ni) Intermetallic compounds formed by reaction with Sn Trade off for Sn content reduces alloy cost

12 B.

PUTTLITZ Lead-Free Definition

There is no universally accepted lead-free definition. In fact, even the EU has not provided (and is not required to provide) a definition until 1 year after the RoHS Directive goes into effect. However, various industry organizations have adopted a 0.1% maximum residual lead content. This value appears somewhat arbitrary (i.e., not based on some health-related threshold value).

C.

Selection Criteria for Suitable Lead-Free Solder Candidates

Lead-free solders must conform to a broad range of properties and characteristics to be considered as suitable candidates to replace eutectic Sn–Pb solders for use in electronic assemblies [31]. Lead-free solder candidates must also be compatible with product packaging trends of everincreasing densification to meet the demands for more functionality and portability. Decreases in package size correspondingly result in smaller solder joint interconnections. While undergoing size reductions, solder joints of high-density, high-performance electronic packages are expected to withstand increasing mechanical, electrical, and thermal burdens, which can lead to reduced reliability and even failure during manufacture or in the field [32]. There are two basic requirements that all solder interconnection alloys must satisfy. First, they must wet and bond to metallic terminals or features on components and substrates. This implies that the solder must be capable of forming intermetallic compounds with Cu, Ni, and other metals routinely utilized in microelectronic packages. Second, a solder must have a melting point that is sufficiently low to be reflowed as a paste when surface mount reflow attaching components to printed circuit boards, yet high enough to avoid any ill effects due to operating temperatures in the field. The most important characteristics that must be considered in selecting suitable lead-free solder candidates are listed in Table 8. Many of the lead-free solder systems of interest consist of some minor additions of third or fourth elements to binary alloy systems to achieve a desired benefit such as enhanced wettability or reliability, etc. 1. Toxicity Clearly, the toxic metallic elements cadmium (Cd), thallium (Th), and mercury (Hg) can be eliminated immediately. It should be noted that some geographies (e.g., Japan, Scandinavia) regard Sb as toxic [33]. Although the metal itself is generally not thought to be toxic, the trioxide compound—not typically formed at soldering temperatures but can be formed at incineration temperatures—is considered toxic. Because incineration is widely practiced in Europe and Japan to dispose wastes as opposed to using landfills, alloys containing Sb are not likely candidates for widespread application.

TABLE 8

Criteria and Desired Attributes for Candidate Lead-Free Solders

Criteria and desired attributes . . . . . . .

Not toxic Available in sufficient quantities to meet current and future requirements Exhibit sufficient electrical and thermal conductivity Possess adequate mechanical properties: strength, toughness, fatigue and creep resistance Compatible with typical terminal metallizations (e.g., Cu, Ni, Ag, Au, Sn, etc.) Affordable (i.e., economically viable) Have acceptable melt and process temperatures Not too different from eutectic Sn–Pb Avoid deleterious temperature effects on cards, boards, and components Low dressing characteristics

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

13

2. Supply and Abundance The number of viable elements to replace Pb is significantly reduced when factoring in the condition of sufficient abundance and availability. Availability refers to the quantity that is produced and therefore in most cases is available to manufacture products, whereas abundance refers to world reserves in the Earth’s crust. It should be noted that politics and a region’s stability can play an important role (i.e., a particular element can be both sufficiently abundant and be in substantial supply, but not readily available due to manipulation by cartels or restrictions imposed by governments). Reserve implies that, in addition to its state of abundance, the metal can be economically extracted by utilizing current technologies at the posted market price. Availability is an important consideration, as can be deduced from Table 9, which provides a comparison for those elements most likely to be substituted for Pb. Consider, for example, the case of Bi. If the entire world’s supply of Bi were utilized for microelectronics applications, only about 7800 tons of eutectic Sn–Bi could be produced, or only a maximum of 8% Bi could be added to Sn to produce a weight of solders equivalent to the use of eutectic Sn–Pb in electronics, about 60,000 tons. Thus, an alloy that contains a significant amount of scarce elements is not a viable replacement for eutectic Sn–Pb solder [6]. 3. Cost Because lead is one of the least expensive elements on Earth, any substitute will have some impact on cost, particularly so because it is likely that the substitutes will be from the group of elements Sn, Ag, Cu, and In. If either In or Ag is utilized, their contents must be kept low because both are relatively expensive when compared to lead, as noted in Table 10. Besides idium’s high cost (typically in the range of US$200.00 per kilogram), it also has a supply problem. Only about 200 tons of indium are produced globally each year [36]. a. Metal Density Factor. There is a significant difference on a cost-per-pound basis between the elements that comprise most of the leading lead-free alloy candidates and the baseline eutectic Sn–Pb solder. Solders are purchased on the basis of weight, not volume. For example, solder paste is typically purchased by the kilogram, and solder wire or bar is purchased by the pound. The candidate substitute metals are much less dense compared to Pb (Table 11). Similarly, Table 12 shows that the density of the most popular Pb-free candidate alloys is much less than eutectic Sn–Pb solders. For example, the popular lead-free alloy, Sn–3Ag–0.7Cu, is nearly 20% less dense. The implication is that 1 kg of eutectic Sn–Pb yields about 1000 jars of solder paste,

TABLE 9 Cost and World Reserves of Metals Utilized in Microelectronic Assemblies Metal Antimony (Sb) Bismuth (Bi) Copper (Cu) Gold (Au) Indium Lead (Pb) Nickel (Ni) Palladium (Pd) Platinum (Pt) Silver (Ag) Tin (Sn) Zinc (Zn) Source: Ref. 34.

World reserves (thousand metric tons)

Price (US$/lb)

3,200 260 650,000 77 6 130,000 150,000 79,000 — 420 12,000 440,000

0.68 3.50 0.89 4480 85.45 0.44 3.91 5700 8602 76.54 3.70 0.65

14

PUTTLITZ

TABLE 10 Relative Cost of Some Candidate Element Replacements for Lead in Electronic Assemblies Replacement elements for lead Lead (Pb) reference Zinc (Zn) Antimony (Sb) Copper (Cu) Tin (Sn) Bismuth (Bi) Indium (In) Silver (Ag)

Relative cost 1 1.3 2.2 2.5 6.4 7.1 194 212

Source: Ref. 35.

whereas it only requires about 0.8 kg of the Pb-free solder to yield 1000 jars of solder paste [37]. Thus, the increased cost for materials is substantially offset by the differential density factor because a solder joint requires essentially the same solder volume independent of whether it consists of lead-bearing or lead-free solders. b. Metal Processing Factor. The processing costs associated with fabricated products such as solder pastes often dominate over the material cost. Therefore, there is little difference between eutectic Sn–Pb and lead-free solder alloy paste, as is shown in Table 13. The table also shows, however, that the cost of products that do not have significant process costs associated with them, such as a bar stock utilized for wave soldering or a wire for repair operations, is dictated by the cost of raw materials [38]. 4. Melt Temperature and Range Both gallium (Ga) and mercury (Hg) can be eliminated from consideration as base materials just based on their low melting points. The melting point of the elements Ag, Cu, Ni, and Au are too high to serve as base materials and some are also too expensive, only leaving In and Sn. Indium is too expensive to use in quantities required of a base alloy. That leaves Sn, whose solder alloy melt points are typically between about 120jC and 280jC, providing a baseline for lead-free solder alternatives to eutectic Sn–Pb. Eutectic alloys with either Cu or Ag have melt points close to pure Sn, so only a few percent of these elements can be added to Sn before the melting point level is considered unacceptable [31].

TABLE 11 Relative Cost of Candidate Elements to Replace Lead in Electronic Assemblies Based on Volume Element Zinc (Zn) Lead (Pb) Antimony (Sb) Copper (Cu) Tin (Sn) Bismuth (Bi) Silver (Ag) Indium (In) Source: Ref. 37.

Metal cost per pound (US$, approximately)

Density at 25jC(lb/in3)

Cost per standard volume (US$/in3)

Cost factor relative to Pb

0.50 0.45 0.80 0.65 3.50 3.40 84.20 125.00

0.258 0.416 0.239 0.324 0.354 0.354 0.379 0.264

0.129 0.184 0.191 0.211 1.20 1.20 31.91 33.00

0.70 1 1.04 1.15 6.52 6.52 173.40 173.30

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

15

TABLE 12 Relative Cost of Some Candidate Lead-Free Alloys to Replace Eutectic Sn–Pb Solders in Electronic Assemblies Based on Their Densities

Alloy 62 Sn–37 Pb 42 Sn–58 Bi 77.2 Sn–20 In–2.8 Ag 91 Sn–9 Zn 91.8 Sn–3.4 Ag–4.8 Bi 90 Sn–7.5 Bi–20.5 Ag–Cu 96.2 Sn–2.5 Ag–0.8 Cu–0.5 Sb 96.3 Sn–3 Ag–0.7 Cu 95 Sn–3.5 Ag–1.5 In 93.5 Sn–3.5 Ag–3 Bi 96.5 Sn–3.5 Ag 99.3 Sn–0.7 Cu 95 Sn–5 Sb

Melting range (jC)

Metal cost (US$/lb)

Density at 25jC (lb/in3)

Metal cost (US$/in3)

Percent increase cost/in3 compared to 63Sn–37Pb

183 139 179–189 199 208–215 186–212

2.37 3.44 30.06 3.23 6.24 5.09

0.318 0.316 0.267 0.263 0.272 0.273

0.75 1.09 8.02 0.85 1.7 1.39

0 45 970 13 125 85

213–219

5.48

0.267

1.46

95

217–218 218 216–220 221 227 232–240

5.90 8.15 5.92 6.32 3.48 3.37

0.268 0.268 0.269 0.368 0.264 0.263

1.58 2.18 1.59 2.33 0.92 0.88

110 190 110 125 23 17

Source: Ref. 41.

To be compatible with standard surface mount and wave soldering processes, and to avoid PCB and component damage require a solder alloy melting point not higher than about 230jC, whereas operating requirements may be as broad as a 55jC to 155jC. This implies a minimum melting point of at least 175jC [31]. The compositions of some lead-free solder alloys and their corresponding melt temperatures are listed in Tables 14–Tables 16, covering melt temperatures between about 180jC and 200jC. Some higher-temperature compositions are listed in Table 17, but are only of utility for niche applications owing to such issues as cost (Au–Sn) and health concerns (Sb-containing alloys).

TABLE 13

Relative Cost of Some Lead-Free Solders in Bar Stock and Solder Paste Form

Solder alloy

Bar relative cost (US$/kg)

Paste relative cost (US$/kg)

Sn–63 Pb (baseline) 1 Sn–3.5 Ag 2.9 Sn–3 Ag–2 Bi 2.17 Sn–2.6 Ag–0.8 Cu–0.5 Sb 2.06 Sn–3.4 Ag–4.8 Bi 2.26 Sn–3.5 Ag–0.5 Cu–1 Zn 2.27 Sn–4.6 Ag–1.7 Cu 2.25 Sn–3.2 Ag–0.7 Cu 2.21 Sn–3.5 Ag–1.3 Cu 2.28 Relative cost of selected metals Pb=1, Zn=1.7, Cu=3, Sb=3.9, Bi=8.6, Sn=11, Ag=260, Au=15,000 Source: Ref. 38.

1 1.07 1.06 1.05 1.06 1.06 1.08 1.06 1.06

16

PUTTLITZ

TABLE 14 Some Lead-Free Solder Alloys with a Melting Point or Liquidus Temperature Below 150jC Composition Bi–33 In Sn–52 In Sn–50 In Sn–58 Bi Sn–57 Bi–0.1 Ag

Melting range (jC) 109.(E) 118.(E) 118.–125 138.(E) 138.–140

E=eutectic composition.

For lower-temperature applications, 42Sn–58Bi (eutectic, mp=139jC), near-eutectic compositions, and Sn–Bi with Cu, In, Au, Sb, or Zn additions are possibilities, but a special flux may be required because a standard flux may not be sufficiently active at lower temperatures [39]. a. Minimize Pasty Range. A solder with a discrete melting point is preferred (i.e., one with a eutectic composition). However, if this is not possible, the melting range should be no more than 10jC, and less than 5jC is desirable. Alloys with a so-called large pasty or mushy range are known to be susceptible to hot cracking during solidification, causing solders to lift away from annular metallization surrounding the tops of plated through-holes (PTHs), a condition referred to as fillet lifting [40]. Accordingly, eutectic or near-eutectic composition alloys are desired to prevent this condition. 5. Wettability Solder wettability is the capability of an alloy in the molten state to spread over the solid surface of another metal, usually metallization on a terminal pad. It is the precursor of ‘‘solderability,’’ which describes the solder’s ability to form a solder joint on a PCB. Joint formation includes such aspects as fillet formation between a vertical surface and a horizontal surface, and capillary flow of the solder as required to fill holes or gaps. a. Effect of Elemental Additions on Surface Tension of Tin. All high-Sn solder alloys exhibit a higher surface tension and are more chemically aggressive (i.e., greater tendency to form intermetallic compounds with termination pad metallizations) compared to eutectic Sn–Pb solder [41]. Lead has the important effect of lowering the surface tension of Sn–Pb solders. For example, the surface tension of eutectic Sn–Pb at 280jC is about 470 dyn/cm compared with 550 dyn/cm for pure Sn at its melting point of 232jC [42]. Minimizing surface tension improves wetting and solderability. None of the elemental additions made to Sn in forming lead-free alloys duplicates the effects of Pb in terms of lowering surface tension, thus is likely the reason that the

TABLE 15 Some Lead-Free Solder Alloys with a Melting Point or Liquidus Temperature Between 150jC and 200jC Composition Sn–20 Bi–10 In Sn–20 In–2.8 Ag Sn–8 Zn–3 Bi Sn–9 Zn E=eutectic composition.

Melting range (jC) 143.–193 175.–186 189.–199 198.5 (E)

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION TABLE 16 Some Lead-Free Solder Alloys with a Melting Point or Liquidus Temperature Between 200jC and 230jC Composition

Melting range

Sn–3.4 Sn–7.5 Sn–7.5 Sn–3.5

Ag–4.8 Bi Bi–2.0 Ag Bi–2.0 Ag–0.5 Cu Ag–3.0 Bi

Sn–3.5 Sn–2.0 Sn–2.0 Sn–3.4

Ag–5.0 Ag–3.0 Ag–4.0 Ag–4.8

Sn–3.5 Sn–3.0 Sn–3.5 Sn–3.5 Sn–3.2 Sn–4.7 Sn–2.5 Sn–3.8 Sn–2.0

Ag–0.7 Cu Ag–0.7 Cu Sb–2.0 Bi–1.5 Cu Ag–1.5 In Ag–0.5 Cu Ag–1.7 Cu Ag–0.8 Cu–0.5 Sb Ag–0.7 Cu Ag–0.8 Cu–0.5 Sb

Bi–0.7 Cu Bi–0.75 Cu Bi–0.5 Cu–0.1 Ge Bi

Sn–2.0 Ag–0.75 Cu Sn–4.0 Ag–0.5 Cu Sn–4.0 Ag–1.0 Cu Sn–3.5 Ag–1 Zn Sn–3.5 Ag Sn–2.0 Ag Sn–0.7 Cu Sn–2.0 Cu–0.8 Sb–0.2 Ag Sn–5 Sb

205.–210 207.–212 186.–212 206.–213a 216.–220a 210.–215 210.–215 216. 208.–216a 200.–216a 217.(E) 217.–218 218.(E) 218.(E) 217.–218 216.–218 213.–218 217.–219a 213.–219a 216.–222 217.–219 217.–219 217.–219 218.–221 221.(E) 221.–226 227.(E) 226.–228 232.–240a 236.–240a

E=eutectic composition. a Different values reported in the literature.

TABLE 17 Some Lead-Free Solder Alloys with a Melting Point or Liquidus Temperature Above 230jC Composition Sn–5 Sb Au–20 Sn Sn–25 Ag–10 Sb

E=eutectic composition.

Melting range (jC)

Characteristics

232.–240 280.(E) 365.(E)

Poor wetting Very expensive Low ductility Unsuitable thermomechanical fatigue resistance for die attach

17

18 TABLE 18

PUTTLITZ Effect of Temperature on the Surface Tension of Near-Eutectic (60Sn–40Pb) Solder

Conditions (jC) 211 260 260, with rosin (R) flux on Cu 260, with rosin mildly activated (RMA) flux on Cu

Surface tension (dyn/cm)

Reference

489.9 483.3 390 F 40 382 F 8

Schwaneke et al. [43] Schwaneke et al. [43] Vianco et al. [42] Vianco et al. [42]

wettability of lead-free alloys is typically worse when compared to studies that use eutectic Sn–Pb as the benchmark. b. Effect of Temperature on the Surface Tension of Pb–Sn Solder. Surface tension has not been found to be a strong function of temperature over the range of temperatures associated with typical solder assembly operations, as noted in Table 18. The wettability and solderability of lead-free solder candidates must be of sufficient adequacy to form solder joints whose reliability is not degraded relative to eutectic Sn–Pb joints due to wettability-related issues. 6. Minimum Complexity in Materials Fabrication It is significantly more difficult to manufacture powder for a multielement (ternary or higherorder) alloy than for a binary alloy. Consider, for example, that the powder for a Sn–Pb eutectic solder paste is prepared by melting ingots of pure Sn and Pb whose weight ratio is 63% and 37% in a pot, respectively. The molten solder alloy is atomized by a special process to form powder that is sieved to be in compliance with specified particle size limits. However, ternary, quaternary, and higher-order alloys require tight control and accuracy of composition. Manufacture will likely require an additional precomposing step. For example, to prepare an alloy consisting of three elements of weight percentages 96.3, 3.0, and 0.7, respectively (i.e., for a Sn–Ag–Cu alloy) demands a high degree of precision, which will result in additional cost [37]. 7. Minimum Metallurgical Complexity It is generally agreed upon that none of the lead-free binary systems investigated is likely to be a consensus replacement for eutectic Sn–Pb in assembly reflow applications, although this may be the case for wave soldering. Viable candidates typically consist of a small quantity of a third or fourth element added to a lead-free binary alloy compared to the relatively uncomplicated eutectic Sn–Pb solder.

TABLE 19

Required Mechanical Property Enhancements Beyond Eutectic Sn–Pb

Properties Shear strength

Fatigue resistance

Elevated temperature compatibility

Remarks .

Solder joints typically experience shear loading –Often due to coefficient of thermal expansion (CTE) mismatches . Increased number of consumer electronics products –Particularly communication, handheld devices –Experiences many on/off cycles . Increased applications involving harsh conditions (e.g., vehicle engine compartment) –Needs improved creep resistance and microstructural stability

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

19

a. Enhanced Properties Required. There are an increasing number of applications that require significantly enhanced mechanical properties, as listed in Table 19. For example, the operating temperature range for consumer and telecommunications electronics products normally can span the range from about 55jC to 100jC. In addition, there are critical applications such as avionics and automotive where under-the-hood, engine-mounted electronics typically experience many thermal cycles, which in some cases span the range from 55jC to 165jC and higher. Accordingly, there are now numerous demanding applications that require solder characteristics to exceed those of eutectic Sn–Pb [44–48], independent of the need to define a lead-free solder, making it necessary to mutually satisfy both requirements. b. Effect of Elemental Additions. Both Cu and Ag have a low solubility in Sn, forming intermetallic compounds instead, so there is little opportunity for solid solution strengthening. The hypereutectic alloys contain primary IMCs. The elements Zn, Bi, and In all exhibit some solid solubility in Sn, and so can contribute to strengthening the Sn matrix. They must be introduced as higher-order additions because the binary systems with Sn (e.g., Sn–In and Sn–Bi) have melting points too low for mainstream electronic assembly. Whereas Zn additions can cause several concerns (poor corrosion properties), the stability of ZnO necessitates the use of highly active fluxes, and pastes containing fine Zn particles can be expected to be unstable due to interactions between the powder and the medium, resulting in unacceptably short shelf-lives [33]. 8.

Adequate Properties for Operational Requirements a. Eutectic Sn–Pb Solder Was Satisfactory. For most applications, the processing and reliability characteristics of Sn–Pb-based solders have served the electronics industry well for more than 40 years, providing a comfort factor that will not be easy to emulate, although a

TABLE 20 Summary of the Base Metal Selection for Lead-Free Solders in Consideration of Key Elimination Factors Elimination factor Unstable

Too toxic

Abundance

MP too high/cost

Remainder

a b c

Metal

Symbol

Melting point (jC)

Cesium Rubidium Potassium Sodium Lithium Magnesium Mercury Thallium Cadmium Lead Gallium Indium Bismuth Copper Nickel Silver Gold Tina Zincb Antimonyc

Cs Rb K Na Li Mg Hg Tl Cd Pb Ga In Bi Cu Ni Ag Au Sn Zn Sb

29 39 64 98 179 651 39 304 321 327 29.8 157 271 1083 1453 960 1063 630 232 419

Tin (Sn) is the best choice, taking all key aspects into consideration. Zinc (Zn) is very active chemically, causes corrosion and oxidation issues. Antimony (Sb) has some components known to be very toxic.

20

PUTTLITZ

significant number of studies on lead-free alloys, some of which are discussed in Chaps. 8 and 9, have demonstrated more than adequate reliability characteristics [36].

D.

Major Lead-Free Candidate Solder Alloys

Although there has been a significant worldwide effort to identify a drop-in replacement for eutectic Sn–Pb solder, none has been found. Based on elemental selection criteria and considerations such as cost, abundance, toxicity, etc., discussed earlier and summarized in Table 20, the industry has focused on several lead-free solder candidates that appear suitable for the assembly of most electronics products in consideration of the various factors noted in Section C Many of these alloys offer a distinct advantage over eutectic Sn–Pb, such as high joint strength, enhanced fatigue resistance, improved high-temperature life, and harder solder joints [36]; all are noted as desired and sought-after characteristics in Table 19. Both tin and indium would serve as good base materials in that both readily wet the metallizations used for terminations in electronic assemblies and form IMCs with those metals to achieve good bonding. However, there is both a cost and a supply issue associated with In, thus leaving Sn. Another advantage is the existence of substantial experience and database in the use of tin solders compiled over a long history. Tin typically cannot be utilized elementally as a solder owing to its high melting point (232jC). Elemental additions are therefore necessary to reduce the melting point of Sn. Typically, Ag, Al, Bi, Ga, and In are used for that purpose. Other elemental additions such as Ni and Sb, which do not lower the melting point of Sn, are added to enhance the properties of the Sn matrix. The TABLE 21

Major Lead-Free Solder System Candidates to Replace Eutectic Sn–Pb

Alloy system Binary Sn–Ag

Su–Cu

Sn–Bi

Sn–Zn

Ternary Sn–Ag–Cu

Sn–Ag–Bi

Other additions Ag, Al, Bi, Ga, In, Cu Ni, Sb

Remarks Fairly high melt temperature (221jC) Alternative for wave soldering, but cost an issue Some reliability concerns related to Ag3Sn platelet growth Fairly high melt temperature (227jC) Wave solder candidate Only moderate wetting, but sufficient for most applications Not applicable as a general assembly solder due to cost, low melt temperature (138jC), and availability Good for low-temperature end of a solder hierarchy Cheap Lower melt temperature than most Pb-free solders Zn is highly active, presents potential corrosion and process concerns Leading candidate system for reflow soldering Lower melt point than Sn–Ag, Sn–Cu binary alloys Adequate wetting, mechanical properties Much reduced Cu scavenger characteristics compared to Sn–Ag Even lower melt temperature than Su–Ag–Cu Best fatigue characteristics among most popular Pb-free alloys Poses some reliability and end-of-life reclamation concerns Reduce melting temperature of Sn-based solders, and some enhance mechanical properties Do not reduce the melt temperature of Sn-based solders, but added to enhance properties

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

21

most popular Pb-free alloy candidates are listed in Table 21 and are discussed in the following sections. 1. Based on Sn–Ag System The most promising alloys identified are primarily based on the Sn–Ag system, with high tin contents, typically greater than 90%, and with a small addition of either Cu or Bi to create a ternary alloy. Sometimes a very minor addition, less than 1%, of a fourth element is made to enhance a particular desired characteristic, such as strength or reliability. In most cases, the binary eutectic (Sn–3.5Ag) is not adequate for several reasons, which include a high melting point (221jC) that would necessitate subjecting electronic assemblies to high temperatures during manufacture, and exhibits the worst wetting characteristics among high-Sn solder alloys [2]. However, there is a substantial experience base in the use of eutectic Sn–Ag for a variety of niche applications over the years. 2. Bismuth Addition to the Sn–Ag System Several alloys have been created through the addition of 5% Bi or less to lower the melting point of eutectic Sn–Ag. A typical example is Sn–3.4Ag–4.8Bi, which melts between 208jC and 215jC. In addition to lowering the melting point, Bi also improves the wettability of eutectic Sn–Ag. In fact, Sn–Ag–Bi alloys within the V5% Bi range generally exhibit the best solderability among the group of the most popular lead-free solder alloys [38]. The addition of Bi greatly increases the fatigue resistance of eutectic Sn–Ag solder. A fourth element such as Ge or Cu is sometimes added to improve alloy strength characteristics. However, the addition of Bi can also have adverse effects, as it can result in the brittleness of Sn–Ag–Bi alloys, and the presence of Pb on components and board finishes can form a lowmelting (96jC) eutectic, which in turn can result in poor fatigue resistance. In addition, fillet lifting has been observed at through-hole joints as wave-soldered joints cool down. Alloys with a solidification range, such as Sn–Ag–Cu alloys, are susceptible to this condition where the solder separates from the IMC layer. Although favored, even alloys without a so-called pasty range, such as eutectic alloys, are not necessarily immune to fillet lifting, as discussed in Chap. 17. Bismuth also has the tendency to easily oxidize, so fluxes that are active at lower temperatures should be used. 3. Copper Addition to Sn–Ag System The addition of minor amounts of Cu, typically in the range of 0.7%, lowers the melting point of eutectic Sn–Ag. The melting point of Sn–Ag–Cu alloys is slightly higher than Sn–Ag–Bi alloys, but is less costly. The ternary eutectic Sn–Ag–Cu alloy melts at 217jC. The presence of Cu in the solder greatly reduces the Cu dissolution rate from terminal pads with which the solder is in contact. In addition, the yields and reliability of eutectic Sn–Ag–Cu or near-eutectic alloys have been found to be at least comparable to eutectic Sn–Pb. These alloys are also reported to exhibit better solderability and reliability compared to eutectic Sn–Ag or Sn–Cu binary alloys [2]. 4. An Assembly Approach An approach which many in the electronic assembly industry believe is viable in utilizing lead-free solders consists not of a single solder, as in the past with eutectic Sn–Pb, but the choice of a small number of solders depending on the type of soldering operation. An alloy from either the Sn–Ag– Cu or Sn–Ag–Bi system is suitable for solder reflow operations (e.g., the attachment of components to PWBs by reflowing solder paste). The use of Sn–0.7Cu (mp=227jC) has been considered as a candidate for wave soldering applications because it is the least expensive of the lead-free alloy candidates of interest. Material cost is a key consideration in wave soldering because of the need to periodically change the entire contents of the bath due to contamination buildup. The alloy has soldering qualities equivalent to eutectic Sn–Pb. Although the eutectic Sn– Cu alloy also possesses the poorest mechanical properties among the lead-free solder candidates [2], it is sufficient for most applications. The Sn–Ag–Cu and Sn–Ag alloys can be utilized for wave soldering, and so, too, for rework operations.

Assignees

Harris Engelhard Willard Motorola IBM IBM IBM IBM IBM IBM Indium Co. Indium Co. Motorola Touchston Ford Lucent Lucent Lucent Seelig Seelig US Army

4758407 4778733 4806309 5229070 5328860 5344607 5393188 5411703 5368814 5414303 5455004 5580520 5410184 5435955 5429689 5538686 5589433 5698160 5352407 5405577 5520572

87 – 93 92 – 99 90 – 95 90 78 90 93 94 42 70 – 90 82 – 90 71 – 92 92 – 97 79 – 97 80 86 40 – 60 59 – 82 93 – 98 90 – 99 86 – 97

Sn (wt.%)

0.2t– 0.5 2t– 11 1.5t– 3.5 0.5t– 3.5 0.3t– 4.5

40t– 60

0t– 5

0t– 4 0t.5

2t– 4

3

2

0.1t– 0.5 0.05t– 3 0.1t– 0.5

Ag

0t– 1 4t– 15

1t– 4 5. 10 2 2 2 58 2t– 10 1t– 5

Bi (wt.%)

Partial List of U.S. Patents on Lead-Free Solders

US patent number

TABLE 22

0t– 9.3

5t– 15 5t

2 8t– 20 3t– 6 4t– 26

5t 10t 8

In

0.2t– 2 0.2t– 2

1 3

3t– 5

4t– 6

Sb

0.2t– 2 0.1t– 3 0.5t

3t– 8 3t– 15

1t 1t

3t– 5 0.7t– 6

Cu

16t– 30

9

4t– 6

Zn

Interim

Set (1)

Nl (1)

Others

Sn Sb Cu Sn Cu Ag Sn Sb Bi Sn Bi In Sn Bi In Ag Sn In Bi Sn Ag Bi Sb Sn Sb Bi Cu Bi Sn In Sn In Bi Sn In Ag Bi Sn In Ag Sn Cu Sn Cu Ag Sn Bi In Sn Zn Ag Sn Bi Ag Sn Zn Ag Sn Ag Sb Cu Sn Ag Sb Cu Sn Ag Bi In Cu

Major composition

22 PUTTLITZ

Iowa St. U. Mitsui Toyota IBM Lucent Ford

Ford Johnson Samsung KIMM Ford IBM Delco H-Tech Mitsui Hitachi Matsushita Alpha Fry Murata Fuji Elec

5527628 5658258 5733501 5730932 5762866 5755898

5833921 5837191 5843371 5851482 5863493 5874043 5938862 5985212 5993736 5942185 6077477 6086687 6139979 6156132

89 90 65 – 95 80 76 – 96 37 – 57 48 – 58 43 – 58 95 77 – 89 80 91 – 97 70 – 74 84 – 90 >75 91 – 95 72 – 87 81 – 91 >90 92 – 96 40 – 70 3t– 6 0t– 0.25 0t– 5

30t– 58

2t– 4

2t– 5 6.5t– 7.5 2.5t– 3.5

1t– 2 0t.6 3t– 4

3.5t– 7.7 1t– 4 0.8t– 5 3 1t– 6

2t– 3 10t– 23 5t– 10 0t–9.25

6t– 14 1t– 20

0.5t– 1.5 0.1t– 9.5 12 0.2t– 6 37t– 57 40t– 60 38t– 52

0.1t– 1.0 0t– 9.25

12t– 24 7t– 11 Dt-6

2t– 5 0.1t– 3

3t– 4 0.1t– 9.5 5 0.2t– 6 6t– 10 2t– 5 2

0t– 9.25 3t– 5 0t– 5

5t– 15 0.75t– 2

0.1t– 10

0.1t– 2 0t– 9.25 0.7t– 2.0 0t– 1

0.5t– 1.5 0.1t– 9.5

0t– 3

1t– 4 0t.6

1t– 4

Ge

0t– 0.2

0.5t– 2 3t– 5

0t.01

0.2t– 8

Tl (0.2) Nl (0.5) Nl (0.2)

Ga (<5)

Al Mg Ni(0 – 3)

Nl (0.6)

Sn Ag Cu Sn In Ag Bi Sn Sb Bi In Ag Sn Bi In Ag Sn Ag Bi In Zn Bi Sn In Sn Bi Ln Sn Bi Sb Cu In Sn Sb Ag Sn Bi Ag In Sn Bi In Zn Sn Ag Cu Nl Sn In Ag Sn In Ag Cu Sn Cu In Ga Sn Ag Bi Zn Sn Bi Zn Sn Bi Ag Cu Sna Sn Sb Cu Nl Sn Bi Ag Sb

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION 23

24 E.

PUTTLITZ Patented Alloys

A large number of patents relating to lead-free solder compositions and processing originate from Europe, North America, and Asia Pacific, as is evident from the partial list of U.S. patents granted, which is presented in Table 22. Accordingly, one or more patents cover many lead-free candidate alloys, whose composition lies within the general family of alloys in which there is substantial interest. Patented lead-free alloys cover a wide range of Sn, Ag, Cu, Bi, Sb, and other elemental combinations, including many in the Sn–Ag–Cu family of alloys. Interestingly, there are often gaps in the patent coverage of an alloy system, and in some cases, no real performance differences between those that lie within the patented composition range and those that do not. Therefore, users are urged to be aware of the patent coverage as it relates to the lead-free solder system they contemplate on utilizing in a product. The use of a patented alloy is anticipated to add between 2% and 8% of the cost of the alloy [37]. A nonpatented alloy may serve the purpose just as well. A partial list of patented and unpatented alloys in the Sn–Ag–Cu system is listed in Table 23.

F.

Conductive Adhesives Alternative

It is generally agreed upon that conductive adhesives that consist of a polymer material such as an epoxy that is filled with conductive metal particles will play an increasing role in the attachment of components to PCBs independent of the lead-free issue. This is particularly true in the case of games, consumer electronics, and some handheld communication products. The technology is uncomplicated, utilizing stencil printing techniques similar to solder paste deposition and therefore extendable to high production volumes. It is also generally agreed upon that conductive polymer lead-free alternatives are best suited for cost-driven, high-volume applications, as opposed to those where performance and reliability are key priorities. 1. General Categories There are general classifications of conductive adhesive materials based on the method by which conduction is achieved. Isotropic adhesives are the most common, and have been used in flexible connections. They normally contain about 70 wt.% Ag particles, but other metals are used as well [6]. The quantity of metal loading is sufficient to assure that the adhesive is electrically conductive in all directions upon curing, as illustrated in Fig. 4, hence the term isotropic. This is in contrast to adhesives with much lower metal filling, such that there are an insufficient number of particle-toparticle contacts to render the adhesive conductive, as depicted in Fig. 5a. However, if the adhesive is sandwiched and compressed between a pair of terminal pads, the particles between the

TABLE 23

Patent Status of Some Alloys in the Sn–Ag–Cu System

Allow composition (wt.%) Sn

Ag

Cu

Melt temperature (jC)

Patent status

93.6

4.7

1.7

216–218

95 96.5 95.5

4.0 3.0 4.0

1.0 0.5 0.5

217–219 — 217–219

95.5 96.3 95.75

3.8 3.2 3.5

0.7 0.5 0.75

217–219 217–218 —

Ames Labs, covers any alloy containing 3.5–7.7% Ag and 1–4% Cu Ames Labs Harris Brazing Co. Unpatentable, published 50 years ago Unpatented Unpatented Senju

Source: Ref. 2.

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

25

FIG. 4 Illustration depicting an isotropic conductive adhesive alternative.

pads are made to contact each other or the terminal pads. Under these conditions, the adhesive is electrically conductive between the terminal pads, but not to either side of the terminal pads, as depicted in Fig. 5b. Because the conduction path is limited in these types of conductive adhesives, they are referred to as anisotropic. 2. Advantages Conductive adhesive materials are very cost-competitive with solder materials, only require a relatively low-temperature cure, and leave no residues, and thus require no subsequent cleaning step. This is a significant advantage compared to lead-free solders that require a high-temperature reflow and, depending on the flux utilized, may also require a subsequent cleaning step. The print speeds of conductive adhesives are comparable to solder pastes. 3. Disadvantages Conductive adhesives are not anticipated to make significant inroads into high-performance, high-reliability products, however, because of the physical and electrical instability they typically

FIG. 5 Illustration depicting an anisotropic conductive adhesive alternative.

26

PUTTLITZ

exhibit under thermal conditions. In addition, there are joint reliability concerns associated with their adhesive bonding integrity, moisture adsorption characteristics, and typically poor impact resistance. Small passive components attached to PCBs with conductive adhesives typically pass standard drop tests, but higher mass packages do not [21]. There are other issues as well, including a lack of self-centering as occurs during reflow with soldered components, and rework difficulties (i.e., removing and replacing components) [36]. Generally, having a nonsolder surface finish to bond a conductive adhesive improves contact resistance [49]. For example, conductive adhesives perform well with palladium-based terminated components. Lead-free board finishes such as nickel/gold and copper/palladium have been evaluated. Failures at the interface between component terminations and conductive adhesives is a typical failure mode observed in durability tests, and also the cause of an increase in electrical resistance. Oxidation and corrosion of Sn–Pb finishes take place at the interface [21].

V.

ISSUES AND CONCERNS IN IMPLEMENTING LEAD-FREE SOLDER TECHNOLOGY

There are several important issues that arise as a consequence of switching from eutectic Sn–Pb to a lead-free solder technology for microelectronic chips, packages, and assemblies. The key issues are discussed in this section.

A.

Implementation Approach

Implementing Pb-free manufacturing in electronic assemblies will occur over time (as noted in Table 24) in several stages that will have some overlaps. 1. Step 1: Lead-Free Solder There has been a significant worldwide effort to identify suitable lead-free solder candidates to replace eutectic Sn–Pb utilized for both the reflow and wave solder attachment of components to PWBs utilized in microelectronic assemblies. Because this effort has consumed much of the initial focus in the lead-free movement within the electronics industry, it is not surprising that the introduction of lead-free solders will represent the initial implementation step. It must be recognized, however, that electronic assemblies will, for the most part, be hybrids (i.e., while the solder is utilized for assembly, both reflow and wave solder operations will be lead-free). It is possible in some assemblies that nothing else will be lead-free, including the components and board finishes, as illustrated in Fig. 6. Early on, there was little agreement which lead-free alloy(s) should replace eutectic Sn–Pb, a variety of which were utilized in some niche products, as noted in Table 25. The benefits of having a consensus agreement for a very reduced number of alloys is, of course, recognized. In essence,

TABLE 24

Staged Implementation of Lead-Free Materials in Microelectronic Assemblies Lead-free

Implementation stage 1 2 3 4

Reflow solder

Wave solder

Component finish or I/O

Board finish

Component internal

Yes Yes Yes Yes

Yes Yes Yes Yes

Perhaps Perhaps Yes Yes

Perhaps Yes Yes Yes

No No Perhaps No

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

27

FIG. 6 Illustration depicting the combination of solders and finish materials achieved through reflow and wave soldering operations.

TABLE 25

Lead-Free Solders Utilized Based on the Industry Served

Solder Sn–Ag Sn–Ag–Bi Sn–Ag–Bi–Cu Sn–Ag–Bi–Cu–Ge Sn–Ag–Bi–X Sn–Ag–Cu

Melting range (jC)

Industry served

Company

221.–226 206.–213

Automotive Military/aerospace/consumer

206.–213 217

Military/aerospace Consumer Consumer Automotive

Visteron (Ford) Panasonic Hitachi Panasonic Sony Panasonic Panasonic Nokia Nortel Panasonic Toshiba Panasonic Panasonic Nortel NEC Panasonic Toshiba

Telecommunications Sn–Bi Sn–Cu

138 227

Sn–Zn

198.5

Source: Ref. 50.

Consumer Consumer Telecommunications Consumer

28

PUTTLITZ

that has occurred, having selected the Sn–Ag–Cu system with or without the addition of a fourth element as the most popular alternative solders selected. The Sn–Cu system has emerged as a lowcost alternative for wave soldering, and is compatible with most lead-bearing finishes, whereas alloys in the Sn–Ag–Bi system are considered attractive for surface mount reflow attachment applications and as a lower-operating-temperature alternative to Sn–Ag–Cu alloys. However, as noted in Section 4, there is a fillet lifting concern associated with Pb-based through-hole applications. Although the bulk of the industry’s soldering will likely be conducted with just a few lead-free alloys, it is anticipated that a variety of alloy systems will still be utilized to satisfy the needs of specific applications. The initial lead-free implementation step, therefore, is to introduce lead-free wave (bar) solders and solder pastes. 2. Step 2: Lead-Free Finishes The next phase will address eliminating lead from board finishes—the protective coatings applied to termination pads on printed wiring boards to protect metal conductors from degradation (e.g., oxidation, corrosion) and remain solder-wettable. Finishes are applied in a number of ways, including dipping into a molten metal bath (e.g., tin, solder), electroless plating, etc. Alternative finishes must, of course, be compatible with the lead-free alloy selected in Step 1. 3. Step 3: Lead-Free Components Completely eliminating Pb from electronic assemblies will require its elimination from components that utilize lead in some aspects integral to a component’s structure and function. Examples of this are lead used in wiring cable sheaths to provide resistance to degradation, lead used in monitor glass for protection against radiation, lead used as the dielectric material (PbTiO3) in some capacitors, etc. This last stage will require the longest time, owing to the number of alternative solutions that must be identified and demonstrated as suitable.

TABLE 26 Items

Effect of Higher Process Temperatures on Various Microelectronic Board Assembly

Board assembly items Components Ceramic chip carriers Organic chip carriers Specials: electrolytic capacitors, wound components, etc. Printed circuit board material

Plastic overmold material

Fluxes

Effects Little to none Degradation can be substantial, depends on Tg of material Increased moisture sensitivity level Susceptible to damage, typically designed for 230j maximum temperature, some up to 260jC Standard FR-4, Tgf140jC Subject to degradation Higher Tg materials Will survive proposed f240jC Typically will not survive z260jC Thermal plastics may undergo shrinking or warping, or cause critical features to move (creep) Thermal setting compounds, typically not affected by MSL-related effects Must be formulated to be active near and at the alloy melt point temperature Must not create charred masses that hinder soldering, or allow surface reoxidation

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION B.

29

Higher Process Temperatures

Most lead-free alloys of interest have a melt temperature that is about 40jC higher than eutectic Sn–Pb (mp=183jC), which can have a drastic effect on the integrity, reliability, and functionality of printed wiring boards, components, and other attachments (e.g., connectors) [18]. The bulk of lead-free research has focused on identifying suitable solder candidate alloys to replace eutectic Sn–Pb. Solder pastes, wires, and bar stocks are only a part of the issue. The effects of the increase in process temperature necessary for utilizing most Pb-free solders are listed in Table 26 and discussed in the following sections. 1. Effect on Components Electrolytic capacitors are very susceptible to high-temperature damage, as are wound components, such as relays [2]. Surface mount packages are not rated for temperature exposures beyond 230jC, but some survive up to about 250jC, and a few beyond that temperature. Pin-in-hole components are typically rated to survive up to 260jC. In standard reflow assembly practice, the solder liquidus temperature is less than 200jC (e.g., eutectic Sn–Pb, mp=183jC) and the reflow temperature is typically below 230jC. In addition, parametric damage can occur to memory integrated circuits (ICs) if exposed to process temperatures around 250jC [38]. 2. Effect on Organic Packaging Materials The higher processing temperatures required of lead-free alloys are incompatible with most organic-base materials utilized in microelectronic assemblies including laminate components and PWBs. FR-4 has been the laminate of choice for years, whose Tg is around 140jC. There are higher temperature-resistant laminates available, but at a higher cost. a. Plastic Components. Plastic components are also constructed of a laminate base and may experience internal damage in several ways when exposed to excessive temperatures. The most common of these is due to water that diffuses into the body of the component between the time that the seal on a plastic bag that contains a set of components and a dessicant is broken, and the time when it is oven-reflowed. The sudden increase in temperature during reflow causes the water to become converted to steam, which is accompanied by a significant increase in pressure within the package, because the gas (i.e., steam) cannot escape the package quickly enough. The pressure is relieved by causing cracks within the package, a process referred to as ‘‘popcorning,’’ due to the sound created. The cracks most often consist of the mold compound delaminating from the lead frame or die paddle. Cracks created due to popcorning are not unusual even when processing at eutectic Sn–Pb assembly temperatures (e.g., 200–225jC) if the moisture in parts is not properly removed through a bakeout procedure. Popcorning is anticipated to be aggravated at increased process temperatures required of most lead-free solder alloys. Cracks created due to popcorning typically are large and may cover half of the package dimension, so are likely to be revealed by electrical tests (i.e., large cracks usually sever one or more interconnections). Delamination, on the other hand, including small separations along a lead frame or die paddle, will usually not be immediately discovered by electrical testing. Delamination areas become collection points for atmospheric humidity and contaminants. Chemical reactions can cause a trace finger to corrode, metal migration between adjacent fingers, and other electrical problems, resulting in failure [51,52]. b. Package Overmoldings. Plastic molded electronic packages are also subject to thermal degradation, ranging from simple discoloration to a complete meltdown of the packaging material. Many thermal plastic materials utilized may undergo shrinking and warping, or have critical features that move (creep) when exposed to thermal profiles utilized when processing leadfree solder alloys. Molded electronic packages consisting of thermal setting compounds typically do not exhibit visible signs of distress due to lead-free processing temperatures, but still can be compromised due to the effects of popcorning discussed earlier. This, of course, puts the burden of package integrity on the end user, requiring that components be prebaked and/or that tighter

30

PUTTLITZ

process controls be utilized. Ultimately, the solution will be to formulate more robust molding compounds that withstand lead-free process temperatures [52]. c. Printed Circuit Boards. A material referred to as FR-4, whose Tg is approximately 240jC, has long been the PWB material of choice in the industry. There are two key concerns associated with the higher soldering temperatures: z-axis expansion and decomposition of the laminate. Higher process temperatures cause higher levels of z-axis expansion that may affect through-hole reliability. Excessive z-axis expansion can be prevented by utilizing laminate materials whose glass transition temperature (Tg) is compatible with increased process temperatures. Plastic materials typically undergo a very significant increase in expansion and a loss of strength at temperatures beyond the Tg, which can result in warping and mechanical instability problems. Decomposition, or the breaking of polymeric bonds in PWB materials, can also occur during high-temperature solder reflow operations. Some tests on laminate materials require a 280jC and higher exposure for 15–20 min. However, for most laminate materials utilized in microelectronics applications, exposure for a mere few seconds at those temperatures results in breaking of polymeric bonds within the material. Although the percentage of broken bonds is quite small (3– 5%), this can still lead to moisture inclusion, or even a degraded Tg. Higher process temperatures can cause several other PCB-related issues such as warpage, which can cause attachment problems especially with large components; it can adversely affect adhesives, markings, and the adhesion of copper circuitry [50]. 3. Effect on Flux Flux plays several important roles during a reflow operation to assure adequate solder wetting and bond formation between the solder and metallization on a termination pad. Its main function is to remove and prevent oxidation in the molten solder and pad surfaces to assure the exposure and intimate contact between fresh, uncontaminated surfaces, and to provide a protective layer to prevent reoxidation during the soldering process. Fluxes are formulated to exhibit their optimum activity over specific temperature ranges that normally are close to the solder melting temperature to reduce oxides. If the flux is exposed to moderate temperatures for long periods or excessive temperatures for even short periods, it forms a hardened mass of charred residue that prevents soldering and is not protective, allowing the surfaces to be reoxidized. Accordingly, many fluxes utilized for eutectic Sn–Pb solder would not be effective with lead-free alloys whose melting temperatures are substantially higher than eutectic Sn–Pb, causing poor wetting and leading to poor manufacturing yields [32]. For reflow processes, a flux should be compatible with peak temperatures above the liquidus temperature up to 250jC, whereas for wave soldering, a suitable flux for a lead-free solder must be capable of withstanding a 130jC preheat and a 280jC temperature in contact with the solder in the molten state for a period of approximately 3 sec [53]. 4. Dissolution of Terminal Pad Metallization Higher reflow temperatures increase dissolution rates that can result in an increased concentration of termination pads and surface finish elements in solder joints. Copper is the main circuitizing material utilized for PCBs and also some component lead frames (e.g., small outline—SO; quad flat pack—QFP; plastic leaded chip carrier—PLCC), owing to its good electrical conductivity and solder wetting properties. Tin has a high affinity for copper, so the high tin composition of many lead-free solders and their high melt temperatures have a much increased tendency to dissolve copper termination pads compared to eutectic Sn–Pb, as shown in Fig. 7 [20]. The lead-free alloys noted in Fig. 7 all have significantly greater Sn contents and melting points than eutectic Sn–Pb, which both serve to increase the Cu dissolution rate between about two and seven times faster compared to 60Sn–40Pb. Excessive intermetallic compound formation (Cu6Sn5) can adversely affect solder joint reliability due to the brittle nature of this compound, particularly under impulsive load conditions [54]. Nickel/gold is another popular board finish when combined with a gold finish on a component terminal pad. Care must be exercised to not exceed the rule-of-thumb Au threshold concentration in a solder joint (approximately 3%) [55].

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

31

FIG. 7 Dissolution rate of copper for several lead-free solder alloys relative to 60Sn–40Pb solders. (From Ref. 20.)

C.

Finishes

Lead-free manufacturing does not only pertain to solder pastes and solder bars for wave soldering, but also as a substitute for lead-bearing finishes utilized for component leads and solder bumps. Lead-free alloys are also used to pretin and protect the copper features on PWBs against oxidation. 1. Component Finishes In general, the change to lead-free finishes for components is anticipated to be a portion of electronic assemblies to convert to lead-free manufacturing later and is thought to be the most

FIG. 8 Illustration depicting typical wetting characteristics of peripherally leaded components with lead-free solders.

32

PUTTLITZ

difficult to implement. Component manufacturers have reliability concerns both in relation to the impacts of devising suitable processes to provide consistent, high-quality finishes with new materials, and also on the effects these finishes will have on the integrity of solder joints. As noted earlier, there is also the concern of the effect that increased process temperatures will have on the components themselves [53]. a. Peripheral Lead Frames. Many peripherally leaded components, as depicted in Fig. 8, utilize Sn–Pb finishes for peripheral leads. It is likely that lead-free solders will be implemented before component leads with lead-free finishes become available. Nickel/palladium, nickel/gold, silver/platinum, silver/palladium, pure tin, and nickel finishes have been utilized in the past and are likely to be key contenders in completely lead-free systems [26]. Several companies have supplied components with Ni/Pd terminations for several years. The Pd dissolves into the molten solder, wetting the underlying nickel. This works well in the case of eutectic Sn–Pb. Nickel/ palladium finished terminations have been evaluated with several lead-free solders and found to provide solderability comparable to eutectic Sn–Pb [52]. Tin–bismuth-based finishes have also been utilized with component lead frames for some time. These finishes exhibit excellent mechanical properties when used with Sn–Ag–Bi solders [56]. Several plated, tin-based lead-free finishes such as Sn, Sn–Ag, Sn–Bi, Sn–Cu, and others have been found suitable with both Cu and alloy 42 component terminations (i.e., lead frames) [57]. b. Ball Grid Arrays (BGAs). Area array components (i.e., ball grid arrays) typically consist of eutectic Sn–Pb of Sn–Pb–2% Ag solder ball terminations, as depicted in Fig. 9. It appears that the major lead-free solder candidates are also suitable as BGA solder ball materials. However, there are dual-solder versions utilizing a high-melt solder ball (90Pb–10Sn) that do not melt during the ball attachment to the component and component attachment to the card processes, each utilizing low-melting (183jC) eutectic Sn–Pb solder paste. The high-melt solder ball assures a substantial standoff height between the component and the card, which has a direct effect on enhancing the solder joint fatigue life. However, a suitable high-melt, lead-free alloy that meets the condition of low cost and whose melting point is within the range of 250–300jC has not

FIG. 9 Illustration depicting typical ball and column grid array terminations.

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

33

been identified. The only established lead-free alloy in this temperature range is 80Au–20Sn, which is too expensive to be utilized for this application [58]. There are concerns with lead-free terminated BGA components; among them are wetting and self-centering characteristics that impact placement accuracy requirements and manufacturing yields [49]. 2. Card/Board Finishes The PCB industry produces a number of lead-free finishes; among them are nickel/immersion gold (Ni/Imm Au), Imm Ag, Sn, Ni/Pd, and organic solderability protectants (OSPs). a. Hot Air Solder Leveling (HASL). Hot air solder leveling accounts for over 60% of the finishes on printed circuit assemblies (PCA). Tin–lead is the PCA material of choice to attach surface-mounted or through-hole components [58]. The PCB industry has been exploring alternatives to HASL to eliminate thermal shock to boards and components, which will be further aggravated by the increased process temperature required by most lead-free alloys. b. Plated Finishes. The trend is to provide thinner, more uniform, and planar finishes that enable fine-pitch assembly. These conditions are more easily achieved by utilizing plated finish solutions: 1. Nickel/gold—However, the Interconnection Technology Research Institute (ITRI) and others have investigated the failure of SMT parts, particularly eutectic Sn–Pb BGA component solder joint failures associated with electroless Ni/Imm Au metallization on card termination pads. The failures occur under mechanical shock test conditions. Given the root cause theory of failure, it is anticipated that lead-free solders will experience the same failure mechanism. Nickel/gold finishes are often utilized for higher temperatures and multisoldering purposes. The cost of Ni/Au is about 25% higher than for OSP finishes [53]. 2. Electroplated tin—Electroplated Sn exhibits excellent solderability and corrosion resistance, but is prone to Sn whisker growth. Thin, hairlike whiskers grow from the surface and pose as a potential reliability problem due to electrical shorts caused in the field. Heat fusing or reflow is used to eliminate tin whiskers [53]. c. Effect of Higher Process Temperatures. The higher process temperatures typical of lead-free solders may cause some lead-free surface finishes to react differently compared to eutectic Sn–Pb conditions. For example, after several reflow or rework cycles, the degree of protection offered by a finish could be compromised. OSP finishes, typically available at approximately the same price as HASL finishes, are easily processed and relatively free of organic contaminants. Oxidation problems can result upon exposure to several soldering processes, particularly at elevated lead-free soldering temperature conditions. Utilizing an inert atmosphere and reformulating OSPs to withstand higher process temperatures will help [53]. d. Solder Reactions/Process Effects. Another area of concern is the reaction rate between lead-free solders and terminal metallurgy systems. The formation of Ni–Sn IMCs at the interface is much slower compared to the formation of Cu–Sn IMCs. This is an important aspect to take into consideration because Sn content and process temperature are both significantly higher for lead-free solders compared to eutectic Sn–Pb, so the reaction rates are quicker. The formulation of excessive thicknesses of IMC layers is to be avoided to prevent weak solder joints. However, even with a reduced formation rate, a thicker Ni layer is required than for eutectic Sn–Pb solders [52].

D.

Process Challenges

1. Process Window As noted earlier, the preliminary challenge that lead-free solders present to IC and component manufacturers, and electronic assemblers is the higher process temperatures typically required of

34

PUTTLITZ

lead-free solders, as noted previously. The generally accepted limit that IC packages can withstand is approximately 235–240jC and the lower limit to reliably reflow eutectic Sn–Pb solders is about 200–205jC. This provides approximately a 30jC process window, which is sufficient in a properly monitored line to produce a low-defect-rate, high-yield product with little concern for creating process drift-related defects. However, with lead-free assemblies, the process window shrinks dramatically, as illustrated in Fig. 10. With bismuth-containing alloys, whose liquidus temperatures typically range between 206jC and 213jC, the window shrinks by one third, to approximately a 20jC operating range. For Sn–Ag–Cu alloys, whose liquidus is 217jC, the window is reduced by two thirds, with an operating range of only 10jC. Given that assemblers desire to remain within 5jC of their control limits, the true process window is very small. However, it has been determined that the soldering process can be significantly optimized by obtaining and analyzing data on a real-time basis in a reflow oven. Process control can be aided by utilizing a prediction software that customizes a profile for a specific solder paste, product type, and user-defined input process limit [12]. a. Processing at Lower Peak Temperatures. The industry initially assumed that a 260jC peak reflow temperature was necessary, predicated on the following reasoning. The eutectic Sn– Pb melting temperature is 183jC and the working temperature is typically 220jC, which corresponds to a superheat (temperature span above the liquidus) of approximately 40jC. Considering the fact that the most popular lead-free alloys (i.e., those based on Sn–Ag–Cu) have a liquidus temperature in the range of about 215–220jC, and applying a superheat of 40jC as a reflow temperature results in a suggested peak temperature of about 260jC. However, numerous studies [59–62] have reported that metallurgically and physically sound solder joints can be achieved by utilizing peak temperatures of 245jC or less, significantly below the 260jC initial de facto maximum requirement. b. Temperature Uniformity. Although adequate solder joints can be achieved at lower temperatures, the issue then becomes how well temperatures can be controlled to assure that all the solder joints of an assembly achieve the minimum reflow temperature prescribed within a process window. With eutectic Sn–Pb, it was permissible to have a variation of 30jC to accommodate various size components, boards, and board cross-section complexities, and still have a viable process (i.e., a window between 210jC and 240jC). However, owing to the approximately 40jC higher melt temperatures of Pb-free solders, the process window must be reduced to lower the peak temperature to a level where it does not degrade the integrity of the board and some components, and severely impact the moisture sensitivity of laminate materials. It has been determined that the peak temperatures that components experience during reflow

FIG. 10 Process window comparison between eutectic Sn–Pb and Sn–Ag–Bi solder alloys. (From Ref. 56.)

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

35

track best on the basis of their volume compared to their mass [63]. Accordingly, Joint Electron Device Engineering Council (JEDEC) defined a standard, JSTD-020, for the reflow attachment of components with lead-free solders: components whose volume is 350 mm3 or greater must be 245jC (+0,5) whereas those less than 350 mm3 must not exceed 250jC (+0,5). These limits are based on several studies conducted. It was demonstrated using a forced convection production furnace with a 12-zone heating system and two nitrogen cooling zones that assemblies consisting of a very broad range of component volumes populated on various board cross sections could achieve the JEDEC criteria. Table 27 indicates that all the components are maintained between 232jC and 245jC for a typical disc drive assembly (Assembly A) with a 6s2p card cross section. The components on a laptop assembly (Assembly B) consisting of a card with larger dimensions (8.49  8.05 in. vs. 5.3  4.3 in.) and an 8s2p cross section were maintained at a peak temperature between 231jC and 237jC. Two significantly more complicated cases were considered typical of server applications. Assembly C consisted of a large board (19.615.0 in.) with components ranging to 5700 mm3, but whose board cross section was the same as Assembly B (i.e., 8s2p). In this case, the peak temperature range was maintained at 227–238jC. The peak temperature range of a server assembly (Assembly D) with a smaller dimension card (10.6  6.0 in.) but with a 12s10p cross section and component volumes ranging to 12,300 mm3 was determined to be between 226jC and 245jC. The complexity represented in most of the test assemblies is much greater (e.g., card size, card cross section, and range of component volumes) than most typical applications. But the test conditions demonstrate that the DT across an assembly can be held to about 10–15jC in moderate to complex conditions. In many applications, DT can reasonably be held to about 5– 7jC in a manufacturing environment. To do so, however, requires that profiles for lead-free assembly must be appropriately modified. Useful modifications are to increase the preheat temperature and to lower the peak temperature to the extent possible. Rapid heating rates should be avoided (rates greater than 4jC/sec), which cause thermal instability and excessive demands on flux makeup [61].

TABLE 27 Peak Temperatures for a Range of Component Volumes When Attached to Various Assemblies using a Convection Belt Furnace Assembly A: disk drive 5.3  4.3 in., 6s2p Component volume (mm3) Temperature (jC) Assembly B: laptop 8.49  8.05 in., 86s2p Component volume (mm3) Temperature (jC) Assembly C: server 19.6  15.0 in., 8s6p Component volume (mm3) Temperature (jC) Assembly D: server 10.6  6.0 in., 12s10p Component volume (mm3) Temperature (jC)

1.2

3.3

26.4

34.5

50

100

240

274

274

1239

240

240

245

239

245

240

235

237

236

232

3.6

12.8

50.4

62

69

140

429

800

1634

235

237

237

233

233

236

238

234

231

1.2

26.2

68.2

226

226

270

369

1207

1400

2400

5700

5700

235

237

237

233

233

236

238

234

231

227

231

232

2.3

3.3

13.2

36.4

59

134

160

274

550

2143

7600

12,300

238

245

242

239

238

241

241

242

239

235

226

230

Source: IBM Corporation.

36

PUTTLITZ

2. Solder Hierarchy To facilitate the manufacturing of electronic assemblies and to increase both yield and reliability, it is a standard assembly practice that any process step requiring an elevated temperature, such as solder reflow or curing of a polymeric material, does not cause existing solder joints within an assembly to undergo melting. Therefore, if various components are to be added to an assembly at different stages of a process, solders with progressively lower melting points are utilized. For example, flip chip solder bumps (97Pb–3Sn, mpf322jC) are reflowed to ceramic BGA chip carriers, which, in turn, are attached to cards with eutectic Sn–Pb (mp=183jC). The flip-chip solder bumps do not become molten during the component-to-card attachment step. Utilizing a solder hierarchy prevents soldering at successive process steps from inadvertently melting previously soldered joints. Unfortunately, there are no high-temperature, lead-free substitutes other than 80Au–20Sn (mp= 280jC) a situation that causes board design and manufacturing issues. At the low end, 58Bi–42Sn is available for a low-temperature step [58]. 3. Equipment Accommodating the heating profiles of lead-free solders will require that some reflow and wave soldering equipment be modified or reconfigured. a. Reflow Soldering Machines. As noted earlier in Section 5.D.1 (Fig. 10), increasing the melt temperature causes the process window to become narrower. Efficient heat transfer becomes increasingly more important as the process window shrinks. A choice must be made among the common heating methods utilized in reflow ovens, namely radiation, hot air/nitrogen convection, or vapor phase. Radiation (lamps) will not meet lead-free requirements, and the throughput of a vapor-phase process is too slow and expensive due to liquid dragout replacement costs. Convection is the method most suited to reflowing lead-free solders. Meeting the process specifications depends on oven configuration, including heating and cooling lengths, number of zones, and other factors such as target throughput, board dimensions and complexity, loading (i.e., distance between boards), required exit temperature, and, of course, the solder paste composition and component specifications. Several peak zones may be required. The equipment should be configured that the heating set points are capable of achieving a nearly uniform assembly temperature (i.e., small DT across the board) when entering the peak zone. The goal is to maintain a board above the liquidus temperature for a sufficient period to allow the coolest spot on the board to solder properly while not exceeding the maximum limit at the hottest location on the board. Furnace design and configuration are very important. Reflow furnaces that utilize advanced technology capable of operating at high peak temperatures while still maintaining a relatively small DT across fairly large boards with components and attachments that range in size and weight are available, as illustrated in Table 27. Clearly, the alloy melting temperature plays a dominant role in the process, excluding alloys with high melting points. Accordingly, alloys such as Sn–Cu (mp=227jC) will probably not be utilized for reflow applications. Whereas alloys such as Sn–Cu–Bi (whose melting range is typically between 200jC and 216jC), Sn–Ag–Cu (mp=217jC), and Sn–Ag (mp=221jC) are better suited for this purpose. Reflow equipment utilized for lead-free PCB assemblies should be flexible, especially in the ability to manipulate and control heating and cooling rates, because temperature control is critical. In general, lead-free soldering suffers from a lack of standardization [53]. b. Wave Soldering Machines. Due to the high melting points of lead-free solders, wave soldering machines require a higher preheat temperature to assure that the thermal shock upon entering the first wave does not exceed 100jC. Both VOC-free or water-based fluxes are recommended for lead-free soldering because they are compatible with higher-temperature operation and are environmentally friendly. If a VOC water-based flux is to be utilized, it is be necessary to retrofit the fluxing machine with a spray fluxer because flux suppliers typically do not support foam fluxing VOC water-based fluxes. If not retrofitted, a robust, heat-resistant flux must be selected for foam fluxing, and foam tube and density control must be verified with the flux to be dispensed. During preheat, the topside of a board should achieve a temperature of about 110–130jC upon having passed through the last preheat zone to limit the thermal shock upon entering the

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

37

first wave. The board is to be heated to 100jC as quickly as possible to evaporate water in through-holes, areas underneath components, and between pallets and boards. Continue heating with forced convection to simultaneously equilibrate the assembly temperature. Depending on the type of production, the system may consist of a third preheat zone, consisting of hot air, infrared lamps (for mixed component assemblies), or calrod heaters. All water must be removed from these VOC fluxes before entering the first wave to prevent spattering. Achieving the necessary preheat conditions requires a minimum of 180 cm of heating length for conveyor speeds of 120 cm/min and higher. In the case of high-speed soldering (180 cm/min), a preheating length of 240 cm is required. If high throughput is a priority, which requires increased belt speeds, the preheat zone and the soldering temperature must be increased. The solderpot temperature, of course, depends on the alloy melting temperature. Therefore, for Sn–Ag–Cu (mp=217jC), the solder temperature will typically range between 250jC and 270jC. However, for high melting point alloys such as Sn–Cu, the soldering temperature will range between 260jC and 280jC. In general, the higher the solder temperature is, the better is the solderability and the shorter is the required contact time between the solder wave and terminal metallization. Solderability can be improved by using an inerting atmosphere such as nitrogen, a more aggressive flux, or a ‘‘smart’’ wave, as shown in Fig. 11. As the figure illustrates, the main difference between eutectic Sn–Pb and lead-free wave soldering is the increased preheat and solder temperature of lead-free alloys. If the peak soldering temperature is limited, as it typically is, then it is necessary to increase the solder wave contact time to achieve good wetting [53].

E.

Mixed Technology

It is anticipated that there will be a hybrid transition period where some components or board finishes will be a mixture of both lead-bearing and lead-free solders. Some studies have been conducted to evaluate the effort of mixing leaded and lead-free technologies on the same board. Some of these have included fairly complicated assemblies such as double-sided, FR-4 highdensity microvia test boards with an electroless nickel/immersion gold surface finish. Standard off-the-shelf components consisting of lead-bearing terminations were reflow-attached to the boards utilizing a Sn–Ag–Cu–Sb solder paste composition. Various area arrays, ceramic leadless chip carriers, standard leaded components (small outline integrated circuits–SOICs), and a variety of passives (resistors, capacitors, and inductors) were utilized. Mixing lead-free and Pb technologies increases the difficulty of predicting an assembly’s long-term reliability, but the results in this study [59] were encouraging. Good solder joints were observed at reflow temperatures even somewhat below 245jC without board and component damage. In addition, the reliability was superior to eutectic Sn–Pb solder joints based on accelerated thermal cycle (ATC) and drop tests.

F.

Inspection

Visual inspection, refined over the years to identify defects and faults that have been correlated to mechanical property behavior and reliability in the field, is a very important aspect of the electronic assembly process. Accordingly, a set of visual inspection criteria has been identified and generally practiced across the industry for eutectic Sn–Pb solder joints. However, the mechanical properties, visual appearance, and solder joint geometry of lead-free solders, in combination with lead-free and lead-containing terminations, are markedly different from those of Pb–Sn, as listed in Table 28. Therefore, new visual inspection guidelines must be developed [19]. Lead-free alloys typically exhibit a dull, mottled, and matte surface appearance compared to the bright and shiny appearance characteristic of Sn–Pb solder joints. In addition, lead-free solder joints tend to have a higher intermetallic compound content, in part due to the higher reaction temperature during processing. Additionally, lead-free alloys have been observed not to wet various termination metallurgies (e.g., Ni, Cu, Pd, etc.) typically utilized in microelectronic packages as well, compared to Sn–Pb solders. Lead coverage, fillet geometries, and working characteristics differ considerably for lead-free alloys compared to eutectic Sn–Pb [64]. Many pads exhibit ‘‘copper halos’’ because wetting does not take place at the edge of the pads. The

FIG. 11 Comparison of wave soldering profile for eutectic Sn–Pb and lead-free solders. The main differences are increased preheat and soldering temperatures. (From Ref. 53.)

38 PUTTLITZ

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION TABLE 28

39

Comparison of Physical Characteristics Between Sn–Pb and Lead-Free Solders

Features Visual appearance Dispersed intermetallic compound content Wetting characteristics Fillet formation

Pb–Sn solders Bight, shiny Low to moderate Excellent Wicks up lead, full fillet at lead heel and toe

Pb-free solders Matte, dull More due to higher Sn content and higher reaction temperature Fair to good, pads typically exhibit ‘‘Cu halos’’ Does not wick up a lead, partial fillet at lead heel and toe

presence of any of these conditions would be sufficient cause for rejection based on the standard inspection criteria applied to Sn–Pb solder joints. However, many of these differences may not be a cause for improper or unacceptable lead-free solder joints. Considerable learning will be necessary to establish standard inspection criteria and a thorough retraining will be necessary on the part of quality and manufacturing inspectors [59]. The reprogramming of automated optical inspection and x-ray systems is likely. For example, higher voltages may be required to compensate for the poorer image resolution afforded by lead-free solders. There have been reports of only minor software tweaks in some cases. The algorithms used to analyze solder joints are still all applicable. But lead-free alloys yield a lighter image because they absorb about 10% less energy compared to lead-bearing solders. This difference can be compensated for by modifications to the solder thickness tables of the inspection system software. There are other differences compared to Sn–Pb practice that also require resolution, such as darker and more persistent flux residues and fillet lifting in terms of acceptability [58].

G.

Rework

Rework is the term in the electronics industry that pertains to removing and subsequently replacing one or more of the components or other attachments on a PWB, as depicted in Fig. 12. The procedure typically consists of locally heating the solder joints of peripheral or area array

FIG. 12 Illustration depicting the replacement of a component attached to a printed circuit board (PCB) utilizing a localized heating technique.

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leaded components. When the solder joints melt, the component is lifted away from the board either by mechanical means or by a vacuum-activated device. A replacement component is locally reflowed to the same site. Sometimes the board is also heated from the underside to lower the thermal gradient and to help eliminate warping. Rework is an important aspect of manufacturing that helps meet required yields to achieve economic viability—another aspect that is anticipated to be substantially affected by a change to lead-free technology. Rework operators, such as inspectors, will require additional training, and development will be necessary because the properties and melting points of the solders can be significantly altered from component to component across an assembly because of elemental additions introduced to the solder from the PWB and terminal pad metallization systems. 1. Increased Temperature Effects As the melt temperatures increase, so does the difficulty of reworking the assembly without damage to the board, surrounding components, and other attachments. For example, it is estimated that for manual rework operations, it will probably be necessary for the soldering iron tip temperature to be beyond 300jC. Accordingly, rework operators will require training to develop skills to minimize the exposure time of temperature-sensitive replacement components to higher temperatures [64]. Studies (including one by the NEMI) focused on rework practices for removing and replacing through-hole and surface-mount components attached with lead-free solders have been reported. Of particular interest is the local degradation a PCB may undergo when subjected to multiple replacement cycles at the same component location on a board [65]. Repeated exposure to high localized temperatures may cause board delamination, thermal degradation (charring, brittleness), and delamination of copper traces (i.e., loss of adhesion to the board). 2. Reclamation Implications Ease of component removal also affects reclamation efforts. Cradle-to-grave ownership of products by their manufacturers or others in the chain of commerce requires a careful evaluation of the recycling cost of an assembly. The processing required to remove several components from an assembly can be substantially reduced by selecting components with a common termination metallurgy, so they can all be removed at a common removal temperature. The selection of a leadfree alloy for component and board finishes should be with subsequent reclamation in mind [59]. 3. Feasibility Demonstrated The rework of lead-free solder joints has been shown to be technically feasible. Concerns associated with locally heating printed circuit boards or attachments at high temperatures are mostly manifested in reworking heavy and complex components such as BGAs that require a high level of heat to effectively achieve removal and reflow. It appears that rework can be accomplished without damaging temperature-sensitive parts, and board warpage can be minimized if the process is optimized. The same issues apply to rework as general assembly operations, soldering temperatures, and other parameters must be tightly controlled. 4. Required Suitable Marking System Because there is no single lead-free drop-in replacement for eutectic Sn–Pb, there must be a suitable marking system to indicate the particular alloy (and its melt temperature) used for assembly. This is necessary for an operator to subsequently remove a component and select a metallurgically compatible replacement component (i.e., compatible with the rework alloy, component termination finish, and original assembly alloy). Numerous investigations indicating a variety of lead-free finishes compatible with the Sn–Ag–Cu family of alloys have been conducted [64].

H.

Cost

The potential cost impact of implementing lead-free technology is foremost on the minds of both manufacturers and users alike. A study conducted by the National Center for Manufacturing

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Science (NCMS) estimated that replacing Pb in electronic assemblies will result in a material cost of US$140–900 million dollars in the United States alone, with supply chain costs likely to run in the tens of billions of dollars [66]. Cost issues are related to materials (solders, components, boards, protective atmospheres during processing, additional energy consumption due to the higher process temperatures required for most lead-free solders, and operational costs). 1.

Solder materials a. Paste. As noted earlier, the cost of solders purchased as bar stocks depends on the raw material cost (Table 11), but for fabricated products such as solder pastes, the processing cost to manufacture the product is often the dominant factor, so the difference between Sn–Pb and Pb– free solder pastes becomes very small [2]. b. Bar Stock. The situation is different in the case of solder bar stock. Consider, for example, a wave solder pot that requires 760 kg of eutectic Sn–Pb alloy (density=8.4 g/mm3) to fill the pot at a cost of about US$4000. The cost to fill the same pot with any of several lead-free solders is given in Table 29. The Cu–Sn alloy has some cost–benefit over other candidates. c. Components. Changing the finish on peripherally leaded components is not perceived to be a major issue because several lead-free finishes have been available for some time. The process costs for lead-free component finishes are not anticipated to increase dramatically because the techniques to apply them are already in place. However, reliability issues such as tin whisker growth of some newer finishes must be thoroughly addressed. The solder balls for area array components (e.g., ceramic ball grid arrays—CBGAs; plastic ball grid arrays—PBGAs; chip scale packages—CSPs) do have, of course, a processing-related cost that will not have as much effect in blurring the cost differential between lead and lead-free solders as occurs with solder pastes. Like solder bars, the cost of solder will likely track more closely with the raw material cost. 2. Card/Board Materials In addition to converting to lead-free board finishes, the industry also desires to eliminate halogenated flame retardants as a board material due to the negative impact they have on the environment. Thus new boards that simultaneously meet the requirements of higher temperature resistance to be compatible with the higher reflow temperatures necessary for most lead-free solders, alternative flame retardants, and lead-free finishes are necessary. The new board materials will require higher glass transition temperatures (Tg) to maintain integrity and shape at the higher process temperatures. Accommodating all the required changes in the new cards and boards will result in a cost increase; the extent will not be clear until the material sections and their fabrication processes have been finalized and optimized [67]. 3.

Consumables a. Flux. Flux plays a major role in the quality of solder joint formation because it removes oxides and other materials to provide solderable surfaces with fewer defects. A ‘‘green’’ soldering process would typically utilize VOC-free, water-based fluxes, which have some advantages

TABLE 29

Cost Comparison Between Eutectic Sn–Pb and Several Lead-Free Solder Alloys

Solder alloy Eutectic Sn–Pb Eutectic Sn–Cu Eutectic Sn–Ag Su–Ag–Cu

Cost factor relative to eutectic Sn–Pb

Cost to fill solder pot (US$)a

1 1.28 1.35 1.45

3,960 5,068 5,346 5,742

a Assumes a pot capacity of 760 kg of eutectic Sn–Pb solder. Source: Ref. 67.

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compared to alcohol-based fluxes. Some studies have shown that VOC-free fluxes provide better results on lead-free solders than no-clean fluxes, particularly as they relate to residuals left on the board and solderability. There are less residues because the required flux volume is less. The activators and chemicals in the flux react more aggressively in water than alcohol. Although VOC-free fluxes are more expensive, the actual cost is about the same or less because the required flux volume is less. Cost savings also result from the fact that as solderability improves, the amount of rework decreases. In addition, a reduced amount of flux reduces maintenance: Cleaning can be achieved more easily and can be done with hot water rather than chemicals. To achieve the desired VOC-free fluxes for lead-free soldering, flux suppliers are developing ways to dissolve rosin into water-based fluxes [67]. b. Inert Atmosphere (Nitrogen). With the reduced solderability of lead-free alloys compared to eutectic Sn–Pb, the use of an inert atmosphere generally, but not always, improves wettability and has other benefits (Table 30) to improve solder joint reliability. The use of inert atmospheres along with glycol submersion minimizes dross formation in wave soldering machines. Excessive flow rates must be avoided in reflow furnaces to avoid a condition referred to as tombstoning in small passive components (e.g., chip resistors, capacitors, etc.). Tombstoning is a term used to describe a condition whereby a small rectangular component with a terminal at each end is lifted away from a board at one end. It remains soldered at the opposite end, and so is oriented perpendicular to the board, having an appearance similar to a tombstone—hence the term. In the end, a cost–benefit tradeoff analysis must be made for a particular product that considers the advantage of enhanced wettability, thus better yield and reliability, and reduced dross formation compared to the additional nitrogen cost. Use in reflow furnaces—The cost justification for utilizing inerting depends upon the degree of enhancement in wettability and the application’s reliability requirements. Geographic location can be an important consideration as well. For example, the cost of a cubic meter of nitrogen in Switzerland is about 10 times as great as that in Germany. When production flexibility is a priority, a reflow oven should be capable of both air and inert gas (i.e., nitrogen) operation. When the use of nitrogen is a significant cost consideration and it is not needed, it should not be utilized. However, with the trend toward reduced pitches in electronic assemblies and more complex designs, the flexibility to utilize nitrogen, if desired, should exist [67]. Use in wave soldering machines—Lead-free solders, such as Sn–Pb solders, oxidize very quickly in the liquid state. In both cases, the oxides form at the surface immediately after the oxide skin has been removed. Dross, the material formed at the molten wave in a wave soldering machine, consists of solder metal surrounded by an oxide skin. The tendency to form dross in the case of lead-free solders is greater than that in eutectic Sn–

TABLE 30

Considerations in the Use of Inert Atmospheres

Factors Inert atmospheres Benefits

Concerns

Remarks H2, N2, and mixtures of N2, H2, and Ar Increases solder wettability Reduces oxidation of solder and solderable surfaces due to: Increased temperature required for Pb-free solders Increased Sn content of Pb-free solders Reduced dross formation in wave soldering operations Cost, tombstoning of passives due to excessive flow rates

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Pb, owing to their greater tin content and higher melt temperatures to increase the reaction rate. By far, most oxides comprising dross are tin oxides (i.e., SnO and SnO2). Dross also forms at the solder pump shaft, but wave solder machines that seal this area to prevent its formation are available. Dross formation at the wave can be reduced by lowering the wave face height (i.e., the distance from the wave crest to the solder pool level). The use of nitrogen, therefore, provides several benefits in wave soldering applications: enhanced wettability and reduced dross formation. These factors must be balanced against the cost of inerting. It should be noted that losses due to dross formation can be significantly reduced because oxides only constitute a small part of dross. Dross can be compressed to partly separate the solder metal from oxide cells [67]. 4.

Operating Costs a. Energy Consumption. More energy is typically required to heat up a board assembly utilizing a lead-free solder compared to that of a eutectic Sn–Pb solder owing to the former’s high melting and process temperatures. The same is true for wave soldering, where there are two areas that require increased energy consumption. A higher preheat temperature is required to heat an assembly in preparation for wave soldering, up to about 25%. As in reflow ovens, due to the higher melting points of lead-free solders, higher solder pot temperatures are required in wave soldering machines with lead-free solders. The result of one study comparing the energy consumption for a pot operating at 280jC (for Cu–Sn) and 250jC (for Sn–Pb) is given in Table 31, where consumption measurements were made without a load (i.e., boards were not passed through the reflow oven). The higher melting points of lead-free solders necessitate up to 25% more energy consumption during the preheat stage to raise the assembly to an adequate temperature prior to being passed over the solder bath. In addition, because the solder melt point is higher, it is necessary to provide additional power to melt the solder [67]. Additional energy is also required if forced cooling is utilized subsequent to solder solidification. b. Throughput Impact. There are several important factors that contribute to a reduction of throughput with lead-free solders compared to eutectic Sn–Pb. In general, lead-free solders require longer contact times with the molten solder wave to achieve good wetting with the board features (i.e., termination pads, plated through-holes). A wave soldering machine can be retrofitted with a different wave former to increase contact time. If the wetting still is not sufficient, then the conveyor speed must be reduced, which also results in a lower throughput [67]. c. Repair. Typically, higher temperatures are required for repair operations to quickly heat a localized region to remove a component or other attachments mounted to a board, and reflow a replacement without melting the solder joints of neighboring components. As noted earlier, reflow and process temperatures of lead-free alloys are already significantly higher (about 40jC) compared to eutectic Sn–Pb. The temperatures necessary for rework only aggravate the situation because they are even higher.

TABLE 31 Comparison of Solder Pot Power Consumption for Eutectic Sn–Pb (250jC) and Eutectic Sn–Cu (280jC) Factors Power consumption to setpoint (kW hr) Time to setpoint (hr) Power consumption per hour at setpoint (kW hr) Source: Ref. 67.

Sn–Pb (250jC)

Sn–Cu (280jC)

34 3.5 5.0

36 5.5 5.0

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As pointed out earlier, the appearance and shape of lead-free solder joints differ from eutectic Sn–Pb. However, it should be pointed out that this has not caused an increase in defects requiring rework. This is true even though new defects such as fillet lifting occur with some lead-free alloys. Reliability tests have not indicated these defects, including fillet lifting, have lower quality, and thus are believed to not require repair [67]. More experience is required to verify that this is the case. d. Maintenance. Although some aspects of utilizing lead-free solder alloys will result in increased operating costs, maintenance is not anticipated to be a contributing factor. In fact, practices such as utilizing VOC-free, water-based fluxes may even result in reduced maintenance time and increase maintenance intervals compared to no-clean fluxes [67].

I.

Reliability

1. Lacking Sufficient Database Although the mechanical properties of eutectic Sn–Pb are not outstanding, they are adequate for most applications. This is important given the many other attributes a suitable solder must mutually satisfy. Additionally, a very substantial database has been put in place over the last 50 years from which both designers and users alike can draw upon in determining long-term reliability under a multitude of field conditions for eutectic Sn–Pb solder joints. There is no such database for lead-free solders, and even with accelerated testing, it will take years to generate an understanding equivalent to eutectic Sn–Pb. Possessing this level of understanding is admittedly less critical for consumer electronics applications such as games, mobile telephones, or other handheld telecommunication devices, etc.; but for applications requiring service lifetimes that span from a few years to a decade or more, this lack of information is of real concern. Not all leadfree alloys are devoid of a reliability database. For example, both Sn–Ag and Sn–Bi have been utilized in various applications for some time. Numerous studies have indicated that, in general, lead-free alloys including Sn–Ag–Cu exhibit reliability under accelerated testing conditions that are at least equivalent to eutectic Sn–Pb solder joints, and in many cases substantially better. The field data of several lead-free solders utilized in some niche products such as phones (Nortel), minidisc players (Panasonic), laptop and tabletop personal computers available from several manufacturers, etc., have not indicated any major reliability problems. However, more accelerated laboratory test and field data are necessary before drawing conclusions about the longterm reliability of these solder materials that have been used in a limited number of solder joint configurations and field conditions. 2. Information and Comparisons It is important to recognize in assessing and comparing the reliability results of tests, or those reported in the literature, that test conditions, regime, and materials are all taken into account. Contradicting claims regarding reliability can arise from inappropriate comparisons [19]. For example, it has been reported that significant wetting differences have been observed to exist for the same nominal composition lead-free alloy pastes but from different vendors, reflowed exactly in the same manner [68]. The poor wetting observed for an alloy that also exhibited good wetting occurred by utilizing solder pastes from different vendors. The wettability clearly is not related to the composition of the alloy, but rather to another factor, such as the effectiveness of the flux. However, in the early stages of lead-free technology, not all parameters were necessarily optimized in an experiment that resulted in erroneous characterizations and reliability results.

J.

Logistics

Although several lead-free solders have been identified (e.g., Sn–Ag–Cu and Sn–Ag–Bi for reflows; Sn–Cu for wave soldering), no single drop-in replacement has been identified with global consensus suitable for all aspects of assembly soldering: reflow, wave, and repair. The lack of a universally adopted lead-free alternative to eutectic Sn–Pb creates logistical problems in implementing lead-free soldering in electronic assembly operations. Contract manufacturers, who are increasingly assuming OEM solder assembly operations, will be faced with the need to

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run multiple assembly lines to satisfy the range of their customers solder requests. The logistical problem is very evident in the case of wave soldering, where emptying and refilling the solder bath is a nontrivial exercise [19].

VI.

SUMMARY

Although there are no easy answers for the rapid pace of the global electronics industry in moving toward the implementation of lead-free products, there are several reasons that can be identified: The fear of potential legislation and potential trade barriers—The proposed EU RoHS Directive will restrict the sale of lead-bearing electronic products, effective July 1, 2006 with some exceptions. The concern of competition and loss of market share resulting from market-driven pressures—Momentum has gathered due to a voluntary program initiated by many Japanese electronics OEMs to eliminate lead from their products and the very favorable preference for them in some geographies. The need for enhanced properties over those provided by eutectic Sn–Pb to meet the requirements of even more challenging service conditions such as high-temperature capability demanded my many automotive applications. The cost and process impacts in implementing lead-free electronic assembly, although significant, do not appear to be as great as initially perceived in many cases. These concerns are mainly based on the fact that lead is relatively inexpensive, and replacing it will add expense. It should not, however, have a major impact on the final product in most cases. In addition, as is typically the case for any new technology, the cost is eventually reduced. At first, it was also believed that it was necessary to conduct lead-free reflow and wave operations at 260jC temperature and above, which would have had severe implications on the structural and functional integrity of cards and circuit boards, attachments including components, the plastic component moisture sensitivity level (MSL), and activity of fluxes. It is now known that solder joints with adequate wetting, metallurgical characteristics, and mechanical properties can be achieved at much lower temperatures (i.e., in the range of 240jC) for most lead-free alloys of interest. This also greatly reduces the concern that significantly elevated process temperatures have on the infrastructure that is already in place. It now appears that most reflow ovens and wave soldering machines used for eutectic Sn–Pb assembly will be suitable, or only require relatively minor modifications. In fact, from various studies conducted globally by industrial organizations, consortia, government-sponsored laboratories, and universities, it has been demonstrated there are some substantial benefits for the use of lead-free solders over eutectic Sn–Pb such as enhanced strength and fatigue resistance properties, with higher maximum operational temperatures of finished products [58]. There will, of course, be many hurdles and issues to overcome, many of which are discussed in this handbook, in driving to implement lead-free soldering of electronic assemblies around the world. But very significant progress has been made in a relatively short period, which is also discussed in this handbook, with no known ‘‘show stoppers’’ having been identified. Long-term reliability remains a major unknown and is a key concern in the industry, notwithstanding early encouraging results reported in the literature. Given the momentum of green manufacturing and the worldwide support for the movement, it appears that the issue is when lead-free technology (including soldering operations in electronic products) will be implemented, and not if.

REFERENCES 1. 2.

National Center for Manufacturing Science. Lead and the Electronics Industry: A Proactive Approach, NCMS, Ann Arbor, May, 1995. Lee, N.-C. Lead-free soldering—where the world is going, Adv. Microelectron. September/October 1999, 29–35.

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3.

Ellis, B. Environmental issues in electronics manufacturing: a review, Circuit World January 2000, 26 (2), 17–21. Prasad, R. Lead-free solder, Surf. Mount Technol. January 2000, 14 (1), 26–28. Reed, G. The greening of SMT, Surf. Mount Technol. June 2000, 14 (6), 16. Wood, E.P.; Nimmo, K.L. In search of new lead-free electronic solders, J. Electron. Mater. 1994, 23 (8), 709–713. Sax, N.I. Dangerous Properties of Industrial Materials; 6th Ed. Van Nostrand Co.: New York, 1984. Lead-free roadmap—a scenario for commercial application, JEIDA, February 3, 1998. Smith, E.B. III; Swanger, L.K. Are lead-free solders really environmentally friendly? Surf. Mount Technol. March 1999, 64–66. Jones, Greg Are you ready for lead-free assembly? Surf. Mount Technol. June 2000, 14 (6), 60–62. Jorgensen, C. Lead-free electronics: full steam ahead. Surf. Mount Technol. May 2000, 14 (5), 87–89. Jones, G. The effect of lead-free assembly on the semiconductor industry. Chip Scale Rev. March/April 2000, 4 (2), 49–55. Environmental book to benefit the workplace, Surf. Mount Technol. June 2000, 14 (6), 48. Selvaduray, G.S. Is design for the environment strictly academic? Chip Scale Rev. September/October 2000, 4 (5), 15. Feilding, S. What is ISO 14001? Electron. Packag. Prod. April 2000, 40 (4), 22. Peo, M.; DeAngelo, D. Are you ready for lead-free reflow? Surf. Mount Technol. May 2000, 14 (5). Palmer, M.A. Alternative processing for forming solder joints with lead-free solder alloys. SMT August 2000, 14 (8), 61–66. Lead Free Soldering 1, HDP User Group International, Inc., Scottsdale, Project No. 032, Rev. A, June 1999. Linton, J. Asia and the environment. Circuits Assem. August 1999, 10 (8), 22–24. Grusd, A. Connecting to lead-free solders. Circuits Assem. August 1999, 10 (8), 32–38. Crum, S. Targeting lead-free solutions. Electron. Packag. Prod. 28–36. Buetow, M. The latest on the lead-free issue. Technical Source; IPC. 1999, spring/summer publication. Vianco, P.T.; Frear, D.R. Issues in the replacement of lead-bearing solders. July 1993, 45 (7), 14–19. Wolters, J. Zur Geschichte der Lottechnik; Degussa: Frankfurt, Germany, 1978. Racklam, H. Pliny’s Natural History; Heinemann: London, 1968; Vol. 33. Mahidhara, R. A primer on lead-free solder, Chip Scale Rev. Mar/April 2000, 4 (2), 40–41. Groover, M.P. Fundamentals of Modern Manufacturing; Prentice-Hall: Saddle River, NJ, 1996; 777– 779. Puttlitz, K.; Totta, P.A. Area Array Interconnection Handbook; Kluwer Academic Publishers: Boston, 2001 (Chaps. 8, 15, and 17). MacIntosh, R. Tin in cold service. Tin Uses 1966, 72, 7. Holmes, P.; Loasby, R., Eds.; Handbook of Thick Film Technology; Electrochemical Pub. Ltd.: Ayr, Scotland, 290–305. Harrison, M.R.; Vincent, J.H.; Steen, H.A.H. Lead-free reflow soldering for electronics assembly. Solder. Surf. Mount Technol. 2001, 13 (3), 21–38. McCormack, M.; Jin, S. New lead-free solders. J. Electron. Mater. July 1994, 23 (7), 635–640. Vincent, J.H.; Humpston, G. Lead-free solders for electronics assembly. GEC J. Res. 1994, 11 (2), 76– 89. U.S. Bureau of Mines. U.S. Geological Survey, 2000. Knapp, D. NCMS lead-free electronic interconnection program. Proc. Surf. Mount Int. 1994, 425–431. Suraski, D.; Seelig, K. The current status of lead-free solder alloys. IEEE Trans. Electron. Packag. Manuf. October 2001, 24 (4), 244–248. Zarrow, P. The true cost of lead free. Circuits Assem. June 2001, 12 (6), 20–22. Lee, N.C. Lead-free soldering of chip scale packages. Chip Scale Rev. March/April 2000, 4 (2), 42–47. Prasad, R. Lead-free solders and their properties. Spec. Suppl. Surf. Mount Technol. June 2000, 14 (6), 9–10. Zarrow, P. Lead-free: don’t fight a fact, deal with it. Circuits Assem. August 1999, 10, 18–20. Bulwith, R.A.; Mackay, C.A. Silver scavenging inhibition of some silver loaded solders. Weld. J. (Weld. Res. Suppl.) June 2000, 79 (6), 9–10. Vianco, P.; Hosking, F.; Rejent Wettability analysis of tin-based, lead-free solders. Proc. NEPCON WEST (Des Plains, IL) 2000, 3017. Schwaneke, A.; Falke, W.; Miller, V. Surface tension and density of liquid tin–lead solder alloys. J. Chem. Eng. Data 1978, 23. Millor, C.M.; Anderson, I.E.; Smith, J.K. A viable tin–lead solder substitute: Sn–Ag–Cu. J. Electron. Mater. 1994, 23 (7), 595–601. Jin, S. Developing lead-free solders: a challenge and opportunity. J. Met. 1993, 45 (7), 20–24. Winterbottom, W.L. Converting to lead-free solders: an automotive industry perspective. J. Met. 1993, 45 (7), 20–24.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69.

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Melton, C. Proceedings of the IEEE International Symposium on Electronics and the Environment, Month 1993; 94 pp. Kang, S.; Sarkhal, A. Lead (Pb)-free solders for electronic packaging. J. Electron. Mater. 1994, 23 (8), 701–707. Raby, J.D.; Johnson, R.W. Is a lead-free future wishful thinking? Electron. Packag. Prod. August 1999; 81–20. Jorgensen, C. Can you paint a rosy picture with green paint? P.C. Fab October 2000, 23 (10), 38–44. Tonga, Y.; Adams, T. Lead free processing: thermal effect on components. Spec. Suppl. Surf. Mount Technol. June 2001, (Guide to Lead-Free Soldering) 15, 10–11. Yang, L.; Berstein, J.B.; Chung, K. The impact of lead-free soldering on electronics packages. Microelectron. Int. 2001, 18 (3), 20–26. Barbini, D.; Diepstraten, G. Five steps to successful lead-free soldering. Circuits Assem. April 2001, 12 (4), 42–49. Humpston, G.; Jacobson, D.M. Principles of Soldering and Brazing; ASM International: Materials Park, OH, 1993. Glazer, J., et al. Effect of gold on the reliability of fine pitch surface mount solder joints. Circuit World August 1992, 18 (4), 14–46. Baggio, T.; Suetsugu, K. Guidelines for lead-free processing. Surf. Mount Technol. September 1999, 13 (9), 60–66. Parker, R.D. The next no-lead hurdle: the components supply chain, August 2000. http://www. circuitree.com. Bell, R. Lead free: will alternative alloys really work? Circuits Assem. September 2000, 11 (9), 120. Dunford, S.O.; Viswanadham, P.; Rantila, P. On the road to lead free. Circuits Assem. April 2001, 12 (4), 34–40. Tonapi, S., et al. Effects of flux and reflow parameters on lead free chip assembly. Proc. APEX, January 2001; AT5-2. Bartelo, J.; Cain, S.; Caletka, D.; Darbha, K.; Gosselin, T.; Henderson, D.; King, D.; Knadle, K.; Sarkhel, A.; Thiel, G.; Woychik, C.; Shih, D.Y.; Kang, S.K.; Puttlitz, K.J.; Woods, J. Thermomechanical fatigue behavior of selected lead-free solders. Proc. APEX, January 2001; LF2-2. Shina, S., et al. Selecting material and process parameters for lead-free SMT soldering using design of experiments. Proc. APEX, January 2001; LF1-1. Susko, R.; McCullen, J. Peak reflow temperature vs. package volume for lead-free solder data. Presented at JEDEC/JEITA Meeting, Arlington, VA, September 17–19, 2001. Richards, B.; Nimmo, K. Update 2000–an analysis of the current status of lead-free soldering—one year on, UK Department of Trade and Industry (DIT), London, www.lead-free.org/download/dti form 2000 html. NEMI lead free interconnect project: statement of work, National Electronics Manufacturers Initiative, Herndon, VA, May 9, 2000, www.nemi.org/PbFreePUBLIC/index.html/. Kallender, P. Lead a heavy problem at industry meeting. Print. Circuit Fabr. July 2001, 24 (7), 18. Diepstraten, G. Five steps to successful lead-free soldering: step 5. Circuits Assem. August 2001, 12 (8), 47–54. Hart, P.J.; Henderson, D.; Puttlitz, K.J.; Tonsi, N.; Sarkhel, A.; Woychik, C. Characteristics of some lead-free solder pastes for microelectronics package assembly. Semicon West 2000 (San Jose, CA) July 2000. Panasonic Corporation, http://www.semicon,panasonic.co.jp/lead-free/e-index.html#menu1.

2 Health and Environmental Effects of Lead and Other Commonly Used Elements in Microelectronics Edwin B. Smith III Hart InterCivic, Lafayette, Colorado, U.S.A.

I. A.

INTRODUCTION Background

Lead is used in many aspects of human endeavor. The primary use is lead-acid batteries, with lesser amounts used in ammunition, fishing weights, electronic and non-electronic solders and components, cosmetics, stained glass, and medicines [1]. Some uses, particularly as an automotive fuel additive, have already been banned in most industrialized nations. Lead use in food canning has also been banned by many nations. Uses such as electronic solders are also banned, in the process of being banned, or becoming more closely regulated. Other toxic metals used in electronics such as silver and antimony are currently regulated by several nations’ environmental authorities and may ultimately be further regulated or banned [2].

B.

Health Related Factors

Lead and other heavy metals such as silver, copper, cadmium, lead and bismuth have been known to have toxic effects on humans and our environment since ancient time [3]. These metals have become mainstays in the electronic world in which we live. As a result, human and environmental exposure to these metals has increased significantly over the past several decades, leading to significant health concern and environmental degradation. Lead and other metals enter the environment and the human body through several routes, which environmental and health regulations seek to reduce or eliminate. Several metals are essential to human and other life. Table 1 [4–6] categorizes these for humans into macronutrients, for which 100 or more mg/day are necessary for the body to function effectively and efficiently, and into micronutrients, which require only a few mg/day. Intake of less metal than required for the body to function adequately can result in both acute and chronic disease, while intake of excess metal can be toxic to the body, resulting in yet other diseases. The human body, as well as many animal and plant species, has metabolic pathways to remove the excess of some toxic metal elements. Animals and plants have their own essential metal requirements. If there are excesses or deficiencies in nutrient intake or the ingestion of

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TABLE 1

Metals Essential to Human Nutrition

Metal elements and human metabolic needs Nutrient elements (100mg/day or more): Ca, Cl, Mg, P, K, Na, S Micronutrients (no more than a few mg/day): Cr, Co, Cu, F, I, Fe, Mn, Se, Zn Possibly essential micronutrients: Ni, Si, Sn, V Generally recognized as toxic elements: Ag, Al, As, Ba, Cd, Pb, Hg

potentially toxic metals over time, vegetation, animals, and fish can adapt to degrading ecosystem conditions in their growth environment or discriminate against uptake; other species do not. The inability to discriminate can result in disease, diminished growth, and failure to reproduce. Some animals and plants accumulate metal elements in their tissues, referred to as bioaccumulation. Toxic metals bioaccumulated in the food web continue up the food chain to humans and other higher level consumers leading to disease, disabling effects, or death [7].

II.

TOXICITY MECHANISMS

A.

History

The toxicity of heavy metals, particularly lead, has been known since ancient times. In fact, pharmacology owes its early existence to the study of metal poisoning. The term pharmacology derives from pharmakon, which is the Greek word for poison [3]. Assimilation of heavy metals into the human body causes disruption in several biological processes. The mechanisms for these disruptions include heavy metal ions masquerading as beneficial metal ions as well as binding to active sites on biological molecules, thus rendering these molecules unable to perform their function. Toxicology examines the dose and time-dependence of interactions between external agents (chemicals and elements) and normal metabolic agents. The first and most important principle of toxicology is that of dose-response, which states that there is a toxic dose and a safe dose for every external agent. Just as there are no inherently ‘‘safe chemicals’’ (safe under all conditions of exposure), there are also no agents that cannot be used safely by simply reducing the exposure.

B.

General Effects

Lead is a divalent metal and often competes with other divalent ions such as iron, calcium, and zinc with respect to absorption and biochemical physiological processes. It may also substitute for the ‘‘normal’’ roles of some other ions, but with deleterious consequences. Overall, lead appears clinically to exert its toxic effects more in some tissues as opposed to others. The nervous, renal, and circulatory systems appear to be sites where lead has its greatest toxic impact. Furthermore, much like some other minerals, lead can cross the placenta and have consequences upon a developing fetus. Children are much more sensitive to lead’s toxic effects than are adults; and governmental agencies in the United States and other countries have many special departments and programs to aid in reducing lead exposure in children’s environments. Special concern has been given to the influence of even small levels of lead exposure in young children, and the effects it has on learning processes. Lead affects most organs and organ systems in the human body. Table 2 [4–6,8] displays a summary of its effects on human metabolism and organ systems.

C.

Target Organs for Heavy Metals

Neurotoxin problems associated with lead ingestion are well documented as a major danger to young children [9]. Other organs are impacted by toxic exposure to lead and other heavy metals. A principle source is dust in homes that have lead-based paints or soils containing paint scrapings

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Effects of Lead on Human Organ Systems

Lead metabolism and toxicity Organ Central nervous system

Cardiovascular

Blood Kidney Skeletal (Bone) Reproductive

Pathology Memory loss and learning difficulties Brain cellular morphology Neurotransmitter release Encephalopathy Cognitive intelligence Brain edema Hypertension Cardiac electrical function Cardiac calcium use Cardiac Na/K metabolism Hemoglobin synthesis Impairment of overall function Gout Development (skeletal as well as dental) Osteocalcin and protein synthesis Spontaneous abortions Sperm morphology and count Pregnancy, gestation period, implantation influences

that children play on or transfer from hand to mouth. Vegetables grown in contaminated soils provide another pathway to the human population. In Eastern Europe, evidence of lead and cadmium toxicity in humans is found in populations living near smelter areas. This global problem has existed and still occurs from smelter and mining operations where economic development goals supercede environmental sensitivity and the health of at-risk populations [10]. 1. Central Nervous System and Developmental Effects Exposure to low lead levels in children can result in behavioral and learning difficulties. The ability of lead to have such dire effects at relatively low levels gives testimony to its reputation as a potent neurotoxin. Memory loss and learning difficulties have been the subject of numerous reports [9,11]. Several regions of the brain appear to be targeted by lead, including the hippocampus, the region of the brain most often associated with the cellular basis of memory and learning. The developing nervous system is sensitive to lead, even when exposure has ceased the functional effects are long term and may lead to a permanent deficit. The mechanism by which lead may affect brain physiology and biochemistry could be due either to a direct influence upon nerve endings or by influence on neurotransmitter release in some fashion [4,6]. Depending on the level of lead exposure, children have been reported to have symptoms such as ataxia, convulsions, headache, and learning disabilities and tend to exhibit hyperactive behaviors. Encephalopathy has been reported in lead-toxic children. Learning and behavioral deficits in children have been reported. Blood levels of less than 10 micrograms per deciliter, which are comparatively low, could result in such deficits in children. Blood lead levels have demonstrated an inverse relation to the neuropsychological performance of children. Children 2 to 4 years of age had slower mental development as blood lead levels increased [4,6,9]. Lead has been shown to stimulate the release of dopamine, acetylcholine, and gammaaminobutyric acid which are all essential neurotransmitters produced by the brain. Some of these effects may be due to the ability of lead to alter calcium entry into nerve cells or in some way cause

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an increase in the intracellular calcium level. Lead may enter the cells through the calcium channels, while calcium channel blockers may inhibit lead uptake in the same cells. Lead is known to substitute for calcium as a second messenger and can bind to cellular proteins. Many proteins that would normally bind to calcium and other nutrient metals bind to lead when it is present. Calmodulin (a protein in the nervous system) has a greater affinity for lead than for calcium. These lead-protein complexes cannot properly regulate neurotransmitter levels, as do their nutrient metal-protein analogs. Lead activates brain enzymes that regulate voltage levels in the brain. This overactivation impairs learning in children. The encephalopathy induced by lead toxicity is most likely due to a compromise in the bloodbrain barrier. Brain edema occurs in the interstitial area and appears due to compromised blood vessels integrity. The brain capillaries and blood vessels have endothelial cells that contain tight junctions and act as a seal or carrier that excludes many plasma proteins and organic molecules and impedes Na and K exchange. Elevated lead levels disrupt these vessels, and plasma proteins such as albumin enter the interstitial spaces, as do some ions. This increases osmotic pressure, and water accumulates in response. The increased interstitial fluid flows into the cerebrospinal fluid. The edema causes in an increase in intracranial pressure and restricts blood flow to the brain. The direct mechanisms by which the blood-brain barrier and blood vessels that compose the barrier may be compromised may be due to astrocytes appearing to be vulnerable to the toxic effects of lead. The astrocytes cover the walls of the brain blood vessels, and lead can injure these structures. 2. Lead Effects on Blood Exposure to toxic levels of lead leads to blood disease and anemia by: competing for absorption with elemental iron; inhibiting hemoglobin synthesis; and altering the relative composition of cell membranes, including red blood cells, making them more fragile and thus prone to premature breakdown and death. Lead and other heavy metals can also affect white blood cells, leading to a deceased capacity for immune system response. Anemia is a classic indication of lead toxicity. Hemoglobin synthesis impairment results because lead can inhibit the enzyme that creates its predecessor molecules. This reaction occurs in the mitochondria; and is significantly impaired by lead toxicity. It is interesting to note that this enzymatic reaction does requires zinc, another metal element, to continue to completion, illustrative of the need for some metal elements in the human body but the exclusion of other toxic elements [4]. 3. Renal (Kidney) Toxicity [4,8] The kidney is also sensitive to lead exposure, with kidney failure resulting from long-term exposure to toxic levels of lead. Furthermore, toxic levels of lead inhibit other functions of the kidney aside from the primary function of urine production. The nephron of a kidney, which is that portion where blood filtering and urine production takes place, is sensitive to lead. It is well known that the active form of vitamin D3 is produced in the proximal tubules of the nephron. Decreased levels of vitamin D3 in the body results in decreased calcium absorption by the gut, and ultimately affect the strength of bones in a lead-exposed body. Increased levels of lead in blood can lead to protein-lead complexes in the nephron tubules. Gout may be a symptom of such toxicity due to increased reabsorption of uric acid into the bloodstream. Typical measures of renal failure (e.g., blood urea nitrogen, creatine) are elevated as a consequence of lead-induced renal failure. Other metals (e.g., calcium, copper, zinc, etc) have an impact upon kidney damage from lead toxicity. Some mitigate the effects of lead, while others magnify lead’s toxic effects. Other heavy metals such as cadmium also attack the kidney. 4. Cardiovascular Toxicity Lead has been linked with hypertension by some epidemiological studies. Even low levels are associated with elevated blood pressure [4]. Some of lead’s influence upon blood pressure may also be related to renal toxicity discussed previously. Calcium and lead are known to have antagonistic biochemical roles, as previously discussed regarding brain protein binding. However, the influence of calcium upon lead-induced increases in blood pressure appears paradoxical.

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Feeding rats a high level of calcium does not appear to block or dampen the effect of lead hypertension. [5,8,9] 5. Bone Integrity Lead has a high affinity for bone, partly due to its antagonistic relationship with calcium. The greatest amount of lead in the body is the skeleton, which acts as a reservoir for lead. The influence of lead upon bone must be considered from the perspective of vitamin D3, the metabolism of which is significantly altered in lead toxicity. Bone structure is in a constant state of self-renewal, with continual controlled destruction, reduction, and rebuilding. Lead is known to influence the various biochemical and physiological events involved with this process. Lead may accumulate in bone beginning in fetal life. Young children with growing bones are apparently more susceptible to the toxicity of lead than are adults. Impaired dental development and delayed skeletal maturation have been reported in congenital lead poisoning cases. In children, the NHANES II (second National Health and Nutrition Examination Survey) study documented decreased height and chest circumference with increased blood lead levels for children 7 years and younger [9]. Levels of plasma osteocalcin, a bone protein, are lower in children exposed to toxic levels of lead [4]. Bone matrix synthesis is impaired with low-to-high lead exposure. Bone formation rate and healing from fractures are both decreased with exposure to lead. Bone structures may also be less dense. Many of the processes of bone turnover are not only attributed to the direct influences of lead on bone dynamics; but also indirect mechanism, as lead may influence hormones targeting calcium and bone metabolic processes. The active form of vitamin D3, 1,25-dihydroxycholcalciferol, appears to be lower in children with elevated blood lead levels. 6. Reproductive Toxicity Toxic quantities of lead in the human body impact the reproductive processes of both men and women. An increased incidence of spontaneous abortions has been documented in female lead workers and in the wives of male lead workers, i.e. persons working in lead smelting or manufacture of lead containing products. Male lead workers with blood lead concentrations of 53 to 75 ug/dl have been reported to have decreased sperm counts as well as altered morphology of the sperm. [4].

D.

Elimination from the Human Body

Metals are elements- they cannot be detoxified. They are already in their simplest state and cannot be changed, other than their valance. The toxic activity of metals is thus not mitigated in the body by biotransformation as it can be with organic pollutants such as pesticides. Biotransformation includes processes that change the toxin into another chemical agent, which tends to be less toxic. Excretion is therefore the major process by which the body removes the threat from metal exposure. Sequestration is the most effective for mitigation of metals toxicity in the body. It is a process whereby a metal ion is moved to a site and placed in ‘‘metabolic storage’’ so that it cannot move to and act upon a vulnerable metabolic site. The human body also has several processes with which to excrete or otherwise remove heavy metals. Once having gained access to the human body, metals undergo diverse but sequential mechanisms of absorption, distribution, sequestration, biotransformation and elimination. These considerations apply irrespective of the mode of entry to the body. This whole chain of events in its entirety, designated the biodynamics of a substance, is depicted in Figure 1 [9]. Central to processes for toxin removal from the body is the blood stream. It serves as the medium both for uptake of metals for distribution through bodily tissues and for removal by excretory routes. Removal of foreign substances and metabolites already absorbed into the body is achieved in a variety of ways. The principal routes of exit are via the kidney and the bile, while excretion into feces, sweat, saliva and breast milk represent minor routes of elimination. Exercise of a toxic or nutrient effect of a metal primarily depends on its ability to enter the body and then move to sites within the body where it can have an effect (i.e. replacement of calcium in bone). The ability to reach a site is mitigated by the balance between the relative rates

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FIG. 1 Biodynamics of lead in the human body.

of opposed biodynamic processes to remove metals and other exogenous substances from the body. Some processes in the body improve absorption of lead and thus facilitate the ease which any compound reaches metabolic sites (moisture on mucous membranes). 1. Respiratory Tract Metals can be removed from the body while still in the respiratory tract and not yet absorbed. The walls of the nasal passages are lined with mucus membranes. Nose blowing, sneezing, and coughing each act to remove particles from the body. In the lungs both scavenger cells and cilia help to move metal particles up the respiratory tract to where the physical actions mentioned above then remove the particles. 2. The Renal System Three separate processes in the kidney, consisting of ultrafiltration, passive reabsorption, and active secretion regulate kidney excretion. Kidney function is efficient for polarized molecules such as salt (sodium chloride). Non-ionized foreign substances such as metal ions bound to proteins are subject to passive reabsorption into the blood stream after delivery into the kidney. This results in a low net rate of excretion for these substances, in contrast to more polar, lipidinsoluble molecules. Consequently, metabolic conversion of a compound into one with a higher polarity, favors excretion. Manipulations of this kind may be beneficial in the prevention of poisoning by certain substances. Protein binding constitutes another factor determining the speed of removal of foreign compounds by the kidney. When lead is tightly bound to proteins, as is the case with the previously discussed brain proteins, excretion by the kidneys is reduced.

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3. The Liver and Bilary System Toxic metabolites formed in the liver may be secreted into the bile. A portion of these may enter the intestinal tract and eventually undergo fecal excretion. A larger portion is usually reabsorbed by the intestines to be transferred to the blood, and thence to the kidney. Extra-hepatic circulation, which consists of passage of the compound via the liver back to the gall bladder, and its ultimate degradation by gut bacteria, also takes place. Thus, biliary excretion may lead to three distinctive terminal processes: fecal excretion, enterohepatic circulation and enterobacterial metabolism. 4. Other Species Metal Elimination and Synthetic Aids to Elimination While the processes for excretion of heavy metals are reasonably well developed in humans, they do not exist in all species; and have limitations in humans as well. In fact, the active processes undertaken by organs such as the kidneys to excrete metals also causes a breakdown of sensitive tissues and structures within the kidney, leading to chronic disease if exposure to the metal continues. Many animal and plant species simply store the metal elements as ions in their tissues. When these animals and plants die or are taken by a higher-level consumer as live prey, the stored metals enter the food web. As higher-level consumers ingest greater quantities of the metal, metal elements become magnified in the food web. Excretion can be assisted in humans and many other animal species. Ethylenedinitrilotetraacetic acid (EDTA) is a common chelating agent, which is infused via the bloodstream into patients in order to remove metal ions from the body. Chelation refers to the formation of metal complexes by bonding a chelating agent or ligand with a metal to form a heterocyclic ring. Initially, the medical use of chelating agents was to treat heavy metal poisonings, for example, British Anti-Lewisite (2,3-dimercaptopropanol) for arsenic poisoning. EDTA was also used to treat lead poisoning in a young child [12] and following its initial approval by the Food and Drug Administration (FDA) in 1953, EDTA was utilized to treat other heavy metal poisonings. EDTA chelates various divalent metal ions with differing affinities. It is excreted mainly by the kidney, with about 50% excretion in one hour, and more than 95% within 24 hours. Almost none of the compound is metabolized. This inability to be metabolized coupled with its excretion by the kidney makes EDTA an excellent agent for removal of heavy metals from the human body [13].

III.

EXPOSURE ROUTES

Lead and other heavy metals enter the human body as well as the ecosystem at large via several routes. Human exposure to lead typically occurs through ingestion rather than inhalation or other routes. United States Environmental Protection Agency (USEPA) and other regulatory agencies’ air quality standards, solid waste disposal regulations, and drinking water standards are based in part on this proportion of exposure in the general population. These routes have some amount of crossover exposure such as airborne lead falling onto plants and other surfaces from which subsequent ingestion occurs.

A.

Air

The human respiratory system has the two primary functions of delivering oxygen to the bloodstream for delivery to tissues and removal of carbon dioxide, a waste product of cellular metabolism. The respiratory system is vulnerable to contaminants in the air including heavy metal particles. In addition to loading the lungs with metal, inhaled metal particles make their way into the digestive system as mucus from the respiratory system is swallowed. Lead in the atmosphere and in soils near heavily traveled roads has been eliminated as a source in many countries by the use of unleaded gasoline; however, lead contaminated soils adjacent to these roads are still in place. Inhaled dusts from these roads provide a means for lead to be inhaled. Another source is lead smelter operation. Much smelter-originated atmospheric lead has been eliminated from the atmosphere by terminating operation of some smelters and by utilization of chemical scrubber-electrical precipitator systems in chimneys of modern or modified operations.

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Many older smelter operations still load the atmosphere and the surrounding soil with lead and other elemental species such as arsenic, cadmium, chromium, mercury, nickel, vanadium, and zinc. This source of inhalable metals is mostly associated with old mining operations, heavy metal industries, oil refineries and energy generation facilities. However, in highly environmentally regulated societies such as Western Europe, the United States, and Japan, airborne exposure to lead has decreased significantly over recent past decades. An excerpt from the USEPA performance standards for lead smelters is found below. This portion of USEPA regulations moderates the amount of particulates, which would be expected to contain lead that can be discharged into the ambient air from a lead smelter [14]. §60.182 Standard for particulate matter. (a) On and after the date on which the performance test required to be conducted by §60.8 is completed, no owner or operator subject to the provisions of this subpart shall cause to be discharged into the atmosphere from any blast furnace, dross reverberatory furnace, or sintering machine discharge end any gases which contain particulate matter in excess of 50 mg/dscm (0.022 gr/dscf).

B.

Ingestion

Ingestion of lead-based paint is the primary cause of lead poisoning of children in the United States, according to the Centers for Disease Control [15]. The mouth, stomach, and intestines are all capable of lead absorption, which ultimately transport the metal to the bloodstream. Other toxic metals can also be absorbed after ingestion, to greater or lesser degrees. 1. Route and Body Destinations Once absorption is completed, the substance enters the circulatory system, leading to its distribution throughout the body. Access into the tissues of the body is not gained until the substance has passed across various tissue boundaries and cell membranes. These boundaries and barriers behave as typical fat-containing membranes; therefore, in order for substances in the environment, such as metals, to pass through these barriers, they must possess certain chemical properties such as non-ionization on an atomic level and low polarity on a molecular level. From an ingestion standpoint the barriers in the lining of the stomach, remainder of the gut and placenta are of greatest importance [16]. 2. Uneven Distribution Various parts of the body have a greater or lesser affinity for various metals and this affects the extent and areas of distribution in the body. Generally speaking, foreign compounds and metals are unevenly distributed among the various tissues, and selective accumulation at particular locations is the rule rather than the exception. The target tissues or organs where the compound exhibits its characteristic activity often are not those with the greatest accumulation of foreign compounds or metals. In fact, foreign compounds are usually segregated in non-sensitive tissues such as fat layers, which function as storage depots. The fat layers allow for the accumulation of toxic compounds and metals that it then releases. Fatty tissues, connective tissues, and the skeleton are specifically adapted to serving as reservoirs for toxic agents. 3. Exchange and Transport Materials deposited in storage areas are constantly exchanged with circulation. Blood serum proteins function as major carriers for the transport of foreign molecules, especially in the case of fat-soluble materials such as DDT and its metabolites (pp’ DDE), and fat-soluble compounds of toxic metals. For metals in particular, transport requires preliminary binding between a protein and a metal compound to be stored or transported. Protein binding, also termed sequestration, is a reversible process the degree of which is governed by the affinity of a foreign compound for specific receptor sites on a protein. Since only the unbound (free metal) form is freely transported across a cellular membrane, dissociation of the sequestrated molecule is necessary to effect passage through cellular membranes. In short, metals must be bound to a protein in order to be

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carried to a target site or storage depot, but must disassociate in order to pass through a cellular membrane and actually enter the cell once at the site. As mentioned above, metals that remain in the bloodstream are excreted at a slow rate, or stored and thus rendered harmless. 4. Drinking Water Drinking water is another avenue for the ingestion route of exposure to lead and other metals. Lead occurs naturally in some drinking water supplies. Lead also enters the drinking water supply through both natural and human sources. Treated drinking water leaches lead from the pipes of the drinking water distribution stream. This is believed to have been the primary means for lead poisoning of citizens of the Roman Empire. Lead pipes distributed drinking water to baths, fountains, and other end uses. Lead-containing solders have been banned from commercial and household piping in many countries since the 1970s to prevent lead exposure through drinking water. In addition, the USEPA and other environmental agencies have put into place regulations which require drinking water treatment facilities to ensure that drinking water has an appropriate pH and other chemical factors to assume a low propensity to leach lead from pipes. These regulations are a portion of the Lead and Copper Rule [17], whose purpose is to help minimize the levels of lead and copper in drinking water. Other environmental authorities worldwide have adopted similar rules and regulatory levels of these metals that are listed in Table 3. The USEPA has set action levels of 0.015 mg/L for lead and 1.3 mg/L for copper, or Maximum Contaminant Level Goals [17,18] established by the Lead and Copper Rule (‘‘the rule’’), which are 0 mg/L for lead and 1.3 mg/L for copper. This rule includes basic requirements for drinking water treatment facilities to optimize corrosion control (lead and copper leaching from the water distribution pipes) and, if appropriate, treat source water, provide public education, and replace lead service lines. Appropriate actions must be taken if the drinking water fails to meet the action levels cited. Metal content measurements are taken at end user’s (homes, schools, etc.) tap. Over the past several years, many worldwide environmental agencies have lowered the allowable lead concentration in drinking water from 0.050 mg/L to 0.015 mg/L. A few agencies have continued to lower the maximum allowable level, and with many setting a lead concentration goal of zero, or undetectable in drinking water. Lower allowable levels are being put into practice, as the effects of lead, especially on children, are now better understood. A large percentage of the worldwide population obtains its drinking water from underground sources rather than lakes or streams. Pollution of these groundwater sources by metals

TABLE 3

Worldwide Regulated Levels of Lead in Drinking Water

Element

Limit, mg/L

Indium Bismuth Tin Silver Antimony Antimony Antimony Copper

None found None found None found 0.10 0.006 0.002 0.005 1.0

Copper Lead Lead Lead Zinc Zinc

2.0 0.015 0.05 0.010 1.0 5.0

Source

USEPA 40 CFR 141 USEPA 40 CFR 141 Japanese legislation 98/83/EEC USEPA 40 CFR 141; Japanese legislation; Thai legislation 98/83/EEC USEPA 40 CFR 141 Japanese legislation; Thai legislation 98/83/EEC Japanese Legislation Thai Legislation

58

SMITH

such as lead causes potential harm to these populations. One source of this pollution is improper solid waste disposal. World Health Organization (WHO) and USEPA scientists estimate groundwater accounts for more than 95% of all fresh water available for use worldwide. Approximately 50% of the United States population obtains all or part of their drinking water from groundwater, with nearly 95% of rural residents relying on groundwater for their drinking supply. About half of irrigated cropland on United States farms uses groundwater and approximately one third of industrial water needs are fulfilled by using groundwater [19]. The issues related to solid waste disposal and contaminated drinking water are discussed in detail in Section 2.4. 5. Food In most cases, food is not a significant avenue of lead exposure. Lead-free solder is used in canneries to ensure that lead is not leached from the canning materials by acidic foodstuffs. One area that lead can pose a hazard to the food supply is lead-coated cookware and tableware. In many countries lead continues to be used as a pottery colorant and glaze. Acidic foods can leach and absorb lead from these decorated surfaces. Crops can absorb lead and other metals into their edible parts. Table 4 lists some examples of metal contamination in crops.

C.

Direct Contact

Lead can also become a health issue through direct contact, which then leads to ingestion and to a much lesser extent, absorption through the skin. The primary route of exposure by direct contact has been chips and dust from lead-based paint. Infants and children make contact with lead chips and dust as lead containing paint flakes off painted surfaces and onto the floor and other horizontal surfaces. Infants and children collect the chips and dust on their hands, feet, clothing, and hair. When the children eat or place their hands in their mouth, lead ingestion occurs. Children exposed to lead are at risk to several health problems including developmental and neurological impairment. Lead poisoning has been linked to mental retardation, poor academic performance and juvenile delinquency. It is estimated that nearly one million children in the United States alone have dangerously elevated levels of lead in their blood [9]. Because of the potential dangers, any exposure to deteriorated lead-based paint presents a hazard. Lead, to a much lesser degree, can absorb through the skin. Direct contact is a minor exposure route, and effects primarily industrial lead workers rather than the public. These workers are normally protected against lead exposure, at least in industrialized countries.

D.

National Standards

In the late 1990s, the USEPA published new national standards to identify dangerous levels of lead in paint, dust and soil. These standards provide homeowners, school and playground administrators, childcare providers and others with guidelines to protect children from hazards posed by lead. The standards provide the general public benchmarks for remedial actions to safeguard children and the public from the dangers of lead. The standards also apply to other Federal lead provisions, such as the EPA’s real estate disclosure requirements when selling or

TABLE 4

Metal Contamination in Crops

Metal element Cadmium Lead Copper Nickel Zinc

Edible plants that accumulate this metal Lettuce, Spinach, Celery, Cabbage Kale, Ryegrass, Celery Sugar Beet Sugar Beet, Ryegrass, Turnip Sugar Beet, Spinach

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD TABLE 5

59

Lead Standards for Real Estate (United States)

Media Dried paint, applied, surface area Dried paint, applied, by weight Dust, on residence floor Air Soil, bare, childrens’ play areas Soil, bare, surrounding a residence (average concentration)

Maximum lead concentration allowed 1.0 mg/cm2 0.5 percent by weight 40 ug/foot2 1.5 ug/m3 400 mg/kg (parts per million) 1200 mg/kg (parts per million)

renting a home or apartment. These hazard standards also provide guidance for other EPA programs engaged in toxic waste cleanups. These standards also provide landlords, parents, and childcare providers with specific levels on which to make informed decisions regarding lead found in their homes, yards, or play areas as noted in Table 5. Under the new standards, lead is considered a hazard if there are greater than: 40 micrograms of lead in dust per square foot on floors; 250 micrograms of lead in dust per square foot on interior window sills and 400 parts per million (ppm) of lead in bare soil in children’s play areas or 1200 ppm average for bare soil in the rest of the yard. The standards allow inspectors and risk assessors to assist property owners in deciding how to address problems which may include lead paint abatement, covering or removing soil or professional cleaning of lead dust. Lead in dwellings is normally abated by sealing it, which may be as simple as repainting the area from which lead dust is flaking, or by removing and disposing lead-containing building materials. Both methods significantly reduce or eliminate direct contact with lead-containing paint chips and dusts.

E.

Mobility

The mobilized form determines the route (s) of exposure to plant and animal life: gases or vapors that evaporate or sublime from liquids or solids, dust or fumes, solid metal or metal oxide particulates on or in the soil, and dissolved in aqueous media. Whether the ions or ionic and anionic complexes are mobilized or precipitated and then perhaps remobilized, is dictated by changes in physical, chemical and biological conditions in source areas and areas where the metals are ultimately deposited. In soils and sediments the controlling parameters that influence mobility and deposition of metals are many. These include: pH, redox potential, concentrations, chemical gradients, cation exchange capacity (CEC) of the soil/rock, presence of clay minerals, Fe/Mn oxyhydroxides, adsorption-desorption, and bioaccumulation. The interaction of these parameters and varying responses of metals to them can make prediction of environmental influences complex. In fact, somewhat random factors such as animals can have a significant impact on the transport of metals and other pollutants. No models have yet been devised to consider all these factors. As with all pollutants, it is best to minimize migration from the source into the environment. The element chromium, for example, is tightly bound in the residual phase and not easily mobilized, whereas cadmium has more distribution in easily soluble and exchangeable phases than the other elements considered.

IV.

SOLID WASTE DISPOSAL AND CONTAMINATED WATER

A.

Landfill Issues

Research indicates that other metals currently used in electronics, as well as those contemplated as replacements for lead in electronics, are leachable into landfill leachates [20,21]. The clean up of metal-element polluted aquifers is a very costly affair and yet necessary to prevent present-day and future metal exposures. Landfill leachate escapes through poor landfill construction and

60

SMITH

design, breaks in the landfill liner, or overflow due to excess leachate in the landfill, typically from rainfall (Fig. 2). All modern landfills have leachate treatment systems, but these can be overloaded through heavy short-term rainfall or treatment system malfunction. If not quickly corrected the escaped leachate may migrate into the surrounding surface soil or below the surface. This can lead to the pollution of private and communal water wells, or the seepage of polluted groundwater into surface water where it can directly cause visibly detrimental environmental effects. Aquifer remediation may take a long time, since the pore volume of an aquifer must be flushed a number of times to remove metal pollutants, which adsorbs to the soil and rock.

B.

Contamination Mechanisms

The United States Environmental Protection Agency (USEPA) has found waste sites in which lead-containing wastes disposed in landfills did leach and reached environmental receptors located beyond the landfill area [22]. Leaching from landfills is the primary means by which human activity causes lead and other toxic metals to enter groundwater. When solid wastes are disposed in landfills, rainfall and chemical reactions within the wastes release metal elements within the waste. These reactions occur because rainwater is acidic after dissolving nitrate and sulfuric radicals from the air as it falls, and because putrification reactions (rotting) of food wastes produce acidic byproducts such as acetic acid. These acids leach the metal elements from waste items, such as lead from electronic wastes, and the rainwater carries the metals into the leachate that collects within landfills. Thus, groundwater and solid waste disposal are grouped together in this chapter. A search of the USEPA Superfund NPL Assessment Program database for USEPA Region Two (northeastern United States) shows leachate from three landfills, which contains

FIG. 2 1999.)

Release of toxic metals via landfill breakage or flooding. (After Smith and Swanger, Ref. [20],

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

61

lead and/or other heavy metals; and this leachate has reached groundwater or surface water. The data also showed two landfills where leachate is heavy metal contaminated, but has not yet reached groundwater or surface water sources. Sites in Harris County, Texas also show groundwater lead contamination via improperly managed activities over the area of the aquifer [23]. As a result of these sites with contaminated groundwater, USEPA and State environmental agencies have enacted regulations regarding disposal of hazardous materials as well as recycling. Both regulations and recycling tend to minimize exposure of groundwater to potential contaminants released from a landfill. When released, leachate may move across the land surface or into the subsoil, or both. If the leachate is contaminated with metal ions or compounds, these may deposit into the surface soils or move through the subsurface soils and rock. The rate of metal movement depends upon the geology of local soils and rock. If we look closely in the pores of topsoil (Figure 3), we see that many reactions are occurring while groundwater is generated from rainwater, and while leachate moves over the soil. 1. Leachate Reactions The most important leachate reactions are dissolution and precipitation of solids and minerals, redox (reduction/oxidation) reactions with organic matter, and ion exchange and sorption on clay minerals and organic matter. Metal element concentrations in surface and near-surface earth systems and the atmosphere are the result of mobility, precipitation, and deposition of free ions, ionic compounds and anionic complexes of those metals. a. Effect of pH. The pH factor is critical in the geomovement of metal contaminants [24,25]. Under acidic conditions, metals such as Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn become more soluble and mobile. This enhances their uptake by grasses and other vegetation. Increased acidity (decreased pH) also increases the amount of dissolved organic carbon in soils, swamps, or wetland environments. Under oxidizing conditions in nature, decomposing organic matter releases associated metals and causes greater acidity, in much the same manner as man-made waste disposal landfills. The acidity increases the mobility of the metals cited above. Conversely, increases in pH to basic conditions establish a geochemical barrier, so many toxic metals with low solubility constants (Ksp) are immobilized and their uptake by vegetation is restricted. For example, a pH rise from 7.4 to 9.0 in aquatic systems favors the adsorption of Zn, Cr and Cd onto suspended matter. Indeed, liming of agricultural soils increases pH and dissolved organic carbon; and can immobilize metals, including lead, that are potentially harmful to crop growth or because of their movement through the food chain into feed and food crops. At the same time, however, other metals such as molybdenum can be mobilized and made bioavailable to fodder. b. The Redox Factor. Metals may have different mobility responses when a reductionoxidation (redox) factor is combined with pH as summarized in Table 6 [24]. As examples arsenic

FIG. 3 Topsoil structure and porosity allows pollutants to enter groundwater. (After Smith and Swanger, Ref. [20], 1999.)

62 TABLE 6

SMITH Metal Mobility at Various pH and Redox Levels Mobility Favored at pH

Mobility Favored Under Conditions

Best Combination for Mobility

Cadmium

lower

oxidizing

Chromium

lower

oxidizing

Copper

lower

oxidizing

Manganese

lower

reducing

Nickel

lower

oxidizing

Lead

lower

oxidizing

Molybdenum

higher

reducing

slightly lower (pH = 6) lower

oxidizing

near pH = 6, mildly oxidizing low pH, highly oxidizing pH = 4, highly oxidizing low pH, highly reducing near pH = 6, mildly oxidizing pH = 4, highly oxidizing low pH, highly reducing near pH = 6, mildly oxidizing pH = 4, highly oxidizing

Metal Element

Mercury Zinc

oxidizing

Precipitates in Reducing Environments with Sulfide Slightly Slightly Yes Slightly Yes Yes Yes Yes Yes

and manganese, increase in mobility with decreasing pH and greater reduction potential (to As3+ and Mn2+); Fe2+ and Mo as Mo2+ react similarly. Under weakly acidic conditions (about pH 6) the elements Cd, Ni and Hg possess increased mobility. With decreasing pH to four or less, soil borne zinc, copper, and lead mobilize as does Fe and Mn at pH of less than two. Under reducing conditions and in the presence of sulfide, chalcophile elements such as zinc, copper, iron, mercury, nickel, lead and others precipitate as sulfide mineral phases. Changes in oxidation or reduction potentials in a subsurface environment can lead to the separation of elements. The separation may be geographic due to changing reduction potential from decaying organics and bacterial activity. Thus, in many subsurface environments some metals are immobilized and precipitated under conditions where others are still mobile and transported to precipitate elsewhere. Natural pH values for most soils are in the range of about 3 to 8.5 and redox potentials from 0.35 to +0.8. Shallow sub surfaces waters have more restricted ranges with pH from 5.5 to 8.5 and redox potentials from 0.08 to +0.6 resulting in a wide range of metal element transport behaviors. c. Cation Exchange Capacity Factor. The concentrations and chemical gradients of metals in the soil and rock have an effect on their speed of transport. Just as in any free liquid medium, diffusion of metals occurs at some maximum rate. Concentrations of ions that compete for adsorption sites or in other ways hamper the diffusion process, can impact the speed of transport through a soil or rock medium. Cation Exchange Capacity (CEC) also impacts a soil’s ability to adsorb and retain metal ions. The CEC of a soil refers to the amount of positively charged ions a soil can hold. When dissolved in water, the resulting metal ions have a positive charge. Soils have a slight excess of negative charge sites due to the presence of clay particles and organic matter. Therefore, the higher the clay and organic matter content, the higher the CEC of the soil. Soils with a high CEC tend to hold onto positively charged nutrients better than soils with a low CEC. The CEC of a soil can be increased somewhat by increasing the soil’s organic matter content and by increasing the pH of a soil. A higher CEC is an indicator that a soil can hold and retain a higher concentration of metals compared to a soil with a lower CEC.

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

63

d. Adsorption and Retention. Adsorption is a function of the density of unsatisfied surface charges on colloids and clay-size particulates. The charges may be negative in the case of clay minerals, manganese oxyhydroxides and organic colloids (especially humic compounds), thus providing absorption sites for cations including heavy metals. Unsatisfied charges may also be positive as is the case for iron oxyhydroxides and provide capture sites for anionic complexes such as arsenate, molybdate, chromate, vanadate, and phosphate. Like pH and redox, adsorption capacity varies with environmental conditions. For example, oxyhydroxides have the strongest adsorption sites followed by particulate organics and clay minerals. The presence of clay minerals can have a great impact on metal transport, especially in soils in which clay minerals dominate. The clay minerals act on metal transport in two ways. One is the adsorption of the metal ion; the other is the physical barrier that high-clay soils create to the movement of liquid contaminants. It is a common practice to use clay soils in landfill construction, and for temporary barriers in construction projects. This is due to the poor permeability of high-clay soils, especially those with little-to-no sand particle content. These soils exhibit permeability as low as 1  106 milliliters of water per day per inch of soil compared with rocky soils may have a permeability of several milliliters of water moving through an inch of soil per day. In addition, adsorption of most metals increases with increasing pH as is predictable from the mobilization of metals in soils or sediments discussed earlier. There is a finite capacity of soil or rock to store metal contaminants under any set of conditions. When this capacity is exceeded, the contaminant may diffuse into the surrounding soil or rock. If the conditions change to decrease the level of adsorption or increase the level of desorption from the soil or rock, there may be a release of contaminants into the surroundings. For example, peat is a known accumulator of metals. Peat is acidic and has sulfide present, so precipitates metals such as lead as lead sulfide. For example, in one case adjacent river waters, with a basic pH, flowed into a peat deposit area and allowed the precipitated metals to be released. This resulted in declines of forests and fish stocks in the adjacent wetlands. In addition, the impact of metal contamination on some parasites and predators lessened the natural control of forests pests [25]. e. Biomethylation and Bioaccumulation. Biomethylation or the microbial mediated formation of organometallic compounds from inorganic precursors is an important process that affects the toxicity potential of several metals. It creates the toxic species of Hg that enter the food web as CH3Hg+ or (CH3)Hg2 after its release from a microbe cell. Other metals which undergo biomethylation in minor amounts include As, Cr, Sb, Se, Sn and T1. Biomethylation is stimulated in minor waters, which are slightly acidic, rich in organic matter and have high bacterial activity. Related to biomethylation is bioaccumulation. Both plants and animals bioaccumulate metals under certain conditions. In this process, metal ions undergo uptake by an organism. This organism may or may not suffer ill effects from the metal intake. If this organism is later ingested by another organism higher in the food web, then the metal may bioaccumulate in this second organism, as more food contaminated with metal is eaten. Ultimately the metal concentration per unit of body weight increases until at some point on the food web the organisms at that level suffer an ill effect. These may be low-level effects such as stunted growth, reduced ability to reproduce, or higher-level and more visible effects such as death.

V.

TOXICOLOGY OF ELECTRONIC MATERIALS

Management of waste in industrialized countries ranges from highly regulated, specific, government-controlled operations to practical non-existence. Most industrialized countries that significantly regulate industrial/commercial hazardous waste fall within this range. This includes those that do or do not regulate household hazardous waste disposal. Hazardous waste is defined as any waste that may ‘‘pose a substantial present or potential threat to human health and the environment when improperly treated, stored, transported, or otherwise managed’’ [26]. When waste is disposed, it is weathered by rainfall and reactions with other wastes, which allows metal elements and their salts to be leached from the metal surfaces of the waste. If the metal bearing

64

SMITH

leachate is allowed to contact storm water, groundwater, or to migrate into groundwater, local drinking water supplies are threatened with contamination.

A. 1.

Definition of a Hazardous Substance United States Government Agencies

a. USEPA. The USEPA identified four characteristics of hazardous waste: 1) toxicity, 2) corrosivity, 3) reactivity, and 4) ignitability. EPA also developed standardized procedures and criteria for determining whether a waste exhibits one of these characteristics. These characteristics and criteria are codified at 40 CFR Part 261 [26]. Testing procedures are detailed in SW-846 [24]. The USEPA has developed a set of assumptions that describe possible ways in which a waste can be disposed improperly. The USEPA intended this so-called ‘‘mismanagement scenario’’ to simulate a ‘‘plausible worst case’’ of mismanagement. Under this worst-case scenario, USEPA assumes that potentially hazardous wastes would be co-disposed with municipal solid waste in a landfill with actively decomposing material overlying a groundwater aquifer. b. USDOT. The United States Department of Transportation (USDOT) has published its own hazardous classification system based on impact to human safety and health in the event of a mishap in the transportation system. The hazard class of dangerous goods is indicated either by its class (or division) number or name. For a placard corresponding to the primary hazard class of a material, the hazard class or division number must be displayed in the lower corner of the placard. However, no hazard class or division number may be displayed on a placard representing the subsidiary hazard of a material. For other than Class 7 or the OXYGEN placard, text indicating a hazard (for example, ‘‘CORROSIVE’’) is not required. Text is shown only in the U.S. The hazard class or division number must appear on the shipping document after each shipping name. [Appendix I] 2. European Union and Other Organizations Worldwide The European Union as well as many governments worldwide utilizes United Nations regulations for definition and shipment of hazardous materials. These requirements are developed by the United Nations Committee of Experts for transport of all types of non-radioactive dangerous goods. Other organizations such as the International Airline Transport Association (IATA) turn these requirements from the United Nations into operational and technical requirements and practices.

B.

Electronic Materials Toxicity

The study of the relative toxicity of various metal elements used in electronics is important as the industry evaluates new solder alloys and new component materials. The Surface Mount Council reported various elements comprising lead-free solder alternatives [27] (Table 7). Based on this and other data cited in the report, toxicity ranking assigned to the common lead-free solders alloying elements are as follows: Bi < Zn < In < Sn < Cu < Sb < Ag < Pb Every cited environmental, health, and safety organization worldwide classifies lead containing materials as ‘‘dangerous,’’ ‘‘hazardous,’’ or by some other similar term. Some organizations place a maximum allowable concentration limit for lead on materials above which they are noted to be ‘‘dangerous’’ or some similar classification. These classifications form the basis for environmental, occupational safety, and other regulations pertaining to lead. Other metal elements used in electronics are regulated for waste disposal and/or occupational exposure in many jurisdictions worldwide as discussed in the following sections. As metal exposure to humans is discussed, it is important to remember that environmental contamination is a significant variable. For example, rural populations that would see little to no metal exposure can be given

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

65

TABLE 7 Summary Results from the Surface Mount Council Study. (After Allenby, et.al., Ref. [27],1999) Metal Element Bismuth Zinc Oxide fume Tin (inorganic) Tin (organic) Antimony Copper (dust) Copper (fume) Indium Silver (metal dust and fume) Silver (and soluble compounds) Lead (inorganic)

OSHA PEL or ACGIH TLV (mg/m3) None 5 2 0.1 0.5 1 0.1 0.1 0.1 a 0.01 b 0.05 c

Note a: OSHA PEL; Note b: ACGIH TLV; Note c: ACGIH TLV is 0.15 mg/m3.

harmful doses of metal through drinking water contamination arising from nearby mining operations with improper mine waste disposal.

C.

Lead

1. Sources from Electronic Assemblies Lead exposure and use in electronic assemblies is mainly associated with printed wiring boards (PWB), solder joints, and the electronic components themselves. a. Components. Several classes of electronic components contain lead either as part of their active and functioning elements, or as part of their interconnection structure. Piezoelectric devices often use lead in their piezo elements inside the device casing. Most devices produced today use a metallized surface finish on their solderable surfaces. This surface finish has traditionally been tin-lead solder, but sometimes consists of palladium, pure tin, or other leadfree alloys. b. Printed Wiring Boards. Most printed wiring boards produced today use tin-lead surface finishes on their solderable surfaces. Although alternative surface finishes are increasing in use, tin-lead solders finished pads and through holes are still the norms worldwide. Thus, a finished printed circuit board may have lead in the printed wiring board, the components, and to attach the solder joints themselves if lead is used as part of the solder paste in the wave machine to attach surface mounted components or pin-in-hole components. c. Cable Sheathing. Some cable sheathing utilizes lead to enhance thermal stability, ductility, increase weight for density intense applications, or to decrease susceptibility to corrosives. If the sheathing is exposed, lead is easily leached from the sheathing surface. d. Monitors. Portions of computer monitors and other cathode ray tube driven devices contain lead. Cathode ray tubes (CRTs) are often referred to as being a hazardous waste because of the leaded glass of which they are constructed. A study involving 36 CRTs was conducted at the University of Florida [28] to quantify the leachable lead from CRTs using the USEPA’s toxicity characteristic leaching procedure (TCLP). The study revealed that the color CRTs leached approximately 18.5 mg/L, well above the USEPA limit of 5.0 mg/L of leachable lead, thus characterized as hazardous waste. None of the monochrome CRTs exceeded this limit. Most jurisdictions in industrialized countries have put recycling programs into place for television sets and computer monitors to reduce this source of leachable lead in municipal landfills. e. Batteries. By far the most prevalent use of lead worldwide is in the production of batteries for electrical energy storage [29]. Over half of worldwide lead consumption goes into

66

SMITH

battery production. In contrast, approximately 0.6% of worldwide lead consumption goes toward electronics manufacture. Lead batteries do undergo one of the oldest and most successful recycling programs worldwide, thus their impact to the environment is not as great as their proportional use relative to other lead uses such as ammunition. Ammunition by its nature and use is non-recyclable. 2. Blood Levels Lead in the human body can be measured through several media and organs. Lead blood level is the best measure of impact to a particular person or population. Analysis of metals in blood is well understood and characterized in terms of accuracy and precision, which are both quite good relative to use of other sample media (i.e. teeth and hair). a. Action Levels. Action levels for lead in various environmental media are provided by several sources. These are primarily environmental regulatory bodies. Researchers also promulgate action levels. Tables 8 and 9 provide example action levels across various media. b. Levels over History. Man’s use of lead has been the only significant variable in the environmental availability and thus exposure to lead. In prehistoric times, a seventy-kilogram human had about 0.3 mg of lead in the body. Humans in North America currently contain about 500 times that amount or about 150 mg per seventy kilograms of body weight. When lead was utilized as common gasoline additive, airborne lead concentrations were roughly one thousand times prehistoric levels [1]. c. Exposure Routes. The human population is exposed to lead via all three principal routes: ingestion, inhalation, and direct contact. The proportion of lead intake from these routes varies significantly depending on the subject’s age, social status, and occupation. The primary route of lead exposure in children is lead based paint dust and chips [9]. As lead based paint ages it becomes dislodged from the painted surface and transforms from a sheet to dust and chip sized particles. Children, especially children who are crawling, gather the paint particles on their hands and clothing. These particles are then ingested as the child places his or her hand in the mouth. Adults are also impacted by lead ingestion from lead glazed cookware and by traditional medicines containing lead [1,9]. Awareness of lead poisoning, both acute and chronic is lessening

TABLE 8 Element

Action Levels for Metals in Liquid Media. (After Smith, Ref. [21], 1999) Media

Limit, mg/L

Indium Bismuth Tin Silver Antimony Antimony Antimony Copper

All All All Drinking Drinking Drinking Drinking Drinking

Copper Lead Lead

Drinking Water Drinking Water Drinking Water

2.0 0.015 0.05

Lead Zinc Zinc

Drinking Water Drinking Water Drinking Water

0.010 1.0 5.0

Water Water Water Water Water

None found None found None found 0.10 0.006 0.002 0.005 1.0

Source

USEPA 40 CFR 141 USEPA 40 CFR 141 Japanese legislation 98/83/EEC USEPA 40 CFR 141; Japanese legislation; Thai legislation 98/83/EEC USEPA 40 CFR 141 Japanese legislation; Thai legislation 98/83/EEC Japanese Legislation Thai Legislation

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

67

TABLE 9 Action Levels for Metals in Solid Media. (After Smith and Swanger, Ref . [20], 1999, and Smith, Ref. [21], 1999) Element

Media

Limit, mg/L unless noted

Source

Antimony Copper

TCLP Leachate TCLP Leachate

1.0 500

Lead Lead

TCLP Leachate Soil, bare, children’s’ play areas Soil, bare, surrounding a residence (average concentration) TCLP Leachate

5.0 400 mg/Kg

TNRCC 30 TAC 335a various State (USA) regulations USEPA 40 CFR 261b TSCA section 403

1200 mg/Kg

TSCA section 403

5.0

USEPA 40 CFR 261

Lead

Silver a

TNRCC 30 TAC 335 refers to State of Texas statutes. Some jurisdiction observe a 1.5mg/L limit, based on a multiple of the 0.015 mg/L drinking water limit.

b

the incidence of this route of lead exposure. In contrast, the primary exposure route for the adult population is inhalation. Lead particles in the air derive from the burning of leaded gasoline, and to a lesser extent from lead smelting and other lead processing in industry. With the ban on lead containing gasoline in many countries, this route of exposure has much less risk than in the past. Lead exposure via inhalation is geographically related, favoring those near roadways and lead smelters with a higher airborne lead concentration. Lead workers, persons involved in the actual processing of lead materials such as battery plates, are the only significant population impacted by direct contact with lead. The adult population, regardless of age, has little opportunity for direct contact with significant quantities of lead. Children are impacted by direct contact via the ingestion mechanism described above. d. Various Populations. The Center for Disease Control (CDC, Atlanta, Georgia, USA) has conducted several blood-lead studies. These studies sampled both children and adults. Various State and Local health departments have also studied blood lead levels. Table 10 shows a compilation of some of these studies. Blood lead levels in children ages one to two years have dramatically decreased. In 1976, 88% of children in this age group had blood lead levels greater than 10 ug/dl, but that number declined to 5.9% by 1994[1,9,30]. e. Steady Decline and Reasons. After it was banned as a gasoline additive, average population blood lead levels fell precipitously. From 1976, where one study [9,30] measured an average blood lead of 100 ug/dl in a population near roadways, the study measured 40 ug/dl in 1980 in the same population of subjects. Other contributors to the reduction of blood lead levels include reduced lead levels in plumbing and canning solders, bans on lead in residential paints, education and screening programs by public health agencies, and destruction of older dwelling structures which contained lead-based paint.

D.

Zinc

1. Uses Zinc is used primarily in galvanizing metals and metal alloys, but zinc compounds also have wide commercial applications as chemical intermediates, catalysts, pigments, vulcanization activators and accelerators in the rubber industry, UV stabilizers, and supplements in animal feeds and fertilizers. They are also used in rayon manufacture, smoke bombs, soldering fluxes, mordents for

68 TABLE 10

SMITH Compilation of Lead in Blood Studies. (After US-EPA, Ref. [9], 1998)

Attribute

Sample Size

Selected Percentiles (95% Confidence Interval)

Geometric Mean (95 % Confidence Interval)

10th

25th

50th

75th

90th

Total, age 1 and older Gender Males

3,189

1.6 (1.4–1.8)

0.7 (0.6–0.7)

1.0 (0.9–1.1)

1.5 (1.4–1.7)

2.3 (2.2–2.6)

3.7 (3.2–4.3)

1,594

Females

1,595

1.9 (1.7–2.1) 1.3 (1.2–1.5)

0.8 (0.7–0.9) 0.6 (0.4–0.7)

1.2 (1.1–1.4) 0.8 (0.7–0.9)

1.8 (1.7–2.0) 1.2 (1.1–1.4)

2.7 (2.5–3.1) 1.9 (1.7–2.1)

4.3 (3.7–5.3) 3.0 (2.6–3.5)

1.7 (1.5–2.0) 1.8 (1.6–2.0) 1.5 (1.4–1.7)

0.8 (0.6–0.8) 0.7 (0.6–0.8) 0.6 (0.5–0.7)

1.1 (0.9–1.3) 1.1 (0.9–1.2) 1.0 (0.8–1.1)

1.6 (1.4–1.8) 1.6 (1.4–1.9) 1.5 (1.3–1.6)

2.5 (2.2–3.0) 2.8 (2.3–3.3) 2.3 (2.1–2.5)

4.2 (3.3–5.2) 4.1 (3.8–5.2) 3.5 (3.1–4.1)

2.0 (1.7–2.3) 1.3 (1.0–1.6) 1.0 (0.8–1.2) 1.4 (1.2–1.5) 1.9 (1.7–2.0) 2.5 (2.2–2.8)

0.9b (0.5–1.1) 0.6 (0.5–0.7) 0.4 (0.2–0.5) 0.6 (0.5–0.7) 0.9 (0.7–1.0) 1.2 (1.1–1.3)

1.3 (1.1–1.5) 0.8 (0.7–1.0) 0.6 (0.5–0.8) 0.8 (0.8–1.0) 1.2 (1.1–1.3) 1.6 (1.5–1.9)

1.9 (1.6–2.1) 1.2 (1.0–1.5) 0.9 (0.8–1.1) 1.3 (1.1–1.5) 1.8 (1.6–1.9) 2.3 (2.1–2.7)

2.7 (2.2–4.4) 1.7 (1.4–2.2) 1.4 (1.2–1.6) 2.0 (1.7–2.2) 2.7 (2.4–3.2) 3.5 (3.0–4.3)

4.7b (3.5–9.8) 2.7 (1.9–4.7) 2.1 (1.9–2.4) 2.8 (2.5–3.2) 3.8 (3.6–4.4) 5.0 (4.5–6.4)

Race/Ethnicity Black, non-Hispanic Mexican American White, a non-Hispanic Age group 1–5 years

693 1,289 1,207

254

6–11 years

419

12–19 years

868

20–39 years

595

40–59 years

471

60+ years

582

Numbers in parentheses are 95% confidence intervals. a Includes other race/ethnic groups. b Estimate meets minimum standards of reliability but should be interpreted with caution.

printing and dyeing, wood preservatives, mildew inhibitors, deodorants, antiseptics, and astringents. In addition, zinc phosphide is used as a rodenticide. Zinc is an essential element with recommended daily allowances ranging from 5 mg for infants to 15 mg for adult males [31]. 2. Ingestion In humans, acutely toxic oral doses of zinc cause nausea, vomiting, diarrhea, and abdominal cramps and in some cases gastric bleeding. Ingestion of zinc chloride can cause burning in the mouth and throat, vomiting, pharyngitis, esophagitis, hypocalcemia, and elevated amylase activity indicative of pancreatitis. Zinc phosphide, which releases phosphine gas under acidic conditions in the stomach, can cause vomiting, anorexia, abdominal pain, lethargy, hypotension, cardiac arrhythmias, circulatory collapse, pulmonary edema, seizures, renal damage, leukopenia, and coma and death in days to weeks. The estimated fatal dose is 40 mg/kg of body weight. Gastrointestinal upset has also been reported in individuals taking daily dietary zinc supplements for up to 6 weeks. There is also limited evidence that the human immune system may be impaired by sub chronic exposures. Chronic oral exposures to zinc have resulted in hypochromic microcytic anemia associated with hypoceruloplasminemia, hypocupremia, and neutropenia in some individuals.

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Gastrointestinal absorption of zinc is variable (20–80%) and depends on the chemical compound as well as on zinc levels in the body and dietary concentrations of other. In individuals with normal zinc levels in the body, gastrointestinal absorption is 20–30%. Information on pulmonary absorption is limited and complicated by the potential for gastrointestinal absorption due to the swallowing of zinc-containing mucus from the respiratory tract. Zinc is present in all tissues with the highest concentrations in the prostate, kidney, liver, heart, and pancreas. Zinc is a vital component of many enzymes such as carbonic anhydrase, which regulates CO2 exchange. 3. Inhalation Under occupational exposure conditions, inhalation of zinc compounds (mainly zinc oxide fumes) can result in a condition identified as ‘‘metal fume fever,’’ which is characterized by nasal passage irritation, cough, headache, altered taste, fever, weakness, hyperpnea, sweating, pains in the legs and chest, leukocytosis, reduced lung volume, and decreased diffusing capacity of carbon monoxide. Inhalation of zinc chloride can result in nose and throat irritation, dyspnea, cough, chest pain, headache, fever, nausea and vomiting, and respiratory disorders such as pneumonitis and pulmonary fibrosis. Although ‘‘metal fume fever’’ occurs in occupationally exposed workers, it is primarily an acute and reversible effect that is unlikely to occur under chronic exposure conditions when zinc air concentrations are less than 8–12 mg/m3. Gastrointestinal distress, as well as enzyme changes indicative of liver dysfunction, have also been reported in workers occupationally exposed to zinc fumes. However, it is not clear to what extent these effects may have been caused by pulmonary clearance (swallowing respiratory mucus) and subsequent gastrointestinal absorption. Consequently, there are no clearly defined toxic effects that can be identified as resulting specifically from pulmonary absorption following chronic low level inhalation exposures to zinc. 4. Low Toxic Concern/Regulation No case studies or epidemiologic evidence is known to suggest that zinc is carcinogenic in humans by the oral or inhalation route. Zinc has received little environmental regulatory attention as it is a necessary nutrient and is ubiquitous in the natural environment. It is regulated at high levels in the TCLP test (500 mg/L) in some jurisdictions in the United States.

E.

Bismuth

1. Uses Used in medicines for decades, bismuth has many medically beneficial uses. At higher doses chronic bismuth poisoning can occur. Symptoms of chronic bismuth toxicity in humans consist of decreased appetite, weakness, diarrhea, fever, and metal line on the gums, foul breath, gingivitis and dermatitis. Jaundice and conjunctival hemorrhage are rare, but have been reported. Bismuth nephropathy with proteinuria may occur, as the kidney is the site of highest concentration, with the liver being considerably lower. Bismuth does pass into the amniotic fluid and into the fetus. Bismuth and/or bismuth compounds have been suggested to be a carcinogen or a co-carcinogen in rats. In addition, some studies have shown that bismuth can cause chromosomal aberrations in rats. More epidemiological studies are required for a more complete determination. Little has been studied as to the potential toxic effects of bismuth in the natural environment. 2. Human Exposure Bismuth has similar human exposure pathways as lead. Its overall exposure opportunity to the human population is much less than lead since bismuth has seen many fewer uses in industry. The opportunity for exposure from ingestion is low, except in its use in medicines. The general population is not subject to bismuth exposure by direct contact. Bismuth is a byproduct of lead smelting and thus there is human exposure from airborne lead smelting emissions.

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3. Regulation Bismuth currently sees little regulation from any governmental authority worldwide. This is due to its perceived low toxicity and generally low industrial use.

F.

Copper

Copper occurs naturally in elemental form and as a component of many minerals. Because of its high electrical and thermal conductivity, it is widely used in the manufacture of electrical equipment. Common copper salts, such as the sulfate, carbonate, cyanide, oxide, and sulfide are used as fungicides, as components of ceramics and pyrotechnics, for electroplating, and for numerous other industrial applications. Copper can be absorbed by the oral, inhalation, and dermal routes of exposure. It is an essential nutrient that is normally present in a wide variety of tissues. 1. Ingestion In humans, ingestion of gram quantities of copper salts may cause gastrointestinal, hepatic, and renal effects with symptoms such as severe abdominal pain, vomiting, diarrhea, hemolysis, hepatic necrosis, hematuria, proteinuria, hypotension, tachycardia, convulsions, coma, and death. Gastrointestinal disturbances and liver toxicity have also resulted from long-term exposure to drinking water containing 2.2–7.8 mg Cu/L. The chronic toxicity of copper has been characterized in patients with Wilson’s disease, a genetic disorder causing copper accumulation in tissues. The clinical manifestations of Wilson’s disease include cirrhosis of the liver, hemolytic anemia, neurological abnormalities, and corneal opacities. 2. Inhalation Acute inhalation exposure to copper dust or fumes at concentrations of 0.075–0.12 mg Cu/m3 may cause metal fume fever with symptoms such as cough, chills and muscle ache-similar to zinc fumes. Among the reported effects in workers exposed to copper dust are gastrointestinal disturbances, headache, vertigo, drowsiness, and hepatomegaly. Vineyard workers chronically exposed to Bordeaux mixture (copper sulfate and lime) exhibit degenerative changes of the lungs and liver. 3. Direct Contact Opportunities for direct contact with copper are minimal in the general population. Occupational exposure is possible, particularly in the electrical equipment manufacturing industry. Dermal exposure to copper may cause contact dermatitis in some individuals. 4. Regulation Copper and copper compounds are regulated under Superfund, SARA 313, Clean Water Act Toxic Pollutant, California State Superfund Hazardous Substances, CAL-OSHA Director’s List of Hazardous Substances, and California HWCL Hazardous Wastes.

G.

Silver

Silver is a relatively rare metal that occurs naturally in the earth’s crust and is released to the environment from various industrial sources. Human exposure to silver and silver compounds can occur orally, dermally, or by inhalation. Silver is found in most tissues in small amounts, but has no known physiologic function. Other than occupational exposures, the general population is not at extensive risk of exposure to silver by any route. 1. Ingestion In humans, accidental or intentional ingestion of large doses of silver nitrate has produced corrosive damage of the gastrointestinal tract, abdominal pain, diarrhea, vomiting, shock, convulsions, and death.

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2. Inhalation Respiratory irritation was noted following acute inhalation exposure to silver or silver compounds. Silver nitrate solutions are highly irritating to the skin, mucous membranes, and eyes. Ingestion, inhalation, or dermal absorption of silver may cause argyria, the most common indicator of long-term exposure to silver or silver compounds in humans. Argyria is a gray or blue-gray, permanent discoloration of the skin and mucous membranes that is not a toxic effect per se, but is considered cosmetically disfiguring. Chronic inhalation exposure of workers to silver oxide and silver nitrate dusts results in upper and lower respiratory irritation, deposition of granular silver-containing deposits in the eyes, impaired night vision, and abdominal pain. 3. Direct contact Mild allergic responses have been attributed to dermal contact with silver. 4. Regulation Silver and silver compounds are regulated under Superfund, SARA 313, RCRA, Clean Water Act Toxic Pollutant, California State Superfund Hazardous Substances, CAL-OSHA Director’s List of Hazardous Substances, and California HWCL Hazardous Wastes.

H.

Antimony

Antimony (Sb) is a naturally occurring metal that is used in various manufacturing processes. Antimony is sparingly absorbed following ingestion or inhalation. Both gastrointestinal and pulmonary absorption are a function of compound solubility. Antimony is transported in the blood, its distribution varying among species and dependent on its valence state. Antimony is not metabolized but may bind to macromolecules and react covalently with sulfhydryl and phosphate groups. Excretion of antimony is primarily via the urine and feces, and is dependent upon valence state. 1. Ingestion Acute oral exposure to high doses of antimony or antimony-containing compounds may cause gastrointestinal disorders (vomiting, diarrhea), respiratory difficulties, and death at extremely high doses. Subchronic and chronic oral exposure may affect blood cell production and blood chemistry. 2. Inhalation Acute inhalation exposure of humans may cause gastrointestinal disorders, probably due to ingestion of airborne antimony. Long-term occupational exposure of humans has resulted in electrocardiac disorders, respiratory disorders, and possibly increased mortality. Antimony levels for these occupational exposure evaluations ranged from 2.2 to 11.98 mg Sb/m3. Based on limited data, occupational exposure of women to metallic antimony and several antimony compounds has reportedly caused alterations in the menstrual cycle and an increased incidence of spontaneous abortions. 3. Direct Contact Exposure to antimony from direct contact is unlikely except under occupational conditions. 4. Regulation Antimony and antimony compounds are regulated under Superfund, SARA 313, Clean Air Act Hazardous Air Pollutant, Clean Water Act Toxic Pollutant, California State Superfund Hazardous Substances, CAL-OSHA Director’s List of Hazardous Substances, and California HWCL Hazardous Wastes. It is interesting to note that where a set of several metals are regulated in drinking water by a given jurisdiction, antimony typically carries the lowest maximum allowable concentration. This attests to its chronic toxicity relative to other toxic metals.

72 I.

SMITH Indium

The information available on the toxic properties of indium in humans is limited due to its low use in industry and its rarity in the earth’s crust. It is known that soluble indium salts are extremely toxic when injected into laboratory animals with a direct effect on the heart, liver, kidneys and blood. However, indium salts are far less toxic when administered orally or by inhalation. Teratogenic effects have also been reported in laboratory animals injected with indium but its applicability to human exposure is unknown. 1. Toxicity in Humans The inhalation route is by far the most significant route in occupational settings. Indium metal is not irritating to the eyes or skin other than by direct abrasive action of metal particles on eye or skin tissue. Soluble indium salts; however, are very irritating to the eyes. Inhalation of indium fume or dust may cause irritation and damage to the respiratory tract. It may also irritate the gastrointestinal tract if ingested. The mechanisms of these irritations are probably similar to the irritations caused by other toxic metals used in electronics. Prolonged exposure to indium fume or dust may cause irritation and damage to the lung. Workers exposed to indium compounds during the production of indium complained of tooth decay, pain in joints and bones, nervous and gastrointestinal disorders, heart pains, and general debility. 2. Regulation Indium and indium oxide are not listed as human carcinogens by the Occupational Safety and Health Administration (OSHA), the National Toxicology Program (NTP), the International Agency for Research on Cancer (IARC), the American Conference of Governmental Industrial Hygienists (ACGIH) or the European Union (EU). Indium and its salts are not heavily regulated by worldwide environmental authorities. If bismuth and indium alloys are selected by the industry, their use will dramatically increase. Regulation may follow as environmental agencies deem them as having an adverse impact on the environment as the electronics industry solidifies its commitment to a given lead-free alloy or alloys.

J.

Nickel

Nickel is a naturally occurring element that exists in various mineral forms. It is used in a wide variety of applications including metallurgical processes and electrical components, such as batteries. Some evidence suggests that nickel may be an essential trace element for mammals. 1. Ingestion Most nickel enters the human body via food and water consumption. The absorption of nickel is dependent on its physicochemical form, with water-soluble forms being more readily absorbed. The metabolism of nickel involves conversion to various chemical forms and binding to various ligands in the body. Nickel is excreted in the urine and feces. 2. Inhalation Inhalation is the primary route for occupational nickel exposure. Inhalation exposure to some nickel compounds cause toxic effects in the respiratory tract and immune system. Acute inhalation exposure of humans to nickel may produce headache, nausea, respiratory disorders, and death. Asthmatic conditions have also been documented for inhalation exposure to nickel. Data on nickel-induced reproductive/developmental effects in humans following inhalation exposure are mixed, with some studies pointing to reproductive effects from inhalation exposure to nickel, while other studies found no observable effects. The primary target organs for nickelinduced systemic toxicity are the lungs and upper respiratory tract for inhalation exposure and the kidneys for oral exposure. Other target organs include the cardiovascular system, immune system, and the blood.

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3. Direct Contact Exposure from contact with nickel is not a significant pathway in humans.

K.

Tin

1. Ingestion Tin compounds have variable toxicity. Elemental tin and organic compounds have low toxicity and are poorly absorbed when ingested Tin and tin compounds can cause biological effects such as severe digestive tract irritation with abdominal pain, nausea, vomiting, diarrhea, and permanent tissue destruction of the esophagus and digestive tract. Tin salts may cause systemic toxic effects on the nervous system, heart, and liver. Tin may interact with the absorption of biologically essential metals such as copper, zinc, and iron and can inhibit various enzymes and metabolism. Ecotoxicity, reproductive effects, and mutagenicity have been observed in laboratory studies. 2. Inhalation Some inorganic tin salts are irritating or can liberate toxic fumes on decomposition. Inhalation of tin dusts over a period of years may cause pneumoconiosis. Like other toxic metals, inhalation typically causes irritation to the nose and throat. Prolonged or repeated exposure may cause pneumoconiosis. 3. Direct Contact Exposure from direct contact with tin is not a significant pathway in humans. 4. Regulation A literature search uncovered no regulation on elemental tin. There is regulation of organic tin containing materials used to kill and remove barnacles from ship hulls (tributyltin) due to their extreme toxicity to shellfish.

L.

Cadmium

Cadmium is a naturally occurring metal that is used in various chemical forms in metallurgical and other industrial processes including the production of pigments. 1. Ingestion Cadmium exposure can occur via the diet and drinking water, accumulating primarily in the liver and kidneys. An acute oral exposure to 20–30 g has caused fatalities in humans. Exposure to lower amounts may cause gastrointestinal irritation, vomiting, abdominal pain, and diarrhea. Chronic exposure to cadmium primarily affects the kidneys, although other conditions such as ‘‘itai-itai’’ disease may involve the skeletal system. Cadmium is excreted primarily in the urine. 2. Inhalation Cadmium is absorbed more efficiently by the lungs (30 to 60%) than by the gastrointestinal tract. For inhalation exposure, both the lungs and kidneys are target organs for cadmium-induced toxicity. Inhalation exposure to cadmium and cadmium compounds may result in effects including headache, chest pains, muscular weakness, pulmonary edema, and death. Renal toxicity (tubular proteinosis) may also result from inhalation exposure to cadmium. 3. Direct Contact Exposure to cadmium for the general population via direct contact is not significant. 4. Regulation Due to its use in industry, cadmium containing solid waste disposal is regulated in many industrialized countries. These countries also have regulated the acceptable levels of cadmium in drinking water and wastewater.

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VI.

TOXICITY TEST METHODS AND ISSUES

A.

Basis for Toxicity Testing

Materials are tested for toxicity using standardized laboratory methods. These methods are believed to produce results that correlate to the perceived risks of their use in the environment, and particularly their potential harm if released into the environment. Many environmental authorities worldwide have published test methods. These vary from jurisdiction to jurisdiction, but each has the same goal stated above. Additionally the results of the laboratory testing undergo a variety of interpretive steps to classify materials and wastes as hazardous and non-hazardous. The results leading to these classifications also vary depending on the media under test, i.e. drinking water has the lowest allowable contaminant levels while solid wastes typically have the highest allowable concentrations before declared ‘‘hazardous.’’

B.

Total vs. Leachate Concentration

There are two primary groupings related to materials testing for environmental contamination potential. These are total concentration and leachable concentration methods. Total concentration methods require the analyst to use the sample material in its ‘‘as received’’ state, with no physical or chemical modifications before analysis for the contaminants of interest. Leachable concentration methods require that the sample material be taken through a leaching process before the leachate is analyzed for the contaminant of interest. With leaching methods, one takes a sample of material and creates a leachate sample that is analyzed. The leached material is normally disposed after the leaching process is completed. 1. Comparison and Usefulness to Estimating Exposure Total concentration methods are useful in estimating exposure to the environment from a contaminant of interest, as the exposure cannot exceed the total concentration in the material. If the environmental characteristics (such as weathering) of a material are well characterized, then a total concentration analysis allows researchers to estimate the impact to the environment as the material releases or retains a contaminant of interest. Leach methods serve to simulate the action of weathering on a material. When acids are used in the leachate fluid, these methods may also simulate the effects of acid rain (nitric and sulfuric acids are used) and landfill leachate exposure (acetic acid is used). The USEPA has determined experimentally that the leaching fluid pH is a significant variable in the variability and thus the usefulness of leaching method results [24].

C.

General Test Methodology for Solid Wastes

1. Procedure Table 11 lists a summary of the primary leach methods published by environmental agencies [32– 34] In general, the waste is sampled using techniques to ensure that the sample represents the waste stream or lot. After sampling and transport to the laboratory, the sample is analyzed for characteristics such as ignitability, reactivity, and corrosivity, that many jurisdictions consider a hazardous characteristic if a substance exhibits any one or more of them. After these preliminary tests, the waste sample is checked for particle size. Most leach methods require that the waste material be ground so that the maximum particle size does not exceed some dimension. Leaching is a surface phenomenon; and sizing the waste allows for an adequate surface area to volume of waste material to provide sufficient conditions for leaching to occur. The leaching fluid of choice is added to the sized waste sample in a prescribed sample weight to fluid weight ratio. The sample is then tumbled (for the USEPA TCLP leach) or shaken and then allowed to settle without further agitation. Agitation is used to simulate weathering and makes the leaching process more aggressive. After the prescribed leaching time, the leachate is filtered typically through a 0.45 um filter to simulate the inability for solids to percolate through soil or rock in the case of a landfill liner breach or leachate overflow across a soil surface.

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD TABLE 11

75

Leach Methods Used Worldwide. (Information from Refs. [32–34]) Method Name

Leach Media

pH of Leach Media

Dilution Factor

United States United States California (USA) European Community Japan

TCLP SPLP STLC PrEN JST-13

Acetic acid, buffered Nitric + Sulfuric Acids Citric acid, buffered Deionized water Deionized water

4.88 5.00 5.00 Neutral neutral

20 20 10 10 10

PrEN 12457 parts 1–3. Compliance test for granular waste materials (CEN TC292 2002)

Two-step (ls2, ls2–10) or one-step (ls10) agitated leaching with deionised water on granular (<4mm) materials. Tested on equivalent of 100 g dry solids after removal and quanitification of non-crushable portion.

Maximum availability leaching test (NEN 7431, 1995)

Two step agitated leaching test at pH 7 and 4 on ground material

Tank test (NEN 7345, 1995)

Sequential extraction of solidified materials at pH4, 8 or 12.5 over a maximum of 64 days. 24 hour extraction of granular materials at controlled pH. Replicate samples are tested over a range of pH values between pH 4 and 12. Sequential flushing of column of granular (<2mm) material with acidified water at increasing liquid to solid ratios (0.1–10).

Jurisdiction

pH dependence test (draft)

Column leach test (NEN 7343, 1995) and draft percolation simulation test

BS 6920 Part 2 Section 2.6 (extraction of metals)

Sequential extraction of cubes of solidified materials in test water over 9 days.

NRA interim test for contaminated soils (NRA 1994)

Two step or one-step agitated leaching with deionised water on granular (<5mm) materials. Tested on equivalent of 100 g dry solids.

To assess leachability under mild extraction conditions for waste disposal or material reuse options. Two step test indicates of relative timescales for release particularly when compared with availability for leaching. As required by Environment Agency RGN2 (2001) for waste acceptance to landfill. Threshold values for inert wastes have been published. Values for hazardous wastes are expected during 2002. To determine the potential (maximum) availability of leaching under worst case environmental conditions. To assess leachability of products which have been solidified for reuse or disposal. To determine the effect of falling or increasing pH conditions on leachability, once a material has been placed in situ. To determine the rate of leaching of various contaminants as a function of time or solid to liquid ratio, particularly at low ratios prevailing in disposal scenarios. To examine leaching of metals from non-metallic materials used for the supply of potable water (e.g. ready-mixed concrete, plastic pipes, coatings and epoxy resins). To examine leachabilty of soils under mild extraction conditions which mimic rainfall infiltration, to assess the potential for groundwater contamination during site development.

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This filtered leachate is then subjected to a variety of tests. When metals are the constituents of interest, the filtered leachate is digested by adding nitric acid and perhaps other oxidizing agents. The leachate undergoes digestion and then is brought to a prescribed standard volume for analysis. The digestion process ensures the complete breakdown of metal complexes, dissolution of metallic solids smaller than 0.45 um but larger than atomic size (provide dimension), and stability of the metal ions in solution while the digested leachate awaits analysis. Undigested filtered leachates can result in unreliable results because the metals may be non-uniformly dispersed through the leachate, and unstabilized metals may leave the liquid solution and plate onto the container wall, causing low readings in the metal analysis. Therefore, the analysis for the metals of interest follows digestion. The filtered leachate is ready to be analyzed for metals, volatile organic compounds, or non-volatile organic compounds such as pesticides. It is interesting to note that in the environment, the transport of metal elements in leachate is determined by the anions (sulfate, sulfide, chloride, bromide, etc.) in the leachate. The laboratory tests discussed do a poor job of emulating the effects of these anions on the toxic metals’ ability to migrate through the environment. The presence, concentration, and types of anions present in landfill leachates varies significantly from landfill to landfill [22] and thus no standardized test could hope to mimic the range of these conditions. Specialized (non-standard) tests can seek to better model the behavior of toxic metals from a particular leachate; however, these tests are only approved by environmental authorities on a case-by-case basis. 2. Analyses The analyses for metals in the leachate are undertaken primarily by Atomic Absorption Spectroscopy (AAS) or by Inductively Coupled Plasma Emission Spectroscopy (ICP). These methods make use of the unique absorption or emission spectra that all metal elements possess to quantify their presence in the leachate. The analytical instrument is standardized using multiple concentration levels of the metal elements being tested in the leachate. The absorption or emission is proportional to the concentration of the metal in solution. After standardization, the leachates are aspirated into the analytical instrument, where the metal concentrations are determined by the correlation with the absorption or emission spectra.

D.

Toxicity Characteristic Leaching Procedure (TCLP)

The Toxicity Characteristic Leaching Procedure (TCLP) was developed by USEPA for determining whether a waste is hazardous by virtue of its toxicity. In this context, waste is hazardous if it presents a threat to human health and the environment when ‘‘improperly managed.’’ The USEPA reasons that the most likely pathway for human exposure to toxic constituents is through drinking water contaminated by leachate from a landfill. Accordingly, the TCLP defines the toxicity of a waste by measuring the potential for the toxic constituents present in a waste to leach out and contaminate groundwater and surface water (and by extension, drinking water sources) at levels of health or environmental concern. Electronic devices containing tin-lead solders are disposed in municipal landfills. These landfills allow lead to leach from the disposed items. The TCLP specifically requires analyzing a liquid waste extract to determine whether it contains unacceptably high concentrations (Table 12) of any of eight toxic metal constituents identified in the USEPA National Interim Primary Drinking Water Standards. These are: arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver. Some states have added copper, nickel, and zinc to this list. 1. Extraction Solution Consistent with the mismanagement scenario, a leachate is obtained from solid waste by exposing the waste to organic acids as these are the acids likely to be found in a landfill containing decomposing municipal wastes. Prior to leaching the alkalinity of the sample must first be determined in order to properly select which of two different extraction fluids should be used. This determination is accomplished by reviewing the waste sample’s pH, or experimentally by adding acid or base until a pH change occurs. Samples with a low alkalinity utilize a sodium acetate extraction solution with a pH of 4.93, whereas those with a high alkalinity (often alkali stabilized

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD TABLE 12

77

STLC and TTLC Values. (Information from Refs. [35,36]) STLC, TTLC, TCLP METALS-MAXIMUM LIMITS

Analyte Antimony Arsenic Barium Beryllium Cadmium Chromium, total Cobalt Copper Lead Mercury Molybdenum Nickel Selenium Silver Thallium Vanadium Zinc

STLC max. limit mg/L (ppm)

TTLC max. limit mg/Kg (ppm)

15 5.0 100 0.75 1.0 5.0 80 25 5.0 0.20 350 20 1.0 5.0 7.0 24 250

500 500 10,000 75 100 2,500 8,000 2,500 1,000 20 3,500 2,000 100 500 700 2,400 5,000

TCLP max. limit mg/L (ppm) 5.0 100 1.0 5.0

5.0 0.20

1.0 5.0

wastes with a pH greater than 11) utilize a dilute acetic acid extraction solution with a pH of 2.8. Samples are tumbled in the appropriate extraction fluid for 18 hours. However, the choice of extraction fluids does not apply to volatiles, where the sodium acetate solution is always used. Analysis for volatile organics also requires a pressurized device to maintain these volatile compounds in the liquid leachate. This device is called a Zero Headspace Extraction (ZHE) apparatus. Results from the leachate are compared against the published TCLP regulatory limits. The TCLP is considered a relatively aggressive leaching procedure relative to other methods in use worldwide.

E.

Synthetic Precipitation Leaching Procedure (SPLP)

Aside from the deionized water leaching methods, there are methods that utilize leaching fluids containing various acids. These methods are used to simulate both acid rain and improper waste disposal mixing household and industrial waste in the same disposal unit. The Synthetic Precipitation Leaching Procedure (SPLP), developed by the USEPA, is one such test. The leaching fluid in the SPLP contains nitric and sulfuric acids, diluted to a pH of 5.00. Some European environmental agencies use a similar leaching fluid. These fluids contain either sulfuric acid or sodium nitrate.

F.

Distilled Water Methods

Many members of the European Community, including France and Germany, as well as Japan utilize deionized (demineralized) water-leaching tests. A European Community test method also specifies deionized water leaching. The State of Texas and other environmental regulators, utilize seven-day, but less aggressive leach method than the TCLP. Methods based on deionized water leaching are used to demonstrate the contamination potential to drinking water and groundwater from a waste that is exposed to potable (drinkable) water. The storm-water contamination potential can also be predicted using the seven-day leach method. In this method, a portion of the material under study is mixed with some multiple of its weight in distilled or deionized water,

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shaken, tumbled, or left to leach undisturbed for some prescribed period. The leachate is then filtered and analyzed for the constituents of interest. In jurisdictions where this test is used, if the leachate shows constituents of interest higher than local drinking water standards, the material is considered to have failed the test. 1. Distilled Water with Salts Methods Other leach methods, used in the European Community, call for chloride salts and nitrate salts to be added to the deionized-water leaching fluid. These leachates are believed to give a more accurate simulation of actual leaching processes in waste disposal areas in these geographic areas. Indeed, acid rain consisting of primarily nitrate anions, and soils with high chloride contents are better represented by these fluids than by the USEPA or other leachate compositions.

G.

Total Threshold Leaching Concentration (TTLC) and Soluble Threshold Limit Concentration (STLC)

The State of California, in addition to its use of the STLC [35] and the USEPA TCLP test, also provides regulations for the Total Threshold Limit Concentration (TTLC). The TTLC is simply a measure of the physical composition of the substance under study, with no regard for its leachability. Substances containing regulated elements or compounds over the TTLC value are deemed hazardous by the State of California. Table 12 lists the STLC and TTLC values, along with the associated maximum percentages a solder alloy can contain without failing the TTLC value. TTLC and STLC are used when determining a hazardous waste characterization under California State regulations [36]. 1. Soluble Threshold Limit Concentration (STLC) this analysis determines the amount of each analyte that is soluble in the ‘‘Waste Extraction Test’’ (W.E.T.) [35] leachate. This W.E.T. leachate procedure is used for solid samples or those containing > 0.5% solids. The sample is tumbled in 10 times its weight of a 0.2M sodium citrate buffer for 48 hours. This leachate is then analyzed to determine the soluble concentrations. 2. Total Threshold Limit Concentration (TTLC) this analysis determines the total concentration of each target analyte in a sample. Samples are analyzed using published EPA methods. When any target analyte exceeds the TTLC limits, the waste is classified as hazardous and its waste code is determined by the compound(s) that failed TTLC. The results of this analysis can be used to determine if analysis for STLC level is necessary by comparing 10 times the STLC limit to the TTLC results. This is done to compensate for a 1:10 dilution factor that is present in one analysis but not the other. If the TTLC results do not exceed 10 times the STLC limit, then normally no further analysis is required. The concentration of analyte in the leachate is compared against the STLC and TTLC regulatory values. These elemental allowances are much smaller than the lead-free alternative alloys typically considered. In fact, the limit values are much less, than allow concentrations expected to materially change the properties of tin-based solders. This presents a significant concern for California-based assemblers and fabricators. It appears unlikely that there will be a lead-free alloy that will pass the TTLC values (thus non-hazardous under California law). Any alloying elements added to tin will likely be in an amount greater than the TTLC limit in order for the alloying element to have any noticeable effect on the alloys physical properties such as melting point.

H.

General Interpretation of Results

The results of the leachate tests are compared against local regulatory limits. If these are exceeded then either the waste is declared ‘‘hazardous’’ or the waste is designated for further treatment to reduce leachable concentrations. In jurisdictions where total concentrations rather than leach-

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able concentrations are used, it is presumed that if the total concentration of a contaminant in the material is less than the leach concentration limit, then the material is ‘‘non-hazardous’’ since it is impossible to have a leachable concentration higher than a total concentration. Along with the results from the sample come various quality control analyses. These include matrix spike samples analyzed in the same analytical run as the actual leachate sample. A matrix spike is an aliquot of sample to which a known amount of the contaminant has been added before analysis. This spiked sample aliquot is subjected to exactly the same sample preparation, instrument calibration, and the like as an actual sample aliquot. It serves at the control for the actual sample test. The recovery of the known amount of contaminant is determined during analysis; and if an inadequate amount is recovered, or if the apparent amount recovered is excessive, it may invalidate the leachate sample results. In some cases, other metals interfere with the metal of interest. In these cases, the results are reported with the matrix spike recovery noted as an exception, or a different analytical technique is chosen—one without the sensitivity to the analytical interference of the first analytical method. A ‘‘blank’’ sample is also prepared and analyzed along with the waste samples of interest and spiked samples. A blank sample theoretically has an undetectable amount of the contaminant of interest. If the blank sample shows elevated levels of the contaminant of interest, it causes the associated analytical run to be suspect. After removing the source of the unwanted contamination of the analytical system, the waste sample is re-analyzed.

VII.

SUMMARY

Human exposure to lead had increased significantly from prehistoric times to the present. Man’s activities have released lead into the air, the soil, and into the various water supplies. Lead and other heavy metals used in electronics also have entered the environment and given rise to human exposure. Lead and other metals enter the environment and the human body through several routes, the most prevalent being ingestion by children. Ingestion of lead causes neurological dysfunction. This effect is especially marked in children, whose nervous systems are developing. Other organ systems are also affected by lead exposure, including the skeletal system, cardiovascular system, and the renal system. The human body, as well as other animal species, has metabolic processes that can remove lead. Drugs such as EDTA can augment these metal removal processes in the body. When solid wastes are disposed in landfills, rainfall and chemical reactions within the wastes release metals contained in the wastes’ solder, components, and circuit boards. As these metals are leached from the waste, the metals may enter the local groundwater. In many communities, groundwater is the source for drinking water. The metal-contaminated leachate liquid may also deposit into surface soils and rocks where humans, animals, and plants come into contact with the toxic metals. Plants may absorb toxic metals and thus enter the food web as animals eat the plants. The heavily industrialized countries regulate the disposal of wastes to prevent contamination of the environment as discussed above. These regulations include landfill construction requirements, landfill leachate monitoring requirements, and inbound waste screening for hazardous and toxic constituents. The inbound waste screening normally involves laboratory tests on the waste material. The tests vary from country to country but typically simulations of landfill leaching, often characteristic of a particular region. The laboratory-generated leachate is then tested for toxic constituents and if present at predefined levels, results in the waste material being classified as ‘‘hazardous’’ by regulation. Hazardous waste is more closely monitored after disposal than is waste classified as ‘‘non-hazardous.’’ Controls on waste disposal including the removal of lead compounds from automotive gasoline, building paints, and food containers have combined to cause a decline in blood lead levels in the United States’ general population. The dramatic reductions observed provide conclusive evidence that efforts taken to remove lead and other toxics from our environment has had a corresponding significant effect on our uptake, thus a very positive impact on our health.

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ACKNOWLEDGMENTS This chapter would not have been completed had it not been for the help and patience of my wife, Donna. I dedicate the chapter to her, my son, Samuel, and my father, without whose guidance on the environment would not have sparked my interest in it. The reader is also directed to: The Lead-Free Soldering Forum at IPC PCEXPO’99, Long Beach, California; March 1999. J. Miller, J. The Environmental and Health Risks of Lead in Electronics; Proceedings of IPC Works ’99, Chicago: IPC, 1999. Stern, Arthur Cecil. Fundamentals of Air Pollution; Academic Press: London, 1984.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

Christian, B. ‘‘Lead Pollution, A Good Story,’’ Proceedings of IPC Works ’99; IPC: Chicago, IL, 1999. Code of Federal Regulations, Chapter 40, Parts 136 (wastewater), 141 (drinking water), 261 (hazardous and solid waste); United States Government Printing Office, 2001. Nriagu, Jerome O. Lead and Lead Poisoning in Antiquity; John Wiley and Sons: New York, 1983; 378–382. Massaro, Edward, Ed.; Handbook of Human Toxicology; CRC Press: Boca Raton, Florida, 1997. Tortora, Gerard, J.; Grabowski, Sandra Reynolds. Principles of Anatomy and Physiology; 9th Ed.; John Wiley and Sons: New York, 2001. Rose, John. Environmental Toxicology: Current Developments; Overseas Publishers Association: New Delhi, 1998; 10, 81–100. Mason, R.P.; Sullivan, K.A. ‘‘Mercury in Lake Michigan’’. Environmental Science and Technology 1997, 31 (3), 942–947. Jacob, S.W.; Francone, C. Elements of Anatomy and Physiology; W. B. Saunders and Co: Philadelphia, 1989. United States Environmental Protection Agency. Risk Analysis to Support Standards for Lead in Paint, Dust, and Soil; EPA-747-R-97-006, Washington, D.C., 1998. Legge, Allan H.; Krupa, Sagar. Air Pollutants and their Effects on the Terrestrial Ecosystem; John Wiley and Sons: New Jersey, 1986; 197–205, 354–359. Prasad, A., Ed.; Essential and Trace Elements in Human Health and Disease: An Update; Wiley-Liss, Inc.: New York, 1993. National Institutes of Health, National Center for Complementary and Alternative Medicine. ‘‘EDTA Chelation Therapy for Coronary Artery Disease’’; RFA-AT-01-004, 1994. Morgan, Jean ‘‘Chelation Therapy in Lead Neuopathy.’’ Southern Medical Journal Aug. 1975, 168 (8), 1001, 1003. Code of Federal Regulations, Chapter 40, Part 60. Centers for Disease Control. ‘‘What Every Parent Should Know about Lead Poisoning in Children’’; Centers for Disease Control: Atlanta,GA, 1997. Bruin, A. Environmental Toxicology of Environmental Agents; Elsevier: The Netherlands, 1976. Federal Register: January 12, 2000 (Volume 65, Number 8), 1949–2015. Code of Federal Regulations, Chapter 40, Parts 9, 141, 142. United States Environmental Protection Agency. Water on Tap; EPA 815-K-97-002, 1997. Smith, Edwin B. III; Swanger, Kristine L. ‘‘Lead Free Solders: A Push in the Wrong Direction?’’ Proceedings of IPC PCB Expo ’99; Long Beach, California, pp. F-10-1 to end, March 1999. Smith, Edwin B. III. ‘‘Environmental Impacts and Toxicity of Lead Free Solders, Including Japanese and European Union Regulations,.’’ Proc. IPC Works ’99; Minneapolis, MN, Oct. 1999. Saleem, Zubair. ‘‘Summary of Comments on: Management and Disposal of Lead-Base Paint Debris; Proposed Rule’’; United States Environmental Protection Agency, Docket No. F98-LBPD-FFFFF, September 1999. For documents concerning an example site, see Texas Register, (12 TexReg 3858-3859) and Texas Register, (13 TexReg 427-428) for items pertaining to the ‘‘Houston Lead’’ site, and displaying groundwater impact statements. United States Environmental Protection Agency. SW-846: Test Methods for Evaluating Solid Waste, Physical/Chemical Methods, 2001. Appelo, C.A.J.; Postma, D. Geochemistry, Groundwater and Pollution; A. A. Balkema: Rotterdam, 1993. Code of Federal Regulations, Chapter 40, Part 261.

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Allenby, B.R.; Ciccarelli, J.P.; Artaki, I.; Fisher, J.R.; Schocnthaler, D.; Carroll, T.A.; Lyons, A.M.; Dahringer, D.W.; Degani, Y.; Gradel, T.E.; Plewes J.T.; Freund, R.S.; Gherman, C.; Solomon, H.; Melton, C.; Munie, G.C.; Socolowski, N. An Assessment of the Use of Lead in Electronic Assembly; Surface Mount Council, Northbrook, Illinois, 1999. 28. Townsend, Timothy G. Characterization of Lead Leachability from Cathode Ray Tubes Using the Toxicity Characteristic Leaching Procedure; State University System of Florida, Gainesville, Florida, December 1999. 29. Thornton, Iain; Rautiu, Adu; Brush, Susan. Lead, the Facts; IC Consultants Ltd: London, UK, December 2001. 30. United States Environmental Protection Agency. A Summary of Studies Addressing the Source of Soil-Lead. Technical Summary 1998, Volume I, EPA 747-R-98-001A, 1998. 31. Centers for Disease Control (CDC). Agency for Toxic Substances and Disease Registry, ‘‘ToxFAQ’s’’, available for all metals discussed in this chapter from the CDC, Atlanta, GA, and National Institutes of Safety and Health (NIOSH). Registry of Toxic Effects of Chemical Substances (RTECS), 1999. 32. Jackson. L.P.; Rohlik, A.R.; Conway, C., Eds.; ‘‘Hazardous and Industrial Waste Management and Testing: 3rd Symposium’’; ASTM Special Technical Publication 851, Publication Code Number 04851000-16, Philadelphia, PA, 1983. 33. Test Method PrEN 12457 parts one through three, published by the European Union; available from British Standards Institute, 389 Chiswick High Road, London, England W4 4AL. 34. Test Method BS6920, available from British Standards Institute, 389 Chiswick High Road, London, England W4 4AL. 35. California Environmental Protection Agency. Waste Classification Regulation Guidance Manual, August 1994. 36. California Code of Regulations, Title 26. 37. International Air Transport Association. Dangerous Goods Regulations; 40th edition; International Air Transport Association: Geneva, 1999.

3 Environmental Impact of Lead and Alternatives in Electronics Laura J. Turbini University of Toronto, Toronto, Ontario, Canada

Dennis Bernier Kester Northrop Grumman, Des Plaines, Illinois, U.S.A.

I.

INTRODUCTION

The environmental impact of lead dates back to 1887 when U.S. medical authorities diagnosed lead poisoning in children and later, (1904), linked this poisoning to lead-based paint. In 1921 Thomas Midgley discovered that tetraethyl lead curbs engine knock when added to gasoline. The following year the public health service warned of the dangers of leaded gasoline. In spite of that warning, leaded gasoline went on sale in 1923 [1] and continued to be sold in the USA until the late 1990s when it was banned. With the Clean Air Act of 1970, the U.S. Environmental Protection Agency (EPA) proposed the phase-out of lead in gasoline in 1972. Eleven years later, the EPA reported that the amount of lead in gasoline had dropped 50% and lead levels in blood dropped 37% between 1976 and 1980. By 1991, there was a 78% reduction in lead-levels in blood [1]. In the United States, the Clean Water Act of 1986 banned the use of lead alloy solders for potable water systems, and lead in household paint has been prohibited since before 1970. The dramatic decline in blood lead levels in adults and children since 1970 has been the result of regulatory and voluntary elimination of lead in gasoline, drinking water plumbing connections and fittings, and paint. In the past, the health and safety concerns with lead poisoning have focused on lead-based paint and lead in gasoline. However, the increasing quantity of scrap electronic products disposed into landfills has raised the question of the environmental impact of this source of lead. Studies in 1991 by Allenby, et al. [2] examined the potential for replacing lead-based solder and concluded that there were no viable alternatives at that time. They also suggested that the total environmental impact of lead and its alternatives, from mining, through manufacture, use, and end-oflife should be considered. In the early 1990s, legislation was proposed in the US, but no regulatory action was taken. The movement toward lead-free electronics emerged into concrete legislation in Europe on June 13, 2000 when the European Commission adopted two proposals: A Directive on Waste of Electrical and Electronic Equipment (WEEE), and a Directive on the Restriction of Hazardous Substances (ROHS). The WEEE requires Member States to set up take-back centers for end-oflife recovery at no cost to the consumer. The initial WEEE included the ROHS requirements and

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set 2004 as the date for lead-free electronics (lead in cathode ray tubes (CRTs) is exempt). Since then the deadline was established as July 01, 2006. In Japan, the Japan Electronic Industry Development Association (JEIDA) developed the Roadmap 2000 for Commercialization of Lead-free Solder. Thus, the environmental impact of lead from electronics and lead-free alternatives is an important issue to explore.

A.

Overview of Issues/Concerns

The regulatory actions banning the use of lead for a variety of applications, such as plumbing, household paint, gasoline, solder in food cans, shot, etc. was based on scientific evidence that lead from these sources can cause harm to human health and to the environment. The action to prohibit lead in electronic products appears to be guilt by association. There is no evidence that lead in electronic products is causing human lead poisoning or is leaching into ground water from obsolete products placed in landfills. It is not known whether alternative solder alloys incorporating tin, silver, copper with additions of other metals such as bismuth, antimony, indium, and nickel, may be even more harmful to the environment. Consideration must be given to the total cycle of metal usage, including mining, smelting, manufacturing, energy requirements, reliability, and leaching potential. The issue is not only a ban on the use of lead solders in electronics, but also a plan to recycle electronics at end-of-life instead of discarding them into landfills.

B.

Lead Utilization

Lead storage batteries represent the major use of lead in the United States, according to data from the U.S. Geological Survey [3] noted in Table 1. The use of lead in electronic solder is only 0.3% of the total lead usage. If one considers consumer electronics, which includes television sets and computers, approximately 90% of the lead utilized in electronics is contained in cathode ray tubes (CRTs) which are exempt under the ROHS Directive.

C.

Obsolete Electronics

The proliferation of electronic technology, particularly for communications and computer applications, is increasingly prevalent in business and home applications. Consumers want smaller, faster, more capable and convenient electronic products. The amount of electronic waste is expected to increase rapidly with most of it destined for landfill.

TABLE 1 Major Uses and Consumption of Lead in the United States (After U.S. Geological Survey, Ref. [3], 2000) Product Storage batteries Ammunition Paints, ceramics, pigments, chemicals Casting metals Sheet lead Cable covering Solder (excluding electronic solder) Electronic solder Brass, bronze billets and ingots Pipes, traps, extruded products Miscellaneous

U.S. consumption (%) 86.6 3.6 3.0 2.0 1.4 0.2 0.4 0.3 0.1 0.1 2.3

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1. Quantity/Rate of Growth Technological obsolescence is creating a stockpile of used electronic products. Computers are a prime example. Every year in the United States 36.7 million computers are manufactured [4]. In 1998, 20.6 million computers became obsolete and only 2.3 million were recycled. The remaining computers are sitting in closets, garages, attics, etc. awaiting future disposal. These obsolete computers, TV sets, VCRs, radios, compact disc players, etc. contain important natural resources which must be considered when evaluating their end-of-life scenarios. In Canada, 1,356 tons of lead from computers and monitors were discarded in landfills in 1999. A report from Enviros RIS for Environment Canada predicts that 67,324 tons of computers, laptops, peripherals and monitors will be disposed in 2005, and another 91,219 tons will be reused or recycled [5]. In Europe [6] the rapid growth in WEEE is projected to be three times higher than the growth of average municipal waste. While 6 million tons were generated in 1998, the volume is expected to increase at least 3 to 5% annually resulting in 16 to 28% more WEEE by 2003 and a doubling of this amount by 2010. Thus, the recovery and reuse of the material resources represented by WEEE must be addressed. 2. Composition of Landfills and Electronic Waste Household trash in the U.S. is normally sent to a Municipal Solid Waste (MSW) facility for placement into a landfill site. Fig. 1 shows graphically the content of this waste stream prior to recycling, with metals representing 8%. The amount of MSW generated in the U.S. in 2000 was nearly 232 million tons (Fig. 2) [7] with the average amount of solid waste generated daily per person increasing from 2.7 pounds in 1960 to 4.5 pounds in 2000. While recycling has increased from 5.6 to 69.9 million tons/year during the same period, the recycling rate also increased from less than 5% to about 30% (Fig. 3) [7]. Though 96.4% of automobile batteries are recycled, their presence in landfills (Table 2) represents the major source of lead at these sites [8].

D.

Replacing Lead-Bearing Solders in Electronics

The solder alloys most commonly used for electronics assembly contain tin (Sn) and lead (Pb) with a standard composition being Sn63Pb37. Compared to the tin-lead solder alloy, most leadfree alloys under consideration melt at much higher temperatures, while only a few melt at lower

FIG. 1 Graphical representation of waste streams prior to implementing recycling. (After U.S. Environmental Protection Agency, Ref. [7], 2000.)

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FIG. 2 Trends in municipal solid waste generation from 1960 to 2000. (After U.S. Environmental Protection Agency, Ref. [7], 2000.)

FIG. 3 Trends in the weight and percentage of recycled solid waste materials. (After U.S. Environmental Protection Agency, Ref. [7], 2000.)

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TABLE 2 Major Lead-Containing Municipal Solid Waste Discards and their Content (After Industry Economics Company, Ref. [8], 1991) Lead-containing MSW discards

Percent by weight

Lead acid batteries Television picture tubes Other consumer electronics Glass and ceramics Plastics Cans and other shipping containers Other

48.1 35.8 4.4 5.5 2.5 1.4 2.3

temperatures. Some solder replacement candidates are eutectic alloys melting at a single temperature, while others are non-eutectic alloys that melt over a temperature range. Table 3 lists some of the many alloys that have been investigated along with their melting points or melting range. 1. Major Sources in Electronics In addition to computer monitors (which account for almost 90% by weight of the lead (Pb) in electronic waste found in landfills) printed wiring assemblies are the other major source of Pb in electronic products. The tin-lead solder alloys used in components and on solderable terminations, and as a preservative coating on cards and boards, are the applications for lead-free solder compositions. Solder is used to join electronic components and other attachments to printed wiring boards. Soldering can be accomplished by hand with soldering irons or with automation using wave soldering machines or reflow ovens. Automation allows the soldering of hundreds of electrical connections per second. Wave soldering machines convey assembled printed wiring assemblies across fluxing and preheating stations, and then over the wave of molten solder. The lead-free solder alloys selected for hand or wave soldering applications include tin-copper (Sn-Cu), tinsilver (Sn-Ag), and tin-silver-copper (Sn-Ag-Cu).

TABLE 3 Partial List of Lead-Free Alloys and their Melting Temperatures Alloy Sn-Pb37 Sn-Bi58 Sn-Zn9.0 Sn-Cu0.7 Sn-Ag3.5 Sn-Ag3.8-Cu0.7 Sn-Ag3.0-Cu0.5 Sn-Ag2.5-Cu0.8-Sb0.5 Sn-Ag3.5-Bi4.8 Sn-Ag2.0-Bi7.5Cu0.5 Sn-Ag2.0-Bi4.0-Cu0.5-Ge0.1 Sn-Ag3.5-In1.5 Sn-Ag2.8-In20.0

Melting point (jC) 183 138 198.5 227 221 217 217.–220 217.–219 205.–210 217.–218 210.–217 218 175–187

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Reflow soldering involves the selective deposition of solder paste at termination pads or other features on printed wiring boards. Then, the electronic components and other devices are placed on the board, and the bond is completed by heating and melting the solder. The most common lead-free alternative alloys considered for solder paste are tin-silver (Sn-Ag), tin-silver-copper (Sn-Ag-Cu), and tin-silver-bismuth (Sn-Ag-Bi). Tin bismuth (SnBi), tin indium (SnIn) and silverfilled conductive epoxy are also considered for certain applications. In addition to the solder used to join parts together, a solder coating may be used to preserve or improve solderability when solder is applied as a metallization or so-called finish on printed wiring boards (PWB). An estimated 60 to 70% of PWB copper circuit features are coated with tinlead solder, usually applied by a hot air solder leveling (HASL) process. The printed wiring boards are passed over a molten solder wave, and the excess solder is removed by hot air jets or knives. Alternative lead-free coatings utilized on copper features of PWBs include thin films of tin (Sn), silver (Ag), or tin-bismuth (Sn-Bi), which are applied by electroless plating. Also, organic solderability preservative (OSP) coatings can be applied to protect the copper and avoid oxidation of the copper during storage. However, these chemical coating materials do not provide corrosion protection equal to solder coatings. Some lead-free work has been done using Sn96.5Ag3.5 (eutectic composition) applied by the HASL process. Many component terminations are either plated or dipped into molten tin-lead solder to preserve their solderability, providing yet another source of lead. This is the normal procedure for component terminations that are difficult to solder, such as nickel-iron or nickel plating. Alternative lead-free finishes for components include palladium (Pd), and gold (Au) applied by electroless plating over nickel (Ni), and hot-dipped coatings of Sn-Ag, Sn-Cu, or Sn-Ag-Cu alloy. Some component metallizations are coated with Pd-Ag, Pd-Au, or Pd-Ni. These coating alternatives for tin-lead add new materials to the electronics waste stream, and potentially make recycling or reclamation of metals in electronic products more difficult and costly due to the increased number of separations required.

E.

This Chapter

This chapter explores the environmental implications of lead-free vs. lead-based solder. It presents an environmental approach known as industrial ecology and analyzes the present lead-based vs. lead-free systems using this approach. The sources, availability, and extraction of required alternative metals are explored. The quantity of lead utilized for electronics is put into perspective relative to other lead applications. Disposal concerns regarding landfill and incineration are addressed. Finally, the value of recycling electronics at end-of-life is illustrated by the anticipated copper recovery required under the European Directive on Waste from Electrical and Electronic Equipment (WEEE).

II.

ENVIRONMENTAL APPROACH

Population growth patterns have occurred as the result of technological developments. Three major technological events–the development of tools, the Agricultural Revolution and especially the Industrial Revolution–have led to large population increases resulting from society’s perception that these new developments would lead to unlimited resources. The relationship between the increase in population and the demand for products and resources, and the subsequent degradation of the environment seem obvious. Nevertheless, as people become more affluent, consumption patterns and resource use become less sustainable. Population growth coupled with affluence has a strong negative impact on the environment. Paul and Anne Ehrlich [9] defined this impact as follows: I¼PAT where

I = Environmental impact P = Population

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A = Affluence T = Technology As the population grows, technology further develops to fill the needs of the increasingly affluent society for products and resources. This situation results in an inefficient consumption of resources and the creation of waste. The Industrial Revolution triggered by the discovery that coal burns better than wood, resulted in the invention of the steam engine for pumping water out of the mines as well as operating other machines to fill the need for products. Iron and steel production developed as major industries. The development of electric generators and distribution of electricity for industrial and residential use created a demand for new metals such as copper which has high electrical conductivity. This led to increased mining, where as much as 90% of the ore residues extracted from the ground were discarded.

A.

Responses to Environmental Problems

In the past, environmental concerns have been reactive by focusing either on wastes from manufacturing processes or end-of-life disposal. The approach was based on remediation and compliance in response to environmental problems that already existed. Today we must move toward Industrial Ecology (IE) and Design for the Environment (DFE). Table 4 shows this historical trend of environmental regulations, the focus of these activities, the geographic and temporal scale, the method of regulation, and the countries leading the actions. When considered globally, the movement generally is from command-and-control of waste substances and emissions, to life-cycle-impact on the environment.[10] For a complete environmental approach, the total life-cycle impact of lead-free solders as well as lead-based solder must be examined. Various terms are used to describe this holistic approach to environmental impact. a)

Green engineering [11] focuses very narrowly on materials management and waste prevention in the manufacturing stage of a product’s life. b) Sustainable development [12] takes a broader approach to assure that development meets present needs while not compromising the ability of future generations to meet their needs. The use of nonrenewable resources is minimized and the capability to produce renewable resources is enhanced as much as possible. c) Industrial ecology [13] integrates engineering and ecological principles with production and consumption processes to minimize the environmental impact. It is the multidisciplinary study of industrial systems and economic activities, and their links to fundamental natural systems [12].

TABLE 4 Three Stages of Environmental Regulation Time

Activity

Past

Remediation

Present

Compliance

Future

IE/DFE

Focus

Geographical/ temporal scale

Waste substances Emitted substances

Local/immediate Point source/ immediate

Products and services over the lifecycle

Regional and global systems/ All time scales

Regulation Command and control Command and control, end of pipe Establish boundary conditions

Leader USA More developed countries European union, especially The Netherlands and Germany

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TABLE 5

The Environmentally Responsible Product Assessment Matrix Environmental concerns

Life stage

Materials choice

Energy use

Solid residue

Liquid residue

Gaseous residue

Resource extraction Product manufacture Product packaging and transport Product use Refurbishment, recycling, disposal

‘‘The concept of industrial ecology is one in which economic systems are viewed not in isolation from their surrounding systems but in concert with them. . . . As applied to industrial operations, it requires a systems view in which one seeks to optimize the total industrial materials cycle from virgin material, to finished material, to component, to product, to waste product, and to ultimate disposal. Factors to be optimized include resources, energy, and capital.’’ Industrial ecologists are drawn to avoid potential environmental impacts. Graedel and Allenby [10] divide these impacts into (a) Global scale concerns–global warming, ozone depletion, loss of habitat and reduction in biodiversity; (b) Regional scale concerns–surface water chemical changes, soil degradation, acid rain, smog and air pollution leading to reduced visibility, and herbicides and pesticides; (c) Local scale concerns–photochemical smog, ground water pollution, radio nucleotides, toxic sludge, oil spills, toxics in sediment and hazardous waste sites. Life cycle assessment is a tool the industrial ecologist uses to assess the environmental impact of materials, product, processes, technology, services or activities. ‘‘Life cycle assessment is an objective process to evaluate the environmental burdens associated with a product, process or activity by identifying and quantifying energy and material usage and environmental releases, to assess the impact of those energy and material uses on the environment, and to evaluate and implement opportunities to effect environmental improvements.’’ [10] Life cycle assessment involves three stages: inventory analysis, impact analysis and environmental improvements. During inventory analysis, the inputs and outputs of an electronic product, for example, throughout its life cycle are examined and quantified. The environmental impact of these outputs is examined in stage 2. Options for material, product or process changes to yield environmental improvements are included in stage 3. Ultimately, new products are designed using DFE considerations. Table 5 shows the complexity of environmental concerns that must be considered. A life cycle analysis follows the resources through every stage of development the end-of-life where the resource is recycled or disposed. Environmental impact, including energy, waste, and effluent, is considered part of the ecological system.

III.

RESOURCE EXTRACTION

Many metallic elements are contained in electronic assemblies either as terminations or coatings for component devices or as the electrical circuit, terminations, or coatings on PWBs. The elimination of lead in electronic products requires substitution by other metals that can provide the performance and reliable properties characteristic of traditionally used lead-bearing alloys. There are a number of metal resources that can be selected as substitutes for lead in solders and as coatings for lead-free electronic assemblies. These metals include tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), antimony (Sb), gold (Au), indium (In), nickel (Ni), palladium (Pd), platinum,

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(Pt), and zinc (Zn). Table 6 lists the world reserves, and price of these metals compared with lead (Pb) based on United States Geological Survey data [14]. Although metal prices vary based on availability and market demands, the prices shown can be used as a comparative guideline. Locating and mining mineral ores, concentrating ores, transporting the concentrates to a smelter, extracting metals from concentrates, and waste generation all have an effect on the environment. When one metal, such as lead, is replaced with another metal, a different set of environmental issues is expanded.

A.

Ores

According to Clarke and Washington [15] the earth’s crust is composed of 98.3% igneous rocks. Compared to the most common metal elements, such as iron and aluminum, found in the earth’s crust, the metals used for electronic products are among the more scarce elements. Fortunately, much of the metal is contained in concentrated areas in the earth’s crust in varying amounts in the native state, such as gold, silver, platinum, and copper; or as salts, such as sulfides, carbonates, or oxides. The metals are incorporated in worthless rock, or gangue, and the ore is often found in very remote locations in the world. Transportation of the ore to a centralized smelter to remove the metals would not be practical, especially when the ore may contain only a fraction of a percent of a desired metal. So, the ore must be concentrated at the mining site to remove most of the gangue. The resulting product is referred to as concentrate, and the waste is gangue or tailing. With much of the waste ore removed, the remaining concentrates are transported to a smelter for more economically refining the product into metals. Mineral dressing, also known as ore processing or beneficiation, is the process of separating the metals from the waste products by crushing, grinding, screening, gravity separation, and flotation methods. There are many variations and proprietary practices, but a brief, generic description of those processes is given below to help provide an understanding of the environmental impact of mining and ore processing. a)

Crushing is usually accomplished in two or three stages, depending on the physical nature of the ore and the size reduction necessary to remove the metal. Jaw and gyratory crushers are used for primary and secondary crushing, depending on the size of the ore feedstock. b) Grinding of the crushed ore further reduces the agglomerate size and may be required for extraction of metals present in only a very small percentage in the ore.

TABLE 6 World Reserves and Unit Price of Metals Utilized in Electronic Assemblies (After U.S. Geological Survey, Ref. [14], 2000) Metal Antimony (Sb) Bismuth (Bi) Copper (Cu) Gold (Au) Indium (In) Lead (Pb) Nickel (Ni) Palladium (Pd) Platinum (Pt) Silver (Ag) Tin (Sn) Zinc (Zn)

World reserves (thousand metric tons)

Price ($/lb)

3,200 260 650,000 77 6 130,000 150,000 39,500 39,500 420 12,000 440,000

0.68 3.50 0.89 4480.00 85.45 0.44 3.91 5700.00 8602.00 76.54 3.70 0.65

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Screening of the ore removes fine particles that do not have to pass through the crusher or grinder, often to increase the production capacity of the crushers. d) Gravity Separation is a method of separating grains of high-specific-gravity metals from the lower-specific-gravity gangue in which they are contained. After the ore is ground and screened to particles of about the same size, the ore is mixed with water in a machine called a jig. This apparatus achieves separation through the principle of Archimedes that the rate of fall of fragmented solids through liquids is proportional to their specific gravity and size. e) Flotation is a method of concentrating fine-particle minerals. The process is similar to gravity separation, except a variety of chemicals, known as collectors, are continuously added in small amounts to the ore, water, and air mixture. The collector bonds to the desired mineral surface, making it hydrophobic, while the other minerals present are unaffected. Air agitation creates a froth containing the desired mineral that rises to the surface of the apparatus for removal. The location of the mineral dressing plant is dependent on the required amount of bulk concentration, the availability of power, water, labor, and the transportation cost. Mineral dressing or concentration is normally performed at a location in close proximity to the mine making it easy to deposit the gangue back where it originated, because it represents the bulk of the ore.

B.

Energy Consideration

The amount of energy expended to produce the metals from ore is related to the abundance of the ore in the earth’s crust, the percentage of the metal that is available in the ore, and the methods for extracting and purifying the metal. Transportation from the mines to the smelter to the marketplace is another energy factor. 1. Abundance The abundance in the earth’s crust of metals that are used in electronic products is shown in Table 7 [16]. Extracting metals contained in seawater is not economically feasible. The major supply of metals is extracted from mineral ore in the earth’s continental crust. There is considerable variation in the cost of extracting the ore and processing it into elemental metals. As the readily available resources become depleted, the process of extracting metals from mineral ores is increasingly more difficult and expensive. For example, in 1900 in the U.S. the copper content in rich ore was more than 4% by weight. By 1950, the available ore contained less than 1% copper and has continued to decline such that the yield is currently less than 0.5%. There are many examples of mining operations suspended since they are no longer profitable because the metal concentration levels are so low. In the past when ore was simply scraped off the ground surface or when rich veins were plentiful, smelting was typically conducted at the mine site. That luxury soon ended. It became necessary to reduce costs for transporting the ore to the mill or smelters by removing the waste material (gangue) from the mineral ore, i.e., concentrate the ore prior to transportation. The tailings are most often dumped into a holding pond for settling of the rock. 2. Pyrometallurgical Roasting of the Ore Because concentrated ore typically contains 20 to 30% water, concentrates must be heated to evaporate the water prior to roasting. Water does not evaporate easily because the process is endothermic. Evaporation of the water prior to roasting is very energy intensive, requiring 10.5 kcal/mole of water (44 kJ/mole). The objective of roasting an ore in air is to convert the sulfides to metal oxides. Roasting typically is performed at 500 to1500jC. The burning of sulfides is exothermic, providing much of the required energy after the process is started. However, the sulfurous gases and dust emitted by the roasting furnace must be caught by dust collectors or an acid washing processor.

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3. Reduction Depending on the metal compound, a reducing agent is used to reduce the metal oxide produced as a result of the roasting operation to produce the desired metal. Most metal oxides are reduced with carbon (coke). The oxide is reduced to metal with the resulting emission of carbon monoxide and carbon dioxide. As an example of these processes, the original creation of lead metal probably resulted from the observation of lead metal flowing out of fires when galena (lead sulfide) was used as the rocks to surround fire pits. The galena rocks were roasted to form lead oxide. This oxidation product and charcoal in the fire reacted to form lead metal while emitting sulfur dioxide, carbon dioxide, and carbon monoxide. 4. Hydrometallurgy After pretreatment by roasting to form metal oxides, the next general purification step for many metals is to chemically treat the metal oxides by using aqueous solutions to leach out the metal. The chemical solution selected for treatment depends on the metal oxide. As examples, zinc oxide will dissolve into an acidic solution, copper oxide forms a water-soluble complex ion when mixed with ammonia and ammonium chloride in water, and tin oxides can be dissolved in fluoroboric acid. The water solutions can then be reacted to obtain specific metal salts, or electroplating techniques can be used to plate out the desired metal. Plating solution waste can either be destroyed or reclaimed.

C.

Waste Products

Surface mining, also known as strip or open pit mining, generated a well-deserved bad reputation because of past practices of poor mine management. Quarrying of rock and the extraction of surface coal are two examples of strip mining. Open pit mining methods are also used to extract minerals, such as iron and copper, that lie near the surface of the earth. Mining of placer or alluvial deposits, such as those containing tin and gold, has similar environmental implications. Dredging from a boom-equipped barge, and hydraulic mining using large amounts of water, have high potential to pollute water and disrupt the aquatic and terrestrial environment. To reduce the environmental damage of open pit mining and coastal dredging, or at least to improve the situation, modern reclamation practices remove the topsoil that is subsequently replaced. The mining tailings (gangue, waste) that make up the bulk of the ore is placed back into the hole and covered with topsoil. New vegetation is planted, and the area is returned to the eco-

TABLE 7 The Abundance of Elements Found in the Earth’s Crust and Oceans that are Commonly Utilized in Electronic Assemblies (After Lyde, Ref. [16], 1999) Metal Antimony (Sb) Bismuth (Bi) Copper (Cu) Gold (Au) Indium (In) Lead (Pb) Nickel (Ni) Palladium (PD) Platinum (Pt) Silver (Ag) Tin (Sn) Zinc (Zn)

Earth’s crust (mg/kg, ppm)

Ocean (mg/L)

2.  101 8.5  103 6.1  101 4.  103 2.5  101 1.4  101 8.4  101 1.5  102 5.  103 7.5  102 2.3 7.0  101

2.  104 2.  105 5.  103 4.  106 2.  102 3.  105 5.6  104 — — 4.  105 4.  106 3.  105

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system. This does not always happen in developing countries because of the costs involved, but much progress has been made in recent years. To extract minerals from lode deposits requires digging holes and mining in tunnels underground. Removal of water from a flooding mine could result in contamination of surface or ground water with heavy metals. Metal-bearing ore is often high in sulfides. When the discarded rock, especially the waste after grinding or milling the ore, is exposed to air, the water seeping through the rock creates sulfuric acid. This acidic water can also leach other heavy metals from the pulverized rock and may contribute to contamination of the ground water. Mineral ore must be crushed and concentrated. The percentage of metal elements in the ore is very small, as shown in Table 8, and most often the metals are recovered as a mixture. To concentrate, grind, and further process the ore to recover the individual metals, it is necessary to use large volumes of water and electrical energy. The environmental issues are related to the stability of the tailing piles and the chemical pollution remaining in closed or abandoned mines. Chemical contamination can result when extracting precious metals by utilizing cyanide salts, mineral acids, and chlorine gas. Ore purification by flotation requires the use of toxic cyanide, silicates or sulfuric acid. In April 1998, a dam of a tailing pond in Sweden, which held the liquid residues from the mining of zinc, silver, lead and copper, caused 3 Mm3 of sludge and 4 Mm3 of acid water to be discharged, polluting a local river. In February 2000, failure of the pond retention dam for a gold mining process in northwest Romania caused 80% of the fish in the Tisza River in Hungary to die and pollution to spread as far as Yugoslavia and Bulgaria [17]. While the mining and concentration processes create environmental concerns, subsequent smelting and roasting processes for copper purification have the potential to emit large amounts of evaporated sulfuric acid vapors into the atmosphere, creating the potential for acid rain. Finally, dust can be a major problem associated with both mining and smelting. As more metals are extracted, the amount of metal in the ore becomes less, and the corresponding quantity of dusty waste becomes greater. There are several hundred thousand abandoned metal-mining sites in the U.S. that have not been assessed for acid runoff, erosion by weathering, and metal dust emission potential. Any increased demand for metals–such as tin, silver, bismuth, and copper–that might be used as alternatives for lead in solder, will require significantly increased use of water and energy to extract the metals from ore that already is becoming depleted.

D.

Key Metallic Elements Utilized in Electronic Assemblies

Besides the plastics, glass, and steel used to fabricate mechanical parts, housings and packaging, the major metallic constituents of present-day electronic assemblies are copper, lead, and tin.

TABLE 8 Percentage of Metals in Ore/Concentrates and the Water/Energy Required for Extraction (After U.S. Geological Survey, Ref. [14], 2000) Metal Tin Silver Copper Zinc Bismuth Gold Nickel Lead a b

% in ore 2 0.03 0.5 2 0.05 0.0007 0.5 3

to to to to to to to to

5 0.2 7 4 0.2 0.002 6 10

% in concentrate 45 <2 30 45 12 0.001 20 70

to to to to to to to to

75 4 70 65 20 0.03 30 80

Metric tons of watera 20 2 2 1 6 2 1 4

Used to process a metric ton of ore. Required to process a metric ton of ore into metal.

to to to to to to to to

30 6 7 5 8 9 3 9

Energy (kWh)b 16 11 9 8 7 8 11 9

to to to to to to to to

25 15 30 10 11 45 15 13

% of consumption from recycled 30 to 22 to 10 to 30 to <5 45 to 40 to 60 to

35 27 15 35 55 50 70

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Copper is used as the electrical conductor for wire, component terminations, and printed wiring boards. Lead is contained in the cathode ray tubes in computer monitors and televisions, and, to a smaller extent, in the solder that either coats the copper circuitry and components or joins these parts together. Tin is alloyed with lead to provide solder alloys because of the ability of tin to wet and bond to other metals used as terminations for components and the printed wiring board. The lead plays a significant role in reducing the melting temperature of the tin and providing a lower surface tension of the solder alloy to improve wetting to other metals. Historically, the electronics assembly industry was developed around the ability of tin-lead solder alloy to make highly reliable connections between components and the printed wiring board. The component terminations are often made of difficult-to-solder metals, such as nickeliron or beryllium-copper, but they can be coated with more easily soldered metals, such as silver, tin, tin-lead, nickel, palladium, or gold. The copper circuitry is usually covered with a solder mask except for the termination pads that are coated with tin-lead solder, either by electrolytic plating or by hot dipping. The solder alloy utilized for joining may also contain up to 3% silver and/or less than 1% antimony. Lead-free solders typically contain tin, silver, and copper with some compositions also containing additions of antimony, bismuth, or indium. 1.

Tin (Sn, Stannum)

a. Utilization. Objects made of bronze (tin and copper), found at the mouth of the Euphrates River, date back to 3500 B.C. There is a long history, exceeding 3000 years, where tin and tin alloyed with other metals have been used for tools, jewelry, food and drinking containers. The greatest use of tin today is as tin-plate (steel sheet coated with tin), accounting for almost 40% of the world’s tin consumption. It is also widely used in bearings, utensils, type metal, bronze and pewter, die-casting, biocides, and dental amalgams. Solders account for the largest use of tin in the United States. The electronics and electrical industries have typically used lead-alloy solders with tin contents in the range of 40 to 65%. Most lead-free solders of interest have higher tin contents, 95% or more. Solder alloys with a high tin content have been used where electrical conductivity is a priority, being better than high-lead solders. Tin alloyed with 3.5 to 5.0% silver is a commonly used solder alloy for applications requiring high tensile and creep strengths. Lead alloys with low (5 to 10%) tin content are used as high temperature (>260jC melting point) solders for high fatigue resistant flip-chip solder bumps, and also in electronic components soldered to printed wiring boards with conventional low melting tin-lead or lead-free solders. b. Source. The most important tin ore is cassiterite (SnO2) with small amounts of tin recovered from sulfide minerals containing copper (Cu) and iron (Fe) such as stannite (Cu2FeSnS4). The main sources of tin (>80%) are associated with granitic rocks or sand in placer (aluvial or glacial) deposits. After concentration, the ore is shipped to the smelter to obtain tin by roasting the ore to oxidize sulfur and to remove arsenic, then reducing with coal in a reverberatory furnace, or by smelting in an electric furnace. c. Abundance. Tin has an average abundance in the earth’s crust of about 2 parts per million (ppm). The world’s major tin producers are China, Indonesia, and Peru. Secondary or recycled tin comes from tin-plated steel, which represents over 30% of the tin consumed. d. Environmental Considerations. Tin metal has a low toxicity, and therefore is commonly used as plating on metals that come into contact with food and drink. Ironically, of all the metals, tin has the greatest number of organic compounds in commercial use, most being toxic if released into the environment. However, none of these tin-containing organic compounds are formed as part of electronic assembly processes. 2.

Lead (Pb, Plumbum)

a. Utilization. Lead is believed to be the oldest metal known to man, having become familiar with it approximately 7,000 to 5,000 years B.C. The applications for lead and lead compounds are more varied than any other metal. Table 1 shows some product uses for lead. Its major use is storage batteries (approximately 87%), electric cable sheathing, and chemical

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processing and plumbing [18,19] due to its low cost and excellent corrosion resistance. Other applications include radiation shielding, ammunition, type metal, vibration damping in heavy construction, and bearing alloys. b. Source. There are three general classes of lead ore: (1) those containing lead alone; (2) lead-silver ore, and; (3) lead-zinc ore. Although there are more than 60 lead-containing minerals, the most important by far is galena (lead sulfide, PbS). The lead metal is extracted by crushing, grinding, and then using flotation methods. The extracted galena is processed by roasting and then reducing the oxide. A variety of metals may be recovered during smelting and refining of lead ore, including copper (Cu), silver (Ag), gold (Au), arsenic (As), antimony (Sb), tin (Sn), and bismuth (Bi) [20, 21]. c. Abundance. The abundance of lead in the earth’s crust is about 14 parts per million (ppm). The major lead producing centers are the U.S., Australia, Mexico, Canada, Russia, and Peru. The total annual world production is about 3.5  106 Mg (metric tons). Recycling of lead scrap (from batteries, lead sheet and cable sheathing) is a considerable accomplishment, providing 60 to 70% of the lead used in the U.S. d. Environmental Considerations. Organic lead (tetraethyl lead) was an air and terrestrial pollutant when it was used in gasoline as an anti-knock additive. Though solders for plumbing applications, especially for potable water systems, have not contained lead since 1986, there is the potential for contamination if acidic or very soft water passes through older lead containing pipes. Lead could potentially get into the water effluent when lead-containing solders are used for printed wiring assemblies that are subsequently cleaned with water to remove soldering flux residue. Though a small percentage (0.3%) of the lead consumed is used in electronic assemblies, it could be better recycled. There is very little probability that lead can be leached from landfill into ground water because lead salts are not soluble in water. However, if electronic products are simply shredded and then incinerated, the lead dust from shredding or the ash from the incinerator will be finely divided and more available for environmental contamination. 3.

Copper (Cu, Cuprum) a. Utilization. Copper has been known since prehistoric time and has been mined for more than 5000 years. It is the main component in bronze (copper-tin) and brass (copper-zinc). Copper is a versatile metal that is used in plumbing applications as pipe and fixtures, heating, roofing and building construction, coins, and plating under other metals such as nickel, chromium and zinc. Copper is used in numerous electrical applications, such as wiring, switches, printed circuit board conductors, heat sinks, and other components. Copper is found rather widely distributed in nature. b. Source. Though copper occurs as elemental metal, oxide (cuprite), and carbonate (malachite), the primary ore mineral of copper is chalcopyrite (CuFeS2) and, to a lesser extent, deposits containing chacocite (CuO2). Sulfide ores are concentrated by floatation and leaching, then roasted to form copper matte (40 to 50% copper), and further to blister (96 to98% copper). Electrolytic refining further purifies the copper. c. Abundance. Copper is present in the earth’s crust at about 61 parts per million (ppm). About one-third of the world’s copper comes from the western U.S. and South America. Other important areas are the Central African plateau and Canada [21]. Recycled materials provide 10 to 15% of the consumption. Much of the copper is consumed as electrical wiring and ‘‘disappears’’ into the environment for decades when demolition of buildings frees the copper for recycling. At this time, a low 10 to 15% of the consumed copper is supplied from recycled copper. d. Environmental Considerations. The main environmental consequence is the low metal content of the ore from which the copper is obtained. To produce a ton of copper metal from lowgrade ore requires processing 150 to175 tons of ore. Also, the copper exists with iron as sulfides, so, during the extraction process, sulfur emissions must be controlled and recovered. In some less

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developed countries, the sulfur is released as the atmospheric pollutant sulfur dioxide (SO2). Residual sulfides in crushed tailings or remaining in the mine can oxidize when exposed to water and air to form sulfuric acid that can dissolve other metals into water runoff. 4.

Antimony (Sb, Stibium)

a. Utilization. The sulfide form of antimony was known since ancient times, and antimony metal is described in the literature since the 15th century. Antimony is added to lead in amounts varying from 1 to 20% (antimonial lead) to harden the lead for such applications as type metal, storage battery grids, battery terminals, pipe, and cable sheathing. High-lead solders usually contain a maximum 0.5% Sb. However, for high-tin solders, the general rule is the Sb should not exceed 6% of the Sn concentration. Antimony has also been added to lead-tin solder alloys to eliminate the allotropic transformation from beta-tin to alpha-tin (so-called tin pest) wherein the accompanying large change in volume results in a drastic loss in solder strength [22]. b. Source. Antimony exists in nature as elemental metal, but primarily as the sulfide. The most important antimony mineral is stibnite (Sb2S3, antimony sulfide) that is directly reduced with iron scrap. Antimony is closely associated with arsenic and bismuth minerals. However, much antimony is obtained as a by-product of lead, copper, and silver ores. c. Abundance. Antimony is present in the earth’s crust less than 0.2 parts per million (ppm), about the same as indium, but more concentrated in locations with other metals. The principal producing countries are Bolivia, China, Mexico, Russia and South Africa. About half of the antimony used in the U.S. is recovered from lead-acid battery scrap. d. Environmental Considerations. The soluble salts of antimony are toxic, and stibine (antimony hydride, SbH3), a highly toxic gas, can be liberated at high temperatures when soldering with a gas torch. Nevertheless, as a minor ingredient in solders for electronics, antimony is not a hazardous material problem. No appreciable increase in antimony usage is expected with the implementation of lead-free solders for electronic assemblies. 5.

Bismuth (Bi, Bismutum) a. Utilization. Bismuth was thought by the Austrian miners in the 16th century to be unevolved silver. Alchemists were familiar with the bismuth metal in the mid 16th century. Paracelsus was the first to describe the properties of bismuth about 1550. Bismuth solder alloys are selected because of their low melting points. For example, an alloy of 42% tin, 42% lead, 14% bismuth melts 144–163jC, 20 degrees Celsius lower than the melting point of the eutectic tin-lead alloy solder. For applications requiring a low-melting temperature solder, such as soldering heat sensitive components, this alloy compares very favorably with the eutectic tin-lead solder in strength and ductility. However, without the lead, the alloy becomes more brittle. A lead-free bismuth solder, 42% tin, 58% bismuth that melts at 138jC, has been used for wave soldering or reflow soldering of electronic assemblies. There is also the potential for adding bismuth to high-tin content, lead-free solders to reduce the alloy melt temperature and enhance mechanical properties. However, the bismuth concentration typically is kept low, ranging between about 3 to 6% [23]. b. Source. Although some bismuth is obtained from ores, such as bismite (bismuth oxide, Bi2O2) and bismuthinite (Bi2S3), bismuth is also recovered as a by-product during the smelting and refining of tin, silver, and gold ores. Copper ores provide another major source of bismuth. Bismuth is found in most lead ores and accompanies the lead through the smelting and refining processes. Bismuth is mined with tungsten ores in China and North Korea, and with copper/gold ores in Australia. c. Abundance. Bismuth constitutes less than 0.01 parts per million (ppm) of the earth’s crust, about one order of magnitude less than silver. Major base reserves are in Australia, Canada, China, Mexico and Peru. Bismuth fusible alloys are recovered for reuse, generally on site. Less than 5% of the bismuth consumed is derived from scrap.

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d. Environmental Consideration. Bismuth metal is considered to have low toxicity and is non-carcinogenic. Though the bismuth solder alloys melt lower than other potential lead-free solders, such as tin-silver-copper alloys, and would require less energy to use, a large increase in the amount of bismuth used for soldering electronic assemblies is not anticipated. An increase in a requirement for bismuth would require processing more lead because about half the bismuth is produced as a by-product of lead production. There is some concern in the recycling industry about the use of bismuth for soldering copper circuitry, because of the difficulty in removing bismuth from copper when recycling. Bismuth is obtained while smelting copper where bismuth is collected in the flue dust. Recovering bismuth is then accomplished by chlorination, leaching with acidic ferric chloride, and cementation with iron powder. Any additional use of bismuth for electronic solders could result in liberation of these air pollutants. 6.

Indium (In, Indium) a. Utilization. F. Reich and H.T. Richter at the Freiburg School of Mines in Germany discovered Indium rather recently, in 1863. It remained a laboratory curiosity until the 1920s. However, as early as 1935 it was noted that the addition of indium to low-melting alloys containing bismuth, lead, tin, and cadmium caused the melting point to drop 1.45jC (2.6jF) for each 1% indium added, with a minimum melting temperature of 47jC (117jF) at an indium content of 19.1%. These solders, known as fusible alloys are used in safety links, fuses, and sprinkler plugs. Indium is used as a base material in low-melting solders, providing enhanced properties, such as reduced gold scavenging, and resistance to thermal fatigue and alkaline corrosion. Indium-based solders used in microelectronics packaging and assembly applications include In-Pb50 for flip chip solder bumps to take advantage of the fatigue resistance of the solder. The In-Sn48 is a low-melting temperature (117jC) solder alloy that is useful for glass-tometal sealing and for the assembly of devices that may be susceptible to temperature-induced degradation when using conventional solders. Indium has also been considered as an addition to lead-free solders, to take advantage of these same properties, i.e., reduced melting temperature, and enhanced fatigue properties [23]. Indium semiconductor compounds are used in many applications such as photovoltaic cells and transistor devices. b. Source. Historically, indium has been produced almost exclusively as a by-product of zinc refining. It is also mined with and extracted from copper, lead and tin. The ores are generally of low indium concentration, containing less than 0.001% indium; many contain less that 0.0001% indium, but become concentrated during the recovery of major metals [23]. c. Abundance. The content of indium in the earth’s crust is 0.25 parts per million (ppm), which is more abundant than silver. However, indium is more widely distributed in nature. Important sources are the western U.S., Canada, China and Russia. Very little of the consumed indium metal is from recycled material, though internal recycling of scrap is done in semiconductor manufacturing plants. d. Environmental Considerations. Because indium is a costly metal, equal to or much more than the cost of silver, usage in electronic-grade solders remains limited except for special lowmelting temperature alloys. Extraction of more indium would require an increase in zinc and lead mine processing with consequential environmental impact. 7.

Gold (Au, Aurum) a. Utilization. Like copper, gold has been known since prehistoric time. Gold is used in coins and jewelry, usually alloyed with other metals such as copper, silver, palladium, platinum, rhodium, or nickel to increase the hardness. Gold wires are often bonded between chips and the next level of electronic assembly. Gold plating is widely used as a protective coating and to promote solder wetting on nickel for electronic component terminations and copper pads on PWBs. Gold is soft and has good lubricating properties, so it is used on circuit board fingers that are plugged into gold-plated connector sockets. Gold has no oxide and solders easily, but it dissolves very rapidly into both tin and lead in molten solder. Dissolution of the gold into the

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solder results in the formation of tin-gold intermetallic compounds (IMC) in the solder matrix. The solder bond takes place with the underlying metal. Solder joints with more than about 2 to 5% gold content exhibit brittleness during thermal cycling. b. Source. Gold is found in nature in lode metal veins and is mostly associated with quartz or pyrite. Gold is also found in placer deposits and is separated from ore that is crushed by sluicing and panning processes. The ore is treated with cyanide solution, and the dissolved gold is recovered by precipitation with zinc dust, aluminum, or by hydrolysis. Further refining is achieved through electrolysis. c. Abundance. Gold is present in the earth’s crust in about 0.004 parts per million (ppm). The largest reserves are in South Africa with additional major reserves in the U.S., Australia, Canada, Indonesia and Russia. In the U.S. about 50% of the consumed gold is reclaimed from scrap. d. Environmental Considerations. Gold is used as a plating to protect other metals such as copper and nickel from environmental corrosion. The benign nature of gold is well known. Nevertheless, the extraction of gold from ore involves the use of cyanide salt solution to leach the gold from ore, soil, and remnants (tailings) of mining operations. Uncontrolled disposal of the waste into mines or ponds of runoff water can result in serious environmental pollution. 8.

Nickel (Ni, Nickel) a. Utilization. German miners in the early 1700s named the mysterious ore that resembled copper ore Kupfernickel, literally Old Nick’s Copper (copper bewitched by the Devil, or false copper). In 1751 the Swedish mineralogist Cronstedt isolated nickel from the kupfernickel ore. It was not until 100 years later that the U.S. introduced nickel into its copper one-cent coin. The primary uses for nickel are as an additive to make stainless steel, cupronickel and nickeliron alloys, for electroplating other metals, and in batteries. Electronic applications include alloys with iron (42% nickel) that are used as leadframe material in ceramic packages for semiconductors, and as plating on component leads (terminations). Nickel is often plated onto copper to prevent copper from leaching into gold plating or melted solder. b. Source. Nickel is primarily obtained from nickel ore that is associated with iron, mainly as sulfide and oxide, with the nickel sulfide accounting for two-thirds of the world’s source. Sulfide ores are refined by flotation and roasting to produce sintered nickel oxide, which is either sold as such or further reduced to metal. The cast metal anodes are electrolytically refined. Oxide ores are treated by hydrometallurgical refining, such as leaching with ammonia. c. Abundance. Nickel is contained in the earth’s crust as 84 parts per million (ppm), about as abundant as lead, zinc, and copper. Major reserves are in Australia, Canada, Cuba, Indonesia, New Caledonia, Philippines and South Africa. Recycled scrap provides 40 to 50% of the nickel consumption. d. Environmental Considerations. Nickel plating has been used for decades as a plated protective coating for electronic parts. As tin-lead protective coatings on the printed wiring board copper circuits are eliminated, a replacement coating is electroless nickel covered with immersion gold (ENIG). Increased usage of nickel in electronic applications may require special recovery techniques when extracting the copper on which the nickel is plated. The increase in nickel consumed will not significantly affect the materials/energy used for coating printed wiring board circuits, because the process is replacing the hot solder coating process. Nevertheless, as with other sulfide ores, nickel sulfide remaining in the tailings and mines can oxidize in a moist environment and liberate sulfuric acid into the environment. 9.

Platinum Group Metals a. Utilization. The platinum group of metals includes platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os), and iridium (Ir). Palladium is a noble metal discovered in 1803 by Wollaston, decades after platinum was described (1741) by Wood. Of these

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transition metals, palladium, platinum, and rhodium are used as plated metals either on circuit board termination pads (rhodium) or on component terminations (palladium and platinum) in conjunction with nickel, gold, and silver. For example, palladium plating on nickel is used as a solderable metallization on component terminations that are soldered with tin-lead solder. For lead-free soldering applications, the palladium-plated terminations on components are overplated with gold to help improve solderable storage life. Palladium and platinum are used as oxidation catalysts for automobile exhaust purification and in fuel cells. b. Source. Palladium is recovered from alluvial ore deposits with platinum, gold, and copper. Concentrated ores are dissolved in aqua regia, gold and platinum are removed by chemical treatment, and palladium is precipitated by treatment with ammonia, followed by hydrochloric acid. c. Abundance. Palladium is about three times as abundant as platinum or gold in the earth’s crust (Table 7). Major reserves are in the U.S., Canada, South Africa, and Russia. About 22% of the platinum group metals (primarily from the automotive industry) were obtained from recycled scrap. d. Environmental Consideration. The amount of palladium used for metallization on electronic component terminations is increasing as more components are being made available with lead-free terminations. While more palladium is being used for electronic applications, significantly less palladium is being used for automotive catalytic converters. If electronic components are not recycled, or at least if the metallization is not removed and recycled, the palladium used will ultimately go to landfill. 10.

Silver (Ag, Argentum) a. Utilization. In slag dumps on islands in the Aegean Sea there are indications that man learned to separate silver from lead as early as 3000 B.C. It is known that officers of Alexander the Great used silver water cups during the campaign to invade India in 329 B.C. The Romans produced substantial quantities of silver to mint coins in 269 B.C., fifty years before minting Au coins. Like gold, silver is used as a light reflector, such as mirrors, and for electrical conductors, jewelry, and dental applications. Silver is also used in coins, batteries, solar cells, photographic emulsions, brazing alloys, and in conductive epoxy. Silver and palladium-silver are used as metallizations on electronic component terminations for soldering. Silver for many years has been added to tin (3.5 to 5.0% silver) and to tin-lead (1 to 3% silver) to improve tensile and creep strengths, and to minimize dissolution during soldering of silver-coated surfaces. Many lead-free solder alloys include a silver content of 3.0 to 4.0%. b. Source. Silver is obtained from precious metal ore with about half coming as a byproduct from processing copper, lead and zinc ores. The main silver ores are native silver, argentite (Ag2S, silver sulfide), and cerargyrite (AgCl, silver chloride). The most important ore is the silver-containing lead ore in which the heavy ore is argentiferous galena. The recovery of silver ranges from 160 ounces, to a few thousandths of an ounce per ton of ore. The typical yield from a ton of lead-silver ore is less than 7 ounces of silver and 400 pounds of lead. c. Abundance. Silver is contained in the earth’s crust in less than 0.1 parts per million (ppm). Mexico is the world’s largest silver producer, followed by Peru, Australia, U.S. and Canada, totaling more than half of the world’s production [14]. About 45% of the consumed silver is recovered from scrap silver products. d. Environmental Considerations. Silver has no known physiological effects in humans, but has a high aquatic toxicity and accumulates in the body tissue of aquatic organisms. One consideration is the replacement of Sn-Pb solders with lead-free solders containing silver will still require the production of lead, as silver is a by-product. 11.

Zinc (Zn, Zink) a. Utilization. Zinc ore was used to make brass centuries before it was recognized as an element. Zinc was known in the 13th century in India and was described by Paracelsus in 1520.

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The primary use for zinc is in alloys (brass, bronze, die-casting metals) and coating (galvanizing) on iron and other metals. Other applications are fuses, storage and dry cell batteries, zinc-air batteries, roofing, rain gutters, and engraving plates. An alloy of zinc with tin is used to solder to aluminum. Zinc oxide is used in the manufacture of ointments, rubber goods, pigments, cosmetics, and as an astringent and protection of skin. b. Source. Zinc exists in nature as sulfide, carbonate, and silicate. Zinc is primarily extracted from sphalerite (zinc sulfide, ZnS) ore by roasting the ore to form zinc oxide. The two standard methods for reducing the zinc oxide to metal include (1) pyrometallurgical process where the zinc oxide is reduced with carbon in retorts, then the zinc is distilled and condensed; and (2) hydrometallurgical process where the zinc oxide is leached from the roasted ore with sulfuric acid to form zinc sulfate solution, then the zinc metal is electrolytically plated out of solution. c. Abundance. Zinc has an average abundance in the earth’s crust of 70 parts per million (ppm), about the same as copper. Major reserves are located in the U.S., Australia and China with smaller amounts in Canada and Peru. More than 30% of the zinc consumed is from recycled material. It is difficult to reclaim zinc from zinc-plated steel, as typically used in the automotive industry. However, zinc has a low boiling point (907jC) compared to other metals, and can be recovered as condensation dust from electric arc furnaces used to process scrap steel. d. Environmental Considerations. Though zinc is not very toxic to humans or animals, it is phytotoxic (toxic to plants). Zinc salts, such as zinc sulfate and zinc chloride, are water-soluble and capable in a water solution of leaching into the ground water. Soil can be poisoned by careless zinc smelting or if zinc-contaminated sewage sludge is put onto the soil. Zinc sulfide remaining in mine tailings, when exposed to water and air, can produce sulfuric acid. The acid, in turn, can dissolve other metals into the water runoff. Zinc has some possibilities for use as a solder in electronic assemblies, but generally the tin-zinc solder alloy corrodes too easily.

E.

Comparative Environmental Impact

Lead alloy solders may be the preferred choice after consideration of the environmental costs of extracting alternative metals, primarily tin, silver, and copper. The elimination of lead and leadbased solder in electronic products will not have a significant impact on the amount of lead being mined. The increased amount of silver needed for lead-free solder alloys will still require extraction of silver from lead-containing ores. The substantial increase in tin required to replace lead in solder has the potential for a significant impact on the environment, such as an increase in dredge and strip mining. Copper also is extracted by strip mining. Analytical models at the present time do not allow assignment of a value to the removal and dispersal of these limited metal resources. With due consideration of the environmental impact and cost of controls, if precautions are taken, the actual cost of replacing lead compounds and lead-solder in electronic products would be significantly higher than the prices in Table 6 indicate. In the present paradigm, the cost of a metal-containing product is based on the world market price, which depends on the availability of the alloy, the location of the deposits, and the cost of extraction, purification and shipping. A new paradigm is needed where the environmental burden associated with resource extraction is factored into the cost. There should also be a cost associated with depleting metals with scarce world reserves. Finally, there should be a credit associated with the use of recycled metal. Including these environmental impact factors represents the true cost.

IV.

PRODUCT MANUFACTURE

The manufacturing processes for soldering electronic assemblies were developed over decades as electronics technology advanced. The substitution of lead-free solders for the standard tin-lead alloys has a considerable deleterious effect on manufacturing methods, the entire supply chain of materials and parts, and the environment. The main contributing factor is the higher temperature required to melt and use the lead-free solder alloys. Development and performance testing of

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temperature-resistant electronic components and materials must include vigorous reliability testing.

A.

Effects of Increased Temperatures During Assembly

As can be seen from Table 3, most lead-free alloys under investigation melt at temperatures that are 30 to 40jC higher than that of eutectic Sn-Pb solder. Higher process temperatures will result in an increased scrap rate of FR-4 epoxy-glass printed wiring boards due to board warpage, delamination and material degradation (charring). In addition, boards processed at higher temperatures are prone to conductive anodic filament failure (CAF) [24]. In many cases new chip carrier materials will be required. Many electronic devices, such as electrolytic capacitors and plastic-encapsulated components, are not rated to experience the high temperatures required to process with lead-free solders. The resulting heat-induced degradation can result in early field failures. Also, the higher temperatures required of lead-free solders are not compatible with optoelectronic components. The increased heat can cause a variety of conditions with these components, among them: electrical variances, changes in silver-epoxy die attach properties, delamination between plastic and lead-frame parts, deformation of plastic encapsulants and plastic lenses, damage to lens coatings and changes in the light transmission properties. In addition, four and five-component lead-free solder alloys, many of them patented, will present manufacturing problems because it will be very difficult to maintain the exact alloy composition in wave soldering machines. Solder powder manufacture for these complex solder alloys will also be complicated.

B.

Additional Energy Consumption Impact

Total energy consumption in the manufacturing process is a major factor to consider in an environmental impact assessment. As previously noted, most of the proposed lead-free replacement alloys require higher processing temperatures, giving rise to a significant increase in energy use. Estimates of the energy used for soldering vary widely because of the differences in methods and temperatures employed. However, calculations based on the tons of solder consumed globally, density of the alloys, and specific heats indicate 115 to 125 MWh (megawatt-hours) is being consumed annually with tin-lead solders. This agrees fairly closely with estimates made by soldering equipment manufacturers for the more than 20,000 soldering machines being used worldwide. For the lead-free solder alloy SnAg3.0Cu0.5, considering the lower density and higher melting/operating temperatures, the energy required is 135 to 145 MWh, about 18% increase in energy. The U.S. Department of Energy estimates about 0.62 metric tons of carbon dioxide (CO2) is emitted per MWh; so, if there is complete conversion to lead-free solder alloys for electronic assembly soldering, about 12 metric tons of additional CO2 could be emitted. However, soldering temperature is not the only consumer of energy. Another example is the additional baking of printed wiring boards that may be required so they can withstand the higher soldering temperatures without delaminating. Nortel’s lead-free Meridian Desktop Phone [25], soldered with eutectic Sn-Cu0.7 (melting point 227jC), required baking of the double-sided boards for four hours at 100 to 125jC prior to soldering, thus significantly increasing energy usage.

V.

END OF LIFE DISPOSAL

End-of-life disposal has been cited as a driving factor for the elimination of lead. When the WEEE legislation regarding the elimination of lead from electronics is put into effect, the ability to recycle products at the end-of-life may be compromised. The technology exists to recycle Sn-Pb soldered assemblies, but it is anticipated that the mixed metal stream resulting from various replacement metallizations and solders will cause the amount of recyclable materials to decrease and thus increase the amount that goes to landfills [26].

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS A.

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Landfills

A common misconception is that trash or waste destined for placement in a landfill is simply being dumped into a hole in the ground. This may be the case in old trash dumps, but a landfill is a controlled disposal environment. 1. Construction and Maintenance of Suitable Landfill Sites Unlike a dump, which is an open, exposed area where trash is deposited, a landfill provides a more enclosed system. A municipal solid waste (MSW) landfill is constructed with a plastic liner to isolate trash from the environment and is covered daily with a layer of soil. Trash in landfills is only minimally exposed to oxygen and moisture and thus it remains there for a long time. Leachate that may percolate through the trash is collected by a pipe that moves it into a leachate pond. The leachate from the pond is tested for contaminants, and then treated like any other sewage/waste water. A storm drainage system is used to minimize water inclusion during heavy rains by directing the water to a ditch surrounding the landfill, where the runoff water can be collected and tested before letting it run off-site [27]. 2.

Leaching a. US EPA Test Procedure. One of the main concerns regarding lead-solder in electronic scrap has been the leaching of this heavy metal from landfill sites. The US EPA has developed a Toxic Characteristic Leaching Procedure (TCLP) [28] to determine if a waste is hazardous. In this procedure a sample is extracted with a buffered acetic acid solution, the sample is diluted and the metal concentration is measured. Since lead is soluble in acetic acid, it is more likely to leach using this procedure. Actual material in landfill sites may be more basic and under these conditions, lead will precipitate out and not be bioavailable. b. Laboratory Test Finding. The US EPA has set regulatory limits for many of the proposed lead-free solder alloy constituents (Table 9) [29]. Leachate from a landfill is diluted by ground water. Thus, the allowed concentration limits for the leachate are higher than those for drinking water. Several lead-free alloys that are under consideration for electronic assembly applications were evaluated [30] according to the leachate test (Table 9). These include Sn-Ag3.2Cu0.5, SnAg3.5, SnAg2, and SnAg0.7, and SnSb5. The findings have important implications for evaluating the environmental effect of these alloys. When solder spheres were tested, the SnAg

TABLE 9 Regulatory Limits for Drinking Water and TCLP Leachate for Several Elements that Comprise Lead-free Solders (After Smith and Swanger, Ref. [29], 1999) Element Tin Silver Silver Antimony Antimony Copper Copper Lead Lead a

Media

Limit, mg/L

Source

All Drinking water TCLP leachate Drinking water TCLP leachate Drinking water TCLP leachate Drinking water TCLP leachate

None found 0.10 5.0 .006 1.0 1.0 500 .015 5.0

EPA 40 CFR 141 EPA 40 CFR 261 EPA 40 CFR 141 TRCC 30 RAC 355a EPA 40 CFR 141 Various state regulations EPA 40 CFR 141 EPA 40 CFR 261b

Texas statutes Some jurisdictions observe a 1.5 mg/L limit based on a multiple of the 0.015 mg/L drinking water limit.

b

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alloys (Sn-Ag3.2Cu0.5, SnAg3.5, SnAg2) leached silver above the allowed TCPL limit, and the lead leachate from Sn63Pb37 solder wire was 200 times the allowed level. Leachate from circuit boards soldered with Sn63Pb37 alloy showed about 40 times the allowable level of Pb [31]. In Boeing studies, circuit boards soldered with Sn-Ag-Cu or Sn-Cu, no silver was detected (detection limits 0.01 mg/l) [32]. In 1999, researchers at the Florida State University [33] examined lead leachability from CRTs, and reported average lead concentrations of 18.5 mg/l (above the 5.0 mg/l regulatory limit). Colored CRTs produced higher levels because the glass funnel in the CRT contains 24% lead, and the frit contains 70% lead. These two components exhibited the highest average lead concentration, 75.3 mg/l. Monochrome CRTs were found not to leach lead above the regulatory limit. c. Landfill Test Results. Actual landfill data presents a different story regarding lead. In 1988, the US EPA commissioned a study of municipal solid waste (MSW) by the NUS Corporation [34] to determine if the leaching of lead from landfills presents a problem. Sources of lead in landfills [31, 35] are listed in Table 2. It is clear from this table that electronic products other than TV picture tubes in MSW represent only 4.4% by weight. The NUS Corporation reported the lead concentrations from 139 leachate samples taken from 45 MSW landfill sites. None of the samples were found to have lead concentrations that would designate them as hazardous wastes under the Resource Conservation and Recovery Act (RCRA) Toxicity regulation. An additional report [36] also identified the principal contaminants for each of 146 landfills. Only 11 landfills reported lead as a major contaminant, which were later determined to be of no consequence as noted in Table 10. 3. Geo-movement of Lead and Other Toxic Metals In 1991, an article [33] reported on a study of the Smuggler and Molly Gibson Silver Mines in Aspen, Colorado, which had been active from 1880 until World War I. Examination of the soil revealed 20,000-ppm lead. The EPA allowed-level for most soils is 500-ppm; but because of the mining, Aspen levels of 1000-ppm were allowed. Despite these high levels of lead in the soil, no lead was found in the ground water and lead particulates in the air were 10 times lower than U.S. national standards. Blood levels of lead were tested from 125 volunteers. The Center for Disease Control (CDC) considers 10Ag Pb/dl toxic. The children in the study had an average of 2.77Ag/dl, while the adults had 3.4Ag/dl. The average lead levels for Cincinnati, Ohio are 17 Ag/dl. Only those who lived near old mine sites equipped with smelters had high levels of lead. Similar results were observed in a Butte, Montana study of copper mines where high levels of lead were found in the soil. However, the levels of lead in the blood were found to be low among trailer park residents who lived near old mine tailings with no smelter. Those who lived in the older part of the city, that had housing containing lead plumbing and lead paint, had high levels of lead in their blood. The

TABLE 10 Cases where Lead from Electronic Assemblies was Mistakenly Identified as a Major Contaminant in Landfills (After NUS Corp., Ref. [33], 1988) Landfill leaching observations Number of sites examined Lead as a major contaminant in landfill -Low levels of lead in groundwater -Mistaken reports of lead problem -Could not confirm elevated lead levels in groundwater -Industrial lead-bearing waste deposits

Number of sites 146 11 4 3 2 2

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only explanation for these results is that lead locked in the soil does not contaminate the ground water.

B.

Municipal Solid Waste Combustors

An alternative to placing electronic products waste in dumps and landfills is to destroy the waste by incineration. One type of incinerator is the rotary kiln that uses the energy from liquid, combustible hazardous waste to increase the temperature of the kiln for drying cement. The chemical industry disposes of high energy-capacity, flammable liquids in this manner. Incinerators that burn hazardous chemical waste must be monitored and achieve a destruction removal efficiency (DRE) rate of at least 99.99 percent of the principle organic hazardous constituents of the waste. Municipal Solid Waste Combustors (MSC) are incinerators that burn household, commercial or institutional wastes. Waste is usually shredded and fed with a controlled rate into the incinerator that uses high temperature, turbulent agitation, and an oxidizing (air) atmosphere to destroy the waste. The objective is threefold – to reduce the volume of the waste, to process hazardous waste, and to convert the organic materials into energy. MSCs reduce the volume of waste by up to 90 percent, and 75 percent in weight, depending on the percentage of organic material input. Once the incinerator is fired up, the organic waste will burn to provide the combustion energy to continue the burn. The energy generated from burning the waste, depending on the incinerator location and use, can be used to generate steam and electricity for commercial or residential applications. 1. Stack Emissions Due to Incineration The composition of stack emissions, mostly fly ash, depends on the materials being burned. Incinerator chemical reactions are very complex, so it is advantageous to sort out materials for input into the incinerator. A notable problem with any incinerator is the inclusion of organochlorine materials, such as polyvinyl chloride (PVC) in the waste. Products of incomplete combustion (PICs) in incinerator emissions can include aromatic chlorinated toxins such as dioxins, hexachlorobenzene and furans, all well-known toxins. Dioxins form with incomplete combustion of the chlorine-containing organic materials when the incinerator is improperly charged and temperatures drop below the 1000jC (1832jF) temperatures required to destroy the organics. It is not unusual to see an improperly charged incinerator burning at 300 to 600jC for some time while emitting the toxins. This is especially a problem when the waste also contains copper or iron that act as catalysts to form dioxin. The potential exposure to lead solder from stack emissions of a MSC is questionable. The National Ambient Air Quality Standard allows ambient lead concentrations up to a maximum of 1.5Ag/m3 [37]. Modeling studies for the EPA (1987) predicted that lead concentrations would not exceed 60% of that value. A revised study [38] (1989) put the value even lower. Indirect exposure due to stack emissions and fugitive emissions were also ruled unlikely to provide adverse exposure to lead. Air pollution control methods to reduce particulate and fume emissions include bag filters, venturi scrubbers, packed tower condensation, multiple cyclones, and water wash filters. The collected incinerator fly ash and wash water may include hazardous materials that must be processed out of this waste or buried in a controlled landfill. 2. Bottom Residue from Incineration The bottom residue in an incinerator is the inorganic material that would not burn or vaporize. The presence of glass in the waste stream can turn the bottom residue into slag that is difficult to further process. The accumulation of metals and metal oxides in the bottom residue must then be sent to a controlled landfill or processed to reclaim the metals. Eliminating the metals from incinerator feed by dismantling electronic products can significantly reduce the amount of metals in the bottom residue.

106 VI.

TURBINI AND BERNIER END-OF-LIFE RECOVERY

Electronic waste is an issue gaining more attention as electronic products, such as computers, mobile phones, palm devices, televisions, radios, CD/VCR/DVD players, toasters, microwave ovens, clocks, toys, etc. are being discarded into dumps. Other large appliances, such as refrigerators, washing and drying machines, and air conditioners, are included in the reported electronic waste, and may amount to more than 70% of the weight. The consumer wants electronic innovations that are smaller, faster, lighter, more stylish, and greater functionality. New electronic products are replacing older, technologically obsolete products. Recyclers of electronic products waste broadly estimate that 30 to 50 million personal computers, either at home or in the office, become obsolete each year. A viable policy and plan must be implemented to properly disposition this obsolete equipment.

A.

Collection/ Take Back Programs

The collection of electronic waste is confused by the definition of electronic waste as hazardous waste because it contains hazardous materials, such as lead. Hazardous materials cannot simply be placed in municipal waste for collection. However, the consumer does not want to pay the high cost of shipping old electronic products back to the manufacturer. Also, if the consumer must pay to get the electronic product recycled, the electronic waste will wind up in the trash. Municipal governments can collect household electronic products through curbside pickup for proper dismantling and reclamation. A regulatory exclusion, not by individual states, but rather by the overall government agency, would apply to collection, transportation, accumulation, storage, dismantling, and recycling the waste. Standards for collection are only voluntary in many communities in the U.S. Sony, for example, conducted a pilot program in Minnesota to take back its televisions from consumers as part of a municipal waste program. IBM and Hewlett Packard are two companies that offer to take back computers for a fee. Some retailers have agreed to take back nickel-cadmium batteries (Radio Shack and Sears), and electronic products (Best Buy). 1. Collection Schemes and Burden Who is going to pay for the collection, processing and recycling of electronic products waste? Ultimately, the cost will be borne by the consumer. The collection plan proposed by the European Union specifically states that the manufacturer is responsible for the costs. However, one can only assume that the price of new electronic products will reflect the cost of recycling. Upfront payment when the electronic product is purchased does not completely solve the problem of who pays for the recycling when the electronic product is eventually discarded. It may be easy to identify the manufacturer of electric shavers or mobile phones. However, the requirement that electronic product is returned to the manufacturer will not work in the case of computers, where the constituent, add-on peripheral parts come from several manufacturers. Also, as much as 40% of the electronic product waste may have been manufactured by a company no longer in business. The political solution appears to be to set up some type of government funding to handle recycling or processing of electronic products waste. However, the cost of the collection system bureaucracy may run several billion dollars a year in a 900 billion dollars global electronics industry. The subsidy supporting recycling is not likely to go away in the near future. The collection and recovery of valuable elements of scrap electronics must be done in an environmentally responsible fashion. At the present time, 50 to 80% of electronic waste from the U.S. is sent to China, India, Pakistan or other developing countries. This can lead to environmental problems not anticipated [39]. In the Guiyu region of Guangdong province in China, approximately 100,000 workers are involved in recycling. Some extract plastic, while others melt circuit boards to extract precious metals. The processes which include open air burning of plastics, river bed acid works for gold extraction, and cracking and dumping of lead-bearing CRTs have caused

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pollution of the ground water and hazardous conditions for the workforce, often young children [40]. 2. Special Transportation Considerations Unlike other programs to recover material, such as paper and aluminum, the value of recovered electronic components, metals, plastics, glass, etc. may not offset the cost of transporting the waste to the dismantling or recycling facility. To encourage recycling of electronic products, the management of end-of-life electronic product waste must be excluded from hazardous waste restricting regulations. This is somewhat similar to the need for applying a special exemption to contaminated solder pot dumping and drosses. Though considered hazardous waste because of the lead content, if the used solder or dross is being sent for recycling, the shipment is exempt from manifesting as hazardous waste. It took many years for the government action to exempt transportation from generators to recyclers. 3. Scrap Composition Electronic products are composed of many potential waste streams, some more valuable than others. Small appliances, including computers and printers, typically contain a mixture of about 35% steel, 20% other metals, and the balance a wide assortment of plastics, glass, wood, and textiles. While electronic waste is becoming more voluminous, the amount of non-functional electronic products sitting in storage in businesses and households is increasing rapidly. End-oflife electronic products can begin recovery by being disassembled or shredded, or some combination involving disassembly and shredding. Disassembly is the most effective method to recycle, if not also the most labor intensive and possibly the most expensive. Disassembly allows separation of materials, organic (plastic) and inorganic (metals and glass) for easier and less costly processing for recovery and disposal. The tradeoff is where the money is spent, upfront or at the end of the recovery process. Disassembly and recovery for re-use of components, metals, glass, and plastics is a problem when the electronic product has not been designed for efficient recovery. The cost of recycling, no matter how well intentioned, is the controlling factor when processing discarded electronic products. a. Re-usable Components. Recyclers increase profits by testing and refurbishing waste such as computer monitors and printers, when practical to do so. They also ‘‘harvest’’ components, e.g., microprocessor chips, power supplies, fans, and the more valuable components, intending to improve the return on the cost of recycling. Component parts may be used to assemble new products, or at least be used as replacement parts for electronic products that are not yet obsolete. Re-use of components also has associated costs for disassembly, refurbishing, and testing. b. Metals. Certainly there are valuable metals such as gold, silver, and palladium in electronic products; and some not so valuable, such as copper, steel, and solder (tin and lead). Occasionally, there are smaller amounts of chromium, cadmium, and zinc. In the early 1990s recyclers of electronic circuit board assemblies were able to remove much more gold and silver from scrap. By the year 2000, manufacturers of the printed wiring boards and components drastically reduced the amount of precious metals to reduce the cost of materials. However, recovery of non-precious materials is an important resource issue and environmental consideration. For example, electrical powder cords can easily be separated for recovery of the copper. Also, a typical desktop-computer central processing unit (CPU) contains more than 80% by weight steel. Shredding the CPU and separating the steel from 10-million scrapped personal computers can result in reclamation of more than 100,000 tons of steel. A typical printed wiring board assembly for a computer or television contains a mixture of metals including about 55% copper, 20% steel, 15% tin-lead solder, 5% nickel, 4% aluminum, less than 3% zinc, and less than 1% other metals such as gold, silver, and palladium.

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c. Glass. The cathode ray tube (CRT) glass in computer monitors and televisions represents about 30% of the weight and contains 3 to 8 pounds of lead, depending on the size of the screen. While the lead contained in the glass cannot be easily extracted, it is in a nonhazardous form, so the glass can be recycled. However, the old, recycled glass is darker than the quality of glass in current monitors, so only a small percentage (<15%) is blended with new glass. While the CPU can pay its own way from reclaimed metals, it costs up to $10 to process a CRT, and there is not a sufficient market for used glass. d. Plastics. Plastics can be shredded and sent to landfill, separated for re-use, or used as fuel for cement kilns and incinerators. The energy value for incineration of one ton of plastics is equal to 1.3 tons of coal. Methods are improving for separating the large variety of plastics used in electronic products. Even with good separation techniques, the problem is finding outlets for the output. The plastics recycling marketplace is already swamped with plastic containers of all sorts, and many communities have resorted to incineration of the plastics that have been collected for recycling. Even if all waste electronic products could be processed for recycling, there still remains the question of what to do with the reclaimed material. For example, the number of old tires generated exceeds the capability to recycle them, and used asphalt is not as durable as new asphalt. It is too expensive to process used paper, glass bottles and cardboard, and only 15% of the CRT glass can be used. Nevertheless, if uses for reclaimed materials can be encouraged, the volume going into landfill or incinerators will be reduced. The ban on certain hazardous materials, such as lead, in the waste stream is intended to make shredding and incineration safer and easier. 4. Components and Materials Recovery Recovery of the components and materials is being accomplished to varying degrees from only removing and re-using certain components, such as monitors and printers, to complete orderly disassembly, harvesting of good components, and reclaiming metals and plastics. The evolution of the electronic assembly to surface mounted components, rather than through-hole attachment, makes disassembly easier. Components can selectively be removed by heating the solder joints, or the whole assembly can be heated with all components removed simultaneously for secondary separation. Energy is required for melting the solder joints and, when required to attach new spheres to components such as ball-grid-arrays, and to re-tin (solder dip) component terminations to renew solderability. a. Components. Removal of electronic components is justified only when there is a market where the component parts can be sold. It is easy to see how a computer monitor or printer can be tested, refurbished and sold if slightly old technology is acceptable to customers. However there is the perception that used or reconditioned parts are lower quality, so testing and warranties are required. Profits made by selling components or cost-reductions made by re-using components are offset by environmental controls on emissions, energy expended, disassembly costs, and testing. The reduction in disposal costs and energy obtained from incineration also contribute to cost/profit. b. Metals. As mentioned, the steel in electronic products is the easiest to process either by disassembly or after shredding the products, using magnetic collection. Extracting metals other than steel is challenging and does not always provide enough financial return to justify the effort. Nevertheless, metals in their pure form, or at least their metallic form if mixed with other metals, are the most recyclable materials. Metals can be purified or refined and made into new products. In the electronic products, metals are often plated onto other metals or, in the case of solder, bonded to other metals. Dissolving metals such as tin and lead in acid, or using cyanide salts to dissolve gold, silver, and palladium, for reclaiming by precipitation or plating, certainly adds cost and potentially could contaminate the environment if proper controls are not in place. Recovery of metals from electronic products requires energy for processing the waste (separating, shredding) and chemicals for extraction and for plating our metals. Recovery of

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109

precious metals is a diminishing opportunity as electronic components and assemblies contain less of these metals. The future will tell if the increased amount of silver in lead-free solder, or palladium on component terminations is enough to encourage salvaging of the metals.

B.

Environmental Benefits

Estimates place the amount of electronic product waste in municipal landfill at 2 to 4 percent. This small percentage would certainly seem to reduce any motivation for reducing the waste volume, but the weight amounts to 5 to 10 million tons per year, and is expected to increase. Environmental damage from electronic products waste is not clearly quantified, but people do realize that disposing of millions of tons of electronic products waste into dumps or trying to burn away the problem in incinerators is not the responsible solution. The objective is to reduce the amount of hazardous materials going into landfills and to encourage reuse and recycling. The importance of end-of-life recovery is illustrated in the Explanatory Memorandum of the WEEE [41] document, which demonstrates the potential environmental savings associated with the recycling of copper. Whereas it takes 1,176 kg primary ore resource to create 1 kg copper, the projected recycled 522,000 tons of regenerated copper from annual WEEE collections would lead to a savings of 600 million tons of primary raw material each year. Though there is very little gold to recover from electronic product waste, a personal computer printed wiring board assembly can yield about one ounce of gold per 3 to 5 tons of electronic waste. This is equivalent to mining and processing about 8 tons of gold-bearing ore.

VII.

INDUSTRIAL ECOLOGY ASSESSMENT

An environmental assessment for comparing lead-based vs. lead-free solders should be based on industrial ecology considerations. This involves a cradle to grave assessment of the material choice, energy use and environmental impact to air, water and soil.

A.

Resource Extraction

As indicated in the Product Assessment Matrix (Table 5), an environmental assessment of mining metal must consider the energy consumption and the solid, liquid and gaseous residues that result from the various mining processes. Availability of the metals is an additional consideration. Table 6 lists the world reserves of the major metals typically utilized in electronic assemblies. SnAg-Cu alloy is one of the major compositions proposed as a replacement for Sn-Pb solder. Silver is 300 times less available than lead. Half of the silver available comes as a by-product from mining lead, copper and zinc. Antimony and bismuth are also obtained as by-products of lead, copper and silver mining. The lead used in solders for electronics is less than 1% of the total usage [42]. As indicated in Table 1, the major uses of lead are for storage batteries (86.6%), paint, glass and ceramic products, pigments and chemicals (3.0%), ammunition (3.6%), and sheet lead, cable covering, casting metals, brass and bronze (3.7%). Most of these applications are expected to continue. Thus, it is important to realize that the mining of lead will decline only marginally or not at all if lead-based solders are banned. Mining of tin, however, will increase. Most of the lead-free alloys under consideration have significantly higher tin concentrations than traditional Sn-Pb solder. Many tin mines have closed during the past few years due to a reduction in the price of tin. An increase of 50% in the amount of tin to replace some of the lead used for electronic assembly could result in shortages as lead-free solders are required in Europe by mid-2006.

B.

Product Manufacture

To evaluate product manufacture, the industrial ecologist must consider the process window of the material chosen, the amount of energy required for the process, cost of materials, amount of

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scrap produced, as well as solid, liquid and gaseous residues produced. Material compatibility and energy consumption are key concerns. In the case of most lead-free solder alternatives, the processing temperature is higher, so energy usage will be increased, which can be significant. Electronic components and printed wiring boards (PWBs) absorb moisture during storage. The moisture can suddenly boil away during soldering, resulting in consequential damage of the components or PWBs. Baking the components and PWBs may be necessary, and, because of the higher soldering temperatures with lead-free solders, the baking time must be longer to ensure thorough moisture evaporation. This results in a significant increase in energy. Some components will require a separate assembly/soldering step since they are not sufficiently heat resistant to withstand the higher soldering process temperatures required of most lead-free solders. Scrap levels will increase as well, especially in the early days of implementation when processes are not yet optimized. These factors, plus the increased cost of raw materials, increased energy and scrap, are important environmental concerns. Thus, the application of lead-free soldering will negatively affect the environment in the manufacturing operation.

C.

Transportation and Shipping

There is no change anticipated in environmental impact related to transportation and shipping of lead-free electronic products.

D.

Product Use

Reliability of lead-free electronics in the field is still unknown. Studies of some alloys have shown improved mechanical properties over eutectic Sn-Pb, but more reliability testing remains to be TABLE 11 Life-Cycle Assessment of Lead-Free Solder Alternatives (After Graedel and Allenby, Ref. [10], 1995) Factor Energy consumption Resource consumptions Availability Cost Air impact Water impact Soil impact

Materials and component compatibility Energy consumption

Cost Impact on air Impact on water Impact on soil Waste impact Total environmental impact

In

Bi

Pb

Ag

Epoxy

2 1 3 0 2 2 2

3 3 4 4 3 4 4

3 3 4 4 3 4 4

Electronic assembly (manufacturing) 1 1.a 1

3

2

2

3

1

3 3 2 2 2

2 3 4 3 2

4 3 2 2 1

27

45

40

Resource extraction 4 4 4 2 4 4 4 3 3 3 4 4 4 4

1

4 3 2 2 1 41

1 Reuse and disposal 4 3 2 2 1 38

a Assumes the use of lead-free metallization on components and board. If lead is present, the factor is 3.

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done. The interaction of lead-free alloys with surface finishes on components and printed wiring boards must also be assessed. Based on the facts known at present, it is too early to comment whether there is an increased or decreased environmental impact during product use.

E.

Refurbish, Recycle, Disposal

From an industrial ecology perspective it appears that what makes more ‘‘sense’’ environmentally than a ‘‘lead ban’’ is a re-thinking of the manufacturing and reclamation processes. For example, an alternate approach to eliminating the potential hazards of lead is to improve the infrastructure and methods for metal recovery and reuse in electronics. This approach has the advantages of eliminating any concerns about end-of-life disposal and the potential to reduce energy consumption. Reuse and recycling are less costly than disposal, and ultimately have less impact on the air, water and soil.

F.

Industrial Ecology Assessment

Expanding the factors listed in the Product Assessment Matrix (Table 5) enables a quantified analysis of the environmental cost of replacing lead-alloy solders with tin alloyed with bismuth, indium, silver, or with silver-filled epoxy. The assigned values used in Table 11 assess environmental impact from 1 to 4, low to high. Rankings of the environmental factors have been taken predominantly from Graedel and Allenby [10]. The energy required to extract resources, the relative impact on the environment, comparable amount of energy to use the alternative metals, and the cost and impact of recycling all contribute to the total environmental impact. A life-cycle assessment from the University of Stuttgart, Department of Life-Cycle Engineering, [43] came to similar conclusions, based on the increased potential for atmospheric acidification, global warming, and human toxicity. Lead-based solder has a lower environmental impact over its entire life cycle than all the other replacement materials being considered.

VIII.

SUMMARY

This chapter reviewed the increased growth of obsolete electronics and the environmental impact in consideration of the WEEE and ROHS legislation in Europe. The impact of materials choice, resource extraction, manufacture, use and end-of-life are discussed. The use of an industrial ecology approach concludes that banning lead in solder for electronics is not necessarily an environmentally sound approach. New lead-free solders will not reduce the amount of lead ore that is mined, but will require an increased use of less available metals. The manufacturing processes involved for electronic assembly will result in increased energy usage and generation of increased scrap, both having a negative effect on the environment. Exploiting scarce resources to obtain solder alloys that may not be as reliable or environmentally-friendly as conventional tin-lead solder is a questionable solution. There is limited experience with the higher-melting temperature, lead-free solder alloys, and evidence that lead-free solders do not function equal to the standard tin-lead alloys in some applications, particularly those requiring high reliability. The amount of lead entering the environment from discarded electronic products is apparently not measurable because there is no evidence from any environmental studies of landfills to indicate that is the case. Recycling and reuse of electronic products is just beginning to become a reality as people realize resources are not unlimited and waste can be reduced. This is the recommended approach and the area where future research should be directed.

IX.

FUTURE

The issue for the future is not just that lead or other materials are toxic or hazardous, or that electronic products should be recycled, disposed in landfills, or incinerated. The overall approach

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should be to reduce the amount of waste being created, the resources that are being consumed, and the energy that is being expended. Designing for upgrades is possible for some electronics products, but advancing technology will generally result in an increased rate of product obsolescence and disposal. We are increasingly living in a world of disposable products. Nevertheless, disposable does not have to mean incinerating unwanted electronic products or discarding the waste into landfills. Preferably, electronic products can be collected for disassembly where feasible to reclaim and reuse valuable components. Designing for recycling can increase the value of the disassembly process with minimal effect on the cost of building a product. The materials that remain after disassembly can be shredded, separated, and reclaimed for their metal, inorganic (glass), or organic (plastic) value. The residual, uneconomical waste can be burned for its inherent energy. The amount of recycling will depend, not only on the environmental concerns, but also on the financial viability of conducting recycling. If disassembly and recycling are beneficial to the environment, but not profitable, nothing short of legislation will be required to initiate a recycling program. However, manufacturers of the electronic products cannot long afford the consumer perception that their companies are environmentally unfriendly. Products need to be manufactured with fewer different materials, components with value that can be easily removed, materials that can be separated for reclamation, and residual materials that can be safely incinerated. While the recycling of single-material products, such as paper, glass and aluminum, is increasing rapidly, in some local areas the amount of available recycled material exceeds the ability of the marketplace to use these materials. The multi-materials electronic products, which are more difficult to recycle into individual materials, are being submitted for recycling at a much slower rate. As consumers or users become more aware of recycling possibilities, electronic products that have been stored away will become more available for reclamation. Recycling of electronic products by disassembly, reuse, separation and reclamation is expected to create thousands of new jobs with worldwide growth. Key cost factors are now associated with collection of the obsolete electronic products and transportation to the companies doing the disassembly and reclamation. As recycling of electronic products becomes more common, the major costs will be associated with the transportation of the recycled parts and reclaimed materials to users of these materials. Of course, recycled components and parts carry the added cost burden of testing, assuring quality and reliability, and extension of warranties. Repair and reuse of old-technology televisions and computer monitors will certainly end as high-definition televisions (HDTVs) and flat-screen displays become more prevalent. The challenge will be for all stakeholders–government, electronic product manufacturers, retailers, environmental groups, and recyclers–to agree on and establish a viable system for financing, collecting, recycling, and transporting electronic products waste. This is done successfully for automobile batteries, but not so effectively for soft drink cans and bottles. An up-front fee would be more effective than an after-use or recycling fee, such as paid for used automobile oil.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Kitman, J.L. The Secret History of Lead: Special Report, August 17, 2001, http://past.thenation. com/isue/000320/0320kitman.shtml. Allenby, B.R., et al. An assessment of the use of lead in electronic assembly. Proc. Surface Mount International: San Jose, CA, 1–28. Smith, G.R. Lead. U. S. Geological Survey 2000, 2002. Lowry, J. Junked PC’s a growing environmental problem. Atlanta Journal Constitution: Atlanta, Georgia, August 8, 1999. Ross, R. Where Old Computers Go to Die. The Toronto Star; Toronto, Ontario, Canada, 2000; E1– E3. Proposal for a Directive on Waste from Electric and Electronic Equipment 76/769/EEC, Fourth Draft, European Commission Directorate General XI: Brussels, Belgium; May 10, 2000. Municipal Solid Waste, U. S. Environmental Protection Agency, 2000; Vol. 2002. Potential Human Exposure from Lead in Municipal Solid Waste; Industry Economics, Inc. (IEC) for Lead Industries Association, May 21, 1991. Ehrlich, P.R.; Ehrlich, A.H. The Population Explosion; Touchstone Books, Simon and Schuster, 1990. Graedel, T.E.; Allenby, B.R. Industrial Ecology; Prentice Hall: Englewood Cliffs, NJ, 1995.

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.

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Green Products by Design, Choices for a Cleaner Environment, Office of Technology Assessment OTA-E-541, September 1992. Our Common Future, World Commission on Environment and Development, 1987 Socolow, R.H. Industrial Ecology and Global Changes; Socolow, R.H., Ed.; MIT Press: Cambridge, MA, 1992. U.S. Geological Survey. http://minerals.usgs.gov/minerals/pubs/mcs/, 2000. Clark, F.W.; Washington, H.S. The data of geochemistr. Bulletin 770, U.S. Geological Survey, 1924. CRC Handbook of Chemistry and Physics. Lyde, D.R., Ed.; 80th ed.; CRC Press: Boca Raton, FL, 1999; 14 pp. Gec, J. Damage claims fly over cyanide spill. Atlanta Journal Constitution, (Atlanta, GA), 2000. Petty, E.R. Physical Metallurgy of Engineering Materials; American Elsevier Publishing Co.: New York, 1968; 97–58 pp. Martin, J. Materials for Engineering; The Institute of Materials: London, 1996; 97–98 pp. Worcester, A.W.; O’Reilly, J.J. Lead and Lead Alloys. In: Metals Handbook, 10th ed.; Ohio: American Society for Materials, International: Ohio. Vol. 12. 543–556 pp. Newton, J. Extractive Metallurgy; John Wiley and Sons: New York, 1963; 1–27 pp. Hampshire, W.B. Tin and Tin Alloys. In: Metals Handbook, 10th ed.; American Society for Materials, International:Ohio, Vol. 2, 517–542. Stevens, L.G.; White, C.E.T. Indium and Bismuth. In: Metals Handbook, 10th ed.; American Society for Materials, International:Metals Park, Ohio, Vol. 2. 750–757 pp. Turbini, L.J.; Bent, W.R.; Ready, W.J. Impact of higher melting lead-free solders on reliability of printed wiring assemblies. Journal of Surface Mount Technology 2000, 13, 10–14. Lead-Free Solder: No Longer a Paper Tiger, Prismark Partners; February 1999. Lehner, T. Boliden Mineral AB, private communication. Freudenrich, C.C. How Landfills Work, http://www.howstuffworks.com/landfill.htm, 2002. Method 1311 toxicity characteristic leaching procedure (TCLP). Federal Register May 29, 1990, 55, 11863–11877. Smith, E.B.; Swanger, K. Are Lead-Free Solders Really Environmentally Friendly? Surface Mount Technology, May 1999; 64–66 pp. Smith, E.B.; Swanger, K. Environmental impact of lead-free solders. Surface Mount Technolog, July 1999; 74–79 pp. Yang, G.C. The environmental threats of discarded picture tubes and printed circuit boards. Journal of Hazardous Materials 1993, 34, 2325–2430. Pinckert, R.E. Boeing’s Lead-Free Soldering Efforts, 1st European Union/United States Lead-Free Solder Interface Meeting, London, UK, Vol. 2002. Bernstein, J. Report from aspen. The New Yorker; November 25, 1991; 121–136 pp. Summary of Data on Municipal Solid Waste Landfill Leachate Characteristics, NUS Corporation (for U.S. EPA), 1988. Potential Human Exposures from Lead in Municipal Solid Waste, Industry Economics, Inc. (IEC) for Lead Industries Association. Case Studies on Groundwater and Surface Water Contamination from Municipal Solid Waste Landfills, NUS Corporation (for U.S. EPA), 1988. Municipal Waste Combustion Study: Assessment of Health Risks Associated with Municipal Waste Combustion Processes, Radian Corporation (for U.S. EPA), 1987. Morrison, R.M. Baseline Risk Analysis to Support Municipal Waste Combustor New Source Performance Standard and Emission Guideline Development, U.S. EPA report, November 22, 1989. Markoff, J. Third world a dumping ground for west’s ‘e-trash’. The Toronto Star; Toronto, Canada, 2002. Aubrey, A. Activists push for safer e-recycling; National Public Radio: Washington, D.C, April 6, 2002. Proposal for a Directive on Waste from Electric and Electronic Equipment 76/769/EEC, Third Draft. European Commission Directorate General XI (Brussels, Belgium), July 5, 1999. Lee, N.C. Lead-Free Soldering - Where the World is Going. Advancing Microelectronics, 1999; 29 pp. Warburg, N. Life-Cycle Assessment. Presentation at University of Stuttgart, Stuttgart, Germany, 2002.

4 Environmental Stewardship with Regional Perspectives and Drivers of the Lead-free Issue Susanna Pelzel Studio Legale Paggi, Italy

Minna Juuti IBM Environmental Affairs, Stockholm, Sweden

Yukio Sugimoto IBM Corporation, Tokyo, Japan

I.

INTRODUCTION

Lead and lead compounds are classified as toxic chemicals that persist and bioaccumulate in the environment. Persistent, bioaccumulative toxic chemicals (PBTs) are of particular concern as they remain in the environment for significant periods, and accumulate in organisms exposed to them. Lead can cause damage to several systems in the human body. Children are particularly at risk from damage to the nervous system, endocrine system, cardiovascular system, and the kidneys. In addition, lead can have adverse effects on the environment, such as acute and chronic toxic effects on plants, animals, and microorganisms.

A.

Lead Restrictions

The potential impact that lead exposure can have on health and the environment has brought worldwide initiatives to reduce lead sources by restricting the use of lead in manufacturing processes or product applications. In the United States, the Environmental Protection Agency (EPA) has taken regulatory action to ban the use of lead from automobile fuel, plumbing solders, paint products, food packaging, and cans. Although there is no general definition of a lead-free application, the EPA guidelines consider plumbing solders and fluxes to be lead-free if they contain less than 0.2% lead [1]. In the European Union (EU), the use of lead has been restricted in products such as paints, fuels, and vehicles. The EU has not agreed to an official definition of leadfree applications. However, an alloy is generally considered lead-free in Europe when the level of lead contamination is less than 0.1% [2,3].

B.

Lead Use in Electronics and Waste Impact

The main applications of lead by the electronics industry, namely leaded-glass cathode ray tubes (CRTs) in monitors for computers, televisions, and video games; have not been restricted thus

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far. This is attributable to the limited environmental and health exposure associated with these applications. It is generally acknowledged that only a very small percentage of lead is found in electrical and electronic equipment. Similarly, it is typically accepted that virtually no risks for consumers arise, even over long periods, from the use of lead in the form of solder or suspended in glass, as the lead is normally sealed within the final product. However, significant advances in technology have resulted in an increased reliance on electronic product that are utilized in our everyday lives such as televisions, computers, wireless phones, copiers, fax machines, VCRs, just to cite a few. However, as innovation multiplied and electronic product use increased, product life has decreased. Although many of these products are still operational, they are nevertheless quickly deemed obsolete because of the rapid rate at which more advanced versions are introduced to the market place. How to properly dispose obsolete electronics is a matter that is becoming more acute owing to the rate at which it is accumulating. It was reported that a billion personal computers were sold by mid-year 2002, and the expectation is that the figure will reach 2 billion sometime in 2008 [4]. A study conducted by the National Safety Council estimated that, between 1997 and 2007, nearly 500 million personal computers are projected to become obsolete in the United States—almost 2 computers for each person in the population [5,6]. Technological advances are creating a similar situation for other consumer electronics as well. For example, a large number of televisions will become obsolete and disposed when high-definition television becomes widely available. These obsolete electronic products, including monitors, printers, etc. are sometimes referred to as ‘‘closet waste’’ because they are accumulating in attics, basements, and warehouses, kept by businesses and households because of the belief that they still have value [5]. However, ultimately they will be discarded, some recycled; but most placed in landfills. Many contain lead and other heavy metals that are toxic if released into the environment. They also contain glass, other metals, and plastics that can be utilized to make new products, rather than wasted, if recycled [6]. The continued increase of obsolete electronic products entering waste streams is a worldwide concern. In 1998, 6 million metric tons of electronic and electrical equipment [waste electrical and electronic equipment (WEEE)] discarded on the European continent, accounted for 4% of the municipal solid waste (MSW) stream, according to the European Commission (EC). The EC expects the WEEE volume to increase by at least 3–5% per year [7].

C.

Potential Health and Environmental Impacts

The increasing amount of end-of-life electronics that may end up in landfill sites or be incinerated without proper pretreatment has caused a reconsideration of the impact that electronic equipment may have on health and the environment. Although lead is likely to remain sealed in equipment, with no significant contribution to exposure during its use, contamination may potentially take place when discarded as waste. One of the main concerns relates to the potential to leach lead from electronic waste in landfills and contaminate drinking water supplies. It has been found that ‘‘significant amounts of lead ions are dissolved from broken lead-containing glass by the acidic groundwater often found in landfills’’ [8]. Therefore, contamination by lead from cone glass in landfills is likely. The Commission argues that end-of-life consumer electronics constitute 40% of lead found in landfills. In addition, waste electrical and electronic equipment (WEEE) represents a relevant amount of total lead emissions from incinerators, with dispersion into the environment. The European Commission argues that WEEE incineration represents about 50% of the total lead emissions from incinerators.

D.

Regulation

Steps have been taken to prevent lead and other hazardous substances from entering the environment from end-of-life electronic equipment destined for landfills. In June 2000, the European Commission proposed legislation to increase WEEE recycling and reduce the amount dispositioned by landfill or incineration, and introduced pretreatment requirements for WEEE

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disposal and recovery operations. In parallel, the Commission proposed restrictions on the use of certain hazardous substances that are considered particularly problematic during the waste management phase, including lead, in electronic equipment. In addition to the European Commission’s initiatives, another major driver for lead restrictions in electronics is the voluntary action taken by Japanese electronics companies. Accordingly, the combined legislative (EU) and competitive (Japan) pressures have motivated United States and European electronics companies to also investigate lead-free initiatives.

E.

This Chapter

This chapter is divided into three sections. The first section focuses on planned or adopted lead restrictions that may affect the electronics industry. The second section addresses waste management initiatives aimed at increasing WEEE recycling to minimize the disposal volume. The third section is an overview of industry and other interested parties’ positions on the phaseout of lead use in electronic equipment.

II.

PRODUCT STEWARDSHIP

Product stewardship means those who design, make, sell, or use a product take responsibility for minimizing its environmental impact. This responsibility spans the life cycle of a product, that is, from the selection of raw materials, through its design, manufacture, packaging, distribution up to its end-of-life management. Importantly, it shifts the traditional responsibility from the public sector (government and taxpayers) to a shared responsibility with the private sector (manufacturers and consumers). The key objectives of a product stewardship program are to encourage environment-friendly design, recycling, and reduce the flow to landfills [9]. To achieve long-term waste reduction and resource conservation, manufacturers, retailers, and consumers must view the products they produce and purchase as resources rather than as future waste [6]. Product stewardship consists of several elements that are briefly described in the following subsections.

A.

Life-Cycle Perspective

An environmental and health impact that a product has must be based on all aspects of creating the product including raw material production, manufacturing/assembly, use/service, and not just end-of-life management (recycling, disposal, etc.), which determines the economic viability of the product system [10]. To account for potential detrimental impacts a product may possess, it is necessary to consider energy and material consumption, emissions to the air and water, toxin levels in the product, worker and user safety, and waste disposal issues [9].

B.

Design for Environment

Product designs and processes must be consistent with environmental consciousness covering the entire product cycle. Design for environment (DfE), also referred to as EcoDesign or Green Design, addresses the global impacts and habitat disturbances, emissions and effluents, chemical releases, inefficient use of water and energy, solid waste, and more. Application of DfE improves efficiency, allows for simplified disassembly at the end of life, and produces less waste, typically resulting in substantial savings. Pollution and waste are considered to be avoidable inefficiencies. Among the things considered are: ways to increase product life, incorporating recyclable materials into the product and its manufacturing processes; reducing the number of polymer types; ease of disassembly for recycling and removal of potentially harmful materials; the substitution of environmentally benign materials for ones of greater risk, to note a few [9]. Introducing changes through design is often more effective at reducing environmental burdens and reducing costs than traditional end-of-life strategies and approaches. However, as

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is typically the case with design considerations, trade-offs are normally necessary because rarely is one product or approach more beneficial in every respect compared to others. Designers are faced with numerous competing requirements and constraints. For example, to meet consumer expectations, handheld products must be light, highly functional, and low cost. To be environmentally friendly, products must also be energy-efficient and easily recycled. Thus, design for environment makes the overall task more difficult than normal because consumers consider energy efficiency and recyclability to be less important product attributes, which means those that relate to cost and functionality cannot be compromised. An important element of DfE is easy disassembly for subsequent recycling, parts reuse, and removal of potentially harmful materials as part of disposal. The economic cost of physically disassembling products to remove valuable components and materials often exceeds the overall value of the materials. DfE can help reduce the time and complications, thus cost, of disassembly that can reverse this balance. Therefore, DfE is a driver for recycling and reuse [11]. Designers, managers, and others in a decision-making position cannot be expected to be environmental experts capable of knowing the environment and sustainability implications of their decisions. As a result, companies often incur high costs because they use materials or processes that are subsequently the cause of a significant environmental problem, when an environmentally benign version exists. Many companies are integrating DfE into their environmental management systems (EMS) that reveals the cost of company decisions regarding materials, products, and manufacturing processes. Incorporating DfE as part of the EMS also helps to position a company at the forefront of environmental consciousness rather than just being compliant [11].

C.

Green Purchasing

Making environmentally sound procurement choices by manufacturers and consumers is a key element in a total life-cycle perspective. Products should be selected based on the following factors: (1) on the basis of their recycled content, (2) they can themselves be recycled, (3) expectation of having a longer life than similar products, and (4) consist of less packaging. Those engaged in volume purchases, such as businesses and government, have the opportunity to be particularly effective in driving so-called environmentally preferable purchasing (EPP) practices by requiring suppliers to conform with prescribed principles and guidelines [9].

D.

Take-back/Recycling

There are two approaches in treating end-of-life materials: (1) dispositioning it in the traditional manner as waste in landfills or incineration, or (2) viewing used materials as valuable industrial materials [12] obtained through recycling. The mechanism by which this can be achieved and who is responsible is steadily evolving. The lines were clearly drawn in the past; producers were responsible for environmental impacts associated with production facilities, but downstream impacts were largely ignored, that is, they were not held accountable for issues that arose as a result of product use, as well as end-of-life issues such as toxic waste [13]. 1. Extended Producer Responsibility The concept of extended producer responsibility (EPR) has been steadily evolving owing to the growing environmental concerns related to product use and end-of-life issues. That is, producers are made responsible for environmental effects extending over the entire product cycle. The concept is often implemented through product take-back programs. Producers are not likely to be involved in the actual physical collection of discarded items, a task that will be handled by a third party [14]. The concept was initiated in 1991, when Germany, a country with a serious landfill shortage, adopted a law making producers responsible for product packaging when consumers discarded it. The German industry adopted a ‘‘Green Dot’’ system in response to the law wherein manufacturers pay a nonprofit organization a license fee for the right to place their logo on their packages. The fee pays for the collection and recycling of waste bearing their logo

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throughout Germany. The concept was quickly adopted throughout the industrialized world for products and not just packaging materials, with the notable exception of the United States. The European Union included the concept as a part of its WEEE Directive, discussed later in this chapter, and it spread to Asian countries such as Japan, Korea, and Taiwan. Although it is not anticipated that extended producer responsibility will be mandated in the United States very soon, at least not at the federal level, American companies will encounter the concept in numerous ways, at home and abroad. Implicit in shifting the responsibility for waste to the producer is the belief that it is one of the best ways to force changes all along the product life cycle. If companies must concern themselves with their product at the end-of-life stage, it will affect the materials selection and design decisions. They, in turn, affect all the potential environmental impact factors such as material extraction and processing, manufacturing, and end-of-life disassembly [14]. 2. Takeback Schemes The paradigm shift of product take-back implies a liability for products over their entire life cycle, including disposal. Producers are responsible for the collection, recycling, and disposing of endof-life products [15]. Waste management and associated costs are borne by those most capable of reducing end-of-life costs by making suitable materials and design changes to reduce toxicity [i.e., lead (Pb)-free electronic products] and volume of waste generated. The products targeted by product take-back regulations are those with serious disposal problems in terms of both volume, and hazardous and toxic material content. Hence, electronics products, including information technology (IT) products, have been targeted because they typically contain Pb-bearing solder as part of their circuit board assemblies. a. Collection. The initial issue that requires resolution is by who and how electronic waste is to be collected. Several schemes were considered, each one having its strong and weak points. 1. Individual system—Each manufacturer would arrange for the pick up of only their products from designated collection points, then have them transported to a recycling facility where they would be appropriately treated (disassembled to remove materials or components to be recycled, or make them unacceptable for landfills and incineration), and then appropriately disposed of. Manufacturers only pay for the costs associated with their products. Consequently, manufacturers such as Hewlett-Packard, which have heavily invested in DfE products, are pushing the individual take-back option because they want to benefit from enhanced recycling capability a (easier disassembly, less toxic materials, reduced mass, fewer polymer types, etc.) of their products. They want to avoid paying the same take-back fees as companies that have not made DfE investments [16]. There is also the belief that this approach would provide an economic incentive for companies not investing in DfE to do so. Individual collection systems may be suitable for manufacturers of low-volume, high-cost products, but questionable for manufacturers of consumer products requiring a significant sorting effort at the collection points. 2. Collective system—Collectively, manufacturers would contract to have waste collected, transported, recycled, and disposed of. This approach is clearly more efficient, thus less costly than treating waste on an individual brand basis. The issue here is to determine the fair share of the cost for each participating company. Several methods have been devised.


Weight based—One method is to assess manufacturers on the basis of the weight of the products they sell. Although the cost of recycler services are typically based on weight, it is also true that there is no actual correlation between weight and ease with which a product can be recycled. In fact, there are many examples of where a heavy product is easier to recycle compared to lighter ones.
Fixed Fee—Another method is to share take-back and recycling fees on a fixed fee per unit basis. But, because as already noted, actual collection and

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PELZEL ET AL. recycling costs vary widely given the broad range of electronic products. Therefore, a scale is utilized based on product category. Products in difficult-to-recycle categories are assessed more than those products in less difficult categories.
Sales Revenue—Yet another method of assessing take-back and recycling fees is base on sales revenue. The concept here is that manufacturers with the highest revenue and highest-priced products can best afford to finance the program. Similar to product weight, product cost is not reflective of the ease with which a product can be recycled. However, applying this metric does have the advantage of only adding an incremented cost to the product, thereby eliminating the concern of overpricing low-cost products. A fee would be added to the purchase price to finance the program. There is a debate if the fee should be visible to the consumer, that is, noted separately from the price of the product, making it clear what the take-back cost is [4,17–19]. 3. Deposit and Return—It has been suggested that the most efficient way to conduct product take-back systems is through deposit–return systems. They have been successfully utilized to collect a wide variety of products around the world such as beverage containers, car batteries, pesticide containers, car bodies, etc. It is suggested that the concept could easily be extended to other products including durables and electronic equipment. Consumers return their used products to the point-of-purchase retailer to redeem their deposit. Retailers are compensated for their storage and handling of returned items [12].

b. Front-end Financing. Independent of the method used to assess waste collection and recycling fees on manufacturers of electronic equipment, ultimately it is the consumer who will bear the cost. That cost will likely be charged at the front end, not at the time when the product is ready for disposition [18]. The National Electronics Product Stewardship Initiative (NEPSI), composed of participants from the electronics industry, government, environmentalists, recyclers, and retailers, is developing a nationwide front-end financing system for electronics collection, reuse, and recycling. It is generally agreed that a nationwide program would be more effective than state-by-state initiatives. Environmentalists have long advocated that frond-end financing is an essential step in establishing an effective electronic waste management system. 3. Scrap Reuse and Recycling Reuse and recycling electronic equipment is a beneficial alternative to disposal. An additional benefit is the reduced amount of hazardous and toxic substances entering the incineration and landfill disposal systems. Reuse serves to extend product life, thereby conserving the energy and raw materials required for new products, while avoiding the pollution and contamination associated with the energy utilization and processes to manufacture them [5]. Beyond reuse, the next best option is recycling to derive a partial benefit from some components, parts, or reclamation of materials for use in new or refurbished computers, monitors, printers, scanners, and other electronic products. Among the materials that can be reclaimed in electronic products are plastics, glass, steel, aluminum, copper, nickel, and precious metals including gold, silver, and palladium. Off Shore Recycling: If directly dealing with a recycler, be advised to check that the firm meets all applicable state and local regulatory requirements and properly manages the recovered materials. An international agreement was put in place because it was estimated that 50–80% of electronic waste collected by U.S. recyclers was exported to developing countries, including China and India, owning to their lower processing and labor costs. The International Association of Electronics Recyclers developed a certification program for recyclers, rendering them more accountable about the destination of their electronic waste [4,5]. The National Recycling Coalition web site (http://www.nrc-recycle.org/resources/ electronics/links.htm) can be con-

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sulted relative to state contacts, and information about transporting and recycling electronic equipment [5].

E.

Waste Disposal

In addition to the benefits derived from take-back programs discussed in the preceding sections, yet another key benefit is the significantly reduced volume of waste disposed in landfills or by incineration. However, an integral part of take-back systems is the removal of harmful and toxic materials prior to disposal. This chapter discusses legislation and other initiatives to achieve these objectives at various geographies worldwide. 1. The Ban on Lead (Pb) In concert with take-back programs, there are also outright bans on the use of certain hazardous and toxic materials, among them lead (Pb). Therefore, it is the focus of this handbook to discuss, in detail, the materials serving as alternatives to lead (Pb) in solders, where its major application by far is electronic assembly. In addition, this handbook discusses the process implications of these alternatives, unique materials characteristics, and their potential reliability impacts.

III.

LEAD USE RESTRICTIONS: EUROPEAN UNION

The European Commission proposed legislation on the management of waste electrical and electronic equipment (WEEE) on June 13, 2000, following a drafting process, which lasted more than 3 years. This legislation aims to significantly reduce waste from electrical and electronic equipment through an increase in reuse, recycling, and other forms of recovery, resulting in a reduction of landfill and incineration.

A.

Materials Restrictions

In addition to the management of WEEE, the first draft for a WEEE Directive released by the Commission’s Environment Directorate General also included restrictions on the use of certain hazardous substances, including lead in electronic equipment. These restrictions are considered ‘‘functional’’ to the waste management provisions. That is, according to the Commission, health and environmental exposure problems linked to current management of WEEE can be significantly reduced by diverting waste from landfill and incineration to separate collection, treatment, and recycling. For the near future, it is nevertheless generally believed that a significant portion of WEEE will continue to be found in existing disposal routes. In addition, even if WEEE were separately collected and recycled, European Commission policy maintains that their heavy metals content still poses risks to health and the environment because recycling does not eliminate hazardous substances, but only delays their return to the environment through new products.

B.

WEEE and ROHS Directives Created

The European Commission’s proposal was split in two separate initiatives. The first proposal, based on EU Treaty, Article 175 (‘‘Environmental Protection,’’ ex-Article 130r), deals with the management of WEEE, the so-called WEEE Directive. The second proposal, based on EU Treaty, Article 95 (‘‘Internal Market,’’ ex-Article 100a), provides for restrictions on the use of hazardous substances in electrical and electronic equipment, the so-called Restriction of Hazardous Substances (ROHS) Directive. A third proposal, also under consideration, addresses ecodesign rules, the so-called EuP (Energy using Products) Directives (ex EEE Directive), that would defines a framework for electronic equipment concerning design, manufacturing, use, and waste management. Equipment which is in conformity with ‘‘European harmonized standards’’ published in the EU Official Journal, the Eco-label award scheme, or equipment designed by companies registered under the EU Eco-auditing and management System (EMAS) would all

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be presumed to be in conformity with the ‘‘essential requirements’’ stated by the Directive. Alternatively, manufacturers could be required to perform a conformity assessment.

C.

Hazardous Substances Restriction In Electronic Products (ROHS Directive)

The Commission’s proposal for a directive restricting the use of hazardous substances in electrical and electronic equipment (the ROHS Directive) was adopted on June 13, 2000 [20], and subsequently modified in June 2001[21]. The proposed directive applies to large and small household appliances, information technology (IT) and telecommunication equipment, consumer equipment, lighting equipment, electrical and electronic tools, and toys. Some categories of lighting equipment will not be covered by the European Commission’s WEEE proposal. Accordingly, these products will be included within the ROHS proposal. The categories concerned are light bulbs, household luminaries, and compact fluorescent lamps. Contrary to the proposed WEEE Directive, the ROHS Directive does not cover medical equipment systems, monitoring and control instruments, or automatic dispensers. Electrical or electronic equipment is defined as any equipment that depends on electric currents or electromagnetic fields; and equipment used for the generation, transfer, and measurements of electric currents and fields. Also included is equipment designed for use not to exceed a predefined voltage, 1000 V for alternating current and 1500 V for direct current. A Producer is defined largely as anyone who manufactures and sells electrical and electronic equipment under his own brand, resells equipment produced by other suppliers under his own brand, or imports equipment. 1. Banned Substances The ROHS Directive would impose substitution on the use of lead, mercury, cadmium, hexavalent chromium, and brominated substances polybrominated diphenyl ethers (PBDE), and polybrominated biphenyls (PBB) in electrical and electronic equipment. The Directive would allow the Commission to establish maximum concentration values, where the presence of specific materials or components is tolerated. Industry welcomes this option because heavy metals covered by the Directive are present in most materials as naturally occurring substances. As for the economic implications, the Commission recognized that substantial costs would be incurred for replacing lead in solders and estimated the additional operating costs of using tin-based solders or other alternatives would be about 150 million Euros per year. An amended Commission proposal makes provision for the list of hazardous substances to be revised by 2003, with the possibility of extending the complete ban to additional substances. When scientific evidence is available, the European Parliament and the Council will decide on whether to prohibit other hazardous substances and require their substitution with more environmentally friendly alternative substances that ‘‘ensure at least the same level of protection.’’ 2. Exemptions The Annex to the ROHS Directive contains a list of applications of substances exempted from the substitution requirement. The Annex will, in the future, be adapted in light of technical progress and scientific evidence, and take into account the precautionary principle, which has several different legal and policy meanings, both at the EU and international levels. For the present purposes, it is understood as the justification for imposing regulatory intervention where, even if there is incomplete or imperfect scientific knowledge, there are reasonable grounds to believe that a hazard is caused to the environment. There will be technical adaptations in order to (a) exempt new applications if the use of concerned substances is technically or scientifically unavoidable, or where the negative environmental and/or health impacts caused by substitutions are likely to outweigh the environmental benefits, and (b) delete applications included in the Annex if the use of the substances concerned becomes ‘‘avoidable.’’ This is predicated on the fact that the negative environmental and/or health impacts caused by a substitution do not outweigh its environmental benefits. These amendments would be adopted by the European Commission, in close cooperation with the Member States, under the so-called Comitology procedure. Comitology refers to the use of regulatory, advisory, or management committees to assist the European Commission

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in implementing the legislative powers it holds under the EU Treaty. In practice, particularly in relation to product regulation, regulatory committees made up of Member State representatives consider (and vote upon) draft Commission legislation. However, the Commission would consult electronics manufacturers, through representative trade associations, before amending the Annex to the Directive. Lead applications the Commission proposes to exempt from the substitution requirements immediately after the Directive enters into effect include lead contained in high-melting-temperature solder used within electronic components for internal connection purposes between a functional element and wires, terminals, heat sinks, etc., and for mounting electronic components onto printed circuit boards. In particular, the applications to be exempted from the ban include those listed in Table 1. 3. Timeline and Penalties July 1, 2006 is the compromise starting date for the phaseout of hazardous substances. The Commission clarified that the ban on the use of hazardous substances for electrical and electronic equipment would go into effect on the date the Directive is published. Member States must transpose the Directive into national legislation within 18 months of the Directive becoming effective (February 13, 2003). Therefore, electrical and electronic equipment produced after the Directive goes into effect but placed on the market before July 1, 2006 would not be covered by the restrictive provisions of the proposed Directive. An important aspect is that this also applies to spare parts for the repair of electronic equipment placed on the market after 2006. In addition, there is a provision that provides for the introduction of penalties for breaches of national law adopted pursuant to the ROHS Directive. National laws are adopted that transpose the terms of the Directive into laws of individual Member States’. All EU directives establish a deadline, typically 18 months, within which such transposition must take place. The European electronics industry is lobbying for an extension of exempted lead applications listed in the Annex. The new applications would include (a) lead contained in high-meltingtemperature-type solder used within an electronic component for internal connection purpose between functional element and wires, terminals, heat sinks, etc. and (b) lead contained in highmelting-temperature-type solder used for mounting electronic components onto printed wiring boards. The Commission noted that there would be ‘‘substantial cost’’ implications on the electronics industry, because of the restricted use of lead (Pb), as noted in the accompanying proposal. According to the Commission’s calculations, the additional operational costs of using tin-based or other alternative solders are estimated to be about 150 million Euros per year. The substitution would have to be implemented by July 1, 2006. Under the current wording of the proposal, it is not clear whether electrical and electronic equipment containing hazardous substances produced following the Directive going into effect prior to July 1, 2006, and placed on the market following that date, would be covered within the scope of the Directive. It is likely

TABLE 1

Proposed Exemptions to the Substitution Requirement in the ROHS Directive

Items exempted . . . . . . . .

Glass of cathode ray tubes, light bulbs, and fluorescent tubes Lead in electronic components Lead used as an alloying element in steel containing up to 0.3% lead by weight Lead used as an alloying element in aluminum containing up to 0.4% lead by weight Lead used as an alloying element in copper alloy containing up to 0.4% lead by weight High-melting-temperature solders (i.e., tin–lead solder alloys containing more than 85% lead) Piezoelectric devices Lead (Pb) containing solders in servers, storage, and storage array systems (exemption available until 2010)

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that equipment produced after the Directive goes into effect and placed on the market after July 1, 2006, will be covered by the Directive.

D.

End-of-Life Vehicles (EOLVs) Directive

Before proposing legislation on end-of-life electronic equipment, the European Union adopted provisions on the management of end-of-life vehicles [22]. The Commission shaped the proposed WEEE and ROHS Directives along the lines of the EOLVs Directive; the latter concerning the prohibition of lead-use in motor vehicles. However, under the EOLVs Directive, provisions relating to the management of waste and restricting the use of hazardous substances in vehicles are both included in a single piece of legislation. The Directive covers both vehicles in used and end-of-life vehicles, including components and materials, and applies to manufacturers and importers of a vehicle into an EU Member State. As regards substance restrictions, the EOLVs Directive requires that materials and components of vehicles placed on the market after July 1, 2003 are not to contain lead, mercury, cadmium, and hexavalent chromium. Considering that this provision refers to the date a vehicle is placed on the market, the ban is likely to apply retroactively to vehicles produced before July 1, 2003—or even before the date the Directive is effective—if placed on the market after that date. Similarly to the proposed ROHS Directive, the EOLVs Directive empowers the Commission to set maximum concentration values for substances in specific materials or components. 1. Exemptions Annex II to the EOLVs Directive contains a list of applications of the concerned substances that are exempted from the substance ban. The lead applications exempted appear in Table 2. The Commission may adapt the Annex, if necessary, in the light of technical and scientific progress, in order to (a) exempt new applications if the use of the concerned substances is unavoidable, or (b) delete applications included in the Annex if the use of the substances concerned becomes avoidable. Amendments to the Annex, and tolerated maximum concentration values for banned applications, will be adopted by the Commission, in cooperation with the Member States, under the Comitology Procedure discussed in Section 3.3.2.

E.

EU Member States’ Initiatives

Aside from the proposed EU legislation, a number of Member States took the initiative to improve the management of electronic waste in parallel with, or even before, the Commission’s proposal. None of these planned or adopted provisions includes measures to restrict the use of lead in electronic equipment. These initiatives concerning the management of waste are discussed below. Nevertheless, Denmark has adopted a general ban on the import and marketing of

TABLE 2

Applications Exempted from the End-of-Life Vehicles (EOLVs) Directive

Items exempted .

. .

Lead as an alloying element in –steel containing up to 0.35 wt.% lead –aluminum containing up to 0.4 wt.% lead –aluminum (in wheel rims, engine parts, and window levers) containing up to 0.4 wt.% lead –copper alloy containing up to 4 wt.% lead –lead/bronze bearing shells and bushes Lead and lead compounds in components, including solder in electronic circuit boards and other applications Other lead applications examined by the European Commission as a matter of priority for a possible amendment of Annex II to the Directive, including electrical components which contain lead in a glass or ceramics matrix compound

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virtually all lead-containing products. Along these lines, Sweden has included the phasing out of lead from consumer products as part of its Environmental Quality Objectives. These measures are examined below. 1. Danish Order to Phase Out Lead In December 1998, Denmark notified the European Commission and the other EU Member States of a draft Order (Table 3) to prohibit the import, sale, and manufacture of lead and virtually all products containing lead by November 1999. The European Commission and the other Member States must be notified of legislation that may act as a barrier to cross-border trade within the European Union, pursuant to Directive 98/34. The notification allows a period for comments and objections to be made, relating to the free circulation of products, before national legislation is adopted. Twelve EU Member States and the European Commission objected to the ban as ‘‘disproportionate.’’ In essence, the objections argued that Denmark would achieve the same environmental objectives through less trade-restrictive measures than a complete ban. Lead- industry groups also raised concerns about the generalized ban. a. Evaluation Phase. The Order was temporarily suspended to allow the EU Committee on Toxicology, Ecotoxicology and the Environment (CSTEE) to publish its opinion on the Danish planned lead ban. The Opinion stated that such a wide-ranging ban on lead use in products would not be ‘‘scientifically justified’’ [23]. The CSTEE argued that ‘‘the limited available evidence indicates that the additional body burden of lead from products in current use is probably small,’’ and those main sources of exposure derive from historic uses, i.e., those that cannot possibly be influenced by product regulation. According to the CSTEE, in some specific cases, lead piping and lead shots are important sources. In air, the main source of lead exposure is derived from its use in gasoline for automobiles. It was also noted that no attention was given to adverse impacts that could occur in humans or the environment from the introduction of substitutes for lead. The Committee concluded that remedial measures should be targeted on specific sources of high lead exposure, such as lead solder in contact with pipes used for drinking water, or a ban on lead in gasoline. However, the Committee also believed that the maximum allowable level of lead in blood endorsed by the World Health Organization might be too high. b. Order Adopted. Despite the CSTEE’s opinion, the Danish Government adopted a general ban on lead in products on November 13, 2000, on the basis that it was ‘‘the best way of reducing future contamination.’’ The Statutory Order prohibiting the import and marketing or products containing lead, became effective December 1, 2000 [24]. However, following discussions with other EU governments and the lead industry, several aspects of the original draft Order have been modified to extend some phase-out timetables and introduce several exemptions to the ban.

TABLE 3

Summary of Key Aspects of Danish Lead (Pb) Phase-Out Order

Key aspects Evaluation phase

Comments . .

Order adopted

.

Ban scope

.

Products containing Pb compounds Products with metallic lead (Pb) Compliance

. . .

CSTEE concluded a wide ranging ban is not scientifically justified Target high exposure applications –e.g., leaded solder contacting drinking water, lead in gasoline Despite CSTEE opinion, Danish Government adapted a general ban on Pb, November 2000 Bans import and marketing of products containing > 100 ppm Pb –does not apply to products involving implementing EU laws Order distinguished between metallic Pb and compounds –products with Pb compounds banned since March 2001 Use in solder applications is okay except for plumbing and sanitation Fine, possible detention

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c. Scope of the Ban. The Danish Order covers the import and marketing of products containing more than 100 ppm lead, where the original threshold level was 50 ppm. The Order does not apply to products imported and marketed exclusively for exports, second-hand products that complied with Danish legislation when initially sold, raw materials, and semifinished goods. In addition, the ban does not apply to products regulated by Danish legislation implementing EU laws, including legislation on leaded gasoline content, batteries, and accumulators containing hazardous substances. d. Products Containing Lead Compounds. The Order distinguishes between products containing chemical compounds of lead and those products containing metallic lead. Import and marketing of products containing lead compounds has been prohibited since March 1, 2001. Annex I to the Order contains a list of exempted product categories until further notice. The prohibition on the marketing and import of products containing metallic lead would not cover products for repairing existing products, including repairs, rebuilding of, and extensions to houses. In the Order, cathode ray tubes are not expressly listed in the product category ‘‘glass for special uses’’ exempted from the ban under Annex I. The industry has argued that cathode ray tubes used in monitors for computers are not covered by the more general category of ‘‘picture tubes’’ which replaced the specific cathode ray tubes category. No definitive ruling has been made in this regard, however. e. Products Containing Metallic Lead. The marketing and import of products containing metallic lead will only be prohibited for certain categories of products listed in Annex II. The only applications of soldering alloys listed in Annex II are those for plumbing and sanitation uses. Therefore, other applications of lead in alloys remain permitted. f. Compliance and Enforcement. The Danish Environmental Protection Agency (EPA), empowered to oversee compliance with the provisions of the Order, may make allowances in special circumstances. Violations of the Order are to be punished with a fine, but detention could be inflicted in serious cases. 2. Swedish Environmental Quality Objectives The Swedish Parliament has established 15 objectives describing required environmental qualities in order to be ecologically sustainable. Among those that the Committee of Inquiry on Environmental Objectives proposed to the Swedish Parliament is the ‘‘Non-Toxic Environment Objective.’’ The Committee noted that the environment must be free from man-made or extracted substances, and metals that represent a threat to health and biological diversity. The Committee set defined targets that manufactured products be free from hazardous substances, including lead, by 2010 at the latest. However, the Committee favored measures prohibiting the use of hazardous chemicals in consumer products at the European Union level. Recourse to national requirements, however, is not excluded if ‘‘measures are not taken sufficiently quickly at the EU level.’’

IV.

ELECTRONIC WASTE MANAGEMENT: EUROPE

A.

WEEE Directive

Much of the electronic waste management in Europe is based on the European Union Commission’s WEEE Directive that aims to reduce environmental problems caused by end-of-life electrical and electronic equipment through its separate collection, sorting of hazardous components, and high degree of recovery and recycling. The WEEE Directive defines the main WEEE management and financing principles. This allows the Member States the flexibility to take national and regional conditions into account when implementing legislation. 1. Scope The WEEE Directive virtually covers all end-of-life electrical and electronic equipment including large and small household appliances, IT and telecommunication equipment, consumer equipment, lighting equipment, electrical and electronic tools, toys, medical equipment systems,

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monitoring and control instruments, and automatic dispensers such as cup dispensers for drinks. The WEEE Directive also covers all components (e.g., screens, keyboards, circuit boards, etc.), subassemblies (i.e., parts of the equipment without which the original piece could not operate as intended by the manufacturer), and short-term disposable parts, (i.e., consumables, such as toner cartridges, that are part of the product at the time of discarding). 2. Responsibility In its WEEE Directive proposal, the European Commission embraces the ‘‘producer responsibility’’ principle by making producers financially responsible for the take-back, recovery, and environmentally sound disposal of WEEE. The idea behind this principle is that making producers responsible for the environmental impact of end-of-life products greatly encourages them to undertake eco-design that facilitates their recycling and environmentally sound disposal discussed in Section 2. The proposal does not require Member States to make producers responsible for the collection of WEEE from households. ‘‘Producer’’ means any entity that manufacturers and sells electrical and electronic equipment under their own brand, resells equipment produced by other suppliers under their own brand, or imports that on a professional basis, such as providers of electronic equipment under finance agreements (e.g., lease). 3. Collection The proposal contains only general requirements for national WEEE collection schemes, and Member States are free to allow producers to set up and operate individual and/or collective takeback systems. a. Requirements. First, Member States are obliged to establish systems which provide sufficient and appropriately located collection points where private households can return WEEE separately from other waste streams without charge, so that WEEE are not disposed together with household waste. Member States are free to leave the responsibility for the collection of WEEE from private households to municipalities, or pass the responsibility to producers. A Member State can also adopt specific measures to encourage consumers to participate in the collection system, including deposit schemes. Second, any collection system must involve distributors under the so-called ‘‘old for new’’ rule. That is, when consumers buy new electrical or electronic equipment they can turn in or retire similar end-of-life equipment to the supplier without charge. Although the proposal does not clarify which equipment is considered similar within the scope of the Directive, it does state that distributors must only take-back equipment that is contaminant. Producers are also responsible for the collection of WEEE from nonprivate households, the proposed Directive makes producers responsible for collection. Lastly, note that on June 7, 2000, the Council agreed that collection systems are to be set up 30 months after the WEEE Directive goes into effect. However, Member States may, for a period not exceeding 5 years from the Directive’s entry into effect, set up or facilitate take-back systems. b. Targets. The target of separately collected WEEE is 4 kg on average per inhabitant per year of WEEE from private households. Member States shall ensure that by 31 December 2006 the target is achieved. 4. Recovery A key objective of the proposal is to increase the recovery of WEEE, and thus reduce disposal. Binding category-specific recovery targets have been set for all separately collected WEEE. Recycling targets that Member States must achieve within 46 months of the Directive’s entry into effect have been defined. Because of their national circumstances, Greece and Ireland have a 24month extension to this deadline. Recycling operations consist of WEEE reprocessing, including the reuse of components (not reuse of entire equipment), with the exclusion of energy recovery. The purpose of clear, aggressive recycling targets is to reduce the recovery of WEEE through incineration for energy use reduction. Recovery targets, which will be revised for the years following 2006, appear in Table 4.

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TABLE 4 Recovery, Reuse, and Recycling Targets by December 31, 2006 based on WEEE Directive Draft (January 27, 2003)

Product .

.

.

.

1 and 10 of Annex 1A –Large Household Appliances –Automatic Dispensers 3 and 4 of Annex 1A –IT, Telecommunications Equipment –Consumer Equipment 2, 5, 6, 7, and 9 of Annex 1A –Small Household Appliances –Lighting equipment –Electrical and Electronic Tools (exception: large-scale stationary industrial tools) –Toys, Leisure, and Sports Equipment –Monitoring and Control Equipment Gas Discharge Lamps

5.

Minimum recovery based on average weight per appliance (%)

Minimum reuse/recycling based on average weight per appliance (%)

80

75

75

65

70

50

–

80 (by weight of lamps)

Financing a. Producer Responsibility. A distinction is made between WEEE from private households and from holders other than private households such as businesses. Similarly, a distinction is also drawn between ‘‘new’’ WEEE, which originates from products placed on the market after the Directive goes into effect, and ‘‘historical waste’’ that originates from products previously placed on the market. For ‘‘new’’ WEEE from private households, Member States will have to ensure that producers bear financial responsibility for its collection, treatment, recovery, and environmentally sound disposal from designated collection points. The Commission believes consumers should not be charged with recovery and disposal costs because it discourages returning WEEE to collection points. It is thought that shifting responsibility from municipalities to producers will serve as an incentive for producers to improve the eco-design of their products. b. Extent and Rationale. Originally, there was a question whether producers should only be financially responsible for WEEE originating from products placed on the market under their own brand (i.e., individual responsibility) or for all WEEE collectively (i.e., collective responsibility), and leaves this choice to the Member States. In addition, the proposal leaves producers the choice to fulfill their financial obligations by means of either individual or collective systems. These issues have been clarified, and individual financing is preferred to collective systems unless unfeasible or too costly. The underlying reason is that individual responsibility encourages producers to improve the eco-design of their products. The situation is, of course, different for ‘‘historical waste’’ because producers cannot influence the eco-design of products already on the market when the Directive goes into effect. The directive also takes ‘‘orphaned’’ equipment into account, i.e., equipment whose original manufacturer or importer no longer exists when the product becomes waste. The proposal imposes a shared responsibility for historical waste according to the market share producers have at the time the collection costs arise. A mechanism for dividing these shared costs must be defined. c. Other Than Private Households. As regards WEEE from holders other than private households, a financing agreement is required between the producer and user at the time of purchase. However, the proposal does not include provisions relating to financing of WEEE

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collection from private households, or its transport to collection points. Accordingly, Member States are also free to make producers responsible for the financing of WEEE collection from private households. It is proposed that financing obligations go into effect 30 months after the WEEE Directive goes into effect, although small independent manufacturers (those manufacturing equipment under their own brand with fewer than 10 employees and a turnover of less than 2 million Euros) may benefit from a transitional period in this regard. 6. Information Requirements The participation of consumers in the collection system is considered fundamental for collecting WEEE separately from other waste streams. Member States must ensure that consumers are informed about return and collection systems available to them, and must take the necessary measures to encourage their participation in the system. Producers will be required to mark electrical and electronic equipment normally disposed of in rubbish bins, to avoid its disposal via municipal waste collections. 7. Selective Treatment Affecting Lead-Containing Electronics Separately collected WEEE must be transferred from collection points to authorized facilities, where it is submitted to treatment operations, with the aim of achieving the highest conversion rate for reuse and recycling. Producers must provide state-of-the-art recovery and recycling systems individually or collectively to guarantee the environmentally sound treatment of WEEE. Minimum standards have been set for treatment operations that must be met prior to any disposal or recovery operation. These include removal from collected WEEE all fluids and certain components (listed in Annex II to the Directive), including PCB-containing capacitors, printed circuit boards, cathode ray tubes, and toner cartridges, and minimum treatment requirements for cathode ray tubes, equipment containing CFC, HCFC, or HFCs, and gas discharge lamps. Table 5 summarizes the selected treatments required for specific materials appearing in WEEE that have been designated for selected treatment. With specific attention to electronic equipment, this implies that (a) printed circuit boards and cathode ray tubes must be removed from any WEEE destined for landfill, incineration, or recovery, and (b) the fluorescent coating must be removed from cathode ray tubes. It is further required that proper consideration be given to meet the requirements for storage and treatment sites of WEEE. Treatment operations may take place outside the Member State

TABLE 5 Substances and Components that must be Removed from Separately Collected Electrical and Electronic Equipment Waste Substances and components requiring selected treatment PCB containing capacitors Mercury-containing components, such as switches Batteries Printed circuit boards Toner cartridges, liquid, and paste, as well as color toner Plastic containing brominated flame retardants Asbestos waste Cathode ray tubes CFC, HCFC, or HFCs Gas discharge lamps Liquid crystal displays –surface area >10 cm2 –those back-lighted with gas discharge lamps

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where the WEEE is generated, and even outside the European Union, provided that this is in compliance with Regulation 259/93 concerning the shipment of waste [25].

B.

National Legislation

In addition to the WEEE proposals at the EU level, there has been considerable legislative activity concerning end-of-life electronics at the national level. The Netherlands, Sweden, Denmark, and the Belgian Flemish region have enacted legislation on the management of WEEE. Furthermore, Austria, the Belgian regions of Walloon and Brussels Capital, Germany, Finland, Italy, France, and Spain have introduced draft legislation or voluntary agreements for end-of-life electronic equipment. Below is an overview of European national drafted or adopted measures in the EU and non-EU Member States. Some key aspects of these programs are listed in Table 6. 1. Austria In Austria, take-back and recovery obligations have been in place for end-of-life refrigeration equipment since 1992 [26]. To ensure recycling and environmentally sound treatment of end-oflife refrigeration equipment, the Austrian legislation requires manufacturers and importers to levy a deposit and take-back fee on equipment sold. As an alternative, manufacturers and importers may be exempted from collecting this sum from consumers by setting up nationwide disposal systems ensuring proper collection and treatment of discarded equipment. In practice, all manufacturers and importers to the Austrian market participate in one of five existing collective schemes. The electronics industry is assessing a voluntary agreement on the collection, disposal, and recovery of electronic waste with the Austrian Environment Ministry. The provisions of the voluntary agreement are to serve as a basis for future national legislation implementing the WEEE Directive. This agreement extends existing manufacturers’ take-back obligations to large and small household appliances and office equipment. Industry and the government would commit to establish collection facilities in each of Austria’s nine states (Bundesla¨nder). The takeback system proposed by industry is likely to be a collective system, with financing solutions tailored to each product category. Two major pilot projects on collection of household WEEE were also carried out in Austria, namely the ‘‘Bregenz Project’’ and ‘‘Weiz Project.’’ Both projects were jointly funded by the National Environment Minister, local government, and industry. These projects provided for take-back of WEEE free of charge except refrigerators, televisions, and visual display units and collection from households through municipalities, public drop-off sites, or retailers, on a voluntary basis. As for the treatment of WEEE, the 1998 Austrian Federal Waste Management Plan set minimum requirements for the environmentally sound disposal of refrigerators. In addition, in 1998 the Austrian Normalisation Institute published two nonbinding technical standards containing guidance for the collection and treatment WEEE, and for the dismantling and processing of discarded electronics, including separation of ‘‘dangerous components.’’ 2. Belgium In 1998, the Flemish region of Belgium adopted a Regulation on Waste Management and Prevention (VLAREA.) This Regulation imposes take-back obligations for ‘‘white goods’’ (such as refrigerators, microwaves, etc.) and ‘‘brown’’ goods (such as televisions). Since July 1999, producers, importers, distributors, and retailers have been required to take back WEEE, free of charge, when consumers purchase new similar products (‘‘old for new’’ rule.) These take-back obligations will be extended to cover all end-of-life products commencing July 2004. On February 19, 2001, 15 associations representing producers, importers, and retailers concluded an environmental policy agreement with the Flemish Region. Similar agreements have been concluded with the Walloon Region and Brussels Capital Region. Under the agreement, collection of WEEE from households may take place either under the ‘‘old for new’’ rule or through municipalities. As regards further disposal of WEEE, associations of manufacturers and importers will, in the future, need to set up ‘‘Management Authorities’’ for further collection and treatment of WEEE.

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TABLE 6 Some Key Aspects and Features of End-of-Life Product Programs of Various Countries Country

Comments

Austria

Take-back and recovery obligations since 1992 of refrigeration equipment Assessing voluntary plan for disposal and recovery of electronic waste Producers, importers, distributors, and retailers required to take back WEEE free of charge since July 1999 All end-of-life covered starting July 2004 Adopted an end-of-life management policy including IT equipment making municipalities responsible Take back obligation not placed on producers or importers Proposal under consideration close to EU WEEE Directive Plan to go into effect with EU WEEE Directive, placing collection responsibility on municipalities ‘‘Old for new’’ take-back required of distributors Electronics manufacturers associations working on voluntary schemes to manage WEEE Schemes likely to provide recycling of WEEE paid by producers A regulation regarding WEEE in effect since July 2001 places collection responsibilities on municipalities and manufacturers The same is true of disposing WEEE Minimum treatment standards have been defined Enacted an ordinance in 1998 governing the take-back and disposal of WEEE Retailers and importers must take back products they normally stock or import respectively No government mandated schemes for WEEE take-back and recycling pending the adoption of the WEEE Directive Electronic industry association works to promote responsible management of WEEE at end-of-life Producers, importers required to set up management authorities for collection/recycling of WEEE 1998 regulation disposal of WEEE Manufacturers and importers are responsible for the recycling of WEEE collected by municipalities A system is under consideration A decree went into effect April 1998 dividing the responsibility for the collection of various products including IT equipment between suppliers and municipalities Requirements for the treatment of WEEE are also defined A 1998 ordinance obligates distributors to take back virtually all electronic equipment Manufacturers and importers must arrange for the free collecting of WEEE WEEE must be sorted and sent to collection points or treatment centers

Belgium

Denmark

Finland France

Spain

Sweden

Switzerland

United Kingdom

Germany

Italy Netherlands

Norway

The activities of these authorities will be financed by contributions from their members for electric and electronic equipment placed on the market. It will be required that WEEE be pretreated by authorized companies by removing hazardous materials (e.g., PCB, batteries, mercury, etc.). Nonreusable WEEE must be separately dismantled and treated. 3. Denmark In 1998, Denmark adopted legislation on the management of the following categories of end-oflife equipment: audio and video recorders, IT equipment, radio and telecommunications equip-

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ment, control and monitoring equipment, equipment for medicinal use, laboratory equipment, capacitors, and transformers [27]. In contrast with the proposed WEEE Directive, Danish legislation does not impose take-back obligations on producers and importers. Overall responsibility for the management of WEEE in Denmark lies with municipalities. Each municipality is requested to draw up detailed regulations on how collection and further handling of WEEE is to be dealt with locally. Producers, importers, and distributors wishing to take-back and recycle their end-of-life products may do so at their expense upon authorization from the municipality. An application for authorization must include a comprehensive description of the type of waste taken back and how it will be managed. Once an authorization is granted, information on handling of WEEE must be provided. As regards the ‘‘old for new’’ rule, distributors are not obliged to accept similar old products from customers when selling new products, but may do so if they wish. The WEEE Danish legislation also prescribes pretreatment requirements for audio and video equipment, computers, radio, and telecommunication equipment. In addition, components such as print circuit cards, mercury-containing contacts and relays, Ni–Cd batteries, mercury dry cell batteries, and flame retardant plastics, must be separated and recovered or disposed according to minimum requirements. 4. Finland The Finnish Environment Ministry drafted an order regulating the take-back of WEEE. This proposal made producers financially responsible for the separate collection of all WEEE, both from private households and other holders and for the further recovery and disposal of household WEEE. The draft proposal set a target of 8 kg per inhabitant for the separate collection of WEEE, and differentiated recycling targets for each category of WEEE. The Finnish authorities decided not to go forward with this Order but to resume discussions for a new proposal, which is expected to follow closely the requirements of the WEEE Directive proposed by the European Union. 5. France At the end of 2000, a number of trade associations representing electronics producers, and the Association of Mayors of France (Association des Maires de France) agreed to a nationwide plan for the collection and recycling of certain categories of WEEE, including computers, refrigerators, televisions, and mobile phones. Under the agreement, collection of WEEE from private households would be the responsibility of municipalities, while distributors are obliged to takeback WEEE under the ‘‘old for new’’ rule. Producers, importers, and distributors of electronic products would be required to set up management authorities (Socie´te´s de Gestion) for the collection and recycling of WEEE. Recovery of WEEE is to be carried out by companies authorized by the Environment Ministry and the Environment and Energy Conservation Agency (ADEME). This plan is expected to go into effect in parallel with the WEEE Directive. 6. Germany A 1994 Act governing the avoidance, reuse, and disposal of waste provides powers to impose a duty on manufacturers to take back end-of-life products [28]. An official proposal [Used Electrical Goods Ordinance (EAV)] to specifically regulate the disposal of IT equipment was circulated in 1998. This proposal was preceded, in 1995, by a policy document from the Federal Environment Minister containing a first draft regulation for end-of-life electronics. The scope of EAV was subsequently extended to include office and telecommunication equipment, voice communication equipment, large household devices, small devices, and equipment put in circulation before the ordinance goes into effect. Under the proposed ordinance, municipalities would be responsible for separate collection of WEEE from private households. Retailers would not be obliged to take back WEEE from consumers under the ‘‘old for new’’ rule, but could do so if they wish. The draft defines ‘‘retailer’’ as anyone who introduces electrical goods into circulation, including mail order businesses. As for further disposal of WEEE, manufacturers and importers would be responsible for taking back WEEE collected by municipalities from local public disposal units, retailers and commercial customers; and for ensuring its recycling. Responsibility would be shared among manufacturers. In practice, this means that public

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disposal units will pass on to manufacturers both WEEE of their own brand as well as a share of WEEE of a similar kind, but of other brands. 7. Italy The Italian Waste Management Decree of 1997 [29] contains specific provisions on the management of WEEE, which must be implemented with framework agreements, so-called ‘‘accordi di programma.’’ These agreements are concluded between the central government, and producer companies responsible for placing electronic products on the market, importers, and all operators involved in the collection and management of WEEE. Categories of priority products to be targeted include refrigerators, televisions, computers, washing machines, and air conditioners. As for collection of WEEE from households, the Decree gives consumers the option to return end-oflife equipment to a retailer under the ‘‘old for new’’ rule; or to return it either through the municipality system, or at collection points to be determined by the framework agreements. Similar agreements must be concluded for the treatment of collected WEEE to increase its reuse, recycling, and other forms of recovery. Associations representing electronics manufacturers were to negotiate framework agreements with the government. Under the Decree, a compulsory deposit system on household appliances may be introduced 3 years after the Decree goes into effect if ‘‘necessary for the protection of public health and the environment with regard to disposal of electronic waste [30].’’ Because the 3-year period expired in March 2000, and there was failure in defining the framework agreement, the Environment Ministry is considering the introduction of such a deposit system. 8.

The Netherlands

a. Scope. The ‘‘White and Brown Goods Decree,’’ adopted on April 21, 1998, was the Decree entered into effect in stages: June 1, 1998, for refrigerating and freezing equipment, washing and drying equipment, equipment for preparing food by heat, image receiving equipment, computers, paper printing equipment, and telecommunications equipment; and January 1, 1999, for the remaining categories of equipment. It establishes rules for taking back and processing the categories of end-of-life equipment, listed in Table 7. The Decree covers equipment intended for use by households, or similar use by companies (e.g., small refrigerators used in canteens.) The Decree also applies to three categories of equipment normally used in a commercial environment, namely computers, paper printing, and telecommunications equipment. In contrast to the proposed WEEE Directive, the Decree does not apply to medical equipment. The categories covered by the decree are listed in Table 8. b. Duties/Responsibilities. Under the decree, collection of WEEE is the shared responsibility of suppliers under the ‘‘old for new’’ rule and local authorities. When a consumer or company purchasing a new product returns a ‘‘similar’’ old one, the supplier has three options. The Explanatory Memorandum to the Decree specifies that a ‘‘similar’’ product is one that is intended for the same purpose; and corresponds in nature and features with the new product, such as an old refrigerator for a new one, irrespective of the size. Option 1: The product taken back may be sold to a third party, e.g., a retailer of used goods or a recycling company. Refrigerators and freezers containing CFCs or HCFCs may not be sold,

TABLE 7 Required Treatment of Some Components Removed from Separately Collected Electrical and Electronic Waste Components Cathode ray tubes Equipment containing CFCs, HCFCs, or HFCs present in foam Gas discharge lamps

Treatment/action required Fluorescent coating must be removed Must be properly extracted and destroyed Mercury (Hg) must be removed

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TABLE 8 Categories for Product TakeBack at End-of-Life Established by the White and Brown Goods Decree Product categories Refrigerating and freezing equipment Heating equipment Hot-water equipment Washing and drying equipment Equipment for preparing food by heat Sound equipment Image receiving equipment Computers Paper printing equipment Telecommunications equipment Electric and electronic charging equipment Electric and electronic kitchen appliances Electric and electronic tools Electric and electronic domestic appliances

and must be processed by manufacturers and importers in a way that the emission of CFC and HCFC is minimized. Option 2: Alternatively, the supplier may pass the product to the municipality if it originated from a private household. If the municipality can pass the product concerned to the manufacturer or importer, this means that the manufacturer or importer in question still exists at that time, or manufacturers and importers set up joint collection schemes under which all products are taken back. Option 3: As a third option, the supplier may pass the product to its manufacturer or importer. Municipalities are responsible for separate collection of WEEE from private households that have not been returned under the ‘‘old for new’’ rule, and must also establish facilities where suppliers can deliver products taken back from households. In addition, municipalities have the duty to accept irreparable equipment that consumers have left with repair companies, if the latter require so. Repair companies have the same options as suppliers, i.e., sell the product to a third party, return it to the manufacturer, or importer, or to the municipality. In contrast with the proposed WEEE Directive, the Decree does not fix a target for separately collected WEEE. Further disposal of WEEE is the responsibility of manufacturers and importers that have the duty to take back and dispose end-of-life products that they placed on the market in the Netherlands. Also, municipalities and repair companies must return to the manufacturer any equipment bearing its brand, i.e., individual responsibility for brand-related products. As for historical waste, the Decree states that manufacturers have collective responsibility, shared with municipalities. An ‘‘old for new’’ scheme also applies to manufacturers, importers, and suppliers until January 2005. In practice, manufacturers or importers, when delivering a new product to a supplier are required to take-back a similar product, irrespective of its brand. Thus, suppliers will not have to cover disposal costs for ‘‘orphaned’’ equipment taken back from end users. Municipalities are responsible for the disposal of historical waste which they collected. Manufacturers and importers must be authorized by the Environment Ministry as to how they will perform their take-back obligations in practice. The Decree allows manufacturers and importers to fulfill their obligations jointly by (1) setting up a joint disposal scheme via a joint notification subject to ministerial approval) or (2) concluding a covenant with the Environment Minister. The Explanatory Memorandum to the Decree specifies that if a covenant contains agreements that conflict with national and European competition rules, these covenants will not

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be approved. Signatories to a covenant are exempted from the duties imposed by the Decree. At the time of writing, two covenants have been approved: ICT-NL for office and telecommunication equipment, and NVPM for ‘‘white and brown’’ goods. The latter is financed, although a levy is charged to consumers at the point of sale. c. Other Aspects. The White and Brown Goods Decree also contains minimum requirements for the treatment of WEEE. The incineration of separately collected WEEE in plants used for the incineration of waste is prohibited, according to in the Dutch Establishments and Permits Decree, Environmental Management Act. In parallel, a landfill ban has come into effect for WEEE under separate legislation (Besluit stortverbod afvalstoffen). The Decree also prohibits the stocking for commercial purposes end-of-life refrigerators and freezers containing CFCs and HCFCs. 9. Norway In Norway, which is not an EU Member State, the 1998 Ordinance imposes take-back obligations for virtually all end-of-life electronic equipment [31]. The Ordinance covers only equipment discarded after it went into effect, even if originated from equipment that was placed on the market before the Ordinance went into effect. a. Collection. Under the Ordinance, distributors are required to take back WEEE from consumers, free of charge, under the ‘‘old for new’’ rule when a purchase of similar equipment is made. It also requires that the WEEE is sorted, properly stored, and sent to collection points or treatment facilities. In parallel, municipalities must also ensure adequate facilities for the reception of household WEEE, and for the proper sorting, storage, and transportation of WEEE to collection points or treatment facilities. In addition, municipalities are obliged to receive business WEEE, but they may charge for this service. Manufacturers and importers must arrange for the collection, free of charge, of WEEE from distributors and municipalities; and of WEEE taken to designated collection points by companies that collect WEEE on a professional basis. Take-back obligations are in proportion to an entity’s sales market share, but limited to equipment of the type that a manufacturer or importer sells, even if of another brand manufacturer. In addition, manufacturers and importers must state, in their sales information material, that WEEE may be returned to distributors, and that equipment sold are covered by a return and recycling scheme. Trade associations agreed to fund take-back and recycling systems via a visible charge on the retail price of affected products. b. Disposition. As for treatment, manufacturers and importers must ensure that hazardous materials and components within the meaning of the Norwegian Hazardous Waste Regulations [32], are dismantled, sorted, and subjected to proper treatment in accordance with the Regulations. These components include circuit boards, fluorescent lighting tubes, televisions, and computer screens. Other materials and components must be reused, recycled, and recovered if justified after considering environmental, resource, and economic factors. This means that recycling costs should not exceed socioeconomic costs, including environmental costs. 10. Spain Similar to Austria, Spanish associations representing manufacturers of small household appliances, white goods, and IT and telecommunications equipment are currently working on voluntary schemes for the management of WEEE. These would be the basis for future national legislation implementing the WEEE Directive. The association take-back schemes are likely to provide for the recycling of WEEE through the public waste-management system, with additional costs to be paid by producers. The associations will conduct take-back studies on the issues, including the volume of WEEE produced, its transport, financing options, collection, disposal, and recycling management. 11. Sweden A Regulation on Manufacturer’s Responsibility for Electrical and Electronic Products went into effect on July 1, 2001. The Regulation notes a ‘‘manufacturer’’ to be any producer, importer, and

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dealer of electrical and electronic equipment in Sweden. The Regulation defines ‘‘electrical or electronic products’’ along the same lines as the proposed WEEE Directive. The equipment categories covered by the Regulation are listed in Table 9. White goods such as refrigerators and freezers are not covered by the Regulation because municipalities are responsible for their management at end-of-life. a. Collection. Responsibility for the collection of WEEE from households is shared between municipalities and manufacturers. The latter must accept, free of charge, WEEE returned by consumers when buying a similar product, under the ‘‘old-for-new’’ rule. A product is considered ‘‘similar’’ when ‘‘in normal use performs essentially the same function as the product sold.’’ If the product is secondhand, the take-back obligations apply only if a manufacturer in Sweden has not previously sold the product. Suitable collection places to accept WEEE from consumers must also be provided. Municipalities must agree with the collection places to coordinate with the municipality’s own obligations relative to refuse collection. b. Disposition. Responsibility is also shared between manufacturers and municipalities for the further disposal of WEEE. Manufacturers must ensure that the WEEE previously accepted back is reused, recycled, or dealt with in an environmentally sound way. In other cases, municipalities are responsible for the disposal of WEEE they collected. Manufacturers are required to provide information to the municipality, the National Environmental Protection Agency (EPA), treatment facilities and consumers, as is necessary and appropriate. In addition, Swedish legislation sets minimum requirements for the treatment of WEEE. The Swedish Ordinance on waste collection and disposal in effect since January 1, 1999 (section 25 of the waste ordinance) contains a provision forbidding the landfill, incineration, and shredding of electronic waste unless it is treated by a certified pretreatment facility. The Regulations, which went into effect July 2001, lay down minimum requirements for treatment facilities, including separate handling of CRT, LCDs, and plastic containing PBB or PBDE. 12. Switzerland Switzerland, which is not an EU Member State, enacted a 1998 Ordinance on the Return, Takeback, and Disposal of Electrical and Electronic Appliances that covers the following product categories of electronics: (1) leisure electronics, (2) information technology appliances, (3) household appliances, and (4) ballast from lamps containing PCBs. Under the Ordinance, anyone who disposes electronic equipment must return it to a retailer, manufacturer, importer, or disposal facility. Return via public collection is also permitted. Retailers must accept back from consumers WEEE of the kind they normally stock, under the ‘‘old for new’’ rule. Importers and

TABLE 9 Equipment Categories Covered by the Regulation of Manufacturer’s Responsibility for Electrical and Electronic Products Product categories Household appliances Tools and garden equipment IT and office equipment Telecommunication equipment Video, audio, and television equipment Cameras and photo equipment Clocks and watches Games and toys Medical equipment Laboratory equipment Lightning equipment

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manufacturers must take back WEEE of the kind they manufacture or import. The Ordinance prescribes that WEEE disposal be carried out according to state-of-art technology. In particular, cathode ray tubes and metal compounds must be recycled ‘‘in so far as this is commercially acceptable.’’ The precise meaning of this caveat is not further explained in the Ordinance. The Swiss Environment Agency completed its first review on the functioning of the Ordinance in July 2000, reporting that it had led to a significant increase in recycling, and to employment growth in companies dealing with WEEE. The Agency identified two main failings. First, industries producing small kitchen equipment, games, and television sets had not agreed to a general system for financing the collection and recycling of WEEE. Second, consumers knew little about take-back opportunities for old equipment, with the consequence that many small electrical and electronic appliances were still being disposed of as household waste. 13. United Kingdom The British Environment Act of 1995 contains ‘‘enabling’’ clauses allowing the government to adopt legislation implementing the principle of ‘‘producer responsibility’’ for waste streams including electronics. There are, at the time of writing, no plans for the adoption of national WEEE take-back and recycling schemes, pending the adoption of the WEEE Directive. Nonetheless, a report on the status of WEEE in the UK was issued by the Electronic and Electrical Equipment Recycling (ICER). ICER was founded in 1992 by the British electronic industry and has over 50 members from the UK and other European countries. It works to promote ‘‘responsible management of WEEE at the end of its life.’’ The ICER report noted that, of 915,000 tons of WEEE entering into the waste stream in 1998, about half (448,000 tons) went to recyclers. Large household appliances represented 77% of the total amount, and IT/office equipment 21%. In 1997, ICER published guidelines on designing for recycling electronic and electrical equipment, outlining ways to increase the recovery of electronics and providing design concepts for dismantling electronic equipment.

V.

LEAD USE PROVISIONS: UNITED STATES

At the federal level, no specific legislation exists, or, is pending, in the U.S. Congress, to restrict the use of lead in electronics. In 1991, the U.S. Senate presented the ‘‘Lead Exposure Reduction Act’’ [33] that would have had significant repercussions on the use of lead in electronics. However, this Act was never finalized. At the state level, there is also no specific legislation requiring the phaseout of lead from electronic equipment. However, a number of states are considering initiatives on waste management to encourage recycling end-of-life consumer electronic products. These initiatives are examined later in this chapter.

A.

The Reid Bill

The Lead Exposure Reduction Act, known as the Reid Bill, aimed at amending the Toxic Substance Control Act by prohibiting the manufacture, import, processing, or distribution in commerce of a number of product categories containing more than a specified percentage of lead. With specific regard to the use of lead in solders, the Bill empowered the Administrator to adopt regulations (a) banning the manufacture, import, processing, sale, and distribution in commerce of lead solders commonly used in plumbing systems, and (b) restricting the sale and display of lead solders not commonly used in such systems. Although the use of lead solders by the electronics industry was not expressly mentioned in the Bill, this latter provision could have provided the basis for the adoption of regulations restricting the use of lead solders in electronics. However, because of the limited environmental exposure associated with the use of lead in electronics, the following bills focused on restriction on the use of lead solder in plumbing, which is the sole restricted application of lead in solders in the United States. In addition, the Reid Bill contained other provisions that could have potentially affected electronics. Manufacturers, processors, or importers of products containing more than 1% lead by dry weight would have been required to notify the government as to the type of products

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produced or imported, their quantity, etc. The government would have devised a list of products containing more than 1% lead by weight, and also products for which any manufacturer or processor used more than 100 lb of lead per year to manufacture a specific product. The maximum permissible lead concentration would have been specified as well. Manufacturers, processors, and importers of products not yet listed, but who intended to manufacture, import, or process products containing more than 1% lead by dry weight would be required to notify the government prior to doing so. Finally, labels would have been required for products to which lead was intentionally introduced during manufacturing, processing, or distribution, or in which the incidental presence of lead exceeded 0.01% by weight.

B.

New Reporting Thresholds for Lead and Lead Compounds

On January 8, 2001, the U.S. EPA issued a final rule under Section 313 of the Emergency Plan and Community Right to Know Act of 1986 (EPCRA) and Section 6607 of the Pollution Prevention Act of 1990 (PPA) lowering thresholds for lead and lead compounds that are subject to reporting under the Toxic Release Inventory (TRI) Program. This rule is part of the Agency’s effort to educate the public about toxic chemicals, especially PBT chemicals. Potentially affected categories may include facilities that manufacture electronic components and accessories. 1. Toxic Release Inventory (TRI) The TRI Program requires certain facilities, manufacturing, processing, or otherwise, using certain toxic chemicals in amounts above fixed thresholds to report specific information about these chemicals to the state and federal government by July 1st of each year in reference to the previous calendar year, as noted in Table 10. The affected facilities must also report recycling and other waste-management data and source reduction activities for these chemicals [34]. Collected data on toxic emissions is then made available to the public through various sources, including the Internet. Under Section 313 of EPCRA, the U.S. Congress set initial parameters of TRI, but also gave EPA the authority to modify reporting in various ways, including amending the list of toxic chemicals subject to reporting, facilities required to report, and thresholds for reporting. Accordingly, the EPA has undertaken a number of actions in this respect, including expanding the number of reportable toxic chemicals in 1994, adding new categories to the list of covered facilities in 1997; and in October 1999, the EPA enhanced reporting requirements for a group of PBT chemicals. The Agency also set criteria to be used for evaluating whether a chemical listed under EPCRA, Section 313, persists or bioaccumulates in the environment. On the basis of these criteria, the EPA concluded that lead and lead compounds are highly persistent and highly bioaccumulative. Because PBT chemicals may contain lead, lead and lead compounds have the potential to expose humans and the environment over a longer period of time. Thus, the nature of

TABLE 10

Several Aspects Surrounding the Toxic Release Inventory (TRI) Program

Key aspects Nature of Pb and Pb compounds

Lower reporting levels [Toxic Release Inventory (TRI)]

Reporting date Exemptions

Comments .

Persist and bioaccumulate in the environment –potential to expose humans for s longer period and provides for increased concentration levels . Previously not required to report Pb or Pb-compound emissions to air, water, unless: –manufactured or processed >25,000 lb annually, or –used >100,000 lb annually . As of January 2001, reporting threshold lowered to 100 lb for each facility emitting Pb or Pb compounds July 1, relative to previous calendar year . Studies to consider use of Pb in certain alloys: e.g., some stainless steels, brasses, and bronzes

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PBT chemicals indicates that even small quantities of lead and lead compounds are of concern, and that low reporting thresholds under EPCRA, Section 313, may be appropriate. Accordingly, the January 2001 rule complements the October 1999 PBT final rule by lowering reporting thresholds for toxic chemicals in general for lead and lead compounds. 2. Lower Reporting Thresholds Under the original EPCRA, Section 313 reporting thresholds, covered facilities were not required to report lead and lead compounds emissions to air, water, and land unless they manufactured or processed more than 25,000 lb annually or used more than 100,000 lb annually. However, the January 2001 rule dramatically lowered the reporting threshold to 100 lb or more annually for each facility emitting lead and lead compounds. The new TRI reporting requirements went into effect starting 2001. 3. PBT-based Modifications In addition, the new rule includes the following modifications to reporting under EPCRA, Section 313 for lead and lead compounds, based on their status as PBT chemicals: (1) eliminating the de minimis exemption for lead and lead compounds. This exemption was introduced in 1998 for facilities newly covered by EPCRA, Section 313, regarding low concentrations of toxic chemicals in mixtures that are imported, processed, or otherwise manufactured as impurities; (2) excluding lead and lead compounds from the eligibility for the alternate threshold of 1 million lb and Form A; (3) eliminating range reporting, as it could misrepresent data accuracy; (4) reporting of all releases or other waste management quantities greater than 1/10 of a pound of lead and lead compounds. At present, the EPA allows facilities to report whole numbers and to round releases of 0.5 lb or less to zero when reporting on EPCRA, Section 313, listed chemicals; (5) exempting reporting obligations for lead in certain alloys: a final decision on lower reporting thresholds for lead contained in stainless steel, brass, bronze alloys will be deferred until completion of ongoing scientific studies.

VI.

ELECTRONIC WASTE MANAGEMENT: UNITED STATES

A.

Federal Standards for Lead in Landfills

The Resource Conservation and Recovery Act (RCRA) [35] of 1976, as amended in 1984, gave the U.S. EPA the authority to regulate hazardous waste at all stages, including its generation, transportation, treatment, storage, and disposal. To this end, EPA issued Regulations which set forth criteria for identifying the characteristics of hazardous waste, its listing, and requirements to which it must comply with. Waste can be classified as hazardous either because it is listed as hazardous by EPA Regulations, or because it exhibits certain characteristics. The ‘‘toxicity’’ characteristic applies to a list of 40 substances, including lead. If a waste leachate derived from a test called the Toxicity Characteristic Leaching Procedure (TCLP) contains any listed substances at or above the defined regulatory levels, the waste is considered hazardous. Chapter 2 provides a more complete discussion of the TCLP and other tests utilized to determine toxicity levels. Hazardous waste must be treated in accordance with EPA standards before being placed in a landfill. Discarded printed wiring board and cathode ray tubes usually qualify as hazardous because they commonly exhibit the toxicity characteristic for lead. That is, the test extract contains more than 5.0 mg Pb/L.

B.

State Recycling Programs

At the state level, the management of end-of-life electronics, especially computers and cathode ray tubes, is an emerging environmental issue on which a number of initiatives have been taken.

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These states, at the time of this writing, include California, Connecticut, Florida, Massachusetts, Minnesota, New Jersey, New York, South Carolina, and Wisconsin. In general, state programs tend to promote the recycling of end-of-life electronics, and to reduce disposal by landfill or incineration. Household and commercial users are encouraged to bring end-of-life electronic equipment back to designated collection points, free of charge. Recycling facilities determine its further destination (e.g., whether reused, upgraded, donated to nonprofit organizations or schools, recycled, etc.) A brief overview of state-level activities on WEEE recycling in the United States is discussed in the following sections. 1. California A pilot project was initiated in San Francisco to collect discarded computers. Residents were afforded the opportunity to drop off end-of-life computers at a number of electronics stores and recycling facilities. The collected units were dispositioned for reuse, upgrading, donation to charities, or recycling. 2. Connecticut The Connecticut Department of Environmental Protection is encouraging a system of regional and municipal drop-off sites for recycling of electronics. The project is financed by individual towns. In addition, the agency is considering introducing a general permit for the collection of certain recyclable equipment, including consumer electronics. 3. Florida and Massachusetts Both Florida and Massachusetts focused on strategies aimed to encourage recycling of cathode ray tubes (CRTs). Although the shared goal is to encourage recycling and decrease the number of CRTs placed in landfills or incinerated, the regulatory framework chosen by the two states is different. The Florida Department of Environmental Protection (DEP) emphasizes that CTRs are hazardous waste under the Resource Conservation and Recovery Act (RCRA) when discarded into landfills or waste incinerators. Under the DEP’s interpretation of RCRA hazardous waste regulations [36], CTRs are considered as materials, rather than wastes, when reused as a substitute for commercial purposes, i.e., when utilized to manufacture new CRTs, or as a fluxing agent at a secondary lead smelter. Only CTRs disposed in landfills or incinerated are regulated as hazardous waste under RCRA. The state of Massachusetts has exempted unbroken CRTs from hazardous waste requirements under RCRA. Disposed of broken or crushed CTRs are subject to full hazardous waste requirements. However, intact CRTs that can still be reused and those which will be crushed and recycled (i.e., as spent material being reclaimed), will be treated as nonhazardous waste under solid waste regulations. In addition, the state of Massachusetts has enacted a rule that prohibits the disposal of CRTs in landfills and municipal waste-combustion facilities by both households and commercial users, as of April 1, 2000. If the screen is intact, a CTR can be safely recycled, but lead is released in the environment when the tube is crushed or recycled. The EPA supports the Massachusetts landfill ban for CTRs, which goes beyond federal rules. 4. Minnesota Minnesota’s Office of Environmental Assistance is developing proposed legislation for electronics product stewardship, and is collaborating with Sony Electronics, Panasonic, and the American Plastics Council to evaluate collection strategies and recycling market opportunities. 5. New Jersey The U.S. EPA has funded a grant for an educational pilot project coordinated by the New Jersey Solid Waste Policy Group at Rutgers University. Under this program, students produced a video addressed to children between ages 3 and 6 on electronics recycling.

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6. New York Rockland County launched a program to collect discarded electronics from both households and business. This program is funded through a grant from the New York Department of Environment. 7. South Carolina South Carolina’s legislature considered a Bill establishing a statewide electronic equipment recycling program. The program would be funded by a tax on the sale of electronics products containing cathode ray tubes. However, the Bill did not survive the 1999 session. 8. Wisconsin The Department of Natural Resources (DNR) has instituted an end-of-life computer collection program. As a consequence, three cities in Wisconsin have taken initiatives for the collection of discarded computers.

VII. A.

ELECTRONIC WASTE MANAGEMENT: JAPAN Recycle Focus

Although much of the initial activity surrounding the elimination of lead in processes and products was in Japan, there are no laws banning its use. The emphasis and governmental policies have been on recycling and reuse, with both government and private sector plants coming on board for those purposes. It is anticipated that the ‘‘Law for Recycling of Specified Kinds of Home Appliances,’’ enacted in 1998, will be expanded to include discarded PCs. Japan, being a relatively small country, must take the appropriate steps to efficiently utilize available real estate. Projections have indicated that the amount of end-of-life disposal exceeded available landfills, and would place a significant burden on available space in the future. Among the corrective measures put in place was an ambitious recycling and reuse strategy to significantly reduce the quantity of material to be placed in landfills. Japanese industry responded to this objective by putting voluntary product roadmaps in place to start reducing the use of lead in their products in the late 1990s. (See Chapter 1)

B.

Lead-Free Drivers

The migration toward a voluntary ban on lead was motivated by several factors: to reduce or eliminate costs associated with the treatment of end-of-life products that contained Pb to be in conformity with requirements to deposit materials into landfills. At the same time, Japanese companies were receiving the benefit of presenting a positive image of their organizations and products to consumers by taking a leadership role to eliminate lead, an issue that has been receiving worldwide attention and support. Japanese companies were among the first to market lead-reduced products, typically utilizing a lead-free solder to attach components whose peripheral leads (e.g., Cu, Alloy 42, etc.) were finished with a thin, plated, Pb-free coating to Pb-free features on a printed circuit board (PCB). The Japanese Pb-free approach was further encouraged and enabled through the Law on Promotion of Green Purchasing that went into effect April 2001.

C.

Key Program Elements

There are several key aspects of Japan’s environmental consciousness program that are briefly discussed in the following sections, particularly as it relates to the electronics industry. 1. PC Green Label A ‘‘PC Green Label’’ is intended to indicate that a PC manufacturer’s overall efforts and activities in introducing computer products are environmentally sound. PC manufacturers must disclose

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TABLE 11 Substance Group List of Prohibition-ranked Substances Level 1 [38] Legal limitsa

No.

Substance group name

1

PCB (substances that contain three or more chlorine atoms), PCT, and their alternatives

2

Asbestos

0.1 wt.%

3

Specific organic tin compounds

0.1 wt.%

4

Aliphatic halogenated hydrocarbons (Specific CFCs, halon, chlorine-based organic compounds) Specific bromine-based flame retardants Penlachlorophenols

5 6 7

Azo compounds, Aromatic amines

8

Polychlorinated naphthalene (Substances that contain three or more chlorine atoms) Fluoroacetic acids

9

11

Organic phosphorous compounds Hexachlorobenzene

12

Benzene

13 14

Yellow phosphorus 2,4,6-tri-tertbutylphenol

10

Applicable laws and regulations Law Concerning Examination and Regulation of Manufacture and Handling of Chemical Substances, EU Hazardous Substances Regulation, ChemG EU Hazardous Substances Regulation, ChemG, Labor Safety, and Hygiene Law EU Hazardous Substances Regulation, ChemG, Law Concerning Examination, and Regulation of Manufacture and Handling of Chemical Substances Ozone Layer Protection Law, Montreal Protocol, ChemG, EU Hazardous Substances Regulation German Dioxin Ordinance

0.01 wt.% 0.1 wt.%

0.1 wt.%

EU hazardous Substances Regulation, ChemG German Consumer Goods Ordinance, ChemG, EU Hazardous Substances Regulation, Law Concerning Examination, and Regulation of Manufacture and Handling of Chemical Substances, Labor Safety, and Hygiene Law Law Concerning Examination and Regulation of Manufacture and Handling of Chemical Substances Poisonous and Deleterious Substances Control Law Poisonous and Deleterious Substances Control Law Law Concerning Examination and Regulation of Manufacture and Handling of Chemical Substances EU Hazardous Substances Regulation ChemG, Labor Safety, and Hygiene Law Labor Safety and Hygiene Law Law Concerning Examination and Regulation of Manufacture and Handling of Chemical Substances

Major use Insulation oil, heat medium

Heat insulation materials Coating materials, antiseptic agents

Cleaning agents, refrigerants, foaming agents

Resin-based flame retardants Insecticides, insect repellents Dyes

Antiseptic agents, insecticides, lubricants

Hygiene materials, agrichemicals Agrochemicals Insecticides, antifoulants

Solvents, cleaning agents Matches Antioxidants

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TABLE 11 Continued No.

Substance group name

15

4-Nitrodiphenyl and its salts Octamethyl pyrophosphoramide Bis-(chloromethyl) ether Aluminum phosphide

16 17 18

Legal limitsa

Labor Safety and Hygiene Law

100 ppm (packaging materials)c

Poisonous and Deleterious Substances Control Law Labor Safety and Hygiene Law Poisonous and Deleterious Substances Control Law EU Hazardous Substances Regulation, EU Directive, ChemG Chemical Substance Control Laws (Netherlands, Denmark), Packaging Material Heavy Metal Regulation ChemG, EU Hazardous Substances Regulation, EU Directive, Proposition 65, Packaging Materials Heavy Metal Regulation EU Directive, Packaging Material Heavy Metal Regulation

100 ppm (packaging materials)c

EU Directive, Packaging Material Heavy Metal Regulation

19

Cadmium and its compounds (limited to inclusion in resins, coating materials, ink, plating,b and packaging)

75 ppm, 100 ppm (packaging materials)c

20

Lead compounds (limited to inclusion in coating materials and packaging materials) Hexavalent chromium compounds (limited to inclusion in packaging materials) Mercury compounds (limited to inclusion in packaging materials)

100 ppm (packaging materials)c

21

22

Applicable laws and regulations

Major use Synthetic intermediates Insecticides, insect repellents Insecticides, insect repellents Resins, packaging materials, coating materials ink

Coating materials, packaging materials

Packaging materials

Packaging materials

a Legal limits show the maximum contents designated by the EU Hazardous Substances Regulation and others. b Use for production of air planes, electrical contacts and glass materials are excluded. c Regulation on packaging materials: Requires to reduce total weight of heavy materials (lead, cadmium, mercury and hexavalent chromium) contained in packaging materials to 100 ppm or below.

environmental information helpful to consumers in selecting (or using) a computer. For example, chemical or other materials that can adversely impact health and/or the environment are not to be intentionally added to their products during manufacturing or recycling processes. Plastic frame components weighing more than 25 g, such as the PC enclosure, display, keyboard, and mouse, are not to contain lead (Pb), cadmium (Cd), or specific flame retardants (PBBs, PBDEs). Also, Pb, Cd, chromium (Cr), and mercury (Hg) are not to be added to inks used in owner’s manuals and shipping containers. The types and use of eco-labels are discussed in Chapter 5. 2. Guidelines for Standardization of Material Declaration The Guidelines for Standardization of Materials was developed by the Japan Green Procurement Survey Standardization Initiative (JGPSSI) to provide a common set of requirements for companies that implement green procurement surveys. The guidelines aid in reducing the burden on suppliers inundated with these types of surveys, and improve the value and accuracy of the responses provided. The guidelines were drafted to address both the ‘‘Chemical Substance Survey’’ and ‘‘Survey Response Format.’’ Nonparticipating companies are free to utilize the guidelines that are published in both English and Japanese. If other languages are required, the surveying companies that require them must create them.

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3. Company Policies, Standards, and Guidelines Many Japanese companies have an internal standard that addresses environmentally hazardous substances. Among these are Sony, Pioneer, Matsushita, Canon, Mitsubishi, and NEC, to name a few. The standard serves as a policy guideline for dealing with environmentally hazardous substances in products and processes, defines a lead-free implementation timeline, and defines lead-free procurement goals for materials, parts, and products. In addition, the standard serves to clarify abolition/reduction plans as they relate to materials and substances being removed voluntarily, such as Pb and Pb-containing substances, and those that are prohibited or banned. The standard sets monitoring checkpoints to assure the implementation of company goals are on schedule. 4. Materials Clarification A materials classification system has been defined by many Japanese electronics manufacturers that usually consist of three categories [37,38]. a. Banned or Prohibited Substances. These are substances for which there are no conditions that relate to use, purpose, content, or phaseout date. These substances are strictly prohibited from any intentional use, and whose use must be immediately halted if found to be present in a substance, Also, these substances are typically subject to a so-called nonuse warranty that guarantees no intentional use of the banned substances. This category was created based on the domestic and foreign laws and regulations as typified by those substances listed by Matsushita Electric Group In Table 11. There is also a subcategory group of substances that some Japanese companies ban but for which there are extenuating circumstances. If their use in confirmed, an alternative technology must be explored so these materials can be discontinued at some point. If, however, there are no viable alternatives, a supplier is required to notify the customer and must obtain an exemption (i.e., off-spec) [38]. b. Reduced Use or Restricted Substances. Japanese companies have placed restrictions on certain substances, some of which are used in volume quantities, when manufacturing their products, if they have been identified as having potential harmful effects on humans or the environment. Each company sets their own targets for phasing out these restricted materials, or significantly reducing their utilization. c. Managed Substances. There is yet a final class of materials whose use is not banned nor restricted, but subject to being managed and accounted for by properly channeling these materials through recycling and prescribed end-of-life processing procedures and operations. 5. Green Procurement Standards Many Japanese companies have committed to procuring materials and assemblies that have a minimal impact on the environment, and thus have aggressively pursued green procurement objectives. These policies have received a high priority among Japanese electronics manufacturers because environmentally related issues have gained universal attention. It therefore behooves corporate entities to take a leadership role in issues involving social responsibility.

TABLE 12

List of Supplier Actions and Activities for Favorable Selection Consideration [39]

Action/Activity . . . . . .

Established an environmental management system (EMS) Corporate philosophy and policy in place focused on global environmental preservation Organizations dedicated to the pursuit of environmental conservation Established a system to evaluate and manage product environmental assessment Have established environmental preservation educational programs for employees Address streamlining physical distribution for resource and energy conservation, and reduced exhaust emissions

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However, green procurement goals cannot be achieved without the support, cooperation, and active participation of suppliers being full partners with manufacturer’s environmental programs. To foster supplier progress, manufacturers now include environmental considerations as an integral part of their supplier and materials selection criteria. a. Selection of Suppliers. Electronic manufacturers, like other manufacturers, traditionally select their suppliers on the basis of price, quality, on-time delivery, service, technical development capability, etc. Environmental aspects are now considered as well as part of the terms and conditions for doing business. Key among these conditions is the earliest possible implementation of environmental preservation activities, such as those listed in Table 12. Compliance with ISO 14001 is generally considered a precondition for selection. Additionally, suppliers are favored that aggressively pursue continuous improvement plans. Suppliers are expected to conduct periodic self-evaluations that measure their progress, and are expected to make these results available to manufacturers. b. Selection of Materials. In a manner similar to suppliers, Japanese manufacturers now include environmental impact considerations as part of their materials solution procedure. Part of the information suppliers provide to manufacturers includes the development of new materials whose environmental impact is less than those they replace [39]. D. Examples of the Impact The beneficial impact of converting the solder used on printed circuit board (PCB) assemblies is clear from data provided by several Japanese electronics manufacturers, who are among the first to convert their lines to lead (Pb)-free assembly. For example, Matsushita Electric (Panasonic) noted by the end of March 2003 that Pb-free solder was utilized in about 12,000 models manufactured in 22 domestic and 79 overseas facilities. Up to that point, Matsushita produced a total of about 70 million Pb-free solder PCB assemblies. In the prior year (2002), before completely switching over to lead-free, Matsushita utilized 2400 tons of solder for its products [40].

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

64 Fed. Reg. Directive 76/769 (on the restrictions on the marketing and use of certain substances and preparations, Annex I). Directive 85/210 on the approximation of the laws of the Member States concerning the lead content of petrol. Alterio, J.M. ‘‘Billion PCs Add Up To Just Too Much Garbage,’’ Toxic Trade News, URL: http:// www.ban.org/../ban_news/billion_pcs.html. National Recycling Coalition. ‘‘Electronics Recycling Initiative,’’ URL http://www.nrc-recycle.org/ resources/electrnics/managing.htm. Minnesota Office of Environmental Assistance. ‘‘Product Stewardship,’’ URL: http://www.moea. state.mm.us/stewardship/policy2001-ps.pdf. U.S. Environmental Protection Agency. ‘‘Electronics Take Back Policies,’’ URL: http://www.epa. gov/jtr/netshare/elect.htm. Explanatory Memorandum to the European Commission’s proposals, page 11, refers to a Study of the Nordic Council of Ministers of 1995. PRRDy, ‘‘Product Stewardship,’’ URL: http://www.prrdy.com/hous-productstew.htm. National Pollution Prevention Center. ‘‘Sustainable Product Systems: Life Cycle Tools and Applications,’’ URL: http://www.umich.edu/fnppcpub/shortcourse97.html. Hendrickson, C.; Conway-Schempf, N.; Lave, L.; McMichael, F. ‘‘Introduction to Green Design,’’ Green Design Initiative, (Carnegie Mellon Univ.), URL: http://www.gdi.ce.cmu.edu/GreenDesign/ gdedintro.html. ‘‘Putting Used Materials Back to Work - A Strategy for the Long Term,’’ URL: http://www.web.net/ fjjackson/usedmat.html. ‘‘An Introduction to Product Takeback,’’ URL: http://www.ce.berkley.edu/fhorvath/NATO_AWR/ FILES/klausner.pdf. Excerpted from, Environmental Manager: Environmental Solutions that Make Good Business Sense, ‘‘Extended Producer Responsibility;’’ John Wiley and Sons, Aug. 1998; 10(1) URL: http://www. informinc.org/eprenvman.php. Klausner, M.; Hendrickson, C. ‘‘Product Takeback Systems Design, Green Design Initiative, URL: http://www.ce.cmu/GreenDesign/research/takeback.pdf.

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Michele Raymond. ‘‘EU Moves on Electronics Mandates,’’ URL:http://www.wasteage.com. Wood, D.; Bender, M.; Murry, M. ‘‘Computer TakeBack Campaign,’’ GrassRoots Recycling Network, URL: http://www.grrn.org/e-scrap/e-scrap_release_03-19-02.html. 18. Northwest Product Stewardship Council, ‘‘Electronics Policies and Legislation,’’ URL: http:// www.productstewardship.net/PoliciesElectronics.html. 19. Jackson, D. ‘‘Industries Agree on Computer Recycling,’’ Waste Age, May 1, 2002, URL: http:// www.wasteage.com/magazinearticle.asp?magazinearticleid=148084&magazineid=12&siteid= 27&releaseid=10166moc. 20. COM (2000) 347 final. 21. COM (2001) 316 final. 22. Directive 2000/53 on end-of life vehicles, O.J. L 269/00. 23. Opinion of the Scientific Committee on Toxicology, Ecotoxicology and the Environment, May 5, 2000. 24. Statutory Order No. 1012 of Nov. 13, 2000. 25. Council Regulation (EEC) No. 259/93 of 1 February 1993 on the supervision and control of shipments of waste within, into and out of the European Community, O.J.L 30/93. 26. The Ordinance on the Take-back of Refrigerating Equipment, Fed. Law Gazette No. 408/1992, as amended by Fed. Law Gazette No. 168/1995. 27. Statutory Order No. 1067 on Management of Waste from Electrical and Electronic Products, December, which went into effect December 1999. 28. Kreislaufwirtschaftsgesetz, in effect since 1996. 29. Waste Management Decree, February 5, 1997, No. 22 (as amended by Decree No. 389 of Nov. 8, 1997, No. 389), Article 44. 30. Waste Management Decree, Art. 44, Par. 4. 31. Ordinance on Collection and Recovery of End-of-Life Electronics, Mar. 16, 1998, in effect since July 1, 1999. 32. Regulations on Hazardous Waste, May 19, 1994. 33. Lead Exposure Reduction Act, S. 391, Oct. 8, 1991. 34. Section 6607 of PPA. 35. 42 U.S.C. s/s 6901 et seq. (1976). 36. 40 CFR 261.2 (e) (ii). 37. Pioneer Corporation. Environmental Hazardous Substance List, (Ver. 4.0), Feb. 2003. 38. Matsushita Electric Group. Chemical Substances Management Rank Guidelines, Ver. 2.1(For Products, Kadoma, Osaka, Japan, Dec. 2002. 39. Matsushita Electric Group. Green Procurement Standards, Ver. 2.0, Dec. 2002. 40. Matsushita Electric (Panasonic) news release, May 15, 2003.

16. 17.

SUGGESTED READING Kester Solder Lead Free Update, ‘‘A Concise Snapshot of the Lead-free Soldering Issue,’’ 2000. Electronics Industry Environmental Roadmap: ‘‘Activities in Europe and Japan, Microelectronics and Computer Technology Corporation’’ (MCC), 2000. National Physical Laboratory (NPL). Electronics Interconnection Lead-free News, UK, 2000. Lead-Free Legislation, UK–Europe, Brian Richards, 2000. Richards, B.P., et al. ‘‘An analysis of the current status of Lead-Free Soldering,’’ Apr. 1999. Richards, B.P.; Nimmo, K. ‘‘Lead-free Soldering—Update 2000,’’ Apr. 2000. Charles, H.K.; Sinnadurai, N. Microelectronics, Rising to the Environmental Challenge? Denmark plans a Generalized Ban on Lead. Environment Daily (ENDs), Mar. 19, 1999. Danish Bid to ban Lead ‘‘Unjustified’’. Environment Daily (ENDs), June 7, 2000. Auken defends proposed Danish Lead Ban. Environment Daily (ENDs), June 15, 2000. Swedish Environment Protection Agency. Swedish Environmental Quality Objectives, 2000. The Future Environment—our, Common Responsibility, Final Report of the Committee on Environmental Objectives; Stockholm, June 2000. U.S. Environmental Protection Agency, 40 CFR Part 372, Lead and Lead Compounds, Lowering of Reporting Thresholds; Community Right-to-know Toxic Chemical Release reporting Final Rule, Federal Register, Jan. 17, 2001. Cathode Ray Tubes banned from Disposal at Transfer Stations, Landfills, Incinerators. BNA Daily Environment, Apr. 6, 2000. EC to get the Lead Out, IPC says U.S. will too. U.S. Tech Interactive, Apr. 2, 1999. An assessment of the use of lead in electronic assembly, Part I. Circuit World, 1993; 19 (2). Price, J.L., Ed.; Reclaiming End-of-life Cathode Ray Tubes (CRTs) and Electronics: a Florida Update. Hazardous Materials Management Conference, Tucson, Arizona, Nov. 15, 1999. National Recycling Coalition, Electronics Recycling Initiatives, 2000. Florida’s Strategy for the Management of End-of-life Cathode Ray Tubes (CRTs), Computers and Other Electronic Equipment, Discussion Paper, Sep. 2, 1999. IPC Roadmap, a Guide for Assembly of Lead-free Electronics, draft June 2000.

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EACEM, European Association of Consumer Electronics Manufacturers. Position Paper on the Commission’s Proposed Directive on Waste Electrical and Electronic Equipment; Jan. 2001. American Electronics Association (AEA). Position Paper on Lead; Aug. 1999. Euro Coop’s Comments to the Commission’s Proposal for a Pair of Two Directives on Waste Electrical and Electronic Equipment, COM (2000) 347, Brussels, Feb. 14, 2001. APME’s Position Paper on the Proposal for a Directive on Waste Electrical and Electronic (E&E) Equipment, Sep. 18, 2000. EICTA Statement on the Proposal for a Directive of the European Parliament and the Council on Waste Electrical and Electronic Equipment, and the Proposal for a Directive on the Restriction of the Use of Certain Hazardous Substances in Electronic and Electrical Equipment, COM (2000) 347 final, Brussels, Sep. 22, 2000. AEA–EIA Joint Principles on Proposed European Waste and Substance Restrictions Directives, Nov. 2000. Orgalime’s Position Paper on Commission’s Proposals for Directives of the European Parliament and the Council on Waste Electrical and Electronic Equipment, and Restriction of the Use of Certain Hazardous Substances in Electronic and Electrical Equipment, Sep. 5, 2000. European Environmental Bureau (EEB)’s Comments on the Commission’s Proposal for a Directive on Waste from Electrical and Electronic Equipment (WEEE), and the Restriction of the Use of Certain Hazardous Substances in Electronic and Electrical Equipment (EEE), Sep. 2000. NEMA (Japanese National Electrical Manufacturers Association), Statement of Principles on EU Directives Regarding Waste in Electrical and Electronic Equipment and Substance Bans. AMCHAM (American Chamber of Commerce)’s Discussion Paper on the Commission’s Proposals for a Directive on Waste from Electrical and Electronic Equipment (WEEE), and a Directive the Restriction of the Use of Certain Hazardous Substances in Electronic and Electrical Equipment (EEE), Oct. 5, 2000.

5 Market, Product, and Corporate Policy Trends E. Jan Vardaman TechSearch International, Inc., Austin, Texas, U.S.A.

I.

INTRODUCTION

The United States is regarded as the first country to discuss regulations on the usage of lead after numerous studies on the toxicity of lead in 1989. Legislative attempts ensued; the lack of a drop-in replacement for electronics resulted in abandonment of the movement. In the late 1990s, Japan undertook a self-imposed restriction with many electronics manufacturers aiming to gain brand awareness by claiming ‘‘environmentally friendly operations.’’ In 1997, Japan’s technology research gained world recognition when Matsushita Electric introduced a consumer product using lead-free solder that gained market share over competitors. A variety of European Committee legislation has been proposed prohibiting the use of toxic substances containing lead by 2004, accelerating the movement for lead-free solder. Largely ignored in North America, Japan declared support for total elimination of lead and started work on legislation to ban lead. While timelines have been revised for European legislation limiting lead use, the date is proposed to be July 1, 2006 with various exemptions for industrial segments such as military and aerospace electronics [1]. The environmentally friendly manufacturing movement has demonstrated that it is not primarily driven by legislative activity, rather, by consumer emotions. The issue is being pulled by market forces, not pushed by technology. Given consumer demand, a great number of consumer products with eco-labels are expected to be introduced in the market over the next few years— especially in Europe and Japan. The green manufacturing wave will be concentrated in the segment of the electronics industry with products that have a short life such as mobile phones, personal computers, television sets, radios, etc. that typically are discarded in landfills. However, lead-free adoption is not limited to these products. Some automotive and computer products have adopted lead-free solder materials, and future telecommunications and additional computer products are anticipated on the horizon. While the exact definition of ‘‘lead-free’’ is debatable, there are numerous examples of products being manufactured and sold that are either completely lead-free or are manufactured with reduced amounts of lead. Many products are consumer or portable products such as consumer electronics; however, lead-free workstation servers and high-end mainframes have been introduced in Japan.

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150 II.

JAN VARDAMAN ENVIRONMENTALLY PREFERABLE PURCHASING TRENDS

Environmentally preferable purchasing insures that environmental considerations are included in purchasing decisions along with traditional factors such as product price and performance [2]. The U.S. Environmental Protection Agency (EPA) encourages that a product’s environmental impact be based on its life cycle, that is, from acquiring the raw material, through manufacture, packaging, distribution, use, and disposal. The impacts can include a number of factors, among them: energy and water efficiency, toxic material content, recycled material content, resources required, adverse effect on humans and animals, adverse effects on plants, water, air, and soil to cite a few. These types of assessments can be both costly and lengthy. So for practical reasons, it is the normal practice to address only the most important factors for their potential of impacting the environment during a product’s life.

A.

Public vs. Private Sector

Environmentally preferable purchasing has been practiced in the public sector for sometime. The government can utilize such tools as federal statutes and Presidential Executive Orders to help promote social and economic programs in achieving Congressional or Presidential policy objectives. That is, in addition to the normal considerations, government contracts would also include environmental considerations. An Executive Order, Federal Acquisition Recycling, and Waste Prevention (Oct. 20, 1993), required the EPA to define the environmentally preferable purchasing guide for federal agencies. Later (Sept. 1998), Executive Order 13101, Greening the Government through Waste Prevention, Recycling and Federal Acquisition, mandated that the federal agencies purchase ‘‘. . .products and services that have a lesser or reduced effect on human health and the environment when compared with competing products and services that serve the same purpose.’’ Many state and local governments practice similar incentives through the EPA’s Environmentally Preferable Purchasing (EPP) Program, in hopes that some of the experiences in the public sector will carry over to the private sector. Although public and private sector procurement share some similarities, they play distinctively different roles in the market place. Government agencies typically buy products designed and manufactured by the private sector, and as purchasers are concerned about price, performance, and availability. Unlike the government, companies are also suppliers and must take actions that make their products competitive, such as cost, time to market, performance, and other factors that affect sales and profit. Typically, these are not matters that play a role in government purchasing decisions.

B.

Move Towards Environmental Concerns

Private sector purchasing decisions typically do not take into account economic, social, or political factors; but this is changing, especially as it relates to the issue of the environment. Companies are rapidly becoming aware that it is good business to be environmentally conscious. Given the worldwide interest in the environmental movement, companies feel the need to respond by providing more environmentally friendly products and utilizing this as a method to differentiate their products from competitors.

C.

Cost Savings

Some companies have realized significant cost savings in implementing environmentally friendly processes and products. For example, the elimination of flux cleaning postsoldering operations of electronic circuit boards by the introduction of no-clean fluxes has resulted in enormous savings by the industry (see Chap. 11, Sec. 6). A broader perspective is often required to take advantage of the cost savings afforded by environmentally friendly approaches. That is, an assessment must be made on the basis of all aspects of production and sales, not just a side-by-side comparison of an individual aspect. As in the flux application case, there is only a modest cost advantage of a no-

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clean flux over a standard flux, which is lost in the more thorough preparation required to assure adequate solder wetting of parts to be soldered with no-clean flux (i.e., low resin content, thus lower activity) compared to a mildy activated rosin (RMA) flux. However, all of this is overshadowed by the significant cost savings in eliminating the flux clean step and the costs associated with treating the cleaning solutions or meeting the legal requirements for their proper disposal.

D.

Selection Guides

In order to supply products that are environmentally friendly requires procuring environmentally friendly parts to manufacture those products. There are organizations such as the Green Purchasing Network (GPN) in Japan which provides a list of vendors that supply environmentally preferable products and compares their environmental attributes through their guidebooks. The nonprofit organization, Green Seal, publishes monthly the ‘‘Choose Green Report,’’ which identifies green products, as well as a buying guide. The aforementioned guides help procurement organizations in meeting the challenge of determining products available in the market that are environmentally preferable. Many large companies generate lists of approved vendors, i.e., those that supply environmentally approved products. This serves to streamline the procurement process in only having to interact with acceptable sources.

E.

Procuring and Providing Environmentally Friendly Products

Depending on the product and the target customer audience, a product may be focused on just one environmental attribute or several attributes. Utilizing products made of recycled materials is a popular single attribute focus. For example, virtually all containers in which fast-food restaurants serve food prominently indicate they are constructed of recycled materials, as a means of impressing their customers that they are environmentally conscious and responsible corporate citizens. The focus may be on environmental attributes that are based on the company’s culture, what it deems important. For example, it is reported that a producer of high-quality skin and hair-care products avoids the use of animal-derived and petroleum-based chemicals whenever possible. Synthetic alternatives are utilized if performance requirements are satisfied since they are more environmentally preferable than those derived from natural sources.

F.

An Industry Consensus Required

In order to turn the entire electronics industry around from its more than 40 years of lead–tin solder assembly practices, a close working relationship is required with the supply chain, i.e., the vendors that provide materials, parts, and assembly services. The industry must come to some consensus concerning lead-free materials, particularly solders and component lead finishes. Vendors cannot be expected to provide multiple versions of individual part numbers. In addition, consensus will allow price reductions that result from economies of scale. As always, companies that desire products from vendors that lie outside the de facto standards (i.e., the industry consensus) must be prepared to bear the burden of additional costs, sourcing issues, and, in some cases, long lead times.

III.

USE OF ECO-LABELS FOR PRODUCT DIFFERENTIATION

Given the interest in environmental issues, many companies are taking serious steps to utilize environmental capability to differentiate their product in the marketplace as an integral part of their marketing strategy. This approach is often referred to as Design-for-Environment or DfE and others such as green design, life-cycle engineering, environmentally conscious design, etc. where the primary intent is to prevent the potential environmental impacts through prudent

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design choices rather than attempting to mitigate them at some point in their life cycle [3]. Manufacturers utilize the practice of eco-labels affixed to their products as a means of communicating to consumers that they have met an approved environmental standard as a method of differentiating their products in the market place.

A.

Types of Labels

Eco-labeling is a voluntary method of certifying environmental performance that is practiced around the world. It identifies a specific product based on life-cycle considerations. There are many different voluntary environmental performance labels with the International Organization for Standardization (ISO) having identified three general categories, whose characteristics are summarized in Table 1. Briefly, they can be described as [4]:


Type I—a voluntary, multiple-criteria-based, third-party program that awards a license authorizing the use of environmental labels indicating environmental preference of a product in a particular category based upon life-cycle considerations
Type II—environmental self-declaration claims
Type III—voluntary programs that provide quantified environmental data of a product relative to parameters set by a qualified third party based on a life-cycle assessment that has been verified by that or another qualified third party Understanding that there must be guiding standards and investigations by independent third parties to assure consumers that a labeled product does, in fact, meet certain prescribed criteria, there are both public and private organizations that assure credibility by serving as impartial

TABLE 1

Several Key Aspects of Eco-Label Categories Categories

Key aspects

I

II

Criteria Definition

Independent 3rd party

Manufacturers self-declarations

Scope

Specific products (e.g., computers, consumer products, etc.) Officially designated third party, (e.g., test laboratory) Environmental organization label or seal Japan: Eco-Mark

Single or multiple environmental attribute(s)

Certification

Designation

Examples

European Union: Eco-Flower Sweden: TCO Label Germany: Blue Angel

III Quantitative life cycle impact assessments of products—difficult to obtain meaningful data Specific products

Use of standardized formats defined by industry organizations Industry associations

Officially designated third parties

Japan Electronics Information Technology Assoc. (JEITA) has an eco-label for PCs

Not gained much acceptance to date

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third-party monitors. Several organizations engaged in eco-label activities such as providing guidance, information, setting standards, and granting certification are given in Table 2.

B.

Benefits

The ultimate gain and benefit of eco-label programs is to improve some aspect(s) of environmental quality. However, in doing so, there are also benefits derived by both producers and consumers. The eco-labels are typically well recognized. For example, the Nordic Swan label is recognized by over 90% of people in Sweden and thus is a very cost-efficient way of communicating that the products bearing that label are environmentally friendly, and the companies that produce them take responsibility for the environment. In addition, 77% of Swedes consider that if a product bears the Swan label, it is likely to be reliable and of good quality [5]. There is anecdotal information that suggests there is a widespread awareness and embracing of environmentally labeled products and a willingness to pay a premium for these products. Even early on (1988), 68% of German households were familiar with the Blue Angel label and its connection to environmental protection. Similarly, in a 1994 survey, 78% of households in Singapore recognized the Green Label and approximately a third considered it as part of their purchasing decision. In a 1993 poll, more than 50% of the population in Japan was familiar with the

TABLE 2

Several Organizations Engaged in Eco-Label Activities

Organization/address Environmental Management Market (http://www.environmarket.com/edp)

Global Ecolabelling Network c/o Terra Choice Environmental Services, Inc. 2781 Lancaster Road Ste. 400 Ottawa, ON., Canada K1B-1A7 Phone: 613-247-1900, 613-247-2228 ([email protected]; http://www.gen.gr.jp) Green Seal 1001 Connecticut Ave. NW Ste. 827 Washington, D.C. 20036 Phone: 202-872-6400 Fax: 202-872-4324 (http://www.greenseal.org) International Organization for Standardization 1, rue de Varembre’ Case Postale 56 Geneve 20, Switzerland CH-1211 Phone: 4122-7490-111, 4122-7333-430 ([email protected]; http://www.iso.ch) U.S. Federal Trace Commission

Source: Ref. 6.

Information/services Environmental product declaration —Product specific requirements —How to calculate and report environmental performance of particular products —GEN is an association of international eco-labeling organizations —Provides information on various organizations —Source for comprehensive news and information on eco-labeling —Independent certifying eco-label organization in the U.S. —Sets the standards and awards the Green Seal eco-label to qualifying products

—ISO Standards 14020 through 14025 established guidelines for both first- and third-party environmental labeling

—Govern environmental marketing claims —Set standards for the use of language on products

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EcoMark. One in five Canadians indicated someone in the household purchased a product with the Ecologo label in 1996 [6]. In addition to providing producers the potential for increasing their market share, public recognition and acceptance has the added advantage of driving producers to continuously search for new and better manufacturing practices to benefit the environment and provide enhanced environmentally sound products.

C.

Eco-Labeling Practice

1. Products Must Meet Standards Independent of whether the product is certified by a first or third party, it must be tested to demonstrate compliance with environmental standards. Most eco-labeling programs have clearly defined standards. Additional guidance in general eco-labeling standards and marketing is available from a variety of organizations, some of which are listed in Table 2 [7]. Prospective applicants must retain a test facility approved by the eco-labeling organization to confirm that the applicant’s product(s) to be covered by the license conforms with the criteria defined by the ecolabels organization. The findings of the tests are kept on file for the period that the license is in force. Licenses are required to maintain records that demonstrate that the eco-labels criteria are met on a continuing basis for all products covered. In addition, to further assure the validity that eco-labels criteria are met, eco-labeling organizations conduct periodic on-site inspections of licensees, subcontractors, importers, etc. without prior notice [8]. If an inspection or test reveals noncompliance, the licensee is required to take corrective action relative to the product, manufacturing process, or quality system. If the noncompliance persists, the license to use the eco-label is revoked. A revocation can be permanent or temporary until noncompliance issues have been appropriately remedied. Revocations are typically announced by eco-label-granting organizations [8]. 2. Term An eco-label license is typically valid for prescribed period, normally 3 years because it is anticipated that product environmental requirements will gradually increase. Companies must then reapply and an assessment must be conducted to determine if product(s) conforms to the new standards or requirements. If not, the license is withdrawn, thereby ensuring that companies pace with technological progress in providing increasingly environmentally friendly products [9]. 3. Fees Applications are typically responsible for an initial application fee and all costs associated with tests and inspections during the application phase [8]. The programs are usually financed in part

TABLE 3 Survey Ranking of Audio Equipment Attributes Rank 1–3 1–3 1–3 4–5 4–5 6–8 6–8 6–8

Item Price/cost of ownership Reliability/durability/build quality Design Sound quality Technical specification/feature Brand name Ease of use Output power

Source: Ref. 10.

MARKET, PRODUCT, AND CORPORATE POLICY TRENDS TABLE 4

155

Survey Ranking of Environmental Attributes

Rank

Item

1 2 3 4 5 6

Energy consumption Hazardous substances Material application Recyclability Life-cycle impact Packaging

Mentioned by (%) 100 60 53 47 23 17

(100) (53) (37) (23) (37) (30)

Source: Ref. 10.

through government subsidies and fees based on a product’s annual sales. For example, in the case of Sweden’s Nordic Swan, the assessment is 0.4% [9].

D.

Green Marketing

Producers such as automakers have not seen a strong correlation between customer’s words and actions. That is, there are claims that environmental performance is important in their purchasing decision, but postpurchase surveys place environmental concerns well below other factors such as a vehicle’s comfort, size, color, engine performance, etc. [2]. Studies have been conducted that if environmental characteristics are the sole focus, the marketing will be relatively unsuccessful. However, if other product benefits are linked to the environmental attributes, most consumers will give such products favorable consideration [10]. 1. Coupling Environmental with Other Parameters A study determined that customers typically do not make buying decisions on their own but take the advice of relatives, friends, and particularly the sales force who exercise a strong influence in approximately half the sales decisions irrespective of product price. The first step is to determine key parameters of interest for a particular product. Consider, for example, audio equipment. From a table (similar to Table 3), selected are the top three or four attributes (e.g., price, cost of ownership, reliability, quality) mentioned most frequently. These selections represent the best candidates to link with environmental attributes to enhance the marketing value of an audio product. Similarly, the environmental factors were ranked by customers as listed in Table 4. Based on the results, it was concluded that energy consumption, hazardous substances, materials application, and recyclability were the most important environmental aspects to couple with other product attributes to help differentiate them from competitor products and increase their marketability [11].

IV.

MATTERS IN JAPAN

Japan’s official policy for dealing with end-of-life electronics products is focused on recycling. Recycling plants in Japan began operation as the home appliance recycle law (Law for Recycling Specific Appliances for Home Use) took effect on April 1, 2001. However, many existed before the legislation was enacted [12]. This legislation provided Japanese manufacturers additional motivation to embrace lead-free technology because it eliminated or reduced disassembly costs associated with removing lead-containing components prior to being dispositioned. By adopting this approach, Japanese manufacturers were well positioned in the marketplace with some introducing products in niche applications that contained little or no lead quite early. Many Japanese companies indicate that it is not just a matter of appearing to be environmentally friendly, but rather a necessity for survival in the world market [13]. Lead-free materials have typically been utilized in consumer electronics such as pagers, televisions, laptop computers, minidisk players, etc. Almost all of these products are eco-labeled to attract environmentally

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conscious consumers. For example, Matsushita’s minidisc players sold both in and outside Japan under the Panasonic brand bare a green leaf logo, which states, ‘‘Produced for the Environment.’’

A.

Japanese Corporate Policies

The committee of the Japan Institute of Electronics Packaging (JIEP) defined Japan’s lead-free soldering roadmap on January 30, 1998. Following the roadmap introduction, several major Japanese consumer electronic manufacturers initiated their own road map and publicly announced accelerated plans to eliminate lead solder completely by 2001. Deadlines for all products have moved out, but many virtually lead-free products have been introduced. Companies such as Matsushita (Panasonic), Sony, Toshiba, and Hitachi pushed forward and met this aggressive timeline. Numerous Japanese companies have announced lead-free goals in concert with other environmental objectives. Japanese firms have been driven partly by the threat of pending European Union legislation and partly by the goal of introducing eco-labeled products to gain market share. Every major Japanese electronics manufacturer has a lead-free product plan and an annual environmental report. While many companies announced goals of making all their products lead-free by 2001, total lead reduction has not been achieved. Nevertheless, some products have been introduced and corporate policies for lead elimination remain in place. Hitachi’s plan was to make all its products lead-free by 2001. The company has been heavily involved in the examination of potential leadfree solders for use in Hitachi’s various product lines. Matsushita (Panasonic) planned to eliminate all lead from major electronics products by 2001. NEC started its lead-free solder task force in April 1998, and in May 1998, a lead-free solder promotion committee was established. With its corporate goal of lead reduction, all NEC divisions were researching lead-free solder for product applications. While the workstation division finds the development challenging because of the extensive use of ball grid array (BGA) components and flip chips that both use solder balls, other divisions are shipping products. By 2003, NEC’s pager and mobile phone division will purchase 50% less solder for use in its factories as compared with 1998 purchase levels. NEC is moving ahead with its lead-free plans and has proposed Sn–Ag–Cu as the industry standard because it can be used with existing components and equipment [14].

B.

Japanese Consortia

Active since the early 1990s, the Lead-Free Soldering Committee of the Japan Institute of Electronics Packaging (JIEP) includes more than 40 companies and three academic institutions. The group has focused its efforts on the evaluation of alternative alloy systems. A project similar to the National Center for Manufacturing Sciences, lead-free project was initiated in Japan (1998) under the New Energy and Industrial Technology Development Organization (NEDO). Oversight for NEDO falls under Japan’s Ministry of International Trade and Industry (MITI). The project goal was to establish a database on lead-free soldering, to select suitable lead-free solder candidates, and to establish the process for lead-free solders. The 2-year (April 1998 to March 2000) w350 million project was composed of members from major electronics, device, and solder companies, in addition to several universities. The database was established, solder compositions including Sn–Ag–Cu have been proposed, and lead-free soldering processes have been implemented. In January 1999, NEDO initiated the Research and Development Program for Lead-free Solder Standardization in an effort to draw together and standardize various lead-free studies and independent corporate projects. The Japanese Welding Association (JWA) was assigned to study the basic performance characteristics of leadfree solder. Numerous conference papers have been published detailing performance characteristics of various lead-free solders.

C.

Japanese Products

Many Japanese companies have introduced lead-free products and others are planned in the area of consumer electronics, computers, and automotive applications.

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1. Hitachi Hitachi initiated production of several lead-free products in 1999, including a laptop computer and camcorder. In its FLORA220CX notebook computer, which started production in February 1999, Hitachi used both a Sn–2Ag–2Bi–0.5Cu alloy solder paste and a Sn–3.5Ag–0.75Cu board finish for an audio board. An Au–Sn or Ag–Sn alloy was used for soldering optical components, and a Sn–Ag–Bi alloy was used to attach the audio substrate portion of the notebook computer. In March 1999, Hitachi began mass production of a lead-free camcorder (VMH845L) that employed a Sn–3.2Ag–2.7Bi–0.5In solder paste and a Sn–3.5Ag–0.75Cu board finish. 2. Fujitsu Fujitsu developed two lead-free solders for use in the GS8900 global server series, introduced in October 1999 and also in the VPP5000 supercomputer. The flip chip solder bumps for the system are a Sn–Ag alloy (melting point of 220jC). The flip-chip-bumped ICs are mounted on a thin-film AlN multichip module. Fujitsu reports less concern with alpha particles, a general concern with lead (Pb)-based solders [15,16]. The CPUs are packaged in a pin grid array (PGA) and mounted on the server motherboard using a pin-in-paste method or a microconnector method. Fujitsu developed a Sn–Bi–Ag alloy with a melting point of approximately 130jC to mount components on the motherboard (Fig. 1). Components are mounted on a preflux-treated bare copper motherboard and then reflowed. Fujitsu has licensed its lead-free alloy to several solder manufacturers. The company is working to introduce additional lead-free products such as workstations, personal computer, hard disk drives, and mobile phones. 3. Matsushita Matsushita introduced its first lead-free solder minidisc player in October 1998, the MJ30. The minidisc player was assembled using surface mount technology and a Sn–Ag–Bi–Cu lead-free solder paste. For through-hole assemblies, the eutectic Sn–Cu alloy was selected. The company

FIG. 1 Components mounted on a Fujitsu System Board with a Sn–Bi–Ag alloy. (Courtesy of Fujitsu, Ltd.)

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reported that lead-free solder could be introduced into manufacturing without any major problems regarding material properties and product quality and reliability. The company introduced two other lead-free minidisc players in 1999, the MJ70 and MR1000, and retired the MJ30 model [17]. Between October 1998 and the end of 1999, Matsushita manufactured 1.6 million portable minidisc players and 1.8 million video recorders with lead-free solder [1]. The minidisc player uses a proprietary Sn–Ag–Bi–X alloy, also used in Matsushita’s phase change optical disk (PD) drives. Matsushita also began producing lead-free professional videotape recorders in August 1999, which used a Sn–Cu solder. The company also introduced leadfree headphone stereo sets manufactured in Taiwan. Additional product introductions included mobile phones, rice cookers, medical equipment, and personal computers. In its effort to develop a lead-free product, Matsushita spent 4 years evaluating approximately 50 lead-free solder alternatives and several component finishes including Sn–Pb, Sn–Bi, Pd–Ni, and Au–Ni [17]. Matsushita announced the availability of lead-free semiconductor products starting in April 2000. Matsushita utilized three solder alternatives, depending on package type, to achieve lead-free soldering for semiconductor products. By late 1999, Matsushita stated that it had started using its lead-free solder in its Panasert line of surface-mounting equipment. 4. NEC NEC introduced lead-free solder pagers and mobile phones in 1999. The company began research in 1997, testing Sn–Ag–Cu alloys, then in December 1998, selected a Sn–3.5Ag–0.75Cu material with a melting point of 220jC and an electroless Ni–Au board finish. In cases where largecomponent leads are soldered with a soldering iron or to repair SMT-mounted components, NEC uses Sn–Ag solder wire. The NEC lead-free mobile phone project started in December 1998, with trial production in September 1999. The same lead-free solder material and board finish combination was selected for the pager and used for the DoCoMo cellular phone produced for NTT. Test results for NEC’s mobile phones indicate that the use of lead-free solders have provided improved solder joint reliability. NEC’s personal computer division introduced a lead-free laptop computer in October 1999. NEC adopted lead-free solder materials in its portable notebook computers, the VersaPro NX series, models VA33H-BS and VA33H-BC. Mobile PC model numbers PC-LB33H12C, PCLB33H12D, PC-LB33H12CW, PC-LB33H12DW, PC-LB400J12C, and PC-LB400J12D are marketed as lead-free. The lead-free solder (Sn–Zn–Bi alloy) was used for both main and daughter boards, such as the card/bus interface boards. With a 3% Bi addition, the solder melting point is between 187jC and 197jC. The personal computer division selected Showa Denko’s solder paste and NEC Niigata manufacturers the boards. The solder volume requirement is onethird less than with previous notebook computers, and the board finish is a flash of nickel. An improved flux and nitrogen reflow is also used. NEC envisions several issues with the use of lead-free solders. There is concern of possibility of overheating small chips because the reflow temperatures are typically higher than with eutectic Sn–Pb solders. Because many semiconductor packages are purchased outside NEC, there is concern about the component finishes on these packages and the resulting brittle intermetallics that could be formed. 5. Others In April 1999, Sony announced that it would change the material composition of its lead-free solder from Sn–4Bi–2Ag–0.5Cu–0.1Ge to Sn–2.5Ag–1Bi–0.5Cu. Sony released a television manufactured with lead-free solder in 2000 and introduced the DCR TRV-30 digital camcorder using Sn–2.5Ag–1Bi–0.5Cu lead-free solder and a halogen-free PCB. Toshiba announced plans to adopt lead-free solder for all digital products, including PCs, televisions, DVD players, and digital cameras, by March 2003. Toshiba’s plans included eliminating lead entirely from its mobile phones by 2002. Toshiba developed a CDMA mobile phone that utilized a Sn–Ag alloy and a lead-free board finish based on an Alpha Metals silver electroless plating.

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The Oki Group launched environmental protection programs in 1993 and set a goal of achieving lead-free solder for all Japanese products by the end of March 2004. Nagano Oki implemented lead-free solder in 20% of all new products by 2001 and set a corporate deadline to achieve 100% by 2004. Japanese automotive electronics makers have also introduced lead-free solutions. Toyota introduced lead-free wiring harnesses in all new cars starting in 1998. Wiring harnesses typically contain between 150 and 200 g of lead.

D.

Japanese Eco-Labels

Many Japanese companies are using eco-labels in product marketing efforts. These eco-labels range from leaves in various shades of green to images of trees or fields of flowers. Examples of eco-labels and eco-seals are shown in Fig. 2. While there has been no reported measure of marketing success with these labels, their use continues. In addition, many major Japanese companies such as Matsushita, NEC, and Sony publish full reports on their corporate environmental policies.

E.

Lead-Free Roadmaps

Japanese companies are ahead of the rest of the world in the introduction of lead-free products. The Japan Institute of Electronics Packaging’s Technology Roadmap for Electronics Packaging in 2010 documents the path toward lead-free product introduction. Lead-free component production is noted to have started in 2000 with rapid progress anticipated between 2003 and 2005. The use of lead-free solder is expected to become pervasive in Japan. While there are a number of products with lead-free solder today, many additional products are expected. The roadmap predicts that the use of lead-free solder will surpass conventional lead-based solders by 2003. While conventional lead-based solders will still be utilized until at least 2010, the volume is

FIG. 2 Examples of eco-labels and eco-seals: a) TerraChoice (Canada); b) Sony eco-info label; c) GreenPlus (Japan); d) KELA eco-label (Korea); e) NEC eco-logo; f) Ecoflower (European Union); g) Environmental Choice (New Zealand); h) Green Cross International; i) Panasonic Panaleaf1; j) Panasonic Panaleaf2; k) Green Seal (USA); l) TCO ’03 display label; m) EPA (USA); n) Nordic Swan; o) Blue Angel (Germany); p) Terra Choice (Canada); q) Eco Mark (Japan).

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FIG. 3 Lead-free replacement time line depicting the components shift to lead-free over time. (From Ref. [18].)

anticipated to sharply decline. The use of conductive polymers, as a substitute for solder, is expected to emerge particularly for consumer electronics applications [18]. Projections for solder use are shown in Fig. 3. While many companies lament that it will be difficult to move to totally lead-free manufacturing, most companies anticipate that lead-free manufacturing will soon become a reality (Table 5) [19].

TABLE 5

Japan Jisso Technology Roadmap

Application Desktop PC

Notebook PC

Handheld PC

PDA

Cellular phone

AV equipment

Source: Ref. 20.

Environmentally friendly characteristic Pb-free materials Percentage of Pb-free utilization (%) Pb-free materials Percentage of Pb-free utilization (%) Pb-free materials Percentage of Pb-free utilization (%) Pb-free materials Percentage of Pb-free utilization (%) Pb-free materials Percentage of Pb-free utilization (%) Pb-free materials Percentage of Pb-free utilization (%)

1999

2000

2005

2010

— 0

Pd, Ag 10

Pd, Ag 70

Pd, Ag 100

Sn–Ag —

Sn–Ag 50

Sn–Ag 50–100

Sn–Ag 100

Sn–Pb 0–20

Sn 50–100

Sn 100

Sn 100

— —

— 0

— 50

— 100

— —

Pd–Sn Application

Pd–Sn —

Pd–Sn —

— —

Sn–Ag–Cu Begin 1999

Sn–Ag–Cu 80

Sn–Ag–Cu 95

MARKET, PRODUCT, AND CORPORATE POLICY TRENDS V.

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MATTERS IN EUROPE

European mobile phone and consumer electronics makers have introduced lead-free products, as have some telecommunications and automotive electronics manufacturers. Many additional European companies have indicated that they are testing lead-free materials in their products and will eventually introduce them for mass production.

A.

European Corporate Policies

Mobile phone companies such as Ericsson and Nokia have made lead-free and halogen-free product plans part of their environmental policies. While some consumer electronics companies, such as Philips, favor product recycling as the best environmental choice, most European companies have lead-free policies driven by legislation initiatives.

B.

European Consortia

The European Community (EC) has many lead-free programs dating back to the early 1990s. The European Brite-Euram Consortia started work in April 1996 under the Improved Design Life and Environmentally Aware Manufacture of Electronic Assemblies by Lead-Free Soldering (IDEALS). The consortium recommended Sn–Ag–Cu as the general purpose solder. The BriteEuram IDEALS program included Siemens, Marconi, Philips, Cookson, and the University of Ireland. The project, which ran from 1996 to April 1999, investigated many potential alternative alloys. It was determined that Sn–Ag–X lead-free solders offer superior creep resistance at room temperature and 100jC compared to Sn–40Pb. Results of this work have been presented, in addition to factors to consider when developing and implementing lead-free alloys such as manufacturability, availability, and cost. The International Tin Research Institute, ITRI Ltd., based in Uxbridge, England, has been involved in solder research since 1932 and has conducted lead-free solder studies since the late 1980s. ITRI opened Soldertec, a lead-free soldering technology center supported by more than 50 industrial members in 1999. The center studies a variety of aspects related to the development of lead-free technology, including production and reliability concerns. The European Cooperative administrated by the Science and Technical research (COST) is studying three lead-free solder alternatives, including two for wave soldering applications recommend by the U.S. consortia, NEMI. Sweden’s IVF Industrial Research and Development Corporation is conducting reliability testing with support from member companies such as Volvo, ABB, and Ericsson. A collaborative effort between Infineon Technologies, Philips Semiconductors, and ST Microelectronics is focused on establishing an upper lead concentration limit for semiconductor packages and common approaches for solderability testing and assessment of moisture-sensitivity levels [20].

C.

European Products

Alcatel is currently producing a line of lead-free mobile telephones and boasts the first lead-free assembly line in France. A line of lead-free PBX systems for telecommunications is offered worldwide. The company also has the distinction of having the second lead-free production line in Europe after Marconi Communications. Alcatel evaluated two lead-free alloys and determined that Sn–3.8Ag–0.7Cu provided the best results. In addition to conducting lead-free solder alloy research, Alcatel evaluated four PCB finishes—OSP, silver, palladium, and NiAu. As with other manufacturers, Alcatel depends upon the termination choices suppliers make available. Many lead frame terminations contain lead, but some suppliers are providing components with leadfree finishes such as pure tin or Ni/Pd. Bosch is involved with lead-free activities including solder paste alternatives. The likely candidates are alloys within the Sn–Ag–Cu or Sn–Ag–Bi solder systems. Some automotive electronics products at Bosch are already lead-free. According to material suppliers, Bosch introduced conductive adhesives in its hybrid devices to a significant extent.

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Ericsson has produced a mobile phone that is both lead-free and uses a halogen-free PCB. Ericsson has a joint venture with Sony for mobile phone design and production that may more closely link lead-free product development in Europe and Japan. In April 1999, Marconi Communications initiated production of a telephone manufactured with lead-free solder, hailed as the first lead-free line in Europe. The telephone utilized a Sn–Ag– Cu alloy that was introduced to Marconi’s existing surface mount assembly line. The only modification required was a slight alteration to the convection reflow profile [21]. In collaboration with Multicore Solders, Nokia developed a lead-free solder alloy suitable for use in mobile telephones. The alternative Sn–3.8Ag–0.7Cu alloy utilizes existing manufacturing processes to replace lead-based alloys in the assembly of surface-mount PCBs. In production trials, 20 Nokia 2110 GSM digital cellular handsets were assembled using the lead-free solder. The alternative alloy was found to exhibit better solderability than the traditional Sn–Pb solder. Siemens initiated lead-free solder development activities in the mid-1990s. The first Siemens demonstrators using lead-free solder included industrial PCs, automotive, and telecommunications products. These products all have double-sided SMT boards with complete lead-free solutions. The smallest component is a 0402 device and the largest is a QFP with a 0.5-mm pitch. After analyzing 162 different alloys, Siemens selected three. The melting point for the Sn–3.8Ag– 0.7Cu alloy was too high; therefore Siemens added bismuth or another element to lower the melting point.

VI.

MATTERS IN NORTH AMERICA

In the absence of legislation requiring the reduction or elimination of lead, either pending or enacted, North American electronics companies have not introduced many products that employ lead-free solders and materials. However, with an eye on both market activities and legislative actions in Europe and Japan, many North American companies have begun lead-free development programs that include assembly evaluations and testing of commercially available and inhouse-produced lead-free solder materials in their products. While many companies are involved in investigations and development of lead-free electronics and manufacturing, other than components, few have immediate plans to introduce systems in North America. Many North American companies are concerned that lead-free components and assemblies will not meet the reliability or functionality requirements necessary for high-end equipment supplied to banks, air traffic control systems, web-based businesses, and other mission-critical applications. Accordingly, manufacturers of aerospace and military electronics have no plans to introduce lead-free solders. There are too many reliability concerns to utilize lead-free solder materials in high-reliability equipment related to the mechanical characteristics of the materials themselves and the effects of high temperatures to process them. The effect of new and modified intermetallic compound phases within solder joints and at the interfaces is yet an additional concern, and there are many more.

A.

North American Corporate Policies

Lead-free programs are being adopted by a number of companies, especially original equipment manufacturers (OEMS) involved in multinational activities. Companies such as Hewlett-Packard are expected to introduce lead-free consumer products before adopting more widespread application. Personal computer makers are addressing the introduction of lead-free systems consistent with market demand. Network communications companies such as Cisco, Lucent, and 3COM are also investigating lead-free manufacturing for future products, whose introduction will also be based on market demand. Personal communications companies such as Motorola have announced lead-free portable products. Automotive makers, who have the additional problem of dealing with harsh conditions for electronics mounted outside the firewall, have been investigating lead-free solders capable of high-temperature stability and mechanical shock. Additional companies such as Hewlett-Packard, IBM, and Sun Microsystems have substantial

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163

lead-free programs. Numerous component suppliers such as AMD, National Semiconductor, Texas Instruments, and others offer components with lead-free finishes.

B.

North American Consortia

The National Center for Manufacturing Sciences (NCMS) backed by a consortium of North American industrial corporations, national laboratories, and academic institutions conducted an evaluation of alternatives to eutectic Sn–Pb solder between 1992 and 1996. Corporate participants included ATT-Lucent, Ford Motor Company, GM-Delco Electronics, GM-Hughes Aircraft, Hamilton Standard, National Institute for Standards and Technology (NIST), Rensselear Polytechnic Institute, Rockwell International, Sandia National Laboratories, Texas Instruments, and the U.S. Navy’s Electronics Manufacturing Productivity Facility. The goal of the project was to determine whether safe, cost-effective, nontoxic, reliable lead solder alternatives could be identified. With over 79 alloys examined, the study results indicated that while a number of possible lead-free solders were functionally possible, there was no universal drop-in replacement for lead-based solders in electronics applications. This study is discussed in detail in Chap. 17. The North American Electronics Manufacturing Initiative (NEMI) is a consortium of more than 50 U.S. electronics manufacturers, suppliers, associations, government agencies, and universities. A NEMI task force, the Lead-Free Assembly Project, was formed with the goal to enable North American companies to produce lead-free products by 2001 and achieve the total elimination of lead solder by 2004. The group recommended an alloy in the Sn–Ag–Cu system as an industry standard and utilized it in their study. The NEMI lead-free roadmap called for an increase in component tolerances to 260jC based on the initial belief that the same degree of superheating (i.e., the heating above the melting point or liquidus) would be required in the case of lead-free solders as is the practice for Pb–Sn solders. It is now understood that acceptable lead-free solder joints can be made at much lower temperatures (approximately 235–240jC), so this criterion was reduced. Even with this reduction, it is probable that most components will eventually be built to be more temperature- and moisture-resistant. However, this transition may take many years. Electronics assemblers implementing lead-free processes may be faced with serious issues during this transition, issues that may continue to linger well past the time when market and legislation forces have made leadfree assembly essential or required, respectively. The High-Density Packaging User Group (HD-PUG) is a nonprofit organization of users and suppliers in the telecommunications and computer industries. HD-PUG includes members from North America, Europe, and Japan. The group, which endorsed NEMI’s recommended alloy, is involved in lead-free issues among other environmental concerns such as halogen-free electronic materials.

C.

North American Products

Lucent Technologies introduced a power system that produces SMC-SMW series d.c.-to-d.c. converter power modules using its patented SnTech pure-tin electroplating process. Components for the power device are supplied by National Semiconductor and Fairchild with a SnTech lead finish. Motorola’s first lead-free product was a two-way radio. Other lead-free products include pagers and mobile phones. Nortel Networks began its lead-free research in 1991 as part of an industry consortium with ITRI Ltd., Multicore Solders, and GE-Marconi. The company selected the desktop telephone for its lead-free product redesign because it allowed the company to test the lead-free manufacturing process on an assortment of lead shapes [22]. Prototype phones were constructed on existing manufacturing lines in 1996 and intended to be operationally equivalent to those manufactured with traditional Sn–Pb solder. When introduced in 1997, it was the first desktop telephone produced entirely with a lead-free interconnection technology. A commercially available (Multi-

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core Canada, Inc.) Sn–Cu alloy, with a melting point of 227jC, was used on the PCB of the Meridian 9316 office telephone. Nortel also employed lead-free component leads and used an organic surface protective (OSP) film on the substrate. Several factors motivated Nortel to produce the lead-free telephone, including public concern over the effect of lead on the human body and the defeated U.S. Lead Exposure Reduction Act in 1994. In addition, efforts in Europe to ban lead-containing electronics waste initiated by the Netherlands served as a driving force to develop the telephone [23]. Other contributing factors to Nortel’s lead-free work included cost issues: lead-free products would incur lower handling and disposal costs, and product end-of-life liability and disposal costs would be avoided. Visteon participates in external lead-free programs and has internal activities to develop lead-free solder materials. Visteon’s first lead-free application is for a mass airflow sensor used in vehicle engine compartments. Production started in the fall of 2001 with Sn–Ag–Cu alloy bumps fabricated by stencil printing by business partner Hitachi. The first application is a single chip with eight bumps mounted on a ceramic carrier.

VII.

FUTURE TRENDS

While an increasing number of environmentally friendly products continue to be introduced in Japan and Europe, some North American companies have taken a ‘‘wait and see’’ approach while others are pacing themselves based on market demands and supply line availability to the lead-free issue. Many Japanese companies have adopted the use of lead-free solder and lead-free components. The use of ‘‘green labels’’ helps to promote lead-free product introduction using guidelines established by the Japan Electronics and Information Technology Industries Association (JEITA). The combination of proposed legislative action, particularly in Japan and Europe, and the market appeal of environmentally friendly products has driven many leading electronics makers to begin lead-free solder implementation in their products [24]. Ultimately, the move to lead-free products will be market-driven. While the issue of lead elimination in electronic assembly remains a major topic at international conferences, alternatives are still debated, and there is no universal consensus choice. Higher processing temperatures, component compatibility, and reliability remain major concerns for lead-free alternatives. The establishment of the Global Environmental Coordination Initiative (GECI), which combines the efforts of nine consortia, is a promising development in assessing the activities of an ever-growing number of organizations generating lead-free-related information.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Asakura, H. Lead-free solder needed for the LSI industry to survive: worldwide trend is accelerating, Pb-free is a condition for participation in the LSI business. Nikkei Microdevices July 2001, 193, 56– 69. United States Environmental Protection Agency. Private Sector Pioneers. Environmentally Preferable Purchasing Program; EPA742-R-99-001, June 1999. Johnson E. Select Committee on Environment, Transport, and Regional Affairs Report; http:// www.parliament.the-stationary-office.co.uk/pa/cm199899/cmse/ect/cmenvtra/149, Dec. 1998. Global Ecolabeling Network, http://www.gen.gr.jp.eco.html. Why Label a Product with a Swan? http://www.svanen.nu/Eng/producer. United States Environmental Protection Agency. Overview of Labeling Programs Worldwide. Environmentally Preferable Purchasing; EPA742-R-98-009, Dec. 1998. GreenBiz.com, Eco-labeling, http://www.greenbiz.com/resources/marketing/tools.cfm?LinkAdvID= 7839. Nordic Ecolabeling Board. Regulations for Nordic Ecolabeling of Products; http://www.svanen.nu/ Regelverk/regelverk_eng.pdf, Dec. 2001. Get to Know the Nordic Swan, http://www.svanen.nu. Stevels A. Green Marketing of Consumer Electronics. Proc. Electronics Goes Green Conf.; Berlin, Germany, Sept. 2000; 539–542.

MARKET, PRODUCT, AND CORPORATE POLICY TRENDS 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

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Stevels A.; Agema, R.; Hoedemaker, E. Green Marketing of Consumer Electronics II. Proc. EcoDesign 2001; Tokyo, Japan, Dec. 2001. Ohkubo, H. Technology Topics: Tokyo Eco-Recycling Plant Begins Operations. Denshi Zairyo June 2001, 40 (6), 101–103. Asakura, H. Lead-free solder needed for the LSI industry to survive: worldwide trend is accelerating, Pb-free is a condition for participation in the LSI business. Nikkei Microdevices July 2001, 193, 56– 61. Asakura, H. NEC cautions about problems with Sn–Zn solders. Nikkei Microdevices September 2001, 195, 141–143. Higashi, O., et al. Lead-free Assembly Technology for High Reliability Server Computer, presented at the EIAJ Conference, Japan, December 1999. Ito, M. Fujitsu and Fujitsu lab developed low cost mounting technique with Pb-free solders. Nikkei Electronics December 13, 1999, 759, 188–189. Baggio, T.; Suetsugu, K. Guidelines for lead-free processing. Surf. Mount Technol. September 1999, 60. JIEP Technology Roadmap for Electronics Packaging in 2010, translated by Semiconductor Equipment and Materials International, 1999; 32–36. Japan Jisso Technology Roadmap, EIAJ, August 1999; 51. BradleyE. Environmental work goes on. Electron. News January 21, 2002. Grusd, A. The Future is Now: No Longer Creeping towards Lead-Free. Proceedings of IPC Works International Summit on Lead-Free Electronics Assemblies; p. S-03-11-1, Minneapolis, Minnesota, October 27, 1999. Trumble, B. Getting the lead-free telephone to market. IEEE Spectrum, May 1998; 58 pp. www.NortelNetworks.com-corporate-community-habitat-commsol-green.html., November 1999. Krishna, J., et al. Thermal and Bending Fatigue of PBGA Assemblies with Lead-Free Solder Pastes. International Symposium on Microelectronics Proceedings; October 9–11, 2001; 553–558.

6 The Metallurgical Aspects, Properties, and Applications of Solders from the Lead–Tin System Paul T. Vianco Sandia National Laboratories, Albuquerque, New Mexico, U.S.A.

I. INTRODUCTION A. History of Soft Solders Containing Lead and Tin The history of joining with the metals lead (Pb) and tin (Sn) as well as their alloys dates back several thousand years [1]. Unfortunately, archeological evidence of the use of Pb and Sn is lacking for largely two reasons. First, these elements are degraded by corrosion under long-term exposure to groundwater and soil. Therefore, actual artifacts are rare. Second, there is an absence of written records of Pb and Sn use because such practices were generally performed by slaves and the lower-class artisans. Therefore, istorians or upper-class individuals having the ability to read and write had very little interest in these materials and associated processes [1]. The first of these elements used for joining was Pb. Lead was used to join copper (Cu) by the Mesopotamians around 3000 BC because of its availability in the region as well as its low melting temperature (327jC) that was compatible with wood-fired stoves and kilns. Tin was exploited around 1900 BC by the Celtic and Gaul cultures of Northern Europe where ore deposits were plentiful. It was not long before alloys of Sn and Pb, that is, the soft solders* were used by these cultures to join Cu or Cu-alloy tools and cooking utensils. The Romans, whose empire extended into Northern Europe, learned of Sn and Pb–Sn alloys and subsequently used them to seal the Pb liners of their aqueducts. It was not until the Industrial Revolution of the 19th century did the use of soft solders extend beyond light tool and jewelry-making applications. The availability of portable heat sources that used combustible gases or electricity enabled soldering to be performed at remote sites. Soon, Pb–Sn soldering found use in the plumbing of water and gas conduit; the construction of heat exchangers; the sealing of food containers; and the assembly of light-duty tools as well as sheet metal construction [2].

* The joining together of metal objects with liquid or ‘‘filler’’ metals and alloys actually began well before the use of Pb and soft solders. The Au-based filler metals or hard solders were used to construct jewelry items by the Mesopotamians as early as 4000 BC. 167

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B. Electronic Applications The 20th century ushered in the era of electronics in which Pb–Sn solders had their most farreaching role. The first electrical connections in early radio and telecommunications equipment were mechanical attachments. The application of solder to those connections prevented them from loosening, thereby reducing static in signal transmission and reception. The low melting temperature eutectic and near-eutectic compositions, 63Sn–37Pb (wt.%), 50Sn–50Pb, and 60Sn– 40Pb were used for these applications. Whether through-hole solder joints or advanced surface mount technology (Fig. 1), soldering with Pb–Sn alloys has remained as the primary assembly methodology in the electronics industry today. The materials systems used in electronic devices, components, and subassemblies have been specifically developed for processing with one or more of the Pb–Sn alloys. The plastic molding compounds for component packaging as well as the organic polymers used in circuit board laminates have been engineered to accommodate eutectic Pb–Sn solder assembly processes. Similarly, paste formulations and flux technologies as well as furnaces, soldering irons, and cleaning processes have been optimized for Pb–Sn interconnections. The versatility of Pb–Sn solders is exemplified by their use in the low-cost, high-volume assembly processes associated with consumer electronics, as well as in low-volume, highreliability avionics and military electronics. Advancements in Pb–Sn soldering in each of these arenas required research and development activities that have elevated a ‘‘garage practice’’ to the level of a critical materials technology. An appreciation of the mechanical properties of a Pb–Sn solder, such as yield and fatigue strength, requires an understanding of the microstructure of the particular alloy. Interface interactions between the Pb–Sn solder and a base metal result in the formation of intermetallic compounds. The intermetallic compound composition as well as both liquid-state and solid-state growth kinetics are critical factors in the long-term reliability of solder interconnections.

FIG. 1 A typical surface mount soldering technology printed wiring assembly. (Courtesy of Sandia National Laboratories.)

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C. Attributes of Pb–Sn Solders It is also important to appreciate the role that Pb–Sn solders have had in the miniaturization and high-volume manufacturing of electronics assemblies. It is these two attributes that have resulted in both low-cost, consumer electronics (e.g., cell phones, personal computers, etc.) as well as highly functional, military products (‘‘smart’’ weapon systems) available today. The highly functional devices, which are the backbone of consumer or military electronics, can be rendered inoperative by the failure of a single solder joint, that solder joint being one of several hundred or thousand interconnections on a package or assembly. The technical attributes of Pb–Sn solders have been accompanied by availability and cost advantages of the Sn and Pb metals. The ores of Sn and Pb are abundant. In addition, the Sn and Pb metals are readily extracted from those ores. The fact that Pb and Sn ores are located in numerous countries around the world serves to stabilize their respective price structures. Finally, the ease of reclaiming these metals from scrap keeps their secondary market prices low as well. Despite the many attributes exhibited by Pb–Sn solders in electronics applications, these materials are being highly scrutinized, primarily because of the potential impact of their Pb content on human health and the environment. An important action taken to limit Pb exposure to the U.S. population was banning Pb–Sn solders from plumbing conduit for potable water [3]. Today, the electronics industry is addressing the technical and cost issues that would accompany the elimination of Pb–Sn solders from both commercial and military products.

II. PHYSICAL METALLURGY ASSOCIATED WITH THE LEAD–TIN SYSTEM A. Microstructures and the Binary Alloy Phase Diagram of the Pb–Sn System 1. The Phase Diagram The importance of understanding the physical metallurgy of the Pb–Sn alloy system stems from the microstructure–properties relationship. That is, the microstructure of a Pb–Sn alloy, or any material for that matter, determines its mechanical properties. In the case of Pb–Sn solders, this relationship is particularly critical because (1) the microstructure of these materials is sensitive to the cooling rate used in an assembly process, and (2) the microstructure can be readily altered during the course of the service life experienced by an interconnection. The microstructure of Pb–Sn solders can be described by the binary alloy phase diagram. The phase diagram is a two-dimensional construct that predicts phase development as a function of the temperature and material composition. An important caveat is that the phase diagram represents the material at equilibrium. Strictly speaking, equilibrium is achieved when the material has been cooled at an infinitely slow rate such that no further changes occur to the microstructure at the target temperature. Unfortunately, an equilibrium microstructure is often not realized in Pb–Sn solders because of the conditions that prevail in manufacturing processes and applications that utilize these materials. First, manufacturing processes result in cooling rates that are relatively fast. Therefore, the microstructure is far from equilibrium. Second, applications, even those at room temperature (25jC) are close to the melting temperatures of Pb–Sn alloys. As a result, there is sufficient thermal energy to support solid-state diffusion processes, which will, in turn, alter the microstructure of the solder and thus the performance of interconnections. 2. The Pb–Sn Binary Alloy Phase Diagram Shown in Fig. 2 is the binary alloy phase diagram for the Pb–Sn system [4]. This rendition is based upon weight percent quantities of Pb and Sn. A calculation of the corresponding atomic percent Sn can be performed with Eq. (1). The atomic percent of Pb is (100%  Sn at.%). Atomic percent Sn ¼ ½A=ð57:3 þ 0:427AÞ  100%

ð1Þ

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FIG. 2 The Pb–Sn binary alloy equilibrium phase diagram. (From Ref. 4.)

In Eq. (1), A is the weight percent of Sn. The abscissa in Fig. 2 is the alloy composition, beginning with 100% Pb and progressing to 100% Sn. The ordinate axis is temperature (jC). The various curved and straight lines separate areas referred to as phase fields. The nomenclature for these lines and phase fields will be illustrated by the following examples. 3. Lead–Tin System: Example 1—15Pb–85Sn The Pb–Sn phase diagram is reproduced in Fig. 3 [5]. A line has been drawn, representing the hypereutectic composition, 15Pb–85Sn. A hypereutectic alloy composition has a Sn concentration that exceeds the eutectic value of 61.8 wt.%, that is, a Sn concentration in the range of 61.8 < Sn (wt.%) < 97.5. The equilibrium phases are described as a function of decreasing temperature along that line. The line begins in the liquid phase (L) field at point A; the corresponding temperature is 275jC. The 15Pb–85Sn solder remains entirely liquid as the temperature is decreased to point B. At point B, the alloy enters the two-phase field designated L + h, where L is the remaining liquid phase and h is the newly formed, Sn-rich solid phase; the latter is referred to as the proeutectic solid phase. The temperature at point B is referred to as the liquidus temperature, which, in this example, is 210jC. The composition of the liquid phase remains that of 15Pb–85Sn. The composition of the proeutectic solid phase is represented by point BV, which is 1.5Pb–98.5Sn as determined by the projection of BV down to the x-axis. A further temperature decrease causes two phenomena to take place. First, there is an increase in the quantity of solid phase and a decrease in the quantity of liquid phase. The fluidity of the alloy is reduced and the material takes on the consistency of a paste or mush (i.e., slush); hence, this two-phase region is often referred to as the pasty range or the mushy zone. Second, there is a change in the respective compositions of the liquid and solid phases. For example, when the temperature of the alloy drops to that of point C, 195jC, the composition of the newly formed

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FIG. 3 The Pb–Sn binary alloy phase diagram reproduced to show equilibrium phase evolution of the 15Pb–85Sn composition as a function of temperature. (From Ref. 5.)

solid phase is CV and that of the remaining liquid phase is CW. The composition of the solid phase tracks along the line M–DV; this line is referred to as the solidus line. The composition of the liquid phase tracks along the line M–DW; this line is referred to as the liquidus line. At any temperature within the two-phase field, the horizontal line CV–CW connecting the liquid and solid phase compositions is referred to as the tie line. It was noted that within the two-phase field, the alloy is comprised of a mixture of solid and liquid phases. At any particular temperature, the weight fraction of solid and liquid phases can be determined by the lever rule. The weight fraction of solid phase is calculated by dividing the length C–CW by the total tie line length, CV–CW. Likewise, the weight fraction of liquid phase is computed by dividing the length C–CV by the length, CV–CW. a. Reaction at the Eutectic Temperature. The temperature is decreased further to that of point D, 183jC. The alloy is still two phased, with the solid phase having the composition of point DV (2.5Pb–97.5Sn) and the liquid phase having the composition associated with point DW, that is, 38Pb–62Sn or, the more commonly cited composition, 37Pb–63Sn. The alloy is 65% solid phase and 35% liquid phase. When the temperature is decreased to slightly less than 183jC, the remaining liquid phase solidifies and the alloy enters the two-phase field (a + h). The microstructure is a mixture of two phases, a and h. The composition of the Pb-rich a phase is determined by point Dj (19Pb–81Sn). The composition of the Sn-rich h phase remains as that of DV (2.5Pb–97.5Sn). The a and h phases are each solid solutions of Pb and Sn. Below 183jC, the a or Pb-rich phase exhibits a greater solubility for Sn than is the solubility of Pb in the Sn-rich h phase. b. Cooling Below the Eutectic Temperature. Further cooling of the 15Pb–85Sn composition results in relatively small changes to the compositions of the a and h phases. At point E (125jC), the compositions of the a and h phases correspond to points EW and EV, respectively. At

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room temperature (25jC), the phase diagram indicates that a phase is 99Pb–1Sn and the h phase is effectively 100Sn. c. Microstructure Depends on Cooling Rate. An optical micrograph of the 15Pb–85Sn solder is shown in Fig. 4. This particular sample was fabricated at a cooling rate of 10jC/min. A homogeneous, cellular microstructure had developed. The light regions are the Sn-rich h phase and the dark areas outlining the Sn-rich phase are the Pb-rich a phase. Faster cooling rates can produce proeutectic Sn-rich dendrites dispersed within the microstructure [6]. The microstructure depends upon cooling rate. Extremely fast cooling rates may not allow the Pb and Sn atoms to rearrange themselves to allow precipitation of the solid proeutectic, Snrich phase (point BV). The alloy remains entirely liquid for several degrees below the liquidus temperature. This phenomenon is referred to as undercooling. The faster the cooling rate, the greater is the difference between the liquidus temperature and the temperature at which solidification initiates and thus, the greater the deviation of the microstructure away from equilibrium. The nonequilibrium microstructure is retained after solidification because solid-state, mass transport (diffusion) processes are not sufficiently fast to rearrange the elemental atoms into an equilibrium configuration. The characteristics of the nonequilibrium, Pb–Sn solder microstructure are threefold. First, the compositions of the phases are not accurately represented by the phase diagram boundary lines. Second, the relative quantities of the phases are not accurately described by the lever rule. Third, the spatial distribution of phases in the microstructure are sensitive to the cooling rate. For example, in terms of individual phase compositions, excessive Sn may be retained in the Pb-rich a phase, causing a supersaturation condition. In effect, the phase boundary line between the a single-phase field and the (a + h) two-phase field is shifted to the right. (A more detailed discussion of approximating a nonequilibrium microstructure from the equilibrium phase diagram can be found in Refs. 5 and 7.) A consequence to the solder microstructure caused by a supersaturated Pb-rich phase is the precipitation of Sn particles in the Pb-rich phase. The above discussion targeted the 15Pb–85Sn composition. A similar analysis would be applicable to the hypoeutectic compositions that have Sn contents less than the eutectic value of 61.8 wt.%, that is, Sn concentrations in the range of 19 < Sn (wt.%) < 61.8. In the latter case, a Pb-rich (a), proeutectic phase forms as hypoeutectic Pb–Sn compositions are cooled through the pasty range (L + a) as would occur, for example, between the points F and G for the composition 70Pb–30Sn in Fig. 3.

FIG. 4 Optical micrograph of the 15Pb–85Sn solder microstructure. The cooling rate used to produce this microstructure was 10jC/min. (Courtesy of Sandia National Laboratories.)

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4. Lead–Tin System: Example 2—38.1Pb–61.9Sn (Eutectic Composition) Referring to Fig. 3, the 38.1Pb–61.9Sn alloy is the eutectic composition. However, more commonly, the eutectic composition is identified as 37Pb–63Sn as it is done in this handbook. Solidification of the eutectic composition occurs at a single temperature as the alloy cools from the liquid phase (L) to the (a + h) two-phase solid. The pasty ranges, (L + a) and (L + h), are avoided during solidification. The as-cast (i.e., as-solidified) microstructure of the Pb–Sn eutectic composition depends strongly upon the cooling rate [6]. A lamellar microstructure develops, as is illustrated in Fig. 5. A slower cooling rate produces dispersed Pb-rich phase particles in a Sn-rich phase matrix. The micrograph shown in Fig. 6 very effectively illustrates the latter microstructure, albeit it was produced from a sample isothermally aged at 70jC for 400 days. In general, time-independent strength properties (i.e., the stress/strain curve) increase as the interlamellae or interparticle spacing decreases. However, time-dependent strength behavior (creep) exhibits the reverse dependence. Because typical applied stresses and service temperatures can activate both timeindependent and time-dependent deformation mechanisms in eutectic Pb–Sn solder, the impact of the microstructure on solder joint performance cannot always be easily predicted. 5. Lead–Tin System: Example 3—95Pb–5Sn The third and final alloy used to illustrate the features of the Pb–Sn binary system is the 95Pb–5Sn composition. The composition line is shown in Fig. 7 [5]. The unique property of this composition, which represents all alloys having a Pb content that exceeds 81 wt.%, is that it solidifies without a eutectic reaction. a. Solidification Without a Eutectic Reaction. At point Z (350jC), the alloy is entirely liquid. At the liquidus temperature designated by point Y (315jC), solidification begins as the alloy enters the two-phase pasty range (L + a). The solid phase has the composition represented by the point YV, 97.5Pb–2.5Sn. The alloy remains in the (L + a) until the temperature has dropped to just above point X, 307jC. Then, the solid phase has the composition 95Pb–5Sn and the liquid phase has a composition 92Pb–8Sn. When the temperature drops below 307jC, the alloy solidifies completely and enters the (Pb) solid solution field. For temperatures between points X (307jC) and W (95jC), the solder is a single-phase solid solution material having the 95Sn–5Pb composition.

FIG. 5 Optical micrograph showing the lamellar microstructure in the 37Pb–63Sn solder resulting from a cooling rate of 10jC/min. (Courtesy of Sandia National Laboratories.)

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FIG. 6 Optical micrograph of the 37Pb–63Sn solder microstructure following isothermal aging at 70jC for 400 days. This micrograph is for illustrative purposes only, as it represents the solder microstructure that would be observed with slower cooling rates. (Courtesy of Sandia National Laboratories.)

FIG. 7 The Pb–Sn binary alloy equilibrium phase diagram reproduced to shown the 95Pb–5Sn composition profile as a function of temperature. (From Ref. 5.)

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b. Solid-State Solubility Adjustment. Decreasing the temperature below point W (95jC) results in the alloy entering the solid, two-phase (a + h) field. Phase separation occurs as the single-phase, solid-solution microstructure separates into the two-phase microstructure via solidstate diffusion processes. The a phase has the same composition as the starting alloy, 95Pb–5Sn (W). The newly appearing h phase will have the composition represented by the projection of point WV on the x-axis, 2Pb–98Sn. The solder microstructure remains in the (a + h) field upon further cooling to room temperature. The high Pb, a phase composition follows the line between the (Pb) and (a + h) fields, while the h phase composition follows that of the line between the h phase (Sn) and the (a + h) fields. The quantity of phases present follows the lever arm rule. Shown in Fig. 8 is the microstructure of the 95Pb–5Sn solder resulting from a cooling rate of 10jC/min. This cooling rate is sufficiently slow such that the resulting microstructure is a reasonable representation of equilibrium. The Sn-rich h phase has precipitated along the grain boundaries of the more prevalent, Pb-rich a phase. Under a very rapid cooling rate, a single Pbrich phase may be observed that is supersaturated with Sn. Subsequent to solidification, the h phase would form as Sn precipitates in the Pb-rich a phase; the rate of Sn precipitation and thus, h phase formation, would depend upon time and service temperature.

B. Physical Metallurgy of Intermetallic Compound Formation with the Pb–Sn System 1. Intermetallic Compounds as Interface Reaction Products The basis of filler metal joining, of which soldering is one of the two primary techniques (the other being brazing), is that the low-melting-temperature filler metal bonds to the base (or substrate) metal. Adhesion is provided by a reaction at the mutual interface, involving some or all of the constituent elements from either the solder filler metal or the base metal. Those reactions can take place during the actual soldering process and may continue afterward when the (solidified) interconnection is placed into service. The interface reaction product formed between Pb–Sn solders and most base metals is commonly referred to as an intermetallic compound layer. Intermetallic compounds exhibit highly directional bonding similar to ceramic materials and, as such, typically have a well-defined stoichiometry, high melting temperatures, high strength, and very low ductility. Unfortunately, the binary alloy phase diagrams provide only an approximate indication of intermetallic com-

FIG. 8 Optical micrograph of the 95Pb–5Sn solder microstructure. The cooling rate was 10jC/min. (Courtesy of Sandia National Laboratories.)

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pound compositions that may develop at the interface. The uncertainty arises due to underlying assumptions of the phase diagram—the constituents are fully intermixed; the constituents are of finite quantities; and the system is at equilibrium—do not prevail in solder/base metal couples. In fact, some solder/base metal couples form metastable phases at their mutual interface during solid-state (aging) reactions [8–10]. The metastable phases can appear and subsequently disappear during the development of the intermetallic compound layer.*

C. Tin as the Reactive Constituent Of the two elements comprising Pb–Sn solders, only the Sn constituent reacts directly with the base metal. The most commonly used base metal is copper (Cu). Two intermetallic compound phases are observed: the DV or Cu6Sn5 phase and the q or Cu3Sn phase [11]. The Cu6Sn5 phase is favored when the high-Sn, Pb–Sn solders (including the 37Pb–63Sn eutectic) are joined to copper [12]. A nominal layer thickness of 0.5–1.5 Am is observed in as-soldered joints [13]. Exposing the Cu base metal to molten Pb–Sn solder for several minutes at temperatures as high as 292jC— clearly, conditions that are well in excess of any automated or manual soldering process—causes only a very slight increase of the intermetallic compound layer thickness [12]. When the Sn content of the solder is reduced to approximately 27 wt.%, the intermetallic compound layer composition favors formation of the Cu-rich, Cu3Sn stoichiometry. The Cu3Sn layer actually develops from an initial Cu6Sn5 sublayer.

D. Liquid Solder Interface Reactions In the presence of molten solder, the interface reactions include the simultaneous process of base metal dissolution or base metal erosion as well as intermetallic compound layer formation. The extent of dissolution depends upon the composition of the base metal as demonstrated in Fig. 9 for the case of molten 40Pb–60Sn solder and the base metals Au, Ag, Pd, Pt, Ni, and Cu [14]. The dissolution rate of Cu, specifically as a function of temperature for several molten Pb–Sn compositions is shown in Fig. 10 [15]. A lower Sn content reduces the extent of base metal dissolution. Dissolved base metal can potentially saturate the nearby molten Pb–Sn solder field. Upon solidification, the base metal, M, precipitates out as MxSny intermetallic compound particles located near to, and at, the interface. For example, dissolved Cu will precipitate as Cu6Sn5 particles, forming a zone of such particles adjacent to a contiguous intermetallic layer. This morphology is shown in Fig. 11. Optical micrographs were taken of the 37Pb–63Sn solder/Cu interface after the base metal had been exposed to molten solder (215jC) for 5, 60, 300, and 900 sec.

E. Solid-State Interface Reactions Intermetallic compound layer growth can continue after solidification of the joint. The Sn component reacts with the base metal. The Pb component is rejected, forming a Pb-rich layer between the intermetallic compound layer and the solder field (see Fig. 12). The reaction kinetics have been studied by isothermal aging experiments. The aging of 37Pb–63Sn/Cu couples results in the Cu6Sn5 layer composition adjoining the solder field [16–18]. The Cu3Sn phase appears as a sublayer at the Cu6Sn5/Cu interface after prolonged time periods and/or higher aging temperatures. The graph in Fig. 13 shows the effect of aging time and temperature on the total intermetallic compound layer thickness for the 37Pb–63Sn/Cu couples [16]. The Cu3Sn sublayer

* The binary alloy phase diagram can provide some insight into potential intermetallic compound compositions [5]. For example, only single-phase fields in the phase diagram can form at the solder/base metal interface. Dual- or higher-order phase fields would not appear, as they cannot develop the concentration gradients required to support diffusion; rather, the latter would exist solely as interfaces.

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FIG. 9 Dissolution rates of Au, Ag, Cu, Pd, Pt, Ni, and Cu in molten 40Pb–60Sn solder as a function of solder pot temperature. (From Ref. 14.)

FIG. 10 Dissolution rates of Cu wire as a function of molten solder temperature for various Pb–Sn compositions. (From Ref. 15.)

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FIG. 11 Optical micrographs of the interface region generated by the exposure of Cu base metal to molten 37Pb–63Sn solder (215jC) for (a) 5 sec, (b) 60 sec, (c) 300 sec, and (d) 900 sec. (Courtesy of Sandia National Laboratories.)

appeared only under aging temperatures of 135j and 170jC. The reaction kinetics of the 37Pb– 63Sn/Cu system are described by Eqs. (2) (3) through (4): Cu3 Sn : x ¼ xo þ 1:82  103 t0:58 expð56; 100=RTÞ Cu6 Sn5 : x ¼ xo þ 3:87  103 t0:31 expð40; 400=RTÞ

xo ¼ 0 m xo ¼ 1:26  106 m

Cu3 Sn þ Cu6 Sn5 : x ¼ xo þ 1:08  102 t0:36 expð45; 500=RTÞ xo ¼ 1:26  106 m

ð2Þ ð3Þ ð4Þ

where x is the layer thickness (m), xo is the initial layer thickness (m), t is time (sec), R is the universal gas constant (8.314 J/mol K), and T is the temperature (K).

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FIG. 11 Continued.

The Cu3Sn intermetallic compound (sub)layer composition is favored under conditions in which Sn availability is limited. A reduced Sn supply occurs when there is only a very thin Pb–Sn solder layer (e.g., an electroplated Pb–Sn coating) present over the Cu base metal, or when the solder composition has a low Sn content. For example, use of high-Pb solders on Cu, such as the 95Pb–5Sn composition, favors the formation of the Cu3Sn composition [19].

F. Interface Reactions with Nickel and Iron Lead–tin solder joints are commonly made to nickel (Ni), the Ni being either a bulk material or a thin solderable finish. The reaction results in the formation of primarily Ni3Sn4 intermetallic compound. The Pb component of the solder has no explicit role in the interface reaction. The reactivity between the Sn component of the Pb–Sn solder and Ni is considerably slower compared to Cu for the liquid-state dissolution reaction (see Fig. 9) as well as the

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FIG. 12 Optical micrograph of the interface between 37Pb–63Sn solder and copper (Cu) showing the intermetallic compound layer that developed after solid-state aging at 135jC for 400 days. (Courtesy of Sandia National Laboratories.)

solid-state interface reactions [20,21]. The low reactivity between Pb–Sn solder and Ni allows the latter to be an effective thin-film barrier layer between the solder and other underlying base metal materials. A very low reactivity has also been observed between Pb–Sn solders and iron (Fe). The observed intermetallic compound layer has the composition FeSn2 [22]. Base metal dissolution data are not available for molten Pb–Sn solders on Fe. Solid-state intermetallic compound layer growth at the 37Pb–63Sn/Fe interface is negligible (< 0.3 Am) [23]. The aging of 37Pb–63Sn/Fe couples at 170jC for 100 days produced a layer thickness of 0.86 F 0.57 Am compared to 17 Am for 37Pb–63Sn/Cu couples exposed to the same conditions.

FIG. 13 Total intermetallic compound layer (Cu6Sn5 + Cu3Sn) thickness as a function of aging time and temperature for 37Pb–63Sn/Cu couples. (From Ref. 16.)

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Interface Reactions with Gold, Silver, and Palladium

An important interface reaction is that which occurs between Pb–Sn solders and gold (Au). Gold is rapidly dissolved into Pb–Sn solders (see Fig. 9). Fast solid-state interface reaction rates between Pb–Sn solder and Au can result in the rapid growth of an intermetallic compound layer; even aging at room temperature. Layer development can degrade solder joint strength, particularly because of the poor adhesion between the Au–Sn intermetallic compound layer and the remaining Au layer (Fig. 14). The intermetallic compound layer compositions that develop between Pb–Sn solder and Au as a consequence of solid-state reactions are AuSn2 and AuSn4 [24]. The AuSn2 appears after relatively short-term aging and is located between the Au-based metal and AuSn4 layer that adjoins the solder field (Fig. 15a). Further solid-state aging causes the AuSn2 layer to disappear in favor of the AuSn4 composition (Fig. 15b). The ternary phase diagram has been constructed for the Au–Pb–Sn system [25,26]. The reaction kinetics for solid-state growth of the total intermetallic compound layer (AuSn2 + AuSn4) in 37Pb–63Sn solder/Au couples is described by the following equation [24]: x ¼ xo þ 3:87  103 t0:31 expð40; 400=RTÞ

xo ¼ 1:26  106 m

ð5Þ

where x is the total layer thickness (m), xo is the initial layer thickness (m), t is aging time (sec), R is the universal gas constant (8.314 J/mol K), and T is the aging temperature (K). The Pb component of the solder, which does not participate in the intermetallic compound reaction, accumulates at the interface between the intermetallic compound layer and the solder field. Two other base metals utilized as finishes in electronics assemblies to form Pb–Sn solder joints are silver (Ag) and palladium (Pd). The predominant intermetallic compound compositions that develop for these base metals are Ag3Sn and PdSn4, respectively [22]. The relative dissolution rates of these base metals in 40Pb–60Sn solder are shown in Fig. 9. There have been no long-term aging studies performed on Pb–Sn/Ag or Pb–Sn/Pd couples from which to predict the solid-state intermetallic compound layer morphology or growth kinetics.

FIG. 14 Optical micrograph showing a crack in a 37Pb–63Sn solder joint made to a Au coating. The crack formed at the interface between the Au–Sn intermetallic compound and the remaining Au layer. (Courtesy of Sandia National Laboratories.)

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FIG. 15 SEM micrographs showing development of the intermetallic compound layer between 37Pb–63Sn and Au base metal following solid-state aging for 90 days at (a) 70jC and (b) 100jC. (Courtesy of Sandia National Laboratories.)

H. Intermetallic Compounds in the Bulk Solder The inadvertent presence of intermetallic compound particles in Pb–Sn solder is most often a result of excessive base metal dissolution into molten solder. In most cases, the base metal is a single element such as Cu, Ni, or Au. In other applications, the base metal may be an alloy of several elements, in which case, some of the elements may exhibit preferential dissolution into the solder [27]. Intermetallic compound particles (e.g., Cu6Sn5, AuSn4, Ni3Sn4, PdSn4, etc.) precipitate upon solidification, when the base metal concentration exceeds its solid solubility limit in either the Pb-rich or Sn-rich phases. The solubility limits are nearly zero for most Sn/base metal combinations as shown in Table 1 [28].

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TABLE 1 Solubility Limits of Common Base Metals in Tin Metal Au Bi Cu Ni Pb

Pd Pt

Temperature (jC)

Solubility limit (wt.%)

0–100 100 50 0–100 0–100 100 50 0 0–100 0–100

f0 10 4 f0 f0 1.5 0.7 f0 f0 f0

Intermetallic compound particles may not develop upon solidification when the solder is cooled at a rapid rate. The solder becomes supersaturated with the base metal element(s). However, the precipitation reaction may occur later on as a thermally activated process, during the service life of an interconnection. Finally, some contaminants simply do not react with Sn. For example, Bi will precipitate as simply Bi particles when it exceeds the solubility limit in Sn (generally 3–5 wt.%) or Pb (0 wt.%).

I.

Effects of Intermetallic Compound Formation on the Mechanical Properties of Pb–Sn Solder Joints

The impact of intermetallic compound layers on the mechanical properties of Pb–Sn solder joints can be described in general guidelines only. The absence of more specific rules is a result of the mechanical and material complexity of a solder joint and, more specifically, the high strain sensitivity of Pb–Sn solders. The mechanical complexity of a solder joint stems from the interactions between gap size, solder joint area, and applied load (tensile, shear, or mixed mode). A thinner gap causes a joint to appear stronger under tensile stress than would be calculated from the bulk strength properties of the solder. The higher joint strength arises from the plane strain constraint imposed on solder deformation by the joint area. In addition, as the intermetallic compound layer occupies a larger percentage of the thickness of smaller joints, its properties have a greater influence on those of the overall interconnection. The material complexity of solder joints arises from the presence of the base metal(s), the intermetallic compound layers, and the ‘‘bulk’’ solder [5]. Each one of those materials has different mechanical properties (e.g., yield strength, ductility, ultimate strength, etc.). In addition, Pb–Sn materials have strength properties that are very strain rate sensitive. The yield strength and ultimate tensile strength values increase dramatically with faster strain rates [29]. Also, there are two interfaces associated with each intermetallic compound layer—the base metal/intermetallic compound interface and the solder/intermetallic compound interface—which can exhibit significantly different adhesion properties. The incorporation of intermetallic compound particles into a solder can cause the strength of the solder to increase via the precipitation hardening phenomenon. The degree of increase depends upon the concentration of those particles. Concurrent with the strength increase is a loss of ductility. However, an excess of intermetallic compound particles will degrade the solder and, hence, solder joint mechanical properties. The Pb–Sn solders and specifically, the 37Pb–63Sn and 40Pb–60Sn alloys, are embrittled when the Au content exceeds approximately 4 to 5 wt.%. A high concentration of AuSn4 particles results in an easy path for crack propagation [30–32]. This point is illustrated in Fig. 16.

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FIG. 16 Optical micrograph showing crack propagation in a 37Pb–63Sn solder joint containing an excess of AuSn4 intermetallic compound particles. (Courtesy of Sandia National Laboratories.)

J. Guidelines In summary, the overall impact of the intermetallic compound layer on the strength of the solder joint is dependent upon the factors and effects listed in Table 2. Given the factors/effects in Table 2, the following guidelines have been developed: Failures will occur preferentially in the Pb–Sn solder under slow strain rate load conditions. As such, an intermetallic compound layer has minimal impact on failures that occur due to timedependent deformation (creep) and low-cycle fatigue (e.g., thermal mechanical fatigue). The failure path occurs in the solder, near to one of the intermetallic compound layers. Cracking in the solder is preempted when one of the interfaces associated with the intermetallic compound layer is inherently weak, such as in the case of the AuSn4/Au structure (Fig. 14).

TABLE 2 Factors that Affect the Role of the Intermetallic Compound Layer on the Strength of Solder Joints Factor Solder intrinsic strength Solder strain rate sensitivity Intermetallic compound material properties Gap size of the solder joint

Rate of applied load (stress)

Effect Solder strength equals or exceeds the strength of the interface structures. Faster strain rate increases the apparent strength of the solder [see (1) above]. Brittle intermetallic compound and/or poor adhesion with the solder and/or base metal. A thinner gap causes an increase in the apparent strength of the solder [see (1) above]. Strain rate sensitive solders will exhibit an apparent increase in strength at faster strain rates [see (1) above].

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As solder strength increases, there is an increased likelihood for failure to occur in an intermetallic compound layer. A study of Mode I failures in several solders joints containing the Cu–Sn intermetallic compound layer resulted in the following trend [33]: Intermetallic compound layer failure was preferred when the local displacement rate was greater than or equal to approximately 2  103 mm/sec and the apparent yield strength of the solder was in excess of 20 MPa. The likelihood of failure in the intermetallic compound layer increases with the inherent brittleness and/or thickness of the layer. For example, the Ni3Sn4 intermetallic compound tends to be more brittle than the Cu6Sn5 compound [34]. A thicker layer increases the likelihood that intrinsic flaws will initiate cracking in the brittle intermetallic compound material.

III. PB–SN SOLDERING PROCESSES A. Liquid Solder Wettability Wettability is the capacity of the molten solder to react with the base metal at their mutual interface and establish a bond there. The wettability of liquid solders is largely an intrinsic property between the specific solder composition and the base metal composition. On the other hand, spontaneous spreading is impacted by several parameters: base metal surface condition, wettability (solder/base metal interface reaction), and the flux. The synergistic roles of the molten solder, base metal, and flux on the combined properties of wetting and spreading, that is, the alloy’s solderability, can be described by Young’s equation: cSF  cSL ¼ cLF cos hc

ð6Þ

where cSF is the solid (base metal)–flux interfacial tension; cSL is the solid (base metal)–liquid (molten solder) interfacial tension; cLF is the liquid (molten solder)–flux interfacial tension; and hc is the contact angle. Young’s equation represents the equilibrium balance of the interfacial tension parameters for both horizontal and vertical surfaces, as shown schematically in Fig. 17 [5]. The contact angle provides a generalized parameter for solderability, as it takes into account the effects of all of the interfacial tensions. As such, solderability is optimized when the contact angle, hc, is minimized. The contact angle is minimized by maximizing cSF; this is accomplished by the removal of contaminants and the oxide layer from the base metal surface. The value of hc can also be minimized by a small value of cSL. The value of cSL is determined by the solder–base metal interface reaction and, thus, can be altered by using a different base metal or placing a metal coating of alternative composition over the base metal surface. Variations among the Pb–Sn solder compositions will have little impact on cSL since the same Sn-based intermetallic compound will be formed at the interface. Finally, the contact angle can also be minimized by minimizing the

FIG. 17 Schematic diagrams of solder spreading on horizontal and vertical open surface configurations. The interfacial tensions balance one another, determining the contact angle as defined by Young’s equation. (From Ref. 5.)

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solder–flux interfacial tension, cLF. The value of cLF is dependent on the compositions of both the flux and the solder.

B. Solderability Performance of Pb–Sn Alloys The excellent solderability of Pb–Sn solders is a result of their good wetting and spreading properties [5]. Wettability is the capacity of a molten solder to react with the base metal at their mutual interface and establish a bond. In the Pb–Sn system, the interface reaction is typically between Sn and the base metal, in the form of an intermetallic compound layer. Spontaneous spreading of molten solder over a horizontal surface ends with the sessile drop profile. On the other hand, spontaneous spreading on a vertical surface results in a meniscus, which then solidifies into a solder fillet. Good spreadability allows molten Pb–Sn solders to cover a base metal surface without necessarily having to be placed over that entire surface. Lead–tin solders also exhibit good solderability in closed solder joint geometries such as gaps and holes. The spontaneous filling of through-hole solder joints is assisted by capillary flow. A contributing factor toward the excellent solderability of Pb–Sn alloys is the low surface tension of molten Pb–Sn solders, which is a direct result of the Pb constituent in the solder. The surface tension of molten Pb is 450 dynes/cm (at Tmelt = 326jC) and that of molten Sn is 550 dynes/cm (at Tmelt = 232jC) [35].* Listed in Table 3 are the solderability parameters, hc, cLF, and (cSF  cSL) of the 40Pb–60Sn and 96.5Sn–3.5Ag solders for the same test conditions [37]. The values of these parameters were determined by the meniscometer wetting balance technique [38]. A lower contact angle is exhibited by the Pb–Sn solder in Table 3, when compared to corresponding 96.5Sn–3.5Ag solder data, a result that is directly attributed to the reduced value of cLF of the Pb–Sn alloy. Also included are solderability parameters for these solders on Au–Ni-plated Kovark.y The further reduction in the contact angle was a direct result of the reduction in cLF which, in turn, resulted from the dissolved Au preferentially locating at the solder–flux interface. The low surface tension (and low cLF interfacial tension) of Pb–Sn alloys also contributes to excellent capillary flow. Molten Pb–Sn solders can readily enter small clearances associated with through-hole solder interconnections and other configurations.z This behavior was documented in a series of capillary flow experiments carried out between parallel Cu plates, using the specimen shown in Fig. 18a; the superior meniscus (capillary) rise of Pb–Sn solders as a function of gap thickness, when compared to several Pb-free solders, is shown in Fig. 18b [5,39].

C.

Inadequate Wetting Conditions

Different conditions of Pb–Sn solderability are illustrated by the stereo photographs in Fig. 19. Adequate solderability results in a solder film that uniformly covers the base metal surface (Fig. 19a). Inadequate solderability can result in two defective conditions: nonwetting (Fig. 19b) or dewetting (Fig. 19c). The condition of nonwetting occurs when the solder simply does not wet and spread over the base metal surface. The solder ‘‘balls up,’’ leaving exposed base metal surface. The second defect, dewetting, is actually a two-step process. Initially, the relatively thick solder readily wets and spreads over a marginally solderable, base metal surface. The film thickness actually supports that initial wetting and spreading action. Then, as the solder film thins out

* Similarly, bismuth (Bi) has a relatively low surface tension, a property that is imparted into alloys with this constituent: 58Bi–42Sn and 91.84Sn–3.33Ag–4.83Bi, thereby improving the solderability of the high-Sn, hightemperature alloys [36]. y Kovar is a trademark of Carpenter Technologies, Inc., Reading, PA. z It is interesting to note that the often-cited equation for capillary rise shows a direct proportion between meniscus rise and interfacial tension, which is opposite to the observed trend. This discrepancy arises from an additional effect caused by the relatively high surface tensions of molten solders and the relatively small gap sizes of solder joints. The consequence is a constraint on the minimum contact angle that can be realized by the molten solder: a high cLF value causes a high hc value, which reduces solderability.

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TABLE 3 Solderability Parameters hc, cLF, and (cSF  cSL) for 40Pb–60Sn and 96.5Sn–3.5Ag Solders on Cu- and Au–Ni-plated Kovar Base metal Cu Au–Ni plated Kovar Cu Au–Ni plated Kovar

Solder alloy (wt.%)

Temperature (jC)

Contact angle (jC)

cLF (dyne/cm)

(cSF  cSL)

40Pb–60Sn 40Pb–60Sn

260 260

17 F 4 11 F 9

380 F 10 330 F 15

370 F 10 325 F 25

96.5Pb–3.5Sn 96.5Pb–3.5Sn

260 260

36 F 3 34 F 4

460 F 30 370 F 30

380 F 40 310 F 40

Source: Ref. 5.

FIG. 18 The parallel plate capillary rise test geometry (a) used to determine the meniscus rise, (b) as a function of gap size for the 37Pb–63Sn, 95Sn–5Sb, 96.5Sn–3.5Ag, 91.84Sn–3.33Ag–4.83Bi, and 95.5Sn–4.0Cu–0.5Ag solders. (From Ref. 5.)

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FIG. 19 Stereo photographs showing the cases of (a) wetting, (b) nonwetting, and (c) dewetting of a 40Pb–60Sn solder film on an Fe–Ni alloy. (Courtesy of Sandia National Laboratories.)

toward completion of the joint, it can no longer support coverage of a marginally solderable, base metal surface and retracts into the localized mounds of solder separated by areas of a thinly coated, base metal surface.

D.

Interfacial Reactions—Coatings on Base Metals

Some applications require the Pb–Sn solder to be used on a base metal that is not readily solderable. In order to provide a processable level of solderability for difficult-to-solder materials, metal coatings are applied to the base metal surface. 1. Protective Finishes One approach uses a single coating—the protective finish. As the terminology implies, the protective finish prevents the loss of base metal solderability due to rapid reoxidation prior to soldering. The role of the protective finish in a soldering process is illustrated in Fig. 20 [5]. The protective finish is initially wetted; therefore, it must be solderable. Then, the coating is dissolved into the molten solder. The solder proceeds to wet and spread over the underlying (pristine) base

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FIG. 20 Schematic diagram showing the processes associated with molten solder spreading over a protective finish. (From Ref. 5.)

metal surface. The protective finish must be sufficiently thick to protect the base metal solderability, but not too thick so as to pose a significant contamination concern for the Pb–Sn solder joint. Common protective finishes include the Pb–Sn alloys (37Pb–63Sn or 95Pb–5Sn) as well as 100Sn. Some ‘‘Sn coatings’’ actually contain a small Pb addition to prevent the formation of tin whiskers, which are of concern with 100Sn electroplated layers. Lead–tin or Sn coatings along with recommended thicknesses are listed in Table 4 [40]. The benefit of fusing, that is, melting a Pb–Sn or Sn electroplated layer, is that the resolidified layer provides a more effective barrier against contamination and reoxidation. The preference for hot-solder dipped coatings is decreasing for microelectronics assemblies due to the variation in film thickness due to (molten solder) surface tension effects and the local geometry (flat surface vs. a corner). The lack of planarity is a concern for fine-pitch applications. However, an attribute of hot-solder dipped coatings is that they provide an on-the-spot assessment of the base metal solderability prior to final assembly. 2. Solderable Finishes In those cases in which it is impractical to sustain base metal solderability, a metallic coating is applied to the base metal. The molten solder wets and spreads over the coating and, ultimately, makes a bond to it. This coating is referred to as the solderable layer. Typically, a protective finish

TABLE 4 Commonly Used Pb–Sn and Sn Protective Finishes Composition Electroplated Pb–Sn (37Pb–63Sn or 40Pb–60Sn) Electroplated Sn (100Sn) Electroplated and fused Pb–Sn Electroplated and fused Sn Electroplated Pb–Sn and Sn Source: Ref. 40.

Thickness (Am) 7.6–23 7.6–13 2.5–13 2.5–13 >51

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FIG. 21 Schematic diagram depicting the processes associated with molten solder spreading over a solderable finish/protective finish combination. (From Ref. 5.)

is added on top of the solderable layer to protect the solderability of the latter after its deposition, but before the soldering process. The functional limitations placed on solderable finish/protective finish systems are illustrated in Fig. 21 [5]. As noted above, spreading of the solder front requires that the surface of the protective finish be solderable (free of contamination or oxidation). The protective finish is sacrificial as it is dissolved into the solder. Therefore, the protective finish must be sufficiently thick to protect the solderability of the underlying solderable layer, but not excessively thick to pose a significant contamination concern for the subsequent solder joint (e.g., Au in Pb–Sn solder). The solder then wets to the surface of the solderable layer. The requirements placed on the solderable layer are the following: it must be solderable; it must have adequate thickness so as not to be completely dissolved by molten solder during the initial assembly process or during subsequent process steps (including repair or rework cycles); and the layer must adhere to the base metal surface. Several solderable and protective finishes along with their thickness as specified by MILSTD 1276D are listed in Table 5 [5,40]. One of the most commonly used solderable-plusprotective layer combinations is Ni and Au. The likelihood of Au embrittling Pb–Sn solder joints is reduced by a hot-solder dipping operation that replaces the Au with a Pb–Sn finish [41,42]. Copper is a frequently used solderable coating; it is often coated with a Pb–Sn or Sn protective finish (electroplated, plated and fused, or hot-solder dipped). Other protective layers include

TABLE 5 Commonly Used Solderable Finish/Protective Finish Combinations Solderable layer (Am)

Protective layer (Am)

Ni (1.5–3.8)

Au (1.3–2.5)

Ag (3.8–8.9)

None or hot solder dipped None or hot solder dipped

Ni (1.5–3.8)

Ni–Pd (1.3–3.5) Source: Refs. 5 and 40.

None or hot solder dipped

Types of base metals Cu, Cu alloys, Zn, Al, Fe–Ni alloys, steels Ni and Ni alloys, stainless steels Ni and Ni alloys, Cu, Cu alloys, Al and Al alloys* Stainless steels

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electroplated Ag and Pd–Ni alloy. The solderabilities of both the Ag and Pd–Ni coatings can degrade due to tarnishing (formation of Ag2S) and oxidation (PdO), respectively.

E.

Flux Technology and Controlled Atmospheres

1. Function of the Flux The flux provides three functions in a soldering process: (1) removal of light oxide layers from the base metal (or coating) surface; (2) prevention of base metal reoxidation and excessive oxidation of the molten solder surface; and (3) reduction of the surface tension of the molten solder. Current flux technology and associated solder paste materials have been developed almost exclusively for eutectic, near-eutectic, and high-Pb, Pb–Sn solders. The coupling of flux technology to Pb–Sn solders grew in response to the need for high-volume, low-defect soldering processes for the electronics industry. Those processes include furnace reflow, wave soldering, and hand soldering. The three flux functions must be accomplished within narrowly defined process cycles. 2. Flux Categories Soldering processes associated with Pb–Sn solders have been performed with fluxes from each of three categories, the (natural) rosin-based fluxes, resin-based fluxes, and the organic acid (watersoluble) fluxes.* a. Natural Rosin-Based Fluxes. The natural rosin-based fluxes have been the primary chemistries used with the 37Pb–63Sn eutectic and 40Pb–60Sn off-eutectic compositions. These fluxes thermally degrade at temperatures in excess of approximately 275jC, thereby limiting their use to the eutectic and near-eutectic Pb–Sn alloys [43]. There are three categories of rosin-based fluxes distinguished by activity level [44]. The accepted classification framework for fluxes utilizes an alphanumeric code described in the ANSI/J-STD-004 specification [45]. (1) R (‘‘rosin’’) fluxes are the least active, containing pure water-white rosin with a solids content in the range of 10% to 40%, depending upon the desired strength level. (2) RMA (‘‘rosin-based, mildly activated’’) fluxes are moderately activated with 0.1% to 0.3% activator and contain a 10% to 40% solids content. (3) RA (‘‘rosin, activated’’) fluxes have 1.0% to 1.5% activator to maximize corrosion activity and are composed of 10% to 40% solids. Two types of rosin-based fluxes were developed for Pb–Sn solders to eliminate the use of solvents for removing residues. Those flux types are the no-clean fluxes and the low-solid (residue) fluxes. The no-clean fluxes have solids contents typical of the rosin-based flux groups (35% to 45%). However, they have been engineered so that when exposed to Pb–Sn soldering temperatures (typically 215j to 260jC), the organic molecules polymerize and harden, effectively sealing the activators from outside moisture to render the residues noncorrosive. A processing advantage of no-clean fluxes is that they perform like the traditional rosin-based fluxes. Low-solid fluxes have, as the term implies, chemistries with solid contents reduced to low levels, typically in the range of 2% to 8%. Their activities range between a typical R flux and an RMA flux, depending upon the level of activator(s) added to the base chemistry. The reduced solid content leaves little or no residue remaining on the assembly after completion of the soldering process. The consequence of a lower solid content is that the process window is considerably narrower when compared to rosin-based or no-clean fluxes. Low-solid fluxes have been specifically engineered for 37Pb–63Sn and 40Pb–60Pb soldering processes. The process window of low-solid fluxes can be widened to some degree through the use of an inert gas (e.g., N2) in the reflow furnace. b. Resin-Based, Synthetic Fluxes. The resin-based fluxes are synthetic materials having activator additions that give them activities between the RMA and RA categories of natural rosin-based fluxes. The designation for these fluxes is resin-based, synthetic, activated (RSA). The

* There is also a fourth category of soldering flux, the inorganic acid fluxes. These highly active fluxes are not used in electronic packaging or assembly applications because of potential corrosion damage posed by their residues. They are typically used in structural soldering applications such as plumbing and jewelry repair.

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particular attribute of RSA fluxes is that they are more compatible with higher processing temperatures; thermal breakdown occurs at temperatures of 350j to 375jC [43]. These fluxes can accommodate the high-Pb, 90Pb–10Sn, and 95Pb–5Sn solder processes. c. Organic Acid or Water-Soluble Fluxes. The organic acid fluxes, also called watersoluble fluxes or ‘‘intermediate fluxes’’ have strength levels commensurate with RMA and RA fluxes, and have been used extensively with eutectic and off-eutectic Pb–Sn solders. Thermal breakdown for these materials occurs at temperatures in the range of 250j to 275jC so that they are of marginal use with the high-Pb solder processes. The organic acid fluxes utilize aqueous or semiaqueous cleaning materials, thereby eliminating the need for environmentally damaging, solvent-based cleaning processes. 3. Controlled Atmospheres a. Passive Atmospheres Passive controlled atmospheres assist the flux by excluding oxygen (O2) during the heating cycle, thereby limiting reoxidation of the base metal and molten solder surfaces. A secondary benefit of passive atmospheres is that they reduce the rate of thermal degradation of the flux components (that is also a type of ‘‘oxidation’’ process but, which occurs to polymer chains). Typical passive atmospheres include nitrogen (N2) and argon (Ar). Nitrogen is preferred for controlled atmosphere soldering [46,47]. It is the least expensive gas to produce. Nevertheless, the use of a N2 atmosphere extols a significant cost penalty on the assembly process. Specially equipped furnaces are required to support a N2 reflow environment. Additional plant costs are incurred for providing the N2 supply as well as to transport N2 gas to the furnace location. Residual O2 levels less than 100 to 200 parts per million (ppm) and, preferably, in the range of 20 to 50 ppm, can significantly reduce Pb–Sn solder defects that occur during assembly. Of course, the controlled atmosphere that most effectively excludes O2 from the process is a vacuum. However, the use of vacuum furnaces for large-volume soldering assembly processes is usually cost prohibitive. b. Reactive Atmospheres. Reactive, controlled atmospheres actually reduce base metal or molten solder surface oxides. Strictly speaking, inert and vacuum atmospheres can be reactive by reducing metal oxides due to the low O2 partial pressure above the metal surfaces. Unfortunately, the thermodynamics of the reduction reaction are not favorable for relevant base metals such as Cu, Ni, Sn, or Pb at the relatively low temperatures used in soldering processes.* Similarly, the effectiveness for hydrogen gas (H2) or carbon dioxide (CO2) to reduce metal oxides is extremely low for common base metals exposed to Pb–Sn solder process temperatures. An alternative approach to reactive, controlled atmospheres for Pb–Sn solder processes has been to mix acid vapors with an inert carrier gas (N2) [48–50]. The acid vapors provide the fluxing action that reduces oxides on the base metal and molten solder surfaces. The inert gas also prevents reoxidation of those new, pristine surfaces. Gas mixtures of 5% formic acid or 3% acetic acid in Ar have been shown to provide adequate solderability of 37Pb–63Sn solder on Cu with little remaining residue. Although residues are considerably less than those that remain after the use of conventional Pb–Sn solder pastes, cleanliness tests should be performed to assure that residue levels meet product acceptance criteria.

F. Heating Methods Used for Assembly with Pb–Sn Solder Generalized time and temperature parameters have been identified for a number of Pb–Sn solder assembly processes. Those parameters are listed below for reflow furnace soldering, vapor phase soldering, wave soldering, and hand soldering.

* The two elements having a thermodynamic potential for oxide reduction under partial pressure O2 at soldering temperatures are Ag and Pd. However, the common surface tarnish (film) that forms on Ag is the sulfide, Ag2S. In the case of PdO, the rate kinetics for oxide reduction are too slow for a vacuum to improve solderability under typical processing conditions.

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FIG. 22 Schematic diagram of the generic time–temperature profile used for the in-line furnace processing of printed circuit assemblies.

1. Furnace Heating The preferred technique for large-volume soldering is the in-line reflow furnace. A schematic diagram showing the time–temperature heating profile is shown in Fig. 22. The process parameters are the heating and cooling rates (DT/Dt) as well as the preheat and reflow temperatures and times (Ti, ti). Some typical parameters for 37Pb–63Sn solder reflow using an RMA flux are listed in Table 6. The parameters will vary with the size of the work piece. However, they are similar for processes conducted in air or inert atmospheres. Also, the same process profiles are targeted for batch-type furnaces, albeit the time–temperature profile is governed by power changes to the heating (and/or) cooling coils rather that the multiple heat zone temperatures and conveyor speed of an in-line furnace. 2. Vapor Phase Heating The second process used in the assembly of printed wiring boards with Pb–Sn eutectic or neareutectic solder is referred to as vapor phase or condensation soldering. This method uses the heat of vaporization released when a vapor condenses into its liquid phase, to heat the assembly and reflow the Pb–Sn solder. Vapor phase reflow is a batch process [5]. The Pb–Sn solder is printed as a paste on the circuit board; the components are placed on the solder paste deposits; then, the assembly is placed into the equipment. A preheat step is performed on the assembly, typically in a separate chamber. Entry of the assembly into the vapor results in relatively fast heating rates for the solder joints, approximately 7j to 10jC/sec. Similarly, the joints experience fast cooling rates upon withdrawal from the fluid. The working fluid vapor is not a very efficient barrier against O2. 3. Wave Soldering The third process that was developed for Pb–Sn eutectic and near-eutectic solders is referred to as wave soldering. Wave soldering was developed as a large-volume process for through-hole printed wiring assemblies; it continues to be used for mixed assemblies that have both throughTABLE 6 Typical In-Line Furnace Process Parameters for 37Pb–63Sn Solder and an RMA Flux per the Diagram in Fig. 22 Process parameter DT/Dt T1 t1 T2 t2

Typical values 1–10jC/sec 150–170jC 60–180 sec 214–245jC 30–60 sec

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hole and surface mount components [5,51–53]. The printed wiring assembly is coated with flux and then passed through a preheat zone. Then, the assembly is exposed to the molten solder wave. The time–temperature cycle resulting from contact with the solder wave closely matches that of a hand soldering process. That is, there is a very rapid temperature rise followed by a short contact period (5 to 10 sec) between the assembly and molten solder. The subsequent cooling rate after a joint has left the wave is likewise, very fast. The wave soldering process has been precisely engineered for Pb–Sn eutectic solder. The type of flux, the take-off angle of the assembly as it exits the wave, the horizontal (belt) speed, and the shape of the wave, have all been specifically engineered to minimize ‘‘skips’’ and ‘‘bridges,’’ the common defects associated with wave soldering. 4. Hand Soldering The hand soldering of electronic assemblies, whether performed as the initial manufacturing step or as a subsequent repair or rework operation, is performed with an electric iron. The soldering step is typically completed in a matter of 2 to 7 sec. The relatively high temperature of the soldering iron (370j to 480jC), and the capability of the iron to concentrate heat into the immediate joint region, allows for the rapid heating of the solder and base metal so that solder flow, gap filling, and fillet formation are quickly completed without thermal damage to nearby materials.

IV.

INTERCONNECTION LEVELS

A.

Level 1 Interconnections

Level 1 interconnections refer to connections made between an integrated circuit (IC) die and the surrounding package. There are two variations to Level 1 soldered interconnections. In the first technique, high-Pb solder (97Pb–3Sn or 95Pb–5Sn) is typically deposited by evaporation or electroplating on the silicon chip bond pads in the wafer state. Upon singulation (dicing), the chips are placed and then reflow attached to a chip carrier. These solder joints are often referred to as controlled collapse chip connections, or ‘‘C4’’ solder joints. These high-Pb solder joints do not remelt during subsequent eutectic PbSn soldering processes. This direct, z-axis solder attachment of a chip to the next level of assembly is referred to as flip chip technology. This technique is used to attach the silicon chip to ceramic carriers because the ceramic carriers can withstand the high reflow temperatures required of the high-Pb solders (340j to 360jC). Alternatively, a low-melting-temperature solder (e.g., the eutectic Pb–Sn alloy) may be utilized to attach a silicon chip (having high-Pb, Pb–Sn solder bumps) to organic printed circuit boards. The circuit boards are not damaged by the low-temperature solder attachment process. This version of flip chip assembly is referred to as direct chip attach (DCA) or flip chip on board (FCOB) technology. A second flip chip technique is realized by the use of eutectic Pb–Sn or other low-meltingtemperature solder bumps in place of high-Pb solder bumps. These chips can be directly reflow attached to the chip carrier using low temperature. In this application, the chip carrier can be either ceramic or an organic laminate.

B.

Level 2 Interconnections

The widely recognized use of Pb–Sn solders in electronics assembly is in the Level 2 interconnections. Level 2 interconnections are solder joints that attach device packages to the printed circuit board. The solderability and, ultimately, the reliability of Level 2 interconnections are determined by the materials used for the package input/output (I/O) and those used for the circuit board. 1. Package Input/Output Materials The materials used for the leads of component packages for through-hole assembly are typically Cu, or Fe–Ni alloy having a Cu or Ni solderable finish with a Sn or Pb–Sn protective layer.

SOLDERS FROM THE LEAD–TIN SYSTEM

195

Peripherally leaded, surface mount packages use lead materials of Cu or an Fe–Ni alloy with a Ni solderable finish and an electroplated Pb–Sn or Sn protective layer. Leadless chip devices have terminations comprised of a fired-on Ag thick film conductor that is overplated with a Ni or Cu solderable coating, followed with an electroplated Pb–Sn protective finish. Leadless ceramic chip components (LCCC) use castellated terminations with a thick-film Au finish. 2. Organic Laminate Substrate Materials Typical organic laminate substrate materials include epoxy–glass (FR-4), polyimide–quartz, and PTFE (Teflonk)–glass composites. The circuit board bond pad (base metal) is typically Cu (electrodeposited Cu over a Cu foil). The Ni (solderable finish)/Au (protective finish) system over Cu has been used in numerous applications to improve solderability, limit Cu dissolution under prolonged process cycles, or to prevent excessive Cu–Sn intermetallic compound layer growth during long-term service at elevated temperatures. The two Pb–Sn solders used exclusively for Level 2 interconnections are the eutectic 37Pb–63Sn and near-eutectic 40Pb–60Sn compositions. Their liquidus temperatures allow for process conditions that are compatible with organic circuit board laminates and the molding compounds used for packages. Flux and solder paste technologies have been so well designed around eutectic Pb–Sn solder processes that defect rates have been reduced to ppm levels, even for the low cost, high-volume assembly of consumer electronics. 3. Ceramic Substrate Materials A number of applications require the use of ceramic substrates. The ceramic materials include several grades of alumina–glass composites designated by the alumina content (94%, 96%, and 99%) as well as low-temperature, co-fired ceramic (LTCC) materials. Ceramic-based assemblies are referred to as hybrid microcircuits (HMCs). The conductor features on HMCs that form the base metal of Level 2 interconnections are thick film layers. The thick-film conductor structure between 37Pb–63Sn solder and a 76Au–21Pt–3Pd thick film layer is shown in Fig. 23. The thick

FIG. 23 (a) Hybrid microcircuit (HMC) assembly. (b) A scanning electron micrograph showing the cross section of a thick film solder joint from the HMC. The solder is 37Pb–63Sn and the thick film composition was 76Au–21Pt–3Pd. (Courtesy of Sandia National Laboratories.)

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film layer begins as a screen-printed paste composed of metal particles, glass particles, and some thixiotropic components for printability. The printed pattern is fired onto the ceramic substrate at temperatures of 800j to 900jC in air. The metal particles sinter together to form a 7- to 15-Amthick conductive layer that serves as the base metal of the solder joint. The glass component migrates to the metal/ceramic interface where it bonds the sintered metal layer to the ceramic chip carrier. Thick-film metal compositions are elemental metals Ag, Pt, Au as well as alloy compositions: Au–Pd, Au–Pt, Au–Pt–Pd, Ag–Pd, and Ag–Pt. (A limited number of HMC assemblies use Cuand Ni-based, thick-film systems.) Lead–tin solder joints made to these metal compositions present several challenges. First, the elemental films, Ag and Au, exhibit relatively fast dissolution rates in Pb–Sn solder. Excessive dissolution can risk complete consumption of the thick film and loss of the conductor pad. Second, rapid solid-state intermetallic compound layer growth occurs between the thick-film compositions and Pb–Sn solders, even at modest service temperatures [54– 57]. The consequences of solid-state aging are a thick, brittle intermetallic compound layer and loss of the thick film layer.

C.

Level 3 Interconnections

The Pb–Sn solders, and primarily the eutectic 37Pb–63Sn and near-eutectic 40Pb–60Sn alloys, are used in the fabrication of Level 3 interconnections. Level 3 interconnections include cable– connector assemblies used to transmit signals between printed wiring assemblies as well as hard wired solder joints such as lead-to-turret or lead-to-eyelet interconnections. Examples of connectors are shown in Fig. 24. The Pb–Sn solder can be used in the actual construction of the connector, such as in the case of the coaxial connector shown in Fig. 25, or in the attachment

FIG. 24 Assortment of connector assemblies that represent the challenges of Level 3 interconnections. (Courtesy of Sandia National Laboratories.)

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197

FIG. 25 Cross-section view of a cable–coaxial connector assembly illustrating the use of 37Pb–63Sn solder joints in its construction. (Courtesy of Sandia National Laboratories.)

of a connector to the printed wiring assembly. Because the connector structures are Au plated for corrosion protection, assurances must be made to prevent the Au embrittlement of associated Pb–Sn solder joints [42].

V.

PHYSICAL AND MECHANICAL PROPERTIES OF LEAD–TIN SOLDER AND SOLDERED JOINTS

This section reviews the physical and mechanical properties of Pb–Sn solder and Pb–Sn solder joints.* As expected, the majority of Pb–Sn properties have been obtained for the 40Pb–60Sn and 37Pb–63Sn compositions due to their predominance in Level 2 interconnections. The low solidus temperatures of Pb–Sn solders in general, and the 40Pb–60Sn and 37Pb–63Sn compositions in particular, causes these alloys to readily undergo microstructural changes that, in turn, alter their mechanical properties. Tensile strength and hardness data from bulk 37Pb–63Sn solder, which illustrate a decrease in both properties as a consequence of room temperature aging, are shown in Fig. 26 [5,58]. Although the elastic modulus is not significantly impacted by microstructural changes, the time-dependent properties (e.g., creep and stress relaxation) are sensitive to these factors. An annealing treatment of 100jC, 16 to 24 hr will often ‘‘stabilize’’ the microstructure against further changes. Finally, it must be reiterated that the mechanical properties of Pb–Sn solders are extremely stain rate sensitive. Therefore, it is preferred that strength data be accompanied by testing rate parameters (strain rate or displacement rate and sample geometry).

A.

Physical Properties

The room temperature physical properties of (1) density, (2) electrical conductivity, (3) thermal conductivity, (4) coefficient of thermal expansion, and (5) elastic modulus are shown in Table 7 of selected Pb–Sn compositions [59]. The Poisson’s ratio of 40Pb–60Sn solder is 0.4 and is not highly temperature sensitive [29]. This value also provides a reasonable approximation for other Pb–Sn solders [60]. The surface tension of molten Pb–Sn solders at 300jC are provided in Table 8 [61].

* A more detailed compilation of these properties and other solder properties can be found in the third edition of the Soldering Handbook (American Welding Society, 2000).

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FIG. 26 The effect of room temperature aging time on (a) lap shear strength (0.2-mm gap, rate unknown) of 37Pb–63Sn/Cu joints subsequent to a soldering operation; and (b) Vickers hardness (100g load) for bulk 37Pb–63Sn solder after solidification. (From Ref. 5; excerpted from Ref. 58.)

The temperature dependence of the modulus of elasticity has been determined for 40Pb– 60Sn solder using acoustic techniques [60]. The behavior can be represented by the following equation: E ðGPaÞ ¼ C0 þ C1 T þ C2 T2 þ C3 T3

ð7Þ

where Co = 3.8305, C1 = 0.01105, C2 = 0.000112, C3 = 0.345  107, and T is temperature (jC).

TABLE 7

Physical Properties of Selected Pb–Sn Solders

Tin content (wt.%) 10 20 40 60 70 Source: Ref. 59.

Density (g/cm3)

Electrical conductivity (% of IACS)

Thermal conductivity (W/m-K)

Coefficient of thermal expansion (K1)

Elastic modulus (GPa)

10.5 10.0 9.28 8.52 8.17

8.2 8.7 10.1 11.5 12.5

35.8 37.4 43.6 49.8 —

27.9 26.5 24.7 21.6 20.7

19 20 23 30 35

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199

TABLE 8 Surface Tension Values at 300jC for Selected Pb–Sn Solders Solder (wt.%) 80Pb–20Sn 70Pb–30Sn 40Pb–60Sn

Surface tension (dyne/cm) 460 470 503

Source: Ref. 61.

B.

Monotonic Strength Properties

1. Time-Independent Deformation (Stress–Strain Response) The 0.2% offset, room-temperature yield strength values for selected Pb–Sn compositions are listed in Table 9 [5]. The strength of Pb–Sn alloys increase with Sn content. Both the yield strength and ultimate tensile strength increase with lower test temperature and decrease with higher test temperature [59,62]. The data provided in Ref. 62 demonstrate the strain rate sensitivity exhibited by the ultimate tensile strength of Pb–Sn solders, even at cryogenic temperatures. Other investigations have similarly illustrated the strain rate and temperature sensitivities of the time-independent deformation shown by the eutectic and near-eutectic Pb–Sn solders [63,64]. Fewer mechanical properties studies have been performed on the high-Pb, Pb–Sn solders (e.g., 90Pb–10Sn and 95Pb–5Sn). Constant strain rate (6.2  105 sec1) tensile properties were measured for 90Pb–10Sn and 97Pb–3Sn solders by Cole and Caulfield [65]. Selected 0.2% offset yield stress, the ultimate tensile stress, and the static Young’s modulus were excerpted from that study and listed in Table 10. The yield and ultimate tensile strengths were generally less than values corresponding to eutectic Pb–Sn solder when the tests were performed at 0j and 50jC. However, the high-Pb solder provided comparatively better performance at temperatures exceeding 100jC. The mechanical properties of bulk Pb–Sn solders are generally reproduced at the solder joint level. However, the actual strength values of Pb–Sn solder joints will differ from those of bulk solder. In particular, solder joint strength is affected by the gap size. An increase in joint strength with smaller gap sizes is most evident when joints are loaded in tension [66]; this trend is mush less pronounced when joints are loaded in shear [67]. The shear strengths recorded for Pb–Sn solder joints made to Cu base metal are listed in Table 11 [62]. The typical deformation mode that accompanies time-independent deformation in the eutectic and near-eutectic Pb–Sn solders is grain boundary sliding [68]. Fracture occurs in the solder, near to the intermetallic compound layer at the solder/base metal interface.

TABLE 9 Room Temperature 0.2% Offset Yield Strength of Selected Bulk Pb–Sn Solders Solder (wt.%) 90Pb–10Sn 40Pb–60Sn 37Pb–63Sn 20Pb–80Sn

Yield strength (MPa) 13.9 14.2 16.1 29.6

The testing rate was not specified. Source: Ref. 59.

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TABLE 10 Bulk Properties of 90Pb–10Sn and 97Pb–3Sn Solders at 0j, 50j, and 100jC Tested Under a Constant Tensile Strain Rate of 6.2  105 sec1 Solder composition (wt.%)

Test temperature (jC)

Elastic modulus (GPa)

0.2% Offset yield strength (MPa)

Ultimate tensile strength (MPa)

90Pb–10Sn

0 50 100 0 50 100

10 9.4 5.6 14 9.5 5.8

14.3 12.6 9.4 6.6 5.8 6.0

23.2 17.4 6.6 13.0 9.8 7.6

97Pb–3Sn

Source: Ref. 65.

2. Time-Dependent Deformation (Creep Response) The use of Pb–Sn solders at high temperatures (relative to their solidus temperature) causes timedependent deformation, or creep, to be a significant contributor to mechanical degradation in electronic interconnections. Creep studies have been performed on bulk eutectic and neareutectic Pb–Sn solder in tension, torsion, compression, and by indentation [69–73]. Minimum compression creep rate (de/dtmin) data can be represented by a ‘‘sinh law’’ expression over the temperature range of 55j to 125jC [72]: de=dtmin ¼ 2:48  104 ½sin hð0:0793rÞ3:04 expð56; 944=RTÞ

ð8Þ

1

where de/dtmin is the minimum creep rate (sec ), r is the applied stress (MPa), R is the universal gas constant (8.314 J/mol K), and T is the temperature (K). The microstructural features resulting from creep deformation in 37Pb–63Sn and 40Pb–60Sn solders are a combination of Sn– Sn grain boundary sliding, Pb–Sn interphase boundary sliding, and coarsening of the Pb-rich phase [74–76]. The creep behavior of Pb–Sn solder joints show similar trends as are observed in the bulk material [77,78]. A study examined the effect of cooling rate on the subsequent creep deformation of 40Pb–60Sn solder [79]. A faster cooling rate resulted in a finer starting microstructure. The resulting impact on creep behavior was most significant under low stress (low strain rates) where the steady-state strain rate increased with the finer microstructure.

C.

Fatigue and Microstructural Effects

The importance of understanding fatigue in soldered interconnects was brought to light by the premature failure of some surface mount solder joints made to organic laminate, printed wiring

TABLE 11 Effect of Pb–Sn Solder Composition on Double Lap Shear Strength at a Constant Strain Rate of Approximately 0.33/ sec Solder composition (Wt.%) 90Pb–10Sn 80Pb–20Sn 40Pb–60Sn Source: Ref. 62.

Shear strength (MPa) 20 23 27

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201

FIG. 27 Optical micrograph showing a fatigue failure of 37Pb–63Sn solder joining a surface mount, ceramic package to an organic laminate, printed circuit board. (Courtesy of Sandia National Laboratories.)

assemblies early in the technology [80–82]. The fatigue failure morphology of 37Pb–63Sn solder joints is illustrated in Fig. 27. The source of the fatigue failures was the strain generated in the solder during thermal cycling due to the coefficient of thermal expansion mismatch between ceramic packages and the organic laminate, printed circuit boards to which they were attached.* In fact, it became clear that time-dependent deformation had a critical role in the fatigue cycle; this was the so-called creep-fatigue phenomenon [83,84]. The term thermal mechanical fatigue (TMF) was introduced to describe this ‘‘temperature cycle plus thermal expansion mismatch’’ degradation mode in surface mount solder joints. This phase emphasizes that the source of the cyclic deformation as being temperature variations rather than applied cyclic loads (e.g., vibration), thereby distinguishing TMF from mechanical fatigue. The commonly observed, microstructural signature of TMF deformation is a localized coarsening of the Pb-rich phase [85]. This phenomenon is illustrated in Fig. 28 for a chip resistor component. The development of the coarsened Pb-rich phase particles is subsequently followed by microcracks that form along the Sn–Sn grain boundaries and Sn–Pb interphase boundaries. The microcracks join into a single discontinuity at which point, the load bearing capacity of the solder is lost. Studies have also examined the TMF behavior of high-Pb solder (97Pb–3Sn and 95Pb–5Sn) interconnections. These joints are typically made between a silicon chip and a ceramic chip carrier since the materials have similar coefficients of thermal expansion. As such, they do not develop a large degree of fatigue damage under temperature cycling conditions. In addition, the chips are of a sufficiently small size that shear strains developed in the joints are likewise of a lesser magnitude. The intrinsic fatigue resistance of these solders appears to be similar to that of eutectic solder [78].

* Thermal mechanical fatigue was not a prevalent in surface mount, hybrid microcircuit products because of the closer match between the coefficients of thermal expansions of ceramic packages and ceramic substrates. Also, TMF was not intensely investigated for through-hole, Pb–Sn solder joints. Although fatigue damage occurs in the latter, the larger bonding area reduces the impact of degradation below levels sufficient to cause electrical failure.

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FIG. 28 (a) Optical micrograph of a surface mount, 37Pb–63Sn solder joint having undergone TMF. (b) High-magnification view of the solder fillet showing the Pb-rich phase coarsening that accompanies TMF. (Courtesy of Sandia National Laboratories.)

D.

Fatigue Failure Studies and Models

1. Coffin–Manson Equation A number of low-cycle, isothermal fatigue studies have been performed on 37Pb–63Sn and 40Pb– 60Sn solders and their solder joints. The data were found to be well represented by a Coffin– Manson-type relationship: Dcp ðNf Þa ¼ h

ð9Þ

where Dcp, is the plastic strain range (shear or tension–compression), Nf is the number of cycles to failure, a is the fatigue exponent, and h is a constant. The Coffin–Manson equation was determined for 40Pb–60Sn solder tested in shear at temperatures in the range of 55j to 125jC, and cycle frequencies greater than or equal to 0.3 Hz [86]. The underlying failure criterion for Nf was a 50% load reduction. This resulting equation is: Dcp N f0:51 ¼ 1:14

ð10Þ

2. Frequency-Modified, Coffin–Manson Equation At frequencies below 0.3 Hz, the slower ramp rates allow a greater extent of damage to occur, resulting in the acceleration of degradation. Then, the frequency-modified, Coffin–Manson equation becomes applicable: Dcp ðNf =mÞ0:51 ¼ 1:14

ð11Þ

where m is the cycle frequency (cycles/sec). A more complex Coffin–Manson equation was developed by Engelmaier for predicting the fatigue life of 37Pb–63Sn solder under an applied shear deformation [87]. The cited work placed the frequency dependency of fatigue damage in the fatigue exponent, a. The resulting modified Coffin–Manson equation had been determined to be accurate for frequencies in the range of 0.00001 V m V 0.01 Hz. 3. Coffin–Manson Approach Based on Plane Strain Levels Isothermal fatigue predictions using the Coffin–Manson relationship have also been developed, but categorized according to the plastic strain levels to which the solder is exposed [88]. Guo et al. developed two equations to predict cycles to failure at 0.5 Hz, based upon a 50% load reduction criterion. The equations refer to two, tension–compression plastic strain ranges, Dep (%): ¼ 12:2; Dep N2:08 f

0:07 V Dep V 0:22%

ð12Þ

SOLDERS FROM THE LEAD–TIN SYSTEM Dep N1:35 ¼ 162; f

0:28 V Dep V 2:5%

203 ð13Þ

Plane strain values in the range of 0.22% to 0.28% would likely require the use of the equation for Dqp closest to the particular case at hand. 4. Strain Energy Approach Energy-based methodologies have also been used to predict fatigue damage in eutectic and near-eutectic Pb–Sn solder interconnections. This approach uses a generalized scalar, the strain energy, as a metric describing the extent of fatigue damage that has occurred in the solder. As a scalar quantity, fatigue degradation resulting from various conditions are simply be added together to define the total amount of degradation. The strain energy approach was utilized by Clech et al. in their development of the comprehensive surface mount reliability model (CSRM) [89]. The strain energy approach allows for the superposition of local thermal expansion mismatch strains, that is, those strains due to the thermal expansion mismatch between the package material, base material, and solder, on traditional global thermal expansion mismatch strains. These strains result from the thermal expansion mismatch between the microelectronic package and the printed circuit board laminate structure. The inclusion of the local expansion mismatch degradation improved the accuracy of TMF predictions compared to other methodologies. 5. Other Factors Each of the aforementioned fatigue prediction techniques are based upon the presumption that the solder in a joint undergoes uniform shear. This condition is most closely met in the gap region of solder joints for surface mount chip resistors or leadless chip carrier packages. However, the impact of the solder fillet and its particular geometry—concave or convex (‘‘bulbous’’)—is not addressed by these approaches because the stress and strain state is much more complex there. Unfortunately, the fillet accounts for the majority of the fatigue life of the solder joints. Predicting fatigue degradation in the fillets of solder joints, or other complex joint geometries, requires the use of computational models based upon constitutive equations and finite element techniques. The modeling approach is discussed in the following section. Generally, voids do not have a predictable impact on the fatigue life of Pb–Sn solder joints. First, the high ductility of Pb–Sn solders often mitigates any ‘‘stress concentration’’ effects caused by a void. Second, the solder deformation of a joint undergoing TMF occurs largely as a straincontrolled event. As such, the extent of plastic strain damage (or strain-energy damage) is the same per unit volume of material. However, voids do represent a loss of material through which a fatigue crack must propagate and thus, may detract from the fatigue life of a joint if suitably located in the crack path.

E.

Computational Modeling to Predict Solder Joint Reliability

1. Constitutive Models for Predicting Thermal Mechanical Fatigue Deformation It was recognized within the electronics community that given the variety of Level 2 solder joint geometries, package materials, and chip carrier materials used in surface mount interconnections, it was necessary to move beyond the Coffin–Manson-based fatigue prediction methodologies for Pb–Sn solder interconnects, and explore the use of computational models. The computational modeling approach requires that two tasks be addressed. The first task is to develop the appropriate constitutive equation that relates strain, e, or strain rate, de/dt, in the Pb–Sn material to the applied stress (r) and temperature (T) conditions: de=dt ¼ fðr; TÞ

ð14Þ

A number of material properties (e.g., elastic modulus, Poisson’s ratio, yield stress, activation energy of creep, etc.) are included as parameters within such an equation. The second task is to map the magnitude of fatigue deformation throughout the solder joint configuration. The latter requirement is accomplished by setting up finite element meshes of the various solder

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joint geometries. Finite element meshes for surface mount, through-hole, and connector (Level 3) interconnections are illustrated in Fig. 29. The critical task is the development of an appropriate constitutive equation for the Pb–Sn solder that is of interest. Current efforts in constitutive equation development have assumed the solder to be a continuum. Deformation is represented by a unified viscoplastic (or creep plasticity) constitutive equation [90,91]. The advantage of the unified viscoplastic approach is that both time-dependent deformation (creep) and time-independent deformation (plasticity from the stress–strain curve) are included in a single equation, thereby greatly facilitating the subsequent numerical computations. Recent advances in the development of TMF predictive models have included the incorporation of solder microstructure as a state variable in the constitutive equation [92,93]. A contour map (Fig. 30) shows the Pb-rich phase coarsening predicted in a leadless chip solder joint subjected to six thermal cycles having temperature limits of 50j and 80jC, ramp rates of 6jC/ sec, and dwell times of 10 min at both limits. The microstructure-based, viscoplastic constitutive equation for Pb–Sn solder improves the accuracy of solder fatigue strain predictions throughout the interconnection geometry. It provides a ‘‘real-time’’ adjustment of the local solder mechanical properties resulting from local changes in the solder microstructure (i.e., the Pb-rich phase). 2. Damage Models of Predicting TMF Crack Development The aforementioned constitutive equations predict the ‘‘continuum’’ deformation in the solder. However, the total functional lifetime of a solder interconnection also includes the initiation and propagation of fatigue cracks. Crack development is addressed by so-called damage models. The

FIG. 29 Examples of finite element meshes for Level 2 and Level 3 soldered interconnections: (a) gullwing leaded package, (b) chip device, (c) pin-in-hole, and (d) connector. (Courtesy of Sandia National Laboratories.)

SOLDERS FROM THE LEAD–TIN SYSTEM

205

FIG. 30 Contour map of Pb-rich phase coarsening in a 37Pb–63Sn chip capacitor solder joint predicted by a computational model. The Pb-rich phase size (coarsening) is the metric of fatigue deformation in the solder. The cycling conditions were: 6 cycles; temperature limits of 50jC and 80jC; ramp rates of 6jC/sec, and dwell times of 10 min. (Courtesy of Sandia National Laboratories.)

goal of reliability modeling is to couple continuum models and damage models so as to predict the total fatigue lifetime of an interconnection. The development of damage models is difficult. Cracks are discontinuities in the solder microstructure and, as such, present formidable challenges for identifying an appropriate constitutive equation as well as numerical code development. Nevertheless, significant progress has been made by investigators toward developing a computational representation of damage in Pb–Sn solders [94–96].

F.

Corrosion Resistance

Both Sn and Pb exhibit relatively good corrosion resistance. Tin is slightly more susceptible to general corrosion in seawater and acid media than is Pb [97,98]. As such, a higher Pb content usually improves the corrosion resistance of the solder. Fundamentally, galvanic-assisted corrosion should be of particular concern for soldered joints because the latter are comprised of dissimilar metals. However, by and large, Pb–Sn solder electronic interconnections do not exhibit a particularly strong susceptibility to galvanic corrosion. Flux residues do not cause severe corrosion, general or galvanic assisted, on Pb–Sn solder or their solder joints, respectively. The limited corrosion activity results primarily from the tenacious oxides of Pb and Sn, in particular, those of Pb. However, the presence of an applied voltage can potentially accelerate corrosion activity in Pb–Sn solder joints. The predominant phenomenon is electromigration, which is illustrated in

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Fig. 31 [99]. Electromigration occurs when a voltage difference between neighboring solder joints sets up a cathode/anode pair. A corrosion cell is formed when an electrolyte is present. For example, the combination of moisture and flux residues can comprise an electrolyte. Metal ions generated by an oxidation reaction at the anode travel to the cathode where they are reduced to their elemental metal. The elemental metal forms filaments that grow back toward the anode. Once the filament contacts the anode, a short circuit occurs between the two joints. The tenacious oxides of both Pb and Sn slow the electromigration phenomenon compared to other metals that do not form tenacious oxides, the most notorious being Ag.

VI.

SUMMARY

The Pb–Sn solders continue to play a critical role in the Level 1, Level 2, and Level 3 interconnections of commercial and military electronics. The widespread availability of Pb and Sn ores and the relative ease of winning the elements from these ores have contributed to their low primary metal costs. However, the real attributes of the Pb–Sn system lie in the physical and mechanical properties of these alloys:


Low liquidus temperatures allow for reduced processing temperatures that limit thermal damage to organic materials such as printed circuit board laminates and device molding compounds.
The simple eutectic Pb–Sn system minimizes the sensitivity of the liquidus temperature to slight compositional variations and contamination effects. The Pb–Sn system provides

FIG. 31 Schematic diagram depicting the electromigration process. Metallic filaments grow from the cathode towards the anode. (From Ref. 5.)

SOLDERS FROM THE LEAD–TIN SYSTEM













207

both high-melting-temperature alloys (90Pb–10Sn and 95Pb–5Sn) for Level 1 interconnections, and low-temperature compositions (37Pb–63Sn and 40Pb–60Sn) for Levels 2 and 3 interconnects. Together, these alloys comprise a step-soldering process methodology or hierarchy used in a variety of electronic assemblies. The low surface tension of the Pb–Sn solders provides excellent solderability (wetting, spreading, and capillary flow behaviors) by the molten Pb–Sn solders in surface mount and through-hole interconnections. The Pb–Sn solders exhibit a limited susceptibility to all forms of corrosion thereby minimizing the need to implement corrosion mitigation measures such as coatings and desiccation. An in-depth understanding of the physical and mechanical metallurgy of Pb–Sn solders and associated interconnections has been established. In particular, the numerous studies on the TMF of Pb–Sn solders has allowed for the development of computational models that can predict the long-term reliability of their interconnects under various service environments. Lastly, an 80-year history of Pb–Sn solders in electronics has culminated into an unprecedented data base of information that supports both design and manufacture activities for today’s electronic products as well as the development of the next generation of advanced electronic packages and assemblies.

ACKNOWLEDGMENTS The author wishes to thank A. Kigo, R. Wright, and G. Bryant, whose optical metallography expertise provided the micrographs found in this chapter; G. Zender and B. Ritchy for the SEM work; and especially J. Rejent as the chief technologist in the soldering program at Sandia National Laboratories. The author also wishes to thank E. Lopez for his careful review of the manuscript. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DEAC04-94AL85000.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

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7 Physical Basis for Mechanical Properties of Solders Morris E. Fine Northwestern University, Evanston, Illinois, U.S.A.

I. A.

INTRODUCTION High Homologous Temperature

Solders are high-temperature materials as their homologous temperatures at room temperature (TR/TM, where TR and TM are the room and melting temperatures, respectively, in Kelvin), are above 0.5. For example, the homologous temperature of Sn at room temperature is about 0.6. Tin–silver-, tin–zinc-, tin–indium-, and tin–bismuth-based solders have even lower homologous temperatures. Therefore, the mechanical properties at ambient and elevated temperatures are highly dependent on strain rate and dwell times. This impacts the design of solder interconnects in electronic devices for high reliability. The models for reliability prediction are based on the hightemperature behavior of solder. Thermally activated plastic flow (e.g., creep) dominates the stress–strain curve except at very low strains and very rapid strain rates. Elastic strains and plastic strains that are not thermally activated are very small compared to thermally activated plastic flow. Thermally activated plastic flow dominates both isothermal and thermomechanical fatigue (TMF).

B.

Coefficient of Thermal Expansion Mismatch

Components interconnected in electronic devices have different coefficients of thermal expansion and this leads to thermal stresses during thermal cycling of a device on turning it on and off. For example, the coefficients of expansion of Si and Al2O3 differ by about 6 ppm/jC. The difference between Al2O3 and the usual printed circuit board material is about 15 ppm/jC. In interconnection arrays such as flip chip and ball grid array joined components, the thermal stresses are almost pure shear. Tensile testing results of bulk solders can be converted to shear, as plastic deformation in crystalline materials (except for such processes as grain boundary sliding) occurs by motion of dislocations on shear planes. However, testing of actual joints in shear and in tension may give different results because fracture at interfaces between solder and an intermetallic compound or an IMC and the metallization it is in contact with will occur differently in shear and in tension.

211

212 C.

FINE Microstructural Stability

High homologous temperatures also result in microstructural instability. The grain and phase structures may coarsen with time even at room temperature. For example, the yield and flow stresses of cast Sn–Pb eutectic solder at constant strain rate decreases with time for many years [1]. These stresses rapidly decrease at first but the rate of decrease slows with time. The flow stress at 105 sec1 strain rate decreases from the 7-day value of 25–15 MPa after 7 years. Plastic deformation that occurs during fatigue from thermal cycling increases the rate of such aging. Further, as solute solubilities decrease with a decrease in temperature, a stable microstructure at ambient temperature contains a larger volume fraction of a second phase than at the extreme temperature of the thermal cycle in a device.

II.

THEORY OF THERMALLY ACTIVATED PLASTIC FLOW

This chapter discusses the theory of thermally activated plastic flow including creep theory, components of the stress strain curve vs. strain rate and temperature and, isothermal and thermomechanical fatigue. Finally, the physical basis for modeling reliability of interconnects under thermal cycling is discussed.

A.

Movement at the Atomic Level

Plastic flow by dislocation motion in crystalline materials occurs by thermally activated processes in the sense that an energy increase is needed to move a dislocation from one stable position to another as discussed below. There are a variety of energy barriers that need to be overcome. The energy of thermal vibrations of the atoms assists this process. The probability that a thermally activated process occurs is proportional to me raised to the (DG*/kT ) power, where m is the thermal vibration frequency (about 1013 times per second), DG* is the activation Gibbs free energy for the process, k is Boltzmann’s constant, and T is the absolute temperature. First, there is the Peierls stress; that is, the dislocation sits in a low-energy crystallographic location and energy is required to move it to the next location. This energy is vanishingly small in face-centered cubic crystals such as lead, but that is not true for body-centered tetragonal tin. Mechanical twinning transformation, which is a thermally activated process, occurs during plastic deformation of tin. This gives rise to a creaking sound, referred to as tin cry, which is heard when bulk tin is bent near one’s ear. Except in perfect crystals, there are many dislocations present and many of these go through the slip plane of an active dislocation, and must be cut by the active dislocation to achieve plastic flow. This is also a process of small activation energy and only impedes plastic flow at ordinary strain rates at very low temperatures. Thus a pure metal such as Cu has its highest flow stress at absolute zero temperature. The flow stress at constant strain rate rapidly decreases on cooling, reaching a plateau where the effect of changing temperature or changing strain rate (except at extremal values of strain rate) is small. This is the case for Cu at 0jC (homologous temperature of about 0.2). There are still obstacles to dislocation motion present that are longrange or with high activation energies. The long-range stress field from neighboring dislocations, for example, must be overcome by the applied stress before a dislocation can move. Any increase in dislocation density is the source of work hardening. The theory of the plateau yield stress is outside the scope of this chapter.

B.

Critical Normal and Shear Stresses

Fig. 1 from the seminal work of Kloske [2] shows the critical resolved shear stress (CRSS) of some Ag dilute Au alloys at temperatures down to 4.2K. The decrease in CRSS on heating is evident. The change in CRSS particularly for the 1 at.% Ag alloy at room temperature is quite small. The temperature dependence of CRSS at low temperatures increases with alloying. This is a general behavior in fcc alloys and is less in Au–Ag alloys than, e.g., Cu and Al alloys. There are interactions between solute atoms and dislocations giving rise to another energy barrier that must be overcome. The result is an increase in the CRSS as the temperature is lowered.

MECHANICAL PROPERTIES OF SOLDERS

213

FIG. 1 The critical resolved shear stress of silver-based gold alloys is plotted vs. temperature. Note that the slope at low temperatures increases with gold content. (From Ref. 2.)

Plastic flow in crystals occurs by dislocations moving on slip planes that slide over one another. Most experiments with single crystals to determine the CRSS are performed with tensile specimens. The shear stress on the active slip plane is computed by resolving the tensile stress onto the active slip plane. With polycrystalline tensile specimens, plastic flow still occurs on slip planes in the individual grains and these are grouped into wide shear bands that traverse the specimen at 45j from the tensile axis. The shear stress is usually calculated approximately for the whole specimen by dividing the tensile stress in half. Failure of a ductile tensile specimen usually occurs by initiation of a shear crack that is followed by a mode I crack that is normal to the tensile axis. Sudden final failure occurs when the true tensile stress exceeds the mode I fracture stress.

C.

Tin Embrittlement Due to Allotropic Modification

The flow stress of body-centered tetragonal Sn cannot be explored at cryogenic temperatures because it is not thermodynamically stable below 13jC where tin transforms to the brittle diamond cubic structure. This transformation is the cause of the well-known condition referred to as ‘‘tin pest.’’ The tin breaks into powder form because of the high internal stresses that result from the large volume change during transformation. The activation energy for nucleation of diamond cubic Sn is high, giving rise to significant undercooling. Tin pest usually occurs at much lower temperatures than 13jC and is suppressed by additions of small amounts of elements such as antimony. A detailed discussion of tin pest and other degradation phenomena involving tin are presented in Chapter 22.

214 D.

FINE Strain Rate Effects on Flow Stress

At high homologous temperatures, there is a decrease in yield stress from the plateau because the rate of diffusion of atoms from one place to another in the lattice becomes appreciable. This allows dislocations to climb over obstacles such as second-phase precipitates. Recovery then occurs, resulting in the reduction of dislocation density and work hardening. Recovery is a time-dependent process because it is determined by the rate of diffusion and the time allowed for diffusion, such as the period of a fatigue cycle. The flow stress at a given strain rate is a balance between recovery and work hardening. The flow stress, which increases with strain rate, is thus very much a function of strain rate. Stress–strain curves for eutectic Sn–Ag and Sn–Pb solders are shown in Fig. 2 (a) and (b), respectively [3] for strain rates varying from 0.001 to 0.0001. For these alloys, the true elastic limit or yield stress is very small. An apparent linear region after yielding is observed. The slope of this apparent linear region increases with strain rate and has caused confusion in values of modulus for these alloys reported in the literature. The low-stress value for eutectic Sn–Pb solder is 35 GPa and for eutectic Sn–Ag solder is 56 GPa. There are many mechanical properties related to thermally activated plastic flow at high homologous temperatures besides reduction of flow stress and the large dependence of flow stress on strain rate. Creep, i.e., plastic flow at a constant stress, and stress relaxation at a constant strain are those of most interest to solder applications. These are all discussed later in this chapter.

E.

Effect of Brittle Phases in Interconnections

Intermetallic compounds that form between solder and metallized substrate features such as terminal pads have much higher melting temperatures than the solder. Room temperature is not a high homologous temperature for them. Further, their crystal structures are much more complex than fcc, bcc, or bct. They have much higher Peierls stresses. The yield stress is so high that it is close or even higher than the fracture stress. Bismuth in Sn–Bi eutectic solder has a complex rhombohedral structure; the Peierls stress is so high that Bi is brittle at strain rates usually used in a stress–strain test or on impact loading. However, because Bi has a low melting temperature, at very slow stress rates such as during thermal cycling of an electronic component during application, thermally activated plastic flow is sufficiently large so that brittle fracture does not occur. Brittle materials at interfaces are susceptible to mode I fracture under tensile loading, but much less susceptible to brittle fracture under shear loading as a result of the coefficient of thermal expansion (CTE) mismatch between the components connected by an interconnection. Dispersed intermetallic compounds such as Ag3Sn in eutectic Sn–Ag solder and SbSn in tin– antimony solders increase the strength and creep resistance of the solder, but the alloys are quite ductile as subsequently discussed. The interface between these intermetallic compounds and the tin-based solid solution is not a source of brittle fracture.

III.

THEORY OF CREEP

A.

The Three Stages of Creep

The theory of creep will be briefly reviewed. The reader is referred to texts [4,5] on the subject for deeper coverage. When a static constant stress (single or combined) is imposed on a metal or alloy component or specimen and the temperature is sufficiently high (i.e., a homologous temperature greater than 0.5), the specimen or component undergoes deformation that increases with time. If the constant stress on the specimen is tensile, the specimen continues to elongate as time goes on. This phenomenon is called creep. A plot of specimen gage length with time shows three stages. An initial transient stage called primary creep, a steady state stage called secondary creep where the rate of elongation is constant with time, and finally tertiary creep where the rate continues to

MECHANICAL PROPERTIES OF SOLDERS

215

FIG. 2 (a) Stress–strain curves for Sn–3.5Ag solder and (b) Sn–37Pb solder are plotted for three strain rates (0.001, 0.0001, and 0.00001) per second. The deviation from Hooke’s law occurs at lower and lower stress as the strain rate is reduced. The plastic flow stress also decreases with reduction in strain rate. (From Ref. 3.)

increase as the specimen elongates toward failure. The three stages of creep are schematically shown in Fig. 3. Most of the published research has been on steady state creep, which is the main interest in this chapter.

B.

Mechanisms

There are a number of creep mechanisms that may be active at the same time. If they are acting in parallel, the slowest essentially controls the creep rate; if they are acting in series, then the most

216

FINE

FIG. 3 The three stages of creep are schematically shown as they occur over time. The symbols en and tn are the corresponding rupture strains and time, respectively. Subscripts 1 and 2 represent the ends of stages I and II, respectively.

rapid is rate controlling. Creep mechanisms in crystals may be divided into two categories depending on whether dislocation motion is involved or not. 1. Diffusional Creep Mechanism At low stresses, creep may be due to diffusion of atoms alone. Under a tensile stress, the grains tend to elongate, setting up a stress gradient. In response, the atoms diffuse to relieve the stress gradient. In the Nabarro–Herring mechanism, bulk diffusion is rate controlling. In the Coble creep mechanism, grain boundary diffusion is rate controlling. In two-phase alloys, diffusion along interphase boundaries may contribute to creep. Diffusion along dislocations is much faster than bulk diffusion and such dislocation–core diffusion may also play a role. Of course, both grains and grain boundaries must deform or the material will begin to fracture. The accommodation may occur by grain boundary sliding, which is sometimes considered to be viscous flow. Because there is no amorphous phase at grain boundaries, the mechanism is closer to bulk plastic flow along slip bands involving defects such as grain boundary dislocations. In applications where the interconnections are under low stresses over a long accumulated time period, they must exhibit a very high degree of dimensional stability. Design against mass diffusional creep must be considered. Accelerated tests at higher stresses cannot be extrapolated into a low-stress regime. The diffusional creep strain rate equation for a pure metal usually used is de A1 rX ¼ Deff dt kTd2

ð1Þ

where Deff ¼ Dv þ

pdDb d

ð2Þ

In these equations, Dv and Db are the bulk and grain boundary diffusivities, respectively, in m2/ sec, X is the atomic volume in m3, r is the applied tensile stress in MPa, d is the average grain size in m2, A1 is a constant, and d is the grain boundary thickness in m. Obviously, a larger grain size results in a smaller diffusional creep rate. In solid solution alloys, Dv for the slowest diffusing

MECHANICAL PROPERTIES OF SOLDERS

217

species controls the creep rate if its atomic fraction is sufficiently large, a few atomic pct. In twophase alloys, an interphase boundary diffusion term must be added. Of course diffusion is an activated process governed by an equation such as Eq. (3) D ¼ ameð kT Þ Q

ð3Þ

where a is a constant including the entropy term and a geometric factor with the same units as D, and Q is the appropriate activation energy in J/mol. The experimentally determined value of Q helps determine whether the diffusion controlling the creep is in the bulk, in an interface or grain boundary, or along the dislocation cores. The activation energy for bulk creep is the largest among these. 2. Dislocation-Related Creep At higher stresses, thermally activated dislocation motion such as dislocation climb out of its slip plane limits the rate of steady state creep. When a lattice vacancy exchanges position with an atom in a dislocation core, a jog is formed and in the process an element of the dislocation moves to a parallel slip plane one atom distance away. Dislocations can get around obstacles by repeated climb events. The direction of climb is controlled by the applied stress state, but, of course, the rate increases with stress as the material acts to reduce its free-energy state. The rate of climb under constant stress is controlled by the rate of diffusion. The theory of dislocation creep mechanisms begins with the well-known Johnston–Gilman equation e ¼ gqbv

ð4Þ

where g is a geometric factor relating tensile strain to shear strain, q is the mobile dislocation density in number per square meter, b is the Burgers vector in m, and v is the dislocation velocity in m sec1, which is a function of stress. The dislocation climb velocity is equal to the probability for a successful climb event times the frequency factor times the applied stress times the area swept out by each climb event (roughly b2), i.e., approximately Q 2 ð kT Þ v ¼ am r lb e

ð5Þ

where a is a constant and l is the shear modulus. The dislocation density is usually considered proportional to (r/l)2. The result is the creep equation
3 Q ´ ð kT Þ eY ¼ A r ð6Þ l e The constant A contains many terms including b, the frequency factor, the entropy term, etc. Experimentally, the exponent on stress is observed for pure metals and solid solution alloys to be in the range 3–8. There is not a general explanation for the variation in n to the author’s knowledge, and discussion of this subject is beyond the scope of the present chapter. The creep equation is generalized allowing the exponent on stress to be a variable giving the well-known Dorn equation
n ´ ð kTQ Þ ð7Þ eY ¼ A r l e At stresses below the flow stress where the dislocation density is independent of stress, q is constant and n equal to one is predicted. This is referred to as the Harper–Dorn creep. a. Threshold Stress. In precipitation- and dispersion-strengthened materials, a much higher value of n is sometimes observed. A useful empirical equation is obtained by introducing a threshold stress into the equation subtracting from the applied stress. The threshold stress is a back stress or a stress that must be overcome before dislocation creep can occur. The resulting equation is  n Q ´ r  rt eð kT Þ ð8Þ eY ¼ A l

218 C.

FINE Tin Creep

Steady state creep data at constant temperature is usually presented as log–log plots of strain rate vs. stress. The slopes of these curves give the values of the exponent n. The activation energy Q is determined from the variation of steady state strain rate with temperature. Because all equations presented contain l, the shear modulus, its variation with temperature must be included in the analysis. The shear modulus and Young’s modulus decrease linearly with an increase in temperature in the vicinity of room temperature. The decrease of the shear modulus of tin is approximately 2000 ppm/jC. The variation with temperature decreases as the temperature is lowered toward 0K and must be zero there, a requirement of the third law of thermodynamics. Log–log steady state creep strain vs. stress curves for tin at three temperatures [6] are given in Fig. 4. There appears to be three stages in the creep behavior. At low stresses, the slope is smaller perhaps because of Harper–Dorn creep, then with an increase in stress there is a liner region where a slope of 8.6 fits the data. At still higher-stresses n decreases to 6. The line drawn for tin in the figure is an extrapolation of the 8.6 slope. Data points are seen to fall below this line. In the midregion, the activation energy is 73 kJ/mol but is 36 kJ/mol at higher temperatures. The activation energy for self-diffusion in tin is about 100 kJ/mol. This suggests that dislocation pipe diffusion may be controlling in the midregion. To the author’s knowledge, there is no good explanation for the high stress behavior. A suggestion is that it is because of the very rapid rate of recovery at high stresses.

D.

Sn–Ag and Sn–Zn Eutectic Alloys

Creep tests were conducted on Sn–Ag and Sn–Zn eutectic solders at 25jC and 80jC over the stress range from 10 to 22 MPa and the creep data of log steady state strain rate vs. log stress were fit to straight lines as shown in Fig. 5. The resulting activation energies were 82 and 68 kJ/mol, respectively [7]. These values are lower than the activation energy for creep of Sn. The exponents n were determined to be 11 and 6, respectively. The value 11 is high and may indicate thresholdstress-type behavior. The creep rates in the alloys are lower than that observed in pure Sn. This is mainly attributed to changes in the preexponential term, A, which is a function of microstructure. The theory for this is not well in hand.

FIG. 4 Steady state, stage II, creep of tin at three temperatures against stress. (From Ref. 6.)

MECHANICAL PROPERTIES OF SOLDERS

219

FIG. 5 Steady state creep rate plotted against stress for Sn–Ag and Sn–Zn eutectic solders at 25jC and 80jC. (From Ref. 7.)

IV.

STRESS RELAXATION AT CONSTANT STRAIN

If a constant strain is imposed on a metal or alloy, the stress relaxes with time as the system reduces its free energy. Dislocations are annihilated and the remaining dislocations move to lower-energy configurations. This is the nature of the recovery process. At higher temperatures, diffusional processes equivalent to creep occur. This phenomenon is very important for solder joints in electronic devices because the device spends much time at the strain extremes when it is turned on and off or put into a sleep mode and returned to active duty. Stress at constant strain vs. time curves for Sn–3.5Ag solder at 25jC and 80jC, and at 0.3 % strain maximum are given in Fig. 6(a,b). The stress as a result of the coefficient of thermal expansion mismatches initially decreases very rapidly with time to a more or less steady state value. At 25jC, the steady state value is about 15 MPa and is rather independent of the initial stress value; however, at 80jC, the stress relaxes to zero. Thus when an electronic device is turned on, the thermal stress will relax to a low value possibly zero during use.

FIG. 6 Stress relaxation for a constant 0.3% strain plotted against time for Sn–Ag eutectic solder at (a) 25jC and (b) 80jC. (From Ref. 7.)

220 A.

FINE Phase Transformation Analog

Because stress relaxation at constant strain may be considered to be a phase transformation from a higher to a lower free energy state, the stress relaxation data may be treated by the standardphase transformation equation F¼

ðr  rf Þ t m ¼ 1  eð s Þ ðri  rf Þ

ð9Þ

Here F is the fraction transformed, r is the instantaneous stress, the subscript i refers to the initial stress, the subscript f refers to the final stress, t is time, s is the relaxation time, and m is a constant depending on the transformation mechanism. This is a good approximation when F is not too close to 1. Letting t equal s, F equals 0.63, thus s may be determined from curves of F vs. t. As determined by Mavoori, all relaxation times were quite small, less than 120 sec, s and m depend on the details of the stress relaxation process. Along with other factors, s is inversely related to the activation energy.

B.

Rate of Energy Reduction Model

Mavoori et al. [7] modeled the relaxation process on the basis of the rate of energy reduction, e(dr/dt)=(constant)(driving energy)(frequency factor)(no. of active sites)(energy reduction per event)(probability of a successful event). This leads to the following equation: e

dr ¼ CðerÞðmqÞðabrÞexpðQ=RTÞ dt

ð10Þ

In Eq. (10), m is the atomic vibration frequency, q is the dislocation density, a is the area swept out by a dislocation during an event, b is the Burgers vector, and Q is the activation energy. [A mistake in the reference has been corrected in Eq. (10).] Following the development of the Dorn creep equation, Eq. (10) can be generalized to dr ¼ Arn eðQ=RTÞ dt

ð11Þ

If a threshold stress is present, then the stress relaxation equation becomes dr ¼ Aðr  rt Þn eðQ=RTÞ dt

ð12Þ

1. Parameters for Eutectic Sn–Ag Alloy Mavoori et al. [7] analyzed their stress relaxation at constant strain data for Sn–3.5Ag solder using Eq. (12) and determined the activation energy, Q, to be 34 kJ/mol and a stress exponent of 6. The threshold stress was found to be 10 MPa. How stress relaxation occurs in this solder is insufficiently known to attempt to interpret these numbers.

V.

STRESS–STRAIN RELATIONS

A.

Effect of Strain Rate

Because of the high homologous temperatures, the stress–strain relations for tin-based solders are much affected by strain rate except at very low stresses and strains. As already discussed, this presents difficulties in determining the modulus from a stress–strain curve. Fig. 3(a) from Ref. 3 gives a set of tensile stress–strain curves for Sn–3.5Ag solder at three different strain rates. The elastic line is also given. Deviation from this line is evident even at e = 0.0002. The deviation becomes less as the strain rate is increased.

MECHANICAL PROPERTIES OF SOLDERS B.

221

Three Strain Components

There are three components to the strain: eel, the elastic strain; epi, the time-independent plastic strain; and ept, the time-dependent plastic strain (creep strain). Plotting the inverse secant moduli vs. inverse strain rate separates these 1 etot eel epi ept 1 1 1 ¼ þ þ ¼ ¼ þ þ Esec Eel Epi Ept r r r r

ð13Þ

The time-dependent plastic strain decreases as the strain rate increases and it vanishes as the strain rate approaches infinity. The inverse secant modulus is plotted against square root of the inverse strain rate in Fig. 7(a,b) for three different stress values for Sn–Ag and Sn–Pb eutectic

FIG. 7 Inverse secant modulus vs. the square root of the inverse strain for (a) Sn–Ag eutectic solder and (b) Sn–Pb eutectic solder. (From Ref. 3.)

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FIG. 8 Stress strain curve for Sn–Ag eutectic separated into its three components for the three strain rates of Fig. 2. (From Ref. 3.)

solders [3]. At de/dt = 0, ept is zero and the extrapolated inverse secant modulus would equal the true elastic modulus if there were no epi. This is the case for bulk Sn–37Pb solder but there is a time-independent plastic strain component for bulk Sn–3.5Ag solder. The elastic strain is, of course, obtained from the elastic modulus. Then stress–strain curve is separated into its three components for Sn–3.5Ag solder in Fig. 8 [3].

C.

Effect of Strain Rate on Flow Stress

The Dorn equation can be inverted to give     T de ðQ=RTÞ 1=n e r¼ A dt

ð14Þ

As shown in Fig. 9, after initial work hardening, the flow stress is independent of strain out to large values for bulk Sn–3.5Ag solder. The actual flow stress is a function of strain rate. Constant flow stress means that the work hardening rate equals the recovery rate. This occurs at increasingly higher stresses as the strain rate is increased. Plots of flow stress vs. strain rate from 105 to 102 at 25jC and 80jC were well represented by straight lines. From these data, n was determined to be 12 and Q, the activation energy, was determined to be 110 kJ/mol. Why there is variation in the n and Q values determined from creep, stress–relaxation, and stress–strain relations is not well known and needs much more study. However, empirically determined values are useful for design purposes.

VI.

ISOTHERMAL FATIGUE

Under cyclic loading or cyclic straining, metals fail after a few cycles (low-cycle fatigue, up to several thousand cycles), and after a large number of cycles (high-cycle fatigue, up to several million cycles). The cycles-to-failure increase as the stress amplitude or strain amplitude are reduced on continued lowering; a value is observed where failure does not occur no matter how large the cycle number—the endurance limit. All metals do not have an endurance limit but the first stress or strain amplitude on lowering amplitude that does not give failure after several

MECHANICAL PROPERTIES OF SOLDERS

223

FIG. 9 Stress–strain curves at 25jC and 80jC for Sn–Ag eutectic solder are shown to approximately 4% strain. Strain rate varied from 102 to 105 sec1. (From Ref. 7.)

million cycles is taken as the endurance limit. Most isothermal fatigue experiments are carried out under constant amplitude cycles and the data is plotted as stress or strain amplitude vs. cycles-tofailure. The cycles can be complex with interruptions or dwell times at set places in the cycle such as at maximum stress or strain. Spectrum loading is important for many applications such as an airplane wing, and spectrum-loading fatigue has been extensively studied. Fatigue failure of a metal or alloy may be divided into three stages: fatigue crack nucleation, microcrack propagation and coalescence, and macrocrack propagation to failure. A macrocrack is a few millimeters or larger in length while microcracks can be submicron is size. Because of the small size of interconnections in electronic devices, only the first two stages are of interest. Of course, if a crack is already present, then the fatigue process consists of microcrack propagation and coalescence.

A.

Fatigue Crack Nucleation and Theory

1. Flaws Act as Stress Risers In the absence of a preexisting crack, the fatigue failure process begins with nucleation of one or more cracks. Stress risers or flaws are not cracks; the stress is larger at defects or stress risers. The latter may be part of a designed interconnection but still a fatigue crack needs to be nucleated. In a study of the effect of flaws and inclusions on the fatigue life of Al alloy castings [8], both pore and inclusion size and distance from the surface were found to be important in establishing a fail-safe criterion. A small pore or inclusion near the surface had more effect than a larger one further away. Fatigue failure of interconnections may sometimes be decreased by paying attention to stress risers in the design of the interconnection. For example, stress concentrators are inadvertently designed into ball grid array interconnects. The ball shape results in a stress concentrator near the top and bottom of the interconnect. A geometry that eliminates these stress risers should give longer lifetime against thermomechanical fatigue. 2. Fatigue Crack Nucleation Energy The energy barrier that prevents spontaneous formation of a crack on the first cycle, such as Griffith’s theory for the fracture of brittle solids, is the energy required to form new surfaces. Mura and coworkers [9,10] were the first to theoretically treat this process. In the Mura analysis, dislocations are assumed to accumulate on parallel slip bands during fatigue cycling. When the energy of the accumulation reaches a critical value, a fatigue crack is spontaneously assumed to nucleate. At high homologous temperatures, recovery reduces the rate of dislocation accumu-

224

FINE

lation, but thermal activation aids the nucleation process. This was treated using the concepts of phase transformation theory [11,12]. The energy release or reduction in internal energy when a fatigue crack forms results from the loss of lattice defects at the nucleation site W1 and from the release of stored elastic energy W2. Thus an equation for the free energy change on forming a crack of area A with c being the surface energy required to create a unit crack area can be stated DG ¼ W1  W2 þ cA

ð15Þ

a. Activation Free Energy. Because two surfaces are created when a crack forms, c equals 2cs, twice the surface energy per unit area. For a penny-shaped crack, A is pa2. Because of recovery balancing, steady state work hardening is achieved after a few fatigue cycles. At a sufficiently high homologous temperature, the dislocation density becomes constant and a defect energy density function, cd, assumed constant, can be defined. For a penny-shaped crack, W2 ¼

8ð1  m2 Þ 2 3 r a 3E

ð16Þ

In Eq. (16), m is Poisson’s ratio, E is Young’s modulus, and r is the maximum stress in a cycle. The change in free energy is then DG ¼ pa2 ðc  cd Þ 

8ð1  m2 Þ 2 3 r a 3E

ð17Þ

Differentiating DG with respect to a and setting the result equal to zero gives the critical crack size a* a* ¼

Epðc  cd Þ 4ð1  t2 Þr2

ð18Þ

Substituting a* into Eq. (17) gives the activation free energy DG* DG* ¼

p2 E2 ðc  cd Þ3 48r4 ð1  m2 Þ2

ð19Þ

If c equals cd, a crack spontaneously nucleates because DG* is zero. This is the nucleation condition in the Mura theory. From nucleation theory, the number of cracks, n, nucleated per unit volume per cycle, N, is given by   dn DG* ¼ gtðkT=hÞexp  ð20Þ dN kT where D is the number of potential nucleation sites per unit volume, t is a time in seconds equal to that portion of the cycle where the stress is near maximum, k is the Boltzmann constant, h is Planck’s constant, and kT/h is the frequency factor. Solving for DG* gives   h dn ð21Þ DG* ¼ kT ln kTgt dN b. Other Considerations. The kinetics of fatigue crack nucleation can be studied without knowing the critical crack size as long as the cracks are not interacting and the crack propagation rate is fast compared to the nucleation rate. This concept was used to calculate the unknowns in these nucleation equations [11,12]. This is subsequently discussed. At lower homologous temperatures, cd is not constant but increases with fatigue cycling; there is an increase in dislocation density. Because cd is not measurable, it needs to be eliminated from the equations. This is also subsequently discussed.

MECHANICAL PROPERTIES OF SOLDERS B.

225

Fatigue Crack Morphology

Fatigue cracks in bulk solder specimens and in interconnections are observed to form in shear bands [7,13–15]. Fig. 10 shows the surface of a fatigued Sn–Ag bulk solder specimen where crisscrossing shear bands are observed and consist of arrays of small cracks and slip steps. The small cracks lie along grain boundaries and interfaces between the intermetallic compound and the Sn solid solution. Lin et al. [10] observed that in near-eutectic Pb–Sn solder joints fatigue tested in shear, the plastic deformation was restricted to a shear band located in the solder close to the substrate. The microstructure in the shear band was much coarser than elsewhere in the solder joint, further illustrating the instability against plastic deformation of the initial microstructure. Coarsening of the microstructure in Sn–Ag and Sn–Zn eutectic solders during isothermal or thermal cycling is expected to be less because Ag3Sn or zinc phases are more effective in stabilizing grain size than a lead-based solid solution phase.

C.

Macrocrack Propagation

Macrocrack propagation itself is divided into three stages as described below. On fatigue loading at low values of crack intensity range, DK, such cracks remain constant in length; however, on increasing DK, a threshold is reached. Above this value of DK, the rate of crack propagation per cycle rapidly increases with increase in DK to a region where da/dN = A(DK )n, the Paris relation, holds. Here N is the cycle number, a is the crack length, n and A are constants depending on the material. Microcracks propagate intermittently, i.e., not during every cycle, at values of DK below the threshold. Thus it is not valid to assume a ‘‘large’’ crack if an interconnection is already present and predict fatigue lifetime of the interconnection using the Paris relation with A and n determined for long cracks in bulk specimens of an alloy. Even if a crack is already present, if it is below the macrocrack size, the Paris relation does not hold.

D.

Definition of Failure

Most relatively large scale fatigue tests define failure when the specimen becomes completely separated into two pieces. With electronic interconnections, failure is defined on the basis of unacceptable electrical resistance values or unreliable electrical connections. The sequence of events for fatigue failures in parts whose dimensions are in millimeters, as already discussed, are nucleation of microcracks, usually at flaws or stress risers, propagation, and link up of microcracks to form a macrocrack that grows following the Paris relation. Typically, interconnection sizes are so small that failure occurs in the microcrack regime. At some small size, but large compared to the size of the interconnection, the crack becomes unstable at the peak stress of a

FIG. 10 Isothermal fatigue damage at room temperature on the surface of bulk Sn–Ag eutectic solder showing criss-crossing shear bands. Strain range was 0.3% and the ramp time was 1 sec. (From Ref. 7.)

226

FINE

cycle and catastrophic failure occurs. This may cause only intermittent electrical failure because of the mechanical connection between the crack surfaces. A failure criterion is needed that is early in the fatigue process. 1. Reduction in Maximum Stress Details of the stress–strain hysteresis loops in mechanical fatigue tests are helpful. A schematic hysteresis loops for solders [7] is shown in Fig. 11 for constant tensile strain varying from zero to a maximum value. Important parameters describing the hysteresis loop are defined in the figure. Although the strain is controlled to vary from zero to a maximum tensile value, the stress goes into compression. The maximum stress rmax in a strain-controlled fatigue cycle varies during fatigue straining for several reasons. Work hardening occurs because of an increase in dislocation density from plastic strain cycling. However, recovery as a result of reduction in dislocation density also takes place. In addition, there is slow reduction in rmax as a result of microcracking and rapid reduction when a dominant crack begins rapid growth. Microstructural instability also affects the maximum stress. The sizes of the phases and the grain size in solders increase with time as the homologous temperature is high even at room temperature. A relatively stable microstructure is established in tin-based eutectic solders early in the fatigue process. A balance between work hardening and recovery to give a steady state at the maximum stress in a cycle is also established after a few cycles in these solders. The increase in rate of reduction of maximum stress when the final stage of cracking is reached is a useful criterion. Many other parameters describing hysteresis loops may also be used as failure criteria. Some of these will be shortly discussed. Initially, hysteresis loops are rather symmetrical but as fatigue failure is approached, a kink occurs in the compression part of the cycle as a result of crack closure effects. As subsequently discussed in lead-based tin solders such as 96.5Pb–3.5Sn, work-hardening continues to increase the maximum stress for many cycles although extensive cracking is observed. Eventually, the reduction in rmax due to cracking overcomes the contribution due to work hardening, causing the rmax vs. N curve to go through a maximum. This provides another failure criterion [16]. The variation in maximum stress of the fatigue cycle vs. cycle number for bulk Sn–Ag eutectic solder is shown in Fig. 12 [7]. After an initial transient, the negative slope of the rmax vs. N curve is rather constant for many cycles followed by a rapid decrease in rmax. The initial transient is attributed to establishing a nearly steady state dislocation structure at every stage in the cycle. Because microcracks were observed early in the fatigue test, the constant negative slope stage was attributed to microcracking. Establishment of a fatal crack, which then rapidly propagates, results in a rapid decrease in drmax/dN.

FIG. 11 Important fatigue parameters are defined on a schematic hysteresis loop. rmin is the maximum compressive stress, rmax is the maximum tensile stress, Det is the total strain range, and Deh is the hysteretic strain range.

MECHANICAL PROPERTIES OF SOLDERS

227

FIG. 12 Failure criteria are defined as abrupt decreases in (a) rmax and (b) rmax/rmin. The abrupt changes are noted by an arrow in each case. Note that the former is a more conservative criterion at this total strain range. (From Ref. 7.)

2. Abrupt Change in rmax/rmin Ratio In strain-range controlled fatigue tests, the ratio of rmax/rmin is often used as a failure criterion and is also plotted vs. N in Fig. 12. This ratio is roughly constant but then abruptly decreases when a significant portion of an interconnection has cracked. The maximum compressive stress is larger because the crack closes in compression but opens in tension. In iron, this ratio has been observed to decrease when 15% of the final fracture surface had cracked [17]. For convenience, the 50% drop in maximum stress has been used as a failure criterion in Sn–Pb eutectic-type solders. Fig. 13 compares these two failure criteria to the 50% reduction in rmax. The data were linearized to fit an equation of the form Y = Ax + B. In the high-cycle regime, both are more conservative than the 50% reduction in rmax criterion. As predicted by the empirical equation, when the strain range achieves a 50% reduction in load at 50,000 cycles, the abrupt change in slope occurs at about 40,000 cycles, and the drop in the ratio occurs at about 44,000 cycles. 3. Higher Homologous Temperature Systems Bulk tin–lead solders with high lead content, such as 97Pb–3Sn cyclically harden for many cycles before the maximum stress in the cycle decreases, although cracking is observed early in the fatigue experiment [16] as shown in Fig. 14. The dislocation density increases with N; therefore cd in Eq. (17) is not constant. Thermal energy still assists crack nucleation except at extremely low homologous temperatures. The rate of recovery and the decrease in maximum stress from cracking are less than the rate of hardening. The maximum stress in a cycle when plotted against N goes through a maximum that indicates the beginning of rapid cracking leading to failure. This

228

FINE

FIG. 13 The rmax and rmax/rmin failure criteria are compared to the 50% reduction in rmax criterion. The equations Y = AX + B are plotted, where X is Nf for the 50 rmax criterion and Y is Nf for another criterion. For abrupt decrease in rmax, () A=0.72 and B=977. For the drop in rmax/rmin(o) criterion, A = 0.72 and B = 1915. The correlation coefficients are better than 0.98. The temperature was 25jC and the ramp time was 1 sec. (From Ref. 7.)

type of behavior is observed in many metals and alloys with low homologous temperatures. The maximum in the curve was taken as the failure criterion as is the practice with other metals and alloys [16]. It indicates the presence of a fatal crack or crack array. 4. Electrical Criteria It would be advantageous to establish an electrical criterion for final failure. Precise monitoring of the electrical resistance should provide this needed criterion because the resistance slowly increases as the cracking increases.

E.

Effects of Strain Range and Cycle Period

Fatigue life is the total time-to-failure including ramp times and dwell times in the cycle. It is equal to the number of cycles-to-failure times the period of the cycle. Thus the number of cycles-tofailure must be examined as a function of the cycle period including the ramp and any dwell times as well as the strain range. This was examined in great detail in bulk 96.5Pb–3.5Sn solder [16].

FIG. 14 Peak tensile stress is plotted against cycle number for 96.5Pb–3.5Sn solder. Failure was defined as the cycle number that exhibited the maximum peak stress as indicated by the arrow. The total strain range was 0.75% and temperature was 25jC. To stabilize the microstructure, the sample was heated to 150jC for 2 hrs and then aged at room temperature for 9 days. (From Ref. 16.)

MECHANICAL PROPERTIES OF SOLDERS

229

FIG. 15 Cycles-to-failure vs. plastic strain range for temperatures varying from 5jC to 100jC. (From Ref. 16.)

1. High Lead–Low Tin Solder Increasing the strain range decreases the number of cycles to fatigue failure in materials. Such data for bulk 96.5Pb–3.5Sn solder is given in Fig. 15 for temperatures from 5jC to 100jC. The tensile plastic strain range is plotted vs. Nf on log–log scales. There was no hold in the cycle and the ramp time of the saw-toothed cycles varied from 0.1 to 2.5 sec. On the scale plotted, there is very little difference in the numbers of cycles-to-failure over this range of ramp time. The data were best fit by two straight lines with a break at about 0.3% plastic strain except for 5jC where there were few data points. The empirical Coffin–Manson relation, Nfhep = C (b and C are constants), does not hold over the full tensile strain range tested. Different constants are needed above and below the transition near 0.3% plastic strain. This was attributed to a change in fracture mode from intergranular at low strain ranges to mixed intergranular–transgranular at high strains. This points up the danger of using accelerated tests under extreme conditions that causes failure modes that do not occur under application conditions of the solder to predict reliability. There must be no difference in failure mode between an accelerated test and application conditions for the solder under consideration. As the temperature is increased from room temperature to 80jC, the cycles to failure at a given strain range decrease; however, there is little change on increasing the temperature to 100jC, an unexpected effect. Also unexpected, the cycles to failure decreased on cooling from room temperature to 5jC. 2. Lead-Free Solders The number of cycles to failure vs. total strain range are compared at room temperature for Sn– Pb, Sn–Ag, and Sn–Zn bulk eutectic solders in Fig. 16 over the strain range 0.2–2% [7]. Over most

FIG. 16 The isothermal cycles-to-failure for Sn–Ag, Sn–Zn, and Sn–Pb eutectic solders compared at 25jC vs. strain range. (From Ref. 7.)

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FIG. 17 Cycles-to-failure vs. total strain range in Sn–Ag eutectic solder at 25jC and 80jC. Ramp time was 1 sec. (From Ref. 7.)

of the strain range investigated, the lead-free eutectic solders have longer cycle-to-failure values than Sn–Pb eutectic solder. The bulk eutectic solders are much less sensitive to a change in temperature from 25jC to 80jC than bulk high-lead solder. Data for bulk Sn–Ag eutectic solder are shown in Fig. 17. 3. Effects of Frequency The effects of frequency or cycle period, 1/f, on cycles to failure and fatigue lifetime, tf, are examined in Fig. 18 for bulk 96.5Pb–3.5Sn solder [16]. The cycles-to-failure increases as the frequency increases but a plateau is reached at about 0.05 Hz. There is little increase in Nf as the frequency is further increased. The time-to-failure of course changes in the opposite direction and rapidly decreases at higher frequencies where a plateau in cycles-to-failure occurs. If Nf is constant as frequency is increased, then tf must decrease. The increase in Nf as frequency increases is expected because of the decrease in time for activated events to occur as fatigue failure is an activated process. When the period is long enough for the activated event to occur with high probability, then a further increase in time is less effective. In bulk eutectic solders, Nf changes little as f is increased from 0.002 to 0.5 Hz (Fig. 19) [7]. The plateau begins at lower frequencies for these solders than the high-lead solder. Of course, the time-to-failure decreases as the cycle time decreases. As shown in Fig. 20, the cycles-to-failure decreases in the bulk eutectic solders to a plateau value as the hold time at maximum strain increases [7]. There is a similar behavior in the bulk high-lead solder. The plateau, as in the previous paragraph, occurs because at some value of tc,

FIG. 18 Increasing the fatigue cycle frequency of 96.5Pb–3.5Sn solder at 25jC increases the life (cycles-to-failure) but decreases the time to failures. Strain range was 0.75%. (From Ref. 16.)

MECHANICAL PROPERTIES OF SOLDERS

231

FIG. 19 Cycle number to failure is plotted vs. frequency for Sn–Ag and Sn–Zn eutectic solders. Data for 62Sn–36Pb–2Ag solder is also shown. The temperature was 25jC and total strain range was 1%. (From Ref. 7.)

the cycle time, the probability for occurrence of the activated event approaches 1. The cycles-tofailure moves in the opposite direction to the time-to-failure (Fig. 21). The time-to-failure is given as tf ¼ Nf ð2tr þ th Þ

ð22Þ

where (2tr + th) is the cycle time. Except for an initial transient, the relation between hold time and time-to-failure is linear [12] tf ¼ A þ Bth

ð23Þ

FIG. 20 Effect of hold time at maximum strain on cycles-to-failure is shown for eutectic Sn–Ag, Sn–Zn, and Sn–Pb solders. Temperature was 25jC and Det was 0.6%. Ramps time was 1 sec. (From Ref. 7.)

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FIG. 21 Ref. 7.)

F.

The same data in Fig. 20 is plotted as time-to-failure instead of cycles-to-failure. (From

Determining Cycles-to-Failure Utilizing Fatigue Crack Nucleation Theory

In eutectic Sn–Pb solder, fatigue cracks initiate at Sn–Pb interfaces and Sn–Sn grain boundaries and grow to the extremities of the individual facets with little interaction between neighboring cracks [15]. Based on micromechanics, Stolkarts [18] defined the microcrack density per unit volume, N = n/r3, where n is the number of cracked facets and r is the average radius of the microcracks. The change in microcrack density per unit volume per cycle, DNc, is related to the ratio Eeff/E, where E is Young’s modulus and Eeff is the effective Young’s modulus of a solder with an assembly of cracks. As the total strain amplitude in the cycles was constant in the tests conducted, Eeff/E is approximately equal to r/reff. Here r is the maximum stress during a cycle in the uncracked solder and reff is the maximum stress in the solder with an array of cracks for a given cycle number. There is also a relation between the strain amplitude and the increase in density per cycle. Cutiongco’s [15] data for Sn–Pb eutectic solder fit the relation DNc = 133De3 and dn/dN then equals r3(133De3). To estimate DG*, it is necessary to estimate D, the number of potential nucleation sites per unit volume, in Eq. (21). The number of shared interfaces or potential nucleation sites was estimated from the microstructure to be 1.4  1016 m3. In the model, the just-initiated cracks are assumed to rapidly grow to the extremities of the facets and the reduction in stress is due to facetsized cracks about 3 lm in radius. With a strain range of 0.006, the number of cracks initiated in one saw-toothed cycle from 0 to maximum strain is estimated to be 1016, which is about 10,000 times smaller than the number of sites. Site saturation is not expected for several thousand cycles. Taking t, the time during the cycle that the stress is near maximum, to be 0.1 sec or a tenth of the cycle time, DG* was computed to be 0.4  1020 J at room temperature. Substituting this value into Eq. (19) and with E = 35 GPa, r = 25 MPa, m = 0.37, ccd computes to 104 J/m2. Because the surface energy, c, is approximately 1 J/m2, cd, the energy of the dislocations, must essentially equal this value. If cd were zero, DG* computes to 2.5  109 J. This is so much larger than kT at room temperature that dn/dN would be zero [see Eq. (20)]. A substantial dislocation pile up at an interface is needed to nucleate a fatigue crack. As already mentioned in the Mura theory, which neglects thermal activation, the fatigue crack nucleates when the dislocations pile up at a singularity until c = cd. The present theory assumes that at high homologous temperatures, a

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steady state dislocation distribution is established after some cycling. This dislocation distribution must be very high to give the dn/dN value estimated from the data. Using ccd = 104 J/m2, a*, the critical crack radius, computes to 6 nm from Eq. (19). Lin [10] observed fatigue cracks as small as 200 nm along slip bands in copper, and Kwon [19], using replicas, observed fatigue cracks that were only 100 nm deep along slip bands in copper. The justnucleated cracks in copper could have been smaller because of the limitations of the observation techniques. If ccd = 103 J/m2, a* is predicted to be 60 nm. The model thus predicts a ‘‘ball park’’ value for the critical crack radius from the observed decrease in rmax per cycle.

VII.

THERMOMECHANICAL FATIGUE

The thermomechanical fatigue (TMF) effect is the change in fatigue life by superimposing a temperature cycle onto a mechanical cycle. As already discussed, this arises in electronic components from turning them on or off or putting them into a ‘‘sleep cycle.’’ The components, being joined electrically and mechanically, have different coefficients of thermal expansion (CTE), which cause thermal strains and the associated thermal stresses. These, of course, are reduced if the mismatches in CTE of the components attached to each other are reduced. This should be a first consideration in designing a device where reduction in interconnection thickness is a goal. The temperature–strain cycles are complex because there are hold times at each temperature extreme in a cycle whether it is a full cycle between on and off, or a minicycle between the ‘‘sleep temperature’’ and the maximum temperature in a full cycle.

A.

Thermomechanical Behavior in Solders

1. High Lead–Low Tin Solder During a cycle, both temperature and strain are in phase because the temperature differential between the materials that are joined causes the strain. Their instantaneous temperatures determine the expansion or contraction of each material and thus the instantaneous strain. While the thermomechanical fatigue effect is a function of phase angle at least in high lead–tin solder [20], the concern here is mainly with zero phase angle. The thermomechanical fatigue of bulk high lead–tin solder has been studied much more thoroughly than any other solder [20]. The TMF effect is very complex. With in-phase cycling between the 25jC and 80jC and a cycle period of 200 sec, the TMF effect was substantially negative, but the effect diminished with cycle time. The TMF cycles-to-failure (saw-tooth waveform) increased with cycle time while the 80jC isothermal values slightly decreased. The two curves crossed at a 500-sec cycle time; for longer times, the TMF effect is positive, as shown in Fig. 22. Introducing a hold time at the maximum strain and temperature reduced the cycles-to-failure as with isothermal testing. Certainly more work is needed to fully explain all of this. Lawson [20] hypothesized that crack growth occurs by vacancies interacting with an oxide layer and he developed a model that was consistent with experimental observations of the effects of frequency, hold time, strain range, and phase angle changes. The complexity of the effects certainly means that design of devices and microelectronic components utilizing lead-free solders requires an in-depth investigation of the bulk and joint solder characteristics to understand the reliability of these materials. The many years of experience with Sn–Pb solders does not exist for lead-free solders. 2. Lead-Free Solders The thermomechanical cycles-to-failure for bulk specimens of Sn–Ag and Sn–Zn eutectic solders are compared in Fig. 23 over the total strain range from approximately 0.2% to 2% [7]. The thermal cycle range was between 20jC and 80jC without a hold time in the cycle and a ramp time of 50 sec. Two failure criteria were used, 50% reduction in load and abrupt drop in load. At least under these conditions, the Sn–Zn eutectic solder was more resistant to TMF failure over all of the strain ranges tested. The overall strain range is almost equal to the total strain range because the elastic strain range is very small. Further, as the plastic strain is inhomogeneous, the plastic strain range in the shear bands is very much higher.

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FIG. 22 Cycles-to-failure for thermomechanical (TMF) 25–80jC and isothermal fatigue at 80jC in 96.5Pb–3.5Sn solder vs. cycle period. The TMF effect is negative for periods less than 60 sec but positive at longer cycle periods. (From Ref. 20.)

The TMF effect for the 50-sec ramp time can be evaluated by comparing the lead-free solder curves in Fig. 23 with isothermal cycles-to-fatigue failure data at 20jC and 80jC (Fig. 17) [7]. For both Sn–Ag and Sn–Zn eutectic solders at 1% total mechanical strain range, the cycles-to-failure in bulk samples are about 4000 cycles at 25jC, and 3000 cycles at 80jC. These results are little affected by a change in ramp time from 0.5 to 500 sec. The TMF cycles-to-failure at 1% strain and 50-sec ramp time is about 200 cycles for bulk specimens of Sn–Ag eutectic solder, and about 300 cycles for Sn–Zn eutectic solder. The TMF effect is in the range of 100–200%. A similar effect occurs with Sn–Pb eutectic solders but appears to be somewhat smaller [15]. Predicting the cyclesto-failure by strain-range partitioning, i.e., integrating over a cycle using the isothermal data and time spent at each temperature, gives a value of cycles-to-failure for thermomechanical fatigue much larger than the true value at the 50-sec ramp time investigated.

FIG. 23 Cycles-to-thermomechanical-fatigue failure (20–80jC) plotted against total strain range for Sn–Ag and Sn–Zn eutectic solders. The ramp time was 50 sec. The mechanical and thermal cycles were in phase. The plot also allows a comparison to be made between the 50% reduction from smax and abrupt load drop failure criterion. (From Ref. 7.)

MECHANICAL PROPERTIES OF SOLDERS B.

235

Superimposed Hysteresis Loops

The isothermal hysteresis loops for Sn–Ag eutectic solder at 25jC and 80jC and the thermomechanical fatigue cycle between 25jC and 80jC are superimposed on each other in Fig. 24, which represents the first complete cycle for 1% mechanical strain. The TMF heating-load cycle initially follows the 25jC curve but then drops smoothly to the 80jC curve; then, during the cooling–unloading cycle, it initially follows the 80jC curve but eventually returns to the 25jC cycle curve. Both Sn–Zn and Sn–Pb eutectic solders exhibit similar behavior [7,15].

C.

Thermomechanical Fatigue in Joints and Components

There have been several published studies of TMF of led-free solders utilized in joints and components. Choi et al. [21] characterized the cracking in Sn–Ag eutectic solder and also Sn– 4Ag–0.5Cu solder joints between alumina and copper that were cycled between 40jC and 125jC. The joints were first examined after 250 cycles. Grain boundary cracks adjacent to the Ag3Sn intermetallic layer and adjacent to the alumina substrate were the ones that led to failure. They propagated along grain boundaries in the solder and not at the interfaces. There was no evidence of grain coarsening as has been observed with Sn–Pb solders. Palmer et al. [22] recorded hysteresis loops during TMF of bulk specimens of Sn–Ag, Sn–Bi, and Sn–Pb. The strain during the temperature cycle was applied from the attachments to the specimens having different coefficients of thermal expansion. It was suggested that the accumulated energy absorption could give an insight into the failure process. All but a tiny fraction of the mechanical work is stored as damage in a specimen, the rest is heat loss. However, a moving crack driven by an applied strain and the associated stress does plastic work at the crack tip. The associated thermal energy is lost to the surroundings. The accumulated summation of the plastic work of crack propagation is a measure of the extent of cracking. The accumulated hysteretic energy obtained from the hysteresis loop areas is a measure of the heat loss, and it should be a measure of the extent of cracking. This is a promising approach.

D.

Solubility Effects

Thermomechanical fatigue of solders may be further complicated because the compositions and volume fractions of the phases present vary with temperature. In contrast to Sn–Pb solders, this is likely to only be a minor problem with Sn–Ag eutectic solder because the solubility of silver in solid tin is very small at all temperatures. This is also true for Sn–Cu-based solders. With Sn–Znbased solders, the effect of temperature on the microstructure is minimal because the maximum solubility of Zn in Sn is only 0.6 at.% at the eutectic temperature. Although the phase

FIG. 24 Thermomechanical fatigue at 25–80jC thermal cycle and isothermal fatigue hysteresis loops at 25jC and 80jC are compared. Mechanical strain range was 1%. There was no hold in the 50-sec ramp time cycle. (From Ref. 7.)

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composition and volume fractions are constant, continued temperature cycling of high homologous temperature solders with added strain cycling significantly coarsens their microstructures. As with isothermal fatigue, the plastic deformation in TMF is inhomogeneous. In bulk specimens of Sn–Ag and Sn–Zn eutectic solder, criss-crossing shear bands are seen and fatigue cracks form in these shear bands [7]. The particle sizes of the minor phases coarsen, and there is some evidence for cavitation particularly in Sn–Zn eutectic solder during 25–80jC and return. As with joints made with Sn–Pb solders [13,14], Sn–Ag and other Sn-based, Pb-free solder joints are expected to develop thin shear bands parallel to the shear direction. However, because the solubility of zinc or silver in tin varies little with temperature and the microstructure is expected to be more resistant to coarsening than Sn–Pb eutectic solder, grain coarsening and thus shear band formation is expected to be less of a problem.

VIII.

FINAL COMMENTS

While much is known about the mechanical properties of the Sn–Ag and Sn–Zn eutectic solders, the knowledge base on fatigue both isothermal and thermomechanical has many gaps. An interdisciplinary research program including theory, science-based modeling, microstructure, and properties is needed for whatever lead-free solders are selected for application. The many years of experience with Sn–Pb solders allows confidence in utilizing them in design but this is lacking for lead-free solders. The research must address actual solder joints themselves including size effect as well as studies on bulk specimens to obtain fundamental properties. Thermomechanical fatigue studies of solder joints where the strain ranges, temperature range, dwell times, ramp times, specimen size, and phase angle are varied and controlled are time-consuming, difficult, and costly, but necessary. However, such information correlated with microstructural studies is needed for a science-based methodology for reliability prediction. Otherwise, each selected solder and processing procedure will require a separate and unique evaluation.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Vaynman, S.; Ghosh, G.; Fine, M.E. Effects of palladium and solder aging on mechanical and fatigue properties of tin–lead eutectic solder. J. Electron. Mater. 1998, 27, 1223–1228. Kloske, R.A. Solid solution strengthening and yield drop effects in Au–Ag alloy single crystals containing 1 to 5 and 95 to 99 at. pct. Ag. Trans. Metall. Soc AIME 1969, 245, 217–225. McCabe, R.J.; Fine, M.E. Athermal and thermally activated plastic flow in low melting temperature solders at small stresses. Scr. Mater. 1998, 39, 189–195. Garofalo, F. Fundamentals of Creep and Creep—Rupture in Metals; Macmillan Series in Materials Science: Macmillan, New York, 1965. Weertman, J. Microstructural mechanisms in creep. In Mechanics and Materials; Meyers, M.A., Armstrong, R.W., Kirchner, H., Eds.; Wiley: New York, 1999 (chapter 13). McCabe, R.J. Creep of InSb and SbSn Tin-based Alloys: Effects of Precipitate Size, Shapes and Spacing, Ph.D. Thesis, Northwestern University, Dec. 2000. Mavoori, H.; Chin, H.; Vaynman, J.; Noran, B.; Keer, L.M.; Fine, M.E. J. Electron. Mater. 1997, 26,783–790. Seniw, M.E.; Conley, J.G.; Fine, M.E. The effect of microscopic inclusion locations and silicon segregation on fatigue lifetimes of aluminum alloy A356 castings. Mater. Sci. Eng. 2000, A285, 43–48. Mura, T.; Nakasone, Y. A theory of fatigue crack initiation in solids. J. Appl. Mech. 1990, 57, 1–6. Lin, M.R.; Fine, M.E.; Mura, T. Fatigue Crack Initiation on Slip Bands: Theory and Experiment, Ph.D. Thesis, Northwestern University, 1984. Acta Metall. 1986, 34, 619–628. Fine, M.E. Phase transformation theory applied to elevated temperature fatigue. Scr. Mater. 2000, 42, 1007–1012. Fine, M.E.; Stolkarts, V.; Keer, L.M. Fatigue crack nucleation assisted by thermal activation. Mater. Sci. Eng. 1999, A272, 5–9. Morris, J.W., Jr.; Mei, Z. Solder Mechanics, A State of the Art Assessment, EMPMD Monograph Series; Frear, D.R., Jones, W.B., Kinsman, K.R., Eds.; TMS: Warrendale, PA, 1990; Vol. 1, 239–270 (Chapter 6: Toward New Solders with Improved Fatigue Resistance). Frear, D.; Grivas, D.; Morris, J.W. A microstructural study of the thermal fatigue failures of 60Sn– 40Pb solder joints. J. Electron. Mater. 1988, 17, 171–180.

MECHANICAL PROPERTIES OF SOLDERS 15.

16. 17. 18. 19. 20. 21. 22.

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Cutiongco, E.C. Fatigue of Near-Eutectic Lead–Tin Alloy for Surface Mount Technology, Ph.D. Thesis, Northwestern University, 1991; Cutiongco, E.C.; Jeannotte, D.A. Proc. Morris E. Fine Symposium; Liaw, P.K., Weertman, J.R., Marcus, H.L., Santner, J.S., Eds.; TMS: Detroit, Oct. 1990; 435–444. Vaynman, S. Isothermal Fatigue of 96.5Pb–3.5 Sn Solder, Ph.D. Thesis, Northwestern University, 1988; Vaynman, S.; Fine, M.E.; Jeannotte, D.A. Isothermal fatigue of low tin lead based solder. Metall. Trans., A 1988, 19A, 1051–1059. Cooper, C.V.; Fine, M.E. Fatigue microcrack initiation in polycrystalline alpha–iron with polished and oxidized surfaces. Metall. Trans. A 1985, 16, 641–649. Stolkarts, V.; Keer, L.M.; Fine, M.E. Damage Evolution Governed by Microcrack Nucleation with Application to the Fatigue of 63Sn–37Pb, PhD Thesis, Northwestern University, J. Mech. Phys. Solids 1999, 47, 2451–2468. Kwon, I.B; Fine, M.E.; Weertman, J. Microstructural Studies on the Initiation and Growth of Small Fatigue Cracks at 298, 77 and 4.2 K in Polycrystalline Copper, Phd Thesis, Northwestern University, Acta Metall. 1989, 37, 2927–2936. Lawson, L.R.; Fine, M.E.; Jeanotte, D. Thermomechanical Fatigue of a Lead Alloy, PhD Thesis, Northwestern University, Jour. of Materials Research 1993, 8, 745–757; Metall. Trans. A 1991, 22A, 1059–1070. Choi, S.; Subramanian, K.N.; Lucas, J.P.; Bieler, T.R. Thermomechanical fatigue of Sn–Ag solder joints. J. Electron. Mater. 2000, 29, 1249–1257. Palmer, M.A.; Redmond, P.E.; Messler, R.W. Thermomechanical fatigue testing and analysis of solder alloys. J. Electron. Packag. 2000, 122, 48–54.

8 Sn–Ag and Sn–Ag–X Solders and Properties Karl J. Puttlitz IBM Corporation, East Fishkill, New York, U.S.A.

I. A.

INTRODUCTION Use and Compatibility

Lead–tin (Pb–Sn) solders have been utilized in electronic packages and assemblies for more than 40 years to attach components to printed circuit boards (PCBs) owing to their unique combination of properties of low melting point, wetting, and mechanical characteristics, and are relatively inexpensive. In addition, these materials are compatible with component lead and finish materials, and allow processing in a temperature range in which card materials and components maintain their integrity even when exposed to multiple thermal excursions as happens during rework operations. Lead–tin solders have demonstrated a high level of compatibility with electronics assembly processing and materials over the years with only a few exceptions, most notable is the effect of even relatively minor gold concentrations to cause embrittlement within solder joints [1].

B.

Migration to Lead (Pb)-Free Solders

There are two main drivers motivating the migration to lead-free solders: legislative-based issues and health concerns related to lead (Pb)-containing materials including solder. Also, there is pressure to support the growing worldwide movement to remove lead from processes and products to demonstrate environmental consciousness and maintain a competitive posture in the market place. However, assuring the reliability of solder joints made with lead-free materials is difficult due to the lack of field experience and lack of fundamental property data upon which to construct reliable service life models [2]. 1. Tin–Silver-Based Systems of Key Interest The lead-free solder alloys attracting the greatest attention are the tin-rich alloys based on the tin– silver (Sn–Ag) and tin–copper (Sn–Cu) systems and their ternary alloys, Sn–Ag–Cu [3–5]. Numerous studies have been reported on Sn–Ag–Cu near-ternary alloys investigating processing and mechanical property behavior, and also their solidification characteristics [6–9].

C.

Challenges for Lead-Free Solder Alloys

Each of the roles and functions of solder joints in electronic assemblies becomes increasingly more difficult to achieve owing to the lighter, faster, smaller, cheaper trend, particularly portable,

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and hand-held electronics products. This is perhaps best exemplified by Moore’s Law that predicts an exponential growth in the number of transistors on a single chip, doubling the number every 18 months. This, in fact, has been the case for over a decade. The impacts, thus challenges, of accommodating the large increase in transistors by traditional methods of increasing the die size and decreasing the interconnection (i.e., solder joint) pitch [10] are discussed in the following sections. 1. Increased Die Size Although a reduction in feature sizes referred to as shrink is one of the ways to accommodate an increased transistor count, it is often not sufficient to provide the required I/O counts, necessitating an increase in die size as well. However, as the strain that a solder joint experiences is directly proportional to the distance from the neutral point (DNP, i.e., the temperature invariant point), this can have a significant impact on the reliability of solder joints located in the outer rows of area-array, flip chip devices. An increased die size may result in a maximum DNP that poses a potential reliability concern for some applications, requiring that additional precautionary steps be taken, among them enhanced cooling capability, chip underfill, and increased inherent fatigue resistance of the solder. 2. Decreased Solder Joint Pitch Another measure to accommodate an increased transistor count is to decrease the interconnection (i.e., solder joint) pitch [10]. A decreased pitch is typically accompanied by a reduction in solder joint diameter. The presence and influence of flaws, such as voids, and particulate debris whose size remains relatively constant becomes more important with a reduced solder joint cross section. Stresses that a solder joint experiences due to a mismatch in the coefficient of thermal expansion (?CTE) between the materials a solder joint is attached to also increase with a pitch reduction. This arises from the fact that the stress is inversely proportional to the height of the solder joint that typically decreases with a pitch reduction as well. 3. Increased Joint Current Density and Temperature Even if the solder joint size did not decrease, chip power requirements would increase simply because of the increased number of transistors on a chip, causing an increased thermal dissipation. Even with cooling enhancements, chip temperatures are anticipated to raise over time. Increasing temperature serves to accelerate the mechanisms that give rise to failure in solder joints. a. Creep-Induced Thermomechanical Fatigue. Thermal fatigue is considered the primary threat to the reliability of solder joints because they typically experience many thermal excursions during the course of their applications due mainly to equipment on/off cycles. Stress is generated during the course of each excursion or thermal cycle due to both CTE mismatches as noted earlier, and inhomogeneities in the pattern of heating that cause thermal gradients. The dominant stresses tend to be in the shear mode. Because solders are utilized at high homologous temperatures (ratio of the ambient or operating temperature to the melting point in absolute temperature, i.e., Kelvin), they tend to undergo significant creep deformation, especially at the hightemperature part of the cycle, resulting in failure by creep-induced fatigue [11,12]. b. Thermo and Electromigration. Attendant with the exponential increase in the number of transistors over time as predicted by Moore’s Law is the near-exponential increase in power as well [10]. Therefore future packages will operate at much higher currents and temperatures, so lead-free solder joints will be required to exhibit good thermo and electromigration resistance properties, and have thermally stable microstructures [13]. Reductions in solder joint cross sections will further aggravate the situation, and particularly so of small area-array components such as flip chips and fine-pitch chip scale packages (CSPs). Severe current crowding can be anticipated in the via region of flip chips with high current flow (i.e., high current flux densities), making them vulnerable to both thermo and electromigration whose mechanism and details are discussed in Chapter 20.

SN–AG AND SN–AG–X SOLDERS AND PROPERTIES D.

241

Response to Stresses

The empirically generated field life prediction relationships that have been utilized for Pb–Sn solders are not automatically applicable to Pb-free solders. Solder joints as noted experience stresses due to thermal excursions, excessive electrical current, and also due to various mechanically applied loads. The tolerance or resistance and responses to stress as a result of these sources are typically different for Pb-free solders compared to the benchmark eutectic Sn– Pb solder. As noted, stresses in a solder joint can result due to a coefficient of thermal expansion (CTE) mismatch between package materials, but can also be due to temperature fluctuations caused by power transitions or environmental changes given the complexity of some modern microelectronic package structures and the environments they are placed in respectively. The ability of solder joints to endure mechanically induced stresses will also be an important factor for Pb-free solder joints due to the increased number and popularity of hand-held electronics continuously introduced into the market place. The solders must be resistant to mechanical influences such as shock impact, bending, and twisting [14].

E.

This Chapter

Taking the foregoing concerns into account this chapter discusses in detail the key metallurgical aspects including as-solidified microstructure, thermal stability characteristics, effects of elemental additions, and thermal aging, etc. as they relate to the Sn–Ag, Sn–Cu and their ternary alloys (Sn–Ag–Cu) into account. These alloy systems are the subject of much attention as solder replacement candidates for solders of the Pb–Sn system, particularly eutectic Sn–Pb. In addition, the chapter discusses the major mechanical properties of interest for electronic packaging applications as a consequence of their metallurgical characteristics.

II.

TIN–SILVER (SN–AG) SYSTEM

Several binary eutectic or near-eutectic alloys have been suggested as replacements for eutectic Sn–Pb, among them are Sn–Bi, Sn–In, and Sn–Ag, which have all been utilized in electronics applications [15–17]. However, the Sn–Ag system has a limited solubility of Ag in Sn, making it more resistant to microstructural coarsening, and the Ag3Sn precipitate is uniformly distributed, resulting in an overall structure that has the attributes of being more reliable [18,19]. Eutectic tin–silver (3.5% Ag) is also attractive because it is free of toxic Pb, offers better mechanical properties than eutectic Sn–Pb [20], is better in high-temperature applications due to its higher melting temperature, and exhibits good wetting behavior on copper and copper alloys [21]. Several higher temperature solutions have emerged in the Sn–Ag system which typically include the elemental additions of Cu, Bi, or Sb [22].

A.

Microstructural Aspects

1. As-Solidified Microstructure The as-solidified microstructure of eutectic tin–silver is relatively coarse and nonuniform. It consists of two phases, large globules of essentially pure tin (less than 0.04% Ag in solution) dendrites ranging in size from about 20 to 200 mm. The second phase is an intermetallic compound (IMC), Ag3Sn, referred to as epsilon (q) phase comprising less than a 5% volume fraction. The eutectic structure consists of the rod-like (q) phase in a nearly pure tin matrix. The lamellar morphology apparently is not routinely observed for eutectic Sn–Ag compared to eutectic Sn–Pb [23]. It has, however, been observed that the region between the Sn dendrites consists of a lamellar two-phase eutectic structure of a Sn matrix and Ag3Sn IMC phase with an elongated morphology [16]. The presence of the Sn dendrites is indicative of a nonequilibrium cooling rate. However, the structure is noted to be stable as the Sn dendrites were observed to be present after months of room temperature aging [16].

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In nonequilibrium cooling, characteristic of solder joints, the rod-like (q) phase which typically forms, tends to minimize the surface free energy. This occurs in phase transformations when the relative volume fraction of one phase is less than 1/p (i.e., approximately 0.3%) [24]. The size and morphology of q-phase depends on the cooling rate during solder solidification.

B.

Effect of Copper Dissolution

In solder joints, molten Sn–Ag solder is typically in contact with Cu termination pads and component leads. The copper dissolves into the solder and forms Cu–Sn IMCs both in the bulk ´ solder and at the Cu interfaces. Two compounds can form Cu6Sn5 (D) or an ordered version D at room temperature, and Cu3Sn (q). However, the addition of a small amount of Cu to a Sn–Ag-based solder alloy can help prevent the dissolution of Cu base metal [25]. At the same time it is possible to seed a fine, uniform dispersion of small precipitates of ternary or quaternary alloying elements to produce fine and homogenous microstructures [26]. 1. Mobility of Copper It has been reported that the diffusion coefficient of Cu at 25 jC along the a and c axes of Sn is about 0.5108 and 2106cm/sec, respectively [27]. These high diffusion rates are an indication that the nucleation and growth of Cu–Sn IMCs can easily occur at elevated temperatures.

C.

Microstructural Stability

The formation, growth, and morphology of the Cu–Sn IMCs formed greatly depend on the soldering process or any process with varying thermal conditions [28]. But it has been demonstrated that the various microstructures that can be generated through several soldering processes (i.e., heating and cooling rates) will not survive an exposure to elevated temperatures, although it has been suggested microstructural integrity can be maintained throughout the service life [29]. As noted in the previous section, the Sn–Ag microstructure remains relatively stable when exposed to relatively low operating temperatures, or for short periods at higher temperatures. However, the as-soldered microstructure becomes significantly altered if exposed to high temperatures, or for long times at moderate temperatures [23].

D.

Intermetallic Compound Layers and Morphology

The formation of Cu6Sn5 at copper interfaces such as lead frames and chip carrier pads occurs more rapidly in the case of eutectic Sn–Ag compared to eutectic Sn–Pb. However, the addition of 0.5% to 1.7% Cu to eutectic Sn–Ag reduces the IMC formation at interfaces and also lowers the melting point of the alloy [30].

E.

Effect of Heating Rate

Heating rate also plays an important role. A study was conducted comparing standard infrared furnace heating to a laser heating process. A peak reflow temperature of 235jC with a 200-sec hold period was conducted with an infrared furnace. The soak period was generally representative of manufacturing practice. It was observed that this heating resulted in (q) phase (Ag3Sn) rods oriented perpendicular to the joint interface, indicative of the solidification direction of the solder. A much faster heating rate was achieved utilizing a Nd:YAG continuous wave laser, resulting in very fine Ag3Sn IMC formed, only visible at a very high magnification (about 4000). The cooling rate was also very rapid, 104jC/sec compared to about 10jC/sec under IR reflow conditions [31]. Rapid cooling promotes nucleation but severely limits growth. The laser heating method superheated the solder to high local temperatures, typically 400–600jC compared to the IR reflow (240–260jC). Superheating increases the copper dissolution rate from termination pads, resulting in an increased copper concentration in the solder. The dissolution of

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243

copper is more dependent on peak temperature than time at temperature, even for short time periods. The significant copper dissolution in the solder results in the precipitation of greater quantities of Sn–Cu IMCs. Those Sn–Cu IMCs formed at the solder/Cu interfaces were observed to be much thinner in the case of solder joints that were created by laser heating compared to standard IR-heated joints. This resulted from the fact that the reaction time (i.e., time above the melting temperature) was so short, coupled with the rapid cooling rate that limited growth.

F.

Melt Temperature

Additions of the elements Cu, Bi, Sb, or In can be used to reduce the melt temperature of eutectic Sn–Ag. Eutectic Sn–Ag has a high melt temperature (221jC), so assemblies must be exposed to temperatures as high as 240jC to achieve adequate wetting. However, typically adding a constituent to lower the melting temperature also results in creating a melting range. Consider the alloy Sn–3.4Ag–4.8Bi that melts between 208jC and 215jC. Although it melts at a somewhat lower temperature than eutectic Sn–Ag, it does so over a 7jC temperature range. This and other lead-free alloys have been found suitable for reflow attaching conventional surface mount components. But fillet lifting in the region of the annular ring of pin-in-hole (PIH) components during wave soldering is a significant concern for alloys that freeze over a temperature range of 5– 10jC or more. Fillet lifting susceptibility also tends to increase with substrate thickness and feature size, and also as CTE mismatch increases [32].

G.

Localized Microstructure Coarsening

Thermal fatigue and mechanical creep in solders occur more rapidly when elevated temperature cycles are involved as discussed in Chapter 19. Accordingly, one of the methods to improve solder joint reliability is to modify the system to achieve and maintain a fine and uniform microstructure. Systems whose major constitutes exhibit a significant mutual solid solubility for each other, especially at elevated temperatures (as is the case with the Pb–Sn system) do not maintain a refined microstructure. That is, these alloys often are not stable over time, resulting in microstructural coarsening. Instability is often confined to localized regions within a solder joint, brought on by strains concentrated near the terminal pads of solder joints. These coarsened bands serve as crack initiation sites from which fatigue cracks propagate and eventually result in failure. An advantage of the Sn–Ag system, which has a limited solubility of Ag in Sn, is that it is intrinsically more resistant to coarsening than Pb–Sn solders [16].

H.

Aging

Aging studies are conducted as an indicator of long-term microstructural stability under field conditions, i.e., how the microstructure evolves with time and temperature. The IMC Ag3Sn that is dispersed throughout the bulk solder coarsens, whereas the dendritic Cu3Sn IMC in the bulk solder was observed to decrease. A layer of Cu3Sn (q-phase) forms nearest to the copper [33]. 1. Temperature Effect Because of kinetic considerations, joints aged at a lower temperatures exhibit slower microstructural development and evolution. That is, both the IMC phases (i.e., Ag3Sn, Cu6Sn5) show little signs of coarsening in the bulk solder even after a year, accompanied by a very moderate increase in IMC thickness at Cu interfaces. The effect of temperature is apparent in a study conducted by Yang, et al. [23] who evaluated Sn–Ag solder joints of 132-pin, quad flat pack (QFP) components that were surface mount attached to FR-4 boards. The components were of the gull-wing type with copper alloy leads spaced on a 25-mil pitch. The solder joints were aged at several temperatures (40jC, 90jC, 140jC, and 190jC) for periods ranging between 1 and 300 days. The results of those aging studies are summarized in Fig. 1, which shows little or no structural changes in solder joints aged at 40jC, but very significant changes in those aged at higher temperatures, e.g., 140jC and 190jC.

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FIG. 1 Effect of aging time and temperature on the microstructure of Sn–Ag solder joints. Test specimens were aged at: a) 40jC, b) 90jC, c) 140jC, and d) 190jC. (From Ref. 23.)

2. High Temperature Stability Based on eutectic Sn–Ag’s higher melting point, and better mechanical properties (discussed in Section 2.J) at room temperature, the alloy is expected to exhibit better high-temperature performance than eutectic Sn–Pb. Of particular interest are applications such as under-the-hood electronics in automobiles, where the service temperature can reach 180jC. Although better than eutectic Sn–Pb, it is apparent from Fig. 1 that the Sn–Ag microstructure is also unstable at higher temperatures. 3. Interface Intermetallic Compounds It is important to note the differences observed in the nature of the IMC layer formed at the copper interface. At room temperature, and when aged at both 40jC and 90jC, the IMC layer only consisted of Cu6Sn but was found to consist of both Cu6Sn5 and Cu3Sn when aged at higher temperatures (140jC or 190jC) on the printed circuit board, but not component leads. Alloying elements such as Ni, Si, and Mg may hinder the formation of q phase, while enhancing the growth of the D-phase [34].

I.

Growth Kinetics of Intermetallic Compound

The growth kinetics of the Cu6Sn5 (D) and Cu3Sn (q) IMC phases has been found to follow a parabolic-growth-kinetics model [34,35], W ¼ wo þ kt1=2

ð1Þ

where W is the thickness of the IMC layer at any time, wo is the initial thickness of an IMC layer, k is the growth rate of the IMC layer, and t is time. 1. An Example The growth rate for D is taken to be, k(D)=8.99109 m/sec 1/2, and q is k(q)=4.43109 m/sec1=2 [35,36].

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Mechanical Properties

The microstructure consists of mostly pure Sn phase, which is much softer than the eutectic structure. The nonuniform distribution of this phase is believed to contribute to the eutectic Sn– Ag alloy’s creep and fatigue behavior. In those cases where a layered IMC structure forms at a copper interface, the presence of the Cu3Sn (q) phase may have an adverse effect on the reliability of solder joints including Sn–Ag, because of the hard, brittle characteristics of that IMC [37]. This can occur as a consequence of strains generated due to a CTE mismatch among the key elements comprising a solder joint: the copper or copper alloy termination pad, component lead, Cu–Sn IMC at the interface, and the solder. If the thermally induced strains are sufficient, cracks are initiated at the hard and brittle Dphase which can grow and propagate.

K.

Wetting Characteristics

1. Effect of Gold The enhanced solder spreading characteristics observed for eutectic Sn–3.5Ag solder on gold surfaces compared to copper are thought to be due to the greater affinity that tin-based alloys have in reacting with gold, as evidenced by how readily Au–Sn IMCs are formed [21]. 2. Effects of Atmosphere The reflow atmosphere has also been found to have a significant effect on the wetting time of Sn– 3.5Ag. It was determined that the wetting time could be reduced by a factor of approximately 1.5 depending on the nature of the reflow atmosphere. However, in the same study it was observed that the benchmark eutectic Sn–Pb solder displayed a greater sensitivity to reflow atmosphere, and that the Sn–3.5Ag alloy generally exhibited better wetting characteristics. Accordingly, it is anticipated that the wetting behavior of eutectic Sn–3.5Ag would be affected less than eutectic Sn–Pb to drastic changes in oxygen content of the reflow atmosphere [21].

III.

TIN–SILVER–BISMUTH (SN–AG–BI) SYSTEM

A.

Microstructure

A wide range of compositions in the Sn–Ag–Bi system has been reported [38–40]. When the Bi content is low (approximately 1%) it is primarily located within the lattice sites of the Sn matrix as a substitutional solid solution. However, when the Bi content exceeds about 1%, it precipitates out as a second phase at room temperature [41].

B.

Effect on Melting Point

Elemental additions of Bi convert the Sn–Ag binary alloy, which melts at a single temperature (221jC), to a ternary alloy that melts over a range of temperatures. For example, Sn–3.3Ag–4.7Bi melts between 210jC and 215jC. The use of bismuth additions is not an efficient way to reduce the melt temperature of Sn–Ag, as it is estimated that a 1% Bi addition results in about a 1.7jC temperature depression for Sn–Ag alloys in the 160jC to 230jC melt temperature range [41].

C.

Presence of Low-Melt Constituents

1. The Concern As the migration toward lead-free microelectronic assemblies moves forward, there will be a hybrid transition period. Some surface-mount-component lead frame terminations will be plated with Sn–Pb solder coatings. The nominal lead (Pb) content in a typical solder joint depending on the plating thickness for Sn–Pb solder compositions is given in Fig. 2. The effect of intentionally adding 3% and 6% Pb on the melting characteristics of several Sn–Ag–Bi-based alloys, on Sn–

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FIG. 2 Effect of component termination pad plating thickness on the nominal lead (Pb) content in surface mounted solder joints for several alloys. (From Ref. 42.)

Ag–Cu, and Sn–Bi is shown in Fig. 3. The concern is the creation of low-melting phases either due to the addition of Bi, Pb, or a combination of the two that would cause a significant reduction in mechanical properties. Of particular concern is a reduction in fatigue resistance resulting from localized deformation due to an increased creep rate relative to the higher melting point material. 2. Effect of Bismuth No low-melting point constituents were detected by differential scanning colorimetry (DSC) studies in any of the alloys shown in Fig. 3 that did not contain lead if the Bi content was less than 10%. This was also reported to be the case by Huang and Lee for Sn–Ag–Bi alloys with less than 7.5% Bi [43]. Bismuth lowers the melting point of Sn–Ag alloys, but a concentration of 5% Bi or more is required to make a reasonable difference in the reflow temperature [42]. It is encouraging that no evidence of the low-melting (138jC) Sn–Bi binary eutectic was noted to have formed due to inhomogenous solidification at the low Bi-content levels (i.e., <12%) typical of Sn–Ag–Bi alloys [43]. 3. Combined Effect of Bi and Pb However, as indicated, the introduction of lead (Pb) into the solders in Fig. 3 resulted in lowering the melting point ternary reactions. There was no change in the temperature at which the lowtemperature reactions occur among the alloys tested whether they contained 3% or 6% lead. However, the onset of first melting was noted to decrease significantly with increasing Bi content. For example, with 2% Bi, melting was observed to start at 155jC for 3% Pb and at 168jC for 6% Pb. The melt temperature was observed to decrease to approximately 135jC with 4% Bi, and then remained relatively unchanged up to 7.5% Bi for both the 3% and 6% Pb content alloys [42]. The 135jC melting constituent was identified to be a product of a peritectic reaction in the Sn–Bi–Pb system [44]. The apparent ternary eutectic and peritectic constituents form at a lower temperature in the case of Sn–Ag–Bi alloys compared to Sn–Ag–Cu alloys [42].

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FIG. 3 Differential scanning calorimetry (DSC) data indicating the onset of the first endotherm on heating for several lead additions to various lead-free solders. (From Ref. 42.)

D.

Mechanical Properties

1. Strength and Ductility Tin–silver alloys have been found to contain a precipitated Bi phase if the Bi content exceeds more than about 1% Bi. As bismuth is a brittle metal it is not surprising that these alloys typically exhibit a lower ductility and fatigue life compared with eutectic Sn–Ag alloy. Accordingly, when

TABLE 1

Attributes and Characteristics of Sn–3.5Ag Alloys with Bismuth Additions Alloy composition

Attributes/characteristics Fatigue resistance

Melt range

Strength properties

Source: From Ref. 41.

Sn–3.5Ag–1.0Bi – For high temperature applications – Exhibits a 130% higher fatigue life than eutectic Sn–Pb . Somewhat better than eutectic Sn–Ag

Sn–3.5Ag–4.7Bi – Slightly better than eutectic Sn–Pb – Slightly lower than eutectic Sn–Ag

– 210jC to 215jC, pasty range is 5jC . Compared to 221jC MP of eutectic Sn–Ag – Exhibits a better yield and tensile strength than eutectic Sn–Pb – 73% and 137% better yield and tensile strengths, respectively, than eutectic Sn–Ag

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the Bi content exceeds approximately 5%, the plasticity is significantly reduced. As might be expected, the ductility decreases with an increasing Bi concentration, the shear strength was found to increase up to about 6.8% Bi, then decreased [41]. 2. Creep Resistance Alloys with a significant Bi concentration possess a superior creep resistance. For example, the steady-state creep rate of the alloy 90Sn–3.2Ag–6.8Bi was found to be consistently lower than eutectic Sn–Pb when tested in the stress range of 8 to 80 MPa. The superior creep resistance is believed due to Bi precipitates blocking dislocation movement, or impeding dislocation movement due to dragging, and reduced vacancy diffusion [41]. a. Optimum Bi Addition. The optimum Bi addition depends upon the intended application conditions as indicated in Table 1.

IV.

TIN–SILVER–BISMUTH–INDIUM (SN–AG–BI–IN)

A.

Applications

Adding In to Sn–Ag–Bi offers some additional benefits for surface mount technology (SMT) manufacturing by relieving some reflow process requirements.

B.

Microstructure

Indium content in the range of 1.7% to 4.0% mainly enters the Sn matrix lattice as substitute solute atoms similar to Bi atoms in concentrations less than 1%. Between 3.3% and 3.5% Ag, there is a limitation in both Bi and In content because of competing metallurgical interactions. Alloys with about 3.0% to 3.7% In, in which there is a deviation in Ag content (between the range 3.3% and 3.5%), can result in generating proeutectic dendrites [45].

C.

Optimizing Composition

If the objective is to lower the melting temperature, the Ag content must be optimized to achieve both a low Bi and In level consistent with the properties desired [45]. 1. Effect of Silver A silver content between 3.3% and 3.5% is optimal because the melting temperature is minimal at those levels, whereas the mechanical properties are at a maximum. Increasing the silver content beyond 3.5% to 4.0% only serves to reduce the melting temperature by a few degrees, but has a very deleterious effect on mechanical properties [45]. 2. Effect of Bismuth As noted earlier, Bi only plays a moderate role in reducing temperature. 3. Effect of Indium Indium plays a major role in reducing the melt temperature. The optimal content is between 1.7% and 4.0% In, because its effect beyond 4% may not be justified due to cost considerations.

D.

Mechanical Properties

The indium-containing alloys investigated (i.e., In content=1.7%, 3.7%, and 4.0%) have an equivalent or greater strength than eutectic Sn–Pb or eutectic Sn–Ag. As noted, the Bi content should be limited because its presence degrades the plasticity of Sn– Ag solder joints. The most important advantage of Sn–Ag–Bi–In alloys from a mechanical properties perspective is that they require a lower Bi content than Sn–Ag–Bi alloys to achieve equivalent performance. For example, 92Sn–3.3Ag–4.7Bi is optimal for the Sn–Ag–Bi system.

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Attributes and Characteristics of Sn–Ag–Bi Alloys with Indium Additions

Sn–Ag–Bi Alloys with In additions Mechanical 92Sn–3.3Ag–3.0Bi–1.7In 90Sn–3.3Ag–3.0Bi–3.7In

91.5Sn–3.5Ag–1.0Bi–4.0In Melting temperature Sn–Ag–Bi–(1.7, 3.7, or 4.0In)

Attributes/characteristics – Most cost effective composition . Low In content – Melt range 206–211jC, only 5jC pasty range – Best balance between melt range and mechanical properties (strength, plasticity, and fatigue life) – Offers the highest fatigue life – All three alloys have a lower melting temp. than: eutectic SnAg (221jC), Sn–Ag–Bi, or Sn–Ag–Cu alloys – Good candidates for SMT manufacture – Each only has a pasty range of 4jC or 5jC

Source: From Ref. 45.

However, with a Bi content greater than 4%, some mechanical properties are compromised on a par with eutectic Sn–Pb. A workable range of Bi content is between about 1.0% and 3.0%. Its use should be minimized to prevent a reduction in fatigue resistance. Accordingly, if the Bi content is held to 1% and the Ag level is between 3.3% and 3.5%, then 4% In can be added to maximize the fatigue life. Indium additions beyond that do not serve to further increase the fatigue life [45]. Sn–Ag–Bi–In compositions with a proper balance of Bi and In content possess superior creep properties. As previously noted for Bi, In solute atoms also cause dragging and impeding dislocation movement, and vacancy diffusion [45]. The attributes and characteristics of Sn–Ag–Bi alloy with In additions are summarized in Table 2.

V.

TIN–SILVER–ANTIMONY (SN–AG–SB) SYSTEM

A.

Microstructure

Antimony additions may result in the precipitation of Sn–Sb IMC particles or react with Ag to form Ag3Sn IMC. That is, there is a competition in the Sn–Ag–Sb system to form Ag3Sn vs. Ag3Sb or ternary Ag–Sn–Sb IMC [46].

B.

Effect on Melt Temperature

Antimony additions of 1% to eutectic Sn–Ag have little or no effect on the melt temperature. It is believed that at this low level, the eutectic Sn–Ag alloy phase transformation still dominates. However, a 3% Sb addition to eutectic Sn–Ag causes the liquidus temperature to increase from 221jC to 225jC [46].

C.

Mechanical Properties

The yield strength and tensile strength were observed to peak at 1% Sb for the Sn–3.5Ag–Sb system and thought to be the optimal composition for mechanical properties. The optimal Sb content depends upon the particular properties of interest. For example, the eutectic Sn–Ag alloy exhibits maximum strength at 1% Sb, but maximum plasticity at 2.0% Sb, and maximum fatigue life at 1.5% Sb. However, the 1% Sb alloy is considered to provide the best balance or compromise of properties. It provides a higher strength and fatigue life compared to eutectic Sn–Ag without a detectable increase in melt temperature [46].

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TABLE 3 Attributes and Characteristics of Sn–3.5Ag Alloys with Antimony Additions Sn–3.5Ag alloys with antimony additions Sn–3.5Ag–1.0Sb

Sn–3.5Ag–2.0Sb

Attributes/characteristics – Compared to eutectic Sn–Pb . Higher flow strength . Lower fatigue life – Compared to eutectic Sn–Ag . Higher flow strength . Lower fatigue life – Maximum plasticity occurs at 2% Sb – Compared to eutectic Sn–Pb . Higher plastic strain at fracture . Lower strength properties

Source: From Ref. 46.

The attributes and characteristics of Sn–3.5Ag alloys with Sb additions are summarized in Table 3.

VI.

TIN–SILVER–ZINC (SN–AG–ZN) SYSTEM

A.

Microstructure

1. Ternary Additions with Specific Solubility Conditions The creep and thermal fatigue resistance of eutectic Sn–Ag alloy can be significantly enhanced without raising the melt temperature through ternary alloy additions. This is best achieved by introducing an element that is not soluble in solid tin, but soluble in silver. The purpose of this type of alloying addition is to assist in seeding a fine, uniform dispersion of precipitates. This condition in combination with the respective solubility of Sn and Ag in each other preserves the microstructural stability of the alloy. Zinc is a reasonable addition to achieve these objectives, because the solubility of Zn in solid Sn is minimal, and substantial in solid Ag [16]. 2. Effect of Minor Zinc Additions As noted earlier, the typical as-solidified Sn–Ag binary microstructure is both coarse and nonuniform. However, if 0.5% Zn is added, although the phase distribution is similar, the Sn– dendrite structure is finer and more uniform. If the Zn concentration is increased to 1.0%, the formation of Sn dendrites is nearly suppressed, and the microstructure is relatively uniform consisting of distinct grains with small finely dispersed Ag3Sn precipitates. The zinc was found to be confined within the Ag3Sn phase. However, unlike the elongated shape of Ag3Sn precipitates observed in binary Sn–Ag alloys, the precipitates in a 1% Zn alloy are both finer (f3000 A) and have a more spherical shape. Zinc apparently has a sphereodizing and refining effect on Ag3Sn precipitates [16]. 3. Effect of Zinc on Melting Point Depression The addition of zinc results in a melting point (liquidus temperature) reduction. For example, the melting point of Sn–3.5Ag (binary eutectic alloy) is 221jC, which is reduced to 217jC for Sn– 3.5Ag–1.0Zn [16].

B.

Zinc Reactivity

Zinc is a very reactive metal in both the liquid and solid state, and care must be exercised even when utilized as a minor constituent addition. When present in liquid Sn–Ag–Zn solder, the Zn

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that is exposed to air readily oxidizes, and even small amounts can result in unacceptable levels of dross (oxide layer at the air/solder interface) formation. This can be circumvented by utilizing an inert (oxygen-free) atmosphere such as nitrogen, argon, or oil submersion. The use of inert atmospheres is discussed in detail in Chapter 15. Zinc additions can also lead to corrosion problems. In fact, zinc coatings are often used as sacrificial anodes, to avoid corrosion of the objects which are coated with zinc for that purpose. However, the corrosion potential of Zn is greatly reduced if it is combined, i.e., not present as a separate and distinct phase. For example, when Zn is combined with corrosion-resistant Ag within the Ag3Sn IMC phase, then the beneficial effect of Zn (i.e., microstructural refinement) can be used with minimal corrosion concern [16].

C.

Mechanical Properties

The very fine and uniform two-phase microstructure created by the addition of 1% Zn is believed to be responsible for both the significantly enhanced strength and good ductility observed for this alloy compared to the binary eutectic Sn–Ag alloy [16]. 1. Effect of Zn Additions on Strength and Ductility The initial additions of Zn (up to 0.5%) have a dramatic effect on the yield and tensile strength of eutectic Sn–Ag as shown in Fig. 4. Those properties are nearly optimized at 1.0% Zn and not significantly altered up to 2% Zn, but reduced beyond that. The ductility, as based on elongation, decreases linearly with Zn additions. The 1% Zn alloy provides a good balance: optimum strength, a minimum loss of ductility (possess an elongation of nearly 35%), and a fine uniform phase morphology [16]. 2. Effect of Zn Additions on Creep Deformation The addition of 1% Zn significantly improves the creep resistance as shown in Fig. 5 and fatigue properties of Sn–Ag. For example, a 1% Zn addition was determined to decrease the creep deformation of eutectic Sn–Ag by an order of magnitude, and three orders of magnitude lower than near-eutectic Sn–Pb (40/60) under the test conditions of 125jC and a 1-ksi load [16].

FIG. 4 Effect of zinc content on the tensile properties of Sn–3.5Ag–XZn solders. (From Ref. 16.)

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FIG. 5 Effect of a 1% zinc addition on the creep deformation of Sn–3.5Ag at 125jC, 1 ksi. (From Ref. 16.)

D.

Wetting

The wetting characteristics of ternary Sn–3.5Ag–1.0Zn alloy appear comparable to binary eutectic Sn–Ag [16].

VII.

TIN–SILVER–COPPER (SN–AG–CU) SYSTEM

The binary systems (SnAg, SnCu, AgCu) that comprise the Sn–Ag–Cu ternary system are given in Fig. 6.

A.

Structures

In binary systems, over a range of compositions in which the eutectic reaction takes place and where one of the solid phases, a, is a nonfaceted phase, and the other, h, is faceted, it often occurs

FIG. 6 Calculated equilibrium binary phase diagram of tin (Sn)–silver (Ag), tin (Sn)–copper (Cu), and silver (Ag)–copper (Cu) systems. (From Ref. 8.)

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that the nonfaceted phase grows more rapidly than the faceted phase. Accordingly, high degrees of undercooling can lead to the formation of dendrites, even in alloys with compositions at or near the h side of the eutectic. To produce a dendritic-free, eutectic-like structure at high degrees of undercooling, a starting composition even further to the h side of the eutectic is required [47]. It has been determined that Sn–Ag [48] and Sn–Cu [49] are characteristic of this type of behavior, because the eutectic compositions of both alloys contain large volumes of Sn dendrites in their microstructures when cooled rapidly. Tin is the nonfaceted phase in both eutectic alloys, and the phases Ag3Sn and Cu6Sn5 are the faceted phases, respectively [50].

B.

Ternary Eutectic Composition and Melting Point

There have been several studies investigating the Sn–Ag–Cu ternary eutectic composition and associated melting point. It was initially reported to be Sn–4.7Ag–1.7Cu (in wt.%) [4], and also Sn–3.24Ag–0.57Cu [8]. However, significant variations in microstructure can occur as a result of cooling rate complicating the determination of the eutectic Sn–Ag–Cu composition and temperature. Loomans and Fine [50] conducted a careful study utilizing several techniques: DSC, metallographic, analytical, and SEM analysis of selected annealed rather than as-solidified alloys. They defined the eutectic composition to be: Sn–3.5Ag–0.9Cu (wt.%). The work of Snugovsky et al. [51] observed the presence of primary Cu6Sn5 in the microstructure of Sn–4.7Ag–1.7Cu, thus demonstrating that alloy to be Cu-rich compared to the eutectic defined by Loomans and Fine. Additionally, it was demonstrated that a small quantity of primary Ag3Sn was present in Sn– 3.5Ag–0.7Cu, but almost completely ternary eutectic, indicating the composition to be only slightly deficient in copper. Shugovsky et al. confirmed that the microstructure of the alloy Sn– 3.5Ag–0.9Cu was entirely ternary eutectic and had a melting point of 217.2jC, thus confirming the values reported by Loomans and Fine. 1. Tin-Rich Corner of the Sn–Ag–Cu System A considerable amount of work has been done to establish the tin-rich corner of the Sn–Ag–Cu ternary system with several isothermal sections given in Fig. 7 that are consistent with those obtained by Loomans and Fine [50]. The results of Moon et al. indicated the eutectic composition to be Sn–3.5 (F0.3) Ag–0.9 (F0.2) Cu at 217.2 (F0.2)jC [52].

C.

Microstructure

1. General Ternary Concepts The solidification of ternary alloys can occur with as many as three distinct steps. Similar to binary systems, solidification is initiated with the formation of primary crystals (L!a) and continues over a range of temperatures until a binary eutectic composition and temperature are attained. However, unlike a binary system, solidification of the binary eutectic (L!a+h) does not occur at a stationary temperature, but over a range of temperatures, often referred to as a pseudo-binary eutectic reaction. It does so based on thermodynamic considerations, a nonvarient binary eutectic reaction, i.e., it has one degree of freedom, and binary in the sense that only two solid phases form from the liquid. Finally, the liquid cools to the ternary eutectic temperature and composition, having the lowest melting point of any alloy in the system. The remaining liquid freezes to form the ternary eutectic (L!a+h+g) at a stationary temperature. The initiation of each stage can be detected utilizing temperature sensing techniques during heating and cooling. Heating is typically preferred. The supercooling characteristic of Sn–Ag–Cu alloys discussed in an earlier section is observed when cooling from the melt. A peculiarity is the difficulty of observing the liquidus temperature during melting [52]. 2. Negligible Solubility The solubility of the solute atoms in the IMC phases (i.e., Ag in Cu6Sn5 and Cu in Ag3Sn) is quite limited. The solubility of both Ag and Cu is also limited in Sn, the matrix phase. As the solidification and melting of alloys in the Sn–Ag–Cu system involve phases with negligible

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FIG. 7 Calculated isothermal sections of the Sn–Ag–Cu system at several temperatures. (From Ref. 52.)

solubility ranges, there is little opportunity for microsegregation to occur within the phases. Predictions based on lever-rule calculation of the equilibrium phase diagram are quite valid during melting and solidification [52]. 3. Formation of Eutectic Phases Due to Simultaneous or Coupled Growth Eutectic microstructures, whether binary or ternary, are typically fine given the nature of their formation. That is, the formation of any phase comprising a eutectic cannot occur too far out of step relative to that of any other eutectic phase due to thermodynamic constraints. These phases form nearly simultaneously, or the time between the formation of an individual particle of one phase and that of another phase is very short to maintain the compositional requirements as dictated by thermodynamics. These short periods for forming individual phases result in creating a fine mixture of the eutectic phases. The simultaneous or near-simultaneous formation of the eutectic phases is sometimes referred to as coupled growth. That is, each of the eutectic phases solidifies at essentially the same rate at the eutectic isotherm (i.e., temperature). a. Coupled Zones. There are factors that can influence the simultaneous formation of all the eutectic phases at the eutectic temperature. Among the most important factors is the nature of the solidifying phases, i.e., whether they are faceted or nonfaceted. A so-called coupled zone is a region located on a phase diagram below the eutectic isotherm. 1. Non-faceted eutectic phases: If the solidifying phases are nonfaceted, the region is symmetrical about the eutectic point (Fig. 8). This is the situation required to have a fine

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FIG. 8 Schematic depicting a coupled zone for a binary system with nonfaceted solidifying phases. (From Ref. 53.)

eutectic mixture that forms at the eutectic temperature, i.e., forms according to the equilibrium phase diagram. 2. Presence of faceted eutectic phases: If one or more of the solidifying phases is faceted, it effectively results in an offset or skew of the coupled region toward the faceted phase(s) as illustrated in Fig. (9). The offset compensates for the increased driving force necessary to create a faceted phase at the same rate as a nonfaceted phase. However, due to the higher driving force requirement (i.e., larger degree of undercooling is required) as indicated in Fig. 10 for compositions at or near the eutectic composition with at least one faceted phase, it may be sufficient to cause departure from the coupled zone. In those cases primary dendrites of the nonfaceting phase are formed, as it is energetically beneficial to do so. This is believed to be the reason for the frequently observed formation of Sn dendrites in Sn–Ag–Cu eutectic solder [51,53]. b. Nature of Eutectic Sn–Ag–Cu Phases. In the case of the Sn–Ag–Cu system, the IMCs Ag3Sn and Cu6Sn5 form, which have orthorhombic crystal structures, but can also be represented by hexagonal unit cells [54,55]. Their hexagonal character is exhibited in the ternary eutectic

FIG. 9 Schematic depicting a skewed coupled zone for faceted–nonfaceted binary eutectic system. (From Ref. 53.)

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FIG. 10 Schematic depicting the effect of faceting on the composition required for coupled growth to take place in a faceted–nonfaceted binary eutectic system. (From Ref. 53.)

microstructure. For example, when a faceted phase forms during solidification from the melt, the faceted planes that form are the close-packed planes of the crystal lattice. In the case of Ag3Sn it is the basal plane of the hexagonal unit cell, hence its plate-like morphology; whereas for Cu6Sn5 the close-packed prism planes define the characteristic hexagonal cross section observed for this phase [51]. c. Growth of Faceted Phases. Faceted phases such as Ag3Sn and Cu6Sn5 project from the solid/liquid interface into the liquid during their formation and growth as illustrated in Fig. 11. Growth takes place preferentially in directions parallel to the facet plane on nonfaceted interfaces, but slowly in directions perpendicular to the facet planes, creating phases whose shapes are much longer in length compared to their width [51]. This accounts for the plate-like and rod shapes observed for Ag3Sn and Cu6Sn5, respectively. 4. Solidification The near-ternary eutectic Sn–Ag–Cu solidifies into three phases: h-Sn, Ag3Sn, and Cu6Sn5, as reported by several investigators [6–8]. It has been determined that Ag3Sn nucleates first as the platelets only require a minimal degree of undercooling [6,9]. However, the h-Sn phase has been found to require a very significant amount of undercooling [6,8,9] to initiate solidification that provides sufficient time for nucleated Ag3Sn platelets to become quite large because they grow rapidly in the liquid phase [56–58]. Undercooling is the temperature difference between the melting point typically measured on a heating curve and the onset temperature for solidification of a phase. Large Ag3Sn platelets were observed to have adverse effects on the fatigue life of some solder joints due to strain localization effects at or near the interface of the platelets and surrounding h-Sn phase [57]. In fact, the Ag3Sn platelets subtended the entire cross section of some ceramic ball grid array (CBGA) solder joints.

FIG. 11 Schematic depicting the growth of faceted phases such as Ag3Sn and Cu6Sn5 into the liquid resulting in shapes that grow much faster (thus longer) in length than the width direction.

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5. Morphology The microstructure between the IMC phases comprises about 98% of the volume and is composed of the dendritic-Sn phase. The dendritic pattern forms due to supercooling prior to the formation of Sn. Interestingly, supercooling is observed with Sn–Ag–Cu although large IMC particles are present, indicating that the Sn phase is not easily nucleated, even when heterogenous nucleation sites are available. It has been noted that the microstructure between preexisting large IMC particles can be thought of as an independent solidification process, whose microstructure resembles that of an alloy whose composition is much more Sn-rich than the actual alloy. The solidification sequence in the region between large IMC phases generally is 1) dendritic Sn, 2) nonvarient liquid (L)!Sn+Ag3Sn, 3) followed by the remaining liquid solidifying as a ternary eutectic mixture, L!Sn+Ag3Sn+Cu6Sn5. Moon et al. [52] investigated alloy compositions that lie along A and B in Fig. 12 and determined that the sequence of solidification for alloy A3 and B3 in Figures 13 and 14 is 1) dendritic Sn, 2) followed by the monovarient liquid (L)!Sn+Cu6Sn5, 3) with the remaining liquid solidifying as the ternary eutectic mixture (L!Sn+Ag3Sn+ Cu6Sn5). The two IMCs can be distinguished in that Ag3Sn appears as a platelet structure, whereas Cu6Sn5 is more blocky in appearance having a hexagonal outer outline with a hollow or pipe-like center. Microstructural observations can be confusing because in cross sections, a plate often appears as a rod or needle. The hollow center feature of the Cu6Sn5 rods allows them to be distinguished from the Ag3Sn phase in cross section [52]. a. Pseudo-Binary Eutectic Solidification. Because the pseudo or monovarient binary eutectics each contain a nonfaceted phase (unlike Sn–Pb), the intermetallic phases do not always grow in perfect coordination with the Sn phase. The size of the intermetallic phases formed is indicative of the solidification stage. Consider for example the IMC phase Ag3Sn in alloys A1, A2, B1, and B2 in Figures 13 and 14 that forms as intermediate size particles during solidification as a pseudo or monovarient binary eutectic, but as very fine particles when solidifying as part of the ternary eutectic. For the alloys A3 and B3, the intermediate-sized Ag3Sn particles are replaced with intermediate-sized Cu6Sn5 particles [52]. b. Absent Ternary Eutectic Microstructure. The solidification path of the liquid between the large-sized IMC particles is not always as noted earlier. That is, the last step, the solidification

FIG. 12 The three monovarient eutetic reaction lines (solid) intersect at the ternary eutetic composition of the Sn–Ag–Cu system. The alloys studied by Moon et al. lie along the Sections A and B (dotted lines).

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FIG. 13 Experimentally determined phase boundaries for section A in Fig. 12. (From Ref. 52.)

of the ternary eutectic mixture, i.e., L ! Sn+Ag3Sn+Cu6Sn5, may not occur due to the competition between the pair of pseudo-binary eutectic reactions [52].

D.

Effects of Cooling Rate

A study was conducted [58] to determine the effect of cooling rate, that varied between 0.012 and 8.3jC/sec, on the microstructure of a series of Sn–Ag–Cu alloys that varied in Ag content (3.0% to 3.9%), but varied little in Cu content (0.5% to 0.7%). A variety of microstructures may be formed depending on the cooling conditions as illustrated in Fig. 15. For example, solder joints

FIG. 14 Experimentally determined phase boundaries for section B in Fig. 12. (From Ref. 52.)

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FIG. 15 Series of scanning electron microscope (SEM) micrographs that show the effect of cooling rate on the microstructure of Sn–Ag–Cu alloys with a silver content of 3.0, 3.5, and 3.9 wt.%. Each alloy was cooled at three rates: rapid cool (RC), medium cool (MC), and slow cool (SC). The micrographs for the various alloys based on Ag content are RC: a) 3.0, b) 3.5, c) 3.9; MC: d) 3.0, e) 3.5, f ) 3.9; SC: g) 3.0, h) 3.5, and i) 3.9. (From Ref. 58.)

may experience slow cooling rates in the case of large components, or thick printed wiring boards (PWBs) that possess a high heat capacity. Solder joints can also experience fairly rapid cooling during their formation if components are small or light and the PWBs are thin. The experimental conditions and their characteristic microstructures are summarized in Table 4. In general, the size of the primary Ag3Sn dendrites was observed to become increasingly large during slow cooling from the melt, and also increased in size as the alloy Ag content increased. It was therefore recommended that the Ag content should be minimized to avoid this situation. Independent of the cooling rate, all the Sn–Ag–Cu alloys investigated in this study, covering the normal composition range of interest, exhibited a fine microstructure consisting of primary h-Sn grains in a eutectic matrix network with Ag3Sn as a precipitated phase. In another study conducted by Kang et al. [9] that involved several Sn–XAg–Cu alloys, where X=2.0 to 3.8, large Ag3Sn plates were observed to form in all alloys cooled at a rate of 1jC/ sec or less. Accordingly, it was determined that the plate formation can be substantially reduced at cooling rates of 1.5jC/sec or greater during a solder reflow operation. Although Ag3Sn plate formation and growth can be kinetically inhibited by controlling the cooling rate, it may not always be a practical approach, especially in cases involving large-mass assemblies and chip carriers resulting in card warpage and other undesirable effects.

E.

Effect of Silver (Ag) Content on Ag3Sn Plate Formation

The effect of silver content in Sn–Ag–Cu solder alloys was investigated [9] with silver varying between 2.0 and 3.8 wt.%. Even with very slow cooling rates (e.g., 0.02jC/sec) Ag3Sn plates were only rarely observed in alloys with less than 3.0% Ag content. But slow-cooled, low-Ag-content alloys were observed to contain rods consisting of Cu6Sn5.

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TABLE 4

Effect of Cooling Rate on the Microstructure of Some Sn–Ag–Cu Alloys

Cooling

Alloys

Rate (jC/sec)

Type Slow cooling (SC)

0.012

Mild cooling (MC)

0.43

Rapid cooling (RC)

8.3

Sn–3.5Ag–0.5Cu

Sn–3.5Ag–0.7Cu

Sn–3.9Ag–0.6Cu

All exhibit a coarse eutectic structure . Particularly characteristic, completely loses fine eutectic network structure Similar microstructure to rapid cool (RC) . Eutectic network is much wider than under RC conditions . Area fraction of coarse eutectic region is much larger than for RC Primary h-Sn is surrounded by a fine eutectic matrix . Presence of coarse eutectic particles . Network microstructure is not uniform

Source: From Ref. 58.

F.

Rate and Effects of Copper Dissolution

Copper is soluble in Sn–Ag–X and other lead-free alloys (as noted in Fig. 7 in chapter 1 but less so than near-eutectic Sn–40Pb. This results in the formation of two IMCs, q phase (Cu3Sn) and D phase (Cu6Sn5). The copper-rich Cu3Sn forms directly adjacent to copper land patterns or component leads and has a planar structure. The tin-rich phase, Cu6Sn5, forms adjacent to the Sn-rich solder and typically exhibits a scalloped-edge appearance. The molten solder often extends all the way to the Cu3Sn/Cu interface between the Cu6Sn5 grains. The Cu3Sn layer is usually thin so the channels serve as a fast diffusion and dissolution path of Cu in the solder to feed the interfacial reaction [59]. The effect of Cu content in Sn–Ag–Cu alloys was investigated by Kang et al. [9] for copper contents ranging between 0% and 0.7% and slow cooled at a rate of 0.02jC/sec. No perceptible difference in large Ag3Sn plates was noted for alloys containing 0.35% or 0.7% Cu. Interestingly, when no Cu was present as in the case of binary Sn–3.5Ag and ternary Sn–Ag–Bi alloys that were part of the study, only a few small Ag3Sn plates were detected, but not very frequently. This suggests that the presence of Cu somehow influences the nucleation of Ag3Sn plates, but is not nearly as important as Ag in causing plate formation. Apparently, only a small number of Cu atoms are necessary to nucleate a Ag3Sn plate. This is believed to account for the reason why there was not much difference observed in the amount of Ag3Sn for a Cu concentration of 0.35 or 0.7 wt.%. This consequence is of some concern, because in real solder joints Cu atoms are dissolved into the molten solder from the terminal pad metallization. The dissolved Cu atoms can also cause excessive precipitation of Cu6Sn5 particles near the termination interfaces, causing local hardening of the solder.

G.

Effect of Ag and Cu Content on Pasty Range

The pasty range is the temperature difference between the liquidus temperature and solidus temperature, DT. That temperature difference has been determined to be very sensitive to Cu, but

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not Ag content in Sn–Ag–Cu alloys in the hypoeutectic Ag composition range. The importance of the pasty temperature range is that, the greater the range the greater the sensitivity to fillet and pad lifting in pin-in-hole joints. It is judged that the Cu content should not be greater than 0.7 to 0.9 wt.%. Also, a high Cu content can lead to the formation of an excessive quantity of Cu6Sn5 rods, especially when Cu metallization is used in solder joints [9].

H.

Microstructural Effects of Aging and Reflow Cycles

A comprehensive microstructural study was conducted by Lee et al. [60] to determine microstructural differences between a standard ball grid array alloy (62Sn–36Pb–2Ag), and two leadfree alloys (Sn–3.5Ag and Sn–3.0Ag–0.5Cu). The processing and stressing temperatures are listed in Table 5. 1. As-Reflowed Solder Ball Microstructure All three solder alloys dissolved the 1.0-Am Au layer to form the IMC AuSn4 throughout the solder ball bulk, and a second IMC, Ag3Sn. The Ag3Sn was distributed evenly in Sn–Pb–Ag alloy, but preferentially along the phase boundary of the Sn-rich grains in the Pb-free alloys. The microstructures are shown in Fig. 16. A very thin layer (approximately 0.5 Am) of Ni–Sn IMC was found to exist at the termination pad/solder interface of the Sn–Pb–Ag and Sn–Ag alloys. But the morphology of the Ni–Sn IMC was much different from Cu6Sn5 at the interface which typically has a scalloped appearance [61,62]. A ternary compound existed at the termination pad/ Sn–Ag– Cu solder interface [60]. 2. Aging of the Pb–Sn–Ag System Significant microstructural changes were observed to occur as a result of aging at 150jC. In the case of Sn–Pb–Ag solder, most of the AuSn4 IMC disappeared with aging time and noted to become part of a ternary IMC at the substrate interface (Au,Ni) Sn4 that has also been reported by others [63–65]. The ternary IMC ultimately evolved into a two-layer structure at the interface after 48 to 90 hr at 150jC as shown in Fig. 17(a). The AuSn4 within the bulk Sn–Pb–Ag solder also converted to the ternary compound, (Au, Ni)Sn4. Also, islands of Pb-rich phase formed near the interface, which were noted to form a continuous layer several microns thick after 1000 hr of aging and was located directly on top of the other two layers, Ni3Sn4 and (Au,Ni) Sn4 as shown in Fig. 17(a). a. Formation of (Au, Ni) Sn4 Phase. The distinctive difference between the Sn–Pb–Ag system and the Sn–Ag or Sn–Ag–Cu system is the formation of the second brittle IMC phase, (Au,Ni) Sn4 upon aging. Ho et al. [64] ruled out the possibility of it being formed as part of a gravity-driven process. Lee et al. [60] suggested that the Pb-rich phase in the bulk solder balls

TABLE 5 Process and Stress Conditions for a Standard Ball Grid Array Alloy (62Sn– 36Pb–2Ag) and Two Lead-Free Alloys (Sn–3.5Ag and Sn–3.0Ag–0.5Cu) Test/features Substrate test vehicle

Reflow temperature, jC

Reflow time, sec Multiple reflows Thermal aging Source: From Ref. 60.

Test conditions – Material, BT – Pad diameter, 0.6 mm – Electroplate, 0.6 ANi/0.1 AAu over Cu trace – 40jC above liquidus temperature – 270jC for Sn–Pb–Ag – 260jC for Sn–Ag, Sn–Ag–Cu – 40 sec. dwell (time above liquidus temperature) Up to 20 Up to 1000 hr

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FIG. 16 Scanning electron microscope (SEM) micrographs depicting the microstructure of a) Sn– 36Pb–2Ag and b) Sn–3.5Ag ball grid array solder balls reflow attached to a Cu/Ni/Au pad metallization. Both systems exhibit the presence of AuSn4 and Ag3Sn intermetallic compounds (IMCs) throughout the matrix. (From Ref. 60.)

plays a role in the resettlement of Au as (Au,Ni) Sn4 at the interface. For example, it was reported that molten eutectic solder can dissolve 10 at.% Au at 220jC [66]. Also, as indicated by the phase diagram, Au and Ni are mutually misable in each other, so any dissolved Au precipitates out and is thought to diffuse to the interface during aging to create a more thermally stable configuration [60]. 3. Effect of Multiple Reflows on Sn–Pb–Ag, Sn–Ag, and Sn–Ag–Cu Systems Multiple reflows of the Sn–Pb–Ag solder resulted in more and longer AuSn4 platelets and tended to accumulate close to the termination pad/solder interface with increased numbers of reflows, and Ni3Sn4 was the only phase found at the interface. This behavior is in contrast to the multiple layers observed after aging. In the case of the Sn–Ag system, the Ag3Sn phase segregated along the Sn-rich phase as observed in aged parts [60]. In the Sn–Ag–Cu system the Ag3Sn phase was found to be similar in both distribution and microstructure within the bulk solder ball as the Sn–Ag system. The AuSn phase was manifested as fine particles evenly distributed within the solder ball. The Cu–Sn IMCs (e.g., Cu6Sn5) tended to segregate along the boundary of the Sn-rich phase [60]. The most distinctive difference was the formation of a Cu–Sn–Ni IMC [Fig. 18 (b)] at the interface of the Sn–Ag–Cu alloy, whereas Ni3Sn4 phase formed at the termination pad/solder interface for both the Sn–Pb–Ag and Sn–Ag solders [Fig. 18 (a)]. The needle-like morphology of the ternary IMC is much different from the Ni3Sn4 phase, protruding into the solder, making the interface rough. a. Growth Rates of Intermetallic Compounds. A comparative study was conducted by Lee et al [60] investigating the growth kinetics of the Ni3Sn4 IMC layer formation in the Sn–Pb–Ag and Sn–Ag systems, and the Cu–Ni–Sn phase in the Sn–Ag–Cu system. The average thickness of

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FIG. 17 SEM micrographs depicting the microstructure of the solder system described in Fig. 16 after thermal aging for 200 hr at 150jC. a) In the Sn–Pb–Ag system, most of the AuSn4 resettled at the substrate interface as a ternary compound, (Au, Ni) Sn4. As can be seen, a Pb-rich phase forms on top of the (Au, Ni) Sn4 phase and becomes continuous with time. b) The lead-free system, Sn–Ag–Cu, formed a quaternary IMC layer (Cu–Sn–Ni–Au), but Au in the bulk solder was manifested as AnSn4 or a ternary (Cu–Sn–Au) compound. (From Ref. 60.)

IMC that formed at the interface of a solder ball and Ni/Au overplated Cu terminal pad with reflow time is given in Fig. 19. The effect of aging temperature and time on IMC growth (thickness) for the three solder systems is illustrated in Fig. 20. The Pb–Sn–Ag system was included as a benchmark. The growth rates of the various IMCs formed with the standard Pb– Sn–Ag BGA solder, and also the Pb-free alloys all followed the classical diffusion controlled relationship, [X=(kt)1/2] as noted in Section 2.9. The growth constant, k, can be determined by taking the square of the slope from a plot of average thickness as a function of the square root of diffusion time. Growth constant measurements given as a function of reciprocal temperature in Fig. 21 show that the most rapidly growing IMC was found to occur in the benchmark Sn–Ag–Cu systems at each aging temperature up to 175jC. Beyond 175jC, the IMC growth rate is faster in Sn–Ag–Cu, which also has a steeper slope, so has a greater temperature dependence. It has been determined that, in general, the ternary phase (Au, Ni) Sn4 grows faster than the Ni3Sn4 layer in Sn–Pb–Ag, and it becomes continuous in a shorter time at a higher temperature, also observed by others [63]. b. Summary. The types of IMC formation both within the bulk solder of the BGA solder balls and at the solder/terminal pad interface for the benchmark Sn–Pb–Ag solder, and lead-free solders (Sn–Ag–Cu and Sn–Ag) are summarized for the as-joined, 150jC aged, and 20 reflowed conditions in Tables 6, 7, and 8. 1.

As-joined—There were no significant differences among the three alloys investigated by Lee et al. [60]. In all cases a thin layer of Ni–Sn IMC formed at the interface, and the Au overplate dissolved to form AuSn4 particulates within the bulk solder. The only

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FIG. 18 Scanning electron microscope (SEM) micrographs depicting the microstructure of the solder systems described in Fig. 16 after having experienced multiple reflows. a) For the Pb–Sn–Ag system the reflows had little effect on AuSn4, but more and larger Ag3Sn platelets accumulated close to the pad interface. Only one IMC layer formed at the interface, Ni3Sn4. b) For either the Sn–Ag or Sn–Ag–Cu systems the AuSn4 remained needlelike in the bulk solder. As shown, Cu–Sn–Ni IMC formed at the interface for the Sn–Ag–Cu system, and Ni3Sn4. (From Ref. 60.)

FIG. 19 Rate of growth of Ni3Sn in Sn–Pb–Ag and Sn–Ag systems, and Cu–Sn–Ni IMCs in Sn–Ag– Cu systems. (From Ref. 60.)

FIG. 20 Effect of aging time and temperature on IMC growth for two lead-free solders (Sn–Ag, Sn–Ag–Cu) compared with the benchmark, Sn–Pb–Ag. (From Ref. 60.)

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FIG. 21 Effect of aging temperature on the growth constant of Ni3Sn4 in Sn–Pb–Ag and Sn–Ag systems, and Cu–Sn–Ni–Au IMCs in the Sn–Ag–Cu system. (From Ref. 60.)

2.

difference was the Ag3Sn phase which was evenly distributed mostly along the Sn-rich phase boundary in the case of the two Pb-free alloys. Thermally aged—There were some significant differences in thermally aged BGA solder balls and their solder joint attachments between the benchmark Pb–Sn–Ag solder which exhibited a two-layer IMC structure: (Au,Ni) Sn4 and Ni3Sn4 at the interface compared to the lead-free solders. For the Sn–Ag solder, Ni3Sn4 increased in thickness with aging time, and a quaternary IMC (Cu–Sn–Ni–Au) was observed to form in the Sn–Ag–Cu system. In general, there were no significant morphological or distribution differences in

TABLE 6 Intermetallic Compounds Formed at the Interface and Bulk Solder of Sn–3.5Ag Alloy Under Various Conditions Condition

At interface

As reflowed

– A very thin layer (f0.5 Am) of Ni–Sn IMC

Thermally aged at 150jC

– Ni3Sn4 layer increases in thickness . f2.3 Am after 1000 hr – No (Au,Ni) Sn4 formed at interface . In contrast to Sn–Pb–Ag solder – Ni3Sn4 layer thickness increases . f1.4 Am after 20 reflows

Multiple reflows, up to 20

Source: From Ref. 60.

Within bulk solder balls – Dissolved Au overplate on pad to form AuSn4 – Ag3Sn distributed mostly along the Sn-rich phase boundary – AuSn4 is present in platelet form – Ag3Sn microstructure and distribution remain basically unchanged – AuSn4 remains in needle form – Ag3Sn segregates along Sn-rich phase boundary

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TABLE 7 Intermetallic Compounds Formed at the Interface and Bulk Solder of Sn–3.5Cu–0.5Cu Under Various Conditions Condition

At interface

As reflowed

– A very thin layer of ternary (Cu–Sn–Ni) phase . About 0.5 Am thick

Thermally aged at 150jC

– No (Au,Ni) Sn4 . In contrast to Pb–Sn–Ag – A quaternary IMC forms (Cu–Sn–Ni–Au) . f1.6 Am thick after 1000 hr

Multiple reflows, up to 20

– The ternary IMC (Cu–Ni–Sn) has a needle-shaped morphology . Protrudes into solder, creating a rough interface

Within bulk solder balls – Au overplate on pad dissolves . Forms AuSn4, needle-like structure – Ag3Sn platelet-like structure forms . Along Sn-rich grain boundaries – Au in the bulk solder present as AuSn4 or Cu–Sn–Au ternary IMC . Both segregate along the boundaries of the Sn-rich phase – Ag3Sn, microstructure and distribution remain basically unchanged – AuSn4 is present as fine, evenly distributed precipitates in bulk solder – Elongated Ag3Sn is distributed along the boundaries of Sn-rich grains – Irregularly shaped Cu6Sn5 tends to segregate along boundaries of Sn-rich grains

Source: From Ref. 60.

relation to the Ag3Sn phase within the bulk solder in comparison to the as-joined state, nor for the AuSn4 phase except for the Sn–Ag–Cu solder where the presence of Au resulted in producing a ternary IMC (Cu–Sn–Au), with both phases tending to segregate along Sn-rich phase boundaries. 3. Multiple reflows—It was observed that multiple reflows cause some very striking differences in comparison to thermally aged solder balls and joints. For example, only Ni3Sn4 was observed to form during multiple reflows, compared to a three-layer structure formed during thermal aging. The only change at the interface with Sn–Ag solder was the increased thickness of the Ni3Sn4 phase with increased reflows. Instead of the quaternary IMC (Cu–Sn–Ni–Au) observed at the interface of the Sn–Ag–Cu solder during thermal aging, a ternary IMC (Cu–Ni–Sn) with a needle-like morphology that protruded into the solder at the interface formed during multiple reflows. No distinguishing differences were noted to exist regarding the morphology and distribution of both AuSn4 and Ag3Sn for Pb–Sn–Ag and the Pb-free alloys compared to thermally aged solder balls and joints. The Sn–Cu IMC, probably Cu6Sn5, tended to segregate along Sn-rich grain boundaries [60].

I.

Effect of Microstructure Due to Cooling Rate

1. Strength Properties The strength of Sn–Ag–Cu alloys depends upon the area fraction of the eutectic network and the dispersion morphology of the fine Ag3Sn within the network. The effect of eutectic network area fraction on the 0.2% proof stress for three rapidly cooled (8.3jC/sec) Sn–Ag–Cu alloys is shown in Fig. 22. The highest 0.2% proof test value is exhibited by the Sn–3.5Ag–0.7Cu alloy that also has the highest eutectic network area fraction. The other two alloys that have approximately the same area fraction of eutectic network also exhibit similar 0.2% proof stress strength values. The same tendency was noted for the three Sn–Ag–Cu alloys cooled at a moderate rate (0.43jC/sec).

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TABLE 8 Intermetallic Compounds Formed at the Interface and Bulk Solder of Sn–36Pb–2Ag Alloy Under Various Conditions Condition

At interface

As reflowed

Thermally aged at 150jC

Multiple reflows, up to 20

– Most AuSn4 disappears over time . Resettles at substrate interface as ternary eutectic (Au,Ni) Sn4 . After aging 48–96 hr forms two-layer structure – Ni3Sn4, (Au,Ni) Sn4 – Islands of Pb-rich phase form at interface, after f500 hr . Several microns thick f1000 hr . Located on top of a two-layer structure (of which NiSn4 is f2.5 Am thick after f1000 hr) – Ni3Sn4 only phase formed at interface . Is <1.0 Am after 20 reflows . Sharp contrast to multiple phases formed during aging

Within bulk solder balls – Dissolves Au overplate on pad to form AuSn4 – Evenly distributed Ag3Sn – AuSn4 has a plate-like structure – Ag3Sn is unevenly distributed in the bulk solder

– AuSn4, more and longer platelets formed . Accumulate near interface with increased reflows – Ag3Sn remains as fine, uniformly distributed particles

Source: From Ref. 60.

When cooled slowly (0.012jC/sec.) it was observed that the ultimate tensile strength and 0.2% proof stress values were about the same for both the Sn–3.5Ag–0.7Cu and Sn–3.9Ag–0.6Cu alloys, and lower for the Sn–3.0Ag–0.5Cu alloy [58]. 2. Ductility The elongation, however, was similar for the 3.5% and 3.9% Ag content alloys, but significantly higher for the 3.0% Ag alloy. The lower ductility of slow-cooled, higher Ag content alloys is explained on the basis that under these conditions large primary Ag3Sn phase is formed, which is brittle and likely to result in cracks; thus reducing elongation. In the case of a low Ag content (i.e., 3.0%) there is less tendency to form large Ag3Sn precipitates, thus exhibiting a higher elongation. Under typical rapid-cooled conditions during solder joint formation, the IMC was not observed, but decohesion occurs at the Ag3Sn and Cu6Sn5 precipitate/matrix interfaces. So the presence of coarse or fine primary Ag3Sn precipitates reduces the elongation. Accordingly, limiting the Ag content in Sn–Ag–Cu alloys eliminates or greatly reduces the tendency to degrade the elongation (i.e., ductility) [58]. 3. Strain Rate Effects Strain rate sensitivity (m) is defined as, r ¼ ce´ m

ð2Þ

where r is the stress, c is a constant, q´ is the strain rate, and m is the strain rate sensitivity factor. The strain rate sensitivity factor for several lead-free alloys and eutectic Sn–Pb solder is listed in Table 9 for various cooling rates at a 3.2% total strain level [58,67–69]. It is typical for the strength of metals, including solder alloys such as Sn–3.1Ag–0.7Cu to increase with increasing strain rate. This was also observed to be the case for Sn–3.5Ag–0.7Cu in the study conducted by

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FIG. 22 Effect of area fraction of eutectic network on the proof stress of three Sn–Ag–Cu alloys. (From Ref. 58.)

Kim et al. at strain rates between 104 and 102 per second for the minimum 5% strain level investigated as shown in Fig. 23. Approximately, a 50% increase in tensile strength was observed independent of the alloy cooling condition, rapid or slow. It was determined that the ultimate tensile strength (UTS) and 0.2% proof stress of Sn–Ag–Cu alloys whose Ag content varied between 3.0% and 3.9% is greater over the strain rate range investigated than either binary alloy, Sn–3.5Ag or Sn–0.5Cu. However, it was determined that lowering the cooling rate decreased strength properties at any strain rate tested, as strength properties depend on the eutectic network area fraction and morphology of the Ag3Sn precipitates in the fine eutectic network. Increasing the eutectic network area fraction results in increasing alloy strength. Based on those criteria, the study recommended the Sn–3.0Ag–0.5Cu alloy as the best candidate among those whose Ag content ranged between 3.0% and 3.9% because this alloy did not result in the formation of large primary Ag3Sn platelets. This alloy exhibited a good balance between strength and ductility over a range of cooling rates and strain rates [58].

TABLE 9

Stain Rate Sensitivities of Several Lead-Free Alloys

Alloy

Strain rate sensitivity (m)

Sn–3.0Ag–0.5Cu Sn–3.5Ag–0.7Cu Sn–3.9Ag–0.6Cu Sn–37Pb Sn–3.5Ag

0.08 (RC)a, 0.08 (SC)b 0.11 (RC)a, 0.06 (SC)b 0.08 (RC)a, 0.08 (SC)b 0.05 0.12, 0.08

Sn–0.5Cu Sn–9Zn Sn–3.0Ag–0.5Cu Sn–3.8Ag–0.7Cu

0.08 0.012 0.08 0.08

a

RC—rapid cool. SC—slow cool. Source: From Ref. 58.

b

Reference Kim et al. Kim et al. Kim et al. Plumbridge and Gagg Plumbridge and Gagg Mavoori et al. Plumbridge/Clark Mavoori et al. Kariya and Plumbridge Kariya and Plumbridge

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FIG. 23 Effect of strain rate on the stress of both slowly and rapidly cooled Sn–3.5Ag–0.7Cu solder alloy. (From Ref. 58.)

J.

Creep Deformation

Creep deformation is the time-dependent plastic flow or deformation a material undergoes at constant load and elevated temperatures. As the so-called homologous temperature (the ratio of the actual temperature to the melting temperature based on the Kelvin scale) increases, the ease with which plastic flow occurs increases as does the creep rate. In general, metals under load are typically not subjected to long-term service conditions where the homologous temperature is more than 0.3 or 0.4. However, solders in solder joints can experience significant creep because the homologous temperature of many solders is greater than 0.5 at room temperature. Creep deformation occurs during the course of every high-temperature excursion. The solder joint experiences strains generated as a result of CTE mismatches between the materials to which a solder joint is attached. Therefore significant strains can result under elevated temperature conditions which also results in an increased creep rate. Creep is a major deformation mode during low-cycle fatigue failure. The solder microstructure morphology is a key factor in accommodating applied strains to resist structural instability and thereby resist fatigue failure. For Sn–Ag–X solders, the strain accommodation occurs through the Sn matrix at individual Sn– Sn grain boundaries [70].

FIG. 24 Creep characteristics of several lead-free alloys compared to the benchmark alloy (Sn–40Pb) at a) 25jC and b) 100jC. (From Ref. 71.)

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1. Creep Rates for Several Lead-Free Solders Tests have been conducted comparing the creep–rupture resistance of several lead-free alloys at both room temperature and 100jC. Under room ambient conditions, as shown in Fig. 24(a), the Sn–Ag–Cu alloy performed better than eutectic Sn–Ag, eutectic Sn–Cu, and significantly better than eutectic Sn–Pb, the benchmark. It is not surprising that both Sn–Ag and Sn–Ag–Cu exhibit similar behavior as they have a similar microstructure. Upon increasing the test temperature to 100jC for the same group of alloys [Fig. 24 (b)], Sn–Ag and Sn–Ag–Cu switched positions, i.e., Sn–Ag is the better performer. In all cases, the time-to-failure is very significantly reduced (approximately 1 to 2 orders in magnitude or more) compared to room-ambient performance. These data suggest that Sn–Ag–Cu and its binary systems Sn–Ag and Sn–Cu exhibit better creep resistance at both room ambient and elevated temperatures (100jC) than near-eutectic Sn–40Pb. Although eutectic Sn–Cu outperforms Sn–40Pb, it does not outperform Sn–Ag–X alloys [71].

VIII.

TIN–SILVER–COPPER–LEAD (SN–AG–CU–PB) SYSTEM

When a lead-free solder and one containing lead (Pb) are in contact during a soldering operation, the lead (Pb) contaminates the lead-free solder, resulting in the formation of a low-melting, leadcontaining compound. This occurs through a secondary alloying process and for practical purposes is instantaneous in standard reflow processes [72,73].

A.

Effect of Lead (Pb) on Melt Temperature

In one study [74] that investigated lead additions ranging between 0.1% and 0.5% to Sn–3.1Ag– 1.5Cu (which served as the unleaded benchmark alloy) it was observed to not have a noticeable influence on the alloy melting temperature. That is, those alloys with Pb additions of 0.1% and 0.2% exhibited the same melt temperature as the unleaded alloy, and an alloy with a 0.5% Pb addition varied by F1jC from the 216 to 217jC melt temperature of the unleaded alloy.

FIG. 25 Effect of lead (Pb) content on the formation of a low-melting temperature phase for several Sn–Cu-based alloys that also contain silver (Ag) and/or bismuth (Bi). (From Ref. 42.)

272 B.

PUTTLITZ Effect of Lead and Bismuth Content on Low-Melt Constituent Formation and Melt Temperature

Similar to Sn–Ag alloys (Section 3.3), the addition of lead (Pb) to Sn–Ag–Cu alloys and those that contain Cu and/or Bi results in the formation of a low-melting constituent whose content increases as the concentration of lead (Pb) increases as shown in Fig. 25. Also noteworthy in the study by Bradley and Hranisavljevic [42] is the fact that Sn–Ag–Cu solder, exhibited the largest fraction of low-melting temperature phase (LMTP) compared to the Bi-containing-alloys studied over the range of Pb content tested (i.e., 6% Pb). Both the bismuthcontaining Sn–Ag–Cu alloys and Sn–Bi–Cu alloy contained significantly less low-melting phase. All three test alloys contained about the same fraction of low-melting phase (approximately 0.02) with 3% Pb, that is about 40% of the LTMP content in Sn–Ag–Cu (Fig. 25). The content of LTMP differed among the test alloys when the Pb content was increased to 6%, but the LTMP percentages remained lower than the Sn–Ag–Cu alloy: 60% for the 2% Bi alloy, which was reduced to 10% by increasing the Bi content to 7.4%, and 23% for the Sn–Bi-Cu alloy, respectively. Although the amount of ternary eutectic phase that forms increases with increased lead content, it was determined that the temperature at which the reaction occurs does not change. Increasing the Bi content was reported to appreciably reduce the onset temperature for melting for Sn–Ag–Cu alloys with and without lead. For example, the Sn–3.8Ag–0.7Cu alloy, the onset of first melting was observed to be 217jC, but the addition of 3% lead (Pb) caused the formation of the ternary eutectic (Sn–36Pb–2Ag) to appear at 178jC as determined by DSC measurements [42].

C.

Mechanical Properties

1. Strength Uniaxial tensile tests (strain rate=6.5104/sec) were conducted on a benchmark (i.e., unleaded) solder alloy, and several containing Pb to a maximum concentration of 0.5% Pb. It was determined that neither the yield strength nor the ultimate tensile strength was affected by the presence of lead for the concentration range investigated as indicated in Table 10. Uniaxial strain was measured in a 25-mm gauge length.

TABLE 10

Composition and Properties of Lead (Pb)-Contaminated Sn–Ag–Cu Solders

Sn

Cu

Ag

Pb

95.4

1.5

3.1

—

216–217

39

49

47

35

1367

94.9 95.2 95.3 63

1.5 1.5 1.5 —

3.1 3.1 3.1 —

0.5 0.2 0.1 37

217–218 216–217 216–217 183

38 40 42 41

47 55 54 47

45 43 44 27

33 29 26 24

9067 5303 3650

Composition Alloys Unleaded Leaded: A B C Eutectic Sn–Pb

Uniaxial strain, 25 Am gauge length. Yield str. and tensile str. (MPa). c Elastic modulus (Ga). d Plastic strain at fracture (%). Source: From Ref. 74. a

b

Propertiesa

Melting point or range (jC)

Yield str.b

Tensile str.c

Elastic modulus

qpd

Fatigue life, Np

SN–AG AND SN–AG–X SOLDERS AND PROPERTIES

273

2. Plasticity While the addition of Pb was determined to have a negligible effect on the strength properties of the base alloy (Sn–3.1Ag–1.5Cu), it can have a significant effect on plasticity. For example, the addition of a small quantity of Pb (approximately 0.1%) causes a nearly one quarter reduction in plasticity as indicated in Table 10. However, further increases in Pb content were observed to steadily increase plasticity such that a 0.5% Pb alloy’s plasticity was found to be nearly equivalent to the unleaded benchmark alloy [74]. 3. Low-Cycle Fatigue Low-cycle fatigue tests were carried out under strain control conditions at a frequency of 0.8 Hz and an R-ratio of 0.8. A characteristic response similar to the effect of lead concentration on plasticity described in the previous section was also observed in the case of low-cycle fatigue. That is, an abrupt reduction in fatigue life was observed with just a 0.1% Pb addition, but then the fatigue life progressively improved with increased Pb additions up to the maximum level tested (0.5% Pb). The fatigue life of the 0.5% Pb alloy was found to be nearly equivalent to the unleaded benchmark alloy [74].

IX.

TIN–SILVER–COPPER–BISMUTH (SN–AG–CU–BI)

Bismuth plays a significant role in reducing the melting point of Sn–Ag and Sn–Ag–X alloys, but the quantity that can be added is limited by its drastic effect on degrading fatigue life and plasticity. This is likely due to the fact that Bi is rather brittle for a metal. Bismuth solubility is limited to about 1% in Sn–Ag–X alloys and precipitates as an elemental phase at concentrations beyond that [75].

A.

Effect of Bismuth on Melting Point

The most important advantage of Sn–Ag–Cu–Bi over Sn–Ag–Cu is a reduced melting temperature that is desired for manufacturing purposes.

B.

Mechanical Properties

It has been stated that an optimum fatigue life is achieved if the composition is maintained within the limits: Sn–3.1Ag–(0.5 to 2.0)Cu–(3.0 to 3.1)Bi, but can be as high as 3.54% Bi [76]. However,

TABLE 11 Mechanical Property Comparison of Sn–3.1Ag–0.5Cu–3.5Bi with Eutectic Sn–Pb and Some Lead (Pb)-Free Solders Comparison alloy Eutectic Sn–Pb Sn–0.7Cu

Eutectic Sn–Ag

Sn–3.1Ag–0.5Cu

Source: From Ref. 75.

Advantage of Sn–3.1Ag–0.5Cu–3.1Bi alloy – – – – –

Offers greater strength, and approximately 200% better fatigue life Significantly higher strength, and better fatigue life Reduced melt temperature by about 15jC But exhibits lower plasticity Melt range is 209jC to 212jC . 9jC lower than eutectic Sn–Ag – Fatigue life is about 1.5 better – Exhibits lower plasticity, but deemed adequate – Addition of 3.1% Bi significantly increases yield and tensile strength – Exhibits a lower fatigue life than alloy with no Bi . Still superior to binary Pb-free alloys

274

PUTTLITZ

higher permissible levels of Bi have been reported [77] such as the alloy composition range: (86 to 97) Sn–(0.3 to 4.5)Ag or (0.0 to 5.0)Cu–(0.0 to 9.3)In–(0.0 to 4.8)Bi. A comparison of the mechanical properties of Sn–3.1Ag–0.5Cu–3.1Bi and several lead-free alloys is given in Table 11. The strengthening observed in alloys containing Bi precipitates is believed to follow the Mott and Nabarro strain field theory wherein the alloy strength increases proportionately with the precipitate volume fraction present in an alloy [75].

X.

TIN–SILVER–COPPER–INDIUM (SN–AG–CU–IN) SYSTEM

A.

Applications

The addition of In to the eutectic Sn–Ag–Cu solder aids manufacturing by reducing the melt temperature, and the alloy exhibits an appreciably enhanced fatigue life, a feature particularly useful for highly strained solder joints. This situation is typical of many components surface mounted to printed circuit boards (PCBs) such as CSPs and BGAs. The material cost of Sn–Ag– Cu solders even with the high cost of indium is largely offset by reducing the cost of final assembly through lower process temperatures. Additionally, a high material cost may be justified when reliability is a priority [77].

B.

Microstructure

Indium additions to Sn–Ag–Cu can result in the several possibilities listed in Table 12.

C.

Effect of Elemental Additions on Melt Temperature

The effect of Ag and In in the Sn–Ag–Cu–In system has been reported by Hwang [77]. Silver—Similar to the Sn–Ag–Cu system the melting point is not affected by increases in Ag content between 3% and 4.3% with a fixed In content (4% or 8%). Indium—The melting point of Sn–Ag–Cu can be significantly reduced through the addition of indium. It has been determined that every 1% In addition up to about 12% results in suppressing the melting point about 1.8jC. For example, the melt range for a Sn–4.1Ag–0.5Cu alloy is 195jC to 201jC for an 8% In addition, and 185jC to 195jC for a 12% In addition. It should be noted, however, that at an approximately 12% In concentration, a low temperature thermal peak has been detected at about 113jC. For indium contents lower than 10%, the alloy liquidus temperature can be tailored to temperatures as low as 198jC. Accordingly, by optimizing the In addition, the melting temperature can be reduced by about 15% below the Sn–Ag–Cu eutectic temperature.

TABLE 12 Interaction Between Indium (In) and Elemental Species Comprising the Sn–Ag–Cu System Reaction species

Result

Tin (Sn) matrix

– In enters Sn lattice sites creating a substitutional solid solution – In reacts with Ag to form Agln intermetallic compound (IMC) – In reaction with Cu similar to Ag, to form f (Cu–In) IMC – The addition of In can result in forming ternary IMCs consisting of Sn–Ag–In and Sn–Cu–In

Silver (Ag) Copper (Cu) Sn, Ag, Cu

Source: From Ref 77.

SN–AG AND SN–AG–X SOLDERS AND PROPERTIES

275

TABLE 13 Comparison of the Mechanical Properties and Characteristics of Sn–3.0Ag–0.5Cu–8.0In with Eutectic Sn–Pb and Some Lead (Pb)-Free Solders Comparison alloy Eutectic Sn–Pb

Sn–3.1Ag–1.5Cu

Eutectic Sn–Ag and Sn–Cu

Advantages of Sn–3.0Ag–0.5Cu–8.0In – Exhibits greater strength and fatigue life – Wetting not as good as eutectic Sn–Pb . However, sufficient for SMT – Exhibits greater strength and fatigue life . While possessing adequate plasticity – Has a significantly lower melt temperature (196–202jC) . Fairly narrow pasty range, 6jC – Exhibits higher strength than either binary eutectic

Source: From Ref. 77.

D.

Mechanical Properties

Hwang [77] reported that for Sn–Ag–Cu–In alloys where the Ag and Cu are 4.1Ag–0.5Cu, or 3.0 to 3.1 Ag–0.5Cu, both the yield stress and tensile strength increase linearly with the In atomic fraction present up to about 8%. With In concentrations above 8%, the yield strength was observed to decrease, but the tensile strength reduced abruptly at about 10% In. Fatigue life was reported to increase exponentially with the In atomic fraction present, attaining a maximum at approximately 8% In. A comparison between Sn–3.0Ag–0.5Cu–8.0In, reported to be the optimal composition for that alloy series, and other lead-free alloys of interest and also eutectic Sn–Pb is given in Table 13.

XI.

TIN–SILVER–COPPER–ANTIMONY (SN–AG–CU–SB) SYSTEM

A.

Applications

This alloy family was an early choice to replace eutectic Sn–Pb, because it was deemed a near drop-in substitution as it appeared suitable for the various soldering processes typically practiced during the assembly of PCBs. Only a slightly higher peak temperature (235jC vs. 215–220jC) is required to reflow attach surface mount components compared to eutectic Sn–Pb, respectively. However, the same equipment utilized for eutectic Sn–Pb solder has been proven acceptable for Sb-doped Sn–Ag–Cu. For wave soldering applications, no change in pot temperature (i.e., 250jC to 260jC) is required, and less dross is created. In addition, Cu pickup from cards and boards is slower than for the binary eutectic alloys of the Sn–Pb, Sn–Cu, and Sn–Ag systems. Also, the same soldering iron tip temperature (343jC to 399jC) utilized for hand soldering to do touch-up operations is also unchanged [78].

B.

Toxicity

There is some debate associated with the toxicity of elemental Sb, but a considerable toxicity issue associated with its salts, oxides (particularly antimony trioxide), and organometallic compounds. However, these products are typically not formed during standard reflow soldering processes. The standard, IPC-J-STD006, allows solder alloys to contain up to 0.6% Sb to avoid leaching Ag or Cu into ground water [78].

C.

Intermetallic Compound Growth Rates

The growth rates of several lead-free solders were investigated both under reflow and 125jC aging conditions with eutectic Sn–Pb as a benchmark. The Sn–2.5Ag–0.8Cu–0.5Sb alloy tested was found to exhibit significantly lower IMC growth rates despite the fact that the Sn–Ag–Cu–Sb

276

PUTTLITZ

TABLE 14 Comparison of the Mechanical Properties of an Antimony (Sb)-Doped Sn–Ag–Cu Alloy with the Benchmark Alloy, Eutectic Sn–Pb Properties Tensile UTS (ksi) Yield strength (ksi) Young’s modulus (msi) Percent elongation Compression Elastic modulus (msi) YS (ksi) Stress 25% (ksi) Hardness

63Sn–37Pb

Sn–Ag–Cu–Sb

4.92 4.38 4.87 52.87

5.73 4.86 7.42 50.00

3.99 4.52 7.17 10.08

4.26 4.33 8.54 18.28

Source: From Ref. 78.

alloy has a similar amount of copper as Sn–0.7Cu. In fact, Sn–Ag–Cu doped with Sb was observed not to grow Cu-containing IMCs when aged at 125jC [78].

D.

Mechanical Properties

1. Strength and Plasticity A comparison of the mechanical properties of an antimony (Sb)-doped Sn–Ag–Cu alloy with the benchmark alloy, eutectic Pb–Sn, is listed in Table 14. The Sn–Ag–Cu–Sb solder was determined to exhibit a superior tensile and compression strength compared to eutectic Sn–Pb, while exhibiting similar yield strength and plasticity (i.e., percent elongation) characteristics. The Sbdoped alloy is significantly stiffer (i.e., Young’s Modulus) than eutectic Sn–Pb in tension, but only slightly stiffer in compression. 2. Fatigue Properties Antimony improves the thermal fatigue properties of solder alloys. For example, some applications have historically required additions of 0.2% to 0.5% Sb to enhance the thermal fatigue properties of Sn–Pb alloys [78].

FIG. 26 Effect of boron (B) on the microstructure of Sn–3.5Ag–0.5Cu solder. (a) Without boron the alloy exhibits a typical coarse and nonuniform solidification structure. (b) With a 0.5% B addition the alloy exhibits a finer and more uniform microstructure. (From Ref. 76.)

SN–AG AND SN–AG–X SOLDERS AND PROPERTIES XII.

277

TIN–SILVER–COPPER–BORON (SN–AG–CU–B) SYSTEM

A fine and homogenous microstructure typically provides enhanced mechanical properties compared to a coarse, heterogenous structure. As discussed in Section 6, when Zn is added to eutectic Sn–Ag it becomes incorporated into the Ag3Sn precipitates, making them finer and spherical in shape. In turn this refines the microstructure and also suppresses the formation of Sn dendrites [16]. Accordingly, having fine, unreactive, and uniformly distributed particles provides sites for heterogenous nucleation to refine the structure. Boron is believed to perform in a similar manner based on the individual binary phase diagrams (B–Ag, B–Cu, and B–Sn); it is not soluble in Ag, Cu, or Sn at room temperature. Also, B has a very high melting point (>1000jC), so will remain solid under typical soldering temperatures [26]. Therefore boron particles are believed to provide sites for heterogenous nucleation resulting in microstructure refinement.

A.

Microstructural Comparison

A study was conducted by Ye et al. [26] to determine the effect of 0.4% boron addition to a Sn– 3.5Ag–0.5Cu alloy. The alloy without boron [Fig. 26 (a)] exhibited the typical coarse and nonuniform solidification structure, consisting of Sn dendrites of average grain size <10 mm and some Ag3Sn precipitates with an elongated morphology (approximately 3 to 5 mm in length). Some of these elongated precipitates form a lamellar structure. However, the microstructure of the alloy with 0.5% B was much more uniform [Fig. 26(b)] and with a finer microstructure. As compared to the standard alloy (i.e., without B) the Ag3Sn precipitates were more or less spherical in shape, and <1mm in size, and distributed homogenously. The presence of B particles provided nucleation sites during solidification, which served to refine the microstructure [26].

B.

Role of Boron as Microstructure Refiner

During the initial solidification of a Sn–Ag–Cu alloy, Ag3Sn and nearly pure Sn start to form prior to cooling to the eutectic temperature (217jC). Ye et al. believed the dispersed boron particles provide heterogenous nucleation sites to both the Ag3Sn and Sn-rich solid solution phase, resulting in their having a finer and more homogenous microstructure compared to the alloy without boron. Interestingly, it was reported that the fine grain size, morphology, and distribution of the Cu6Sn5 precipitates remained unaffected by the 0.5% B addition.

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Huang, B.; Lee, N.-C. Prospect of lead-free alternatives for reflow soldering. Proc. IMAPS Conf, 1999. Yoon, S.W.; Lee, H.M. A thermodynamic study of phase equilibria in the Sn–Bi–Pb solder system. Calphad June 1998, 22 (2), 167–178. Hwang, J.S. Another strong lead-free candidate: the Sn–Ag–Bi–In system. Surf. Mt. Technol. Sept. 2000, 14 (9), 20–23. Hwang, J.S. Lead-free solder: Sb effect on Sn/Ag. Surf. Mt. Technol. Nov. 2000, 14 (11), 20. Kurz, W.; Fisher, D.J. Fundamentals of Solidification; Trans Tech Publications, Ltd.: Aedermannsdorf, Switzerland, 1992; 11 pp. McCormack, M.; Jin, S.; Kammlott, G.W.; Chen, H.S. New Pb-free solder alloy with superior mechanical properties. Appl. Phys. Lett. July 1993, 63 (1), 15–17. Loomans, M.E. Northwestern University, Evanston, IL, unpublished research, 1997. Loomans, M.E.; Fine, M.E. Tin–silver–copper eutectic temperature and composition. Metall. Mater. Trans. A. Apr. 2000, 31A , 1155–1162. Snugovsky, L.; Perovic, D.D.; Rutter, J.W. Solidification of ternary eutectic and near-eutectic alloys in the Ag–Cu–Sn system, presented at TMS Meeting, (San Diego, CA), March 2003. Moon, W., et al. Experimental and thermodynamic assessment of Sn–Ag–Cu solder alloys. J. Electron. Mater. 2000, 29 (10), 1122–1136. Snugovsky, L.; Snugovsky, P.; Perovic, D.D.; Ritter, J.W. Effect of the cooling rate on the microstructure of Ag–Cu–Sn solder alloys, presented at TMS Meeting, (San Diego, CA), March 2003. Daams, J.L.C.; Villars, P.; vanVucht, J.H.N. Atlas of Crystal Structure Types for Intermetallic Phases; ASM International: Materials Park, OH, 1991; Vol. 3, 4596. Gangulee, A.; Das, G.C.; Bever, M.B. An x-ray diffraction and calorimetric investigation of the compound Cu6Sn5. Metall. Trans. 1973, 4 , 2063–2066. Henderson, D.W.; Gosselin, T.; Sarkhel, A.; Kang, S.K.; Choi, W.K.; Shih, D.Y.; Goldsmith, C.; Puttlitz, K.J. Ag3Sn plate formation in the solidification of near eutectic Sn–Ag–Cu alloys. J. Mater. Res. Nov. 2002, 17 (4), 2775–2778. Frear, D.R.; Jang, J.W.; Lin, J.K.; Zhang, C. A metallurgical study of Pb-free solders for flip chip interconnects. J. Met. June 2001, 53 (6), 28–32, 38. Kim, K.S.; Huh, S.H.; Suganuma, K. Effect of cooling speed on microstructure and tensile properties of Sn–Ag–Cu alloys. Mater. Sci. Eng., A 2002, 333 (1–2), 106–114. Kim, H.K.; Tu, K.N. Kinetic analysis of the soldering reaction between eutectic Sn–Pb alloy and Cu accompanied by ripening. Phys. Rev. B June 1996, 53 (23), 16,027–16,034. Lee, K.Y.; Li, M.; Olson, D.R.; Chen, W.T.; Tan, B.T.C.; Mhaisalkar, S. Microstructure, joint strength, and failure mechanism of Sn–Ag, Sn–Ag–Cu versus Sn–Pb–Ag solders in BGA packages. Proc. 51st Electronic Components and Technology Conf. (Orlando, FL); May 2001; 478–485. Kim, H.K.; Liou, H.K.; Tu, K.N. Three dimensional morphology of a very rough interface formed in the solder reaction between eutectic Sn–Pb and Cu. Appl. Phys. Lett. 1995, 66 (18), 2337–2339. Liu, A.A.; Kim, H.K.; Tu, K.N. Spalling of Cu6Sn5 spheroids in the soldering reaction of eutectic Sn– Pb on Cr/Cu/Au thin films. J. Appl. Phys. 1996, 80 (5), 2774–2780. Zhong, C.H.; Yi, S.; Maui, Y.C.; Howe, C.P.; Olsen, D.; Chen, W.T. Missing solder ball failure mechanisms in plastic ball grid array packages. Proc. 50th Electronic Components and Technology Conf., Las Vegas, NV; May 2000; 151–159. Ho, C.E.; Zheng, R.; Luo, G.L; Lin, A.H.; Kao, C.R. Formation and resettlement of (AuxNix)Sn4 in solder joints of ball grid array packages with the Au/Ni surface finish. J. Electron. Mater. 2000, 29 (10), 1175–1181. Minor, A.M.; Morris, J.W., Jr. Inhibiting growth of the Au0.5 Ni0.5 Sn4 intermetallic layer in Pb–Sn solder joints reflowed on Au/Ni metallization. J. Electron. Mater. 2000, 29 (10), 1170–1174. Prince, A. J. Less-Common Met. 1967, 12 , 107. Plumbridge, W.J.; Gagg, C.R. Effect of strain rate and temperature on the stress–strain response of solder alloys. J. Mater. Sci. Mater. Electron. July 1999, 10 (5/6), 461–468. Mavoori, H.; Chin, J.; Vaynman, S.; Moran, B.; Keer, L.; Fine, M. Creep, stress relaxation and plastic deformation in Sn–Ag and Sn–Zn eutectic solders. J. Electron. Mater. 1997, 26 (7), 783–790. Kariya, Y.; Plumbridge, W.J. Proc. 7th Symp. on Microjoining and Assembly Technology in Electronics (JWS); 2001; 383 pp. Tribula, D.; Morris, J.W. Creep in shear of experimental solder joints. J. Electron. Packag. June 1990, 112 (2), 87–93. Grusd, A. Connecting to lead-free solders. Circuits Assem. August 1999, 10 , 32–38. Hwang, J.S.; Guo, G. Lead-free solders for electronic packaging and assembly. Proc. SMT; 1993; 732 pp. Hwang, J.S. Solder materials. Surf. Mt. Technol. March 2000, 14 (3), 81.

280 74. 75. 76. 77. 78.

PUTTLITZ Hwang, J.S.; Guo, Z. Effects of Pb contamination on the material properties of Sn–Ag–Cu solder. Chip Scale Rev. Jan/Feb. 2001, 5 (1). Hwang, J.S. A strong lead-free candidate: The Sn–Ag–Cu–Bi system. Surf. Mt. Technol. Aug. 2000, 14 (8), 20–23. U.S. Patent No. 5,520,752. Hwang, J.S. A fatigue-resistant lead-free candidate: The Sn–Ag–Cu–In system. Surf. Mt. Technol. Oct. 2000, 14 (10), 18–20. Seelig, K.; Suraski, D. The status of lead-free alloys. Circuit Assem. Sept. 2000, 11 (9), 56–62.

9 Bi–Sn, Sn–Sb, Sn–Cu, Sn–Zn, and Sn–In Solder-Based Systems and Their Properties Sung K. Kang IBM Corporation, Yorktown Heights, New York, U.S.A.

I.

INTRODUCTION

Soldering technology plays a key role in various levels of electronic packaging, such as solder joints in printed circuit boards (PCB) formed by either surface mount soldering or platedthrough-hole (PTH) soldering of components, flip-chip connections, attaching heat sinks, and others. Solder joints in electronic packages serve as electrical interconnections and provide mechanical/physical support. When either of these requirements cannot be met, the solder joint is considered to have failed, which can cause the failure of the entire electronic system. Lead (Pb)containing solders, such as 63Sn–37Pb, Pb–10Sn, and Pb–3Sn (all in weight %), have been used in various microelectronic applications. The research and development activities for replacing Pbcontaining solders with Pb-free solders have been intensified due to both competitive market pressures and environmental issues.

A.

Replacement Solder Criteria

An acceptable Pb-free solder solution must satisfy both process requirements and reliability objectives [1]. Ideally, it should be suitable for mass-production applications of both surface mount technology (SMT) and PTH soldering. Furthermore, the process technologies utilized for Sn–Pb eutectic or other Pb-containing solders must be adaptable to a Pb-free replacement solder without major changes in manufacturing processes or the existing manufacturing infrastructure resulting in significant capital investments. The wettability of a new Pb-free solder should be better or equivalent to Pb–Sn solders and yield a lower or equivalent defect rate during assembly. A replacement Pb-free solder should be usable with water-soluble or no-clean fluxes. The melting point of a replacement solder is desired to be equivalent to Pb-containing solders in order to minimize the magnitude of thermally induced stresses, degradation of the package materials, and damage due to moisture sensitivity effects in organic packages. A replacement solder must be capable of producing solder joints with an acceptable joint strength and have a high resistance to thermal fatigue. These characteristics must extend over the projected operating life of a soldered assembly and meet other reliability requirements, such as adequate corrosion, oxidation, and electromigration resistance. Lastly, the material cost of a replacement solder should not be so high as to overwhelm the assembly cost.

281

282 B.

KANG Candidate Solder Systems

Chap. 8 discussed the Sn–Ag and Sn–Ag–Cu based systems, whose alloys are of key interest as substitutes for Pb-containing solders. However, there are several other alloy systems of interest as well. Some lead-free solders have been utilized commercially, while others developed as Pb-free substitutes [2–11]. Several examples are Sn–3.5Ag, Sn–3.5Ag–0.7Cu, Sn–3.5Ag–4.8Bi, Sn–0.7Cu, Sn–5Sb, Sn–20In–3Ag, Sn–8Zn–3Bi, Sn–10In–3Ag–1Cu, and Bi–43Sn among others. The list has been reduced to three or four promising candidates as a result of accelerated development efforts in the microelectronics industry [12–15]. This chapter will review a few promising Pb-free solders and their applications in electronic packaging in terms of their physical and mechanical properties, microstructure, wetting characteristics, interfacial reactions, thermal fatigue behaviors, and reliability concerns. The Pb-free solders discussed in this chapter include the binary systems, Bi–Sn, Sn–Sb, Sn–Cu, Sn– Zn, Sn–In, and their ternary or quaternary compositions.

II.

BISMUTH–TIN SOLDERS

A.

As-Cast Microstructure

Eutectic Bi–Sn solder (57Bi–43Sn) has a melting point of 139jC, which is significantly lower than eutectic Pb–Sn solder, with m.p. 183jC. The as-cast microstructure of a small volume of eutectic Bi–Sn, as occurs in solder joints that are solidified rapidly, is an irregular lamellar structure consisting of the Bi and Sn phases as shown in Fig. 1. In the as-cast microstructure, both the Sn and Bi phases are supersaturated with the other element by an appreciable amount. Upon aging at room or elevated temperatures, the supersaturated solute atoms are precipitated to meet the equilibrium concentration of each solid solution [16]. The as-cast microstructure of eutectic Bi– Sn solder undergoes considerable coarsening upon aging at 100jC, and its hardness increases significantly. The hardness increase in the aged microstructure is explained on the basis of the

FIG. 1 The microstructure of eutectic Bi–Sn solder alloy rapidly solidified. The scanning electron microscope (SEM) micrograph shows an irregular lamellar eutectic microstructure, a mixture of Sn– rich (dark matrix), and Bi-rich phases (light phase). (Courtesy of W. K. Choi, IBM Corp.)

SOLDER-BASED SYSTEMS AND THEIR PROPERTIES

283

increase of the lattice constant of the Sn-rich solid solution, which is caused by introducing Bi into the Sn-rich solid solution resulting in solid solution hardening [16,17].

B.

Wetting Characteristics

A detailed study on the wetting characteristics of eutectic Bi–Sn solder has been reported in terms of impurity effects, fluxes, base metals, and soldering temperature [18]. It was determined that eutectic Bi–Sn solder is far less tolerant of impurities than eutectic Pb–Sn [19]. In particular, the presence of impurity elements which form intermetallic compounds with Bi–Sn solder, such as Cu, Ni, Fe, and Pd, is especially critical, while Sb and Pb appear to be beneficial in terms of promoting wetting characteristics. Table 1 compares the solderability of several low-meltingpoint solders on various surface metallizations [20]. Only the Au/Ni-plated metallization is considered acceptable for Bi–Sn solder, while the Cu, Ni, and Au/Ni metallizations are all acceptable for eutectic Sn–Pb solder when a rosin-base flux is used. Another study confirmed that Bi–Sn solder does not wet Cu-base metallizations as well as eutectic Sn–Pb solder does when a rosin-base flux is used [21]. However, if the Cu surface is pre-tinned, then Bi–Sn wetting is acceptable even with a rosin-base flux [22]. Based on the wettability studies, eutectic Bi–Sn solder can only be considered a viable candidate if a suitable flux system is developed which allows it to be utilized for metallizations other than a Au/Ni overplate.

C.

Interfacial Compound Formation

The interfacial reactions of Cu and Ni metallization with several Pb-free solders including eutectic Bi–Sn solder have been reported [23]. The dissolution rates of Cu and Ni metallization into the molten Bi–Sn solder at 170jC are much less than for eutectic Sn–Pb solder at 215jC. This is attributed to the lower reaction temperature and lower concentration of Sn in the eutectic Bi–Sn solder compared to eutectic Sn–Pb. Consequently, the intermetallic compound growth on Cu and Ni metallizations is much less with eutectic Bi–Sn solder compared to eutectic Sn–Pb [23].

D.

Mechanical Properties

The mechanical and physical properties of bulk eutectic Bi–Sn eutectic solder compare favorably with eutectic Sn–Pb as noted in Table 2. The mechanical properties of the solder alloys are strongly influenced by both the strain rate and their microstructure [24]. This strong strain-rate sensitivity of these materials is recognized by noting the wide ranges of tensile strength and

TABLE 1

Solderability of Selected Solder Alloys Spread factor (%)a

Solder alloy (wt.%) Sn–52In Sn–35In Sn–58Bi Sn–37Pb Sn–40Pb Sn–3.5Ag 100Sn Sn–5Sb a

Cu-clad

Au–Ni plated

Ni plated

Solid Ni

76 73 72 91 89 71 73 66

67 82 89 86 89 80 79 78

62 48 69 99+ 99+ 74 71 68

66 67 69 81 81 72 73 68

Test conditions: temperature at 260jC, flux type RMA, hold time was 5 sec after solder perform melted. Source: Ref. 20.

284

KANG

TABLE 2

Comparison of the Bulk Properties of Solder Alloys

Properties Melting point (jC) UTS (MPa) Elongation (%) Shear strength (MPa) Hardness (HV) Creep strength MPa) (75jC, 1000 hr) Low cycle fatigue life (cycles) Cu dissolution (Am/min) Elec. resistivity (AV cm) Elastic modulus (GPa) a

Sn–37Pb

Sn–57Bi

Sn–3.5Ag

Sn–5Sb

Sn–0.7Cu

Sn–9Zn

183 31–46 [109]a 35–176 [109] 28.4 [111] 12.9 [106] 2 [76] 16,000 [93] 0.085 [23] 17.0 [112] 29 [104]

139 45–80 [24]a 40–200 [24] 48.3 [110] 20 [17] —

221 55 [108]a 35 [108] 32.1 [111] 17.9 [112] 11 [76] —

238 23–42 [107]a 90–350 [107] 31.8 [111] 17.2 [112] —

227 31 [71]a 12 [71]

199 60–65 [82]a 38 [82]

14.4 [117] 3 [76] —

23 [17] —

—

—

10–15 [118] —

10–15 [118] —

8,500 [93] 0.055 [23] 30–35 [117] 35 [105]

> 0.2 [23] 7.7 [112] 37 [104]

6,300 [93] >0.2 [23] 17.1 [112] 58 [116]

—

Numbers in brackets are references.

elongation data of both eutectic Sn–Pb and Bi–Sn solders listed in Table 2. At a given strain-rate (possibly with an equivalent microstructure), eutectic Bi–Sn solder exhibits a higher tensile strength, but less ductility compared to eutectic Sn–Pb. However, the mechanical properties of bulk solders cannot be directly applied to those of solder joints because the latter usually consist of a small solder volume constrained between two rigid surfaces or within a small space. Moreover, soldering processes involving microelectronic packages and assemblies result in complicated interfacial reactions between the molten solder and attachment-pad metallization. As a consequence, the microstructure in solder joints can be markedly different than observed in bulk solder samples of the same solder alloys. The shear strength of solder materials is typically quite strain-rate-sensitive as noted in Table 3. In consideration of the variations in soldering processes and in microstructure, it is not surprising that a large discrepancy often exists between the elongation and shear strength data of bulk solders and those of solder joints even when tested under equivalent conditions.

E.

Fatigue and Creep Properties

Several studies were conducted comparing the thermal and mechanical fatigue behavior of Bi–Sn and Sn–Pb [20,25–27]. Table 4 summarizes a comprehensive study of PTH solder joints made with various solders that have low melting points. The data in the table indicate that under a severe thermal cycling condition (i.e., 54jC to 75jC, 2 hr dwell), both Bi–Sn and Sn–Pb exhibit poor fatigue resistance compared to joints made of Sn–3.5Ag, Sn–5Sb, or Sn–52In solders [20]. It has been reported that the thermal fatigue resistance of PTH Bi–Sn solder joints is comparable to Sn–Pb [27]. Table 5 compares the low-cycle fatigue data of lap-shear joints 10 mils thick made of several low-melting-point solders. At low-cycle testing (1/15 CPM), Sn–Pb solder joints exhibit a superior fatigue life for a wide range of shear strains compared to Bi–Sn, while at a high-cycle testing (5 CPM) and/or at a high temperature (100jC), Bi–Sn solder joints have a much longer life. In addition, at high strains (between 8% and 20%), Sn–Pb solder joints again exhibit a longer

SOLDER-BASED SYSTEMS AND THEIR PROPERTIES TABLE 3

285

Shear Strength of Solder Joints to Copper Substrate

Solder alloy (wt.%) Sn–57Bi Sn–58Bi Sn–57Bi 60Sn–40Pb 63Sn–37Pb Sn–3.5Ag Sn–3.65Ag Sn–3.5Ag Sn–5Sb Sn–5Sb Sn–5Sb Sn–0.7Cu Sn–9Zn Sn–3.8Ag–0.7Cu

Strain rate (mm/min)

Shear strength (MPa)

Elongation (%)

Reference

0.05 0.10 50.0 50.0 0.10 0.05 0.10 50.0 0.05 0.10 50.0 0.10 0.10 0.10

25.3 46.2 19.6 18.0 32.7 26.8 37.2 36.8 21.8 31.6 29.6 27.0 28.8 35.1

4.8 — 3.2 — — 10.2 — 7.8 5.7 — 7.2 — — —

[113] [115] [113] [114] [115] [113] [115] [113] [113] [115] [113] [115] [115] [115]

life than Bi–Sn joints. This is consistent with the fact that Sn–Pb solder joints usually possess a greater ductility or elongation than Bi–Sn joints when tested under similar conditions. Creep behavior of the low-melting point solders has been fully investigated [26,28,29]. Table 6 lists the steady-state creep parameters of several solders, indicating that the steady-state creep rate of Bi–Sn solder joints is smaller than similar Sn–Pb solder joints, although eutectic Bi– Sn solder has a significantly lower melting point than eutectic Sn–Pb solder, 139jC vs. 183jC, respectively.

F.

Applications

1. Assembly There is a substantial history of eutectic Bi–Sn solder utilization for electronic applications where low-temperature soldering is required [25,30]. However, when decreasing the soldering temperature, there is often an adverse impact on flux activation, which results in changing the wetting

TABLE 4 Thermal Fatigue Failure Rate of Plated-Through-Hole (PTH) Solder Joints Encapsulated with Epoxy Thermal cyclesa Solder alloy (wt.%) Sn–52In Sn–58Bi Sn–35In Sn–37Pb Sn–3.5Ag 100Sn Sn–5Sb

1

25

50

75

100

150b

0.0 0.0 8.3 0.0 0.0 0.0 0.0

0.0 100 91.6 100 0.0 0.0 0.0

0.0 100 100 100 0.0 18.2 0.0

0.0 100 100 100 0.0 27.3 0.0

0.0 100 100 100 0.0 27.3 0.0

10.0 100 100 100 0.0 27.3 0.0

Test conditions: temperature cycling between 54jC and 75jC, soak time of 2 hr in each chamber. Thermal fatigue failure rate (in percent) was determined by detecting a 360j crack around the solder fillet at a magnification of 40 at the first cycle and every 25 cycles thereafter. Source: Ref. 20. a

b

286 TABLE 5

KANG Fatigue Life Comparison of Lap Shear Joints of Several Solder Materials

Solder alloy (wt.%)

Cycles to failure as a function of percent shear strain, frequency, and temperaturea

Frequency

1/15 CPM at 25jC

5 CPM at 25jC

5 CPM at 100jC

Strain

20%

8.0%

3.0%

20%

8.0%

3.0%

20%

8.0%

3.0%

Sn–52In Sn–58Bi Sn–37Pb Sn–5Sb

200 1.6 30 65

1,800 110 630 600

17,000 8,500 16,000 6,300

130 3.3 13 28

300 100 180 290

6,700 6,300 3,000 3,600

700 9.3 13 15

150,000 250 160 200

— 8,500 1,900 3,000

a

Failure point was established by the 0.03-mV resistance change. Source: Ref. 93.

characteristics of the molten solder. Special soldering processes and techniques are utilized to counter these effects, such as immersion wave soldering [30]. The immersion wave soldering process was developed for PTH soldering of advanced multilayer printed circuit boards (PCB). In this process, an assembled and fixtured PCB is immersed in a fluxing fluid and preheated to the soldering temperature. After preheating and while still immersed in the fluxing fluid, the PCB is passed over a Bi–Sn eutectic solder wave to form the solder joints. The fluxing fluid typically is gylcerine activated with ethylenediaminetetraacetic acid (EDTA). This flux is water-soluble [30, 31]. Another application of Bi–Sn in a conventional wave soldering process has been reported for a low-temperature assembly of complex PCBs [32]. Furthermore, a two-step soldering process has been demonstrated by using both Pb–Sn and Bi–Sn solders in sequence [33]. In this process, surface mount technology (SMT) components are attached to one side of a PCB using eutectic Sn–Pb solder, while through-hole or SMT components are attached to the opposite side using a Bi–Sn solder wave. However, caution should be exercised in this process in terms of possible Pb contamination. It has been reported that even low levels of Pb contamination in Bi–Sn solder joints can seriously degrade both the joint strength and thermal fatigue life [18,25]. The deterioration of the mechanical properties is attributed to the formation of a low melting point (96jC), 52Bi–32Pb–18Sn ternary eutectic phase [34,35]. A small addition (up to 2%) of In into eutectic Bi–Sn solder has been determined to eliminate the formation of the Bi–Pb–Sn ternary phase [35].

TABLE 6

Steady-State Creep Properties of Several Solder Materials

Solder alloy (wt.%) Sn–52In Sn–58Bi Sn–40Pb Sn–3.5Ag Sn–3.8Ag–0.7Cu Sn–1Cu Sn–5Sb Sn–9Zn

A (per sec)

n

DH (kcal/mol)

Ref.

     

3.2205 4.0514 6.3 6.05 3.69 8.1 5.0 5.7

22.862 16.852 20.0 14.6 8.6 19.0 10.7 15.6

[29] [26] [28] [94] [94] [95] [61] [96]

1.9492 5.5403 1.0927 9.3 2.6 1.41

10+4 107 1012 105 105 108

2.17  102

Steady-state creep rate, dc/dt=Asn exp(DH/RT ), where c=shear strain, s=applied shear stress, A = preexponential factor, n=stress exponent, DH=activation energy, R=gas constant, and T = temperature in Kelvin scale.

SOLDER-BASED SYSTEMS AND THEIR PROPERTIES

287

However, mainstream SMT soldering applications using eutectic Bi–Sn solder and conventional fluxes and surface finishes have not been reported. This may be due to the fact that the wetting characteristics of the Bi–Sn solder are generally not as favorable as Sn–Pb solder with existing flux materials and surface metallizations. 2. Other Attachments A few interesting applications utilizing eutectic Bi–Sn have been reported: electrically conducting adhesives [36], flip-chip interconnections [37], and bonding of thin film structures [38]. For conducting adhesive applications, BiSn-coated Cu powder was used as conducting filler particles in a matrix of polymer resins in Pb-free, low-temperature assembly [39] and via fill applications [36]. In the conducting adhesive applications, the BiSn-coated Cu particles are fusible during lamination or bonding under heat and pressure, which leads to metallurgical bonds among the conducting particles as well as to the conducting pads, resulting in superior mechanical and electrical properties of the joints. For flip-chip applications, the eutectic Bi–Sn solder alloy was electroplated to form solder bumps on various under-bump metallizations (UBMs) [37]. Based on the nature of the interfacial reactions and ball shear strength exhibited when exposed to multiple reflow cycles, Bi–Sn solder bumps were identified as a potential candidate for replacing the Pb–Sn eutectic solder bumps. A fluxless bonding process was demonstrated with a Bi–Sn–Au multilayer, thin film structure to form the eutectic Bi–Sn phase during the bonding of a Si wafer to another Si at a low temperature, 180jC [38].

G.

Utilization

A concern has been raised with regard to the replacement of Pb with Bi metal in current Pb–Sn solder applications. Since the major source of Bi metal is as a by-product of Pb refining [40], the replacement of Pb with Bi would lead to the dilemma of drastically increasing the amount of refined Pb metal. However, since the amount of Pb used in electronics is less than 2% of the total Pb consumption, the mining of Pb for automobile batteries which utilizes approximately 80% of the Pb consumed would provide a sufficient supply of Bi metal. The final report of the National Center for Manufacturing Sciences (NCMS) on ‘‘Lead-Free Solder Project’’ has raised a serious

TABLE 7

Addition of Alloying Elements to Bi–Sn Solders

Alloying elements In Cu, Ag Au, Ag Ag Au, Ag In, others Sb, others Ge, others

In

Typical compositions (wt.%)

Ref.

Bi–Sn–2In Bi–Sn–4Cu–2Ag Bi–Sn–1Au Bi–Sn–1Ag Bi–Sn–0.5Ag Bi–Sn–1Au Bi–Sn–2Ag Bi–Sn–(2–5)In–(Te,Se,Cs,Au,Ni) Bi–Sn–(6–10)In–(Te,Se,Cs,Au,Ni) Bi–Sn–(5–15)Sb–(Te,Se,Au,Ni,Ce,Cs,Co,Si,Mg,Ca) Bi–Sn–0.1Ge Bi–Sn–5Ag–5Sb–0.1Ge Bi–Sn–0.2Ni–1Cu Bi–Sn–5Ag–5Sb–0.2Ni–1Cu–0.1Ge Sn–2Bi–8In Sn–10Bi–10In Sn–10Bi–20In

[41] [41] [42] [43] [44] [45] [46] [47]

[48]

288

KANG

question with Pb-free solder alloys containing high levels of bismuth and/or indium, which are expensive and have limited availability [4]. Based on an economics and availability assessment combined with their property down-selection criteria, the NCMS report recommended limiting Bi usage to no more than 20%, preferably up to 9% of a solder formulation. However, eutectic Bi–Sn solder was included in the manufacturability and reliability assessments as a baseline alloy evaluated in the NCMS study for surface-mount and through-hole applications. Based on a better or comparable thermal fatigue life exhibited by the Bi–Sn solder joints investigated in the NCMS study, eutectic Bi–Sn solder was recommended as one of three promising Pb-free solders for SMT applications [4]. Eutectic Bi–Sn solder has been extensively used for PTH applications in IBM high-performance mainframe computer systems for the past 25 years where low assembly temperatures are required, and the end-use temperatures are well below 100jC [4,30]. Several alloy compositions have been developed to improve the mechanical and physical properties of the eutectic Bi–Sn solder by adding a small amount of alloying elements [41–47], as indicated in Table 7. The Sn-rich, Sn–Bi–In ternary alloys have also been disclosed for high solidus temperature, high service temperature, and high strength applications [48].

III.

TIN–ANTIMONY SOLDERS

A.

Microstructure

Sn–5Sb solder has a near-peritectic composition and a relatively high-liquidus temperature, around 238jC, which is significantly higher than the melting point of eutectic Sn–Pb solder, 183jC. Unlike other alloying elements, the addition of Sb to Sn raises the melting point of Sn gradually as the solid solubility of Sb in Sn increases [49]. The peritectic reaction, which occurs near 246jC, produces Sn–Sb microstructures which are complex. A typical cast microstructure of Sn–5Sb alloy solidified rapidly is shown in Fig. 2. This microstructure exhibits coarse dendrites of Sn-rich solid solution with Sn–Sb intermetallic compounds dispersed between the dendrites.

FIG. 2 The microstructure of Sn–5Sb solder alloy rapidly solidified. The scanning electron microscope (SEM) micrograph shows a coarse dendritic structure of Sn-rich solid solution with the SnSb intermetallics dispersed in between the dendrites. (Courtesy of W. K. Choi, IBM Corp.)

SOLDER-BASED SYSTEMS AND THEIR PROPERTIES B.

289

Benefits

A small addition of Sb to eutectic Sn–Pb solder has long been recognized to impart several beneficial effects, such as the prevention of tin pest [50,51], strength improvements [52,53], and creep resistance [54]. Antimony additions to pure Sn generally provide the same beneficial effects observed with Sn–Pb eutectic solder. In Table 3, a significant increase in shear strength is noted for solder joints made of Sn–5Sb solder in comparison with those made of pure Sn. Table 4 also shows a better thermal fatigue life of PTH joints made with Sn–5Sb than those of pure Sn. However, one drawback is recognized with Sn–5Sb solder in terms of solderability, as measured by the percent spread of a wetted area in Table 1. Its solderability is lower than that of eutectic Sn– Pb, and it is also reduced by adding Sb to pure Sn. Several Sb-containing, Sn-rich solders with multicomponent alloying elements such as Bi, Ag, Cu, and Ni have been invented for high-solidus, high-service temperature and high-strength applications [55,56]. By adding Bi, Ag, and Cu, the melting point of Sn–5Sb can be reduced to 218jC [57]. Bismuth-free, quaternary Sn-rich solders were also reported for electronic applications, with a sufficiently low melting temperature, superior wetting characteristics, and superior mechanical strength compared to Sn–Pb alloys [58,59].

C.

Mechanical Properties

The mechanical properties of Sn–5Sb solder are favorable compared to eutectic or near-eutectic Sn–Pb solders listed in Table 2. The alloy exhibits good creep resistance and thermal fatigue properties. The room-temperature ductility of the alloy is very high. The creep deformation mechanisms of this alloy were investigated with both bulk samples and solder bumps compared to Sn–3.5Ag solder alloy [60]. Power law relationships between strain rate and stress were observed with a similar stress exponent (n = 5) for both alloys, but with a different creep activation energy, Q = 61 and 45 kJ/mol for Sn–5Sb and Sn–3.5Ag, respectively [61]. The ratecontrolling, creep deformation process is dislocation climb as a result of lattice diffusion in Sn– 3.5Ag, while for Sn–5Sb, it is viscous glide of dislocation by pipe diffusion. Therefore Sn–5Sb alloys having a larger activation energy are more creep-resistant than Sn–3.5Ag [61]. 1. Effects of High Sb Concentrations The addition of Sb can be harmful to the mechanical properties of pure Sn when the Sb level approaches the solubility limit in Sn, which is about 10 wt.% at 250jC [62]. This is attributed to an extensive formation of the intermetallic phase, SnSb [63].

D.

Base Metal Dissolution and Intermetallic Growth

A serious concern with Sn-rich solders is the excessive dissolution of the base metal such as Cu into the molten solder. A comparison of the dissolution rate of Cu into molten Sn–5Sb solder with other Sn-rich solders is given in Table 2. Due to its high Sn content and high reflow temperature, the molten Sn–5Sb solder dissolves Cu metallization at a rate greater than 0.2 Am/min and Ni metallization at a rate of 0.07 Am/min at 270jC [23]. The corresponding intermetallic growth rate of Sn–5Sb is also much more than observed with eutectic Sn–Pb or Bi–Sn solder [23]. Fig. 3 compares the interfacial reaction that occurs between Cu metallization reacted with Sn–5Sb at 270jC and Sn–37Pb solder at 215jC. After 20 min, very large, separated Cu 6Sn5 intermetallics were observed to form at the Sn–5Sb solder interface, while the Cu 6Sn5 was much less and well adhered at the Sn–Pb interface.

E.

Application

Several electronic applications of Sn–5Sb solder have been reported: the creation of hermetic seals in multichip modules, bonding a semiconductor device onto a substrate [64], and attachment of I/O pins to multilayer ceramic substrates [65]. The braze alloy, Au–20Sn, used to attach I/O pins to ceramic chip carriers was replaced by Sn–5Sb solder. The replacement provided several

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FIG. 3 Scanning electron microscope (SEM) micrographs showing the interfacial reactions of Cu metallization with a) Sn–37Pb at 215jC for 2 and 20 min and b) Sn–5Sb at 270jC for 2 and 20 min. (From Ref. 23.)

advantages, such as lowering the joining temperature by about 50jC and reducing the stress induced by the joining process. In addition, Sn–5Sb solder joints offer good strength and creep properties which are important characteristics for a pin-attachment application. In another application, Sn–5Sb solder was used as a corrosion protection coating on a steel plate, as well as an electrically conductive area that could be used for subsequent grounding [66].

F.

Utilization

In the NCMS investigation [4], Sn–5Sb was included in the initial list. However, based on the down-selection criteria of toxicity and alloy properties assessment, it was not selected for a manufacturability assessment. It is stated in the report that ‘‘Sb is highly toxic when inhaled or ingested and has the same Lethal Dose, Lower Limit (LDLo) as Cd.’’ Therefore it was recommended to not consider Sb as a possible major constituent in Pb-free solders, although Sb has been present in solders utilized for military applications since the 1940s but only in small amounts, typically less than 0.5% [4].

IV.

TIN–COPPER SOLDERS

A.

Microstructure

Eutectic Sn–Cu solder (99.3Sn–0.7Cu, by weight) has a melting point of 227jC. A typical microstructure of as-cast eutectic Sn–Cu solder is shown in Fig. 4, consisting of large dendritic h– Sn grains surrounded by a network of the eutectic microstructure containing a fine intermetallic compound, Cu6Sn5. The microstructure of Sn–0.7Cu solder bumps used in flip-chip interconnections is composed of large, tin-rich grains (on the order of 20–50 Am) with a fine dispersion of Cu6Sn5 intermetallics [67].

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FIG. 4 The microstructure of Sn–0.7Cu solder alloy solidified at the rate of 5jC/sec. The optical micrograph shows large dendritic h-Sn grains surrounded by a network of the eutectic structure containing fine Cu6Sn5 intermetallics. (Courtesy of P. Lauro, IBM Corp.)

B.

Tin Pest

During the study of low-cycle fatigue life of Pb-free solders, tin pest was observed in as-cast, bulk samples of Sn–0.5Cu alloy [68]. Tin undergoes an allotropic transformation of white tin (bodycentered tetragonal) into gray tin (diamond cubic) at temperatures below 13jC (286jK). The gray tin transformation product is called ‘‘tin pest,’’ and the change to this form of tin is accompanied by an increase in volume of about 26% [69]. Approximately 40% of the specimen surface was transformed into gray tin after aging at 20jC (255jK) for 1.5 years, and this proportion increased to about 70% after exposure for 1.8 years. The degree of transformation observed in work-hardened areas was much higher than other areas suggesting that residual stress or strain may provide an additional driving force for transformation to gray tin [70]. In the same study, tin pest was also observed on the strain-hardened surface of a Sn–3.5Ag ingot which was thermally aged at 20jC (255jK) for 1.8 years. Here the surface growth of gray tin was small compared to the Sn–Cu alloy [68]. This observation suggests a potential reliability issue where certain Sn-base, Pb-free solder joints, which are under stress, are used at low temperatures. Tin pest is discussed in detail in Chap. 22.

C.

Mechanical Properties

The tensile strength of eutectic Sn–Cu solder is lower than eutectic Sn–Pb solder [71], but the hardness is similar to the eutectic Sn–Pb alloy. In a study of temperature-dependent tensile properties of Pb-free solders, the strength of Sn–0.7Cu was found to be lower than Sn–3.5Ag and Sn– 3.8Ag–0.7Cu [72]. But, Sn–0.7Cu exhibited a higher tensile strength at 100jC and 180jC compared to Sn–3.5Ag and Sn–3.8Ag–0.7Cu [72]. The addition of a small amount of Ag (up to 1 wt.%) refines the Sn–Cu eutectic microstructure by introducing another fine intermetallic compound, Ag3Sn [71]. The mechanical properties (both tensile strength and elongation) of corresponding Sn–Cu–Ag alloys that are discussed in detail in Chap. 8 are superior to eutectic Sn–Cu. This is believed to result from the deformation mechanism being more similar to that of eutectic Sn–Ag than the eutectic Sn–Cu alloy. To further improve its mechanical properties and control

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the interfacial reactions during soldering or thermal aging, several modifications of Sn–Cu-based solders have been disclosed: Sn–Cu–Ni [73], Sn–Cu–Ni–Sb [74], and Sn–Cu–Ga–In [75]. 1. Creep The creep behavior of as-cast, Sn–0.5Cu, and several other solders (Sn–3.5Ag, Sn–3.8Ag–0.7Cu) was investigated at 75jC and compared to eutectic Sn–Pb [76]. In terms of stress and time to rupture, the Sn–0.5Cu alloy behaves similar to eutectic Sn–Pb over the range of rupture lives up to 1000 hr. The Ag-containing alloys exhibit a creep resistance which typically is a hundred and a thousand folds greater than observed for Sn–Ag and Sn–Ag–Cu, respectively. The superior creep resistance of Ag-containing alloys is also retained when the alloys are tested at a constant homologous temperature [76].

D.

Applications

Because of its high melting point, eutectic Sn–Cu solder is not recommended for surface mount technology [4]. However, because of its low material cost, it is considered as a good candidate for Pb-free, wave-soldering processes in the mass production of commercial electronics. One example of using eutectic Sn–Cu alloy was reported with the world’s first virtually Pb-free telephone, where eutectic Sn–Cu was chosen for both solder joints and surface finish layers on PCB and component termination pads [77]. Also, the use of a Sn–Cu coating is preferred over pure Sn to avoid the growth of Sn whiskers [78,79]. 1. Flip-Chip Solder Bumps Eutectic Sn–Cu solder has received considerable attention for flip-chip interconnections [67,72,80]. In order to find a best combination of solder composition and under-bump metallurgy (UBM), a variety of Pb-free solder bumps and UBM combinations were investigated for their mechanical properties, thermal/mechanical fatigue life, and failure mechanisms. The Sn–0.7Cu eutectic alloy was identified to exhibit the best fatigue life in both thermal and isothermal mechanical tests regardless of UBM type [72]. The flip-chip parts used in this experiment were bonded to an organic board without underfill encapsulation to accelerate failures, yielding the fatigue life only on the order of 100 cycles under the thermal cycle test conditions performed. In actual flip-chip packages, the solder joints are often underfilled. The underfill encapsulation constrains solder joint deformation and significantly lowers the strain imposed on the flip-chip joints, resulting in a much longer fatigue life than those without underfill encapsulation [80]. Accordingly, it is difficult to assume the failure mechanisms operating in both cases being identical. Hence a caution must be exercised in interpreting the thermal fatigue life data without underfill encapsulation, especially if the application will utilize an underfill.

V.

TIN–ZINC SOLDERS

A.

Microstructure

The Sn–Zn binary system has a eutectic composition of Sn–8.8Zn at 198.5jC [81], which is close to eutectic Sn–Pb solder, 183jC. Because of this reason, the Sn–Zn system has been investigated as a promising candidate in replacing eutectic Sn–Pb solder. The microstructure of eutectic Sn– Zn consists of a needle-like Zn-rich phase uniformly dispersed in the primary Sn-rich matrix as shown in Fig. 5 [82]. Since there is little solid solubility of Zn in Sn, most Zn atoms precipitate out as large needles during solidification.

B.

Replacement Alloys Based on Melting Point

Thermodynamic calculations and experimental work were performed to find a Sn–Zn alloy with a melting point close to eutectic Sn–Pb by adding In or Bi alloying elements, which are known to

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FIG. 5 The optical micrograph shows the microstructure of Sn–9Zn consisting of needlelike Zn precipitates uniformly dispersed in the primary Sn-rich matrix. (From Ref. 82.)

lower the melting point of Sn [83]. A quaternary alloy, Sn–2Bi–5In–6Zn, was identified whose melting point is very close to 183jC [83]. It has been disclosed that adding In to Sn–9Zn creates Sn–Zn–In ternary alloys whose melting points are close to eutectic Sn–Pb: Sn–9Zn–5In (m.p. 188jC) and Sn–9Zn–10In (m.p. 178jC) [84].

C.

High Activity Metal

Although Sn–Zn alloys have been used as solders for many years, primarily for soldering aluminum assemblies [85], they have not been used as solders for microelectronic applications because they readily oxidize during soldering which causes poor wetting. The eutectic Sn–9Zn alloy was included in the original list of 79 alloys considered in the NCMS study. However, it was eliminated early based on significant oxide formation during a wetting balance test [4].

D.

Wettability

The wettability of Sn–Zn alloys was evaluated by a wetting balance test, where a short wetting time and high wetting force were desired to form reliable solder joints [86]. The wetting time of Sn–Zn alloys was found to decrease considerably by adding Bi or In. This is probably due to a lower liquidus temperature of the ternary alloys. The wetting force of Sn–Zn alloys decreased slightly with an increase in Bi content, while it became a minimum at 5% In and then increased with In content. It was also determined that the contact angle was important to evaluate the wettability during the wetting time when measured in the wetting balance test [86]. The addition of Bi or In gradually lowered the contact angle of the Sn–Zn alloys. However, the measured contact angles of the Sn–Zn alloys were much larger than of Sn–Pb or Sn–Ag alloys, and so the wettability of Sn–Zn–Bi or Sn–Zn–In alloys was assessed to be poor compared to eutectic Sn–Pb or Sn–Ag solders. Since zinc is active chemically and metallurgically, its addition to Sn-base solders has the potential to completely change the behavior of a solder, such as oxidation of molten solders and interfacial reactions during soldering.

E.

Interfacial Compound Formation

The interfacial reactions of Sn–8.5Zn–5Bi with brass, copper, nickel, and nickel–iron substrates were investigated [85] which resulted in zinc rather than tin intermetallic compound layers formed

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at the substrate metallization interfaces. During prolonged annealing at 125jC, a zinc depletion layer in the solder matrix together with the formation of interfacial voids was reported. The interfacial voids were attributed to the Kirkendall effect, wherein one component in a diffusion couple (Zn in this case) diffuses more rapidly than the other. The surface chemistry of the reacted Sn–Zn–Bi alloy was also investigated using x-ray photoelectron spectroscopy (XPS). Contrary to the cases of most Sn-based or Sn–Pb solders where tin oxides are exclusively formed [87], the surface chemistry was dominated by zinc segregation and oxidation. This may have implications with regard to flux compatibility and rates of drossing [85]. A similar effect was reported involving the addition of aluminum, also an active element, into Sn–9Zn [88]. Flip-chip solder bumps were produced on Cu termination pads by applying Sn– 9Zn–0.5Al solder using a dipping method. During an extended annealing period at 150jC, Al and Zn diffused to the Cu/solder interface to form a thin layer of AlCuZn intermetallic compound, but not Cu–Sn compounds, which can serve as a reaction barrier layer. 1. Effect of Base Metallization The solder joint reliability of chip-scale packages (CSPs) assembled by using Sn–8Zn–3Bi solder balls and paste materials was evaluated for three different surface finishes: bare Cu, immersion Ag, and Au/Ni [89]. The thermal fatigue life of the Sn–Zn–Bi joints was reported to be longer than Sn–Pb joints on a Au/Ni surface finish, while the combinations with Cu or Ag exhibited much worse results than with Sn–Pb. For the Cu surface finish, a thick Cu–Zn intermetallic compound layer grew at the interface, and a fracture occurred from this layer. On the other hand, in the case of the Au/Ni finish, nickel behaved as a reaction and diffusion barrier and prevented the formation of a Cu–Zn layer, resulting in a longer fatigue life than the other surface finishes tested [89]. However, some large voids were observed in the CSP joints that consisted of Sn–Zn– Bi solder balls and paste that may have resulted from poor wettability due to oxidation characteristics of Zn-containing alloys.

F.

Mechanical Properties

The room-temperature tensile strength of Sn–9Zn is about twice that observed for Sn–37Pb [82,90], and its flow stress is greatly dependent on strain rate. The percent increase in flow stress for strain in the range of 2–6% and a strain rate from 103 to 102 sec1 is reported as 13%, 21%, and 29% for Sn–37Pb, Sn–3.5Ag, and Sn–9Zn, respectively [90]. The strain-controlled isothermal fatigue life of Sn–9Zn is reported to be better than Sn–3.5Ag and Sn–37Pb for the strain range of 0.2–2.0%. The introduction of a hold time at maximum strain, which represents a creep loading condition, causes a drastic reduction in the number of cycles to failure, suggesting a significant interaction of creep-fatigue failure mechanisms.

G.

Utilization

Among seven down-selected alloys for the NCMS manufacturability and reliability assessment [4], only one Zn-containing alloy, Sn–3.5Ag–0.5Cu–1Zn (liquidus temperature of 221jC), was included. The maximum concentration of Zn in the selected alloy was chosen to limit the pasty range, the difference between the solidus and liquidus temperature of a solder, to be less than 30jC.

VI.

TIN–INDIUM SOLDERS

A.

Abundance and Cost

Because of its low melting point, extreme softness and ductility, and excellent wetting characteristics, indium has been widely used as an alloying element for many low-temperature solders and fusible alloys [53,63]. However, because of its high price (several times more expensive than silver), it is not economically feasible to use In-rich alloys for replacing Pb–Sn solders, which are

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cost-effective [63]. In addition, the world production of indium metal is so small that the supply could only meet about 1% of that required as a substitution for Pb-based solders (assuming the alternative solder contains about 50% In) [91]. The NCMS report [4] recommends that a potential Pb-free solder alloy should contain no more than 1.5% indium based on material cost and availability.

B.

Properties of Major Candidates

In this section, three Sn–In solders, 51In–49Sn, 65Sn–35In, and 77Sn–20In–3Ag, are considered as Pb-free solder candidates. The eutectic 51In–49Sn alloy has a melting point of 120jC, while 65Sn–35In is a hypoeutectic alloy with a liquidus temperature of 162jC. 77Sn–20In–3Ag (IndalloyR #227) is a patented Pb-free solder alloy [92] that has a liquidus temperature of 187jC, very close to the melting point of eutectic Sn–Pb, and a solidus temperature of 175jC. Its physical and mechanical properties (including fatigue and creep resistance) compare favorably to eutectic Sn–Pb [92]. 51In–49Sn and 65Sn–35In solders exhibit a low tensile strength and high degree of ductility compared with eutectic Sn–Pb solder [1]. Eutectic In–Sn solder joints also exhibit a low shear strength and large elongation [1]. The solderability of the Sn–In solders is comparable to eutectic Sn–Pb solder with either a Cu or Au/Ni metallization, but are inferior to Pb–Sn on Ni metallization (Table 1) [20]. Thermal fatigue life of eutectic In–Sn PTH joints is found to be much better than Pb–Sn joints tested under identical conditions (Table 4). This is also true of In– Sn lap-shear joints tested at various conditions of strain amplitude, frequency, and temperature, in comparison with eutectic Sn–Pb solder (Table 5) [93]. Table 8 lists two sets of low-cycle fatigue life data obtained from three eutectic solders including eutectic In–Sn. Some discrepancy is noted with regard to the fatigue life of the In–Sn solder joints. In the early work of Wild [93], a superior fatigue life of In–Sn joints was reported compared with Pb–Sn, while the opposite was reported by Mei and Morris [26] under apparently similar conditions. However, a detailed examination of these studies revealed that the results were obtained from solder joints formed differently and tested with a different failure criterion [1]. 1. Creep The steady-state creep rate of eutectic In–Sn solder joints is reported in Table 6, in comparison with Pb–Sn and other solders. Among these solders, it was determined that In–Sn joints display the weakest resistance against creep deformation [26,29,61,94–96]. In contrast, eutectic In–Sn solder exhibited the largest total deformation to fracture among them. Some superplastic behavior of this solder is reported under shear loading at temperatures above 65jC [29]. This finding may explain why In–Sn solder joints possess a superior fatigue life compared to other solders, as discussed earlier.

TABLE 8

Low-Cycle Fatigue Life of Solder Joints Percent shear strain vs. cycles to failure

Solder alloy (wt.%) Sn–52In Sn–58Bi Sn–37Pb a

10% (strain)a

8%( strain)b

20 50 150

1800 110 630

After Mei and Morris [26]. Fatigue life is established as the cycle of 50% reduction in the initial load amplitude. Mechanical fatigue testing at 1/10 CPM, 10% strain/cycle at 20jC. b After Wild [93]. Low-cycle fatigue tested at 1/15 CPM, 25jC; fatigue point established by the resistance increase of 0.03 mV [93].

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In electronic packaging applications, In-rich solders are selectively used where their unique properties are essential, for example, the flip-chip interconnections of Josephson devices [97,98], glass-sealing alloys of 52In–48Sn [99], cryogenic solders for thin film joining [63], thermally conductive die attachments to a multichip module [100], and Pb–In alloys for controlled-collapse, chip connection (C4) applications [101–103]. In-rich solders are also used in place of Sn-rich solders to prevent gold scavenging since gold dissolves at a slower rate in In-rich solders compared to Sn-rich solders [99].

VII.

SUMMARY

Several Pb-free solders were reviewed in light of replacing the conventional Pb–Sn solders particularly the eutectic alloy used for various electronic packaging applications. These candidates include the binary solder alloys of the Bi–Sn, Sn–Sb, Sn–Cu, Sn–Zn, and Sn–In solder systems and their ternary and quaternary compositions. The Sn–Ag system including its ternary and quaternary compositions are discussed in detail in Chap. 8. The topics discussed in the chapter cover the fundamental aspects such as the microstructure and mechanical/physical properties of Pb-free solder alloys, their processing issues (wetting behaviors, interfacial reactions, and joint defects), and their reliability issues (thermal fatigue, creep resistance, tin pest, and others). The existing and potential applications of each solder system for microelectronics have been extensively evaluated. In addition, the recent patent literature for Pb-free solders has been reviewed to discuss the trends in solder alloy development activities. For eutectic Bi–Sn solder, long used in low-temperature soldering, it is desirable to improve its wettability to be adopted for mainstream soldering technology. Although its creep and fatigue resistance are acceptable, its ductility is not adequate for high-performance applications. For Sn–Sb solders, because of their favorable mechanical properties and high melting temperature, it is a good candidate for solders to be used in high-temperature, high-strength applications. However, antimony cannot be considered as a major alloying element in Pb-free solders because of its toxicity and harmful effects on mechanical properties when added at high concentration levels. For eutectic Sn–Cu, its high melting point is not favorable for SMT applications, but it is recommended for wave soldering because of its low material cost. Owing to its low strength and high ductility, it has been widely pursued for various applications including flip-chip interconnects. However, due to its low alloying content, the alloy may undergo the allotropic transformation of white tin to gray tin at low temperatures, suggesting a potential reliability concern with this alloy. For eutectic Sn–Zn, the low melting point of its ternary and quaternary alloys is very favorable compared to other Sn-base alloys in replacing eutectic Sn–Pb. However, since Zn readily oxidizes during soldering which causes poor wetting, their wettability must be improved to be used in microelectronic applications. A proper flux/metallization system compatible with the SMT soldering technology is required. For Sn–In solders, due to its high material cost and limited availability, indium is only recommended as a minor alloying element in Pb-free solder alloys.

REFERENCES 1. Kang, SK.; Sarkhel, A. Lead (Pb)-free solders for electronic packaging. J. Electron. Mater. Aug. 1994, 23 (8), 701–707. 2. Vianco, P.T.; May, C. An evaluation of prototype surface mount circuit boards assembled with three non-lead bearing solders. Proc Surface Mount International Conf; SMI: San Jose, CA; 1995; 481–494. 3. Anderson, E. Tin silver copper: A lead free solder for broad applications. Proc NEPCON West’96 Conf., Reeds: Norwalk, CT, 1996; Vol. 2, 882–887.

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4. NCMS. Lead-free solder project final report. National Center for Manufacturing Sciences; Report 0401RE96, NCMS: Ann Arbor, MI, Aug 1997. 5. Ninomiya, R.; Miyake, K.; Matsunaga, J. Microstructure and mechanical properties of new lead free solder. Proc InterPACK’97; Kohala Coast, HI, ASME: New York, 1997; 1329–1333. 6. Choi, S.L.; Gibson, A.W.; McDougall, J.; Bieler, T. Mechanical properties of SnAg composite solder joints containing copper based intermetallics. Proc TMS Symp ‘‘Design and Reliability of Solders and Solder Interconnections’’; TMS Annual Meeting, Orlando, FL, TMS: Warrendale, PA, Feb. 1997; 241–245. 7. Habu, K.; Takeda, N.; Watanabe, H.; Ooki, H.; Saito, T.; Taniguchi, Y.; Takayama, K. Development of new Pb-free solder alloy of Sn–Ag–Bi. Proc 1997 7th IEEE Int. Symp. Electronics and the Envir.; 1999; 21–24. 8. Ylikoki, T.L.; Steen, H.; Forsten, A. Development and Validation of a Lead-Free Alloy for Solder Paste Applications. IEEE Trans. Compon. Packag. Manuf. Technol. Part C Manuf. July 1997, 20 (3), 194–198. 9. Kang, S.K. Development of lead (Pb)-free interconnection materials for microelectronics. Met. Mater. 1999, 5 (6), 545–549. 10. Seelig, K.; Suraski, D. The status of lead-free solder alloys. Proc 50th Electron Comp Tech Conf; Las Vegas, NV, IEEE: Piscataway, NJ, May 2000; 1405–1409. 11. Snowdon, K.G.; Tanner, G.G.; Thompson, JR. Lead free soldering electronic interconnect: Current status and future developments. Proc. 50th Electron. Comp. Tech. Conf.; Las Vegas, NV, IEEE: Piscataway, NJ, May 2000; 1416–1419. 12. Mawer, A.; Koschmieder, T.; Hodges, D. Can we assemble reliable Pb-free assemblies? Packaging Assembly Symposium: New Millennium Electronics-Manufacturing Challenges; Binghamton, NY, Nov. 2000. 13. Bartelo, J.; Henderson, D.; Knadle, K.; Sarkhel, A.; Woods, J.; Thiel, C.; Caletka, D.; Darbha, K.; King, D.; Woychik, C.; Gosselin, T.; Shih, D.Y.; Kang, S.K.; Cain, S.R.; Puttlitz, K. The effect of temperature range during thermal cycling on thermomechanical fatigue behavior of selected Pb-free solders. Proc. APEX2001 San Diego, CA, IDC: Northbrook, IL, Jan. 2001. 14. Gayle, F.W.; Becka, G.; Badgett, J.; Whitten, G.; Pan, T.Y.; Grusd, A.; Bauer, B.; Lathrop, R.; Slattery, J.; Anderson, I.; Foley, J.; Gickler, A.; Napp, D.; Mather, J.; Olson, C. High temperature lead-free solder for microelectronics. JOM June 2001, 53 (6), 17–21. 15. Frear, D.R.; Jang, J.W.; Lin, J.K.; Zhang, C. A metallurgical study of Pb-free solders for flip chip interconnections. JOM June 2001, 53 (6), 28–32. 16. Hu, J.N.; Tanaka, H.; Narita, T. Aging behavior of a Sn–Bi eutectic solder at temperatures between 233 and 373 K. Mater. Trans. JIM 2001, 42 (5), 769–775. 17. Miyazawa, Y.; Ariga, T. Influences of aging treatment on microstructure and hardness of Sn–(Ag, Bi, Zn) eutectic solder alloys. Mater. Trans, JIM 2001, 42 (5), 776–782. 18. Mackay, C.; von Voss, W. Mater. Sci. Technol. March 1985, 1 (3), 240–248. 19. Ackroyd, M.; Mackey, C.A.; Thwaites, C.S. Effect of certain impurity elements on wetting properties of Sn–Pb solders. Metals Tech. Feb. 1975; 73–85. 20. Jarboe, D.M. Thermal Fatigue Evaluation of Solder Alloys; NTIS, U.S. Dept. of Commerce BDX613-2341, Feb. 1980. 21. Vianco, P.; Frear, D. Issues in the replacement of Pb-bearing solders. J Met. July 1993, 14–19. 22. Felton, L.E.; Raeder, C. H.; Knorr, D.B. The properties of Sn–Bi alloy solders. J. Met. July 1993; 28–32. 23. Kang, S.K.; Rai, R.S.; Purushothaman, S. Interfacial reactions during soldering with lead–tin eutectic and lead (Pb)-free, tin-rich solders. J. Electron. Mater. 1996, 25 (7), 1113–1120. 24. Pattanaik, S.; Raman, V. Proc. Mat. Dev. in Microelec. Packg. Conf.; ASM: Montreal, 1991; 251– 257. 25. Wild, R.N. Proc. Inter. NEPCON Conf.; Briton, Reeds: Norwalk, CT, 1971; 81–91. 26. Mei, Z.; Morris, J., Jr. Characterization of eutectic Sn–Bi solder joints. J. Electron. Mater. 1992, 21 (6), 599–607. 27. Seyyedi, J. Thermal fatigue of low-temp solder alloys in insertion mount assembly. J. Electron. Packag. Sept. 1993, 115 (9), 305–311. 28. Tribula, D.; Morris, J., Jr. Creep in shear of experimental solder joints. J. Electron. Packag. Trans. ASME June 1990, 112 (6), 87–93. 29. Mei, Z.; Morris, J., Jr. Superplastic creep of low melting point solder joints. J. Electron. Mater. 1992, 21, 401–407. 30. Getten, J.; Senger, R. IBM J. Res. Develop. 1982, 26 (3), 379–382. 31. Lazzarini D.; Sarnacki, F. Water Soluble Fluxes, U.S. Patent, 4,000,016, Dec. 28, 1976. 32. Gudul, H.; Calson, R. Low temp Pb-free wave soldering for complex PCBS. Electron. Packag. Prod. Sept. 1992, 29–32. 33. Phillips, J.; Stillahn, K. Eutectic Sn–Bi for two-step PCB soldering. Electron. Packag. Prod. Feb. 1992, 78–80.

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Mei, Z.; Holder, H. A thermal fatigue failure mechanism of 58Bi–42Sn solder joints. Trans. AIME June 1996, 118 (6), 62–66. 35. Mei, Z.; Hua, F.; Glazer, J. Sn–Bi–X solders. Proc. SMTA Int. 1999; 399–403. 36. Kang, S.; Buchwalter, S.; LaBianca N.; Gelorme, J.; Purushothaman, S.; Papathomas, K.; Poliks, M. Development of conductive adhesive materials for via fill applications. Proc. 50th ECTC; Las Vegas, NV, IEEE: Piscataway, NJ, May 2001; 887–891. 37. Jang, S.Y.; Paik, K.W. Comparison of electroplated eutectic Sn/Bi and Pb/Sn solder bumps on various UBM systems. Proc. 50th ECTC; Las Vegas, NV, May 2000; 64–68. 38. Choe, S.; Chang, R.; Lee, C.C. Fluxless Sn–Bi–Au bonding process using multilayer design. Proc. 51st ECTC; Orlando, FL, IEEE: Piscataway, NJ, May 2001; 486–488. 39. Kang, S.K.; Purushothaman, S. Development of low cost, low temperature conductive adhesives. Proc. 48th ECTC; Seattle, WA, IEEE: Piscataway, NJ, May 1998; 1031–1035. 40. Ojebuoboh, F. Bismuth—Production, properties, and applications. J. Met. April 1992, 46–49. 41. Gonya, S.G.; Lake, J.K.; Long, R.C.; Wild, R.N. Lead Free, Tin–Bismuth Solder Alloys. US Pat. No. 5,368,814, Nov. 29, 1994. 42. Melton, C.M.; Beckenbaugh, W.; Miller, D. Tin Bismuth Solder Paste, and Method Using Paste to Form Connection Having Improved High Temperature Properties. US Pat. No. 5,389,160, Feb.14, 1995. 43. Chen, H.S.; Jin, S.; McCormack, M.T. Lead-Free Low Melting Solder with Improved Mechanical Properties. US Pat. No. 5,569,433, Oct. 29, 1996. 44. Mei, Z.; Hua, F.; Glazer, G. Low temperature soldering. Proc. 21st IEMT Symposium, IEEE: Piscataway, NJ, 1997; 463–476. 45. Paruchuri, M.R.; Shangguan, D. Low Temperature Lead-Free Solder Compositions. US Pat. No. 5,755,896, May 26, 1998. 46. Paruchuri, M.R.; Shangguan, D. Lead-Free, Low-Temperature Solder Compositions. US Pat. No. 5,833,921, Nov. 10, 1998. 47. Yamashita, M.; Tada, S.; Shiokawa, K. Solder Alloys, US Pat. No. 6,156,132, Dec. 5, 2000. 48. Gonya, S.G.; Lake, J.K; Long, R.C.; Wild, R.N. Lead-Free, High Tin, Ternary Solder Alloy of Tn, Bismuth, and Indium. US Pat. No. 5.414,303, May 9, 1995. 49. Massalski, T. Binary Alloy Phase Diagrams; ASM: Metals Park, OH, 1986; 2018 pp. 50. Thwaites, C.J. Antimony in soft solders. Brazing Solder 1986, 11, 22–26. 51. Fischer, J.; Lusk, L.; Hinkley, D.; Dowd, M.; Boske, G.; Plott, M.; Moffitt, J. Reliability assessment of antimony removal from Sn–Pb solders. Proc. Natl. Electron. Packag. Prod. Conf. 1992, 2, 1056. 52. Nightingale, S.; Hudson, O. Tin solders. Br. Non-Ferr. Metals Res. Assoc. Monogr., 1. 53. Klein Wassink, R.J. Soldering in Electronics; Electrochemical Pub. Ltd., Ayr, Scotland,1989; 123 pp. 54. Baker, W.A. The creep properties of soft solders and soft-soldered joints. J. Inst. Met. 1939, 65, 277–297. 55. Gonya, S.G.; Lake, J.K.; Long, R.C.; Wild, R.N. Lead-Free, Tin, Antimony, Bismuth, Copper Solder Alloy. US Pat. No. 5,411,703, May 2, 1995. 56. Gickler, A.E. Lead-Free Solder, US Pat. No. 5,837,191, Nov. 17, 1998. 57. Tulman, S. Tin Base Lead-Free Solder Composition Containing Bi, Ag and Sb. US Pat. No. 4,806,309, Feb. 21, 1989. 58. Seelig, K.F.; Lockard, D.G. Lead-Free and Bismuth-Free Tin Alloy Solder Composition. US Pat. No. 5,352,407, Oct. 4, 1994. 59. Seelig, K.F.; Lockard; D.G. Lead-Free and Bismuth-Free Tin Alloy Solder Composition. US Pat. No. 5,405,577, Apr. 11, 1995. 60. Murty, K.L.; Yang, H.; Deane, P.; Magill, P. Deformation mechanisms in Sn-based solder materials. Adv. Electron. Packag. ASME 1997 June 1997, 19 (1), 1221–1231; (Kohala Coast, Hawaii). 61. Mathew, M.D.; Movva, S.; Murty, K.L. Deformation mechanisms in tin and Ti-based electronic solder alloys. Key Eng. Mater. 2000, 171–174, 655–662. 62. Allen, W.; Perepezko, J. Constitution of the tin–antimony system. Scr. Metall. 1990, 24, 2215–2220. 63. Manko, H.H. Solders and Soldering; 3rd Ed.; McGraw-Hill: New York, 1992; 143, 128. 64. Hattori, H.; Kugawata, M. U. S. Patent, 4,480,261, Oct. 30, 1984. 65. Lombardi, T.E.; Miller, W.R.; Monjeau, G.; Srivastava, K.K.; Wild, R. Sn–Sb solder process for packaging I/O interconnections. IBM Tech. Dis. Bull. Dec. 1991, 34 (7A), 308. 66. Rice, L.; Roberti, A. Corrosion-protective solder coating. IBM Tech. Dis. Bull. April 1984, 26 (11), 6092. 67. Frear, D.R.; Jang, JW.; Lin, J.K.; Zhang, C. Pb-free solders for flip-chip interconnects. JOM June 2001, 53 (6), 28–32. 68. Kariya, Y.; Williams, N.; Gagg, C.; Plumbridge, W. Tin pest in Sn–0.5wt% Cu lead-free solder. JOM June 2001, 53 (6), 39–41. 69. Hedges, H.E. Tin and Its Alloys; Edward Arnold Ltd.: London, 1960; 51 pp.

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10 High-Temperature Lead-Free Solders with Dispersoids K.N. Subramanian and Fu Guo Michigan State University, East Lansing, Michigan, U.S.A.

Sunglak Choi Samsung Electronics, Seoul, South Korea

I. INTRODUCTION A. Insufficient Data Base Several recent studies address the numerous challenges faced in the development of lead-free electronic solders since there are no drop-in substitutes for traditional lead-bearing solders. Melting temperature, processing parameters, solderability, mechanical and thermomechanical fatigue behavior, etc. are among those challenges. Unfortunately, the extensive database on leadbearing solders generated over nearly the past several decades is not directly applicable to leadfree solders. As a result, the databases currently available to benchmark reliability models for lead-free solders are incomplete [1].

B. Process Challenges Most lead-free solder studies for electronic applications are aimed at defining suitable alloy compositions [2,3]. The tin (Sn)–silver (Ag) alloy system, with or without small alloy additions such as copper (Cu), is believed to be a suitable candidate [4–6]. The eutectic temperature of binary Sn–Ag is 221jC, while that of the ternary Sn–Ag–Cu is slightly reduced (217jC) [6,7], but both are reasonably higher than the tin (Sn)–lead (Pb) binary eutectic temperature (183jC). In general, the lead-free solders identified as potential candidates have melting points within the range of approximately 200jC to 225jC. They, of course, require higher processing temperatures compared to eutectic Sn–Pb, an important process and reliability issue that must be addressed when changing over to a lead-free solder. Higher processing temperatures will impact existing soldering technology in many key areas including substrate material stability, reliability, equipment modification/reliability, and higher energy costs.

C. High-Temperature Applications None of the typical lead-free solder alloys can be utilized at high service temperatures (f270jC) except 80Au–Sn solder, which would have a limited range of applications since it is expensive [8]. How to increase the service temperatures which existing lead-free solders can endure, or arrive at

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new solder compositions for such applications, is one of the serious problems that had remained largely unaddressed. However, the National Center for Manufacturing Sciences (NCMS) launched a lead-free solder program to explore their use in applications with service temperatures up to about 150jC [9]. For example, in automotive under-the-hood applications, significant performance advantages could be derived if electronic components/circuit boards were mounted on the engine manifold. This would significantly reduce the amount of wiring and minimize several complications in the electrical circuitry. Similar conditions also exist in aerospace and defense applications. Although the processing parameters must be modified to accommodate the temperature increase, these solders may or may not provide solder joints with a higher servicetemperature capability depending on thermomechanical fatigue characteristics. The results of several mechanical and thermal studies of solders whose structures remain stabilized during elevated temperature applications are reported in the literature [10–14]. Solder joints experience even more severe conditions in applications that combine both high current densities and high temperatures as occurs in automotive alternators or rectifiers. At present, there is no substitute, which is both suitable and economical, among the high-lead/lowtin solders used for these applications [15]. Typical solders based on tin (Sn)–gold (Au) alloys that can withstand these service conditions are cost-prohibitive in large-scale automotive-type manufacturing situations. Given the need to satisfy these design requirements, it is likely this issue will be addressed in the near future. This aspect of lead-free solders has not received much attention because solder joints in consumer and handheld communication electronics typically are not required to withstand the harsh environmental conditions encountered in automotive, defense, and aerospace applications.

D.

Use of Dispersoids

Incorporation of dispersoids has been pursued to improve the mechanical and, particularly, thermomechanical behavior of solders and their service-temperature capability without significantly altering the processing parameters [16–19]. This chapter addresses the role of dispersoids, their requirements, methods to incorporate them in a solder, and their influence on the microstructure and properties. Prior studies have dealt with several aspects of dispersoids in leadbearing solders, which will be briefly reviewed as they provide the basis for developments in leadfree solder systems.

II.

GENERAL REQUIREMENTS

A.

Harsh Service Conditions

1. Thermally Induced Table 1 lists typical homologous temperature ranges experienced by solders and some engineering alloys during service. Homologous temperature is the ratio of a test or service temperature to the melting temperature, both in absolute temperature. As can be seen from the data in Table 1,

TABLE 1 Homologous Temperature for Various Alloys Under Service Conditions Alloys Solders Superalloys (Ni and Co alloys) Stainless steel Al alloys in transportation Structural steel Source: Ref. 20.

Homologous temperatures 0.7–0.9 0.7–0.8 0.5 0.3 0.16

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solders are unusual materials because they are typically utilized in applications well beyond half of their melting temperature (i.e., in homologous temperature ranges between 0.7 and 0.9). Very few other materials, such as some so-called refractory superalloys, used in jet engines, are used at high homologous temperatures. Most engineering alloys are rarely utilized in applications which exceed more than half of their melting points (i.e., homologous temperature equals 0.5). Solder joints undergo low-cycle thermomechanical fatigue due to strains that develop as a consequence of the coefficient of thermal expansion (CTE) mismatch between the materials to which solder is attached [20]. Development of an alternative solder for high-current/high-temperature service environments has not been pursued vigorously. Electromigration in flip-chip solder joints is a significant device functionality concern which likely can be controlled by incorporating fine dispersoids in the solder. A fine dispersion of copper atoms in aluminum is already utilized to significantly decrease and often eliminate electromigration in the thin film circuitry of computer chips [21]. 2. Mechanically Induced Mechanical vibration that electronic components may experience, such as attachments to automotive engines, can create fatigue conditions at frequencies which are typically much greater than those which occur under thermomechanical conditions. Solder joints in electronics may also be subjected to impulsive conditions such as when automobiles encounter potholes, during aircraft landings, handling of a laptop computer during baggage handling operations, or by another passenger of carried-on and placed in the luggage rack.

B.

Microstructural/Property Requirements

Although fine-grained microstructures are beneficial to enhance mechanical-fatigue resistance, they can be deleterious for creep resistance. This is due to the fact that solder joints that experience high homologous service-temperature conditions undergo creep by grain boundary sliding. Accordingly, reducing the total grain boundary area reduces the tendency for creep. In addition to these opposing requirements, the highly inhomogeneous microstructure of as-joined solder joints and inhomogeneous strains generated within a solder joint during service result in significant coarsening of the microstructure. Aging causes growth of a solder/substrate interfacial intermetallic layer and coarsening of microstructural constituents within the solder joint. An ever-evolving microstructure correspondingly results in changing mechanical properties of the solder joint. Understandably, these changes present significant hurdles in formulating models for reliability prediction. 1. Presence of Dispersoids The presence of fine, stable, and system-compatible dispersoids located at the grain boundaries can retard coarsening, enhance mechanical fatigue behavior, and reduce creep rate by decreasing grain-boundary sliding tendency. Dispersoids present at a grain boundary represent obstacles which resist sliding between grains which share the boundary. A tradeoff is required between a strong solder joint to provide creep resistance but also one which is sufficiently compliant to dissipate stresses which could damage a device; that is, a solder joint must be capable of dissipating induced stresses, while still maintaining electrical functionality and mechanical integrity. Thus solder joints must exhibit reasonable strength with sufficient ductility to achieve optimal thermomechanical fatigue resistance characteristics. Although these two attributes appear to be mutually exclusive, both can be satisfied by appropriate microstructural modification of a solder with suitable dispersoids.

C.

Process Compatibility

The methodology employed to enhance the service performance of a solder joint should be compatible with the standard processes and assembly procedures practiced by the microelec-

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tronics industry. It is also essential that the incorporation of dispersoids does not significantly alter solderability, melting temperature, etc.

III.

OVERVIEW OF LEAD-FREE SOLDER ALTERNATIVES

A.

Key Studies

There has been significant worldwide effort investigating lead-free solders during the past decade. The details of these studies are presented in Chap. 17. Some significant findings of these studies are presented in this section as an introduction to lead (Pb)-free solders utilized for much of the solder reinforcement studies described in this chapter. 1. NCMS Project In the United States, the National Center for Manufacturing Sciences (NCMS), which is a consortium comprised of 11 industrial corporations, undertook a study to evaluate several alternatives to eutectic tin (Sn)–lead (Pb) solder [22]. Five solder alloys emerged from the downselection process as viable candidates to replace 80% of the standard near-eutectic Sn–Pb solder typically utilized. It was determined that eutectic Sn–58Bi, Sn–3.4Ag–4.8Bi, and eutectic Sn– 3.5Ag solders performed substantially better than eutectic Sn–Pb solder in certain surface mount applications. The project also demonstrated that Sn–58Bi and Sn–3.4Ag–4.8Bi exhibited thermomechanical fatigue lives comparable to or better than eutectic Sn–Pb. These Pb-free alloys exhibit bulk properties similar to eutectic Sn–Pb solder at room temperature, but possess higher tensile and yield strengths combined with good ductility properties (i.e., moderate-to-high elongation). However, the 4-year NCMS study, conducted between 1992 and 1996, did not identify a ‘‘drop-in’’ replacement for eutectic Sn–Pb solder. 2. DTI Project A project funded by the Department of Trade and Industry (DTI) in the United Kingdom [23] focused on the suitability of Pb-free solders for electronic assembly based on melting point, mechanical properties, corrosion resistance, and supply potential. Five solder alloys, namely, Bi– 42Sn, Sn–9Zn, Sn–5Sb, Sn–3.5Ag, and Sn–0.7Cu, were considered. Bi–42Sn, Sn–9Zn, and Sn– 5Sb solders were rejected for various reasons, such as poor mechanical properties, low melting point, corrosion of zinc phases, or an excessive melting temperature. The Sn–3.5Ag and Sn– 0.7Cu solder alloys exhibited the best overall mechanical and wetting performance among the five alloys evaluated. 3. JIEP Project A Japan Institute of Electronics Packaging (JIEP) project focused on the Sn–Ag–Bi alloy family with bismuth (Bi) varying from 7 to 25 wt.% [24]. It was determined that those alloys containing more than 7% bismuth were very brittle, and fillet lifting became a serious concern, although the melting temperature of the solder alloy was relatively low. Addition of a few percent of Bi to Sn– Ag–Cu alloys does not cause them to be brittle. It was also suggested that Sn–Zn and Sn–Cu alloys have a potential for use as lead-free solder alloys. Studies dealing with these alloys are currently in progress.

B.

Potential Candidates Based on Melting Point Considerations

As a result of recent research activities, a large number of lead-free solders have been developed in laboratory studies, and patent applications have been filed for a variety of alloy compositions. Several of these compositions are available from commercial sources. Some examples of lead-free solders (and their melting temperature ranges) which have been considered for application in microelectronic assemblies are given in Table 2. None of these, however, are considered to be a suitable universal drop-in replacement for eutectic Sn–Pb. The most promising lead-free alloys for electronic soldering applications appear to be Sn–Ag and Sn–Ag–Cu, Sn–Cu,and Sn–Ag–Bi [23].

HIGH-TEMPERATURE LEAD-FREE SOLDERS WITH DISPERSOIDS TABLE 2 Ranges

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Examples of Lead-Free Solder Alloys Covering a Broad Melting Temperature

Category Low melting temperature (<180jC) Melting temperature (183jC to 200jC) equivalent to eutectic Sn–Pb solder Mid-range melting temperature (200jC to 230jC) High melting temperature (230jC to 350jC)

IV.

COMPOSITE SOLDERS

A.

Definition and Purpose

Alloy Composition (wt%)

Melting range (jC)

Sn–58Bi Sn–52In Sn–9Zn Sn–8Zn–3Bi Sn–20Bi–10In Sn–3.5Ag Sn–0.7Cu Sn–3.8Ag–0.7Cu Sn–5Sb Sn–80Au

138 118 198.5 189–199 143–193 221 227 217 232–240 280

Solders with intentionally incorporated reinforcements are termed composite solders. The composite approach to solders was developed mainly to improve service-temperature capabilities. This improvement results from a stabilized fine-grained microstructure and an increased uniformity in solder-joint deformation. These factors serve to improve the overall mechanical properties of a solder joint, especially creep and thermomechanical fatigue resistance. An important additional feature is that the reinforcements do not alter the melting point of the solder matrix, but effectively increase the service-temperature capabilities of the base solder materials.

B.

Reinforcement Requirements and Considerations

Reinforcements added to a solder matrix should satisfy certain conditions to enhance the service performance of the solder. The key conditions are noted in Table 3. Reinforcing phases should bond to the solder matrix, and the bonding can be weak or strong depending on the requirements for the specific application. Reinforcements must have minimal or no solubility in molten solder

TABLE 3

Key Attributes for Reinforcements to Be Effective

Key attributes Bonding to matrix Minimum solubility in solder Density matches the matrix Size be optimal Not prone to coarsening Not altering the processing temperature and wettability

Purpose Maintain continuity Stable during reflow Uniform distribution Stabilize microstructure Stabilize microstructure under aging Maintain the advantage of the matrix material

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under normal reflow temperatures to maintain stability of the reinforcements during reflow. Reinforcements and the solder matrix densities should be closely matched to facilitate uniform distribution of the reinforcing phase throughout the matrix. If the densities are not matched, settling or floating of the reinforcements will cause the reinforcements to segregate within the solder, resulting in a nonuniform distribution [25]. Good wetting between the reinforcements and the solder matrix is also essential, although the interfacial strength could significantly alter the overall mechanical behavior of the composite solder. The size of reinforcing phases should be optimal in order to stabilize the microstructure. It has been determined that reinforcement particles of size f1 Am or smaller tend to stabilize a very fine-grained microstructure [18,26]. Reinforcement particles should not be prone to significant coarsening during reflow and service. Due to thermodynamic considerations, fine particles tend to coalesce to reduce the high interfacial energy associated with small particles. Finally, reinforcements should not significantly alter the processing temperature nor alter solderability (i.e., wetting characteristics of a solder with a chip carrier pad).

C.

Reinforcement Types

Reinforcement particles utilized in composite solders can be grouped into two categories. 1. Intermetallic Compounds One type of reinforcement addition involves intermetallic particles. These intermetallic reinforcements can be incorporated into the solder matrix either by adding intermetallic particles (e.g., Cu6Sn5, Cu3Sn, or Ni3Sn4 [18,19,25,27]) or by forming intermetallics from elemental particles (e.g., Cu, Ni, or Ag [16,17,28]) through a reaction with Sn during manufacturing or by subsequent aging and reflow processes. 2. Low Solubility in Tin Another type of reinforcement involves the addition of particles which have a low solubility and diffusivity in Sn or which do not react with Sn. Examples of this type of reinforcement are Fe particles [29] and oxide particles like Al2O3 or TiO2 [27]. Their utilization serves several purposes. Proper choice of reinforcement additions allows hard, uniformly distributed, noncoarsening particles to be introduced in a matrix. Current efforts include incorporation of nanosized particles as reinforcements [26,30]. Well-dispersed reinforcements serve as obstacles to grain growth, crack growth, and dislocation motion to resist solder creep and fatigue deformation [18,26,29].

D.

Reinforcement Addition Methods

The composite solder can be prepared by two methods. In the traditional approaches, the reinforcement is usually added to the molten solder to produce the composite [31]. These reinforcements could be stable intermetallics or metallic particles that can form intermetallics during processing [16–19,25,27,28]. The main aim of these approaches is to introduce compatible intermetallic reinforcements in the solder matrix. It is essential that these reinforcements do not significantly alter their size, shape, and volume fraction during service. This method of producing composite solder is termed as mechanical mixing method. An alternate mechanical mixing method involves addition of metallic particles to solder paste. After significant mixing, this mixture is melted to produce the composite solder. During the melting process, reflow, and aging, these metallic particles react with the solder matrix to produce stable intermetallic reinforcements. A recent development in the production of composite solder, termed as in situ method, introduces the compatible intermetallic reinforcements in the solder matrix during the processing of the solder itself. Details of the procedures used to achieve such an in situ composite solder are not available in the current published literature since the patent application for the process by the inventors is still pending.

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EARLY COMPOSITE SOLDER STUDIES UTILIZING LEAD/TIN SYSTEM

Several attempts have been made to improve the mechanical properties of lead-bearing solders using a composite approach [19,26–51]. Microstructural analyses as well as mechanical properties of the composite solders have been reported [26–51]. Certain composite solders exhibited enhanced mechanical properties sought by the electronic and automobile industries [26,28– 33,38–51].

A.

Microcharacterization of Reinforcement Particles

1. Equilibrium Conditions If there is a thermodynamically preferential intermediate phase between a metallic particle and solder, then the metallic particle reinforcement will be chemically unstable and form the intermediate phase upon processing [31]. However, the extent to which intermetallic formation can proceed depends upon diffusion considerations. Marshall and Calderon [31] performed a microstructural study involving reinforcement particles in eutectic Sn–Pb composite solder prepared by mechanically mixing the particles with eutectic Sn–Pb solder paste. The types of particles utilized were Ni, Ag, Cu, Cu3Sn, and Cu6Sn5 with an average size of 1.1, 1.75, 7.9, 12.72, and 10.8 Am, respectively. After melting of the bulk composite solders at 200jC for a minute in an inert atmosphere, Ni3Sn and Cu6Sn5 intermetallic layers were formed on the surface of Ni and Cu particles, respectively. It was also reported that a Cu6Sn5 intermetallic layer formed around Cu3Sn particles, while no new intermetallic layer formed around Cu6Sn5 particles. This indicates that Cu6Sn5 intermetallic phase is thermodynamically the equilibrium (i.e., most stable) intermetallic phase in the Sn–Pb–Cu system. In contrast, 1.75-Am size Ag particles completely transformed to Ag3Sn intermetallic particles. Microstructural analysis showed that the incorporated particles formed a good bond with the eutectic Sn–Pb solder matrix, suggesting that the reinforcements might enhance the mechanical strength of the solder. 2. Particle Size Effect On the contrary, Ag particles ranging between 4 and 7 Am in size did not completely transform to Ag3Sn intermetallic phase in the study of Wu et al. [32]. The procedure utilized in this study [32] was the same as in Marshall and Calderon except that the reflow was at 250jC for 1 to 1.5 min. Based on the fact that Wu et al. utilized a higher processing temperature, the observed difference in the intermetallic formation is attributed to the initial size of Ag particles utilized in the Wu et al. studies (4 to 7 Am) and Marshall and Calderon studies (average 1.75 Am). Au particles with a size range of 2 to 5 Am transformed completely to AuSn4 intermetallic particles during reflow [32].

B.

Effects of Processing on Porosity

1. Porosity and Mechanical Properties Clough et al. [33] compared the effects of three processing methods (referred as processes A, B, and C) on the microstructure of composite solders. In process A, the Cu6Sn5 particles with a size of 50 Am or less were added to a melt of eutectic Sn–Pb solder covered with flux and stirred for about 10 sec. Process B is the same as process A, with the exception that eutectic Sn–Pb solder paste was reflowed first, and the excess flux was removed. The same Cu6Sn5 particles were then added and stirred in. In process C, composite solder pastes were made by adding flux to premixed powders of eutectic Sn–Pb and about 2-Am Cu particles. This paste was then reflowed and cast. It was found that the volume fractions of porosity in the composite solders were process A<process B<process C. Both processes A and B resulted in less than 10 vol.% porosity for particle additions ranging to 30 vol.%, while porosity in the process C composite increased to more than 60 vol.% when 20-vol.% particles were added. In general, the volume fraction of porosity increased with increasing amounts of incorporated reinforcement particles. Compression and compressive creep tests were performed using cylindrical composite samples. The yield strengths of the composite solders were process A > process B>process C, indicating the direct

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effect of void fraction on those properties. The composite solder made by process C using a composite solder paste did not show any strengthening even with the addition of up to 30 vol.% Cu6Sn5 particles. This suggests that the large volume fraction of porosity in the composite solder prevented strengthening by reinforcements. It was also observed that the average strain rate of the composite solder made by process A can be reduced by a factor of about 8 with the addition of 30 vol.% Cu6Sn5 particles. This composite solder exhibits comparable ductility with Sn–Pb solder with no reinforcements. Marshall and Calderon [31] compared two processing methods and concluded that the processing method significantly affects the microstructure of a composite solder. The first method involved blending powdered reinforcement particles with eutectic Sn–Pb solder paste. In the second method, the particles were slowly added to molten eutectic Sn–Pb solder, while the melt was stirred under an inert atmosphere. The particles utilized in both methods included Ni, Ag, Cu, Cu3Sn, and Cu6Sn5. The blended particle method provided the composite solders with a larger fraction of porosity than the particles addition to molten solder method for all types of particles tested. Increasing the volume fraction of reinforcement particles produced larger volume fraction of porosity in the composite solder. Remelting of the composite solders, produced by the first method, under an argon-purged high-vacuum environment greatly reduced the porosity in the composite solders. 2. Porosity Formation Mechanisms Clough et al. [33] proposed that a possible mechanism for the formation of pores is through flux or gas bubbles trapped due to the viscosity of the melt. Since each particles in the solder paste is coated with flux, flux can be internally trapped when the solder paste melts. On solidification, these bubbles are left as pores within the solder when there is insufficient time for the bubbles to escape. Porosity is generally detrimental to strength and ductility. Pores can serve as sites of stress concentration resulting in the formation of cracks, leading to failure. It is apparent that the method of mixing the particles with the molten solder containing the least amount of flux results in minimal porosity and higher strength compared to the methods utilizing solder paste. Based on buoyancy, the upward velocity (v) of gas bubbles in the melt can be described by the Stokes Law v¼

gðDqÞd2 ; 18g

ð1Þ

where g = 9.8 m/sec2 is the gravitational constant, Dq = qsqb is the difference between the density of the solder and the bubble, d is the bubble diameter, and g is the viscosity of the melt. The viscosity of molten solder at 280jC is about 0.13 Pa sec. The density of the solder is 8880 kg/ m3. When d = 0.5 Am, the velocity of the gas bubble is 9.3  103 Am/sec. On the other hand, if the bubbles are 50 Am in diameter, the velocity will be 93 Am/sec. Smaller bubbles have a lower velocity. Thus smaller bubbles tend to remain and form pores on solidification. The viscosity of the melt has been observed to increase with an increasing amount of added particles [33]. The velocity of bubble motion then decreases due to increased viscosity of the melt as evidenced by the above formula, resulting in larger fraction of residual porosity. There is no quantitative analysis of this phenomenon in solder available in published literature.

C.

Effects of Solubility and Diffusivity of In Situ Formed Reinforcements

The possibility of producing composite solders containing closely spaced, fine reinforcements by rapid-solidification processing of induction melting/gas atomization (IMGA) was investigated by Sastry et al. [34]. First, rapidly solidified powders of Sn–Pb–Cu and Sn–Pb–Ni alloys were produced by induction melting and gas atomization. Powders less than 180 Am in size were coldpressed and extruded into rods at 120jC to consolidate into bulk solder specimen. The extruded rods were reflowed with a maximum temperature of 200jC to produce microstructures simulating those found in solder joints. The dispersoids in Sn–Pb–Cu and Sn–Pb–Ni composite solders had a composition close to that of Cu-rich Cu6Sn5 and Ni-rich Ni3Sn4, respectively.

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In the rapidly solidified and extruded condition, the Sn–Pb–Cu composite solders exhibited 0.1 to 1 Am spherical Cu–Sn dispersoids in Sn–Pb solder matrix with 5–10 Am grain size. Reflow of the composite solders increased grain size of Sn and Pb phases and drastically changed morphology and size of Cu–Sn dispersoids. The Cu–Sn dispersoids underwent significant coarsening and changed their morphology to platelike shape. In the rapidly solidified and extruded condition, the Sn–Pb–Ni composite solders contained Sn–Pb solder matrix with finer grains of 1–5 and 0.05–0.5 Am spherical Ni–Sn dispersoids. Upon reflow, limited coarsening of grains and dispersoids resulted. As compared to Sn–Pb–Cu composite solders, the Ni–Sn dispersoids remained spherical and were less than 1 Am in size. As proposed by Sastry et al. [34], the difference in stability of dispersoids in both these composite solders was attributed to higher solubility and diffusivity of Cu in Sn–Pb matrices than Ni. Sn–Pb–Ni composite solder thus was expected to provide relatively greater strengthening than Sn–Pb–Cu composite solder. When tested in tension at 25jC and with a strain rate of 2  103/sec, these composite solders showed 35–180% higher yield stress and 12–90% higher Young’s modulus than those of eutectic Sn–Pb solder processed by the same method. As expected, Sn–Pb–Ni composite solder exhibited 20% higher yield stress and Young’s modulus than those of Sn–Pb–Cu composite solder.

D.

Effects of Dispersoids on Microstructural Stability

One of the important considerations to increase the reliability of the solder joints used in microelectronic industry is to engineer and stabilize microstructures of solders after reflow and homogenize the deformation during service. The effect of dispersoids on microstructural stability was investigated using dispersion-strengthened composite solders made by mechanical alloying [18]. Particles of Ni3Sn4 and agglomerates less than 20 Am in size were blended with 60Sn–40Pb solder and mechanically alloyed to introduce 2.2 wt.% dispersoids in the composite solder. Solder joints were prepared by reflowing the composite solder between two Cu blocks on a hot plate at 220jC and were subjected to aging at 100jC for 29 hr. The grain size of the composite solder was finer in both the reflowed and aged conditions compared to the Sn–Pb solder without dispersoids. The finer microstructure in the as-reflowed condition was attributed to the Ni3Sn4 dispersoids that acted as heterogeneous nucleation sites in the Sn–Pb solder matrix. The Ni3Sn4 dispersoids are also considered responsible for retarding grain growth during aging by pinning the interphase boundaries between Sn and Pb phases. As a result, the composite solders with finer grain size exhibited higher strength, ductility, and a greater resistance to both mechanical and thermomechanical fatigue damage compared with conventional solder without dispersoids.

E.

Effects of Ultrafine Oxide Dispersions

In optoelectronic packaging, strong creep resistance of the solder joints is required because of the need to maintain positional accuracy of the components over a long period of time. Gradual misalignment of the components due to time-dependent creep deformation results in a reduced intensity or even a complete loss of signal transmission. Higher melting point solders such as eutectic Sn–Au (MP 278jC) are often utilized in applications requiring creepresistant solder joints to ensure dimensional stability. However, the higher processing temperatures can cause deterioration of optical fibers and optoelectronic components. Thus it is highly desirable to use low melting point solders with high creep and mechanical strength for these applications. 1. A Novel Processing Method to Achieve Ultrafine Oxide Dispersions Dispersions of nanosize Al2O3 (10-nm average) and TiO2 (5-nm average) oxide particles in Sn–Pb solders were attempted by Mavoori and Jin [26,34] to develop composite solders with superior mechanical properties. Nanosized Al2O3 and TiO2 oxides particles were chosen because they are nonreacting, noncoarsening, and relatively undeformable. Thus they can enhance mechanical properties by impeding dislocation motion and grain boundary sliding. A uniform distribution of dispersoids in a solder matrix was achieved through a novel approach of particle coating and repeated plastic deformation to rearrange the oxide particles [26,34]. First, powders of eutectic

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Sn–Pb solder (nominally 35-Am size particles) and nanosized TiO2 and Al2O3 particles were dispersed and mixed in ethanol with constant stirring to form a slurry comprised of Sn–Pb particles covered with 3 vol.% oxide particles. After drying, they were compacted into pellets at room temperature. The pellets were flattened due to repeated compressive deformation at 120jC in an inert atmosphere. Such pellets were cut into six pieces, stacked, and sealed in a heattreatment envelope filled with argon, followed by heating to 120jC and pressing to obtain a sixfold reduction of the stack. This process was repeated six times to obtain a calculated interparticle spacing of 0.8 nm. Finally, the specimens were annealed to remove the residual stress. The dispersoid-free Sn–Pb solder control specimens were prepared by the same method, resulting in a 2-Am grain size. It was determined that superior creep resistance of other solder systems such as Sn–Au, Sn– Ag, or Sn–Bi solders can also be achieved by employing this technique. Mavoori and Jin suggested that either a preform or solder paste might be used in applications of these composite solders [34]. Fast soldering operations such as laser soldering were recommended to minimize possible segregation of the dispersoids during melting or solidification. It was also suggested that the choice of proper dispersoid particles density which is matched to the solder matrix is beneficial. 2. Effects of Fine Oxide Particles on Microstructure and Mechanical Properties It is ideal to have interparticle spacing less than the size of dispersoids for maximum strengthening (<5 to 10 nm) [26,34]. The interparticle spacing in Mavoori and Jin’s study [26,34] was larger than the calculated 0.8 nm due to plastic flow of the solder matrix and low dispersoid volume. The particles exhibited some agglomeration in the preparation of the composite and during reflow. The final microstructure of the composite solders consisted of equiaxed grains with 200- to 500nm size dispersoids in the solder matrix. Fine-grained microstructures possess a large grain boundary area which increases the free energy of the system. Reducing the free energy is the driving force for grain growth. The composite solders were aged at 120jC for 48 hr to investigate the effects of fine dispersoids on microstructural stability. The composite solders containing Al2O3 and TiO2 dispersoids virtually retained their initial fine grain sizes, while the grain size in the dispersoid-free control Sn–Pb solder increased by 150% from 2 to 5 Am. This indicates that fine particles served as effective obstacles to grain growth and coarsening of the solder matrix by pinning grain boundaries and dislocations. Compressive creep tests were performed under constant load exerting a stress of 1.724 MPa conducted at 80jC and 100jC. The Sn–Pb composite solders exhibited a significant increase in creep resistance compared to dispersoid-free Sn–Pb control and eutectic Sn–Au solders. The TiO2-containing composite solders exhibited a creep resistance 2 to 3 orders of magnitude greater than the Sn–Pb solder control and increased to an average 60% of eutectic Sn–Au solder. Tensile tests were also performed to examine the effect of various strain rates at 25jC and 80jC on strength and ductility. It was observed that the composite solders were about 4 to 5 times stronger at the lowest strain rate (4  106/sec) compared to the dispersoid-free Sn–Pb solder control, while the effect of strengthening essentially disappeared at high strain rates (>2  103/sec). It was also observed that the composite solders were more ductile than the Sn–Pb solder control at higher strain rates (>103/sec). It was tentatively attributed to the pseudosuperplastic behavior present in the ultrafine composite solders (200-nm grain size). This suggests that the composite solders can be engineered to exhibit high ductility over a wide strain rate range by altering the volume fraction of dispersoids or the grain size to make solders useful for high strain rate applications.

F.

Effects of Reinforcements with Low Coefficient of Thermal Expansion

Higher coefficient of thermal expansion (CTE) of solders as compared to the other entities present in a solder joint is a root cause for producing thermal strain in the solder joints and failure of electronic parts. Thus there is a need for a solder with a low CTE. Solders can be tailored to possess a desired thermal expansion coefficient with proper choice of type and volume fraction of reinforcements.

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1. Carbon Fiber Reinforcements Incorporation of carbon fibers was attempted to develop a composite solder with a low CTE [35]. Carbon fibers were selected because they have nearly a zero CTE with good electrical and thermal conductivities. A continuous form of carbon fibers was also expected to prevent nonuniform distribution of carbon fibers after remelting and solidification. A carbon-fiber reinforced solder was fabricated by squeeze casting 60Sn–40Pb solder with continuous Cu-coated, 10-Am diameter carbon fibers. About 1-Am thick Cu layer was electroplated on carbon fibers to ensure good wetting to the solder and the fiber. Composite solders containing uncoated fibers were also prepared for comparison. The CTEs of the composite solders were measured at temperatures ranging from 25jC to 105jC along directions parallel and normal to the fibers. The CTE values parallel to the fibers were lower than those perpendicular to the fibers. The CTEs in the parallel direction drastically decreased with increasing fiber content, while the reduction of CTEs in the normal direction was not significant. The composite solders showed CTE values close to 0 in the parallel direction at fiber contents above 42 vol.%. Increasing temperature caused an increase in CTE values for a given fiber content and fiber orientation. 2. Mechanical Properties In tensile tests where the load was applied along the fiber direction, the tensile modulus and strength increased with increasing volume fraction of carbon fibers, while the ductility decreased with increasing fiber content. The composite solders containing coated fibers exhibited higher strength and ductility than those with uncoated fibers. This result was attributed to the improved wetting between the solder matrix and fiber provided by the Cu coating. On the other hand, the modulus was not affected by copper coating. Thermal fatigue testing was performed using solder joints made with alumina (Al2O3) and Kovar (Fe–29Ni–17Co) by cycling between room temperature and 100jC. Alumina and Kovar were coated with Au (20 to 40 A˚ thick) to promote wetting prior to soldering. The start of debonding identified by slight cracking between the composite solder and alumina or between the composite solder and Kovar was used for estimation of the fatigue life. Among the fiber volume fractions tested, the composite solder joints containing about a 29.42 fiber vol.% exhibited the longest fatigue life. This result was attributed to the fact that the composite solder containing 29.42 fiber vol.% exhibits a CTE (4.3  106/jC at 25jC) closest to alumina (6.7  106/jC at 25jC) and Kovar (5.3  106/jC at 25jC) parallel to fiber orientation.

G.

Growth of Interfacial Intermetallic Compound Layers

1. Typical Growth Without Reinforcements Two layers consisting intermetallic compound (IC) of Cu3Sn and Cu6Sn5 form at the interface of Sn-based solders and Cu circuit pads. The Cu-rich Cu3Sn layer forms next to the Cu pads and Snrich Cu6Sn5 layer forms adjacent to the solder. Immediately after soldering, only the Cu6Sn5 layer is observed and aging is required to form the Cu3Sn layer. During aging, the interfacial IC layers grow by consuming Sn from the solder and Cu from the bond pad [32]. 2. Effect of Reinforcement Type on Solder Interfacial IC Growth Wu et al. [32] and Pinizzotto et al. [36] compared the growth of interfacial IC layers in eutectic Sn– Pb composite solder joints with similar joints without reinforcements at several aging temperatures ranging to 160jC for periods up to 64 days (1536 hr). The composite solders were prepared by mixing the eutectic Sn–Pb solder paste with particles of Ni, Ag, Au, Cu, Cu3Sn, and Cu6Sn5. The solder joints were produced via reflow on a hot plate preheated to 250jC for 1 to 1.5 min. Although none of the particle additions strongly affected the initial thickness of the Cu6Sn5 layer, they had significant influence on the growth kinetics of the interfacial IC layer. Compared to the standard eutectic Sn–Pb solder (i.e., no reinforcements), additions of Ag and Au particles retarded, and Ni particles completely suppressed the growth of the Cu3Sn layer, while enhancing the growth of the Cu6Sn5 layer. In contrast, Cu-based particles (Cu, Cu3Sn, and Cu6Sn5) impeded Cu6Sn5 layer growth but promoted Cu3Sn layer growth.

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3. Interfacial IC Growth Kinetics Since the Cu substrate/Sn-based solder interface is planar in nature, the growth of Cu–Sn interfacial IC layers in the solid state occurs in a planar manner. Tin and copper diffuse in opposite directions, and then react with each other. The growth kinetics of the interfacial IC layers is modeled in many studies by assuming a diffusion-controlled growth [32]: pffiffiffiffiffiffi x ¼ Dt; ð2Þ where x is the interfacial IC layer thickness at time t and D is the IC layer growth rate constant expressed by D ¼ Do expðQ=RTÞ;

ð3Þ

where Do is the preexponential constant corresponding to absolutely maximum possible layer growth rate (in the absence of an activated process, where Q = 0), Q is the activation energy for the growth of IC layers, R is the universal gas constant, and T is the temperature in degrees Kelvin. It has been determined that the diffusion-controlled growth model is valid for interfacial IC layer growth over the temperature range studied (RT to 160jC) [32,36]. Table 4 lists the activation energy values for intermetallic layer growth in Sn–Pb eutectic and composite solders. The incorporation of Cu-based particles (Cu, Cu3Sn, and Cu6Sn5) increased the activation energy for Cu6Sn5 layer growth while decreasing the activation energy for Cu3Sn layer growth in eutectic Sn–Pb solder. This is consistent with findings of a retarded growth of Cu6Sn5 layer and enhanced Cu3Sn layer growth as a consequence of adding Cu-based reinforcements. On the other hand, Ag increased while Au decreased the activation energy for Cu6Sn5 layer growth compared to eutectic Sn–Pb solder. Tin–lead eutectic composite solders containing Ag and Au particles produced lower activation energies for Cu3Sn layer growth than without those additions, thus exhibit a more rapid Cu3Sn growth rate. Nickel particles drastically increased the Cu6Sn5 thickness with higher activation energy for the growth of Cu6Sn5 layer than that in the eutectic Sn–Pb solder. However, the measured activation energy values for intermetallic layer growth in composite solder containing Ni and Ag reinforcements appear to be in contradiction with the observed enhanced growth of Cu6Sn5 and Cu3Sn layer. 4. Proposed Growth Mechanisms Based on the findings of activation energies for growth, and growth behavior of the interfacial IC layers, possible mechanisms by which reinforcement particles affect the growth of intermetallic layers have been proposed [32]. The proposed mechanisms are based on an overall reduction of tin diffusion through the solder matrix required for growth of intermetallic layers at the solder/Cu interface. First, Cu and Cu3Sn particles act as Sn sinks. During aging, both Cu and Cu3Sn TABLE 4 Activation Energies for the Growth of Cu6Sn5 and Cu3Sn Intermetallic Compounds in Eutectic Sn–Pb and Composite Solders Activation energy (kJ/mol)

Eutectic Sn–Pb and additions

Cu6Sn5 IC layer

Cu3Sn IC layer

Eutectic Sn–Pb 20 wt.% Cu6Sn5 20 wt.% Cu6Sn5 7.6 wt.% Cu 4 wt.% Au 4 wt.% Ag 4 wt.% Ni

77 118 117 96 54 131 209

163 92 114 108 107 114 –

Source: Refs. 32 and 36.

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particles continue to react with Sn until the preferred terminal phase Cu6Sn5 is formed. They remove Sn from the solder and reduce the amount of Sn available to form intermetallic layers. Removing Sn enriches Cu, so the kinetic balance is directed toward favoring the formation of Cu3Sn over Cu6Sn5. However, the mechanism of a Sn sink is not applicable to Cu6Sn5 particles because Cu6Sn5 does not directly react with Sn. It was postulated that Cu6Sn5 particles impede Sn diffusion by serving as a partial barrier, limiting the Sn supply to the solder/Cu substrate interface. This promotes the growth of the Cu-rich Cu3Sn phase and deters the growth of Sn-rich Cu6Sn5. However, the role of reinforcement particles present in a solder matrix on the growth of Cu6Sn5 and Cu3Sn intermetallic layers at the solder/Cu interface is uncertain at present.

H.

Further Investigations and Implementation

Other studies which address the effects of reinforcement particle additions on the microstructure, solderability, and mechanical properties of composite solders have also been reported [16,17,25,27,31,37]. In general, composite solders tend to render improved material properties. All the reported investigations were basically exploratory in nature, and further detailed studies are necessary. However, the driving factors for continued development and ultimately widespread implementation of composite solders are heavily dependent on environmental and economic considerations. Most investigations on composite solders have been focused on leadbearing systems, and studies on lead-free composite solders commenced much later.

VI.

STUDIES ON LEAD-FREE COMPOSITE SOLDERS

Several lead-free solders such as binary Sn–Ag, Sn–Bi, ternary Sb–Ag–Cu, etc. are being considered as potential alternatives for lead-bearing solders. Composite approaches used in lead-bearing solders can also be applied to lead-free solders. Although studies on composite solders utilizing lead-free solders are somewhat limited compared to those for lead-bearing solders, developments of lead-free composite solders are essential based on the reliability considerations.

A.

Bulk Solder Without Reinforcements

Eutectic Sn–Ag solder possesses a microstructure consisting of almost pure Sn phase and the eutectic, a mixture of Sn and Ag3Sn intermetallic phases. The eutectic Sn–Ag solder was found to exhibit microstructural instability during isothermal aging as observed with lead-bearing solders. Fig. 1 shows the microstructures of eutectic Sn–Ag solder joint made with Cu substrate before and after aging at 150jC for 100 hr [5]. During aging, Ag3Sn particles in the solder matrix coarsen, resulting in an increased particle size and a reduction in particle count. After aging, Ag3Sn particles are primarily found along Sn–Sn grain boundaries. It is considered that coarsening of Ag3Sn particles took place along grain boundaries providing a fast diffusion path. Cu6Sn5 intermetallic phase in the solder joint is also shown in Fig. 1, resulting from the dissolution of Cu from substrate and reaction with Sn in solder. The aging temperature has been found to dramatically affect the coarsening of Ag3Sn particles. The instability of the Ag3Sn phase provides the impetus for identifying suitable particle-strengthening strategies.

B.

Solders with Reinforcements Introduced by In Situ Method

Figs. 2 and 3 provide microstructures of eutectic Sn–Ag solder joints containing Cu6Sn5, Ni3Sn4, and Fe–Sn reinforcement particles in the as-joined condition and after long-term aging [38,39]. Intermetallic particles of Ni3Sn4 and Fe–Sn incorporated into the solder matrix by the in situ method were found to be relatively stable even after 1400 hr of aging at 180jC as compared to Cu6Sn5 intermetallic particles, an attribute desirable for a composite approach. The Cu6Sn5 intermetallic reinforcements were observed to coarsen under long-term conditions, 5000 hr at

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FIG. 1 Light photo micrographs depicting microstructural coarsening of the eutectic Sn–Ag solders, where the dark phase is Sn and white particles are Ag3Sn. (a) Before aging, (b) after aging at 150jC for 100 hr. (From Ref. 4.)

FIG. 2 Scanning electron microscope secondary electron emission photographs of the initial microstructure of Sn–Ag eutectic solder joints containing 20 vol.% (a) Cu6Sn5, (b)FeSn2, and (c)Ni3Sn4 reinforcement particles. (From Refs. 38 and 39.)

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FIG. 3 Scanning electron microscope secondary electron emission photographs of the microstructure of in situ composite solder joints after aging (a) Cu6Sn5 composite solder at 150jC for 5000 hr, (b) FeSn2 composite solder at 180jC for 1400 hr, and (c) Ni3Sn4 composite solder at 180jC for 1400 hr. (From Refs. 38 and 39.)

FIG. 4 Scanning electron microscope secondary electron images depicting microstructural stability of Cu6Sn5 particles during short-term aging of a composite Sn–Ag solder (a) before aging, (b) after aging at 150jC for 100 hr. (From Ref. 4.)

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temperatures above 120jC. They did not coarsen during short-term aging consisting of a few hundred hours as shown in Fig. 4. This may indicate different mechanism of coarsening at low and high temperatures. 1. Interfacial Intermetallic Compound Layers The effect of the Cu6Sn5, Ni3Sn4, and FeSn2 particles on the solder/substrate interfacial IC layer growth in the eutectic Sn–Ag composite solder joints was investigated by carrying out aging studies up to a few thousand hours at several temperatures [38,39]. Fig. 5(a) shows an example of the total growth of interfacial IC layers (Cu6Sn5 and Cu3Sn) during aging at 150jC up to 5000 hr. The solder joints exhibited only Cu6Sn5 intermetallic layer at initial condition, and the Cu6Sn5 and Cu3Sn intermetallic layers are clearly visible after aging as shown in Fig. 3. The interfacial IC layers normally exhibited rapid growth at the initial stage of aging, but the growth rate decreased as the layer thickness increased. The reinforcement particles were observed to affect the formation of interfacial intermetallic layers during reflow, resulting in a thinner initial interfacial layer compared with the same solder without reinforcements. Fig. 5(b) shows IC layer growth rate constant of the Cu6Sn5 intermetallic layer in the eutectic Sn–Ag and Cu6Sn particulate-reinforced Sn–Ag solder joints during aging at 150jC up to 5000 hr at five temperatures. The Cu6Sn5 particulate-reinforced Sn–Ag solder joints showed higher IC layer growth rate constants at temperatures above 120jC, while the IC layer growth rate constants were lower below 120jC. A similar behavior was observed for Ni3Sn4 and FeSn2 particulate-reinforced Sn–Ag solder joints. This was a temperature-dependent effect in that the reinforcement particles were effective in retarding the interfacial intermetallic layer growth at temperature below 120jC, but not above this temperature, as shown in Fig. 5(b).

C.

Solders with Mechanically Added Reinforcements

The stability of mechanically incorporated metallic reinforcement particles in the Sn–Ag composite solders was examined after multiple reflows and during aging. The composite solders were prepared by mechanically blending nominally 15 vol.% of micron-sized Cu, Ag, or Ni

FIG. 5 (a) Graph which depicts the effect of aging time at 150jC on the total thickness of interfacial IC layers (Cu6Sn5 + Cu3Sn) for eutectic Sn–Ag and composite Sn–Ag solder joints which contain Cu6Sn5 intermetallic particles introduced by the in situ method. (b) Graph which depicts the IC layer growth rate constants at five different temperatures for the Cu6Sn5 intermetallic layer in the eutectic Sn–Ag solder joints and Cu6Sn5 particulate-reinforced Sn–Ag solder joints during aging at 150jC up to 5000 hr. (From Ref. 38.)

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particles with eutectic Sn–Ag solder to form a mixture with a uniform distribution of reinforcement particles [40,41]. Figs. 6–8 illustrate the microstructure of Cu, Ag, and Ni particles in asfabricated solder joints, after aging at 150jC for 500 hr, and after three reflows at 280jC, respectively. 1. As-Fabricated Microstructure The intermetallic layers formed around the particles due to the reaction of Sn in the solder matrix with the metallic particle during the first reflow can be seen in Fig. 6. The Cu–Sn intermetallic layers consist of Cu3Sn and Cu6Sn5 layers formed on the surface of Cu particles, and Ag3Sn intermetallic layer formed on the surface of Ag particles. In contrast, a ternary Cu–Ni–Sn intermetallic layer formed around the Ni particles. This indicates that Cu diffused to Ni particles from the Cu substrate during reflow and participated in the formation of the ternary Cu–Ni–Sn intermetallic layer. It was also observed that binary Cu–Sn and ternary Cu–Ni–Sn intermetallic

FIG. 6 Scanning electron microscope secondary electron images depicting the as-fabricated microstructure of Sn–Ag eutectic solder joints with (a) Cu reinforcement, (b) Ag reinforcement, and (c) Ni reinforcement, which were mechanically added. In (a), a padal-like layer of CuSn IC with a light appearance surrounding the dark-appearing Cu particles was added. In (b), the added Ag particles have a gray appearance and surrounded by a thin layer if light-appearing Ag3Sn. The Ni particles added in (c) appear dark and surrounded by light-appearing Cu–Ni–Sn intermetallics. (From Refs. 40 and 41.)

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FIG. 7 Scanning electron microscope secondary electron images depicting the thermally aged microstructure of Sn–Ag eutectic solder joints with (a) Cu reinforcement, (b) Ag reinforcement, and (c) Ni reinforcement, which were mechanically added. The solder joints were aged at 150jC for 500 hr. (From Refs. 40 and 41.)

layers were much thicker on Cu or Ni particles than the Ag3Sn intermetallic layer on Ag particles. Similar differences in the thickness of the intermetallic layers around particles were also noted after isothermal aging at 150jC or reflow at 280jC as shown in Figs. 7 and 8. 2. Aged Microstructure As noted in Fig. 7, the growth of intermetallic layers around particles during aging was in the following order: Ni particles>Cu particles>Ag particles. In the study conducted by Guo et al. [40], Cu particles were largely consumed by Sn in the solder matrix after aging at 150jC for 1000 hr, while there were no significant changes in the case of Ag particles under similar conditions. In contrast, Ni particles were completely converted to ternary Cu–Ni–Sn intermetallics after aging for 500 hr [41]. These studies attribute the difference in growth kinetics of intermetallic layers that surrounded Ni, Cu, and Ag particles to their difference in diffusion coefficients. Table 5 gives a comparison of diffusion distances of metallic elements in solid Sn under the same condition. As indicated in the table, the diffusion of Ni, Cu, and Ag in Sn is in the following order: Ni>Cu>Ag under the same conditions. This large difference in diffusion capabilities may account for the observed difference in intermetallic layer growth around reinforcement particles. Since Ag particles consume less amount of Sn, it is expected that change in composition of solder matrix would be least in Ag-reinforced composite solders compared to composite solders containing Cu

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FIG. 8 Scanning electron microscope secondary electron images depicting the multiple reflowed microstructure of Sn–Ag eutectic solder joints with (a) Cu reinforcement, (b) Ag reinforcement, and (c) Ni reinforcement, which were mechanically added. The solder joints were reflowed three times at 280jC with a total time of about 30 sec above the melting point of eutectic Sn–Ag (MP = 221jC). (From Refs. 40 and 41.)

TABLE 5

Comparison of Diffusion Distance for Ag, Cu, and Ni in Sn

Diffusion species and direction Ag in Sn ?c Ag in Sn //c Cu in Sn ?c Cu in Sn //c Ni in Sn ?c Ni in Sn //c a

D0 (cm2/sec)

Q (kJ/mol)

0.18 77 0.0071 55.1 0.0024 33.1 D =2  106 cm2/seca 0.0187 54.2 0.0192 18.1

(Dt)1/2, 100 hr (Am) 25jC

75jC

100jC

150jC

0.454 7.5 369 8,485 — 21,549

4.2 37 964 — — 36,421

— — 1,414 — — 44,915

45 200 2,660 — 369 —

D0 and Q values are not available in the references individually. ?c indicates diffusion in the direction perpendicular to the c-axis of a tin unit cell. //c indicates diffusion in the direction parallel to the c-axis of a tin unit cell. Source: Refs. 4 and 40.

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and Ni reinforcements. Ag particles also exhibit higher resistance to coarsening than Cu and Ni reinforcement particles due to its lower diffusion coefficient in a Sn matrix. 3. Effect of Multiple Reflows on Microstructure On the other hand, Fig. 8 illustrates that the growth of intermetallic layers around particles during multiple reflow was: Cu particles>Ni particles>Ag particles. The intermetallic layers present around metal reinforcement particles appear to control the growth of the intermetallic layers under such conditions.

D.

Effects of Reinforcements on Mechanical Properties/Other Characteristics

1. Examples of Enhanced Creep Resistance a. Fe in Sn and Sn–Bi. Composite solder containing Fe particles introduced in the presence of a magnetic field to achieve uniform distribution of reinforcements in solder was found to possess greater creep resistance than unreinforced solders, as reported by McCormack et al. [29]. The Fe powders were chemically coated with a submicron layer of Sn before processing to improve wetting and prevent oxidation during handling and processing. Powders of Sn and the Sn–Bi alloy were mixed with 2.5 wt.% of approximately 2-Am size Fe powder in hexane to form a slurry. After drying the slurry, the dry powder mixture was melted and solidified in a quartz tube under a magnetic field. The choice of Fe powders as reinforcements is based on the facts that Fe has a low solubility and diffusivity in Sn-based solders, avoiding their coarsening. The Sn composite solders containing 2.5 wt.% Fe powder exhibited a 60% to 100% higher ultimate tensile strength than the dispersion-free control solder. More importantly, the creep resistance of the composite solder at 100jC was 20 times greater than the dispersoid-free solder. In the case of the eutectic Sn–Bi composite solder with Fe particles, the tensile strength was a modest 10% higher, and the creep resistance was 5 times better than the dispersoid-free specimens under similar conditions. b. Cu6Sn5 in Sn–Ag. Results of creep testing performed on Sn–Ag composite solder joints containing 20 vol.% Cu6Sn5 particles introduced by the in situ method in as-fabricated and preaged conditions for 100 hr at various temperatures have been reported by Subramanian et al. [42] and Choi et al. [43]. Creep testing was carried out in an interrupted manner where the specimens were periodically unloaded to obtain micrographs of a particular scratch placed on the side of a solder joint to quantify the shear displacement in the specimen. Fig. 9 illustrates creep behavior of the composite solder joints in as-fabricated and pre-aged condition along with data of eutectic Sn–Ag solder joints from Darveaux et al. and McDougall et al. The creep resistance of the composite solder joints was about 2 to 3 orders of magnitude better compared to noncomposite solder in the as-fabricated condition at room temperature. Furthermore, the composite solders were capable of sustaining a creep strain similar to lead–tin solders, but lower than the dispersoidfree control solders can be sustained [42]. It was also determined that pre-aging of the composite solder reduced its creep strength. Depending on the aging temperature and time, its behavior can approach that of the noncomposite solder. In dispersoid-hardened composite alloys, the particle size and interparticle spacing are important parameters affecting the mechanical properties of composite solders. In the pre-aged condition, the intermetallic particles maintained their initial morphology, while Ag3Sn particles coarsened, resulting in a larger particle size and interparticle spacing. A coarsened microstructure apparently allows dislocations to bypass particles more easily, thus rendering them as ineffective dislocation barriers, resulting in reduced creep resistance. c. Cu6Sn5, Ni3Sn4, and FeSn2 in Sn–Ag. Choi et al. [44] have performed constant-load creep tests on composite solder joints containing 20 vol.% in situ Cu6Sn5, Ni3Sn4, and FeSn2 reinforcement particles. Fig. 10 compares the normalized creep behavior of eutectic Sn–Ag and each composite solder joint, earlier creep data obtained from cyclic creep testing, and the data of Darveaux and Banerji [44]. For the eutectic Sn–Ag solder joints, the activation energy for creep

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FIG. 9 Log plot of normalized shear strain rate vs. normalized shear stress data showing creep behavior of Sn–Ag composite solder joints in the as-fabricated, pre-aged condition, along with data from Darveaux et al., and McDougall et al. (From Refs. 42 and 43.)

was determined to be 60 kJ/mol, which is about half the activation energy for tin self-diffusion. This suggests that grain boundary or dislocation pipe diffusion may control deformation of Sn– Ag eutectic solder joints. A value of 120 kJ/mol was found in composite solder joints containing Cu6Sn5 or Ni3Sn4 reinforcements, but only 38.5 kJ/mol for composite solder joints with FeSn2 reinforcements. Fig. 10 shows that Sn–Ag composite solder joints typically exhibit the same properties as Sn–Ag eutectic solder joints at elevated temperatures, but not at lower temperatures. Cu6Sn5 and Ni3Sn4 composite solder joints appear to be weaker at room temperature, while FeSn2 composite solder joints are effectively stronger than the eutectic solder joints at equivalent normalized stresses. 2. Creep Resistance Factors Data from Choi et al.’s [43] prior work on the same in situ Cu6Sn5 composite solder joints showed substantially better creep resistance even under pre-aged conditions as shown in Fig. 10(a). These data points were obtained by unloading the specimen to obtain micrographs of a particular scratch placed on a solder joint to quantify the shear displacement. Recovery that can occur during these periods of interrupted unloading apparently reduces the mobile dislocation density available for deformation on reloading the sample. Quantitative evaluation of global and localized creep deformation behavior of solder joints has been developed by mapping the distortion of a straight-line ablation pattern imprinted by an excimer laser on the surface of shear lap solder joints [45]. Fig. 11 is an illustration of distortion due to creep deformation of a laser ablation pattern placed on a solder joint [46]. a. Effect of Temperature. Damage based on the onset of tertiary creep was examined in shear creep deformation of eutectic Sn–Ag single shear lap solder joints with and without Cu6Sn5 reinforcements. These studies indicated that at elevated temperature, local tertiary creep commenced at similar strains for both composite and noncomposite specimens, and their fracture surfaces were also similar. However, composite solder joints generally strain more homogeneously than noncomposite counterparts at elevated temperatures. At room temperature, where the creep resistance of the composite solders was found to be much greater than noncomposite parts, the composite solders generally exhibited a more uniform strain distribution. However,

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FIG. 10 Graphical depiction of normalized steady-state creep behavior of eutectic Sn–Ag and composite solder joints. Data are presented for (a) the eutectic and Cu6Sn5 composite solders, (b) the eutectic and Ni3Sn4 composite solders, and (c) the eutectic and FeSn2 composite solder joint. Note that (a) contains data from Choi for solder joints in the pre-aged condition. (From Ref. 44.)

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FIG. 11 Shape of laser ablation patterns placed on a solder joint. (a) Before creep and (b) after creep. (From Ref. 46.)

local tertiary creep was observed to commence at a lower creep strain compared to noncomposite solder joints. It is considered that since the composite material is more creep-resistant at lower temperatures, the strain hysteresis may be smaller during thermal cycling, preventing development of large strains. However, it is not clear whether this benefit outweighs the fact that less strain before the onset of tertiary creep is tolerated locally in a composite solder at higher strain rates and lower temperatures. b. Strain for Tertiary Creep. Eutectic Sn–Ag solder joints exhibited a significantly higher strain for the onset of tertiary creep than all the composite solder joints tested. Among the composite solder joints, the tertiary creep of in situ Cu6Sn5 composite solder joints occurred at strain value comparable to eutectic Sn–Ag solder joints. The relatively higher strain value required for the onset of tertiary creep observed in in situ, Cu6Sn5-reinforced composite solder joints is attributed to the weak interfacial bonding between Cu6Sn5 particles and the solder matrix identified from nanoindentation tests [51]. It was concluded that the weak interface of uniformly distributed Cu6Sn5 particles in the solder matrix provided multiple initiation sites for deformation throughout an entire solder joint. This condition promoted homogeneous deformation, leading to the observed higher ductility of in situ compared to composite solder joints prepared by mechanical mixing. c. Particle Reinforcement Type. A summary of the creep properties of composite solders reinforced with Cu, Ag, and Ni metallic elements introduced by mechanical mixing methods is given in Table 6, as are eutectic Sn–Ag and in situ Cu6Sn5 composite solders for comparison [46– 48]. Solder joints made with Cu-particle reinforced composite solders are noted to exhibit a lower steady-state creep rate at all testing temperatures compared with Ag particle-reinforced composite solder joints. The creep behavior of the Ag particle-reinforced composite solder joints is comparable to eutectic Sn–3.5Ag solder. Ni particle-reinforced composite solder joints are about 5 times more creep-resistant than composite solder joints reinforced with Cu and about 30 times better than eutectic Sn–Ag and composite solder reinforced with Ag. Even at 105jC, the Nireinforced composite solder joints were stronger than Cu- and Ag-reinforced composite solder joints.

E.

Stress Relaxation

Stress relaxation describes the time-dependent deformation behavior of a material under constant displacement, which is evaluated by measuring the stress reduction with time. The evaluation of the stress relaxation process is necessary in order to assess the effect of hold periods in the thermomechanical fatigue behavior of solder joints. Understanding of the stress relaxation mechanisms is important because the deformation processes involved in stress relaxation are fairly representative of the stress history experienced by solder joints at the temperature extremes of thermal cycling experienced in service. Thus the stress relaxation behavior may be a better

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TABLE 6 Summary of Creep Properties of Eutectic Sn–Ag and Composite Solders Prepared by In Situ and Mechanical Mixing Methods Steady-state creep rate (S1) (at 17 MPa nominal stress) Solders Sn–3.5Ag solder paste Cu particle-reinforced composite solder (15 vol.% Cu) Ag particle-reinforced composite solder (15 vol.% Ag) Ni particle-reinforced composite solder (15 vol.% Ag) In situ Cu6Sn5reinforced composite solder (20 vol.% Cu6Sn5)

105jC

Average strain for the onset of tertiary creep (at 17 MPa)

2.62  10

4

1.50  10

1.90  103

0.54

—

4.07  106

2.03  105

3.95  104

0.08

M

1.91  105

8.37  105

1.80  103

0.19

M

8.02  107

1.08  105

1.54  104

0.11

M

7.6  106 (25 MPa)

85jC

125jC

0.45

I

9.8  105 (13 MPa)

5.8  104 (12 MPa)

25jC

65jC 5

Methods M-Mech. I—in situ

Source: Refs. 46–48.

metric rather than steady-state creep behavior in developing constitutive relationships required for modeling thermomechanical fatigue behavior of solder joints. 1. Effect of Plastic Deformation History Stress-relaxation tests were performed using single shear lap solder joints made with eutectic Sn– Ag and Sn–Ag composite solder containing 20 vol.% Cu6Sn5 particles introduced by in situ methods at 25jC and 150jC, as reported by Jadhav et al. [49]. The solder joint thickness was about 100 Am and soldered area was approximately 1 mm2. The Cu half dog bones, about 0.5 Am thick, are with reduced gauge sections of 1 mm in width and 11.5 mm in length. Stress-relaxation tests were conducted after imposing three different displacement levels, which correspondingly created three stress states: before peak stress, at peak stress, and beyond the peak stress. It was determined that the percent stress reduction (%SR) increased with increasing plastic strain imposed prior to relaxation as illustrated in Fig. 12. The %SR also increased with temperature for all the pretest stress levels examined. These trends were observed for both eutectic Sn–Ag and composite Sn–Ag solder joints. It was also observed that the %SR depends on the accumulated plastic strain, indicating a dependence on the prior deformation history. Substrates to which solder joints are attached can significantly affect the stress-relaxation behavior of those solder joints. For example, it was determined that solder joints of the configuration and dimensions described earlier in this paragraph did not exhibit complete stress relief even after several hours, while the stress in bulk solder relaxes to zero with time depending on the temperature [52]. This indicates that constraints imposed by a substrate and specimen geometry play important roles during the stress-relaxation process of solder joints. 2. Creep Mechanism The creep parameters required to assess deformation mechanisms were evaluated utilizing stressrelaxation data [49]. The stress exponent values obtained during relaxation were large, i.e., above 7, which are commonly observed for dispersion-strengthened alloys. The experimental results from the stress-relaxation tests were interpreted on the basis of dislocation recovery processes. During relaxation at high temperatures (>0.5 Tm), dislocations are annihilated by thermally activated rearrangement processes, resulting in a reduced mobile dislocation density during the

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FIG. 12 Stress relaxation curves of three different composite solder joints ranging in thickness from 110 to 115 Am and tested at 25jC. Arrows indicate the beginning of stress relaxation. Cross-head speed is 0.5 mm/min. Initial plastic shear strains of 0.08 (A), 0.48 (B), and 1.5 (C) correspondingly resulted in 10%, 53%, and 77% stress reduction after 1 hr. (From Ref. 49.)

stress relaxation process. Therefore the dislocation density prior to relaxation determines the plastic strain level achievable prior to dislocation annihilation. Accordingly, when a high initial mobile dislocation density is created (e.g., by inducing plastic strain), it correspondingly leads to more plastic strain during relaxation, as these dislocations are annihilated, resulting in a greater %SR.

F.

Solderability

The wettability of in situ composite solders was observed not to degrade up to 20% volume fraction of Cu6Sn5, and the limit was investigated [42]. The contact angle is a measure of the wettability of solder to the substrate. Several micrographs of the spherical caps of the solders produced by reflow of disk-shaped solder preforms on Cu substrate were analyzed with PhotoShopR to arrive at average contact angles [42]. The measured contact angles for composite and noncomposite solders on a Cu substrate are given in Table 7. The eutectic and composite eutectic solders exhibited similar contact angles, indicating no substantial difference in wettability.

TABLE 7 Contact Angle Measurements of Eutectic Sn–Ag Solder and 20 vol.% In Situ Cu6Sn5 Reinforced Sn–Ag Composite Solders on a Copper Substrate Contact angle measurement

Specimen

Actual contact angle

Composite Sn/Ag solder 5-disk stack; disk radius = 1500 Am; disk thickness = 165 Am Eutectic Sn/Ag solder 5-disk stack; disk radius = 1500 Am; disk thickness = 190 Am

#1

16.62, 17.15

#2 Average #1

21.17, 19.55 18.63 18.52, 19.35

#2 Average

19.67, 19.62 19.29

Source: Ref. 42.

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The comparison of average wetting angles of mechanically incorporated Cu, Ni, and Ag particle-reinforced composite solders measured after four reflows is given in Table 8 along with the data for the eutectic Sn–Ag solder and in situ Cu6Sn5-reinforced composite solders [41,47]. The wettability of composite solder with 6 vol.% mechanically incorporated Cu particles is comparable to 15 vol.% Ag particle-reinforced composite solder produced by a similar method and 20 vol.% of in situ Cu6Sn5-reinforced composite solder. This was attributed to the fact that the volume fraction of Cu–Sn intermetallics was nearly doubled after four reflows, resulting in an increased solder viscosity and wetting angle during reflow. On the other hand, the wetting angle of the 15 vol.% Ni-reinforced composite solder (12.5j) was comparable to eutectic Sn–Ag solder (10.5j), but significantly better than 15 vol.% Ag (21.1j) and 15 vol.% Cu (47.3j) reinforced solders. The total effective volume fraction of Ni reinforcements was observed to remain nearly unchanged during multiple reflow processes.

G.

Fracture Characteristics

The fracture behavior of in situ composite and noncomposite solder joints was studied in isothermal mechanical fatigue (i.e., a bending mode) using a mechanical vibration exciter [50]. The fracture surface of the composite solder joints exhibited cleavage of the Cu6Sn5 particulate reinforcements and ductile mode I fracture of the eutectic matrix where fracture initiated at several locations, suggesting homogeneous ductile fracture. Mode I failure refers to fracture in a plane perpendicular to the loading direction. Meanwhile, the fracture surface of noncomposite counterparts exhibited ductile and mixed fracture behavior of modes I and II and step-type fatigue striations that originated at a local region. Mode II failure is due to shear directed parallel to the fracture surface.

H.

Weakly Bonded Reinforcements

Nanoindentation experiments have shown that the interfacial strength between particles and solder matrix can be estimated from indenting particles at an off-center location so that they

TABLE 8 Average Wetting Angles of Metallic Particle-Reinforced Composite Solders, Along with Eutectic Sn–Ag Solder, and Sn–Ag In Situ Composite Solders after Four Reflows on a Copper Substrate

Solders Eutectic Sn–Ag solder paste Cu particle-reinforced composite solder Ag particle-reinforced composite solder (15 vol.% Ag) Ni particle-reinforced composite solder (15 vol.% Ag) In situ Cu6Sn5 reinforced composite solder (20 vol.% Cu6Sn5) Source: Refs. 41 and 47.

Methods M-Mech. I—in situ N/A M

Average wetting angle after four reflows on Cu substrate 10.5j

M

47.3j (15 vol.%) 18.0j (6 vol.%) 21.1j

M

12.5j

I

17.6j

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FIG. 13 Scanning electron microscope secondary electron images depicting the rotation of Cu6Sn5 particle in a eutectic Sn–Ag matrix resulting from an off-axis load applied by a nanoindenter. (From Ref. 51.)

rotate about their initial position [51], as illustrated in Fig. 13 for a Cu6Sn5 reinforcement particle. Weak interfaces between Cu6Sn5 reinforcement particles and eutectic Sn–3.5Ag matrix determined by the nanoindentation method are responsible for increasing the ductility of the eutectic Sn–Ag solder, but without increasing the alloy strength significantly. A similar result was also reported for 60Sn–40Pb composite solders containing Cu9NiSn3 intermetallic particles [27], which exhibited a 40% increase in shear strain to cause failure, while the ultimate shear strength essentially remained unchanged. It is suggested that Cu6Sn5 particles in the Sn–Ag composite solder promote homogeneous deformation by creating strain heterogeneities in the interfacial regions due to weak interfacial strength, resulting in an increased ductility for the in situ Sn–Ag composite solder [42].

VII.

SUMMARY

Use of dispersoids is a viable means to improve the properties and service-temperature capabilities of solders to be utilized in microelectronic packages and assemblies. Composite approaches can provide improvements without significantly affecting current solder-joint fabrication practices. Dispersoids must be compatible with the solder matrix material and remain relatively stable when solder joints are in service. Reinforcement materials can be introduced to the solder by either in situ methods or by converting mechanically mixed elemental metallic particles into stable intermetallic compounds. The conversion into intermetallics occurs due to a chemical reaction when the solder is molten during solder joint formation or during reflow. The presence of dispersoids aids in stabilizing solder-joint microstructures by retarding the aging process. All dispersoids tend to improve solder creep strength by several orders of magnitude. An ideal dispersoid should enhance solder ductility without significantly strengthening it. Since the microstructure of solder joints typically is highly inhomogeneous, the deformation within a joint tends to be highly localized. Dispersoids create weakly bonded heterogeneities which promote the initiation of deformation at many locations within a solder joint, resulting in deformation which is much more homogeneous than similar nonreinforced solders. In other words, reinforcements aid in improving the overall ductility of a solder joint. These features render solder joints more compliant by increasing their capacity to accommodate stresses by relaxation while delaying the onset of tertiary creep. These factors can result in a significant enhancement of thermomechanical fatigue resistance of solder joints. Reinforcements introduced by in situ methods appear to offer the best opportunity for achieving this goal.

VIII.

FUTURE

When the current rush to identify suitable lead-free solder candidates to replace Pb-based solders succeeds, attention will be focused on improving their service-temperature capabilities. It is

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anticipated that activities related to lead-free composite solders will increase dramatically since they show promise in providing properties difficult to achieve but necessary for high-temperature applications. Although there is sufficient evidence at present to indicate the potential of these composite solders, they have not received sufficient attention. The technology to produce leadfree solders with dispersoids either by in situ or mechanical means already exists. However, implementation of these solders to produce joints in large-scale manufacturing settings still needs to be developed. These solders conveniently lend themselves to be utilized in either a preform or paste format. Atomization techniques must be developed to convert in situ composite solders into a powder form to make paste. However, in the mechanical mixing method, the particles can be added to the metal powder during paste-making process. These hurdles are manageable, and lead-free composite solders are likely to gain initial importance for solder joints that experience harsh environments, and then perhaps move globally to provide enhanced fatigue resistance in high-reliability applications.

ACKNOWLEDGMENTS Several of the results presented in the section on lead-free composite solders are based on the research activity at Michigan State University by the authors and their colleagues. The authors wish to thank Composite Materials and Structures Center at Michigan State University for financially supporting this effort.

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Proceedings of the Technical Program, National Electronic Packaging and Production Conference; NEPCON WEST: Des Plains, IL, 1992; Vol. 3, 1276–1277. 28. Marshall, J.L.; Sees, J.; Calderon, J. Microcharacterization of composite solders. Proceedings of the Technical Program, National Electronic Packaging and Production Conference; NEPCON WEST: Des Plains, IL, 1992; Vol.3, 1278–1283. 29. McCormack, M.; Jin, S.; Kammlott, G.W. Enhanced solder alloy performance by magnetic dispersions. IEEE Trans. Compon. Packag. Manuf. Technol. (CPMT), Part A 1994, 17, (3), 452–457. 30. Mavoori, H.; Jin, S. Dispersion strengthening for dimensional stability in low-melting-point solders. JOM 2000, 52 (6), 30–32. 31. Marshall, J.L.; Calderon, J. Hard-particle reinforced composite solders, part 1: Microcharacterisation. Solder. Surf. Mount Technol. 1997, 9 (2), 22–28. 32. Wu, Y.; Sees, J.A.; Pouraghabagher, C.; Foster, L.A.; Marshall, J.L.; Jacobs, E.G.; Pinizzotto, R.F. The formation and growth of intermetallics in composite solder. J. Electron. Mater. 1993, 22 (7), 769– 777. 33. Clough, R.B.; Patel, R.; Hwang, J.S.; Lucey, G. Preparation and properties of reflowed paste and bulk composite solder. Proceedings of the Technical Program, National Electronic Packaging and Production Conference; NEPCON WEST: Des Plains, IL, 1990; 1256–1264. 34. Sastry, S.M.L.; Peng, T.C.; Lederich, R.J.; Jerina, K.L.; Kuo, C.G. Microstructures and mechanical properties of in-situ composite solders. Proceedings of the Technical Program, National Electronic Packaging and Production Conference; NEPCON WEST: Des Plains, IL, 1990; Vol. 3, 1266–1275. 35. Ho, C.T.; Chung, D.D.L. Carbon fiber reinforced tin–lead alloy as a low thermal expansion solder preform. J. Mater. Res. 1990, 5, (6), 1266–1270. 36. Pinizzotto, R.F.; Wu, Y.; Jacobs, E.G.; Foster, L.A. Microstructural development in composite solders caused by long term, high temperature annealing. Proceedings of the Technical Program, National Electronic Packaging and Production Conference; NEPCON WEST: Des Plains, IL, 1999; Vol. 3, 1266–1298. 37. Marshall, J.L.; Calderon, J. Hard-particle reinforced composite solders, part 2: Solderability. Solder. Surf. Mount Technol. 1997, 9 (3), 6–10. 38. Choi, S.; Bieler, T.R.; Lucas, J.P.; Subramanian, K.N. Characterization of the growth of intermetallic interfacial layers of Sn–Ag and Sn–Pb eutectic solders and their composite solders on Cu substrate during isothermal long-term aging. J. Electron. Mater. 1999, 28 (11), 1209–1215. 39. Choi, S.; Lucas, J.P.; Subramanian, K.N.; Bieler, T.R. Formation and growth of intermetallic layers in eutectic Sn–Ag solder and its composite solder joints. J Mater Sci. Mater. Electron. 2000, 11 (6), 497–502. 40. Guo, F.; Choi, S.; Lucas, J.P.; Subramanian, K.N. Microstructural characterization of reflowed and isothermally-aged Cu and Ag particulate reinforced Sn–3.5Ag composite solders. Solder. Surf. Mount Technol. 2001, 13 (1), 7–18. 41. Guo, F.; Lee, J.; Choi, S.; Lucas, J.P.; Bieler, T.R.; Subramanian, K.N. Processing and aging characteristics of eutectic Sn–3.5Ag solder reinforced with mechanically incorporated Ni particles. J. Electron. Mater. 2001, 30 (9), 1073–1082.

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Subramanian, K.N.; Bieler, T.R.; Lucas, J.P. Microstructural engineering of solders. J. Electron. Mater. 1999, 28 (11), 1176–1183. 43. Choi, S.L.; Gibson, A.W.; McDougall, J.L.; Bieler, T.R.; Subramanian, K.N. Mechanical properties of Sn–Ag composite solder joints containing copper-based intermetallics. Design Reliability of Solders and Solder Interconnections—Proceedings of Minerals, Metals Materials Society (TMS) Symposium; Orlando, FL, 1997; 241-245. 44. Choi, S.; Lee, J.G.; Guo, F.; Bieler, T.R.; Subramanian, K.N.; Lucas, J.P. Creep properties of Sn–Ag solder joints containing intermetallic particles. J. Miner. Metals Mater. Soc. 2001, 53 (6), 22–26. 45. McDougall, J.L.; Choi, S.; Bieler, T.R.; Subramanian, K.N.; Lucas, J.P. Quantification of creep strain distribution in small crept lead-free in situ composite and non composite solder joints. Mater. Sci. Eng. 2000, A285, 25–34. 46. Guo, F.; Lucas, J.P.; Subramanian, K.N. Creep behavior in Cu and Ag particle-reinforced composite and eutectic Sn–3.5Ag and Sn–4.0Ag–0.5Cu non-composite solder joints. J. Mater. Sci. Mater. Electron. 2001, 12 (1), 27–35. 47. Guo, F.; Choi, S.; Lucas, J.P.; Subramanian, K.N. Effects of solder reflow on wettability, microstructure and mechanical properties. J. Electron. Mater. 2000, 29 (10), 1241–1248. 48. Guo, F.; Lee, J.; Choi, S.; Lucas, J.P.; Subramanian, K.N.; Bieler, T.R. Creep properties of eutectic Sn–3.5Ag solder joints reinforced with mechanically incorporated Ni particles. J. Electron. Mater. 2001, 30 (9), 1222–1227. 49. Jadhav, S.; Subramanian, K.N.; Bieler, T.R.; Lucas, J.P. Stress relaxation of tin–silver solder joints. J. Electron. Mater. 2001, 30 (9), 1197–1205. 50. Gibson, A.W.; Subramanian, K.N.; Bieler, TR. Comparison of mechanical fatigue fracture behavior of eutectic Sn–Ag solder with and without Cu6Sn5 intermetallic particulate reinforcement. J. Adv. Mater. 1998, 30 (2), 19–24. 51. Lucas, J.P.; Gibson, A.W.; Subramanian, K.N.; Bieler, T.R. Nanoindentation characterization of microphases in Sn–3.5Ag eutectic solder joints. Proc. Mater. Res. Soc. Conf. 1998, 522, 339–345; (Pittsburgh, PA). 52. Mavoori, H.; Chin, J.; Vaynman, S.; Moran, B.; Keer, L.; Fine, M. Creep, stress relaxation, and plastic deformation in Sn–Ag and Sn–Zn eutectic solders. J. Electron. Mater. 1997, 26 (7), 783–790.

11 Solder Wetting and Spreading Timothy J. Singler State University of New York at Binghamton, Binghamton, New York, U.S.A.

Stephan J. Meschter BAE Systems, Johnson City, New York, U.S.A.

James Spalik IBM Corporation, Endicott, New York, U.S.A.

I.

INTRODUCTION

During the Second World War, a situation occurred—in retrospect rife with irony given the current subject of this book—when wartime demands for tin (Sn) led to an effort to develop Snfree solder alloys. The results of this effort were not entirely successful and produced lead (Pb)rich alloys that exhibited poor wetting properties in comparison to their Sn–Pb predecessors that were Sn-rich. A deep appreciation developed for the excellent wetting characteristics with which Sn endows many solder alloys. Now, some 60 years later, a markedly different imperative has served to initiate an effort to identify a set of candidate Pb-free solder alloys capable of replacing the existing Pb–Sn alloy set. This effort is spurred by health concerns over Pb in the waste stream and by economic concerns that those with Pb-free microelectronics products would attain an edge in the market place based on the environmentally benign character of their products. The Pb–Sn system is one of the most extensively characterized low-melting-point metal systems because of its longstanding success in soldering of microelectronic components. This alloy system has a long history based on its adaptability to increasingly sophisticated soldering processes and demanding soldering applications. Current industry processes use the Pb–Sn eutectic alloy (63Sn–37Pb) and higher Pb content alloys (e.g., 90Pb–10Sn, 97Pb–3Sn, etc.) extensively. The popularity of the Pb–Sn system grew out of its many desirable material properties [e.g., low melting point, limited dissolution of copper (Cu) substrates, etc.]. Recently, a variety of Pb-free alternative alloys have been investigated (e.g., Sn–Ag, Sn–Ag–Cu, Sn–Cu, Sn–In, Sn–Sb, Sn–Bi, and Sn–Zn) [1]. There is a general consensus that a single-alloy system may not have sufficient versatility to serve as a basis for drop-in replacements for the present Sn–Pb alloys, particularly eutectic Sn–Pb. However, one characteristic of the new Pb-free solder formulations that has emerged with some measure of certainty is that of high-Sn compositions.

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II.

CONCEPTS OF WETTING

A.

Definition of Wetting

The term wetting is generally applied to the fluid dynamic process of a liquid displacing another fluid (gas or liquid) along a solid surface (Fig. 1). The locus of points where solid, liquid, and fluid intersect is referred to as the contact line. Wetting is generally described by the position of the contact line, its rate of advance, and the contact angle, the local angle formed at the contact line between the tangents to the liquid–fluid and liquid–solid interfaces and measured within the liquid. General categorizations of wetting performance are best illustrated by considering a small drop of liquid in a state of equilibrium on a flat, horizontal surface (Fig. 1). A liquid is said to be nonwetting when hE > 90j (Fig. 1a) and to wet the solid if the equilibrium contact angle hE<90j (Fig. 1b). In some references, the liquid is said to be partially wetting when hE<180j, where the tendency to wet is stronger in Fig. 1a than in Fig. 1b. The liquid is said to completely or perfectly wet the solid when hE=0j (Fig. 1c). In the last case, the drop spreads out to form a thin film whose thickness is a function of interfacial energies [2]. These loose definitions describing the degree of wetting have created some degree of ambiguity in the interpretation of previous wetting studies.

B.

Spreading

Strictly speaking, the large-scale macroscopic motion of a liquid mass that accompanies wetting is typically referred to as spreading. Wetting is an essential precursory process to the process of spreading. Wetting should be considered as a phenomenon local to the contact line, whereas spreading has more global implications. Generally, there exists a strong correlation between the two dynamic processes: good wetting generally leads to substantial spreading by the liquid mass. However, situations arise in which the tendency to wet is strong but spreading is constrained and limited by the arrest of the contact line. Such is frequently the case in microelectronics soldering, where contact line arrest is performed intentionally using so-called resist to confine solder masses, often for the purposes of robust joint formation. Mechanisms of contact line arrest or pinning are discussed in Sec. IV.E.

C.

Inert Wetting vs. Reactive Wetting

When a liquid wets a solid and does not react chemically with that solid, the wetting may be referred to as inert wetting. This strict definition of inert wetting is sometimes relaxed to include those systems in which mass transport occurs between the liquid and the substrate, but has negligible effects on interfacial energies. In strictly inert wetting systems, the compositions of the three phases forming the contact line do not change. The values of interfacial energies can be affected by chemical changes from adsorption, particularly when high energy surfaces are involved. Adsorption of impurities from the gas phase or vapor phase is frequently overlooked in the study of inert systems. These effects can be significant, particularly in reactive systems, and are briefly discussed below. Extensive literature is devoted to the study of inert wetting with emphasis on the relationship between the dynamic contact angle and the rate of advance of the associated contact line, as well as the detailed flow in the region of the contact line, both topics that still await complete physical understanding [2–4].

FIG. 1 Qualitative portrayal of wetting: (a) nonwetting; (b) partial wetting; (c) perfect wetting.

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Reactive wetting generally describes a situation in which the solid undergoes chemical reaction with either the gas phase or the liquid phase, or with both phases. Wetting in metal–metal systems is, in general, highly reactive. To the extent that inert wetting is not completely understood, reactive wetting is significantly less well understood. A broad range of possible chemical interactions between a liquid metal and a solid metal substrate is possible, most importantly the phenomena of dissolution and intermetallic compound formation. The presence of multiple species in reactive wetting allows for diffusive mass transport in a drop, in addition to the convective mass transport present in inert wetting systems. The simultaneity of these physical and chemical mechanisms, in combination with the presence of a contact line, makes wetting in metal–metal systems among the most complex of wetting systems. Furthermore, reactive wetting in metal–metal systems is extremely system-dependent and, consequently, is very difficult to discuss in generality. Despite intensive study, many fundamental problems remain poorly understood.

D.

Interfaces as Surfaces and Surface Energies

It is useful to first review some classical concepts of inert systems. The limitations of the inert theory with respect to reactive systems as well as potential modifications are discussed. As noted in Sec. 2.1, the definitions of both the contact line and the contact angle are predicated on the intersections of interfaces between dissimilar phases. Physically speaking, the term interface refers to a thin stratum—in some cases only several molecules thick—in which the density q varies rapidly from its bulk value in phase 1 to its bulk value in the adjoining phase 2 (Fig. 2), in the direction Z of its gradient. 1. Concept of Surface Excess A common mathematical conception of an interface is due to Gibbs [5] and arises from the idealization of the continuous transition of molecular density between two phases in equilibrium with each other to a fictitious discontinuous transition in which the local densities of the two phases extend unchanged up to a surface within the transition region. The surface may be oriented (but not located) according to the local gradient of the molecular density. The difference

FIG. 2 Density variation in the interfacial region.

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between the actual mass, achieved by integrating the actual density profile across the transition region in a direction given by the local gradient, and the fictitious mass associated with uniform densities extending right up to the interface is ascribed to the interface and termed the excess mass. This idealization may be extended logically to any extensive thermodynamic property associated with local densities. The difference in value of any particular property between the physical situation and the idealized model, integrated over the transition region, is similarly ascribed to the fictitious interface and referred to as the surface excess of the particular property. The motivation for the idealization of a continuous transition to a discontinuous one containing a surface is the mathematical simplification realized in trying to describe the physics of a diffuse (three-dimensional) region vs. the physics of a surface (two-dimensional), to which, in the latter case, the familiar results of differential geometry can be applied. For example, the concept of the curvature of a surface derives from differential geometry and plays a very important role in spreading of solders. 2. Surface Energy and Surface Tension The energy associated with an interfacial region differs from that associated with either of the bounding bulk materials. The origin of interfacial energy is the existence of unsatisfied or missing bonds for atoms in the interfacial stratum. Energy is liberated when bonds are formed, so the most stable low-energy configuration has the fewest missing bonds. The concentration of energy at surfaces thus induces a tendency of surfaces to minimize their areas. In the Gibbs model, interfacial free energy results from the application of the sharp interface model to the free energy of a local region that straddles two distinct phases. In what follows, the concept of Gibbs is implicit when referring to surface energies. Consider an inert system in which the interfaces are ideal and planar. The ways in which chemical reaction and the nonideal nature of interfaces may change the results derived here are discussed below. The surface (area-specific) free energies of the solid/vapor (S/V), solid/liquid (S/ L), and liquid/vapor (L/V) interfaces are denoted as rSV, rSL, and rLV, respectively. These energies represent the work necessary to form a unit area of interface. The units of any r are Newton meters per square meter (i.e., energy per unit area); despite these units, the r will hereafter be referred to as surface energy. As was pointed out by Gibbs [5], surface energies differ from surface tensions for S/V and S/L interfaces because the surface energy is associated with the work performed in forming the surface, whereas surface tension is associated with the work performed in stretching the surface. The quantities differ because altering the surface area by stretching, in general, involves some measure of deformation of the surface (strain energy), a phenomenon that is absent in forming a surface. This may easily be shown as follows. Consider a surface with surface energy r and area A. The change in the energy of this surface (or work performed on the surface) due to a change in area may be written as: dðrAÞ ¼ rdA þ Adr

ð1Þ

The term Adr is zero for liquids because of their atomic mobility (i.e., the equilibrium structure of a liquid surface is unchanged upon change in surface area). This is not true for surfaces comprised of a solid (e.g., S/L and S/V surfaces), for which stretching the surface leads to greater distances between atoms (strain) and a consequent reduction in surface energy (dr<0). However, it may easily be demonstrated that the surface energy and surface tension for the L/V interface are equivalent quantities and may be used interchangeably. Consequently, we may express rLV either in units of Newton meters per square meter or Newtons per meter. 3. Effects of Adsorption The quantity rSV represents the surface energy associated with equilibrium between the solid surface and the vapor of the liquid (wet systems). If vacuum conditions prevail (dry systems), the surface energy of the solid/vacuum interface is represented by the term rSO. rSO may be viewed as the value of rSV in the presence of an infinitely dilute vapor. A solid surface in the presence of vapor differs from a solid surface in vacuum by the degree of adsorption of vapor molecules on the solid surface—a process that can occur spontaneously and thus can result in a lower surface

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energy. There are two widely recognized paths by which adsorption can occur in a wetting process. One path is the evaporation of atoms from a spreading liquid phase and their subsequent condensation on the solid surface over which spreading is occurring. A second path is via the surface diffusion of atoms from the liquid in the contact line region to the solid surface ahead of the contact line. The driving force for such surface diffusion is a gradient of the chemical potential of the diffusing species between the liquid at the contact line and the solid surface immediately in front of the contact line. At equilibrium, the difference in surface energies is equal to an amount pSV referred to as the film or spreading pressure. Thus, the relation between surface energies for the vacuum and equilibrium vapor cases is given by rSO=rSV+pSV, where pSV>0 and is given by an appropriate Gibbs adsorption isotherm (e.g., pSV ¼ RTmP0 0 CS=V =P dP, where CS/V is the Gibbs surface excess of the vapor on the solid, P is the pressure, P0 is the vapor pressure of the liquid, and R is the universal gas constant). The effect of the film pressure can be very significant and the presence of adsorption should not be overlooked, particularly when a solid/gas interface is characterized by high surface energy and a spreading liquid possesses a high vapor pressure. As an example, Bondi [6] notes that rSV for Cu can be reduced from 1.2–1.3 to 0.7 N m/m2 by the adsorption of Pb. 4. Effect of Gaseous Contaminant Absorption The assumption is often made that there exists little difference between rSO and the surface energy of the air/solid interface. This is a reasonable approximation for solid surfaces in the presence of liquids that possess low volatility. The existence of oxides on most metallic surfaces in the absence of a reducing agent should be assumed. Kuhmann et al. [7] have shown that many metals susceptible to oxidation will oxidize even in ultra high vacuum conditions because the threshold partial pressures of oxygen for the oxidation of metals (Table 1) are so small as to be unachievable with the present vacuum technology. Certainly, even in the purest of commercially available ‘‘inert’’ gases, the levels of oxygen are well in excess of threshold oxidation values. In poorly controlled environments, contaminants may also be present in the vapor and may preferentially be absorbed at liquid surfaces and adsorb to solid surfaces, causing a reduction in the respective surface energies. Absorption of limited amounts of oxygen can lower the value of rLV, leading to enhancement of wetting. However, when oxygen reaches its solubility limit in a liquid metal, a solid oxide can form on the liquid surface, a development that inhibits wetting. Adsorption of oxygen onto a solid metal surface at sufficiently high partial pressures typically leads to the formation of an oxide that lowers the value of rSV. This is detrimental to wetting. Both forms of oxide formation require the use of some form of oxide reduction mechanism in commercial soldering processes. Adsorption, as a part of overall system energy minimization, may also occur within the liquid phase when the liquid is a solution. The species with the lowest surface energy can adsorb to the gas/liquid interface, causing a reduction in that surface’s energy. For example, in solder processes employing Pb–Sn solders, a thin layer of solution several atoms in thickness with a Pb concentration exceeding the bulk concentration is typically found at the surface during the flow phase and the (solid) end state of the process [8]. This adsorption occurs over the entire

TABLE 1 Partial Pressures of Oxygen (O2) in Equilibrium with the Oxide of Metal at its Fusion Temperature Metal

P O2 (atm)

Cu Ni Pb Sn

2  106 8  106 2  1028 1050

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composition range of the alloy system. For instance, the eutectic Sn–Pb composition corresponds to a bulk Pb concentration XBPb=26.6 at.%, whereas the measured surface Pb concentration XSPb=67 at.%. Thus, the value of the surface tension of the eutectic solder during the flow phase of the soldering process is less than the expected bulk phase value for the eutectic composition. Beyond the change in surface tension, this surface segregation can have a profound influence on wetting dynamics and is given additional consideration in Sec. IV.D.

E.

Static Contact Angles

A free energy analysis of the region where three two-phase couples (e.g., solid–liquid, solid– vapor, and liquid–vapor) coexist leads, under the above model, to the consideration of equilibrium of the contact line formed by the intersection of three surfaces associated with two-phase couples. The free energy analysis of the contact line region most frequently invoked in wetting discussions is that for an inert system consisting of a liquid resting on a locally flat, solid surface (Fig. 3). The system is assumed to be in mechanical equilibrium so its free energy is at a minimum. Thus, an infinitesimal displacement dx of the contact line toward the vapor should result in zero change in free energy. This change in free energy may be written as: rSV ðdxÞ  rSL ðdxÞ  rLV ðcoshE dxÞ ¼ 0

ð2Þ

Dividing by dx in Eq. (2) leads to the Young equation [9]: rSV  rSL  rLV coshE ¼ 0

ð3Þ

Eq. (3) represents the thermodynamic equilibrium between surface energies and the local orientation of the interface as given by the equilibrium contact angle hE. It is deceptively simple for several reasons. The values of the surface energies are assumed to be well-defined thermodynamic variables. This is only true at distances from the contact line that are several orders of magnitude larger than the atomic distances characterizing the core region at the contact line. The core region may be considered a three-dimensional region where the S/L, S/V, and L/V interfacial layers overlap. Inside the core region, the surface energies and the contact angle h may not be well defined. The contact angle h measured outside of the core is the macroscopic contact angle that would be measured with an ordinary optical microscope. The above equation was obtained by writing the differential of the free energy due to a displacement of the contact line in the direction of the initially undeformed S/V interface. This assumes that the S/V and S/L interfaces remain coincident. The reality and consequences of deformation of the contact line region out of the initial S/V plane due to the stress associated with the L/V interface are discussed later. From Eq. (3), hE is a material property of the system through its dependence on interfacial energies. As thermodynamic variables, interfacial energies are functions of the system temperature and, consequently, hE is also a function of temperature.

FIG. 3 Displacement of the contact line in equilibrium.

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1. Spreading Coefficient The only quantities that are directly measurable with acceptable levels of uncertainty are rLV and h. Accordingly, Young’s equation only affords knowledge of the difference rSVrSL, referred to as the driving force of wetting. The angle hE is a material property of the system. The tendency of a given material system to spread is often described in terms of the equilibrium spreading coefficient, defined as: SE ¼ rSV  rLV  rSL

ð4Þ

which, when combined with Young’s equation [(Eq. (3))], yields: SE ¼ rLV ðcoshE  1Þ

ð5Þ

The equilibrium spreading coefficient can be either negative (partial wetting) or zero (perfect wetting). There are conditions (e.g., when a liquid first contacts a solid, or when a liquid phase is produced via melting) when the solid surface is not in equilibrium with the vapor of the liquid. Setting rSV=rSO+rSV in Eq. (5), it follows that: S ¼ SE þ pSV ¼ rLV ðcoshE  1Þ þ rSV

ð6Þ

where SurSOrLVrSL is referred to as the nonequilibrium spreading coefficient. Thus, in the initial phases of spreading, S can be positive. As adsorption occurs, S decreases until it becomes negative or zero when equilibrium is achieved. For example, Naidich [10] notes that for pure Pb spreading on Cu in a gaseous H2 environment in the temperature interval 4000 so that the Pb should completely wet the Cu. However, the adsorption of Pb on the Cu (see above) occurs before the drop has completely spread and during which time the contact angle increases to the range 40j0 via macroscopic observation or measurement because complete wetting occurs in either case. As discussed in Sec. 4.3.3, there is a scarcity of solid surface energy data for metallic elements and alloys, and S often cannot be evaluated with sufficient accuracy to allow meaningful quantitative comparison between various metal–metal systems. In many cases, values of S for pure metal–metal systems are small negative numbers corresponding to values of hE<<1. A specific exception to this generalization is discussed in Sec. 5.

F.

Solderability

Wettability is a critical factor when evaluating the performance of a solder–substrate system. In general, good wettability is a necessary but not sufficient requirement for good solderability, the latter requiring, as an additional criterion, good adhesion between the solder and the substrate subsequent to wetting. 1. Cohesion and Adhesion To discuss the adhesion between a liquid and a solid, consider separate cylindrical volumes of liquid (L) and solid (S) with unit cross-sectional areas (Fig. 4). If each of these bodies is cleaved in half, the work required in each case is equal to the difference in free energies before and after cleavage. This may be expressed as: WCðLÞ ¼ 2rLV ;

WCðSÞ ¼ 2rSV

ð7Þ

where WC(L) and WC(S) are the work of cohesion of the liquid and the solid, respectively. If a liquid and a solid surface are brought together, the resulting difference in free energies is referred to as the negative of the work of adhesion: WSL ¼ rSL  rLV  rSV

ð8Þ

Note that the spreading coefficient SE is equal to the difference between WSL and WC(L), reflecting the competition between adhesive and cohesive forces during wetting.

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FIG. 4 Diagram illustrating the work of cohesion and the work of adhesion: (a) before cleavage; (b) work performed in creating S–S and L–L surfaces (work of cohesion); (c) work performed in creating S–L surfaces (work of adhesion).

2. Wetting and Adhesion Using the ‘‘nearest neighbor’’ model of atomic interaction [11,12], the above quantities may be expressed in terms of the bulk bond energies eLL and eSS and the interfacial energy eSL. Let n be the number of nearest neighbors in the bulk crystal, m be the fraction of broken bonds per atom at the surface of the liquid or solid, x be the surface area per atom, and DeLL, DeSS, and DeSL be the bond pair energies defined as negative quantities. Then: rLV ¼ 

nm DeLL ; x 2

rSV ¼ 

nm DeSS ; x 2

rSL ¼ 

nm h eLL þ eSS i DeSL  x 2

ð9Þ

Using Eq. (9), the work of adhesion may be written as: WSL ¼ 

nm DeSL x

ð10Þ

From Young’s equation (Eq. (3)), it follows that: coshE ¼

WSL 1 rLV

ð11Þ

which, using the expressions in Eq. (10), can be written as: coshE ¼

DeSL 1 DeLL =2

ð12Þ

Eq. (12) again reflects the competition between adhesion and cohesion during wetting. From Eq. (12), it may be seen that metallic systems for which both cohesive and adhesive interactions are strong (i.e., DeSLfDeLL>>1) yield good wetting. However, organic liquids such as fluxes have weak cohesive forces, but also interact weakly with metallic substrates. In this case, DeSLfDeLL<<1, but wetting is still good. Thus, good wetting may occur in systems that do not manifest good adhesion.

G.

Dynamic Contact Angles

When the contact angle of a spreading liquid is measured, its value typically differs from the equilibrium value hE. The angle associated with a moving contact line is referred to as the dynamic contact angle hD. The dynamic contact angle is generally dependent on time s, and has the characteristic velocity dependence shown in Fig. 5. This dependence reflects the behavior that, as the contact line speed is indefinitely increased in the advancing direction, hD!180j, and as the speed is increased indefinitely in the receding direction, hD!0j. Also, hD!hA as the advancing speed is brought to zero, and hD!hR as the receding speed is brought to zero. The difference hAhR is a measure of the contact angle hysteresis whose origin lies in the nonideality of the solid

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FIG. 5 The relationship between the dynamic contact angle hD and the contact line speed VCL.

surface. Hysteresis is a major source of widely varying contact angle values reported in the literature for a given system; it is discussed in Sec. V. 1. Universal Relationship For inert systems, Kistler [13] suggests that hD is a complex function of fluid properties, solid properties, and physical properties of the L/V and S/L interfaces. However, for systems in which hochE=0, Hoffman [14] proposed an empirical mobility relationship of the general form: hD ¼ fðh0 ; CaÞ

ð13Þ

where h0 is the static contact angle. Curve fitting to data often (but not always) determines that h0 is the static advancing contact angle hA (possibly different from hE when wetting is not perfect); Cau(lLVCL)/rLV is the capillary number (lL is the dynamic viscosity of the liquid, VCL is the contact line speed, and rLV is the surface tension). The implication is that dynamic contact angle data for perfectly wetting systems follow a universal function when plotted against Ca (Fig. 6). Hoffman [14] did not provide an explicit functional form, but a curve was determined to fit his and others’ data, when Ca
FIG. 6 Data showing capillary number hD vs. Ca for silicone oils spreading on glass and displacing air. (From Ref. 13.) Data collapse onto a universal curve. (From Ref. 14.)

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when CaVO(101). However, as noted above, metal–metal systems in general do not exhibit perfect wetting but typically obey hE<<1. In this case, the universal behavior, with a judicious choice of h0, still provides an adequate description of dynamic wetting h3Dh30ccTCa. The choice of h0 for nonideal surfaces is complicated due to nonideal effects such as contact angle hysteresis. 2. Reactive Systems In reactive wetting systems, the material properties of a system change with time due to dissolution and product formation processes. Boettinger et al. [17] suggested a modification of the general equation to reflect the time dependence of various relevant variables: hD ¼ fðh0 ðsÞ; Ca; CðsÞ; TðsÞ; SðsÞÞ

ð14Þ

where C(s) is the concentration field, T(s) is the temperature field, S(s) represents a general change in wettability of the S/V interface (e.g., oxide patches, etc.), and s is the time. The mobility equation (Eq. (13)) is measurable in inert systems and is of practical value, having been used successfully in many analyses of problems involving dynamic contact lines (cf. Erhardt and Davis [18], Erhardt [19]). However, the form of Eq. (14) for reactive systems is largely conceptual as there is presently no way of measuring the temporal dependence of the variables. Moreover, even if the form of Eq. (14) could be measured for a particular system, it would lack generality and would only be applicable to the identical system for which measurements were successfully made.

H.

Thermodynamic Treatment of Wetting

1. Equilibrium Equilibrium thermodynamics can only provide an indication of the direction of a process and, in the present case, this specifically means an indication of whether a system will wet or not. In single-component systems (systems containing one chemical species), thermodynamic equilibrium requires a balance of mechanical forces acting on a system (mechanical equilibrium) and equality of temperature between various phases of the system (thermal equilibrium). In multicomponent systems, such as reactive metallurgical systems comprised of multiple metal species, equilibrium requires the aforementioned two criteria as well as equality of the chemical potential of a given species between different phases (chemical equilibrium). In general, a system that is not in equilibrium will undergo a spontaneous process taking it toward equilibrium. Often a multitude of local equilibrium states referred to as metastable equilibria is accessible to a system but there exists only one state corresponding to minimum energy that represents a global equilibrium for a system. Equilibrium thermodynamics has been used effectively to calculate shapes of normal solder joints and also pathological solder configurations (e.g., solder bridging [20], tombstoning, etc.) by implementing numerical algorithms that minimize free energy under the assumption of mechanical equilibrium. These shapes have subsequently been used in analyzing the structural integrity of solder joints, and the prediction of solder joint fatigue life [21]. 2. Effect of Chemical Reaction It has been known for some time [22] that, from a thermodynamic perspective, the occurrence of a chemical reaction between a wetting liquid and a solid substrate can augment the driving force of wetting. The effects of dissolution and reaction (product formation) can affect wetting performance in metal–metal systems by changing the physical and chemical properties of the bulk liquid as well as the S/L, S/V, and L/V interfaces. The interactions between the liquid alloy and the solid substrate are, in general, the most complex and are not completely understood. The most complex situation occurs when both the solubility and reaction rates are high. Generally speaking, the more violent and rapid the dissolution and reaction are, the larger is the wetting system’s departure from thermodynamic equilibrium. In highly nonequilibrium situations, predictions based upon equilibrium thermodynamic analyses must be interpreted with care. The contribution of a reaction to the driving force for wetting in metal–metal systems that form compounds can be analyzed based on a model first proposed by Yost and Romig [23].

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Consider a binary system comprised of a drop of molten metal B that wets a flat solid metal A and forms a compound C (Fig. 7) concomitantly with wetting. A simple reaction product morphology is assumed to be associated with wetting: A thin flat layer of compound C is formed by consumption of thin flat layers of metals A and B. Let the reaction be represented by the stoichiometric relation xA+yB!AxBy(C). When the contact line (assumed circular) of the drop advances a distance dr, the molar changes associated with the reaction are: dnA dnB ¼ ¼ dnC x y

ð15Þ

where dni is the change in the number of moles of species i. The change in free energy dG associated with the contact line advance dr must reflect the changes in molar composition as well as the changes in the energies associated with areal changes. Thus: C dG ¼ ðlC A  lA ÞdnA þ ðlB  lB ÞdnB  2prdrðrBC þ rAC þ rBV coshD  rAV Þ

where G is the free energy and Eq. (15) leads to:

lji

ð16Þ

is the chemical potential of i in j.

dnA dnB qA q q y qA ¼ Z dVA ¼ B dVB Z B dVB ¼ x MA x y xMA xMB MB

ð17Þ

Furthermore, when wetting ceases, equilibrium is characterized by: rBC þ rAC þ rBV coshE  rAV ¼ 0 Substituting Eqs. (17) and (18) in Eq. (16) yields: qA C dVA þ 2prdrrBV ðcoshD  coshE Þ dG ¼ fxðlC A  lA Þ þ yðlB  lB Þg xMA

ð18Þ

ð19Þ

C where the term {x(lC AlA)+y(lB lB)}uDGF is the Gibbs free energy of formation. Noting that dVA=2prdrlA, where lA is the thickness of the thin layer of A consumed by the reaction, Eq. (18) can be written as:

dG ¼ 2pr½n þ rBV ðcoshD  coshE Þ ð20Þ dr where n=[(qAlA)/xMA]DGF. For spontaneous wetting to occur, the condition dG/dr<0 must be satisfied and for an advancing contact line, hD>hE (see Fig. 5, where hE is taken to be hA) and thus cos hDcos hE<0. The right-hand side of Eq. (19) reveals two contributions to the wetting driving force: the usual capillary contribution rBV(cos hDcos hE) that is present in inert wetting and the contribution due to reaction n, which is new. Yost and Romig [23] provide comparisons of the magnitudes of these two terms for several reactive binary systems. For the Sn–Cu system, when the product C formed is the D-phase intermetallic, 6Cu+5Sn!Cu6 Sn5: jnCu6 Sn5 jc6026 erg=cm2 ;

rSn=V c550 erg=cm2

FIG. 7 Pure molten metal B wets pure solid A and forms a compound C.

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Because the maximum value of either cosine term in Eq. (20) is one, rSn/V represents an upper bound for the capillary term. Wetting was assumed to occur at the melting temperature of Sn and the above calculations were performed for this value of temperature. Thus, it may be seen that the reaction term can dominate the capillary term in the driving force for reactive wetting. However, caution must be exercised in the interpretation of this result because the morphology of product formation was greatly simplified, its rate of formation was assumed instantaneous subsequent to wetting, and the dissolution of the substrate and other phenomena were neglected. Some of these neglected phenomena are discussed in Sec. V.

III.

MEASURING AND QUANTIFYING WETTING PERFORMANCE

Many experiments have been devised to quantify wetting performance. These include sessile drop experiments, dip experiments, wetting balance experiments, and others. All have their strengths and weaknesses, which will not be categorically reviewed here. The present ANSI standards for assessing solderability do not specify sufficiently stringent procedures for experimental results to be repeatable from one laboratory to the next. It is critical now, as the microelectronics industry embarks on establishing new databases for wetting performances of entirely new Pb-free solder alloys, that unambiguous procedures are developed. An essential element in any assessment of wettability is the accurate characterization of the materials involved. The composition of the solder alloy must be accurately known. If the experiment involves forced wetting (e.g., a wetting balance), the rate of advance of the substrate relative to the liquid must be specified and maintained with high precision. If an experiment involving spontaneous wetting is utilized (e.g., a sessile drop), then the initial volume of the alloy must be accurately known. In either case, it is critical that the surface characteristics of the substrate be measured, including both the amplitude and the average asperity slopes associated with roughness. The chemical state of the surface must be known and reproducible, as adsorbed species and oxides can have a profound effect on wetting performance. The two most common techniques for the quantitative and qualitative assessment of the wetting and spreading performance of solders are the wetting balance and the sessile drop technique. Each method possesses certain advantages over the other and is discussed in Secs. 3.4 and 3.5. However, there are certain experimental conditions that are common to either method and merit brief consideration.

A.

Orientation

Surfaces must have precise attitude orientation (e.g., in sessile drop experiments, it is essential that the substrate be horizontal). Any tilt can cause the equilibrium contact angle on the lower side of the drop to be an advancing contact angle and the contact angle on the higher side of the drop to be a receding contact angle. These two angles will have different values depending on the amount of hysteresis exhibited by the wetting system (Fig. 5). Furthermore, because the contact angle is generally measured from a single side view perspective, it is essential that the perspective used be representative of the entire system. This constraint requires that there be no asymmetry of the contact line. For example, in sessile drop experiments, it is necessary to measure both the circularity and concentricity of the contact line. If these measurements reveal asymmetry, the data must be discarded. If the contact angle is measured during wetting balance tests, then translational symmetry of the contact line in the direction of viewing must be verified.

B.

Temperature Control

The temperature of the system must be precisely controlled. The equilibrium contact angle exhibits temperature dependence. If a gradient of temperature exists on the substrate on which wetting is occurring, it can introduce variations in the contact angle. In the case of spreading drops, such variations in the contact angle can cause the drop to migrate up the temperature gradient [8]. A temperature difference between the substrate and the ambient atmosphere can

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induce thermal Marangoni motions (see below) that influence the extent of spreading of the drop. Thus, it is desirable to maintain this temperature difference at a minimum.

C.

Effect of Fluxes in Experimental Measurements

The use of liquid fluxes for oxide elimination is problematic. Liquid fluxes can affect the wetting performance of a metal–metal system in a variety of ways. Fluxes themselves must wet the oxides to reduce them in advance of the spreading molten metal. The wetting of fluxes can potentially be asymmetrical and consequently introduce asymmetry in the wetting of the solder alloy. The molten metal must displace the flux in order to wet the metallic substrate. The displacement of flux by a liquid metal is affected by the volume of flux that is present on the substrate; the control of flux volume and its distribution on the substrate (e.g., thickness of flux layer, etc.) is difficult, even in a laboratory setting. The presence of liquid flux at the contact line obscures the contact angle, making an accurate measurement impossible. Almost all scientific wetting experiments are conducted in carefully controlled gaseous atmospheres or high vacuum to avoid the introduction of uncertainties associated with the use of liquid fluxes.

D.

Wetting Balance Method

1. Introduction The wetting balance is an instrument that measures the wetting forces that a liquid exerts on a substrate. The substrate, often in the form of a thin plate or wire, is initially positioned over a bath of the liquid. The bath is then raised at a specified constant speed whereby the substrate enters the liquid edgewise. After a specified immersion depth of the substrate’s leading edge in the liquid bath is achieved, the bath advance is halted. A dwell time at this immersion depth is preset during which time both the bath and the sample are held stationary. Following the dwell period, the specimen is withdrawn from the bath at a specified speed. Forces may be measured for the entire period that the substrate is in contact with the liquid. The wetting balance is a comparatively new method of assessing wetting. In the microelectronics industry, its use to measure the wetting performance of solders and fluxes is pervasive. It is considered by some to be the most versatile method for evaluating solderability. Its primary advantages are: (a) it provides a near real-time quantitative output of force; (b) substrates in the shape of microelectronic components can be used as specimens; and (c) it can be used equally well to evaluate the wetting performance of solders and fluxes. 2. The Wetting Balance Curve Fig. 8 shows a typical wetting balance curve in which the force on the substrate is plotted vs. time (an upward force on the substrate is displayed as a negative force). The notations (a) through (f) in the figure mark points in the measurement process that are roughly associated with the physical stages illustrated in the drawings above the graph. Point (a) represents the balance output prior to the contact of the specimen with the solder. The weight of the specimen is typically tared and wetting forces are measured relative to this zero datum. a. Forces Acting on Specimen. Point (b) is imagined as associated with a force on the specimen with the meniscus still turned down toward the bath (i.e., an obtuse contact angle). There are two forces on the specimen. One force is buoyancy, given simply by Archimedes’ principle as the immersed volume times the density of the solder times the gravitational acceleration. The second force is due to capillarity, given by the wetted perimeter times the value of the surface tension. At point (b), because the contact angle is apparently (and probably) obtuse, capillarity actually serves to reinforce the buoyancy force. b. Initial Immersion Effects. The reason for the obtuse contact angle is not entirely clear, nor can it be verified if the contact line is obscured, and should not necessarily be interpreted as a universal feature of early immersion times. In an ideal experiment, the bottom of the substrate first contacts the solder everywhere along its leading edge. Assuming that the solder–substrate pair is wetting, no pinning should occur and the contact line should move immediately and

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FIG. 8 Wetting balance force vs. time wetting balance curve. Drawings at the top of the figure correspond to the labeled points on the curve. (From Ref. 26.)

uniformly to the sides of the specimen. The meniscus can be depressed below the free surface of a solder bath if the immersion speed exceeded the wetting rate of the system. Immersion rates for wetting balances are typically 20F5 mm/sec. But Meschter [24] observed wetting rates of pure Sn on pure Au in excess of 20 mm/sec during early times in spontaneous wetting by drops. Even though the meniscus may be depressed below the bath surface, there exists the possibility that the dynamic contact angle is acute, as the essential aspect of this phase of the test is the speed of the contact line and the mobility response (Fig. 5) manifested by the wetting system. For most practical commercial solder–substrate pairs, however, it is likely that the dynamic contact line is obtuse for the duration of the immersion process. c. Prefluxing. Another aspect of the immersion process that must be considered is that the specimen is typically prefluxed before immersion. On immersion, the solder must displace the flux before it can achieve metal–metal contact with the substrate. This displacement process is a complex fluid dynamic process and it clearly depends upon the thermophysical properties of the flux (e.g., density, effective viscosity, surface tension, etc.) and upon the thickness of the flux layer applied. This displacement process must accompany the contact line as it progresses up the specimen. Thus, the presence of a liquid flux can significantly affect observed wetting performance and there is no clear way to control this effect. This is a major reason why researchers doing fundamental metal–metal wetting studies strive to conduct such studies in a gaseous atmosphere that reduces the metal oxides that may be present. d. Buoyancy Force. Point (c) is generally assumed to coincide with zero capillary force (i.e., the solder is wetting the substrate and the liquid solder interface forms a contact angle of 90j). In this case, the only force is the upward force of buoyancy. The pure buoyancy force level is represented on the diagram as a dashed horizontal line. The dynamic contact angle of 90j is often

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assumed to occur when the contact line lies in the plane of the undisturbed solder bath interface. However, based on the prior discussion, this condition need not be true. e. Equilibrium. As the solder continues to wet the substrate, the meniscus moves upward while exhibiting an acute dynamic contact angle. The wetting at this point is entirely spontaneous. The contact angle decreases monotonically as the contact line speed decreases. As the contact angle decreases, the downward force exerted by the surface tension increases. At some point between (c) and (d), the contact angle assumes a value for which the buoyancy force and capillary force are equal and there is a zero net force on the specimen. The approach of the contact line to point (d) is associated with a decreasing contact line speed and decreasing contact angle, and the latter manifests an increasing (downward) capillary force. Past the point (d), the system comes to a quasi-equilibrium and the contact line speed is nominally zero; the wetting force becomes constant and may be estimated from the expression rpcos(h)qV, where p is the wetted perimeter and V is the volume of displaced solder. An upper-bound estimate for the wetting force may be obtained by setting h=0. Between the times corresponding to zero net force and maximum force, many wetting curves can be approximately described by the expression ( F max F(t))/ Fmax=exp(t/t0), where Fmax is the maximum observed force and t0 is a characteristic time. However, this observation is purely anecdotal and provides little insight into the physics of the wetting process. f. Specimen Withdrawal. When the specimen is withdrawn, the value of the contact angle decreases consistent with the dewetting mobility response of the system. The upward motion of the specimen induces an opposing shear force that contributes to the increase in measured force [25]. The contact line may pin on the bottom of the specimen in which case the contact angle could take the value of zero, resulting in the maximum capillary force on the specimen. The specimen withdrawal phase of the wetting balance test is reflected in the force response near the point (e). The detachment of the solder meniscus occurs via capillary instability associated with the necking down of the meniscus until it ruptures. The force during this phase of the test decreases monotonically with time and eventually takes a constant value as shown by point (f). The final constant value is larger than the initial value at point (a) because the specimen retains a certain amount of liquid solder subsequent to detachment. 3. Interpretation Criteria One of the major impediments to using a wetting balance to assess solderability is the lack of a universally accepted criterion or set of criteria to interpret the force–time curve. As pointed out by Lea [26], a major factor in this dilemma is that the wetting flow associated with the wetting balance technique has not been successfully modeled theoretically. The lack of analytical results makes it difficult to translate force–time data into meaningful physical interpretations of wetting performance. This is not to say that there exists a paucity of criteria of wetting performance based on the force–time curve. Paradoxically, the lack of theoretical guidance has led to a proliferation of ad hoc criteria. The result of so many different criteria in the literature has made comparison of studies of wetting performance based on the wetting balance technique difficult. Almost all of the proposed criteria fall into two general categories: (a) various measures of the wetting force; and (b) the time required for the force to reach a certain value. For purposes of comparison, most wetting balance practitioners attempt to eliminate the dependence of the force on specimen size by normalizing the measured force by the perimeter of the specimen. In the first category, either the maximum force or the wetting force that is exerted after a certain time is typically used. In the latter case, a time interval may be selected comparable to the time the solder is molten in a particular commercial process. In the second category, the time elapsed before observation of a force corresponding to a certain wetting event is taken as a measure of wetting performance. Times are typically measured from the zero datum corresponding to the moment immersion is initiated. Times corresponding to force equilibrium between buoyancy and capillarity [zero force, point (d)] and two thirds of the maximum force are commonly used as wetting performance parameters. An existing standard, MIL-STD-833 Method 2022, specifies that acceptable solderability requires a time to zero force less than 590 msec and a time to two thirds maximum force of less than 1 sec.

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4. Negative Aspects of the Wetting Balance Method Certain aspects of the wetting balance method that detract from its ability to accurately assess solderability are rarely noted. Commercial wetting balances possess a bath with a relatively large surface area contained in an enclosure for which precise control of the ambient atmosphere is difficult. Thus, although the specimen is usually prefluxed, oxidation of the solder bath interface is a strong possibility and an oxidized solder surface (often referred to as dross) can have profound influences on wetting. Frequent cleaning of the bath and replacement with fresh solder are discouraged because the volume of the solder bath is typically large. Also, the repeated dissolution of substrate material into the bath as a result of numerous tests introduces contaminant species that can have a significant impact on wetting. In typical industrial environments, this influence is difficult to quantify. a. Disparity Between Measured and Calculated Forces. It has been observed [27] that the measured and calculated (based on static equilibrium) forces are not in agreement during the transient wetting phase before quasi-equilibrium is established. It was determined that the calculated downward force always exceeded the downward force measured during wetting. The force disparity was observed to decrease as the contact line slowed and quasi-equilibrium was approached. The data weakly suggest that the sign of the disparity changes (i.e., the measured downward force exceeds the calculated downward force) in the neighborhood of quasi-equilibrium. The calculations of Moon et al. [27] support the hypothesis that the force disparity away from quasi-equilibrium is, in significant part, attributable to the upward shear force exerted on the specimen because of the upward solder flow attending the advance of the meniscus. However, Moon et al. [27] were not able to solve exactly for the velocity field in the solder and thus could only estimate the shear force. b. Thermocapillary Effects. Moon et al. [27] also noted that the observed meniscus rise at quasi-equilibrium was greater than the predicted rise. They postulated that the additional capillary rise was caused by thermal Marangoni flow along the solder/atmosphere interface toward the contact line. Industrial wetting balances have temperature-controlled baths. However, the specimen, even after it is partially immersed in the solder, does not achieve thermal equilibrium with the bath. The specimen is cooler because it is partly exposed to the ambient chamber atmosphere that is cooler than either the bath or the specimen. Therefore, the solder/ atmosphere interface is warmer than the contact line region, causing the surface tension at the contact line to be greater than along the solder/atmosphere interface. The resulting gradient in surface tension drives a thermal Marangoni flow along the solder/atmosphere interface toward the contact line. The associated mass flux contributes the extra fluid needed for additional capillary rise. In general, the Marangoni motions can contribute in a significant way only when the overall flow is weak; this occurs during the wetting flow when a state of quasi-equilibrium is approached. Furthermore, a Marangoni flow toward the contact line must turn downward along the specimen to complete the Marangoni convection cell (i.e., to conserve mass). The associated weak downward shear force exerted on a specimen may help to explain the possible change in sign between the measured and calculated forces. In general, a complete understanding of the wetting balance force–time curve for solders awaits a rigorous theoretical analysis capable of resolving all the dynamic aspects of the solder flow noted above. 5. Test Conditions vs. Industrial Process Conditions The actual wetting dynamics of a wetting balance test has little in common with most microelectronic soldering processes. The wetting balance test is a hybrid of forced and spontaneous spreading processes. During the immersion phase, the wetting is forced; during the subsequent dwell phase, the wetting transitions to spontaneous wetting from an initial state very much dependent on the preceding forced wetting phase. The various criteria used to assess wetting performance usually span both of these test phases. The more recent industrial soldering applications (e.g., attaching ball grid array components to printed circuit boards, etc.) utilize

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deposited solder paste and subsequent reflow. The nature of solder wetting in processes designed for these newer technologies is almost always spontaneous.

E.

Sessile Drop Spreading Dynamics

The chapter authors feel that the sessile drop technique represents a more precise and versatile tool for quantifying wetting, particularly in high-temperature systems where environmental control is imperative. For solderability assessment, the technique involves placing a solder alloy on a preheated substrate either as a molten (liquid) drop or as a solid mass that subsequently melts. Wetting in the sessile drop method is spontaneous rather than forced. Therefore, it shares a strong similarity with the conditions that prevail in many commercial soldering processes. A liquid mass in the form of a sessile drop on a flat, horizontal substrate changes shape by spontaneously spreading over the substrate. The attending fluid motion is unsteady and is driven by gravity, liquid–solid, solid–vapor, and liquid–vapor interactions so that the system experiences a decrease in its total free energy. As the drop spreads from its initial rest configuration, there is a partial transfer of the initial free energy into kinetic energy that is subsequently dissipated by viscous action and appears as heat energy within the liquid. The heat energy ultimately is transferred to the surroundings that are assumed to have an effectively infinite heat capacity relative to the mass of the drop. 1. Inert vs. Reactive Systems In inert systems, spreading terminates in a minimum energy state marked by mechanical and thermal equilibrium. Equilibrium is typically achieved on the order of seconds for low-viscosity liquids such as liquid metals. However, in reactive systems, spreading terminates in a minimum energy state marked by mechanical, thermal, and chemical equilibria. Attainment of chemical equilibrium in metal–metal systems can take dramatically longer than mechanical and thermal equilibria. Chemical equilibrium is marked by the equilibration of chemical potentials of a given species among all phases within the system. 2. Global vs. Local Equilibrium Warren et al. [28] estimate that a Sn–Bi alloy spreading on pure Bi takes on the order of 108 sec (over 3 years) to achieve chemical equilibrium. Their estimate is for global equilibrium or equilibrium that exists over the entire thermodynamic system. However, local equilibrium is assumed to exist at specific locations (e.g., interfaces, etc.) in thermodynamic systems when global equilibrium does not exist. The concept of local equilibrium is almost essential if any progress is to be made in applying equilibrium thermodynamics to transport problems. The use of the terms transport and equilibrium may seem an oxymoron. However, the criteria for local equilibrium are often approximately met in dynamic processes that are globally not in equilibrium. Global chemical equilibrium in the molten state is assuredly not realized at joining sites in commercial soldering processes because the time spent in the molten state is far too short. Total equilibrium must then be achieved by solid-state diffusion and reaction. 3. Time Duration Measurements of sessile drop spreading are generally made from the instant of drop contact to some point in the spreading when quasi-equilibrium is achieved. The alloy is typically placed on a preheated substrate as a solid sphere, immediately following which sphere undergoes phase change from a solid to a molten state. The time for complete melting may be estimated by calculating the time for thermal diffusion to occur across the sphere diameter tTHfd2/DT, where d is the diameter of the sphere and DT is the thermal diffusivity of the solid alloy. For millimetersized metallic spheres, tTH fO (102) sec. 4. Bond Number The role of gravity in commercial soldering processes for modern microelectronics technology is generally not significant. Because of characteristically small length scales of solder joints and the

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high surface tensions of liquid metals, gravity generally has a negligible effect on the final shape of a solder joint. Consequently, solder wetting studies should be designed so the effects of gravity are negligible. An estimate of the relative importance of gravity and surface tension in joint shape is found in the value of the Bond number Bo given by Bo=L2/(r/qg), where L is a characteristic length scale of the joint (e.g., pad diameter, etc.), r and q are the surface tension and density, respectively, of the liquid alloy, and g is the acceleration due to gravity. When Bo<<1, gravity has a negligible influence or, conversely, surface tension effects play a dominant role. In this case, the joint shape minimizes its surface area to achieve minimum energy. For a typical pad size of 3.81102 cm in diameter, using values for the physicochemical properties of Sn (r=560 dyn/cm, q=6.98 g/cm3) and with g=981 cm/sec2, an estimate for the Bond number is Bo=1.8102. Clearly, gravity has a minimal effect on the final joint shape in the molten state and this shape is nominally unaltered subsequent to solidification. To ensure that gravity plays a negligible role in drop spreading where a characteristic length is LfV1/3 (V is the drop volume), it is important to employ drops of small initial volume. 5. Primary Spreading The description of drop spreading in terms of overall system energy minimization has a mechanical analog. a. Surface Curvature. When a drop spreads on a surface, there are typically variations in the curvature of the L/V surface. A nonzero surface curvature leads to a jump in pressure Dp across the interface given by Laplace’s equation Dp=2rLVj, where j is the mean curvature of the surface. The mean curvature is defined as ju1/2[(1/R1)+(1/R2)], where R1 and R2 are the principal radii of curvature. If both radii are drawn in the liquid, the L/V surface is concave toward the liquid and the pressure jump is such that the pressure in the liquid exceeds the pressure in the vapor phase as given by Laplace’s equation. Neglecting the hydrostatic variation of pressure, the variation of pressure over the surface of the drop will introduce pressure gradients within the drop. These capillary pressure gradients drive flow from regions of high pressure to regions of low pressure. b. Push and Pull Concept. The importance of capillary pressures in droplet spreading was elucidated by Rosenblat and Davis [29]. They note that if the initial configuration of the drop (Fig. 9) was such that the contact angle was equal to its equilibrium value hE but the L/V interface was not an equilibrium shape (spherical cap), then the capillary pressures associated with varying curvature of the surface would introduce a viscous flow, which would ultimately change h from hE according to Eq. (13) in Sec. II.G.1. They describe this mechanism of liquid motion as capillary push. Flow would cease only when the surface took the form of a spherical cap and h=hE again. Alternatively, if the L/V interface of a drop (Fig. 9) had the form of a spherical cap but, say, h>hE, then the contact line would move outward until h=hE while the L/V interface maintained a spherical cap shape. They describe this mechanism as contact line pull. The flow driven

FIG. 9 Sessile drop configurations corresponding to capillary push and contact line pull mechanisms.

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by capillarity toward the contact line in either case is resisted by the action of viscosity. The streamline pattern that results (for an inert system) is shown in Fig. 13d. Either of these mechanisms induces flow referred to as primary spreading, which is typically characterized by a rapid increase in the wetted radius R with time s. Primary spreading is associated with the period of fastest contact line speed during drop spreading. 6. Marangoni Flows Several weaker fluid motions can be present during primary spreading that become apparent and sometimes significant only after motion associated with the primary wetting period has considerably weakened [8,18,30]. One class of motions is referred to as thermal and solutal capillary motions or Marangoni motions, and derives from the fact that the L/V surface tension is a function of the temperature T and concentration C, rLV=rLV(T,C). a. Thermal Marangoni Effects. Consider, for example, a drop spreading on a substrate that is warmer than the ambient atmosphere. In this case, the L/V interface at the contact line is warmer than the L/V interface at the apex of the drop. Because BrLV/BT<0 for most L/V systems, where T is the temperature of the surface, the surface tension at the drop contact line is less than the surface tension at the drop apex and fluid flows along the surface from the contact line to the apex (Fig. 10). The viscous traction across an L/V interface induces motion in the bulk of the drop. The net effect is to transport fluid from the contact line region to the apex region, in a direction opposing the flow associated with the primary flow. The Marangoni flow is typically weaker than the flow associated with primary spreading, but when the primary flow grows less strong, the Marangoni flow can compete with it. In the preceding temperature scenario, the Marangoni flow opposes the primary flow and retards wetting. The same line of reasoning applies to the converse case, that where the substrate is cooler than the ambient atmosphere. Here the thermal Marangoni flow is from the apex to the contact line, tending to reinforce the primary flow and promote wetting. Thermal Marangoni flows can be sufficiently robust to cause partially wetting systems to completely spread, and perfectly wetting systems to spread incompletely [18]. b. Solute Marangoni Effects. Similar surface-driven motions can occur when the concentration varies on the surface of a drop [30]. Consider the case when a liquid metal dissolves the substrate on which it is spreading (Fig. 11). Although the streamline pattern shown in Fig. 13d will be changed due to the erosion of the lower boundary, the essential character of the flow remains the same (i.e., flow downward from the apex and outward along the dissolving interface toward the contact line). Solute from the dissolved boundary is swept toward the contact line

FIG. 10 Diagram illustrating thermal Marangoni effect: (a) retarded spreading due to opposing Marangoni flow when substrate temperature is greater than ambient temperature; (b) enhanced spreading due to reinforcing Marangoni flow when substrate temperature is less than ambient temperature.

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FIG. 11 Micrograph of a metallographic cross section of a pure Sn drop spreading on pure Bi at 250jC. The etched region represents the total dissolved volume of Bi, bounded below by the S/L interface and above by the L/V interface. It is not possible to determine from this micrograph whether the S/L boundary intersects (tracks) the contact line.

where it accumulates. Thus, a higher concentration of solute exists in the contact line region than at the apex region. Because substrate terminal pad metals generally possess a larger surface tension than solder alloys, the accumulation leads to an increase in the surface tension at the contact line, whereas the surface tension at the apex remains roughly that of the original alloy. A Marangoni flow is again induced from the apex toward the contact line region, leading to spreading enhancement. 7. Effect of Dissolution on Contact Angle The spreading of liquid metals on metal substrates is always reactive in the sense that there is always nonzero solubility, however small, of substrate metals in liquid metals or some amount of product formation between constituents in the liquid alloy and a substrate metal. In many cases, both solubility and product formation occur in a given metal–metal system. Visually, there is no obvious difference between spreading in inert systems and spreading in systems that manifest only nonzero solubility. There is also a general similarity between the shape of the wetting kinetic curves, R vs. s, and h vs. s, for these two cases. Fig. 12 shows both kinetic curves for initially pure Sn spreading on pure Bi, a binary system that exhibits dissolution but no compound formation. Several processes that affect the dynamic behavior of the contact line occur simultaneously. The flux of Bi atoms dissolving into the molten Sn causes both R and h to change with time, but the

FIG. 12 The effect of time s on both (a, top view) wetted radius R and (b, side view) dynamic contact angle hD for pure Sn spreading on pure Bi at 250jC (initial solid sphere diameter was 0.889 mm).

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size of these effects is not known at this juncture. In this system, there is only a primary wetting regime, and R increases rapidly with time and asymptotes to a quasi-equilibrium value of RE corresponding to the equilibrium between liquid Sn saturated with Bi and solid Bi at 250jC. As noted above, complete equilibrium for the Sn–Bi system has been estimated to take approximately 108 sec, but this equilibrium has little relevance to practical experiments and commercial processes characterized by much shorter timescales. The rapid monotonic increase of wetted radius R to a final constant value in the primary wetting period also typifies spreading in inert systems. The apparent contact angle hD may be seen to decrease monotonically with time. This behavior is consistent with the contact angle behavior shown in Fig. 5. At early times just after melting, the drop attains its maximum speed when presumably it experiences the maximum wetting driving force. As the drop spreads, these forces diminish monotonically with increasing time as does the contact line speed VCL. This corresponds to a leftward traverse in the positive quadrant in Fig. 5 from larger to smaller VCL, and the contact angle hD decreases accordingly. What an experimenter observes (e.g., Fig. 12) in dissolutive systems is the apparent contact angle, i.e., the angle made by the L/V interface with the original plane of the substrate (original planar S/V interface). However, one possible effect of dissolution is to change the actual contact angle (as strictly defined in Sec. 2.1) by changing the geometry at the contact line (Fig. 11) and making the actual contact angle larger at any instant. However, the observer cannot discern such changes of the S/L interface and observes a relatively normal evolution of the contact angle history. For the apparent contact angle to be the actual contact angle, the S/L boundary as it is shaped by dissolution must track with the contact line, and the S/L interface must orient parallel to the original (undissolved) S/V interface. In situ observations of dynamic contact angle evolution in dissolutive systems utilizing specially designed wetting flow cells will help to determine if this is the case [31]. 8. Effect of Compound Formation in Dissolutive Systems Unlike pure dissolutive systems, those that undergo dissolution and that also exhibit compound formation often exhibit visual signs of the occurrence of compound formation. Fig. 13 shows

FIG. 13 Pure Sn spreading on polished pure Au at 250jC. Inset pictures show (a) side and (b) top views of the spreading drop keyed with the wetted radius data on the plot. (c) Later time data plotted as R2 vs. s suggesting that spreading in this time interval is controlled by diffusion. (d) Computed streamline pattern in a spreading drop on a planar substrate (initial solid sphere diameter was 0.889 mm). (From Ref. 24.)

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spreading of initially pure Sn on pure Au at 250jC. Early in the process, the system exhibits a primary spreading regime in which the spreading kinetics is qualitatively similar to those described above for purely dissolutive systems, and R appears to be approaching an equilibrium value (see point B in Fig. 13), just as in Fig. 12. However, the primary spreading regime transitions (between points B and C) into a secondary spreading regime (between points C and E) that is characterized by a much slower increase in R with time than in primary spreading. It also terminates in a larger (by approximately 20%) equilibrium value RE than that suggested by primary spreading behavior. Similarly, the value of h asymptotically approaches a different equilibrium value of hE. The transition from the primary to the secondary spreading regime seems unremarkable, but close observation of the contact line region between the two regimes (points B and C) reveals the appearance of a film emanating from the contact line. The precise nature of this film is unknown but is discussed in Sec. V.B. The film presents a major impediment to the measurement of the contact angle, which resides at the tip of the film and which cannot be resolved with acceptable accuracy utilizing optical microscopy. 9. A Remark on Systems Exhibiting Primary and Secondary Wetting One interpretation of systems that exhibit primary and secondary wetting regimes is that the primary spreading occurs nominally without physicochemical interaction with the S/L boundary (similar to inert wetting) and approaches a quasi-equilibrium in which hD!hQE, where hQE represents a quasi-Young equilibrium contact angle characterized by the approximately pure molten metal in contact with the pure solid metal. This ‘‘quasi-Young’’ or primary spreading takes O (101–100) sec in metallic systems. The approach to quasi-equilibrium is subsequently altered by physicochemical interaction with the substrate (e.g., the dissolution of the solid substrate, the transport of solute into the molten phase, and the formation of reaction products). As these phenomena become robust, the transition from primary spreading behavior to secondary spreading behavior occurs, and the system dynamics is dominated by physicochemical interactions. This secondary spreading takes O (102–103) sec. In summary, metal–metal systems spread in very short times as if they are inert, whereas their spreading behavior is reflected by their true reactive nature in longer times.

IV.

MATERIAL PROPERTIES

A.

Introduction

The solution to the reactive wetting problem is intimately coupled to the mass transport occurring in the liquid domain. Thus, in the fluid dynamic study of the reactive wetting problem, it is necessary to understand how liquid viscosity, density, and surface tension can change during spreading. In addition, the diffusivity of solute species within the liquid is useful in estimating the magnitude of diffusive transport relative to convective transport during the spreading process. A brief discussion of the theoretical or empirical descriptions of liquid alloy properties is presented below because many of the emerging alloys have not yet been fully characterized experimentally. Empirical data for pure elements and some alloys are also given. In compiling this section, the liquid metal works of Iida and Guthrie [32] and Eustathopoulos et al. [12] have been used. In addition, the reader may find useful the following general references for metal properties: Smithells [33], Hultgren [34], and Murr [35].

B.

Density

Density is defined as the mass per unit volume of a substance. For the present work, it is desirable to have a model from which the density of an alloy mixture can be computed using the (pure) properties of its constituents. Consider first an alloy system where the ‘‘actual’’ molar volume for the mixture VA is defined as [32]: VA ¼

x1 M1 þ x2 M2 þ : : : þ xi Mi qA

ð21Þ

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353

where xi and Mi are the atomic fractions and the atomic weights of component i, respectively, and qA is the actual alloy density. Unfortunately, in many instances, actual alloy data may not be available and an estimate for the density will be needed. In general, for multicomponent systems, the atomic volume may be approximated by: X ð22Þ Vc xi Vi where xi is the atomic fraction for each component i, and where the atomic volumes Vi of the individual species are given as: Vi ¼

Mi qi

ð23Þ

in which Mi and qi are the atomic weight and the density of the species i, respectively. For a twocomponent system:   x1 M1 x2 M2 Vcx1 V1 þ x2 V2 ; or Vc þ ð24Þ q1 q2 Often, the excess volume VE is used in the study of real alloy systems:   x1 M1 þ x2 M2 x1 M1 x2 M2  þ VE ¼ VA  Videal ; VE ¼ qA q1 q2

ð25Þ

Iida and Guthrie [32] state that in a simplistic way, a negative VE value indicates attractive interaction between dissimilar kinds of atoms. In other words, alloy systems showing negative volume changes on mixing have larger mutual, as opposed to self-interaction energies. Negative VE values have been observed for compound-forming alloy systems (e.g., systems typically used in microelectronics soldering). The liquid density data for some of the pure elements of interest in soldering applications are given in Table 2. The density and molar-specific volume at a given temperature are computed using the following expression: q ¼ qm þ KðT  Tm Þ

ð26Þ

V ¼ Vm f1 þ aðT  Tm Þg

ð27Þ

where Tm is the melting temperature and the subscript ‘‘m’’ indicates the value of the property at Tm. The reader is referred to Table 3 for a selection of references, which give the liquid density for TABLE 2

Element Ag Au Bi Cu In Pb Ni Sn Sb Zn

Liquid Density Data for Selected Elements Routinely Used in Electronics Applications

Melting temperature, Tm (jC)

Density at Tm, qm (103 kg/m3)

Density temperature coefficient, K (u(Bq/BT)p) (101 kg/(m3K))

960.65 1063.15 271.1 1083.1 156.55 327.55 1728.15 231.99 121.7 419.62

9.33 17.4 10.5 8.00 7.03 10.67 7.90 6.98 6.48 6.58

9.1 12/17 11.8 8.0 6.8 13.2 11.9 6.1 8.2 9.8

Source: Refs. 32 and 36.

Molar-specific volume at Tm, Vm (106 m3/gatom)

Molar-specific volume temperature coefficient, am (uK/qm) (104 K1)

11.6 11.3 20.80 7.94 16.3 19.42 7.43 17.0 18.8 9.94

0.98 0.69/0.98 1.17 1.0 0.97 1.24 1.51 0.87 1.3 1.5

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TABLE 3

Reference Summary for Liquid Alloy Density Data

Material system Sn–Pb alloys Bi–Sn alloys Ag–Sn eutectic with Bi additions Ag–In

Range of composition (molar fraction) XSn=0.0, 0.30, 0.43, 0.60, 0.73, 1.0 XBi=0.2, 0.4, 0.6, 0.8 (3.8 at.% Ag–Sn) with 0.03, 0.06, 0.09, and 0.12 molar fractions of Bi XIn=0.0, 0.20, 0.35, 0.45, 0.65, 0.80, 1.0

Temperature range (jC)

Reference

236–583 227–877 —

Schwaneke et al. [37] Moser et al. [38] Moser et al. [38]

—

Moser et al. [39]

Sn–Pb and various Pb-free alloys. Depending upon the alloy, the density varies between 7 and 10 g/cm3. In these references, the density values for binary alloys were reported to decrease linearly with increasing temperature. For example, from the experiments of Schwaneke et al. [37], the empirical fits for density in grams per cubic centimeters obtained for pure Sn and 60.4Sn/Pb (wt.%) were 7.16–6.3(104)T and 8.59–7.3(104)T, respectively, where T is temperature in degrees Celsius.

C.

Surface Tension

For the spreading problem, the liquid–vapor rLV, the solid–liquid rSL, and the solid–vapor rSV surface energies are of importance. As noted in Sec. II.A, the use of a mathematical surface to separate a condensed phase and its vapor idealizes the actual three-dimensional nature of the transition region. The surface is in a higher energy state than the bulk liquid phase because coordination among the atoms at the surface is incomplete. This higher-energy state endows the interfacial region with a surface energy that is attributed wholly to the separating surface. An extensive discussion on the topic of surface tension in the context of the spreading problem is given by Eustathopoulos et al. [12]. The surface tension of a liquid metal alloy is dependent upon temperature, liquid metal composition, and the adjacent gaseous or liquid flux composition [40]. The manner in which the surface tension can vary with these parameters is given next followed by a discussion of the solid–liquid and solid–vapor surface energies. 1. Effect of Temperature The dependence of the surface tension on temperature is largely influenced by the fact that the interface between the liquid phase and the gas phase disappears at the critical temperature (i.e., the temperature above which the liquid state no longer exists regardless of pressure). Thus, the surface tension of liquids is reduced to zero at this temperature. Consequently, the surface tension of pure liquids must decrease with increasing temperature. The surface tension data for the elements most commonly used in solder alloys are given in Table 4, where the expression: rðTÞ ¼ rm þ

Br ðT  Tm Þ BT

ð28Þ

is used to calculate the surface tension at temperature T based on its value at the melting temperature Tm. As can be seen from the table, there is greater variation in the reported values of surface tension for elements having higher melting temperatures. 2. Surface Tension of Mixtures (Alloys) The surface tensions of most binary liquid mixtures exhibit negative deviations from the proportional mathematical addition of the pure components’ surface tensions. This is often a result of the surface of the liquid becoming enriched with the component of lower surface tension [32]. In other words, the surface tension of a 50Sn–50Pb mixture would be less than the weighted average of pure Sn and pure Pb surface tensions (0.5565 mN/m+0.5460 mN/m) due to the

SOLDER WETTING AND SPREADING TABLE 4

355

Liquid Surface Tension Data for Selected Elements

Element

Surface tension at the melting temperature, rm (mN/m)

Surface tension temperature coefficient, Br/BT (mN/m/K)

910 996a 1138 1169a 374 378b 1355 1303a 555 556b 1838 1778b 462 458a 570 560a 366 367b 815 782b

0.17 0.19a 0.19 0.25a 0.08 0.07b 0.19 0.23a 0.12 0.09b 0.42 0.38b 0.11 0.13a 0.11 0.09a 0.04 0.05b 0.25 0.17b

Ag Au Bi Cu In Ni Pb Sn Sb Zn

Source: (a) Ref. 41; (b) Ref. 42. All other data are from a compilation in Ref. 12.

tendency of Pb to segregate to the surface. Lead (Pb) segregation during the spreading of Sn–Pb liquid on Cu was observed using in situ Auger measurements [8]. However, recent resonant x-ray reflectivity measurements from the surface of liquid Bi–In found only a modest surface Bi enrichment, with 35 at.% Bi in the first atomic layer [43]. The author states that this observation is in contrast to many adsorption studies to date, which often exhibit surface segregation of a complete monolayer of the component with the lower surface tension. DiMasi et al. [43] suggest that surface adsorption in Bi–In is dominated by attractive interactions that increase the number of Bi–In neighbors at the surface. a. Theoretical Surface Tension Models. At this point, it is desirable to use the surface tension data from pure elements to predict the surface tension of alloy mixtures. Butler [44] extended the Gibbs treatment of the surface tension of mixtures, deriving the following equations for a liquid alloy mixture: rM ¼ r1 þ

RT as1 ln ; A1 a1

rM ¼ r2 þ

RT as2 ln ; . . . ; A2 a2

rM ¼ rn þ

RT asn ln An an

ð29Þ

Here, rM is the surface tension of the mixture having components 1,2,. . .,n; A1,A2,. . .,An are the respective areas occupied in the monolayers (i.e., the molar surface areas); a1,a2,. . .,an are the bulk mole fraction activities of the components; and as1,as2,. . ., asn are the activities of the components in the surface monolayer. This equation is based on the fundamental assumption that a difference in composition of the surface compared to the bulk occurs in the first monolayer of molecules. Butler’s equations have been used to compute the surface tension for Bi–Sn, Ag–Sn [45,46], In–

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Sn, Cu–Sn [45], and Ag–In [39] binary alloys. If an ideal binary mixture is assumed, the activities can be replaced with the molar fractions x1 and x2 and Eq. (29) can be written as: rM ¼ r1 þ

RT xs1 ln A1 x1

ð30Þ

rM ¼ r2 þ

RT xs2 ln A2 x2

ð31Þ

Unfortunately, the aforementioned equations and others (see Refs. 47 and 48) cannot be universally applied and experimental measurement of surface tension is required. For instance, the surface tension of liquid alloys can have local maxima and minima with changes in composition and temperature as seen in the Au–Sn system shown in Fig. 14. 3. Experimental Liquid Surface Tension Data When utilizing the surface tensions given in Sec. IV.C.4, the reader is cautioned that a liquid metal’s surface tension can also change with the presence of surface-active contaminants. Liquid metals are highly reactive, especially with oxygen, and the formation of oxides on the surface tends to lower the measured surface tension. For the case of pure Sn, the surface tension decreases from 0.510 to 0.460 N/m at 800jC [49] as the partial pressure of oxygen is increased. In addition, when the liquid metal is spreading in the presence of a liquid flux, the liquid–vapor surface tension should be replaced by the liquid flux rLF surface tension, which tends to have a magnitude lower than rLV [40]. In addition, the solid–vapor surface energy should be replaced with the solid flux surface energy rSF, which is thought to have a lower magnitude than rSV, but data on this topic are lacking. In most cases, when pure liquid Sn is alloyed with an element having lower surface tension than Sn, the surface tension of the resulting mixture is reduced and the converse is also true. A selection of references for solder alloy surface tension is given in Table 5. It has long been known that the addition of Pb to Sn results in a reduction in surface tension. From the data developed by Schwaneke et al. [37], the addition of 39 wt.% Pb to Sn results in a change in the surface tension magnitude from 0.555 N/m (pure Sn) to 0.513 N/m at T=250jC. In addition, the surface tension of mixtures could be reduced (or increased) by adding elements having surface tensions lower (or higher) than the original mixture. Carroll and Warwick [52] found that the addition of Bi and Sb to 40 wt.% Pb–Sn solder reduced the surface tension, whereas the addition of Ag and Cu increased the solder surface tension. Similarly, Moser et al.

FIG. 14 Surface tension of Au–Sn alloys. (Reprinted with permission from Ref. 50.)

SOLDER WETTING AND SPREADING TABLE 5

Liquid Alloy Surface Tension References

Material system Sn–Pb

Range of composition (molar fraction, unless otherwise noted)

Bi–Sn

Pure Sn; 39, 53.4, and 69.5 wt.% Pb–Sn Pure Pb 40Pb–60Sn wt.% with Bi, Sb, P, Ag, and Cu additions XSn=0.0, 0.3, 0.4, 0.5, 0.65, 0.75, 0.9,1.0 XBi=0.2, 0.4, 0.6, 0.8

Cu–Sn In–Sn Ag–Sn

97–99 wt.% Sn–Cu XSn=0.0–1.0 XSn=96

Ag–Sn eutectic with Bi

(3.8% Ag–Sn) with 0.03, 0.06, 0.09, and 0.12 molar fractions of Bi XIn=0.0, 0.20, 0.35, 0.45, 0.65, 0.80, 1.0

Sn–Pb

Au–Sn

Ag–In

357

Temperature range (jC) 236–583

Reference Schwaneke et al. [37]

215 and 250

White [51] Carroll and Warwick [52]

500–1150

Kaufman and Whalen [50]

227–877

Moser et al. [38], Lee and Lee [46] Park et al. [45] Lee and Lee [46] Lee and Lee [46]

— 250 250–600 (calculated) —

—

Moser et al. [38]

Moser et al. [39], Park et al. [45]

[38] determined that the addition of Bi to Ag–Sn eutectic resulted in a surface tension reduction. Thus, the liquid alloy surface tension can either be reduced or increased depending upon the element being added. Over the years, considerable work has been performed on the surface tension of Sn–Pb alloys and, in recent years, more data have become available for the emerging Pb-free alloys. 4. Surface Energies at Solid–Liquid and Solid–Vapor Interfaces In addition to the liquid–vapor surface tension, wetting analysis requires an understanding of the surface energies at the solid/liquid and solid/vapor interfaces rSL and rSV, respectively. Note that when solid surfaces are discussed, there is a distinction between surface energy and surface tension. For a solid, these terms are not interchangeable because strain energy is developed when a solid surface is deformed [12]. The rSL and rSV data for metal systems relevant to soldering are limited. The sessile drop, frequently used to evaluate rSL, only gives the difference rSVrSL, and independent rSV measurements are still required to determine rSL. At the solid/liquid interface, the density change across the interface is not as great as the density difference across either the solid/vapor or the liquid/vapor interface. Thus, the value of rSL for a given metal is generally lower than either rSV or rLV [12]. The magnitude of surface energy for the solid/liquid interface is usually on the order of 101 J/m2. Further complicating the determination of rLV in solder alloy systems is the possibility that reaction products (intermetallic compounds) are forming at the solid/liquid interface. The rSV magnitudes are generally much greater than the rLV magnitudes. The rSV surface energies for some common substrate metallizations are given in Table 6. The measurement of rSV is made at very high temperatures (T=0.9Tm) relative to most soldering processes. These high temperatures are problematical in the case where one is trying to determine the solid flux surface energy rSF instead of rSV because most liquid solder fluxes decompose at these temperatures. Using arguments similar to those made when the solid–liquid surface energy was discussed [12], it is expected that rSF would be less than rSV because the density difference between the solid and

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TABLE 6

Solid–Vapor Surface Energies for Selected Elements Utilized in Electronics Applications

Element

Temperature (jC)

Solid–vapor surface energy, rSV (mJ/m2)

Reference

950 1000 Near Tm 239 927 Near Tm 1250 1547

1100 1366 1400 457 1568 1650 2072 1500

Murr [35] Russell and Aaronson [53] Eustathopoulos and Joud [54] Russell and Aaronson [53] Russell and Aaronson [53] Eustathopoulos and Joud [54] Russell and Aaronson [53] Murr [35]

Ag Au Bi Cu Ni Pd

the liquid flux is less than the density difference between the solid and the vapor. This same argument seems to be consistent with the fact that the liquid flux surface tension is less than the liquid–vapor surface tension [40]. Because adsorption of species onto the solid surface can also influence surface energies, more data are required to determine how much rSF differs from rSV.

D.

Viscosity

Viscosity is used to describe the momentum transport within a moving fluid. The viscosity of liquid metals is discussed in detail by Iida and Guthrie [32] and Poirier and Geiger [55]. The coefficient of viscosity l for the laminar flow of a Newtonian fluid is defined by: sxy ¼ l

du dy

ð32Þ

where syx is the shearing stress and du/dy is the fluid velocity gradient perpendicular to the shearing stress. Several expressions have been developed for the viscosity of liquid metals [32,55– 57], which are generally based on the idea that momentum is transferred from one layer of atoms to an adjacent neighboring layer of atoms. 1. Temperature Dependence of Viscosity The Andradre [56] equation for the viscosity lm of simple monotomic liquids at the melting point is: 1

lm ¼ 1:6  104

ðMTm Þ 2 2

½mPa sec

ð33Þ

Vm3

where M is the molecular weight and Vm is the atomic volume at melting. An expression for the temperature variation of viscosity was also given by Andradre as:
c 1 ð34Þ lv 3 ¼ Aexp vT where A and c are constants and v is the specific volume. The Andrade equation provides good correlation to experimental viscosity data for many systems. However, there has been no rigorous theoretical explanation that would account for the success of this simple equation [32]. The temperature dependence of viscosity in liquid metals can also be approximated by an empirically based, Arrhenius-type relationship:   Hl l ¼ Aexp ð35Þ RT

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359

where A and Hl are constants, R is the ideal gas constant, and T is the absolute temperature. The parameter A in terms of well-known parameters [58] is given as: 1

5:7  102 ðMTm Þ 2
 ½mPa sec A¼ 2 H Vm3 exp RTlm

ð36Þ

where Tm is the melting temperature, Vm is the atomic volume at the melting point, and M is the atomic weight. An empirical relation for the apparent activation energy Hl [59] is given as 1.2 Hl=5.06T1.2 m [kJ/mol] for normal metals (Au, Ag, Cu, and Pb) and Hl=3.14Tm [kJ/mol] for semimetals (In, Sn, Bi, and Sb). These expressions correlate well to the experimental data for Bi, In, Pb, Sb, Sn, and Zn among others [59]. a. Melting Point Viscosity of Some Pure Metals. The viscosity data for some pure elements used in solder alloys are given in Table 7. Generally, liquid metal viscosity data are not well established and the accuracy of experimentally measured liquid metal viscosities is estimated to be between F1% and F20% except for a few metal systems [32]. 2. Viscosity of Alloys It is desirable to use the data from the pure elements to predict the viscosity of alloys. As yet, no theoretical formula provides reliable or precise data for the viscosities of liquid alloys because little is known of the interaction of dissimilar atoms. Using empirically based equations similar to Eqs. (34) and (35) in combination with thermodynamic data [62,63], Lee and Lee [46] determined the viscosities for the Ag–Sn system at T=1000jC (presumably a high temperature was required in order to allow evaluation of alloys having a fairly large Ag fraction) and for the Bi–Sn system at T=400jC, but had less success predicting the viscosity for the In–Sn system at T=300jC. Lee and Lee [46] also computed the viscosity for the Sn–Ag–Bi and Sn–Ag–Cu ternary systems. Experimental data for the viscosity of binary liquid alloys sometimes exhibit anomalous viscosity changes at compositions corresponding to the eutectics and limits of solid solubility [55]. Viscosities for liquid alloys at compound-forming compositions and eutectic compositions relevant to solder alloys are given in Table 8. Note that the viscosity of the 80Au–20Sn alloy is approximately 50% lower than pure Sn, whereas the viscosity of the 10Au–90Sn alloy is higher than pure Sn. Thresh and Crawley [65] also measured the viscosity for a broad range of Sn–Pb compositions and temperatures. In the temperature range relevant to soldering, more viscosity data are still required for the emerging Pb-free alloys to support solder spreading modeling efforts.

TABLE 7 Viscosities at the Melting Point, Tm, for Selected Elements Utilized in Solder Alloys Element Ag Au Bi Cu In Ni Pb Sn Sb Zn

Viscosity, l (mPa sec) 4.28 5.38 1.63 4.34 1.80 4.5–6.4 2.61 1.81 1.43 3.50

Reference Iida et al. [59] Gebhardt and Wo¨rwag [60] Iida et al. [59] Iida et al. [59] Iida et al. [59] Iida and Guthrie [32] Iida et al. [59] Iida et al. [59] Iida et al. [59] Iida et al. [61]

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TABLE 8

Viscosities at the Melting Point of Selected Liquid Alloys at Eutectic Compositions

Eutectic composition

Viscosity, l (mPa sec)

Reference

0.90 2.31 2.67 3.16 1.86 1.50

Gebhardt et al. [64] Gebhardt et al. [64] Thresh and Crawley [65] Gebhardt and Ko¨stlin [67] Crawley [68] Bodakin et al. [69] in Seetharaman and Sichen [62]

80Au–20Sn 10Au–90Sn 63Sn–37Pb 91.2Sn–8.8Zn 57Sn–43Bi

E.

Diffusivity

Diffusion is the macroscopical transport of a given species from one region to another due to interactions on the atomic scale. In the presence of a concentration gradient, there will be diffusion of species from regions of high concentration to low concentration. The diffusive flux J is defined as the amount of matter transported per unit time through a unit area perpendicular to the concentration gradient. In one-dimensional diffusion, the magnitude of the diffusive flux is defined by Fick’s first law as: J ¼ D

Bc Bx

ð37Þ

where the partial derivative of the concentration c with respect to position x is the concentration gradient of diffusing particles in the direction of diffusion, and D is the diffusion coefficient. Because the positive flow of diffusing particles is in the direction of decreasing concentration gradient, a negative sign is required in the expression to make the diffusivity positive. Generally, diffusivities in the liquid state (Df109 m2/sec) are much faster than the solid state (Df1011– 1014 m2/sec) due to the high atomic mobility of the liquid state compared to the solid state. In liquid metals, diffusivity increases with increasing temperature and this is often approximated using the following Arrhenius-type relation:   HD D ¼ Do exp RT

ð38Þ

where Do and the activation energy HD are constants, R is the gas constant, and T is the absolute temperature. The values of Do and HD for various solutes in liquid Sn are given in Table 9.

TABLE 9 Solute Diffusion Parameters for Bi, Cu, Ni, and Zn in Liquid Sn Solute

Diffusion coefficient, Do (m2/sec)

Bi Cu Ni Zn Source: Ref. 70.

1.3 1.8 2.3 6.2

   

107 107 107 107

Activation energy, HD (kJ/mol) 20.92 17.57 18.95 20.42

SOLDER WETTING AND SPREADING TABLE 10

361

Solute Diffusivities in Liquid Sn

Solute Ag Au Sb

Constant, A (cm2/(sec K))

Diffusivity at the melting point, Dm (cm2/sec)

Temperature range (K)

17.2  108 17.7  108 10.6  108

2.07  105 1.30  105 2.23  105

621–1853 693–1853 589–1705

Source: Ref. 71.

The Arrhenius relationship does not always yield the best fit to diffusivity data. Bruson and Gerl [71] found a linear temperature dependence for the diffusivity of Sb, Ag, and Au solutes in liquid Sn that follows the relationship: D ¼ AðT  Tm Þ þ Dm

ð39Þ

where T is the temperature of interest [jC], Tm is the melting temperature [jC], A is a constant, and Dm is the diffusivity at the Sn melting point (Table 10). Suggested useful liquid binary alloy diffusion references are Roy and Chhabra [72] and Cahoon [73]. Diffusive mass transport is important in many soldering processes, especially when solder flow velocity magnitudes are small (f104 cm/sec) and the resulting convective mass transport is minimal.

V.

DYNAMICS OF WETTING

A.

Introduction

In metal–metal wetting systems, temperature and substrate solubility (in the liquid) affect the wetting and spreading behavior of the liquid metal. Material properties (density, viscosity, and surface tension), diffusive transport, and chemical reactions are all affected by temperature. Furthermore, the manner in which temperature affects the wetting behavior of metal–metal systems cannot be discussed without considering the liquid compositional changes accompanying substrate dissolution. Systems where substrate solubility in the liquid phase is large typically exhibit extensive substrate dissolution during spreading. Generally, metal–metal systems exhibit increased wetting with increased temperature. Unfortunately, increasing the temperature cannot always be used to improve wetting because increased temperature also: (a) increases oxidation kinetics, making soldering more difficult; (b) increases reaction kinetics, leading to thicker intermetallic compound layers that may decrease joint strength; and (c) often conflicts with temperature limits imposed by other parts of a microelectronic assembly. Thus, there is usually a tradeoff between higher-temperature processing and overall reliability of a completed assembly.

B.

Effect of Temperature

Typically increasing the temperature decreases liquid viscosity and decreases liquid surface tension, which promotes spreading. Increasing the temperature also increases species diffusivities, chemical reaction rates, and substrate solubility in the liquid metal. However, there are exceptions. In some instances, the surface tension of liquid mixtures occasionally has been observed to increase, rather than decrease, with increasing temperature [74]. Also, because temperature influences substrate solubility, the change of surface tension with resulting change in liquid composition may be important. Given the complex interaction between a liquid, a solid, and a flux (or gas) phase, an experimental assessment of spreading is usually required. Because high Sn content characterizes the overwhelming majority of emerging Pb-free solders and is the primary reactive element in the solders, it is expected that the spreading behavior of pure Sn will provide insight into the expected spreading behavior of high-Sn content solders. Utilizing the sessile drop technique, Meschter [24] and Meschter et al. [75,76] evaluated

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FIG. 15 The effect of temperature on the contact line morphology and final wetted radius for Sn spreading on 1-Am Au coating on 7-Am Cu (initial solid sphere diameter was 0.889 mm). (From Ref. 24.)

the isothermal spreading behavior of pure Sn on Au, Cu, and Au-coated Cu substrates. Gold coatings are often used to enhance solderabilily and prevent oxidation [77]. Fig. 15 reveals that the contact line morphology of an Sn drop spreading on Au-coated Cu varies considerably with temperature as does the final wetted radius. The contact line morphology is smooth at 250jC and 279jC, becomes more irregular at 338jC, is very irregular and fractal in nature at 388jC, and becomes smooth again at 427jC. The final wetted radius of spread is larger at 279jC than at either 250jC or 338jC. The corresponding kinetic spreading curves are shown in Fig. 16. The spreading rate (indicated by the slope of the R vs. s curve) appears to increase with increasing temperature and the largest spreading rates were observed for the experiments performed at 388jC and 427jC. Examination of the Au–Sn binary phase diagram (Fig. 17) shows that the large wetted radius observed at 427jC is consistent with the large increase in solubility of Au in Sn above 419jC. It is interesting to note that at a temperature of 388jC, a thin film can be observed emanating from a central liquid pool (Fig. 15). The presence of thin

FIG. 16 Parametric plot showing the effect of temperature on the wetted radius R vs. time s for Sn spreading on 1-Am Au coating on 7-Am Cu for 250
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FIG. 17 Au–Sn binary equilibrium phase diagram. (From Ref. 78.)

films must be considered when developing solderability standards. Although thin films yield large areas of spread, there is little volume associated with such films and volume is the critical parameter for robust solder joint formation.

C.

Dissolution

The effects of substrate dissolution become more important as microelectronic components and features become smaller. Barrier layers of low-solubility metals are often intercalated between the coating layer and the base layer to minimize dissolution effects. Nickel is the metal most frequently employed as a barrier metal, but even Ni can dissolve in Sn [79]. Substrate dissolution can alter the surface tension of a liquid metal. All of the substrate metals used in microelectronics soldering (e.g., Cu, Au, Ni, Pd, etc.) have higher surface tension rLV values than pure Sn, which is the primary constituent in Pb-free alloys. The same substrate metals also have higher surface tensions than the Pb–Sn alloys. Thus, dissolution does not contribute a tensioactive (surface tension-reducing) species to the liquid. If the dissolution of the substrate is not extensive, then the value of the solid–liquid surface energy rSL is also not significantly changed. The net effect of the dissolution is then to change the contact angle by changing the geometry at the contact line (Figs. 11 and 18). However, the observer cannot discern such changes of an S/L interface but observes an evolution of the contact angle similar to an inert system. Extensive substrate dissolution is typical of metal–metal systems where there is large solubility of the substrate in the spreading liquid metal. Extensive base metal dissolution is usually not desirable because it requires thicker base metallization, or addition of barrier layers to maintain a reliable interconnect. The dissolution of substrate materials behind an advancing contact line is illustrated schematically in Fig. 18. The solid metal dissolves behind the contact line of the advancing liquid and the possibility exists for intermetallic compounds to form at the L/S interface right up to the contact line. Although equilibrium phase diagrams [80] or metastable phase diagrams [81] are frequently used to determine the maximum concentration of substrates

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FIG. 18 Illustration of an advancing liquid drop on a metal substrate with dissolution and reaction occurring.

that can be dissolved into a liquid phase, such an approach does not include kinetic considerations or the impact of the presence of intermetallic compounds on the dissolution process. In addition, as more constituents are added to a system, the phase behavior becomes increasingly complex. Given that many newly formulated Pb-free alloys are binary, ternary, or quaternary mixtures and that the substrates are often comprised of several species, determining the critical liquid phase composition at which the mixture will undergo transition to a solid phase is a very difficult undertaking. 1. Strong Dissolution—No Reaction Products Case Spreading experiments in the Bi–Sn system, in which strong dissolution is observed but no reaction products are formed, showed that the advance of the contact line was controlled by the dissolution of the substrate into the liquid [28,82]. Fig. 13 shows the results of several isothermal drop spreading experiments in the Au–Sn system at 250jC where dissolution is moderate and intermetallic compound reaction products form. Early in the spreading, the drop spread rapidly and the dissolved substrate material was convected along the S/L boundary toward the contact line region during the primary spreading stage (see streamline pattern in Fig. 13d). Solubility at this temperature is moderate (see Au–Sn phase diagram; Fig. 17) and one can expect moderate dissolution of the substrate before the appearance of either the D-phase (AuSn4) or the q-phase (AuSn2). These phases are expected to form along the S/L boundary, appearing in the vicinity of the contact line due to the strong convection of solute Au to the region during primary spreading. 2. Effect of an Increased Liquid Volume In systems where significant substrate solubility is present, substrate dissolution can substantially increase the liquid volume. From the Au–Sn phase diagram (Fig. 17) at T=427jC, 17 mg of Sn can cause 83 mg of solid Au to be converted to liquid. Substantial substrate dissolution was observed for Sn drop spreading on Au foil after 5790 sec at 427jC, as shown in Fig. 19. The S/L

FIG. 19 (a) Micrograph of a cross section showing the dissolution of a Au foil by Sn spreading at 427jC after 5790 sec. (b) Top view photograph that shows the presence of a well that forms in the central dissolution area early in the spreading process (within 240 sec) (initial solid sphere diameter was 0.889 mm). (From Ref. 24.)

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interface morphology is characterized by a deep central dissolution well, which is connected to the contact line region by a more gently sloped L/S interface. The general shape of the central dissolution well is similar to the L/S interface morphology of the Bi–Sn system [28,82] at later times. The right side of Fig. 19 shows the top view of the spreading drop where the flow from the central dissolution well to the contact line is observed to result in a very thin film 4–10 Am thick. The concentration of this liquid film is estimated to be between 80 and 84 wt.% Au. It is apparent that the liquid film was drawing liquid from the central dissolution well and that the spreading forces were sufficient to cause the liquid level in the center to fall below the surrounding substrate surface. Because this behavior is not observed in the Bi–Sn system and because the Bi–Sn system does not form these films, it is possible that the film formation and the strong film spreading were due to a driving force associated with the formation of the ~-phase (Au5Sn intermetallic compound) that is stable at 427jC. 3. Tin Reaction with Gold-Coated Copper The drop spreading kinetics of Sn on Au-coated copper substrates can be reexamined in light of the high solubility of Au in liquid Sn. Fig. 20 shows R vs. s results for Sn drops spreading on Cu substrates with various Au coating thicknesses at 250jC [24]. The spreading rate decreases with increasing coating thickness, with the most rapid spreading having occurred on the 0.2-Am Au coating. The final wetted radius, however, increases with increasing coating thickness. In comparison, the final wetted radius for pure Sn on pure Au and Cu was considerably less than on any of the Au-coated substrates. Even a very thin coating of Au significantly improves the spreading of an Sn drop. Thin Au coatings are also occasionally used over Pd metallizations to improve wettability. 4. Effect of High Solubility on Spreading The contact line region of a liquid metal drop spreading over a metallic substrate having a highly soluble coating is illustrated schematically in Fig. 21. Mortensen et al. [83] postulated that the diffusion of substrate coating materials into the liquid can control the spreading, and concluded that the time evolution of a drop follows an R4 vs. s behavior under these conditions.

FIG. 20 The effect of various Au coating thicknesses over copper on the wetting radius with time at 250jC (initial solid sphere diameter was 0.889 mm). (From Ref. 75.)

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FIG. 21 Schematics depicting liquid metal spreading on a low-solubility base metallization (Cu or Ni) coated with a metal having high solubility in the liquid. (a) Overview of a liquid metal drop wetted on a solid metal substrate. (b) Details at the contact line.

5. Diffusive Spreading Coefficient Although the Mortensen et al. model is only applicable to flows having a small solutal Peclet number, some of the parameters related to diffusive transport were found to be relevant in describing the rapid spreading of pure Sn observed on Au-coated Cu substrates. In the present case, let Au be the solute and Sn be the solvent. We assume that as the liquid advances, the highly soluble layer completely dissolves into the liquid. The driving concentration difference for diffusion is the difference between the concentration of solute in the center of the liquid drop Co and the equilibrium-saturated solute concentration (the liquidus line on the phase diagram) CE. In order for the material region adjacent to the contact line to remain completely liquid, the solute concentration must remain below CE. A dimensionless diffusive spreading coefficient SD was derived [24] from a mass balance between the Au diffused in the liquid away from the contact line and the Au mass dissolved from a coating. The SD parameter may be written as: SD ¼

tqAu R4 R4 ¼K DVL ðCE  Co Þ s s

ð40Þ

where t is the thickness of the Au coating, qAu is the density of solid Au, D is the diffusivity of solute in the solvent, and VL is the volume of the liquid. As the KR4 vs. s plot shows in Fig. 22, Eq. (40) effectively describes the drop spreading kinetics for Sn spreading on 0.2-, 0.4-, and 1-Am Aucoated Cu substrates at 250jC, and 1-Am Au-coated Cu substrates at 250jC, 279jC, and 338jC. All these conditions collapse to a single line having a slope of 2.35. Note that the line was fit over a time interval of 1<s
D.

Effects of Intermetallic Compound Formation on Wetting

As demonstrated in Sec. II.H, the formation of intermetallic compounds during wetting, at least in the simplest model of product formation, can augment the thermodynamic driving force for

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FIG. 22 Parametric plot showing the effect of reflow temperature and Au coating thickness on KR4 vs. time s for the case of a Cu substrate. (From Ref. 24.)

wetting. The assumption of local quasi-equilibrium in a metal–metal wetting system is generally justified. However, global equilibrium, by which is meant the thermodynamic state of the entire system, (e.g., entire solder joint and surroundings), is generally not realized during wetting. Consequently, predictions from analyses of reactive wetting based on equilibrium thermodynamics must be interpreted with caution. In many cases, depending on the initial differences in concentration between the alloy and the substrate, transient phases can be formed as the system moves toward total equilibrium. In such highly nonequilibrium situations, the equilibrium phase diagram serves only as a crude map of the system evolution and metastable compounds may form and ultimately disappear. When and where intermetallic compounds form in reactive metal/metal systems undergoing wetting remain fundamental but poorly understood issues. Because of the opaque nature of liquid metals, the observation of phase formation and important transport quantities is not presently possible. 1. Thermodynamic Approaches Thermodynamic approaches that account for the effects of reaction in wetting include the work of Aksay et al. [22], Yost and Romig [23], and Eustathopoulos et al. [12]. The energy change associated with the reaction proposed by Eustathopoulos et al. [12] is generalized so that both dissolution and compound formation can be taken into account:  dG ¼ 2pr rSL ðtÞ þ r0LV coshD ðtÞ  r0SV dr

ð41Þ

Here, the 0-superscripted quantities denote surface energies before any reaction occurs. This form of energy expression allows for the S/L interface energy to evolve with time due either to dissolution or compound formation. In the former case, as time increases, the term rSL(t) approaches (in the case of a binary, purely dissolutive system) the value corresponding to the equilibrium between a liquid metallic solution saturated with the substrate metal and the original solid substrate. In the latter case, the term rSL(t) approaches the value rBC+rAC corresponding to the formation of a compound C so that Eq. (41) becomes equal to the expression (Eq. (20)) from Yost and Romig [23]. It is important to emphasize that the S/L boundary is assumed to

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remain flat and morphologically unaltered by either dissolution or compound formation. Eq. (41) may be rewritten as: dG ¼ 2pr½r0SL þ DrðtÞ þ DGðtÞ þ r0LV coshD ðtÞ  r0SV  dr

ð42Þ

where Dr(t) u rSL(t)r0SL, r0SL is the equilibrium solid–liquid interfacial energy before reaction, and DG(t) is the change in free energy per unit area released by either the dissolution reaction or the compound formation reaction, or both. Setting r0SVr0SL=r0LVcosh0, where h0 is the equilibrium contact angle in the absence of a reaction, Eq. (42) may be rewritten as:  dG ¼ 2pr DrðtÞ þ DGðtÞ þ r0LV coshD ðtÞ  r0LV cosh0 dr

ð43Þ

If the assumption of local capillary equilibrium at the contact line is made, then dG/dr=0 in Eq. (43) and the instantaneous contact angle hD(t) is given by: coshD ðtÞ ¼ cosh0 

DrðtÞ DG  0 r0LV rLV

ð44Þ

The reader is cautioned, if attempting to compare Eq. (43) with Eq. (20), to properly account for the differences in the definitions of various contact angles. Because both Dr(t) and DG(t) are negative, the effect of either term is to reduce the contact angle from its value corresponding to no reaction. Eq. (44) suggests that the greater the reactivity of a system is, the better is its wetting capability. Aksay et al. [22] asserted that the contribution from the DG(t) term has its maximum value when a pure alloy first wets a pure substrate and the reaction kinetics is maximum. Thus, the contact angle initially decreases, but eventually, DG(t) decreases and the contact angle increases and asymptotes to some value that depends on the final value of Dr(t). 2. Homogeneous vs. Heterogeneous Reaction It was suggested that the behavior of DG(t) is coupled only with the interfacial reaction rate (heterogeneous reaction), and so is concentrated along the original solid interface. However, Meschter [24] has shown that in some highly reactive systems (e.g., Sn–Au), the reaction contributing to DG(t) can occur throughout the fluid domain. It is not known whether such pervasive compound formation is due to a heterogeneous or a homogeneous reaction. In the latter case, it is possible that the liquid metal becomes supersaturated with the highly soluble substrate metal and homogeneous nucleation occurs. This is further discussed below. Eustathopoulos et al. [12] note that the dynamic contact angle in most systems does not exhibit a minimum but rather decays monotonically to a steady-state value. This behavior is apparently supported by ample experimental evidence. Eustathopoulos et al. [12] further argue that, at least for metals spreading on ceramic substrates, wetting energetics can be described adequately just with the term Dr(t) and the term DG(t) is not necessary. However, dissolution and product formation in such systems are comparatively weak relative to highly soluble and highly reactive systems (e.g., as is the case with many commercial solder/substrate systems). In these systems, the Dr(t) term probably cannot be neglected. In reality, it is difficult to calculate DG(t) because of the complexity of combined heterogeneous and homogeneous reaction kinetics. 3. Product Formation in Highly Reactive Systems It is very unlikely that simple thermodynamic analyses can capture the complexities of reactive wetting in highly reactive systems. Even continuum formulations of transport phenomena known to occur in these types of systems, in combination with thermodynamics, cannot accurately predict the evolution toward equilibrium of the highly nonequilibrium state of an initially pure low-melting-point metal placed on a reactive metallic substrate preheated to a temperature above the melting point of the wetting metal [17,25]. A careful set of experiments of isothermal wetting in the Sn–Au system by Meschter [24] supports the above assertions and reveals complex material interactions at the contact line, the S/L interface, and the bulk of the drop. System wetting behavior was strongly dependent on temperature and it was assumed that the equilibrium phase

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diagram provided a rough map of which of the system’s intermetallic compounds were capable of forming in this highly nonequilibrium situation. 4. Rapid Solidification (Pure Sn on Au) Perhaps the most poorly understood and controversial issue concerning wetting in a given reactive metal–metal system is the extent to which the formation of intermetallics affects the wetting characteristics of the particular system. Because solder wetting performance studies are typically conducted at constant temperature, the possibility of the molten solder metal freezing during wetting is often overlooked. Intermetallic compound formation and freezing are equivalent terms when considered in an isothermal context: At a fixed temperature, sufficient substrate material is available to the molten liquid by dissolution so that the solubility limit is reached, and a solid phase forms. The most dramatical examples of isothermal freezing are the experiments of Meschter [24] for pure Sn spreading on pure Au in the temperature range 279jC
FIG. 23 Rapid freezing sequence of pure Sn spreading on pure Au at 314jC. (a) Side view images showing melting and spreading in the period 0VsV1.29 sec. (b, c) Top and side view images, respectively, showing the dissolution of intermetallic compound with time (initial solid sphere diameter was 0.889 mm). (From Ref. 24.)

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diagram provides some rough clues. In this temperature range, the solubility of Au in liquid Sn is very large (Fig. 17). The drop appears to achieve a supersaturated state in less than 1 sec and rapid solidification ensues. The subsequent reliquification of the drop occurs due to the additional Au enrichment that takes the system to the Au-rich side of the phase diagram. It is not known whether the solidification occurs from homogeneous nucleation throughout the liquid phase or heterogeneous nucleation at the S/L interface. It is also not well understood how the transport of Au necessary for supersaturation can occur so rapidly. Regardless, the important point is that isothermal solidification can occur rapidly and pervade much of the drop volume, and could have a profound effect on wetting experiments in other metal–metal systems. 5. Mechanism Theories It is not clear whether the above rapid solidification phenomenon is possible for metal–metal systems other than Sn–Au [84]. In the above case, rapid freezing is most certainly related to the particular structure of the Au–Sn phase diagram. However, solidification is certainly possible for any system in which a solubility limit can be reached. The logical places to anticipate the formation of compounds are where the concentration of solute is highest. Consider again the streamline pattern during the primary (convectively dominated) spreading regime of a droplet shown in Fig. 11. This pattern is only appropriate provided that dissolution has not yet significantly altered the initial planar shape of the S/L boundary. Although, as discussed above, the dissolution is moderate during the convective regime, the convective flow in the drop tends to concentrate the solute diffusing across the S/L interface into the contact line region. The possibility of freezing in the contact line region raises several possibilities, none of which has been proved or disproved. One possibility is that the narrow wedge of the contact line region freezes completely, momentarily arresting the contact line. Because the resulting solid would not be in global equilibrium, the solute would eventually diffuse into the unsaturated bulk liquid of the drop and the contact line region would revert to a liquid phase and spreading would resume. A stick–slip motion (see below) of the contact line would result. Another possibility is that a volumetrical precipitation of intermetallic phase could create a liquid–solid mixture in the contact line region. Consequently, this region would possess an effectively higher viscosity than the bulk of the droplet and hence the contact line motion would be slowed. In systems where spreading is rapid and solubility is low, it is possible that an intermetallic layer would form along the S/L interface and occupy a part of the wedge of the contact line; however, the top portion of the wedge would be liquid and wetting would be affected but not immediately arrested. This last scenario was studied by Schaiffiono and Sonin [85,86]. 6. Effect of Intermetallic Compound Thickness The effects of intermetallic formation on wetting have been considered from other perspectives. One area of interest is the thickness of the intermetallic layer formed at the S/L interface. The thicker is the layer formed upon wetting, the larger is the contribution to the wetting driving force through the term DG(t) in Eq. (43). Intermetallic compound thickness should be measured in vivo when the alloy is molten. Measurements reported in the literature to date were made from metallographical cross sections of solidified samples. However, it is not known how much the intermetallic compound thickness grew during cooling. No method yet has been devised for measurements of intermetallic thickness in the molten state. 7. Effect of Surface Roughness Yet another potential effect of intermetallic compound formation on wetting is a roughening of the original unreacted S/L interface. Roughening can have a significant effect on the energetics of wetting because the total energy associated with the S/L interface is (cf. Eq. (1)) rA and the change in energy due to areal changes as a result of wetting is included in the term r dA. The dA associated with the formation of a comparatively flat intermetallic can be significantly less than the dA associated with the formation of a rough intermetallic, and the difference in the wetting driving force can be significant. Zuruzi et al. [87] studied both the roughness evolution and reaction layer thickness of Cu6Sn5 and its effect on wettability.

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8. Effect of Coupling Between Surface Segregation and Intermetallic Formation An interesting interpretation of the potential coupling between surface segregation and intermetallic formation during spreading is suggested in the study of the Pb–Sn system by Smith and Lea [8]. Using in situ scanning electron microscopy (SEM) of molten Pb–Sn drops spreading on Cu substrates, they observed five distinct surface composition regions, beginning on the surface of the bulk portion of the drop and proceeding radially outward to the unwetted solid substrate. In sequence they are: (a) the molten bulk of the drop; (b) a dark molten band of width f20 Am; (c) a bright band of width 20 Am; (d) a band of width f80 Am, which the authors referred to as a halo; and (5) a transition region to pure Cu substrate. These in situ data were subsequently found to be consistent with static SEM data taken between 10 and 30 min after wetting. The surface enrichment of Pb persisted across the first three regions. The dark molten band was found to be Sn-depleted. The 20-Am bright band was primarily composed of Cu3Sn intermetallic. The consumption of Sn by this intermetallic may account for the depletion of Sn in the adjacent dark band region. The halo region was primarily a monolayer of Pb with approximately 50% surface coverage. This was followed by a transition to almost pure Cu. Smith and Lea suggested that the halo was formed by the rapid surface diffusion of Pb atoms from the surface-rich Pb layer onto the pure Cu substrate. Thus, wetting occurs over the adsorbed Pb halo and the wetting rate is slower than over pure Cu. It is interesting to examine the effects of the Pb monolayer on the spreading coefficient rSO. Changes depend on the extent of Pb adsorption and may be estimated using regular solution theory for a monolayer with X% fractional coverage as: Cu rSO ¼ XrPb SO þ ð1  XÞrSO

ð45Þ

Cu 2 2 2 From Smith and Lea [8], rPb SO=0.45 N m/m and rSO=1.73 N m/m . Take rLV=0.48N m/m 2 for a 73Sn/27Pb at.% alloy and rSL=0.09 N m/m (estimated using the method of Klein Wassink [88]). Then for the case of no halo, X=0 and SurSOrLVrSL=1.08 N m/m2. Thus, the initial

spreading prior to the formation of the Pb halo is spontaneous. In the presence of a halo, however, S is reduced. In the above case, a value S=0 corresponds to a fractional surface coverage X=0.83; a value of X greater than this value would yield limited spreading. Of course, the value of X is a function of position within the halo, typically being highest adjacent to the liquid. However, the reader is cautioned that the value of S is not correlated with the rate of spreading [2].

E.

Effects of Surface Condition

Many metals employed in commercial soldering processes form surface oxides when exposed to oxygen-bearing ambients. Oxide layers on solid surfaces can be continuous, or can exist as patches. In the latter case, the presence of the oxide layer makes the surface chemically heterogeneous. It is clear that because oxides form spontaneously, they possess values of rSV less than that of a pure metal surface. Therefore, an oxide patch constitutes a free energy mesa, a surface region of low surface free energy surrounded by a surface of higher free energy. A spreading liquid will manifest both different static and dynamic contact angles on the different phases of the heterogeneous surface, and the contact line speed will change accordingly. For example, Weirauch and Krafick [89] report equilibrium contact angle values for liquid Ag spreading on solid Ni at a temperature of 970jC. In a helium atmosphere, they report a hE value of 90j, whereas in air, they report a hE value of 90j, which represents a new interfacial equilibrium between Ag and NiO. In general, differences in contact angles for a metal and its oxides can be as large as tens of degrees. 1. Effect of Oxide Patches on Wetting The wetting behavior induced by oxide patches is complex. Depending on the strength of the free energy jump, the size and geometry of the mesa, and the nature of the wetting flow, the contact line can be pinned or grossly distorted by an encounter with an oxidized surface region. Pinning on a chemical heterogeneity such as an oxide patch is illustrated in Fig. 24, where spreading is driven by the gradual increase in volume of a liquid drop. At (a), the drop is originally on a solid

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FIG. 24 Schematic depicting the pinning and depinning of the contact line on an oxide inhomogeneity. (a) Liquid wets the solid metal surface (a) with the equilibrium contact angle haE. (b) The contact line pins at the intersection of the pure metal/metal oxide interface of the oxide until the contact angle increases to the value of hbE. (c) When this occurs, wetting commences over the oxide surface (b).

surface a where it exhibits an equilibrium contact angle of haE. (b) When the contact line arrives at the line between the solid surface a and the solid surface b, it pins. As the volume is increased further, the contact angle increases continuously until it reaches the value hbE, the equilibrium value on the solid surface b, at which time it unpins and moves along b with a new equilibrium value. 2. Contact Angle for a Chemically Heterogeneous Surface When the oxide heterogeneities exist in the form of patches, the S/L and S/V surface energies can be expressed as an areal average [17]. Let the area fraction of b on a be s. Then rLS=(1s)rLa+srLb and rLV=(1s)raV+srbV, which, upon substitution into Young’s equation (Eq. (3)), yields: coshC ¼ ð1  sÞcoshaE þ s coshbE

ð46Þ

This result is known as Cassie’s equation and gives an estimate for the equilibrium contact angle hC for a chemically heterogeneous surface, but does not allow for contact angle hysteretic effects. Cassie’s equation predicts that the value of the contact angle for a surface comprised of a metal and its oxide(s) lies between that for a pure metal and that for the oxide. Boettinger et al. [17] assessed the validity of Cassie’s equation by measuring contact angles for 60Pb–40Sn (wt.%) solder spreading on prepared heterogeneous surfaces of Cu with stainless steel inhomogeneities between 10 and 20 Am in diameter. Experiments were conducted using both R and RMA fluxes. For both fluxes, the equilibrium contact angle for stainless steel inhomogeneities hSS E was determined to be 180j. Their results are shown in Fig. 25, which plots hC vs. s. There is a reasonable agreement between the data of Boettinger et al. [17] and Cassie’s equation for experiments with both fluxes. 3. Stick–Slip Motion The logical implication of Eq. (46) for dynamic contact angles (i.e., for a moving contact line) is that solders will wet a partially oxidized surface at a slower rate than the pure metal. The rate of advance of the contact line over the patch depends on pinning dynamics. The shape of the contact line is distorted at each point the liquid encounters an oxide patch. Contact line motion on heterogeneous surfaces typically exhibits an erratic behavior often referred to as stick–slip motion. It is possible that a gaseous inclusion will be formed when the solder negotiates a patch if the surfaces of the deformed liquid on either side of the patch coalesce before the entire patch is wetted. 4. Wetting Completely Oxidized Surfaces When the oxidation of a metallic surface is complete (i.e., when the oxide layer is continuous), solders typically exhibit extremely poor wetting. However, Eustathopoulos et al. [12] point out that wetting can sometimes proceed slowly on surfaces with thin oxide layers when the metal–

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FIG. 25 Apparent contact angles for 60Pb–40Sn solder spreading on Cu with stainless steel heterogeneities of area fraction s. Solid lines are Cassie’s equation. (From Ref. 17.)

metal system is highly reactive. In this case, reactions can occur at oxide layer defects or by diffusion of reactive constituents of the solder alloy through the oxide layer. The resulting product formation further disrupts the oxide layer and wetting can thus be sustained. Similar effects can be observed when the oxidation exists on the solder surface and the solid surface is a highly reactive noble metal. For example, oxidized Sn spheres will slowly wet Au surfaces when surface forces generated by the reaction at the Sn–Au contact sites tear or peel the oxide on the Sn spheres, allowing pure metal to escape from the sphere. Spreading in such cases is often asymmetrical. 5. Surface Roughness Surface roughness is an attribute of a surface that represents a departure from the perfectly flat surface assumption under which Young’s equation was derived. There are several manifestations of surface roughness on wetting, which lead to apparent conflict with Young’s equation. a. Contact Angle at Metastable Positions. One such effect of roughness is that the contact line can arrest on many metastable positions. If one considers the rough surface shown in Fig. 26, idealized to be sinusoidal, the actual contact angle must always be given by Young’s equation hE. However, when the contact line lies parallel to the trough, it can locate within any trough, consistent with the constraint of constant volume and uniquely valued Young’s angle. Johnson

FIG. 26 Schematic depicting the side view of a liquid drop on an idealized rough solid surface.

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and Dettre [90] show that two positions within each trough are possible: one that is metastable and the other that is unstable. These positions differ depending on whether the contact line is advancing or receding. Depending on which trough the line resides, the apparent contact angle / with the horizontal varies. This is the contact angle recorded by an observer. There is one position in one trough for which the free energy of the system is a minimum and this position represents absolute stability. Any two adjacent metastable states are separated by an energy barrier. The magnitudes of the energy barriers are proportional to the amplitudes of the roughness asperities. The contact line can be trapped in a metastable equilibrium in a given trough depending on both the thermal energy and the vibrational energy of the liquid. The larger the thermal and vibrational energies are, the less likely the drop will come to rest at a metastable equilibrium whose equilibrium value differs significantly from the minimum energy configuration (i.e., the smaller the hysteresis is). The speed of the contact line also influences how spreading is affected by roughness. Generally speaking, the faster the contact line speed is, the less sensitive the wetting system is to surface roughness. b. Contact Angle on Rough vs. Smooth Surfaces. Wenzel [91] derived a relationship between the apparent equilibrium contact angle on a rough surface hW and that on an ideal (perfectly smooth) surface hE (Young’s angle): coshW ¼ r coshE

ð47Þ

where r is Wenzel’s ratio, defined as the ratio of the actual area of the rough surface to its projected area on its own (average) plane. Thus, r>1. Wenzel’s equation suggests that when hE

p/2 then hW>hE and the observed contact angle increases (wetting degrades) as roughness increases. Wenzel’s equation is derived under a set of assumptions that are typically not strictly satisfied [90], so Eq. (47) is an approximate relationship and does not always yield accurate predictions. For instance, Eustathopoulos et al. point out that experimental observations show that advancing contact angles increase with increasing roughness for both nonwetting liquids (e.g., Hg on SiO2) as well as certain wetting liquids (e.g., H2O on SiO2). Given the fact that there are numerous surface finishes on substrates used in microelectronics soldering processes, each with its own roughness properties, Wenzel’s equation alerts to the possibility of altered wetting characteristics. c. Random Roughness Effect on Wetting. Realistic surface roughness takes the form of randomized sharp peaks and troughs, rather than the idealized sinusoidal rough surface discussed above. It is well known that contact lines can arrest or pin on geometrical eccentricities. Consider a contact line as it advances from point A to point C in Fig. 27. The solid surface has a discontinuity in slope at point B. If the contact line is advanced sufficiently slowly, then the value of the dynamic contact angle along AB remains approximately hE and the L/V interface shape

FIG. 27 Contact line pinning at a geometrical eccentricity on a rough surface.

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remains unchanged as the line moves from A to B. When the line arrives at point B, it becomes pinned at B despite attempts to advance it. However, the L/V interface shape begins to change and the contact angle h increases continuously from its equilibrium value hE to the value hE+/. When h=hE+/, the L/V interface makes an angle of hE with the solid surface BC (measured within the liquid), and this is the condition for the line to unpin and to begin moving along the surface BC, again with constant angle. The pinning in the advancing case produces an observed angle greater than hE. If the contact line had instead been receding with motion from point C to point A, the pinning at B would have caused an observed angle less than hE. In this way, surface eccentricities in randomized roughness can produce hysteresis in the measured contact angle. d. The Effect of Dissolution on Roughness During Wetting. In metal–metal systems, the process of dissolution, particularly of the solid surface in the spreading liquid metal, is conjectured to dramatically alter the influence of roughness on wetting. The interactions of a spreading metal with a rough solid surface are obviously an extremely complex process. However, it is clear, for example, that a liquid metal contact line momentarily pinned on a ridge of roughness can potentially dissolve the ridge. The rate at which the ridge can be dissolved depends on many things, particularly the mass transport near the contact line, which itself depends on how far the liquid is from saturation. Clearly an understanding of how a liquid metal spreads on a rough metal substrate awaits understanding of the much more simple problem of how metals spread on perfectly smooth substrates. e. Effect of Grain Boundaries. Grain boundaries in polycrystalline metallic solids constitute a particular type of roughness. The enhancement of wetting by grain boundaries oriented normally to the contact line seems to be much more significant than the retardation of wetting by pinning on grain boundaries oriented parallel to the contact line. The enhancement of metal– metal wetting by normally oriented grain boundaries has been studied by a number of investigators [12,92,93]. The general idea is that a grain boundary can potentially serve as an open capillary through which liquid metal is wicked away from the contact line (which essentially serves as a reservoir) toward the unwetted solid. Eustathopoulos et al. [12] show that for the infiltration of a grain boundary to occur, the dihedral angle of the grain must be smaller than the value h* given by: h coshE sinhE sin * ¼ 2 hE

ð48Þ

1. Large grains vs. small grains—Given that 090j) liquids. For surfaces with large grains, the result is a spiderlike pattern of liquid-filled grooves adjacent to the bulk liquid and emanating from the contact line. Large grains do not appear to contribute significantly to enhancement of spreading. For surfaces with small grains, the liquid-filled grain boundary network is much more dense. If the grains fill to their tops and there is subsequent spillage, coalescence of adjacent spills can collectively serve to wet macroscopically sized areas and advance the position of the contact line. Thus, surfaces with small grains are much more likely to enhance wetting via grain boundary infiltration than are surfaces with larger grains. 2. Primary vs. secondary wetting—Grain boundary enhancement of wetting is often associated with the phenomenon of secondary wetting discussed in Sec. 3.5. The primary phase of wetting is insensitive to the presence of grain boundaries because there is ample energy associated with the motion to overcome energy barriers presented by grain boundaries parallel to the contact line, and the rate of contact line advance is much faster than the rate of advance in capillaries perpendicular to the contact line. Yost et al. [93] found that the rate of advance of solders in artificially constructed V-grooves (presumably a model of a grain boundary) could be described using a variant of Washburn’s model for flow in a closed capillary. Thus, the relationship between the distance advanced in a groove and time was found to be x2=Ct, and thus xft1/2, a behavior that is consistent with a diffusion rate. For example, Meschter [24] found that

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SINGLER ET AL. during secondary spreading of Sn on Au, Rft1/2, where R is the effective radius of the drop. Meschter hypothesized that an undetermined diffusion mechanism was controlling the wetting kinetics. The contact line in Fig. 11 during the secondary spreading regime appears to be ragged as one might expect with contact line advance due to grain boundary infiltration. Because various surfaces finishes are routinely utilized during the soldering of microelectronics assemblies, further investigation is required to verify the role of their grain boundaries in advancing the solder contact line.

VI.

FLUX CHEMISTRY AND TECHNOLOGY

A.

History

Soldering with fluxes has been practiced for thousands of years by artisans and metal workers, but only began developing as a technology with the industrial revolution [94,95]. Even beyond the middle 20th century, much soldering was practiced as a skilled trade in plumbing, radiator repair, and electronic wiring. It is interesting to note that the metals still primarily involved, such as copper, tin, lead, iron, silver, and gold, were known to the ancients. Tin plating of iron was mentioned by Theophrastus as early as 320 BC and Herodotus discussed the sources of tin and amber, a fossil resin, in his ‘‘History’’ [96]. It is also interesting to note that one of the organic acids common in modern fluxes, succinic acid, was known in the 16th century as a distillation product of amber [97]. So it appears that joining of metals with solders and utilizing natural resins as part of the procedure were practiced in antiquity. Modern solder and flux technology began in the 20th century with the demands of the electronics industry. For most of the century, the most extensive utilization encompassed tin–lead solders and rosin-based fluxes. Early fluxes were used to clean surfaces in preparation for coating metals, but these were highly corrosive and required extensive washing. They consisted of strong acids and inorganic acid salts soluble in water. Electronic requirements brought about the development of rosinbased fluxes whose residues do not cause excessive corrosion. These, however, require organic solvents for postsolder operations that eventually became an environmental concern leading to a demand for ‘‘water-soluble’’ and ‘‘no-clean’’ fluxes.

B.

Definitions

1. Solder The American Welding Society defines soldering as metal coalescence below 427jC (800 jF) [94]. Therefore, the solder must be a metallic element, mixture, or alloy, which will melt, wet, spread at or below this temperature, and serve as a method to bond the metals being joined. Perhaps a more functional definition from an electronics assembly viewpoint is that soldering is a low-temperature, reversible joining process that allows rework without damaging substrates and other temperature-sensitive components [98]. For electronics applications, it is essential that bonding and electrical conductivity be maintained at operating temperatures, and withstand thermal stresses caused by environmental and power on/off cycles throughout the life of the product. Lead (mp 327jC) and tin (mp 232jC) form a eutectic alloy (mp 183jC) that has been universally utilized to attach components to printed circuit boards for more than 50 years. Some of this popularity is likely due to its compatibility with organic packaging because of its low-meltingpoint eutectic temperature, and because the Pb–Sn system allows for a hierarchy of melting temperatures, which is often useful in manufacturing electronic assemblies. With the advent of regulations leading to the elimination of lead (Pb) from solders, there are substantial challenges to obtaining the level of success and reliability that has been achieved with lead (Pb)-containing solders in the electronics industry. The choice of metallic elements in the periodic table from which a substitute solder can be derived is very limited, and a drop-in replacement appears unlikely. Examination of a few lead-free solder alloys in Table 11 reveals that some of the more

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TABLE 11 Comparison of the Melting Point and Range of Several Lead-Free and Common Sn–Pb Alloys Solder Sn–37Pb Sn–90Pb Sn–95Pb Sn–58Bi Sn–9.0Zn Sn–0.7Cu Sn–3.5 Ag Sn–2.5Ag–0.8Cu–0.5Sb Sn–3.5Ag–4.8Bi Sn–2.0Ag–7.5Bi–0.5Cu Sn–2.0Ag–4.0Bi–0.5Cu–0.1Ge Sn–3.5Ag–1.5In Sn–2.8Ag–20In

Melting point (jC) 183 308–312 268–302 138 198.5 227 221 217 213–218 205–210 217–218 210–217 175–187

likely candidates will require soldering melt temperatures 30–40j higher than eutectic Sn–Pb. This poses significant challenges in terms of organic packaging and flux effectiveness. For more examples of lead-free solder candidate alloys, see Refs. 99 and 100. 2. Flux The word flux is derived from a Latin word meaning ‘‘to flow.’’ A flux is usually required to facilitate the wetting of the bonding surfaces and the flowing of solders. Typically, flux is defined as a substance capable of cleaning or removing the oxide from a metal surface [101]. This definition is somewhat simplistic because the metallic oxides are seldom actually removed during soldering of electronic components. Most soldering processes are performed with fluxes and oxides left on the surfaces to be soldered. In fact, the flux–oxide reaction product becomes an integral component in facilitating the wetting and joining process. Removal only occurs if the oxides are reacted (typically as an acid–base reaction with the flux), dissolved in a solvent system, and rinsed away by additional solvents. Except for some special processes, which will be described briefly, the metal oxides are only displaced by flux during the temperature cycle. Flux is sometimes thought of as a catalyst that lowers the surface tension between the molten solder and a metal surface [98]. In reality, the chemistry of flux interactions at oxide surfaces can be very complicated and involve acid–base, oxidation–reduction, and coordination-type and adsorption-type reactions discussed in later sections [102–104]. Spalik prefers to think of most fluxes used for electronic soldering as substances that react as Bronsted–Lowry acids with metallic oxides to form their respective salts and water, and that the salts serve as surfactants that promote solder wetting. 3. Activity Fluxes contain an agent(s) that chemically reacts with the surface tarnish on a metal surface so that it can be wetted by solders. The reaction results in the formation of a chemical compound(s) that is soluble in the liquid flux, exposing the surface so it can be wetted by the molten solder. Some agents are more efficient than others in removing tarnish layers based on their physical and chemical characteristics, generally referred to as a flux’s activity. 4. Thermal Activation The activity of agents added to a flux to remove surface tarnishes is temperature-dependent. That is, a temperature level must be attained before the material becomes active (i.e., the activation temperature). For example, this may coincide with a dissociation temperature that provides

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species that react with tarnishes as is the case with amine hydrohalides. In other cases, as occurs with rosin fluxes, activation is initiated when changing from a solid to a liquid form. The same is true for gaseous fluxes such as hydrogen, which must be heated to elevated temperatures before it can be effective in reducing metal oxides. However, in some cases, particularly with organic activators, the flux activity can be altered due to thermal exposure and become progressively less active and ultimately inactive. A common flux (water-white rosin) utilized in the solder reflow attachment of flip chips to their carriers exhibits this characteristic. Also, evaporation can deplete the activator to the point where its concentration is too low to be effective (i.e., an excessive hold at temperatures just below the solder melting temperature can render a flux ineffective). It is essential that the reflow thermal profile be optimized to take advantage of an activator’s most effective activity temperature range. 5. Solder Wetting The parameters associated with and the mechanism of solder wetting are described in great detail in other sections of this chapter. It is necessary for a flux to chemically clean the mating solder and metal surfaces at their mutual interface to assure intimate contact that promotes chemical bonding between the solder and metal surfaces. Cleaning of the surface at the interface through the action of the flux serves to alter the surface energy equilibrium in a way that favors exchanging the solid/flux and solid/solder interfaces for a solid/solder interface—in other words, driving the reaction in a direction that favors solder wetting.

C.

Purpose and Function

In soldering applications, the primary purpose of a flux is to promote the solder wetting of the interfaces to be joined, and assist in the formation of good solder joints. The interfaces may consist of a molten solder in contact with a solid metal as Cu, or two solders in contact with each other, in which case at least one is molten. To achieve reaction (i.e., wetting and bonding), the flux typically lowers the surface energies by reacting with the oxides on the surfaces of both the solder and the metal the solder is to wet. To promote solder wetting, the flux must first dissolve materials that may be absorbed in the uppermost surface layers of a solid such as surface gases, water vapor, or materials from upstream processes. Even gold surfaces that do not have tarnish can have adsorbed materials at the surface that can inhibit solder wetting. Metal surfaces left in the ambient react to form a tarnish layer that is an oxide(s), but can be carbonates, sulfides, or a combination of these. Flux must free the surface of these contaminants in order for the solder to bond with the underlying base metal. Once the base metal is exposed, the flux must act as a protective layer to prevent the ambient gases from reacting with the surface metal to reoxidize and form a new tarnish layer before the solder wets the surface. In addition to promoting wetting, some fluxes may also facilitate heat transfer to the joint area. Also, the tackiness of flux is utilized to physically maintain the alignment and position of components and devices on electronic assemblies between the time they are placed and the time they are reflowed. Flux is an important ingredient of solder pastes and is a major factor contributing to a paste’s stencil printing characteristics and quality of print. The flux can be designed to support solder microspheres to prevent slumping.

D.

Flux Requirements

In order for a flux to achieve its purpose and function, it must meet or possess certain characteristics. Some requirements are fundamentally common to all flux materials, whereas others are application-specific: 1. Stability at soldering temperature—A flux may possess sufficient activity to remove a tarnish layer from a surface, which is to be wetted by solders. However, if the evaporation or sublimation rate of the active ingredient in a flux is too rapid, the surface may be left unprotected. If there is sufficient time for the surface to reoxidize before solder wetting has occurred due to the reaction with ambient gases (particularly oxygen)

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at elevated soldering temperatures, wetting to the surface will be marginal at best, subsequently when the solder becomes molten. Fluxes must therefore be relatively stable at the soldering temperature to maintain the pristine solder-wettable characteristics of the base metal after the tarnish has been removed. 2. Slow degradation rate at soldering temperature—A flux may have sufficient activity to remove tarnish from a surface to be soldered and good stability at the soldering temperature (i.e., low evaporation or sublimation rate), but still not be adequate. Fluxes typically lose their activity over time at the soldering temperature, and more rapidly at higher temperatures due to chemical (i.e., structural) changes that the active ingredient undergoes. Although some degradation is acceptable, this process must be sufficiently slow at the soldering temperature to allow a flux to remove a surface tarnish in proper preparation for solder wetting to occur. Depending on the flux system, this condition can sometimes be mitigated. For example, if the active ingredient is rosin, a flux material with fairly low activity, it may be possible to add a more active ingredient depending on circumstances. In this way, a tarnish may be removed before the rosin decomposes into a charred mass that prevents wetting and subsequently becomes difficult to clean. 3. Easily displaced by solder—A flux must have the capability to dissolve the tarnish layer from both the solder and any surface to be soldered because the mating interface must be pristine to assure optimum wetting. As noted, the flux must prevent reoxidation of the mating interface surfaces. The solder proceeds to start wetting the clean metal surface below the flux flowing in an outward direction. But in order for wetting to proceed, the flux must be displaced (i.e., pushed ahead of the solder front). The ability of the flux to maintain a viscosity that allows the displacement process to continue, thus enabling complete wetting, is clearly a key requirement. 4. Easily removed if desired—As will be discussed, some fluxes are formulated with the intent of not removing them postsoldering. It is desirable that the raw flux, the vapors given off during soldering, and the residue formed during soldering all exhibit a low tendency to corrode the solder joint, the printed circuit board traces, and the interconnects of components and attachments. This may not always be the case, so in high-reliability and other applications, the capability must exist to easily and thoroughly remove residues and condensed vapors. As discussed in Sec. VI.E, fluxes typically consist of various ingredients, each providing a particular attribute. As noted, fluxes have several key requirements. Typically, no single ingredient provides a flux with all the required characteristics. Conversely, a flux formulated with ingredients that have only some of the required attributes will not provide optimal solder wetting, if at all.

E.

Major Flux Components

Although there are hundreds—perhaps thousands—of flux formulations, most are formulated from ingredients having similar chemical properties, with each ingredient added to meet a basic criterion. There are, of course, many ideas of how to best achieve each of these criteria, hence the numerous flux formulations. An ideal flux would be capable of meeting the manufacturing requirements, be effective in promoting wetting or joining, and leave no residues. No one flux meets these requirements for all applications, so it is necessary to make tradeoffs, or to circumvent some undesirable characteristics. The major classes of ingredients in flux are: Active species—This is usually an acid capable of reacting with the metal oxide to form salt and water. An inorganic acid forms an inorganic salt, and an organic acid forms an organic (carboxylate) salt. Solvents—Because many active species are solids, which must be solubilized for dilution and dispensing, this ingredient is selected for its ability to dilute the active ingredient, and to provide a viscosity suitable for application requirements.

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SINGLER ET AL. Activators—This component generally stimulates the flux’s interaction with the metal oxide and is usually a strong acid in the form of an amine hydrochloride that is released as the soldering temperature is approached. Surfactants—This group of compounds usually comprises polyalcohols that serve to promote wetting by reducing the interfacial surface tension between two liquids, or between a liquid and a metal surface, and helps solubilize the residues. The major categories are: detergents, emulsifiers, and wetting agents. Their function can be compared to detergents whose purpose is to solubilize both polar and nonpolar materials. Viscosity modifiers—These are substances (thickening agents) that increase the viscosity of liquid mixtures. They can gel to maintain stability in solder pastes by their emulsifying properties, have the ability to liquefy with shaking or stirring (thixotropy), and are effective in small concentrations that can be easily removed or leave no corrosive residue. The major classifications of thickening agents are: (a) starches, gums, casein, gelatin, and phycocolloids; (b) cellulose derivatives such as carboxymethylcellulose; (c) polyvinyl alcohol and carboxyvinylates; and (d) bentonite, silicates, and colloidal silica. Tackifiers—These materials help hold the components in position where they were placed before and during reflow.

Several excellent texts devote chapters to fluxes [105–108] and, although somewhat dated, one text [109] lists many formulations from patent literature. Patents and Material Safety Data Sheets (MSDS) are additional sources of flux compositions; however, it should be recalled that many suppliers keep their formulations proprietary.

F.

Flux Types

To accommodate special requirements, numerous flux formulations have been developed, many of which are either proprietary or patented [107]. There are literally thousands of flux formulations based on vendor literature, MSDS, patents, and texts [109] from which to categorize functions and deduce characteristics. Given the availability of choices, it is not unusual that often hundreds of flux formulations are evaluated when developing or optimizing solder processes. Therefore, a fundamental understanding of flux compositions is essential in selecting a flux for process optimization. Several flux classification systems that identify them as inorganic, organic, resin-based, and rosin-based fluxes are utilized. ANSI standard J-STD-004, ‘‘Requirements for Soldering Fluxes,’’ further distinguishes each classification into six more categories of low, moderate, and high activity, with and without chloride, for a total of 24 types as listed in Table 12 (i.e., the designation ROM1 specifies a rosin flux, moderately activated, with a weight percent of chloride of between 0.5% and 2.0%) [110].

G.

Inorganic Fluxes

Inorganic fluxes are composed of inorganic salts and acids. They are very active and generally used to join difficult-to-solder surfaces. These fluxes require thorough cleaning after soldering because they are very corrosive. Some common inorganic materials used as activators in fluxes are listed in Table 13. Because of their corrosive nature, fluxes based on these materials are not to be used for electronic purposes. Some exceptions are: the use of inorganic gases (i.e., dry H2) for solder bump reflow of flip chips in the wafer form after solder deposition by evaporation or plating, and inorganic additives to enhance the activity of organic fluxes. Use of fluxes based strictly on inorganic salts and acids will not be discussed in detail. However, it is instructive to examine the role of chlorides in fluxing applications and to attempt to relate it to the carboxyl group in organic fluxes. There is an extensive history of the development of halide-activated fluxes used in the electronics industry largely based on resins derived from rosin. It was recognized early on that rosin-based fluxes activated with inorganic salts (e.g., zinc and ammonium chlorides) are very effective in providing rapid wetting, but also

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ANSI Flux Classification System and Guidelines

Flux

Symbol

Activity

Type

Rosin

RO

Resin

RE

Organic

OR

Low (0%) Low (<0.5%) Moderate (0%) Moderate (0.5–2.0%) High (0%) High > (2.0%) Low (0%) Low (<0.5%) Moderate (0%) Moderate (0.5–2.0%) High (0%) High > (2.0%) Low (0%) Low (<0.5%) Moderate (0%) Moderate (0.5–2.0%) High (0%) High > (2.0%) Low (0%) Low (<0.5%) Moderate Moderate (0.5–2.0%) High (0%) High > (2.0%)

L0 L1 M0 M1 H0 H1 L0 L1 M0 M1 H0 H1 L0 L1 M0 M1 H0 H1 L0 L1 M0 M1 H0 H1

Inorganic

TABLE 13

In

Some Inorganic Activators Utilized in Fluxes

Active ingredient in flux Inorganic salts (zinc chloride, ammonium chloride, tin chloride)

Inorganic acids (hydrochloric, orthophosphoric acids)

Inorganic gases (dry hydrogen, hydrogen chloride)

Characteristics/requirements Removes heavy tarnishes Fast-reacting Active when molten Stable at soldering temperatures Mostly water-soluble Easily neutralized Must remove residues/condensed fumes Remove any common tarnish Fast-acting Stable/active at soldering temperature Very corrosive before and after soldering Must remove residues/condensed fumes Easily removed with aqueous solutions Only active at elevated temperatures Require clean, debris-free surface Hazardous, require special equipment

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that corrosion can take place almost immediately [111]. Development of organic-based hydrohalides overcame gross corrosion problems experienced with inorganic halides. 1. Counter Ions in Organic vs. Inorganic Fluxes As discussed earlier, the primary reaction of the active species in fluxes is an acid–base reaction resulting in the formation of a salt and the evolution of water. Both inorganic and organic acids share a common feature, the positive hydrogen ion H+, but the counter ions (negative ions that balance the electronic charge) are different. Almost universally, the counter ion in inorganic fluxes is a halide (fluoride, chloride, or bromide) and, by far, the most common is the chloride (Cl) ion. On the other hand, the counter ion (carboxylate) in organic acids is different for each organic acid, yet it serves the same purpose—to form a salt with the metal ions at the surface of the solder and mating-solderable surface. 2. Acid Strength Another difference between inorganic acids and organic acids is the acid strength. Generally speaking, inorganic acids are much stronger than organic acids. The concept of acidity is discussed in Sec. VI.K.1. It is generally thought that acidity or acid strength is an important factor in flux activity, and properties such as acid number and acid strength are often reported relative to flux effectiveness. Sufficient acid (hydrogen ion, H+) is required to react with the oxygen on the surface of the solder to form water. But, with the evolution of water, the counter ion must take the place of oxygen and form an organometal salt. Because the salts are the species remaining at reflow temperatures, the properties of these salts with respect to solder wetting are more important than acidity. 3. Eutectic Systems The phase diagrams (Fig. 28) of the chloride salts of Sn, Pb, Cu, Ag, and Zn depict four pairs of inorganic salts forming eutectic mixtures. It is interesting to note that they are molten and

FIG. 28 Binary equilibrium phase diagrams of Pb, Sn, Zn, and Cu chlorides.

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thermally stable within the temperature range of most soldering operations [112]. This is a significant factor with respect to the high activity of chloride containing fluxes because it confirms the fact that chloride salts remain in contact with solderable surfaces during reflow. Also, the effectiveness of chloride additions to fluxes is well documented [111,112]. Molten solders react with metallic salts in fluxes and influence solder wetting of the base metal. Surface adsorption of heavy metal ions promotes wetting (e.g., copper salts enhance wetting on copper surfaces and nickel salts enhance wetting on nickel surfaces). Because the molten eutectic chloride salts are known to be effective, it is reasonable to expect that carboxylate salts forming binary eutectics with carboxylic acids and organic solvents would also be effective. Copper stearate enhances wetting on copper surfaces [112]. Tin(II) ethylhexanoate has been reported to improve the wetting of copper by tin–zinc eutectic [113]. These salts are formed when the flux reacts with metal oxides. It would be useful if future research addressed carboxylate salts to provide the type information (e.g., melting, boiling, and decomposition temperatures) discussed for chloride salts. 4. Role of Organic Salts Little has been reported on the thermal properties of organic salt mixtures in relation to common solder metallurgy. Several handbooks [114–117] list the physical properties of inorganic and organic compounds and it is easy to find the melting, boiling, and decomposition temperatures of many substances. However, there is scarcity of data on organic salts of common solder metals. Perhaps part of this is due to the tendency of many organic salts to sublime, decompose, or be catalytically altered in this temperature range. There is also the difficulty in obtaining pure samples of these salts for analysis. Salts of organic acids play a role similar to chloride salts. Typically, they are soluble in their respective organic acid, and serve as surfactants to promote solder wetting. 5. Analytical Tools It would be valuable to know the thermal behavior of various flux constituents, particularly those of organic acids. Utilization of techniques such as differential scanning calorimetry (DSC), thermal gravimetrical analysis (TGA), and TGA coupled with Fourier transform infrared spectroscopy (TG/FTIR) or mass spectrometry (TG/MS) provides the opportunity to study flux reactions, and the properties of organic salts during solder reflow operations [118]. It has been shown that evaporation rates and reaction products are different in air than in nitrogen. A brief description of these techniques is given in Ref. 119. Utilizing these and other techniques, it will be possible to apply more scientific approaches to optimizing fluxes and reflow thermal profiles.

H.

Organic (Rosin-Based Fluxes)

1. Description Rosin is a natural resin mixture referred to as colophony [
Gum rosin—This is the residue left after the distillation of turpentine oil from the oleoresin (Latin oleum, oil) from living trees.

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SINGLER ET AL. Wood rosin—This material is obtained by extracting pine stumps with naphtha and distilling off the volatile fraction. Tall oil rosin—This is a byproduct of the fractionation of tall oil (Swedish: tallolja; tall, pine+olja, oil), which in turn is a byproduct of paper manufacturing. It consists of about 50% resin acids and 50% unsaturated fatty acids, mainly oleic and linoleic acids.

a. Rosin Grades. There are several grades of rosin, of the lowest to the highest quality, based on color and are designated as: B, D, E, F, G, H, I, K, L, M, N, W-G, W-W, and X [106,122]. The criteria for rosin quality and acceptability depend upon the application. It is interesting to note that the quantities used for flux applications are so small compared with other industrial applications that they are scarcely mentioned in many rosin information sources. For electronics applications, the key indicators of resin quality are color, softening point, and acid number. Those with the lightest color, highest softening point, and highest acid number are of the highest quality. The grade selected for electronic soldering is a ‘‘gum’’ rosin, lightest in color of all the designations and is called water white (W-W) rosin, which consists of 80–90% abietic acid [105]. The reason this particular rosin is selected as a flux for electronic soldering applications is because of its cleanliness and effectiveness. 2. Unique Properties and Characteristics Much of the history of electronic soldering is based on various rosin fluxes due to their effectiveness in creating good solder joints with termination pad metallizations (e.g., Au, Cu, and Ni), and the benign nature of rosin with respect to corrosion. Rosin is a noncrystalline (glass) solid at room temperature, with a usual softening point in the range of 70–80jC, with the higher end representing better quality. A higher softening point is an indication of higher purity. It does not have a melting point, but starts to become liquid at about 150jC, and is fully molten at eutectic Sn–Pb soldering temperatures. Because rosin is a solid at room temperature, it is usually necessary to dissolve it in a solvent such as benzyl or isopropyl alcohol. a. Excellent Tackiness. Several features of rosin fluxes are worth noting, especially because some of these attributes typically are useful in nonrosin flux compositions as well. They are soluble in a wide range of alcohols and can be diluted for easy application by spray, brush, or syringe. Rosin fluxes exhibit excellent tackiness if not diluted excessively. These fluxes are very efficient in holding chips, solder balls, and components in place during reflow. Even with high dilution and solvent evaporation, these rosin-based fluxes are sufficiently tacky to hold chips and small components in place. Complete solvent evaporation leaves a nontacky protective coating on ready-to-solder surfaces. b. Aids in Self-Alignment. Rosin appears to facilitate the self-alignment of chips, solder balls, and small solder ball-based components with respect to copper termination pads and pins so that exact placement is not critical. A study was conducted by Spalik to define the role of resin in providing self-alignment of components with their mating I/O pads. To observe the effect of flux in correcting the problem, 10/90 Sn–Pb solder balls were purposely misplaced on nonsolderable surfaces (ceramic and polyimide) up to 5 mm away from their intended location adjacent to a Cu pin head I/O connection surrounded by a land pattern on a pinned chip carrier. In practice, the solder wets the pin head and land pattern, providing both good electrical and mechanical connections between them. Because this reflow operation is normally carried out under a nitrogen atmosphere at temperatures greater than 330jC, and it is desirable to make observations in real time, the substrate and solder balls were reflowed under N2 in a microhot stage oven, with a viewing port (Fig. 29). The experimental setup was capable of replicating manufacturing reflow temperature profiles. The solder balls were observed to move toward the intended Cu pin if a flux trail extended between the ball and the pin. Fig. 30a and b presents sequential time lapse photographs, taken as a solder ball was observed to approach a pin and as wetting began. This property may be unique to fluxes with a high concentration of abietic acid

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FIG. 29 An example of a microreflow furnace for observing and monitoring real-time reflows.

because similar results could not be achieved with glycerine and other fluxlike materials that were investigated. It may also coincide with abietic acid’s ability to transport tin through flux without solder wetting, as described Sect. 6.8.2.3. c. Transports Tin. Rosin also has the ability to transport tin (Sn) through a rosin flux over copper surfaces beyond the extent of the solder spread. In other words, rosin can transport Sn from a Sn–Pb or other Sn-based solder surface to a relatively distant copper surface without an intervening layer of solder. The Sn forms a Cu–Sn intermetallic, and no Pb or other species are transported. This phenomenon, described as a halo by some investigators, appears as a grayish cast on the Cu surface, preceding and extending beyond the solder spreading front. Three mechanisms have been proposed: electrochemical deposition, condensation, and surface diffusion. These halos have been reportedly made with very active ammonium chloride–zinc chloride fluxes, but also slowly with urea and pure dimethylammonium chloride [123].

FIG. 30 Sequence of time-lapse photographs: (a) taken prior to reflow, where the solder balls were purposely misplaced; (b) after reflow, illustrating how solder balls follow a flux path to Cu pins.

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d. Phenomenon Verified. A study was conducted by Spalik using the apparatus depicted in Fig. 31 to verify that tin (Sn) can be transported through rosin flux, over a copper surface, without the assistance of any solder spreading. Tin–lead (90/10) solder and flux were heated in the test tube-like apparatus to evaporate the solvent so only the rosin remained and then heated to 350jC in a purged nitrogen atmosphere. A cleaned Cu wire was introduced through the top vent with the end dipped slightly into the molten solder. The dip time intervals ranged from 10 sec to 4 hr. Fig. 32a–d depicts a solder on the tip of the wire, and the appearance of the halolike substance on the Cu wire that was immersed with the rosin above the molten solder. The left side of the wire shown in Fig. 32a is bright and shiny, where it was wetted by the Sn–Pb solder. However, the wire has a dark appearance on the right side, where it was in contact with only the rosin. A SEM backscatter image of the same areas of the wire depicted in Fig. 32a is given in Fig. 32b. A SEM image (Fig. 32c) of the portion of the wire immersed in the rosin (i.e., dark-appearing area of the wire in Fig. 32a) is not characteristic of the structure of copper wire. It was determined via energydispersive spectroscopy (EDS) analysis (Fig. 32d) that the material on the surface of the wire in contact with the rosin consisted of Sn and Cu, but not Pb. Sn could only have arrived at that location on the wire due to the rosin having participated in its transport. e. Reducing Activity Due to Isomerization. Rosin flux increases in activity up to temperatures around 285jC but loses its effectiveness (i.e., reduces in activity) when exposed to elevated temperatures over time. In addition to the loss in activity, the tin transport phenomenon described in Sec. VI.H.2.d does not occur with rosin exposed to elevated temperatures for extended periods. Both are believed to be due to changes that abietic acid undergoes. Rosin used for soldering has been reported to consist of 70–85% abietic acid and 10–15% pimeric acid [124]. Another source reports an abietic acid concentration of 80–90% [125]. Abietic acid undergoes

FIG. 31 Apparatus utilized to demonstrate that Sn is transported through flux and that Sn is oxidized by Pb ions.

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FIG. 32 Series of images that relate to a substance with a halolike appearance on a copper wire. (a) Optical image of Cu wire showing the left, bright-appearing end that was immersed in solder and the right, dark-appearing end with transported Sn. (b) SEM backscatter image of Cu wire showing the same area as in (a). (c) High-magnification image of a section of Cu wire that was only immersed in rosin. (d) Energy-dispersive spectroscopy analysis spectrum from the section of Cu wire shown in (c), showing only the presence of Sn and Cu but not the Sn transported in the region in contact with the resin.

changes due to isomerization at 300jC (Fig. 33). In the case of abietic acid, the rearrangement of double bonds from an alternating double–single–double bond, referred to as a conjugated double-bond structure, in adjacent rings results in a more dispersed structure as noted in the various compounds due to the rearrangement that takes place (Fig. 33). The redistribution due to thermal isomerization results in an equilibrium concentration of only 10–20% abietic acid. f. Disproportion of Abietic Acid. Rosin also undergoes disproportionation [127,128], a proess in which a radical transfers a hydrogen atom to another radical. It occurs more readily on catalytic surfaces, which results in some molecules of abietic acid being dehydrogenated, and other molecules being hydrogenated to give dehydroabietic, dihydroabietic, and tetrahydroabietic acids as depicted in Fig. 34. g. Other Considerations. The conjugated double-bond structure of abietic acid may assist in transporting Sn more readily through coordination—the sharing of electronegative electron pairs on carbon atoms with electropositive Sn ions—because of the higher electron density and mobility of electrons in the conjugated bond region. It is important to note that this type of rearrangement can take place even in synthetic fluxes containing molecules of similar structures, during heating. Therefore, it is possible that they also can exhibit variability in composition and performance depending on their thermal history.

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FIG. 33 Isomerization of abietic acid due to heating.

3. Relationship Between Thermal Stability and Activity Rosin oxidizes in air and darkens when heated to soldering temperatures, but can withstand degradation at temperatures up to 400jC, for several hours, without significant discoloration or charring in very low oxygen (<10 ppm) atmospheres (N2). It appears to evaporate quite rapidly when its surface-to-volume ratio is large, and more rapidly in air than in nitrogen. The difference in evaporation rates is likely due to bonds breaking under the influence of oxygen. During the Spalik study of Sn transport by rosin, the rosin was observed to change in color from light amber to a very pale, transparent yellow as a liquid and became a light yellow solid upon cooling. It could be remelted and held between 350jC and 400jC for several hours without a visible change if a N2 flow was maintained, but charred and blackened quickly if the N2 flow was

FIG. 34 Disproportionation of abietic acid on a catalytic surface.

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interrupted. The evaporation rate during this study was low. However, as discussed in Sec. 6.8.2, isomerization and disproportionation (Figs. 33 and 34) occur as the temperature approaches 300jC, which decreases the activity of the flux. Activity increases with temperature until disproportionation of abietic acid becomes significant. Therefore, it is generally preferred that the temperature profile consist of a steep ramp in the range of solder melting, especially for alloys that melt near or above 300jC. Typically, the ramp of a reflow temperature profile increases slowly to allow solvents to evolve, has a ‘‘soak’’ plateau between the solvent boiling point and solder melting temperature, and has a rapid ramp to the solder melting temperature. Fluxes based on only rosin dissolved in alcohols typically exhibit high surface insulation resistivity (SIR), so flux residues are often left without concern for corrosion and electrical leakage. But disproportionated rosins left after reflow display even higher surface insulation resistivities than normal rosins [129]. This property is advantageous when cleaning is difficult. However, it is important to note that residues resulting from high levels of tarnish, oxidation, and charring accelerated by atmospherical oxygen may have significantly lower SIR values. 4. Acid Number Rosins are organic acids and their fluxing effectiveness depends on their acid number; a high acid number (and saponification number) is an indication of good fluxing action. A higher acid number indicates a greater ratio of acidity to inert material in the rosin. Because it is the acid functionality that provides the flux activity with respect to solder metal oxides, it is very important that the acid concentration in the flux is sufficient to react with all the oxides present. The best rosins to be used as a flux material for soldering usually have an acid number in the range of 160–170. The acid number is the number of milligrams of potassium hydroxide (KOH) required to neutralize 1 g of rosin. Table 14 lists the acid numbers of several types of rosin compared with pure abietic acid [122,130]. Theoretical acid numbers for organic acids can be easily calculated by dividing the formula weight of KOH (56.11) by the formula weight of the acid and multiplying by 1000. Using abietic acid (302.44) as an example gives 56.11/302.441000=185.52. This might appear to be a theoretical limit for rosin, but a rather rare resin acid derived from P. merckussi (Vietnam) can be 190 or more. For the acid number to be higher than 185, the rosin must be comprised of acids with low formula weights. The detailed resin acid composition of rosin is usually of little consequence or interest to the end user, only that the acid number is sufficiently high. It is, therefore, not surprising that rosins are classified by their acid numbers. 5. Rosin-Based Classification System A flux classification system for rosin fluxes that has been in use for many years defines the use of additives to a rosin base to increase the flux activity. Some applications require stronger fluxing action than is provided by rosin alone. Therefore, compositions with an increased activity have been formulated. The system consists of the designations R, RA, and RMA for rosin, activated

TABLE 14 Materials

Acid Numbers of Several Rosin

Rosin Abietic acid Gum rosin Wood rosin Hydrogenated rosin Dehydrogenated rosin Polymerized rosin Dimerized rosin

Acid number 185 168 163 160 154 144 140

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rosin, and mildly activated rosin, respectively. Table 15 provides guidelines for comparing the equivalency of these fluxes to ANSI designations [110,131].

I.

Halogen-Free Organic Fluxes

As was noted with rosin-based fluxes, the activators added typically contain halides such as chlorides and bromides, which are fairly corrosive. There are, however, several categories of milder, halide-free activators such as organic acids and amine compounds. Because these fluxes are less aggressive, they are slower-acting and do not have the ability to remove thicker tarnish layers. For these fluxes to be effective, it is necessary to provide pristine solderable surfaces. 1. Carboxylic Acid Fluxes A carboxylic acid always contains the carboxyl (–COOH) functional group and is the primary active ingredient in OR classification. There can be one or several carboxyl groups on a single molecule, and are designated as monocarboxylic, dicarboxylic, tricarboxylic acids, etc. Many fluxes are composed of simple monocarboxlyic and dicarboxylic acids, as depicted in Fig. 35a and b. The stick figure illustrations indicate that both oxygen atoms are attached to their adjacent carbon atom and that the ionizable hydrogen is attached to an oxygen atom. Resonance or electron sharing (also referred to as delocalization) between the carbon atom and the two oxygen atoms is the reason that the hydrogen is able to be more easily ionized. Although depicted as being attached to one oxygen atom, the hydrogen atom (ion) is really shared by both. These diagrams illustrate the similarity in structure of some organic acids commonly used in fluxes. This class of fluxes typically undergoes what is described as simple acid–base reactions with metal oxides, although chelation, coordinate covalent bonding, (i.e., formation of complexes), and electrochemical interactions (i.e., oxidation and reduction) may also be involved [102,104]. These types of reactions will be explained later. However, there can be subtle yet significant differences in performance related to molecular structure, carbon chain length, melting point, and boiling point. a. Melting and Boiling Points. A flux can consist of a single carboxylic acid or a combination of these acids that exhibit melting and boiling points consistent with the reflow temperature of a particular solder alloy. It is necessary that the acid and/or its respective salt be present in a liquid state during reflow. That is, the molten acid, salt, or both should coat the solderable interfaces through the reflow process in order to achieve good fluxing. For example, if the acid remained solid, the reaction rate would be too slow, and if it evaporated too quickly (before reflow), it could not participate in the reaction. The melting and boiling point trends of the first members of the monocarboxylic and dicarboxylic acid series are illustrated in Figs. 36 and 37. From the graphs, it can be seen that most of the monocarboxylic acids are liquids at room temperature, but all the dicarboxylic acids are solids. Also, the boiling points of most monocarboxylic acids are below reflow temperatures of solders utilized in electronic assemblies but are above solder reflow temperatures for dicarboxylic

TABLE 15 ANSI Flux Designations and their Equivalent R Classification Designations ANSI flux designations L0 L1 M0 M1 H0 H1

Equivalent R classification fluxes All R, some RMA, some low solids no-clean Mostly RMA, some RA Some RA, some low solids no-clean Mostly RA None Mostly water-soluble and synthetic-activated

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FIG. 35 Structural formulas of straight-chain (a) monocarboxylic acid and (b) dicarboxylic acid.

FIG. 36 Plot illustrating the monotonic increase of the boiling points of monocarboxylic acids with increasing carbon atoms in the chain. Their boiling points do not exhibit a trend.

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FIG. 37 Plot that indicates the increase in boiling points, and oscillating trend of melting points with the number of carbon atoms in the chain of dicarboxylic acids.

acids. It is important to remember that for an acid to be active in the role of a flux, it must be molten or in solution. 1. Monocarboxylic acids—It should be noted that the temperature range between melting and boiling is important because a greater range provides more time for fluxing during a reflow operation. For monocarboxylic acid series, there is an initial downward trend in the melting point due to hydrogen bonding, followed by an increase as a result of longer carbon chain lengths. Corresponding with the trend in melting points, there is a monotonic increase in boiling points with carbon chain length. Also, even with the slight oscillation in melting points, the melting point range is noted to generally broaden with increasing numbers of carbon atoms. This suggests that for monocarboxylic acids, a higher-molecular-weight acid is better suited for eutectic solders. 2. Dicarboxylic acids—The boiling points of dicarboxylic acids fall within a relatively narrow range, and it appears that they are all nearly equivalent. Note that the melting points of dicarboxylic acids not only decrease, but oscillate downward—those with oddnumbered carbon chains melting at much lower temperatures compared to adjacent even-numbered carbon acids. These large excursions are not expected based on molecular weight or carbon chain length alone. It is beyond the scope of this text to delve into all of the factors affecting melting points, but it is important to be cognizant of these characteristics because a flux’s activity is initiated when the acid melts. Thus, those dicarboxylic acids with lower melting points allow more time for flux action before the solder melts. Also, evaporation due to sublimation increases with temperature during a typical reflow profile. The vapor pressure [132] and, consequently, evaporation rate vs. temperature are quite similar for several dicarboxylic acids (Fig. 38). For example, adipic, pimelic, and suberic acids will evaporate at approximately the same time during a typical furnace reflow profile, but because pimelic acid melts earlier, it provides more time for fluxing before it is vaporized. 3. Early evaporation concern—Boiling points for the first three dicarboxylic acids are not depicted in Fig. 37 because oxalic acid sublimes at 157jC, malonic acid decomposes at mp 130–35jC, and succinic acid decomposes at 235jC. Sublimation is defined as the process of evaporation directly from the solid state. It is important to be aware of the fact that evaporation of liquids occurs below their boiling points, and that the rate of evaporation increases with temperature. It is possible for a flux to evaporate completely before its boiling point, and for that matter before the solder reflow temperature is attained, in which case the flux will be ineffective. This is illustrated by the TGA plots of Fig. 39a and b that show pimelic acid (bp 342jC) and palmitic acid (bp 351jC)

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FIG. 38 Vapor pressure plots of dicarboxylic acids illustrating that they have very similar evaporation rates.

commencing rapid evaporation at about 200jC and are completely evaporated before 300jC. In no-clean fluxing operations with flip chips it is important to apply sufficient flux to assure its presence and effectiveness during reflow, but not so much that an excess remains postsoldering. Boiling points indicated for the rest of the acids are the temperatures at which their vapor pressures reach 760 Torr, in accordance with the definition of boiling point. However, they evaporate quite rapidly before reaching their respective boiling points and are sometimes described as subliming even though they are in the liquid state. This can be advantageous for no-clean fluxes used for flip-chip attachment because excess flux components evaporate, leaving very little residue. b. Relationship Between Properties and Parameters. Several properties are discussed when trying to correlate the effectiveness of a flux component to solderability. Among these are the activation energy (Ea), as described below; the acidity constant (Ka), described in Sec. 6.11.1; and wetting rates. Wetting rates are determined by measuring the increase in force vs. time using a specially designed analytical balance of solders wicking up on a specified solderable coupon. This procedure is described in detail in this chapter. It would be useful if these and other properties could be related in such a way that flux design and solderability could be predicted on the basis of scientific principles, but, unfortunately, attempts to correlate flux effectiveness with chemical structure, ionization constants, activation energy, and wetting rates have been generally unsuccessful. Kinetics—The activation energy for a reaction is the energy that must be supplied to reacting molecules before they can attain the transition state. For a chemical reaction to take place, it is often necessary for the reacting species to pass through a higher energy state than they are in originally before any product can be formed. This is known as the transition state or activated complex. The products formed will have a lower energy than the maximum energy required to initiate the reaction. Reaction rates are related to activation energy and temperature by the equation:

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FIG. 39 Thermal gravimetric analysis plots illustrating that volatilization begins before 200jC and complete loss begins before 300jC.

Ea

Rate ¼ Ae RT

ð49Þ

where A is a constant which assumes that other variables and concentration are fixed, e is the base of the natural log 2.718. . ., R is the gas constant (8.31 J/mol K), and T is the absolute temperature. Higher activation energies represent greater energy barriers [i.e., the energy that must be added (DHact) to raise the energy of the reacting species to the increased energy level to achieve the transition state] (Fig. 40a and b). Note that according to the equation, the rate increases with temperature and that the reaction rate with respect to temperature is greater for lower activation energies. Therefore, low activation energies predict high wetting rates and more complete reflow within a process time window. Catalysts are substances that can facilitate the formation of a transition state having a lower activation energy than one formed in the absence of a catalyst. It is possible that the metal surfaces, solder alloys, and other flux components provide a catalytic effect. For example, a high activation energy barrier is observed to exist when reflowing solders in a H2 atmosphere. Temperatures over 400jC are required to

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FIG. 40 Potential energy plots illustrating the energy change during the progress of (a) an exothermic reaction, and (b) an endothermic reaction.

overcome the activation energy barrier [157]. Reflowing in the presence of hydrogen with Quadrol has been shown to significantly reduce the required reflow temperature, but it has not been verified whether or not this is a reduction in the hydrogen activation energy or simply a fluxing effect imparted by the Quadrol. However, platinum (Pt) and palladium (Pd) are well-known catalysts that significantly reduce the energy barrier. Hydrogen reflow occurs more easily when these elements are near or in contact with the solder to be reflowed. Electrodeposition and vapor deposition of a seed layer under or over solder bumps reduces the temperature at which H2 reflow can

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SINGLER ET AL. be achieved. Some promising high-vacuum results are believed to have resulted from using Pt on flip chips under bump metallization [66,67]. Lack of parametrical relationships—Table 16 illustrates an attempt to establish a parametrical relationship or trend upon which to predict wettability based on the carbon chain length and other parameters of dicarboxylic acids [133]. The dependence of wettability was measured on copper substrates with Sn–Pb eutectic solder. The correlation of the effectiveness of the flux with activation energy, acid ionization constants, or chemical structure has not been successful. Note in particular that although adipic, pimelic, and suberic acids appear to follow a trend in wetting rates and activation energies, this trend does not hold for the other acids listed. Also, although the acid ionization constants are very similar for all of the acids above n=2, there are substantial differences in wetting rates. Oxalic acid with a pKa of 1.23 and a wetting rate of 1988 dyn/cm/sec is not very different from suberic acid with a pKa of 4.52 and a wetting rate of 1892 dyn/cm/sec. Noteworthy is the fact that adipic acid has the lowest wetting rate and highest activation energy but is used in many fluxes because it performs well in practice. In addition, pimelic acid, which does not have significantly different parameters compared to the other acids listed in Table 15, is the active species in a flux that has provided satisfactory results for the assembly of flip chips to their carriers for many years [134]. Although it is useful to observe fundamentals and attempt to correlate them with soldering effectiveness, such empirical studies are generally unsuccessful at predicting soldering effectiveness when other parameters are changed a priori. This example, which attempts to correlate a series of measurements with chemical structure and other parameters, illustrates the fact that it is very difficult to predict a best candidate for a flux in a particular application, and underscores the need to investigate a variety of formulations using screen tests, but to test the final candidates under actual manufacturing reflow conditions.

2. Amines Amines are organic bases having the general formula R-NH2, R2NH, and R3N referred to as primary, secondary, and tertiary amines, respectively. Lower-molecular-weight primary amines, although water-soluble, have boiling points too low to be useful for soldering. Because hydrogen bonding is mainly responsible for raising boiling points and water solubility, it should be expected that secondary and tertiary amines would be even less soluble because they have less hydrogen. Boiling points and solubility are also affected by the hydrocarbon part of the molecule (e.g., TABLE 16 Acid

Key Parameters of Several Dicarboxylic Acids Structure (n)

(Dibasic) HOOC(CH2)nCOOH Oxalic 0 Malonic 1 Succinic 2 Glutaric 3 Adipic 4 Pimelic 5 Suberic 6 Azelaic 7 Sebacic 8 Fumaric HOOCCHjCHCOOH (Monobasic) CH3(CH2)nCOOH Myristic 12 Palmitic 14 Searic 16

Activation energy, Ea (kJ/mol)

Wetting rate (dyn/sec at 233jC)

13.22 11.19 14.15 16.20 42.84 32.60 12.72 18.54 40.07 12.29

1988 1891 1178 1274 456 930 1892 930 888 1069

46.89 34.23 43.06

340 540 340

pKa1

pKa2

1.23 2.83 4.19 4.35 4.43 4.48 4.52 4.53

4.19 5.69 5.48 5.42 5.42 5.42 5.40 5.40

3.02

4.38

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longer carbon chains increase boiling points and decrease water solubility). Above six carbon atoms, amines have borderline water solubility and become difficult to remove. Compounds that have more than one amine group are called diamines, triamines, and tetraamines. Some examples are ethylenediamine (NH2CH2CH2NH2) and ethylenetriamine (NH2)2CH2CH2NH2. Generally speaking, increasing the number of hydrogen atoms attached to nitrogen atoms infers increased water solubility, and increasing the number of carbon atoms infers decreased solubility. a. Use of Amine Derivatives. Ordinary amines are not employed in fluxes, but their salts and derivatives are. The salts are crystalline materials that decompose before or upon melting and leave a basic low-water-soluble amine residue. Although amines do not contain halides, they are still corrosive and must be removed because they are able to react to form carbonates, chlorides, and sulfides. Due to this characteristic, they increase the potential for failure due to reduced surface insulation resistance (SIR), and difficult-to-remove carbonate and sulfide residues. b. Effect of Sulfides. With respect to sulfides, it is worth mentioning that liquid propane (LP) and natural gas used for heating contain odorants, mercaptans, and organic sulfides that are released in sufficient quantity to tarnish metal surfaces quite rapidly. This is the reason why silverware darkens quickly in homes that use gas for cooking. It is important to be aware of this fact when storing components. Materials such as urea, ethylenediamine, monoethanolamine, and triethanolamine, and derivatives such as aniline phosphate are extensively utilized as activators in fluxes. These materials are also temperature-sensitive and, as noted above, have the potential to leave difficultto-remove residues [102,135]. Structural and empirical formulas of several amine and derivative amine compounds, emphasizing functional features, are given in Fig. 41. c. Charging with HCl. Amines are bases that can be charged with hydrogen chloride (HCl) gas according to the reaction: R  NH2 þ HCl ! R  NH2 .HCl

ð50Þ

to form amine hydrochlorides. Each molecule of an amine can retain one molecule of HCl. Amine hydrochlorides release HCl when heated, which in turn reacts with the metal oxide. Amine constituents of a flux can be fully or partially charged with HCl to achieve a desired level of

FIG. 41 amines.

Structural representations of amines, amine hydrochlorides, amine carbonates, and alcohol

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activity. Although such species highly activate the flux, they must be removed quickly postsoldering because they have the potential for corrosion. It has been demonstrated that the Cl ion can reduce Cr–Cu line adhesion rapidly at elevated temperature and humidity [136]. Amines can also form coordination complexes with metals, thus increasing solubility [102,103]. 3. Amino Acids Amino acids contain both the –NH2 (amine) and the –COOH (carboxyl) groups. They are classified as acidic, neutral, or basic depending on which functional group is predominant, as indicated in Fig. 42. In contrast to amines and carboxylic acids, they are crystalline, nonvolatile solids that melt upon decomposition at fairly high temperatures. The acidity constants of neutral amino acids are extremely low compared with organic compounds of similar structure having only carboxylic acid functionality. Their properties are consistent with dipolar saltlike ion structures depicted below: 2 3 3 2 3 R R R ½OH  ½Hþ  5 4 5f4 5f4 j j j H2 O H2 O H3 Nþ-CH-COO H3 Nþ-CH-COOH H2 N-CH-OO 2

Basic Salt form ðNaþ þ OH Þ

Neutral Salt form

Acid Salt form ðHþ þ Cl Þ ð51Þ

FIG. 42 Structural representations of neutral, acidic, and basic amino acids.

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That is, they are ionic and water-soluble, and can form basic, neutral, or acidic salts. Typically, they are weaker than acids [137], having only the carboxyl group functionality, but can form coordination complexes with metals by the sharing of lone pairs of electrons on nitrogen and oxygen atoms with metal ions. This can help to solubilize metal ions in the flux and help in their removal in postsolder cleaning [102]. 4. Alcohol Amines Alcohol amines contain the –OH (hydroxyl) group in addition to the amine group as depicted by the formulas in Fig. 41. These compounds are bases that rapidly form salts with acids, and have good solubility in both aqueous and organic systems. Therefore, they serve to help solubilize and emulsify flux components and reaction products [102]. An important feature of alcohol amines is their ability to help solubilize residues and form complexes. a. Inductive Effect and Its Role in Acidity. In symmetrical organic molecules such as ethane, a nonpolar, covalent bond exists between atoms of equal electronegativity. But it is also not unusual for covalent bonds to exist between elements of compounds such as methyl chloride, where there is a difference in electronegativity. So in the methyl chloride case, the bonding electrons are closer to chloride than to carbon because the chlorine has a higher electronegativity. In cases where there is a shift in the bonding electrons, the bonds exhibit a slightly polar character. The chlorine end of a methyl chloride molecule has a small negative charge, whereas the carbon has a small positive charge. 5. Increasing Acidity (Activity) The strength of acids and bases is affected by molecular structure, steric effects, resonance, inductive effects, hydrogen bonding, and solvation. a. Inductive Effect and Its Role in Acidity. The inductive effect is illustrated by the fact that acidity is increased by a substituent that withdraws electron density away from the carboxyl group, and is decreased by a substituent that releases electrons to the carboxyl group. A more electronegative atom adjacent to a carbon atom attached to the carboxyl group increases acidity. The effect of halogens on acidity can be observed in Table 17 by noting the trend in pKa values with the electronegativities and number of electronegative substituents attached to the carbon adjacent to the carboxyl group. Acidity and pKa notation are discussed in Sec. 6.11.1, but suffice it to note here that the pKa notation is a logarithmical representation of the degree of ionization of the H+ ion at equilibrium with an integer change indicating a 10-fold reduction in acidity (e.g., an acid with a pKa=1 is 10,000 times stronger than one with a pKa=5). Note the trend with respect to electronegativity (F>Cl>Br) for the monohalide acids (e.g., fluoroacetic is the strongest). The addition of electronegative atoms increases the inductive effect (i.e., induces or causes an increased withdrawal of electron density from the carboxyl group). The size of electronegative substituents is also a contributing inductive factor that results in tribromoacetic acid being the

TABLE 17

Effect of Halogens on Acidity

Acid Fluoroacetic Chloroacetic Bromoacetic Difluoroacetic Dichloroacetic Dibroboacetic Trifluoroacetic Trichloroacetic Tribromoacetic

Formula

pK1

FCH2COOH CICH2COOH BrCH2COOH F2CHCOOH Cl2CHCOOH Br2CHCOOH F3CCOOH Cl3CCOOH BR3CCOOH

2.59 2.87 2.90 1.33 1.26 1.39 0.50 0.52 -0.15

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strongest acid (pKa=0.15), as listed in Table 17. It is preferable to state that increased acidity is due to the stabilization of the conjugate base (i.e., the negative ion) of the acid. b. Effect of Number and Spacing of Carboxyl Groups. Acidity also increases with the number of carboxyl groups; thus, a tricarboxylic acid is stronger than a dicarboxylic acid, and a dicarboxylic acid is stronger than a monocarboxylic acid, etc., as long as the groups are in close proximity to each other. If the carboxyl groups are widely separated, the inductive effect will be mitigated, and the acid strength of the first ionization step will be similar to that of a monocarboxylic acid. c. Effect of Successive Ionization. The notation pKa1, pKa2, pKa3, etc., represents successive ionization constants [i.e., they represent successive stepwise ionization (i.e., H+ ion release) of the carboxyl groups]. It should be noted that successive stages of ionization, as each H+ ion is removed, correspond with an increase in negative charge on the conjugate base. An increase in negative charge further destabilizes the conjugate base, and makes it more difficult to remove successive H+ ions, thus resulting in successively weaker acids. In other words, each successive H+ ion-releasing species becomes a weaker acid. d. Effect of Oxygen. Oxygen atoms increase acidity by the inductive effect and double bonds increase acidity by resonance stabilization. These effects are illustrated by the pKa values listed under the structural formulas of Fig. 43. Compare, for example, propionic acid (pKa=4.874) with similar monocarboxylic acids and note that the influence of an additional oxygen atom attached to the carbon next to the carboxyl group causes a 10-fold increase in acidity for 2-hydroxypropionic (pKa=3.858) and methoxy acetic (pKa=3.570) acids. Resonance and the inductive effect combine in 2-oxopropionic (pKa=2.49) to increase the acidity by another order of magnitude. e. Effect of Solvents and Surfactants. Hydrogen bonding and solvation are additional factors affecting acidity and are important in the selection of solvents and surfactants. Changing the solvent system or surfactant can significantly change flux effectiveness. Typically, the solvent or surfactant in a flux formation is an alcohol, glycol, polyol, etc., having one or more (–OH) groups. In addition, there may be (–CH2–O–CH2– or R-O–CH2CH2–) type functional groups

FIG. 43 Structural formulas illustrating the effect of substituent groups on the acidity (pKa) of carboxylic acids.

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(e.g., Tween, Brij, Igepal) as depicted in Fig. 44. It is beyond the scope of this work to detail the function and importance of solvents, but their importance should not be underestimated. Care must be exercised not to render a flux ineffective due to excessive dilution. Some flux compositions have a small amount of water added to increase effectiveness. It is important to select the right combination of organic acid, solvent, and surfactant for optimum performance. These combinations are very difficult to reverse engineer (i.e., chemically analyze), part of the reason so many proprietary formulations continue to exist. Another factor contributing to flux activity is the ability of some species to form stable coordination complexes with heavy metal ions [103]. Compounds containing nitrogen, classified as amines, ethylenediaminetetraacetic acid (EDTA), and N,N,NVNV-tetrakis(2-hydroxypropyl)ethylenediamine (Quadrol) (Fig. 45), and triethanolamine are able to form multiple bonds with a metal ion (central atom) as depicted in Fig. 46. The species forming the bonds are called ligands, the arrangement around the central atom is the coordination sphere, the number of bonds formed is the coordination number, and the species contributing the pair of electrons are the ligands. Any species having a pair of nonbonded electrons (e.g., nitrogen and oxygen) can be a possible ligand in solution, but to be effective in fluxing, it must be stable at reflow temperatures. Species having nitrogen and oxygen atoms connected by an ethylene (–CH2CH2–) linkage have the preferred structure to enclose the central atom, and to satisfy the coordination number. Synthetic fluxes discussed in Sec. 6.10 are examples that illustrate the application of these concepts.

J.

Synthetic Fluxes

The concept of synthetic fluxes derives from the desire to control the composition of natural fluxes. It is generally conceded that good process control requires that the composition of materials entering a process be consistent. Additionally, synthetic mixtures can be modified to provide chemical and physical properties that meet the needs of specific fluxing requirements. For example, Figs. 47 and 48 illustrate two commercial flux systems formulated for high-temperature stability, sufficient tackiness, and amenability to aqueous cleaning. Note how the formula structures and combination of ingredients reflect the principles discussed in Sec. VI.I.5. That is,

FIG. 44 Structural representations of alcohols. The OH functionality is important in water solubility and surfactant properties.

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FIG. 45 Examples of two important nitrogen-containing molecules that illustrate the electronegativity effect.

FIG. 46 Examples of coordination complex formation by nitrogen-containing molecules.

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FIG. 47 Some components of a commercial, nonvolatile, water-soluble flux.

polycarboxylic amino acids provide good fluxing activity and the possibility of forming stable complexes with metal ions, whereas the polyalcohols provide tackiness, viscosity control, temperature stability, and water solubility for postsolder cleaning.

K.

Chemical Reactions Involving Flux

The most frequently suggested role of a flux in soldering is oxide removal. Except for a few circumstances, that typically is not what occurs. Terminology such as ‘‘the purpose of flux is to dissolve the tarnish’’ or ‘‘remove the oxide,’’ frequently used to describe the role of flux, should be understood to mean a chemical reaction that converts the metal oxide to a species that promotes solder wetting. Flux reacts with oxides, making them more easily displaced by the liquid solder. It is thought that the salt formed remains at the solder surface and acts as a pseudo-surfactant that actually facilitates a reaction at the metal interfaces. Chemical reactions that take place during soldering may be very complex. Usually the reactions can be described as acid–base neutralizations, but also can involve electrochemical reduction and the formation of coordination

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FIG. 48 Some components of a commercial, nonvolatile, water-soluble, tacky flux.

complexes. Also, it should be remembered that metallic tin is very important as a reducing agent, and is known to reduce Pb [138] and Cu oxides. A brief review of relevant chemical principles follows. 1. Acid–Base Reactions, pH and pK Theories on the nature of acids and bases are presented in most general chemistry texts [139]. The Arrhenius theory proposes that an acid is a material that releases protons (H+), and a base is a material that provides hydroxyl (OH) ions in solution. It follows that the combination of these ions produces water: Hþ þ OH ! H2 O

ð52Þ

Because H+ is solvated in water, it is usually shown as H3O+, and the equation becomes: H3 Oþ þ OH ! 2H2 O

ð53Þ

For the reaction of a strong acid (completely ionized) (e.g., HCl) with a strong base (e.g., NaOH), the equation is: Base Salt Water Acid H3 Oþ þ Cl þ Naþ þ OH ! Naþ þ Cl þ 2H2 O

ð54Þ

Because the Na+ and Cl are unchanged on both sides of the equation, they are considered ‘‘spectator’’ ions and may be left out of the equation, which then becomes identical to the previous equation. a. Bronsted–Lowry Theory. Flux–metal oxide interactions are better described by the Bronsted–Lowry theory, which defines an acid as a proton donor, and a base as a proton acceptor. Utilizing this concept, a metal oxide (MOx) reacts with an acid (HA):

SOLDER WETTING AND SPREADING

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Metal Acid Metal Water Oxide Salt MOx þ 2xHA ! MA2x þ xH2 O

ð55Þ

yielding a salt and water. b. Dissociation Constant. The ability of a weak acid to dissociate into its conjugate acid and base is characterized by its acid dissociation constant Ka or pKa, where ‘‘p’’ is an operator log10[x] or log10[1/x] (take the negative of the base 10 logarithm of Ka). The dissociation constant is a measure of the strength of an acid (i.e., its ability to ionize in solution). It is determined by taking the product of the activities of the right-hand species, dividing them by the activities of the left-hand species. Consider the case of water that dissociates into its component species: H2 OWH3 Oþ þ OH ðUse of Gibbs free energy of formation tables and the carbon small positive charge electronegativity added algebraicallyÞ The chemical activity is described as:  H3 Oþ ½OH  Ka ¼ ¼ 1  1014 ½H2 O2

ð56Þ

ð57Þ

Because the activity of H2O in its standard state is defined as unity, the equation may be simplified to: ½Hþ ½OH  ¼ 1  1014

ð58Þ

Applying the operator ‘‘p’’ to all terms gives: pH þ pOH ¼ 14; or pH ¼ pOH ¼ 7

ð59Þ

Consider now the case of weak acids such as acetic acid (CH3COOH) (i.e., those acids that do not dissociate completely in water compared to HCl that does). For convenience, the formula for acetic acid can be abbreviated as HOAc, in which case the equation for its dissociation in water can be written as: HOAc þ H2 OWH3 Oþ þ OAc and the equilibrium constant expression is:  H3 Oþ ½OAc  ¼ 1:8  105 Ka ¼ or ½HOAc½H2 O

ð60Þ

pKa ¼ 4:7

ð61Þ

Extensive tables of dissociation constants for inorganic and organic acids, bases, and amino acids are listed in Refs. 140 and 141. 2. Decomposition Reactions When either the initial components of a flux or the products formed from the interaction with surface oxides are heated during reflow, it is possible that they polymerize, revert to the original species, decompose to carbonaceous (charred) residues, or convert to volatile effluents. Table 18 lists several reactions and possible residues left after peak reflow temperatures for some typical chloride salts and a general organic salt. These residues remain in the vicinity of the solder connection. Organic residues are usually innocuous chemically and electrically, but may interfere with adhesion of underfill materials used with flip chips and some chip scale and ball grid array components. Principle exceptions are amine and chloride residues that must be removed. Highermolecular-weight, thermally stable acids such as abietic acid remain as a mixture of both acid and

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TABLE 18 Residues That Can Be Formed by the Reaction of Tin and Copper Oxides with Chloride Salts and an Organic Salt Reaction

Residue

SnO+2HCl=SnCl2+H2O SnO+2NH4Cl=SnCl2+H2O+NH3 SnO+2RNH2HCl=SnCl2+2RNH2+H2O CuO+2HCl=CuCl2+H2O

SnCl2 SnCl2 SnCl2 CuCl2

CuO+HCOOH=Cu+CO2+H2O

Cu

PbO+2HCl=PbCl2+H2O

PbCl2

SnO+RCOOH=Sn(RCOO)2+H2O

Sn(RCOO)2

Properties Stable (mp 247jC, bp 652jC)

Stable chloride at most soldering temperatures CuO reduced to oxide-free metal by formic acid Stable chloride salt at most soldering temperatures Many possible decomposition products depending on the properties of the particular carboxylic acid and its respective carboxylate salt, ambient oxygen levels, and solvent Possible products may volatilize, polymerize, or char Adipic and pimelic carboxylates can form volatile ketones

FIG. 49 Flux reaction with SnO and decomposition with glutaric acid.

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FIG. 50 Flux reaction with SnO and decomposition with pimelic acid.

salt. An interesting example of dibasic acid decomposition that is very desirable for no-clean, flipchip applications is illustrated in Figs. 49 and 50. In this type of decomposition, the carboxylate salt reverts to SnO, and the organic species is evolved harmlessly. Polyfunctional acids can polymerize, leaving nearly impossible-to-remove residues, but they can also be innocuous. There are ongoing efforts to produce fluxes for flip chip applications that polymerize after solder reflow to serve as underfill adhesives as well. A flux that can serve this dual purpose must possess good solder wetting properties, polymerize after reflow is complete, have high electrical resistivity, and be reworkable. 3. Redox Reactions Reactions that involve electron transfer are defined as reduction–oxidation or, simply, redox reactions. Reduction is defined as the gain of electrons by one species, with a corresponding increase in negative charge; and oxidation is the loss of electrons, with a corresponding increase in positive charge. Because it is impossible to have a gain of electrons by one species without a corresponding loss of electrons by another, it is useful to write the equations as pairs of half-cell reactions indicating the direction of electron (e) transfer. Reduction always occurs at the cathode, and oxidation always occurs at the anode of either electrolytic or galvanic cells. Hence, half reactions are sometimes designated as cathodic for reduction, or anodic for oxidation. Extensive tables of half-cell reactions are readily available [142,143] listing standard reduction potentials (SRPs) compared with a standard hydrogen cell. The SRP for a half-cell reaction is the ‘‘voltage’’ produced by an electrochemical cell in which the species of interest [e.g., a Cu electrode in a solution of copper ions (Cu2+) at a concentration of 1 mol/L at 25jC] is compared with a standard hydrogen cell having a Pt electrode in a hydrogen ion H+ concentration of 1 mol/L under a hydrogen H2 pressure of 1 atm. Strictly speaking, the effective concentration (activity) should be unity. The standard hydrogen electrode potential is defined as 0.000 V, and in this example, the measured Cu electrode potential is 0.337 V. In these tables, the half-cell reactions are tabulated as reductions for convenience, so that the most favorably reduced species is at the top and the least favorable is at the bottom. a. Determining Reaction Viability. The species with a higher (more positive) reduction potential is theoretically capable of oxidizing the lower (less positive) species. The tables are

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useful in predicting the spontaneous direction of a reaction, and in determining the corrosive effect of one metal upon another: 1. Use of reduction potential tables—It can be determined which species is reduced or oxidized in a redox couple from standard tables, as exemplified by the brief listing given in Table 19. The cathode and anode half-cell reactions are noted and added algebraically as noted for Pb and Sn. Note that the algebraic sign of the Sn electrode potential is also reversed (i.e., from 0.136 to +0.136): Cathode

Pb2þ þ 2e 5 Pb0

 0:126 V

ð62aÞ

Anode

Sn0 ! Sn2þ þ 2e

0:136 V

ð62bÞ

Net ionic reaction

Pb2þ þ Sn0 ! Pb0 þ Sn2þ

0:010 V

ð62cÞ

The sum of the two half-cell reactions gives the net ionic reaction and the overall cell potential with a positive algebraic sign, signifying that Pb2+ (from PbO) is spontaneously reduced to elemental Pb0 by elemental Sn0. It is also possible to determine potentials at other concentrations, or to determine equilibrium concentrations by the use of Nernst equation. This will not be discussed here, but can be reviewed in most undergraduate level college chemistry texts (e.g., Ref. 144). 2. Use of Gibbs free energy of formation tables—The Gibbs free energy is also a measure of the tendency of a reaction to proceed in the direction indicated by the equation. It is directly related to the electrochemical potential by DG0f =no¨o˜, where n is the number of moles of electrons transferred, o¨ is the Faraday constant (96,485 C/mol), and o˜ is the potential of the cell in volts. One volt coulomb is equal to 1 J and the Gibbs energies in Table 19 are given in kilojoules per mole at 25jC. To determine if a reaction is feasible, subtract the sum of the energies of formation of the reactants from the products of a chemical reaction. If the numerical value of the result is less than zero, the reaction will proceed. Consider an example based on the oxides of Sn and Pb, and their free energies of formation from Table 19. Free energies of formation of the elements in their standard states are defined as zero, so the example may be stated as: Sn þ PbO ! Pb þ SnO ð0Þ þ ð188:0Þð0Þ þ ð257:0Þ

ð63Þ

Summing the free energies as directed gives: ð0  257:0Þ  ð0  188:0Þ ¼ 69:0 kJ=mol

ð64Þ

indicating that Sn0 can reduce PbO to Pb0. Tin, however, is capable of reducing both

TABLE 19 Table of Reduction Potentials for Contaminants Commonly Encountered in Soldering Electronic Assemblies Half-cell reaction

Standard reduction potential (V)

Ag++e!Ag0 Cu2++2e!Cu0 2H++2e!H02(g) Pb2++2e!Pb0 Sn2++2e!Sn0 In3++3e!In0 Zn2++2e!Zn0

0.779 0.339 0.000 0.126 0.136 0.338 0.763

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copper and lead oxides, whereas hydrogen can reduce copper oxide but not tin oxide. This illustrates the important role that tin plays in soldering copper surfaces. It is useful to be familiar with free energy and standard reduction potential tables [145–147], and how they can be used to predict the direction of reactions. For electrochemical reactions involving metals and metal ions, tables of electrode potentials are most convenient, but for reactions involving organic species, it is generally easier to find free energy data. Many undergraduate college texts provide reviews of the principles of thermodynamics (e.g., Ref. 148). b. Tin Depletion Mechanism. This example illustrates that, during reflow in the presence of flux, Sn0 reduces PbO to Pb0, resulting in a change in alloy composition. This is the basis for the observation that the Sn0 concentration of Sn–Pb solders in C4 bumps and other solder joints decreases with successive flux-and-solder reflows. At the beginning of each reflow, both SnO and PbO are present on the surface of the solder. They react with the flux acid during reflow, forming a flux–salt solution. Although in solution, Pb2+ ions oxidize an equivalent amount of Sn0 to Sn2+ from the underlying base solder. Sn2+ replaces Pb2+ in the salt solution, and Pb2+ is reduced to Pb0 and becomes part of the alloy. The sequence is repeated with successive reflows and the amount of Sn0 depletion depends on the degree of oxidation between reflows. This phenomenon, which also occurs with In–Pb solders, is often misinterpreted as the ‘‘leaching of Sn and In from solders’’ by flux, but is predicted correctly by the application of electrochemical principles [149– 151] (Table 20). Spalik conducted a study to determine whether flux or the oxidation of Pb0 in the solder caused the decrease in tin concentration observed with flip chip solder joints (C4) that experienced multiple reflows. Differential scanning calorimetry analyses made on 90Pb–10Sn solder samples both before and after heating to about 350jC in the presence of a rosin flux for more than 24 hr showed no detectable change in the tin content. This result was consistent with the requirement that in order for a metal to dissolve in an acid, H2 must be evolved. In other words, the following reaction must occur: 2C19 H29 COOH þ Sn0 ! Sn2þ þ 2C19 H29 COO þ H2ðgÞ z

ð65Þ

Hydrogen was not evolved in the Spalik study. It was determined that the reduced tin content observed in flip chip solder joints subjected to multiple reflows was the result of lead (Pb) whose TABLE 20 Gibbs Free Energy of Formation for Common Solder Oxides Reaction 2Ag+1⁄2O2!AgO Cu+1⁄2O2!CuO Pb+1⁄2O2!PbO Pb+O2!PbO2 H2+1⁄2O2!H2O Sn+1⁄2O2!SnO Sn+O2!SnO2 Zn+O2!ZnO 2In+3⁄2O2!In2O3 2In+3SnO!Sn+In2O3 Sn+PbO!SnO+Pb Sn+CuO!SnO+Cu H2+SnO!H2O+Sn H2+PbO!H2O+Pb H2+CuO!H2O+Cu C+1⁄2O2!CO C+O2!CO2 CO+SnO!Sn+CO2

DGof (kJ/mol) 11.2 129.0 188.0 219.0 237.0 257.0 521.0 319.0 832.0 121.0 69.0 128.0 20.0 49.0 108.0 137.2 394.3 3.0

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oxidized surface was reduced by metallic tin, which subsequently dissolved in the excess flux. It is more noticeable when the surface-to-volume ratio is large, as is the case for flip chip solder joints. Spalik provided additional verification of this mechanism by dissolving lead(II) acetate in the flux and repeating the experiments. In this case, Spalik observed a substantial reduction in tin content. Based on this observation, a flux composition that could reduce the concentration of Sn0 in low Sn0 solders (e.g., 5/95 or 3/97 Sn–Pb) by 1–2% was formulated [138], thereby significantly increasing the ductility and resistance to thermal fatigue of flip chip solder joints, while maintaining the wetting characteristics of the higher-Sn-content solder. c. Potential as Gaseous Fluxes. With regard to oxide reduction by the active organic species in a flux, Table 21 shows the Gibbs free energy of formation DG0f of some carboxylic acids, and the results of free energy calculations for their reduction reactions with Sn and Cu oxides. As noted earlier, this information is useful in predicting whether or not a reaction is feasible. Using

TABLE 21 Gibbs Free Energies for the Reduction of Solder Oxides by Carboxylic Acids DG0f (kJ/mol)

Reduction reaction Formic O H-C OH

þ CuOý Cu þ CO2 þ H2 O

361.4

129.0

0

394.3

237.0

140.9

Oxalic O O C-C þ SnO ý Sn þ 2CO2 þ H2 O OH HO 697.9

257.0

0

394.3

237.0

70.7

Formic O H-C OH

þ SnO ý Sn þ CO2 þ H2 O

361.4

257.0

Acetic H

O

H-C-C OH

H 390.0

0

394.3

237.0

12.9

þ 4SnO ý 4Sn þ 2CO2 þ 2H2 O

257.0

0

394.3

237.0

155.4

Succinic H

O

H

O

C-C-C-C HO 747.4

H

H 257

OH

þ 7SnO ý 7 Sn þ 4CO2 þ 3H2 O2

0

394.3

237.0

258.2

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the example of succinic acid as a potential reducing agent for SnO, as depicted in the table, the calculation is: Products  Reactants ½7ð0Þ þ 4ð394:3Þ þ 3ð237:0Þ  ½ð747:4Þ þ 7ð257:0Þ ¼ 258:2 kJ=mol which shows clearly that reduction of SnO by succinic acid is not possible, and thus reaffirming the more likely acid–base type of reaction described previously. This is of interest because gaseous formic and acetic acids have been considered potential candidates for ‘‘clean’’ gaseous fluxing. Based on the large negative free energy (right-hand column), formic acid easily reduces CuO, but the reduction of SnO is not nearly as favorable. Even though some reactions may be feasible, other factors may take precedence. For example, formic acid can also react with SnO as an acid and produce tin(II) formate. So instead of a clean reduction reaction, a carboxylate salt is produced. Oxalic acid can also reduce SnO, but acetic acid and those having more carbon atoms cannot. These data indicate that reduction of oxides is only feasible for several organic acids. Only those reactions for which the calculations result in a negative value are feasible. d. No Relationship. It should be noted that electrochemical potential and Gibbs free energy are not related to activation energy. Activation energy represents an energy barrier to an otherwise spontaneous reaction. A catalyst can lower this barrier, but cannot cause a nonspontaneous reaction to proceed. 4. Typical Flux Reactions The primary reaction of a flux (acid) with a metal oxide (base) is to form a salt at the surface of the solder, and water that is evolved with heat during reflow. A study of the reaction products of abietic acid with SnO, PbO, and CuO identified carboxylate salts [152]. It should be emphasized that the product of this reaction is an organic salt, and should not be referred to as a complex to distinguish it from coordination complexes. The salt acts as a surfactant (i.e., it lowers the surface tension of the liquid solder, much as soap lowers the surface tension of water, and promotes the reaction of the metals). The pairs of chemical equations listed in Table 22 illustrate representative reactions of SnO and CuO with different potential flux activators and the reaction products. In general, other activators and oxides could be substituted into these equations. Observe in (1) and (2) of Table 22 that gaseous reaction products would be evolved at reflow temperatures, leaving only their respective liquid chloride salts to promote wetting. If left on the surface, they could potentially cause electrical failure, but because of their good water solubility, they are easily removed. Example (3), on the other hand, leaves an insoluble residue. Zinc oxide is not electrically conductive, but makes it difficult to completely remove any excess ZnCl2. As noted in Sec. 6.7, these activators are not used in electronics applications. The amine chloride (hydrohalide) noted in Eq. (4) illustrates that these activators are not classified as inorganic and fall under the RMA and RA classifications in Table 15. The respective chloride is water-soluble; however, the amine residues may or may not be water-soluble. The lower-molecular-weight compounds (typically less than six carbon atoms) are water-soluble, but also quite volatile; whereas those with more than six carbon atoms are less water-soluble. In any event, chloride and amine residues must be completely removed. It is only necessary to provide a minimum amount of HCl as an activator. For example, a no-clean paste that contains less than 100 ppm of organic bromide was developed [111]. However, if an amine derivative is selected as the activator in a flux, it must be one that provides good water solubility for the residue. Amino alcohol residues have been shown to be significantly easier to remove [102]. The carboxylic acid (Eq. (5)) illustrates the formation of a general salt (R indicates any hydrocarbon) and water. The salt may be innocuous, as in the case of water white (W-W) rosin, and need not be removed if permitted by the application. Other salts may be water-soluble, as noted in the example fluxes of Figs. 47 and 48. Volatile decomposition products (Figs. 49 and 50) that are desirable for no-clean applications may also form. The redox reactions illustrated in example (6) are the only reactions that truly leave no residue. When applicable, they have the advantage over oxide removal techniques such as reduced oxide

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TABLE 22 Representative Reactions of SnO and CuO with Several Potential Flux Activators Representative flux reactions (1) Hydrogen chloride SnO+HClýSnCl2+H2O(g) CuO+HClýCuCl2+H2O(g) (2) Ammonium chloride SnO+2NH4ClýSnCl2+H2O(g)+NH3(g) CuO+2NH4ClýCuCl2+H2O(g)+NH3(g) (3) Zinc chloride SnO+ZnCl2ýSnCl2+ZnO CuO+ZnCl2ýCuCl2+ZnO (4) Amine hydrochloride SnO+2RNH2HClýSnCl2+H2O(g)+2RNH2 CuO+2RNH2HClýCuCl2+H2O(g)+2RNH2 (5) Carboxylic acid SnO+2RCOOHýSn(RCOO)2+H2O(g) CuO+2RCOOHýCu(RCOO)2+H2O(g) (6) Redox SnO+COýSn+CO2(g) CuO+COýCu+CO2(g) SnO+H2ýSn+H2O(g) CuO+H2ýCu+H2O(g) SnO+HCOOHýSn+CO2(g)+H2O(g) CuO+HCOOHýCu+CO2(g)+H2O(g)

soldering activation (ROSAk) discussed in Sec. 6.13.2 in that the oxide is removed at the time of reflow. 5. Effect of Surface Tension and Surface Oxides To achieve proper solder joints, the molten metal must be in direct contact with the metal surface to be joined. For bonding to occur, some form of interaction must take place at the interface. In the case of Sn–Pb solders bonding to Cu, Sn must diffuse into the Cu surface, forming the intermetallic compounds Cu3Sn and Cu6Sn5 with the bulk of the solder alloy bonded to this layer. Solder-to-solder bonding results from alloy reacting at the liquid interfaces. Oxides and surface tension present a barrier to the spreading and reaction of liquid metals in contact with solid metals. The oxide presents a crustlike layer that restricts the flow of molten metal across the interface. Surface tension is the result of unequal forces acting upon the surface atoms. Both oxides and surface tension act to inhibit the transport of one metal into the other. a. Surface Modification Required. The surface layer must be modified for solder wetting to proceed. In addition, surface energies must be favorable for wetting, such that the total free energy of the system is lowered as a result of solder wetting. In the case of solder-to-solder systems, the total free energy is lowered by interaction and minimization of the surface area of different or identical alloys. For solders with different elemental compositions or concentrations, the lower-melting-point solder becomes liquid first and dissolves the other, creating an intermediate composition or concentration at the interface. The increase in entropy of mixing results in lowering of the total free energy of the system. If the solders are of the same concentration, the joining at the solder surfaces results in a reduced area, and thus lowers the total surface energy. This situation is also encountered when a solder paste melts and interacts with a solder used as a lead finish or solder balls on components (ball grid array), or flip-chip solder bumps when attached to chip carriers.

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The primary role of the flux is to interact with surface oxides to form salts, which in turn either assist the oxide layer to be displaced, or act as wetting agents in conjunction with the remaining liquid flux components. In electronic applications, flux should not be expected to clean contamination resulting from poor handling and atmospherical conditions. An R-type rosin or R-type equivalent synthetic flux, now designated ROL0, REL0 or ORL0, is capable of dissolving small amounts of atmospherical contamination and reacting with thin oxides of Cu and Sn. This type of flux can be quite effective with SnO, but not SnO2. Other metal/metal oxide systems such as nickel require additional activity, such as that provided by an amine hydrochloride. However, HCl-activated fluxes are not typically used in high-reliability electronics applications. In these situations, Ni is overcoated with Au and a nonactivated flux is usually adequate.

L.

Choosing a Flux

1. Activity Based on Application The primary consideration in selecting a flux is the intended application. The Institute for Interconnecting and Packaging Electronic Circuits (IPC) has devised a classification consisting of three categories (Table 23) based on application. These categories range from least critical applications such as consumer electronics to those requiring a very high degree of reliability. In general, as the required reliability increases, the activity and potential for corrosion or reducing surface insulation resistance. In applications requiring reduced level activity, fluxes also require more care to assure that parts are properly prepared for adequate solder wetting. Only tin tarnishes can be removed by low-activity fluxes. In addition to selecting a flux that is prudent for the application, there are other considerations as well. Among these are the method utilized to apply the flux, whether the flux is to be removed, and how it is to be removed. 2. Reduced Rosin Content: No-Clean Fluxes There is an ongoing emphasis to eliminate postsolder cleaning operations, particularly those that contain organic species that have been identified as harmful to the environment. Using cleaning agents that contain harmful species adds significant cost to treat and recycle these solvents to comply with local, state, and federal environmental requirements. Accordingly, the so-called lowsolids-content fluxes have been developed, commonly referred to as no-clean fluxes. There are two categories of no-clean fluxes. The first category contains approximately 10–15% resin by volume. These fluxes are used when aesthetics are not a priority, as they leave a visible residue. They are particularly useful in cases where an inerting atmosphere (N2) is usually required because they typically produce acceptable solder joints without the need for a protective environment.

TABLE 23 Flux Activity Based on Application According to IPC Specification IPS-5-815 Class (allowable activity)

Product type

1 (Moderate to high)

Consumer

2 (Low to moderate)

Industrial

3 (Very low to low)

Critical/uninterrupted operation

Product examples Electronic games Hand-held communications Home entertainment systems Laptop/desktop computers Telecommunications Business/industrial electronics Noncritical military Work station/server computers for industry, business, and government

Reliability level required Low to moderate

Moderate to high

High to very high

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However, a class of fluxes containing 3–5% rosin is the most widely utilized low-solids fluxes (LSFs). It is important to always be cognizant of the fact that no-clean fluxes are not no-residue fluxes; rather, they are fluxes that leave a minimal quantity of residue postsoldering that is also benign in terms of reliability. Even though all of the components of a flux may be volatile at reflow temperatures, the chemical reaction with the oxide surface usually produces salts which remain. Figs. 49 and 50 illustrate the reactions of two dicarboxylic acids with SnO through a reflow profile. Also, most ingredients contain impurities of up to 2%. Therefore, it is preferable to use only a sufficient quantity of flux to assure satisfactory results. 3. Increased Rosin Content and Activity As already noted, one way to increase the activity of rosin-based fluxes is to increase the activity of the activator material, hence the progression of R to RMA to RA fluxes. Another less drastic approach is to increase the rosin content, which is typically 35–45%. Because rosin is mostly abietic acid as noted earlier, increasing the rosin content increases activity. Doing so, of course, increases the potential for corrosion. If a rosin flux was not removed postsoldering for an application, it may be necessary to remove it if the rosin content is increased. 4. Aqueous Clean A cleaning approach between utilizing an organic solvent and a no-clean flux is to utilize a watersoluble flux. It may be desirable to utilize a flux that also leaves residues that are a potential reliability concern, and so must be removed. A class of materials consisting of organic acids that is water-soluble has been formulated. These fluxes were originally dispensed as aqueous solutions, but now as alcohol solutions similar to rosin fluxes to reduce the tendency to form solder balls due to ‘‘spitting.’’ Cleaning essentially involves a wash to remove the residue, a rinse to remove the contaminated wash water and solvent, and a drying step to remove leftover moisture [153]. Like all cleaning solvents, a flux system that is compatible with the product design must be selected (i.e., capable of removing residues from closely spaced components, leads, and from beneath components with a small board-to-component gap height as occurs with flip chips and chip scale packages). For aqueous clean fluxes, isomers of carboxylic acids, solvents, and other ingredients can be selected for heat stability in the temperature ranges of interest [154], tackiness, and aqueous solubility. For no-clean applications, the activators and solvents can be chosen so that by optimizing flux quantity, very little residue remains, and postreflow oxygen plasma treatment can offer an effective nonsolvent method for removing low-residue solids associated with a no-clean flux in assembling flip chips on organic carriers [155].

M.

Fluxless Soldering

The advantage of fluxless soldering is that no residues are present postsoldering. If the only impediment to wetting and spreading required for successful soldering was the presence of an oxide surface layer, the simple reduction with forming gas or organic acid vapor would be sufficient to achieve coalescence. 1. Hydrogen Not Practical Utilizing hydrogen to reduce the metal oxides (MO+H2=M+H2O) would be a good candidate for fluxless soldering. However, the reaction is too slow at the standard eutectic Sn–Pb reflow temperature (approximately 220jC) and only becomes effective above 350jC, which would damage many components and degrade the board material [156,157]. Although it has been reported that a lower-temperature reduction can be achieved in high vacuum [158,160], this would be a cumbersome and expensive approach in high-volume manufacturing. 2. Other Possibilities Reduced oxide soldering activation is another method being evaluated [161,162]. Oxides of Sn, Pb, and Cu are reduced by placing the component in a solution of vanadous ion under a blanket of nitrogen. The half-cell reactions are:

SOLDER WETTING AND SPREADING Reduction Oxidation

SnO2 þ 4Hþ þ 4e ¼ Sn þ 2H2 O 2þ

4V

¼ 4V

3þ

þ 4e



415 ð66Þ ð67Þ

The solution is regenerated electrolytically: Cathode Anode

N.

4V3þ þ 4e ¼ 4V2þ þ

2H2 O ¼ 4H þ O2 þ 4e

ð68Þ 

ð69Þ

Flux Perspectives for Joining with Lead-Free Solders

Ideally, a drop-in replacement for lead-based soldering would be highly desirable. However, finding a lead-free alloy with properties identical to the commonly used lead alloys now appears to be unlikely. Metals do not form an unlimited number of compounds, and in their liquid state are usually just mixtures of metallic elements. The number of solder alloys with melting points low enough to form eutectics near the Sn–Pb eutectic is very limited. The most promising alloys have melting points 30–40j higher than eutectic Sn–Pb and are more difficult to solder. Because all soldering processes require temperatures exceeding the solder melt temperature, sometimes referred to as super heating, it is necessary to raise the peak reflow temperatures as well. For example, Sn–Pb assembly operations are traditionally carried out at temperatures that range from about 210jC to 230jC. For lead-free solders that melt at about 217jC, the reflow temperature must be about 235–245jC. These temperatures are borderline for some components and organic materials utilized in microelectronics assemblies before they start to suffer some degradation. Some maintain their integrity to 250jC and beyond. Most fluxes were developed for the Sn–Pb system, particularly eutectic Sn–Pb. Their chemistry was developed through years of experience and trial-and-error to accommodate a variety of soldering requirements. Many of these fluxes have provided robust processes, and several were utilized during the initial stages of implementing lead-free solders, but typically have not performed as well as they did with Pb-based solders. There is a strong need to develop new fluxes that become active at higher temperatures, are closer to lead-free alloy melting points, and peak at reflow temperatures to improve wetting. Some possibilities involve steps to minimize oxide thickness by methods such as the addition of organic salts based on common solder metal ions and polycarboxylic acids.

O.

Flux Application

1. Methods There are many ways of applying flux depending on flow characteristics and manufacturing requirements, some of which are listed in Table 24. Among these are: brushing, rolling, dipping, spraying, foaming, and wave fluxing, to name a few that are not particularly sensitive to quantity. For some applications, however, it is necessary to dispense an accurate flux volume. For example, an automated tool is used to dispense a predetermined droplet in the center of a microsocket array of a chip carrier for the placement and subsequent reflow attachment of flip chips. The flux spreads out evenly over the surface of the carrier, owing to the excellent wetting properties of the flux/carrier system. It was noted in Sec. VI.L.3 that one method to increase the activity of a rosin-based flux is to increase the rosin concentration in the flux. It should be noted that high-solids-content fluxes typically do not readily form foam. Wave fluxing is often used to dispense flux for high-solidscontent fluxes. A tradeoff analysis is necessary if raising the solids level to increase activity results in the need to change the method of flux application. 2. Quantity Only the amount of flux needed to cover the solderable surfaces at reflow temperatures is generally required. Beyond meeting a minimum requirement, most soldering operations are not overly sensitive to flux quantity. There are some, however (such as flip chips, passives, and other

416 TABLE 24

SINGLER ET AL. Some Commonly Used Flux Application Methods

Method

Description

Dipping

Bring board in contact with flux reservoir

Brushing

Circular brush located between a pan or reservoir containing flux and the bottom of a circuit board Brush dips into flux, rotated to contact board underside as board passes by Action is continuous Very similar in concept to wave soldering Underside of board passes over a flux wave Several variations, but all resulting in a fine mist of flux directed at the board One or a number of nozzles can be utilized to address larger-sized boards, increase throughput Small flux bubbles are created by forcing air through porous tubes housed in an A-shaped fixture Bubbles exit the top of the A-shaped fixture Similar to wave soldering, bubbles contact the bottom of a board they passes over the A-shaped fixture

Wave fluxing

Foam fluxing

Comment Simple Low cost Poor flux volume control Simple Flux volume control is poor

Flux volume can be excessive Typically more uniform and better volume control than dipping, brushing, and wave fluxing Maintenance issues (e.g., clogged nozzles) Simple Low cost Applies thin, very reproducible flux coating

Compatible with no-clean low-solids fluxes

low-mass components), where excessive quantities can create problems (i.e., ‘‘float away’’ the component). Typically, the optimum flux volume must be determined empirically using process equipment and controlled dispense techniques. The optimum amount, of course, depends on the application; but generally speaking, a quantity sufficient to cover the solderable surfaces is all that is necessary. The flow characteristics of a flux are chosen or optimized for a specific application by diluting the base flux with a solvent to provide the desired viscosity, tackiness, and evaporation and wetting properties to assure that the active ingredient is well distributed primarily over the wettable surfaces. It has been suggested that with some fluxes, a thin layer gives better spreading than a thick layer [163]. A flux is optimized not only in relation to a specific set of ingredients but also in relation to the solids content; in other words, specific gravity and viscosity are optimized for various application purposes. Therefore, manufacturing quality procedures must be put in place to maintain these parameters within an acceptable process window. a. Excess Amount. The negative consequences of too much flux are increased residues and floating of components. From a cleaning perspective, excess flux leaves more flux residues, which become increasingly difficult to remove from beneath components, or from flip chips with large numbers of dense (i.e., tight pitches) area array interconnections. In automated lines, excess flux is typically removed by utilizing a low-pressure air knife, similar to the removal of excess solder during wave soldering. In no-clean flux applications, excess flux residue is of particular concern because it has been shown to impede the adhesion of underfill materials that facilitate improved solder joint reliability. b. Insufficient Amount. It is essential that enough flux be present when reflow begins to assure that the joining surfaces have sufficient active material to react with the complete oxide

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surface during the period of time the solder is molten. Too little flux may not assure that all of the solder bumps on an entire flip-chip site are covered, or may result in the active ingredient evaporating before reflow is complete and result in partial or nonwet conditions.

P.

Test on Basis of Manufacturing Conditions

Although much has been interpreted from empirical data about flux action and soldering, it is still an inexact science. In 1958, Milner [164] wrote ‘‘that scientific principles which govern the behavior of fluxes are not known.’’ Even with the knowledge of various flux chemistries, that opinion largely is still true today. In other words, knowing the acidity (pH and pKa) values, wetting rates, wetting angles, activation energies, etc., does not guarantee success in formulating or selecting a flux for a particular application. Although useful, most of the techniques applied in evaluating fluxes are a little more than tabulating and comparing results. With even limited knowledge of the chemistry of fluxes, it is relatively easy to select a few ingredients and formulate a flux that exhibits substantially satisfactory wetting. However, the development of a flux that is optimized for a particular application requires a fairly detailed understanding of chemical principles and requires extensive testing of many formulations. To a large extent, the formulation of fluxes is still conducted as an art, which, perhaps, is why there are so many proprietary formulations. Wetting rates, wetting angles, area of spread, activation energy, etc., provide convenient means of comparing formulations and narrowing the field of candidates. However, reflow profiles, heating rates, oxygen levels in N2 blanketed reflow furnaces, and solder metallurgy all affect performance, so it remains necessary to conduct final testings on manufacturing equipment. An understanding of the chemistries and function of flux ingredients is extremely valuable as a starting point. Testing on manufacturing equipment with the actual product is time-consuming, expensive, and potentially problematic (contaminating equipment), causing downtime and a throughput reduction, so it may be beneficial to conduct tests by offline methods. Although there are numerous methods to evaluate the effectiveness of fluxes, it is still most informative to observe the effect of a particular flux under actual reflow conditions. Hot plate experiments can simulate reflow furnace temperature ramp rates and are rapid, but atmospherical oxygen may cause results to be different from that which would occur in a reflow furnace under a nitrogen atmosphere. Hot plate-type studies allow the observation of component movement due to the action of the flux when heated, whereas these movements could not be observed by just examining the component position after passing through a reflow furnace. Observation in real time also can help to evaluate evaporation rates, solder spitting, and phase changes that occur as the volatile components boil off, as well as determine the effects of different temperature ramp rates. Therefore, hot plate-like studies and those that address manufacturing aspects are both necessary to evaluate the adequacy of a flux for a particular application.

VII. A.

NO-LEAD SOLDERS Pb–Sn Solders (Benchmark)

To most effectively identify candidate no-lead tin-based replacement alloys for the Pb–Sn alloy system, it would be helpful to have an understanding of the effects of Pb on wetting in the Pb–Sn system. However, as was discussed in Sec. V, the role of Pb and other secondary constituents in wetting is not precisely understood. Notwithstanding, a brief review of certain aspects of Pb–Sn wetting is presented here. 1. Tin Addition to Lead (Pb) It is interesting to consider the topic of this subsection from the perspective of the effects of the addition of Sn starting with pure Pb. The focus is on the industrially important ternary system Pb–Sn–Cu. Liu and Tu [165] studied the wetting of the Pb–Sn alloy system on Cu with both molten alloy and solid substrate immersed in RMS flux. The experiments were conducted at 10jC

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of superheat for each alloy. For the case of pure Pb, they measured a value of hEf115j. This value of the contact angle differs strongly from the result quoted by Naidich [10] in Sec. 2.5.1, but the reader is reminded that measurements there were made in a gaseous H2 environment, whereas here measurements were made using an RMA liquid flux environment. They found that hE decreases monotonically with increasing weight percent of Sn to a minimum equilibrium angle at approximately 60 wt.% Pb where hEf8j. However, further increase in Sn content resulted in a monotonic increase to hEf37j for pure Sn; thus, there is a distinct minimum. The value of the contact angle for the smallest Sn addition is hEf39j, corresponding to 95 wt.% Pb [165]. In the absence of wetting data for even lower concentrations of Sn, one could conjecture that smaller concentrations of Sn than 5 wt.% would have a similarly strong effect on the contact angle. The beneficial effects of the presence of Pb may be seen for the particular case of (eutectic) 37 wt.% Pb in Fig. 51, which shows spreading kinetic data for pure Sn at 250jC (18jC superheat) plotted against eutectic Pb–Sn at both 200jC (17jC superheat) and 250jC. The superior wetting of the Pb–Sn alloy over pure Sn is clear and is consistent with the above contact angle results of Liu and Tu [165]. In their result for pure Pb, cos hE<0, so that from Young’s equation, rSF0 and rSF>rSL. If the assumption is made that rSF remains unchanged (this is discussed below), then the apparent reduction in rSL (associated with the change in sign of rSFrSL (=rLF cos hSE)) is consistent with the spontaneous (exothermic) reaction to form an intermetallic compound at the S/L boundary. However, as pointed out in the discussion below, there is no complete agreement that rSL is reduced when pure Cu is replaced by its intermetallic compounds with Sn. 2. Presence of Copper Interface In the case of pure Pb, there is very little solubility of Cu and no product formation. Thus, rSL is approximately that of the Pb/Cu interface and rSF is that of the flux/Cu interface. The addition of Sn significantly alters the surface energies. Prakash and Sritharan [166] assert that for Pb-rich alloys, the formation of the q-phase (Cu3Sn) is expected due to the 3:1 Cu:Sn ratio and the low availability of Sn atoms. They further suggest that D-phase (Cu6Sn5) formation is favored thermodynamically but that the difference in the driving force of formation between the two

FIG. 51 The wetted radius R vs. s for pure Sn and eutectic Pb–Sn spreading on 12-Am sputtered Cu.

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419

phases decreases with increasing temperature, and the q-phase formation eventually becomes favorable due to the low Sn supply. They, therefore, note that the q-phase forms at large values of superheat whereas the D-phase forms at smaller values of superheat. The D-to-q transition occurs at different temperatures for Pb–Sn alloys of different compositions, and they conjecture that at extremely low Sn levels, little or no superheat may be necessary to form the q-phase. As the Sn content increases, however, all forces act to favor D-phase formation. Once Sn is present in the alloy, the value of rSF approximately corresponds to the interface either of Cu3Sn or Cu6Sn5 in equilibrium with the corresponding liquid phase. Unfortunately, there is no experimental data providing unambiguous values of rSL for either of the two intermetallic/ liquid interfaces or for the liquid Pb/solid Cu interface. However, Boettinger et al. [17] used a wetting balance to show that a 60Sn/40Pb weight percent solder wets pure Cu, Cu3Sn, and Cu6Sn5 equally well in the presence of an RMA flux at 235jC. All three cases exhibited nearly identical wetting force vs. time traces, terminating with the same steady-state value of the force. At first, this may seem like a controversial result, implying that the energetics of the three interfaces plays an insignificant role in wetting. However, the reader is reminded that the experiments of Boettinger et al. were carried out on preformed intermetallic compounds, which are quite different from intermetallic compounds formed during and as a consequence of wetting. Furthermore, the initial alloy composition is far from being in equilibrium with the three interface compositions. 3. Effect of Some Fluxes The changes in rSL are not the only possible changes occurring in the system as the Sn content is increased. Recall from the discussion in Sec. V that there may exist a mode of transport of Sn ions in some types of flux. Thus, when Sn is introduced into the system, Sn atoms could possibly migrate from the liquid alloy through the flux to the flux/Cu substrate where they cannot only adsorb to the Cu surface but subsequently react to form Cu3Sn (assuming that the supply of Sn by adsorption is very low). Again, there exists no experimental data for the corresponding change in rSF between the flux/Cu and flux/Cu3Sn interfaces other than that it is clear that the latter must be less than the former. It would also be possible that not all adsorption sites would be filled ahead of the advancing contact line, with the result being a chemically heterogeneous surface whose surface energy could be estimated according to Eq. (45) in Sec. V.D.8. Thus, the contact line may in fact be moving over an Sn–Cu intermetallic compound instead of Cu and this would alter the contact line kinetics. This scenario of Sn–Cu compound formation ahead of the contact line is fundamentally different than the surface diffusion of Pb scenario postulated by Smith and Lea [8] and presented in Sec. V. Evidence of Sn–Cu intermetallic halos or side bands preceding the contact line was presented by Liu and Tu [165]. Apparently, the topic of halos is an area of some controversy but an extremely important one as it has profound implications with regard to wetting and spreading in metallic systems. Several other phenomena associated with increased Sn content should be mentioned. The first is the effect on the surface tension (whether it be of Pb–Sn alloy in contact with liquid flux or inert gas). The surface tension of Pb–Sn alloy increases monotonically with increasing weight percentage of Sn. Thus, from the perspective of surface tension, increased Sn reduces the driving force of wetting. Additionally, Prakash and Sritharan [166] note that there is a difference in the morphologies of the intermetallic compound liquid/alloy interface that depends on the initial concentration of the Pb–Sn alloy. These different morphologies manifested different surface areas and hence affected the total interfacial energy associated with the liquid alloy/intermetallic compound surface. This would also affect the wetting dynamics. Finally, if the intermetallic compound formation is extensive, the original composition of the Pb–Sn alloy could change, particularly in localized regions, due to the consumption of Sn as a reactant. It is the confluence of all of the above phenomena and the uncertainties associated with them that have prevented wetting and spreading, even in an established alloy/substrate system such as Pb–Sn–Cu, from being completely understood.

420 B.

SINGLER ET AL. Pb-Free Wetting

In the identification of a suitable Pb-free alloy, it is desirable to select an alloy, which has wetting performance similar to the Sn–Pb alloy system on Cu-based and Ni-based substrate metallization. Most Pb-free candidate alloys have a high Sn fraction with some secondary constituents such as Ag, Bi, Cd, Cu, In, Sb, and Zn added. From a wetting perspective, it is important to understand the manner in which these secondary constituents influence the physical and chemical properties (e.g., melting point, viscosity, surface tension, density, substrate dissolution, and intermetallic formation). Consider, for a moment, adding constituents to liquid Sn to improve its wetting performance on Cu. Feng et al. [167] investigated the effect of electronic structure on the wetting of Sn-based alloys on Cu. They suggest that spreading of Sn on Cu was expected to improve when Sn was alloyed with an element that reinforces the electron orbital interaction between Sn and Cu. In addition, it may be possible to improve the spreading by adding a species that lowers the overall liquid surface tension. Some possible alloying options are Bi, Pb, and Sb, which have a lower liquid surface tension than Sn. Also, Cu substrate dissolution can be reduced if a small amount of Cu is added to the liquid alloy. Unfortunately, with the simple guidance offered above, it is not presently possible to predict the spreading characteristics of a proposed Pb-free alloy. In fact, there are indications that Ag, Sb, and Cu additions may result in reduced wetting (Figs. 52–54).

FIG. 52 (a) Spreading area measurements for various lead-bearing alloy and lead-free solder alloys spreading on Cu with a rosin flux. (b) Corresponding experimental peak reflow temperature. (From Ref. 167.)

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FIG. 53 Wetting equilibrium contact angles of various Pb-free solder alloys. (a) Cu substrate with RMA (GF-1235) flux. (From Drewin et al. Ref. 169.) (b) Cu substrate with RMA (Alpha 611) flux f40jC superheat over the liquidus temperature. (From Vianco et al. Ref. 170.) (c) Cu substrate with RMA flux at f30jC superheat over the liquidus temperature where the liquid alloy is dispensed onto a heated substrate. (From Pan et al. Ref. 171.)

The many interdependent factors involved in wettability assessment make experimental evaluation of various alloys essential. Reviewing the body of experimental spreading data, considerable variation in wetting performance exists for the various Pb-free alloys tested. These variations arise from differing experimental techniques, experimental temperatures, substrate types, and flux types. In the present discussion, wetting measurements that report equilibrium contact angle and use an RMA (mildly activated rosin) flux on a copper substrate were used wherever possible. Some of the wetting data presented here were obtained from a database being compiled by a National

FIG. 54 Comparison of the contact angles on Cu of Sn–Bi eutectic, and 1% addition of ternary elements. A 200jC peak reflow temperature and Kester 197 rosin flux were utilized. (From Ref. 173.)

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SINGLER ET AL.

FIG. 55 Contact angles on Cu of various lead-free solder alloys utilizing Kester 197 rosin flux at a peak reflow temperature of 250jC. (From Ref. 173.)

Institute of Standards and Technology project [168] and, in those cases, original data references were cited. 1. Spreading Characteristics A comparison of the spreading area measurements for various lead-bearing and lead-free alloy is given in Fig. 52a. The reported peak reflow temperature, which varies with alloy (Fig. 52b), is generally 30 to 50jC above the alloy melting temperature. Fig. 52a shows that increasing Bi content in Sn-Bi solders decreases spreading. In contrast, increasing Pb content in Sn-Pb solders (up to the eutectic composition) improved spreading. The various alloys were ranked according to their wetting characteristics in order from best to worst as: Sn–Pb, Sn–In, Sn–Ag, Sn, Sn–Sb, Sn–Cd, and Sn–Zn. Reviewing the ranking, it is apparent that no one attribute (surface tension, viscosity, reaction with substrate, etc.) can be used to explain the spreading variation observed from alloy to alloy. 2. Contact Angle Fig. 53 is a compilation of contact angle measurements of various lead-bearing and lead-free solder alloys measured by Drewin et al. [169], Vianco et al. [170], and Pan et al. [171], Drewin et al. and Vianco et al. determined the contact angles from a wetting balance, whereas the experiments of Pan et al. were unique in that they deposited a liquid drop of the alloy onto a preheated fluxcoated substrate. Generally, the contact angles of the lead-free alloys tend to be in the 30–50j range, with Ag-bearing alloys tending to have slightly higher contact angles. None of the leadfree alloys have contact angles as low as Sn-Pb culectic alloy. 3. Effect of Ternary Elements Many emerging lead-free solders are comprised of 3 or more alloying elements. Loomans et al. [172] evaluated the spreading of Sn–Bi eutectics with the addition of 1% ternary elements (Fig. 51) and found that the contact angle increased with the addition of each ternary element investigated. As seen in Fig. 55, the presence of Zn always increased the contact angle (i.e., decreases wetting). 4. Effect of Silver Additions Sessile drop experiments, using a formic acid–nitrogen gas mixture as a flux, performed by Goslin et al. [173] on Au-coated Ni substrates (Fig. 56), indicate that the addition of Ag in the alloy reduced spreading. Fig. 57 shows the spreading results for the same alloys except that the

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FIG. 56 Spreading studies carried out at 250jC of Sn, Sn–0.7Cu, Sn–3.5Ag, and Sn–3.5Ag–0.7Cu solders on 0.05-Am Au over 4.5-Am Ni deposited on a Si wafer. All alloys are given in weight percent. A gaseous flux mixture of formic acid and nitrogen was used. (From Ref. 173.) The initial sphere diameters were 0.889 mm and were deoxidized with the ROSA process. (From Ref. 174.)

FIG. 57 Spreading studies carried out at 250jC of Sn, Sn–0.7Cu, Sn–3.5Ag, and Sn–3.5Ag–0.7Cu solders on thin Au over 0.34-Am Pd/3.3-Am Ni deposited on a Si wafer. (From Ref. 173.) All alloys are given in weight percent. A gaseous flux mixture of formic acid and nitrogen was used. The initial sphere diameters were 0.889 mm and were deoxidized with the ROSA process. (From Ref. 174.)

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substrates have a layer of Pd between the Au and Ni. With the presence of the Pd, the addition of Ag to the liquid alloy did not result in as great a reduction in spreading for reasons that are not fully understood at this time. 5. Effect of Flux With the exception of the gaseous flux experiments shown in Figs. 56 and 57, all the spreading experiments discussed utilized a mildly activated rosin flux. As noted in Sec. 6, there appears to be Sn transport in the liquid rosin flux from the spreading drop to the unwet copper ahead of the contact line, so the impact of flux chemistry on solder spreading is clearly significant. Loomans et al. [172] showed that SnCl2 promotes good spreading of solders on Cu not only because it is a good activator but because it also reacts with Cu to give Sn or Sn–Cu intermetallic compounds on a Cu surface. Vaynman et al. [175] were able to reduce the contact angle of Sn–Zn eutectic alloy (Tm=198.5jC) on Cu at 260jC from 89j to 25j by changing from a mildly activated rosin flux to a flux containing Sn organic compounds. For comparison, pure Sn under these conditions had a 37j contact angle. They indicate that x-ray diffraction experiments showed that these Sn organic compounds decompose and form pure Sn on the Cu surface. Although the effectiveness of these fluxes to improve wetting is clear, they must be carefully evaluated to insure that there are no flux residues left behind, which would be detrimental to the long-term reliability of an electronic assembly. 6. Summary In summary, the spreading of Pb-free solders is generally not as good as the heritage Sn–Pb alloys they will be replacing. The Pb-free solders also require higher process temperature. There are a number of factors influencing the spreading behavior of Pb-free alloys and it is likely that several different substrate/alloy/flux combinations will emerge and that manufacturing inspection criteria for ‘‘acceptable wetting’’ will need to be redefined.

ACKNOWLEDGMENTS T. Singler and S. Meschter would like to express their deep gratitude to Liang Yin of the Materials Processing Laboratory of the Department of Mechanical Engineering at SUNY Binghamton for his expert assistance with figures and other aspects of manuscript preparation.

REFERENCES 1. Frear, D.R.; Wang, J.W.; Lin, J.K.; Zhang, C. Pb-free solders for flip-chip interconnects. J. Mater. 2001, 53 (6), 28. 2. De Gennes, P.G. Wetting: statics and dynamics. Rev. Mod. Phys. 1985, 57 (3), 827–863. 3. Dussan, E.B. V On the spreading of liquids on solid surfaces: static and dynamic contact lines. Annu. Rev. Fluid Mech. 1979, 11, 371–400. 4. Wettability; Berg, J.C., Ed., Marcel Dekker: New York, 1993. 5. Gibbs, J.W. The Scientific Papers of J. Willard Gibbs, Thermodynamics; Longman, Green: Dover, New York, 1906; Vol. 1, 1931, 1961. 6. Bondi, A. The spreading of liquid metals on solid surface, surface chemistry of high-energy substances. Chem. Rev. 1953, 52 (2), 417. 7. Kuhmann, J.F.; Maly, K.; Preub, A., et al. Oxidation and reduction of liquid SnPb (60/40) under ambient and vacuum conditions. J. Electrochem. Soc. June 1998, 145 (6), 2138–2142. 8. Smith, G.C.; Lea, C. Wetting and spreading of liquid metals: the role of surface composition. Surf. Interface Anal. 1986, 9, 145–150. 9. Young, T. An essay on the cohesion of fluids. Philos. Trans. R. Soc. 1805, 95, 65–87. 10. Naidich, J.V. The wettability of solids by liquid metals. Prog. Surf. Membr. Sci. 1981, 14, 353. 11. Tabor, D. Gases, Liquids and Gases and Other States of Matter; Cambridge University Press, 1969. 12. Eustathopoulos, N.; Nicholas, M.G.; Drevet, B. Wettability at High Temperatures; Pergamon Press: Amsterdam, 1999. 13. Kistler Hydrodynamics of Wetting. In Wettability; Berg, J.C., Ed.; Marcel Dekker: New York, 1993. Chap. 6.

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12 Lead-Free Finishes for Printed Circuit Boards and Components Rob Schetty Technic Inc., Plainview, New York, U.S.A.

I. INTRODUCTION Tin–lead is used as the primary electronic interconnection material in solder pastes, PCB coatings, and as a finish on the external leads of electronic components such as surface-mount and through-hole devices, or as a solder ball or bump material on advanced semiconductor packages, such as ball grid arrays (BGAs), chip scale packages (CSPs), and flip-chip devices. In order to create a truly environmentally safe Pb-free electronic assembly it is required that Pb-free materials be used throughout the interconnection. As with solder pastes, many options exist for creating a Pb-free circuit board or component finish. To date, most research on Pb-free solder paste materials has identified binary, tertiary, and quaternary alloys of tin with additions of trace metals such as silver, copper, and/or bismuth as the most promising to replace Sn–Pb solder. In the case of solder balls or bumps on advanced packaged components, these are typically formed via a molten extrusion process; therefore any combination of compatible metals can be blended to the desired composition, and as such, this is outside the scope of this chapter. However, in the case of PCBs or ‘‘conventional’’ surface mount or through-hole components, the solder material is often applied via electrodeposition to create a plated deposit or finish. Electrodeposition is a complex electrochemical reaction involving metal ions, electrolyte compounds, and proprietary additives in an aqueous solution. The electrodeposition of more than one metal is difficult, and more than two metals almost impossible in a manufacturing environment. It is not practical for plated finishes on boards and components to match Pb-free solder paste materials. Therefore it is anticipated that Pb-free electronic interconnections that consist of a solder paste material, a board finish, and a component finish will likely not all be the same material, but some combination, i.e., hybrid structure. That is, the board or component finish need not necessarily duplicate the selected solder paste composition; the main requirement is that they be compatible with the Pb-free solder paste material. This chapter will focus on a number of Pb-free board and component finishes such as nickel/palladium and other precious metal-containing coatings, pure tin, and tin alloys. Solderability performance will be demonstrated with both Pb-containing and Pb-free solders, and other important properties will be discussed.

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II. TYPES OF FINISHES AND DEPOSITION METHODS The term ‘‘finish’’ as used in this chapter is defined as a surface coating or treatment that is applied to a substrate or to an underlying layer on a substrate. The function of the finish is to modify, protect, and enhance certain properties of the substrate or underlying layer onto which it is applied. Most finishes for PCBs and components are applied to enhance the solderability of the substrate or underlying layer so that at the board-assembly level, the board or component will solder well and provide a reliable, long-life solder joint. Other functions of finishes include wire bonding, contact switches, press fit connectors, etc. Finishes can be metallic or nonmetallic, organic or inorganic, and fusible or nonfusible in nature. Most finishes for boards and components are metallic. Many common metallic finishes for PCBs and components are applied through a technology called ‘‘plating.’’ Plating processes can be further categorized by the specific deposition method employed: (i) electrolytic plating or electroplating; (ii) electrode-less plating, more commonly known as electroless plating; and (iii) immersion plating also known as displacement plating. These are briefly described below.

A. Electrolytic Plating In electrolytic plating, a substrate is immersed into an aqueous solution containing ions of the metal to be plated (Mn+) and an anode which is a solid form of the metal to be plated (M). In this case the substrate acts as the cathode, and when the anode and cathode are linked through an external power supply and current is applied, electroplating occurs by virtue of the following reactions: Cathodic reaction: Anodic reaction:

Mnþ þ n e ! M M ! Mnþ þ n e

ð1Þ ð2Þ

At the cathode, the metal ions in solution are reduced to their metallic state and metal is electrodeposited onto the substrate; at the anode, solid metal dissolves electrolytically, forming more metal ions that feed the cathodic plating reaction. Electroplating takes place until such time as the current is turned off. The maximum thickness of electroplated deposits is, in theory, only limited by the current utilized to apply the finish, and the length of plating time. However, once the practical thickness limit is exceeded for a specific material, a functional deposit can no longer be obtained. Other than the metallic ions in solution, there normally also are proprietary compounds known as ‘‘additives’’ in solution that highly influence the electrodeposition reaction and the deposit properties. Collectively, this solution is known as the ‘‘electrolyte.’’ Electrolytic plating reactions can be very simple or highly complex, and much of the ‘‘art’’ around electroplating involves the formulation of the specialized proprietary additives that impart specific desired deposit properties. A schematic of a typical electroplating reaction is shown in Fig. 1, which illustrates the interaction of the metal cations (M+), conducting acid ions (A), organic addition agents (O), and electrons () as the metal ions are reduced electrolytically at the cathode surface. Solution pH, operating temperature, applied current, composition and concentration of chemical constituents all play an important role in electroplating.

B. Electroless Plating Electroless and immersion plating are similar in that no external power supply is required to drive the deposition reaction. In electroless plating, also known as autocatalytic plating, the electrons required to force the deposition reaction are supplied by a chemical reducing agent (R) that donates electrons to the catalytic surface of the substrate. As the deposition reaction proceeds, the reducing agent (R) is subsequently oxidized to species (Rox) as illustrated by the reaction below, Mnþ þ R ! M þ Rnþ ox

ð3Þ

LEAD-FREE FINISHES FOR PCBs AND COMPONENTS

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FIG. 1 Schematic illustrating the interaction of various species in a bath during plating.

No power source, electrical connections, or anodes are required for electroless plating, and plating occurs on a continuous basis until the substrate is removed from the plating bath, unless the metal ions or reducing agents are depleted from the solution, and so high thickness deposits are possible. Electroless plating baths are usually complex mixtures of metal ions, reducing agents, stabilizers, chelating agents, conductivity compounds, and proprietary additives. Solution pH, operating temperature, composition and concentration of chemical constituents, etc. all play an important role in electroless plating and careful control of the electroless plating bath is required to ensure consistent, uniform deposition.

C. Immersion Plating Immersion or displacement plating is the deposition of a metallic coating (M) on a metallic substrate (S) from a solution that contains the metal being plated, and the metal on the substrate is displaced by a metal ion from solution that has a lower standard electrode potential than the displaced metal ion. The displaced substrate material enters the solution in ionic form (Sn+) and the metal ion (Mn+) deposits onto the substrate in its place. In immersion plating the electrons required to force the deposition reaction to occur are supplied by the surface of the substrate itself and no chemical reducing agents are required as illustrated in the reaction below, Mnþ þ S ! Snþ þ M

ð4Þ

No power source, electrical connections, or anodes are required for immersion plating although the thickness of the deposited metal in this case is self-limiting because deposition stops when the entire surface of the substrate has been coated; therefore only relatively thin (
D. Other Types of Finishing Application Methods Molten coatings such as hot air solder leveled coatings (HASL) on PCBs are applied via specialized equipment that coats the substrate with molten metal selectively in the areas where the finish is desired. The thickness distribution is not as uniform as with plated finishes. As the molten coating method involves application of a molten metal at very high temperatures, the

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TABLE 1 Summary of the Advantages and Disadvantages of the Various Plating Methods Utilized for Metal Finishes in Electronic Assemblies Finishing method Electrolytic plating Electroless plating Immersion plating Molten coating application Organic coating application

Thickness variation

High thicknesses possible

Difficulty to apply

Cost

M L L H M

Yes Yes No Yes No

L M L M L

M H L/M L L

Key: L=low; M=moderate, H=high.

method has additional drawbacks of safety and health concerns for operators of the equipment, as well as a high thermal loading which the substrate must endure during the coating operation. Organic coatings are often applied via an immersion process. Although in name this process may seem similar to the immersion plating process the fundamental mechanism is quite different in that in place of a metallic layer an organic layer is deposited and no substrate material is displaced during the reaction as occurs during immersion plating. Instead of relying on electron donation from the substrate to drive the deposition reaction, the organic coating immersion process relies more on surface adsorption and chemical agents that are specifically attracted to and/or reacted with the substrate material. A summary of the advantages and disadvantages of the various finishing methods is provided in Table 1.

III. LEAD-FREE PRINTED CIRCUIT BOARD FINISHES Extensive activities are underway in both industry and academia to identify and implement alternatives to Pb for the various aspects of Sn–Pb solder electronic assemblies. In the area of Pbfree solder pastes, a ternary Sn–Ag–Cu alloy has been generally identified as the most promising candidate and so for the solder paste portion of electronics assemblies, the selection of a Pb-free material appears to have a clear direction. For PCB finishes, the choice is not so clear. Board finishes, also referred to as surface finishes or final finishes, can be generally classified into two categories: reflowed or molten coatings, and plated coatings. Plated coatings can further be subcategorized as electrolytically plated (with current) or electroless/immersion plated (without current) as described in the previous section.

FIG. 2 Graphic depicting the percentage of various printed circuit board (PCB) finishes produced in the United States in the year 2002.

LEAD-FREE FINISHES FOR PCBs AND COMPONENTS TABLE 2

435

Summary of Key Criteria for Board Finishes

Board finish criteria . .

. . .

. .

Possess good solderability and wetting characteristics, and be capable of maintaining those characteristics after exposure to a variety of natural and artificial aging environments. Be compatible with the reflow profile and flux types recommended for the selected Pb-free solder paste, and other component and board materials of construction (in general, enabling a reflow profile with a maximum peak reflow temperature of approximately 230–245jC). Be compatible with surface mount technology (SMT) and wire bonding. Be compatible with press fit and high density interconnect (HDI) connectors. Have a robust manufacturability performance, in most cases this means the ability to be applied from a chemistry that is easy to control, does not attack the solder mask, has good stability, and produces a finish that can be verified in production. Resist tarnishing or discoloration during storage and/or exposure to natural and artificial aging/ environments. Be cost-effective.

Board finishes have traditionally consisted of a molten Sn–Pb coating applied via an application method referred to as hot air solder leveling (HASL). End users have been seeking a replacement for HASL for years, due to the inherent aspect of producing a nonuniform film thickness which creates a lack of planarity, which is of increasing importance as component I/O grid dimensions are steadily decreasing. Good planarity is important to assure high yields, and in dealing with high-reliability solder joints with fine-pitch components. The need to eliminate Pb from PCBs has only accelerated the push for HASL replacements. Although HASL is widely sought to be eliminated, it is worthy to note that in 2002 approximately 50% of all PCBs produced in the United States were processed through HASL (Fig. 2).

A. Lead-Free Board Finish Criteria Board finishes must meet the criteria listed in Table 2.

B. Lead-Free Printed Circuit Board Finishes The most common Pb-free PCB finishes under consideration are listed in Table 3. Each surface finish is discussed in detail in the following sections.

TABLE 3 The Most Common Lead (Pb)-Free Printed Circuit Board Finishes Circuit board finishes . . . . . . . .

Pb-free HASL Reflowed tin Electrolytic tin–bismuth Electroless nickel/immersion gold (ENIG) Electroless nickel/electroless palladium/immersion gold (ENEPIG) Organic solderability preservatives (OSPs) Immersion silver Immersion tin

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TABLE 4 Variations Typical of Eutectic Sn–Pb Hot Air Solder Leveled (HASL) Finishes Parameters Poor planarity Solder joint volume variations

Net variability due to HASL

Characteristics Typical range, 10 to 100 A-in. Typical example – 0.050 in. pitch, 0.0260.080 in. pad size – solder volume. Range 104 to 2080 mils3 – theoretical printed volume, 4440 mils3 2.3% to 47%

Source: From Ref. 1.

1. Industry Reference: HASL Eutectic Sn–Pb HASL has the key advantage of excellent solderability because ‘‘nothing solders like solder.’’ In addition, HASL-produced finishes possess an excellent shelf life (at least 1 year), provide a good electrical probe surface, and HASL finishes easily withstand multiple processing steps. Advantages: Notwithstanding its limitations, HASL is a well-understood, mature, low-cost surface finish, and currently >80% of the cards used worldwide have a HASL finish. HASL’s popularity is predicated mostly on two factors: (a) It is much cheaper than a plated finish in most instances, and (b) users do not have to be concerned about wetting (i.e., yields, reliability) because it comelts with the screen printed solder forming an integral mass (i.e., no wetting of an interface is required). This is particularly true if the card or board is used fairly soon after the finish is applied (i.e., Sn still in the 2+ state instead of 4+ state). Disadvantages: The main disadvantage of HASL coatings, notwithstanding the fact that they contain Pb, is the wide thickness variation they typically exhibit, both in terms of pad-to-pad and board-to-board thickness variations. The HASL process is not capable of providing finishes with a planar surface to enable the placement of components during SMT assembly as described in Table 4 and illustrated in Fig. 3. There are other concerns in addition to the effects of solder volume variation, such as oxidation. High Sn content alloys have Sn oxide on the surface. In the case of Sn–Pb, it is Sn oxide as opposed to a Pb oxide that is present on the surface. A fresh HASL alloy coating containing Sn is very wettable as the Sn in the oxide, typically 10–20 A˚ thick, is in the

FIG. 3 Scanning electron microscope (SEM) micrograph depicting a significant variation in solder volume deposited on metal features on a printed wiring board (PWB) by the HASL method.

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TABLE 5 Comparison of Variations Between Finishes Created by Plating (Planar) Compared to HASL (Nonplanar) Device

Pitch (in.)

Pad width (in.)

Solder vol. (mils3)

Paste vol. (mils3)

% Variability finish: paste

104–2080 36–720 25–500 5.7–113

4440 1680 920 303

2.3–47.0 2.1–43.0 2.7–54.0 1.8–37.0

15–22.5

920

0.16–0.24

15–22.5

920

0.16–2.7

f0

920

f0

Nonplanar finish PLCC 0.050 0.026 Lead 0.020 0.012 QFP 0.016 0.010 BGA 0.020 0.012 Planar finish Immersion Ag or Au, 3–4 Ain. thick QFP 0.016 0.010 Flat solderable Sn, 30–35 Ain. thick QFP 0.016 0.010 Organic solderable preservative QFP 0.016 0.010 Source: From Ref. 1.

(2+) state. However, over time due to exposure to air, etc., the Sn2+ is converted to a Sn4+ oxide state that is more difficult to reduce by the flux during soldering, thus making the surface significantly less wettable. The thickness of the Sn oxide is also thicker (50 to >100–300 A˚). There is also a high thermal stress imparted to boards during HASL due to the high temperatures they encounter, and the risk of solder-plugged holes or reduction in hole size that poses potential reliability problems. In terms of manufacturability, HASL can be performed in a vertical or horizontal conveyorized mode, but the HASL process is not ideal because the equipment is difficult to work with, poses safety hazards due to high operating temperatures, and frequent maintenance requirements. However, as the drive to finer pitches continues, HASL coatings will increasingly become a problem due to bridging, and solder volume variations resulting in even larger solder joint solder volume % differences compared to larger size pads. Table 5 indicates that planar finishes achieved through plating result in a much smaller finish variability compared to a nonplanar finish as is created by a HASL deposition. Planar finishes are therefore the choice for tight pitch applications. 2. Lead-Free HASL It is possible to maintain the HASL operation and simply substitute one of the many common Pb-free alloys listed in Table 6 in place of molten Sn–Pb solder in the HASL equipment. The end result is a Pb-free HASL finish that eliminates the Pb problem, but the intrinsic deficiencies

TABLE 6 Several Candidate Lead-Free Alloys for HASL Deposition of Printed Circuit Board Finishes Alloys

MP, jC

93.3 Sn–0.7 Cu 96.5 Sn–3.5 Ag 96.2 Sn–3.2 Ag–0.5 Cu 90 Sn–2.0 Ag–7.5 Bi

227 221 218 186–212

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of the HASL process listed in Table 7 remain unresolved. Based on the potential consequences, it is not likely that Pb-free HASL will be widely adopted. 3. Reflowed Lead-Free Solder or Tin Reflowed lead-free solder or reflowed tin, also referred to as selective reflow or selective solder (or tin) strip, is a process whereby solder or tin is electrolytically plated onto the copper pads and traces on a PCB prior to solder mask application. A solder that does not contain Pb could potentially be plated as an electrolytic Pb-free surface finish. The solder (or tin) must then be selectively stripped to expose those areas of the copper where soldermask will be applied. The coating is then reflowed to produce the final finish. The main advantage of this method compared to HASL is that the coating thickness variation is greatly reduced, as the thickness uniformity is much better with an electroplated deposit compared to a HASL molten solder coating. Plated and reflowed films provide a relatively planar and consistent surface suitable for SMT fine-pitch component attachment. Other advantages of this method include good solderability; long shelf life (at least 1 year); good electrical probe surface; and the reflowed tin or solder coating easily withstands multiple processing steps as the solder-wetting characteristics of tin are excellent. The disadvantages of this method include a limited and decreasing availability of suppliers providing this technology; environmental issues related to tin and tin–lead stripping during the process sequence, an additional high thermal stress condition, and increased risk of via or holes plugged as a result of the tin or tin–lead reflow operation. Also, the registration tolerances for the BGA soldermask is insufficient as the tolerance buildup between the registration of the tin or tin– lead stripping process and the soldermask features can create very narrow soldermask dams with questionable solder mask adhesion around BGA devices. The adhesion of the BGA soldermask dam is a critical feature that prevents solder from being wicked away from the BGA pad and escaping down the adjacent via hole [2]. The additional process steps of this method cause the cost to be higher compared to HASL. 4. Electrolytic Tin–Bismuth (Sn–Bi) Some companies have investigated the use of a eutectic tin–bismuth alloy (58Bi–42Sn) coating deposited electrolytically from a plating bath during the PCB manufacturing operation. This method is similar to the reflowed tin option discussed in the previous section; however, a tin– bismuth alloy is deposited in place of pure tin or tin–lead. A eutectic tin–bismuth finish may be beneficial in cases where low-temperature soldering is required, given eutectic tin–bismuth’s low melting point (138jC). The properties of tin–bismuth finishes are summarized in Table 8. Solder wetting tests indicate that eutectic tin–bismuth coatings reflowed at 170jC provide good wetting characteristics with low wetting angles (<15j), essentially similar to Sn–Pb coatings reflowed at 220jC (Fig. 4). Other than some niche applications where a low melting point is acceptable or even desired, electrolytic tin–bismuth will probably not be widely used owing to potential reliability issues arising out of service ambient temperatures, the presence of Sn–Pb solder, and other factors discussed in Section 4 on Pb-free component finishes.

TABLE 7 Intrinsic Deficiencies of the HASL Process for Board Finishes Issue Higher melting point

Process variations Higher material cost Compatibility

Consequence Stress HASL equipment Stress components Increase dross formation Solder joint volume variation As compared to Sn–Pb Tooling infrastructure Other metallurgical systems (e.g., components)

LEAD-FREE FINISHES FOR PCBs AND COMPONENTS TABLE 8

439

Summary of the Major Aspects of Tin–Bismuth (Sn–Bi) Finishes

Parameters Mechanical properties Plating deposition rate Eutectic (42Sn–58Bi) melting point Melting temperature range Early acceptance Compatibility with Sn–Pb solder

Characteristics Acceptable Acceptable 138jC From pure Sn (232jC) to eutectic composition (138jC) Nikkei Sangyo, Tokyo, Japan, 1998 Forms brittle Bi–Pb IMC. Concerned with hybrid (partially Pb-free) systems

Source: From Ref. 3.

5. Electroless Nickel/Immersion Gold This surface finish is widely used (Fig. 2). The electroless nickel/immersion gold (ENIG) technology involves autocatalytic deposition of a nominal 5-Am coating of nickel–phosphorous alloy (typically 6–10% P) from an electroless nickel process followed by an immersion coating of gold (f0.05 to 0.1 Am Au). The thin gold coating in the ENIG deposit functions simply as a protective layer to prevent oxidation of the underlying nickel, and all subsequent assembly operations (solderability, wire bonding, etc.) take place directly with the nickel surface. Advantages of the ENIG surface finish are summarized in Table 9. Characteristically, ENIG finishes provide an extremely flat, planar surface for attaching SMT components with excellent solderability, a good electrical probe surface, a suitable surface for edge connectors and press fit applications, and a good surface for wire bonding (with aluminum wire only). Handling issues are minimized and the finish easily withstands multiple process steps. The excellent solder wetting performance of ENIG finishes is demonstrated in Fig. 5. This figure shows the wetting force vs. time for an ENIG finish that has been stored for a period ranging from 2 to 18 months at room temperature. Several notable features to this figure are (i) the wetting speed is consistently fast; (ii) high, stable wetting forces are consistently obtained, and (iii) there is minimal degradation in the solderability of the ENIG finish after being stored for 18 months, as evidenced by the very slight decrease in wetting force and speed in the figure. The ENIG process is very complex and involves at least six primary chemical processing steps and greater than 20 overall process steps, each of which requires a very strict level of process control. Both the nickel and the gold plating baths operate at temperatures exceeding 88jC,

FIG. 4 Comparative wetting test results of eutectic Sn–Bi with benchmark eutectic Sn–Pb solder.

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TABLE 9 Some Advantages/Benefits that ENIG Surface Finishes Provide Advantages/benefits . . . . .

Flat, uniform finish Finish wets solder well Relatively good shelf life – Resists tarnishing and discoloration Precious metal electrical contact surface Nickel provides several PTH benefits – Strengthens them – Protects barrels from Cu dissolution during wave soldering

increasing the magnitude of the thermally induced stress due to the coefficient of thermal expansion (CTE) mismatch between the deposited film pad metallization and circuit board material. The higher plating temperature also causes problems for other materials on the circuit board, especially the soldermask. Soldermask lifting was the initial problem encountered by the implementers of the ENIG process, although that has mostly been resolved by improvements in formulations made by soldermask suppliers and improved process control by board fabricators (running processes at the lowest possible temperature, minimizing dwell times, etc.). Other disadvantages of ENIG finishes include the difficulty in measuring the final Aucoating thickness, cost increase relative to HASL, and most importantly, a major reliability issue that is a low-level defect common to ENIG finishes referred to as ‘‘black nickel’’ or ‘‘black pad’’ in the industry. The defect, which exhibits a characteristic interfacial (i.e., flat) fracture (Fig. 6), is believed to be due to corrosion of the underlying nickel layer by the gold plating process under certain conditions. The corrosion causes solder to dissolve gold off a SMT pad and form a weak intermetallic bond to the nickel as depicted in Fig. 7. The weak bond can lead to interfacial fracture of solder joints during storage resulting in components actually falling off the board. It is particularly problematic with large-lead-count ball grid array (BGA) components as shown in Fig. 6. The defect is typically experienced at a low ppm level, although some board types are more susceptible than others. It is believed the problem can be contained through a combination of plating process and board design improvements and process control at the board fabricators. However, the potential reliability risk is a barrier to the wider implementation of ENIG.

FIG. 5 Wetting force for ENIG finishes that have been stored for up to 18 months at room temperature.

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441

FIG. 6 Planar separation characteristic of ENIG ‘‘black pad’’ problem.

6. Electroless Nickel/Electroless Palladium/Immersion Gold The electroless nickel/electroless palladium/immersion gold (ENEPIG) finish is simply an ENIG finish into which an intermediate layer of palladium (0.1 to 0.5 Am thick) is deposited between the electroless nickel and immersion gold coatings. Palladium is deposited from an autocatalytic plating bath. Palladium is a noble metal applied to protect oxidizable Ni. Palladium (m.p. 1552jC) is not fusible but rather dissolves in the molten solder in a manner similar to gold, and

FIG. 7 Conditions that cause the ‘‘black pad’’ problem sometimes observed to occur with ENIGtype finishes. (From Ref. 4.)

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solder wetting is limited by the need for complete dissolution of Pd into the solder. That is, the thickness of Pd must be sufficient to adequately protect the underlying Ni from oxidation, yet not be so thick as to inhibit complete dissolution during soldering. Therefore a trade-off is required as dissolution of Pd into Pb–Sn solder is four orders of magnitude less than Sn. No more than 0.3Am Pd can be dissolved in a few seconds during soldering. Therefore the coating on Pd-finished components must be kept to less than about 0.4 mm to assure dissolution. The oxidation protection for Ni is questionable if the deposit is porous. Other advantages of an ENEPIG finish include excellent solderability, an extremely planar surface (comparable to ENIG) with ‘‘multifunctional capability’’ to accept SMT assembly, wave solder, wire bonding with gold or aluminum wire, and press-fit connectors. These finishes also provide a good probe and contact surface result in minimal handling issues, and withstand multiple process steps. In terms of reliability, the ENEPIG finish resolves the interfacial fracture issue because the palladium layer acts as a barrier to prevent the immersion gold bath from attacking the plated nickel film. Disadvantages include even more complex chemistry and additional process steps compared to ENIG, which results in a higher cost. The high cost of the finish is driven mainly by the palladium (Pd) content. Palladium prices historically were lower than gold until the mid-to-late 1990s when demand increased due to multiple applications causing the supply to severely tighten. Availability was also significantly affected by the political climate and instability in parts of the world that control the supply of palladium. The end result was a very erratic and upward Pd metal price peaking at over US$1000 per troy oz. in the early 2000s, making its use prohibitive. The price of Pd has reduced to the US$300–$400 per tr. oz. range, but given the uncertainties and instability in the supply situation many are uncomfortable with the selection of this finish. 7. Organic Solderability Preservatives Organic solderability preservatives or OSPs rely on a very thin organic coating directly on top of the bare copper on PCBs. Two types of OSPs are available: a ‘‘thin’’ version (30 to 50 A˚), and a ‘‘thick’’ version (500 to 1500 A˚). The organic coating acts as a relatively short-term protection of the copper surface with an anticorrosion or antitarnish function. The coating then dissolves or evaporates during subsequent assembly operations leaving unoxidized copper as the surface for soldering operations. As the name implies, the only assembly operation an OSP finish addresses is solderability, a function it performs very well, providing good solder wetting characteristics, as shown in Fig. 8, which illustrates the wetting force vs. time curves for an OSP coating aged up to 672 days in a room temperature office environment. This figure demonstrates the good wetting behavior of the OSP finish as evidenced by consistently fast wetting times and high wetting forces

FIG. 8 Wetting force for a copper surface coated with an OSP finish aged up to 672 days at room temperature.

LEAD-FREE FINISHES FOR PCBs AND COMPONENTS

443

FIG. 9 Degradation of solder wetting of organic solderability preservative (OSP) with repeated temperature/time cycles. (From Ref. 5.)

over the duration of this test. The finish provides a very flat surface for SMT, is very inexpensive, utilizes a simplified chemical processing sequence, and can withstand multiple process steps when used in the ‘‘thick OSP’’ configuration. Thin OSP coatings degrade with increased temperature and time as shown in Fig. 9. The disadvantages include the inability to function as a contact surface. The coating insulates the copper, so all electrical testing must be performed prior to the coating application. The material has a relatively short shelf life (maximum 6 months) and there are also handling concerns as a thin coating is easily damaged. Other disadvantages are a narrow process window for multiple passes, incomplete pad wetting, and an inability to accurately or easily measure the coating thickness. The advantages and disadvantages of OSP coatings are summarized in Table 10. 8. Immersion Silver (Ag) Immersion silver relies on a thin (0.1 to 0.4 Am) coating of silver deposited from an immersion silver process directly onto copper and is one of several immersion finishes under consideration. As described in Section 2, the immersion deposition process occurs as a result of the deposited metal ion having a lower standard electrode potential than the displaced metal ion. Supplemental reducing agents are not required in immersion plating because the base metal itself acts as the reducing agent. That is not the case in electroless plating. Immersion plating occurs when a base metal is immersed into a solution containing metal ions whose standard electrode potential is lower. For example, the standard electrode potential of silver is lower than copper as shown in Table 11 (i.e., silver is a more ‘‘noble’’ metal than copper); therefore silver will displace copper

TABLE 10 Characteristics of Planar Nonmetallic (Organic) Pb-Free Board Finishes Under Consideration Types Thin imidazoles Thick substituted benzimidazoles Thick phenyl imidazoles Prefluxes

Advantages Availability Low cost Large installed base Compatible with Pb-free solders

Disadvantages Relatively short shelf life Not suitable for contacts or wire bonding Difficult to inspect

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TABLE 11 Standard Electrode Potentials of Some Common Elements Element

Standard electrode potential

Gold Platinum Iridium Palladium Silver Mercury

Au +1.4 V Pt Ir Pd Ag +0.80 V E Hg

Ruthenium Copper Bismuth Antimony Tungsten Hydrogen Lead Tin Molybdenum Nickel

D=0.46 V Ru z Cu +0.34 V Bi +0.32 V Sb W H +0.00 V Pb Sn 0.1375 V Mo Ni 0.25 V

j

from the surface of a printed wiring board (PWB) circuit feature when the PWB is immersed into an immersion silver plating solution by the following reaction: Cu þ 2Agþ ! 2Ag þ Cu2þ

ð5Þ

As silver is prone to tarnishing (i.e., oxidation) during storage, the immersion silver process is usually chemically modified to include an antitarnish agent. The characteristics of a typical commercial immersion silver finish are shown in Fig. 10.

FIG. 10 Ref. 6.)

Steps to form and characteristics of a commercial immersion silver board finish. (From

LEAD-FREE FINISHES FOR PCBs AND COMPONENTS

445

FIG. 11 Wetting force for an immersion silver (Ag) board finish subjected to a precondition of 85jC/ 85%RH/24 hr. (From Ref. 7.)

Similar to OSPs, immersion silver coatings act to protect the copper surface from oxidation and therefore provide a solderable coating even after a period in storage. During SMT assembly, silver dissolves very quickly into the molten solder, so wetting characteristics are very good, i.e., wetting is very quick and exhibits high wetting forces with time as indicated in Fig. 11. As the coating is metallic or organometallic, it is more robust than a purely organic OSP coating, and the solderability performance of an immersion silver board finish after being subjected to a variety of aging conditions as measured by the solder float test is demonstrated in Table 12. In terms of manufacturability, the immersion silver process is relatively simple. The coating is very planar and provides a good electrical probe surface capable of withstanding multiple process steps. The coating is also easily detectable and measurable, and it is compatible with all known Pb-free assembly materials. The benefits of immersion silver compared to HASL are provided in Fig. 12. The disadvantage of immersion silver is the film is not as robust as a HASL coating and the thickness is inconsistent due to the self-limiting nature of the immersion process. The surface of an immersion-silver coating can tarnish during storage, even with the best of precautions. As the immersion silver finish is a relatively recent innovation, the long-term reliability and storage issues are not well understood. TABLE 12 Solder Float Test Results for Immersion Silver (Ag) Board Finishes After Exposure to Various Preconditions Condition As is 8-hr bake, 160jC – 8 hr – 8 hr, OSP – 24 hr Temperature/humidity (85/85) – 7 days – 25 days – 25 days, HASL Steam age – 4 hr – 8 hr Source: From Ref. 7.

% Solderability 100 100 65 100 100 100 85 100 100

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FIG. 12 Benefits of immersion silver (Ag) board finish compared to HASL. (From Ref. 6.)

9. Electrolytic Silver An alternate to immersion silver is electrolytically deposited silver. Although electrolytic silver is not commonly applied to PWBs, it is a very common electronic component finish that is particularly suitable for wire bonding applications. A standard procedure used in the industry (in contrast to a Ni/Pd finish) is to directly wire bond to an electrolytically deposited Ag pad and component lead surface as depicted in Fig. 13. Electrolytic silver is not practical for most PWB applications because of the high cost of plating a relatively thick silver deposit, and because of the reliability concerns associated with silver migration. Under certain conditions of temperature, humidity, and electrical bias, silver migrates from one pad to another as shown in Fig. 14. The silver filaments that form as a result of the migration are conductive and can cause electrical failures due to short circuits. 10. Immersion Tin (Sn) Immersion tin is another relatively new board finish, although it has been utilized in the industry in other forms for many years. Immersion tin relies on a relatively thin (0.1 to 0.5 Am) coating of tin deposited from an immersion-tin process directly over copper. The tin protects the copper from oxidation during storage. As tin is not readily deposited onto copper by an immersion process, chemical enhancing agents known as catalysts must be utilized to enhance the tin

FIG. 13 Schematic depicting a wire directly bonded to an electrolytically deposited layer of Ag on a lead frame finger at one end and on a similar deposit on a chip pad at the opposite end of the wire.

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FIG. 14 Schematic depicting the formation of thin Ag filaments, created through Ag migration, causing electrical shorts between adjacent PCB metal surface pads.

deposition characteristics. The compounds used typically are not environmentally friendly, and the process can be very aggressive in attacking soldermask material. Immersion tin provides a flat, consistent surface for SMT, a good surface for electrical probing and press fit connectors, and provides excellent solderability. An aspect of concern in utilizing immersion-tin coatings is that they can transform into a copper–tin intermetallic compound (IMC) over time that results in inhibiting solderability and shortens shelf life (6 months maximum). Tin deposited on copper begins to form copper–tin IMCs immediately after plating, and over time the tin coating is continuously consumed as it is converted to IMCs. The rate at which the conversion occurs depends upon the time and temperature of storage. While tin is readily solderable, copper–tin IMCs are inherently unsolderable; and if too much of the tin deposit is converted to IMC, the solderability of the finish degrades dramatically. Plating chemical suppliers have modified the immersion-tin process to incorporate organic addition agents and grain refiners in an effort to deter copper–tin IMC formation. These measures have succeeded to some extent in extending the shelf life of immersion-tin board finishes. Another issue that can arise with immersion-tin finishes and a potential reliability problem is the growth of tin whiskers. It has been observed that room temperature storage of boards coated with immersion tin can grow long filaments from the surface referred to as whiskers within a few weeks (Fig. 15). Again, plating chemical suppliers have attempted to address this problem through modifications of their plating formulations, and to some extent have succeeded. The subject of tin whiskers is a concern any time a tin or tin-rich alloy coating is utilized. The tin whisker issue is discussed in detail in Chapter 21. 11. Electroplated Tin (Sn) Electroplated Sn is another finish for both boards and components that is rapidly becoming a leading candidate. Electroplated tin can be deposited with a variety of deposit appearances and characteristics as noted in Table 13. Owing to historical considerations and military specifications on maximum permissible carbon content in tin and tin–lead deposits, matte tin or satin tin is generally plated for rigorous solderability applications such as PWBs and electronic components. Because of its higher hardness and lower susceptibility to staining, bright tin is usually selected for connector

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FIG. 15 SEM micrograph that shows a tin whisker growing from the wall of a PCB via hole.

applications where high insertion forces are experienced. Matte or bright tin is commonly used as an etch resist for PWB applications and as in this case the finish is only a temporary coating, the type of finish selected depends primarily on the fabricator’s preference. Much additional information on electroplated tin deposits is provided in Section 4 on component finishes. 12. Summary It is clear that no lead-free board finish is the perfect choice and the selection of a suitable board finish is highly application dependent. A summary of concerns of the various PCB surface finishes discussed in this chapter is listed in Table 14. A summary of the properties, key issues, advantages, and disadvantages of the board finishes discussed in this chapter is presented in Table 15. Typical process sequences are listed in Table 16.

TABLE 13 Characteristics of Various Types of Electroplated Tin (Sn) Finishes for Both Boards and Component Leads Characteristic Carbon content: Grain size: Reflectivity: Ductility: Hardness: Solder wetting speed: Ease of staining:

Matte (or matt) tin

Satin (or satin bright) tin

Bright tin

<0.1% f1–5 Am <0.2 GAM High Low Slow High

0.005–0.2% f0.5–5 Am <0.2–0.7 GAM Intermediate Intermediate Intermediate Intermediate

0.1–>1% <0.8 Am >0.7 GAM Low High Fast Low

LEAD-FREE FINISHES FOR PCBs AND COMPONENTS TABLE 14

449

Summary of the Issues for Several Lead (Pb)-Free Board Finishes

Finish

Concerns

Pb-free HASL finishes

.

Immersion metal finishes (Sn, Ag)

. .

Immersion Ag

. .

Immersion Sn

.

. .

Ni/Au

.

OSP

. .

Electroless Ni/immersion Au

. .

Electroless Pd NET

. . .

Compatibility challenges – Planarity sufficient for fine pitch – High temperature deposition – Alloy compatible with screen/wave solders Compatible with Pb-free alloys considered – Levels (i.e., amount) not critical Compatible with higher soldering temperatures Interest particularly in North America Silver migration (?) Cheaper than Ag – Early concern with whisker formation – Flat solderable Sn (FST) with small Bi concentration (<1%) reported not whisker prone per testing – Approval at Siemens, Alcatel, others Potential for whiskering Sn baths require careful control Package design considerations critical – Avoid ‘‘black pad’’ problem – May be replaced with Ni/Pd/Au flash Degraded solderability, multiple heat cycles No solder paste rework Au can embrittle solder joint – Requires proper PCB design Process not highly controllable Now, very expensive No universal finish anticipated User (at assembly) lowest cost will be key factor – Assumes reliability expectations are satisfied

IV. LEAD-FREE COMPONENT FINISHES Tin–lead has been used as an external lead finish for electronic components since the 1970s when technical and cost limitations of previously utilized gold plating made the continued use of gold prohibitive. Since that time, the finishing of internal leads and pads of devices with silver and external lead finishing with Sn–Pb has become the standard manufacturing method. The primary purpose of the external lead finish is to protect the component during storage and to preserve its solderability so that when attaching the component to the board during assembly is achieved with high yields, and that the solder joints meet reliability expectations. During the assembly operation, Sn–Pb on the component melts and mingles with the molten solder on the circuit board and solder paste to form a reliable solder joint. However, in practice it is not required that the component finish melt during the assembly operation. As long as the solderability of the component is maintained to provide a reliable solder joint, it is not necessary that the component finish must melt to achieve this. In 1989, Texas Instruments developed a multilayer palladium (Pd) finish [8] for components known today as Ni–Pd. The internal leads and pads of the semiconductor device as well as the external leads are coated with a common finish consisting of an initial Ni and then Pd electrodeposits. This method has been further refined to include a very thin gold layer as a final coating to enhance solderability. An additional benefit of this technology was that all Pb was eliminated from the package. Although the component industry has been relatively slow to adopt Ni–Pd finishes on a mass-produced scale, Ni–Pd (Au) is a serious contender to replace Sn–Pb as a Pb-free component finish.

Yes

Yes

OSPs

Electroless Ni/ Immersion Ag

Electroless

Yes

No

Immersion Sn

Pb–Sn HASL

High

Relatively low cost

Relatively low cost

Relatively expensive

High

Low

Cost

Compared to eutectic Sn/Pb. Source: From Ref. 1.

a

Yes

Immersion Ag

Pd

Yes

Finish

Needs OSP coating to preserve solderability Tarnishes in sulfur bearing atmosphere –

Excellent

.

.

Good, wetting slow

Excellent

Good

Solderabilitya

Yes

Yes

–

Yes

Marginal, requires more active flux Yes

Multiple heat cycle solderabilitya

Excellent

–

–

Good

Good

Good

Shelf life

.

. .

No

No

Yes, Al wire Ni under coating required Au wire with Au flash Yes

Yes

No

Wire bondable

Yes

Yes

–

Yes

Yes

Difficult

Electrically testable

Some solder masks are attached Yes

Good

Good

Aggressive chemistry lifts most resists

Good

Solder mask integrity

Comparison of Various Lead-Free Board and Component Finishes Under Consideration to Replace Tin–Lead (Sn–Pb)

Planar finish

TABLE 15

Yes

Yes

Yes

–

–

Difficult

Visual inspection– ability

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Acid clean Rinses Micro-etch Rinses Acid rinse Rinses OSP Blow dry Rinse Dry

OSP Acid clean Rinses Conditioner Rinses Predip Immersion silver Rinses Dry

Immersion silver Acid clean Rinses Micro-etch Rinses Predip Immersion tin Warm rinse Rinses Dry

Immersion tin

Typical Process Sequences for Various Lead-Free Finishes

Source: From Ref. 1.

Acid clean Rinses Micro-etch Rinses Drain Flux Preheat HASL Flux cleaner Hot rinses Rinse Dry

HASL

TABLE 16

Acid clean Rinses Micro-etch Rinses HCl predip Pd catalyst Nickel Rinses Palladium Rinses Dry

Electroless Ni/Pd

Acid clean Rinses Micro-etch Rinses HCl predip Pd catalyst Nickel Rinses Immersion gold Drag out Rinses Dry

Electroless Ni/immersion Ag (ENIG)

Acid clean Rinses Micro-etch Rinses HCl predip Pd catalyst Nickel Rinses Palladium Immersion gold Drag out Rinses Dry

Electroless Ni/Pd+Au flash

LEAD-FREE FINISHES FOR PCBs AND COMPONENTS 451

452 TABLE 17

SCHETTY Summary of the Requirements for Component Finishes

Property

Criteria

Melting point Conductivity Ductility Toxicity World reserves Solderability

Reliability

Manufacturability

Cost effectiveness

Compatible with reflow profile of the selected Pb-free solder paste Possess acceptable conductivity Sufficient to not cause fracturing of the coating during thermal excursions and/or mechanical assembly operations Meet worldwide governmental requirements Sufficient to supply worldwide electronics industry on an ongoing basis Possess good solderability/wetting characteristics and be able to maintain those characteristics after exposure to a variety of natural and artificial aging environments Not cause any unusual or accelerated reliability risks, such as whisker formation or solder joint fracture and, as such, be compatible with both Sn–Pb and Pb-free solder pastes Have a robust manufacturability performance, in most cases this means the ability to be electroplated from common, robust, commercially available plating solutions Be cost-effective relative to Sn–Pb

Lead-free component finishes must meet a variety of criteria that are listed in Table 17. The properties and reserves of the most common Pb-free component finishes are listed in Table 18. The table indicates that the physical and mechanical properties of the reference eutectic Sn–Pb are, for the most part, outstanding. For example, eutectic Sn–Pb exhibits a relatively low melting point that is compatible with common industry reflow profiles, and also with board and component materials of construction. The eutectic alloy has acceptable conductivity, has acceptable elongation, and there are sufficient world reserves. The only major drawback to eutectic Sn–Pb is its toxicity to humans due to its Pb constituent. As many of the desirable physical and mechanical properties of eutectic Sn–Pb are due to the Sn constituent, pure tin would seem to be a logical Pb-free alternative. Tin exhibits good conductivity, superior elongation, low human toxicity, and there are sufficient world reserves of this element. The melting point of pure tin (232jC), although higher than most lead-free solders, does not pose a problem in that the Sn would dissolve into the solder it is in contact with. The major lead-free component finishes are discussed in the following sections.

TABLE 18

Properties and World Reserves of Various Component Lead Finishes

Component finish

Eutectic composition

Melting point

Electrical resistivity (AV cm)

Sn–Pb Sn Sn–Bi Sn–Ag Sn–Cu Sn–Zn Ni–Pd/ Ni–Pd–Au

63Sn/37Pb NA 42Sn/58Bi 96.5Sn/3.5Ag 99.3Sn/0.7Cu 91Sn/9Zn NA

183jC 232jC 138jC 221jC 227jC 199jC >800jC

14.99 11.5 34.48 12.31 11.67 12.0 V12.0

Elongation

Human toxicity

World reserves of alloying element

28–30% >30% 20% 73% >30% 40% <20%

High Low Low Low Low Low Low

High High Relatively low Relatively low High High Very low

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A. Tin–Bismuth (Sn–Bi) Tin–bismuth possesses adequate electrical conductivity and elongation, moderate ductility, and low human toxicity. Bismuth is hampered by a relatively low amount of world reserves, and a very low eutectic-composition melting point. However, the melting point of tin–bismuth can be manipulated to a higher level by adjustment of the bismuth content. In fact, tin–bismuth is the only finish listed that has a melting point at or below the range of eutectic Sn–Pb and for this reason Bi-containing, Pb-free component finishes were the primary focus in the early stages of Pbfree development. However, there are significant reliability and compatibility issues regarding the use of Bi-containing materials with Sn–Pb solder that have emerged which have limited its longterm prospects.

B. Tin–Silver (Sn–Ag), Tin–Copper (Sn–Cu) Sn–Ag and Sn–Cu possess reasonably low melting points, acceptable electrical conductivity, good ductility, low human toxicity, and a moderate-to-large world supply. These three finishes are excellent candidates to replace eutectic Sn–Pb from a physical and mechanical property perspective, but suffer in other areas. 1. Tin–Silver (Sn–Ag) Lead Finish Tin–silver deposits are ductile and exhibit good solderability. However, the cost of Ag is high (>U.S.$8.00/lb). Also, plating variations have a significant impact on the melting point as noted in Table 19. The deposition rate is less than eutectic Sn–Pb, resulting in reduced productivity. Also, the plating solution waste treatment is more complicated. Tin–silver plating baths require strong complexing agents for deposits with low Ag content. These complexing agents complicate the waste management. Another complicating factor is undesirable immersion deposits on the cathode, anode, and equipment. In this system, Ag is much more noble than Sn, so Ag deposition is preferential, which is yet an additional complicating factor. 2. Tin–Copper (Sn–Cu) Lead Finish A notable attribute of the tin–copper system is its low cost (U.S.$2.50/lb). The typical alloys used are Sn–0.7Cu and Sn–0.35Ag–0.75Cu. The deposition current density is typically in the range of 5 to 30 A/dm2. These films exhibit good to excellent solderability characteristics. They have been determined to also exhibit excellent bend and ductility characteristics, i.e., they do not crack over a wide range of deposition and aging conditions. However, their composition and thus melting points are sensitive to process parameters. They also exhibit several plating-related factors such as undesirable immersion deposits on the cathode, anode, and equipment, a more difficult process to control, and more complicated waste treatment. The tin whisker growth concern is greater in the tin–copper system because small amounts of copper increase the propensity for whisker growth, which is a major reliability issue.

C. Nickel–Palladium (Ni–Pd) Nickel–palladium and its derivatives including Ni–Pd–Au are alternative Pb-free finishes that do not rely on melting of the coating during the board assembly operation. Instead, the top precious TABLE 19 Sensitivity of the Melt Temperature of Tin–Silver (Sn–Ag) Solder Alloys to Silver Content Silver content 3.5 5.0 10.0

Melting point, jC 221 250 300

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TABLE 20 Dissolution Rates of Various Metals in Molten Eutectic Sn–Pb Solder Metal Ni Pd Cu Au

Dissolution rate, Am/sec (215jC)

Dissolution rate, Am/sec (250jC)

<0.0005 0.00175 0.08 1.675

0.005 0.07 0.1325 4.175

Source: From Ref. 9.

metal coating(s) dissolve into the molten solder paste during reflow, and soldering is performed to the underlying nickel coating. Therefore the high melting point of Pd (>1500jC) is not an issue in terms of its applicability to being a Pb-free component finish as the dissolution rates of Pd and Au into molten eutectic Sn–Pb solder are the more important parameters as shown in Table 20. This table indicates that thin coatings of Pd and/or Au are fully dissolved during conventional reflow operations. Other physical and mechanical properties of Ni–Pd and Ni–Pd-Au are acceptable electrical conductivity, marginal ductility (potentially impacting its usage on certain component package types), and low human toxicity, but the most significant issue is the availability of Pd, the most important factor affecting its implementation. In summary, none of the Pb-free component finishes listed can clearly be eliminated due to any glaring deficiencies in physical or mechanical properties. The low melting point of Sn–Bi and the questionable availability of Pd are worrisome but not show-stoppers.

D. Pure Tin (Sn) Electroplated pure tin is emerging as one of the leading candidates to replace Sn–Pb as a component finish. As discussed in Section III.B.11, several different tin deposit types are possible, but for semiconductor component plating only the matte and satin tin deposits are receiving the greatest consideration, although others may be acceptable depending on the undercoat and other factors discussed in Chapter 21. In terms of physical and mechanical properties, pure tin finishes exhibit several characteristics. These finishes are nontoxic to humans, exhibit excellent wetting, are relatively easy to electroplate, have a reasonable cost, and possess good mechanical properties. As already stated, the most notable unfavorable aspect is the potential for dendritic whisker growth that can form even after having been in a field environment for several years. Typically, tin whiskers cannot be eliminated from entering the field by a post-manufacture inspection. These very thin filaments act as ‘‘microwires’’ creating a bridge between leads, thereby causing shorts. The mechanism is not fully elucidated; however, alloying is believed to inhibit growth in some cases while hastening growth in others (e.g., Sn–Cu system). Some additional tin whisker phenomenon discussions are in Section VII.A and very detailed in Chapter 21. Although deposition of tin is also possible through immersion and electroless techniques, these deposition methods are not considered here for component finishing due to the relatively high thickness requirements (typically >8 Am). Also, high productivity is required for component finishing which makes immersion and electroless plating methods not feasible for this application.

V. SOLDERABILITY Solderability is perhaps the most important property under consideration for a component finish, and obtaining and maintaining good solder wetting characteristics are the primary objectives for

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TABLE 21 Comparative Solderability Test Results for a Variety of Finishes Subjected to Various Aging Conditions Component finish Sn–Pb 90–10 Sn–Pb 90–10 Sn–Pb 90–10 Sn Sn Sn Sn–Bi 90–10 Sn–Bi 90–10 Sn–Bi 90–10 Sn–Ag 97–3 Sn-Ag 97-3 Sn-Ag 97-3 Sn-Cu 99-1 Sn-Cu 99-1 Sn-Cu 99-1

Aging condition

ZCT (sec)

% Coverage

As plated Steam aged Heat aged As plated Steam aged Heat aged As plated Steam aged Heat aged As plated Steam aged Heat aged As plated Steam aged Heat aged

0.29 0.46 0.32 0.58 0.83 0.68 0.33 0.41 0.37 0.42 0.54 0.53 0.39 0.83 0.79

>95% >95% >95% >95% >95% >95% >95% >95% >95% >95% >95% >95% >95% >95% >95%

any component finish. Many studies [10,11] have shown that fundamental solderability performance of all the finishes mentioned with both a Sn–Pb and a Sn–Ag–Cu solder is acceptable when evaluated by either the ‘‘dip and look’’ or wetting balance methods as shown in Table 21. Solder wetting is discussed in detail in Chapter 11. Previous research [12] has demonstrated to achieve good solder wetting that it is critical to maintain an adequate degree of superheat, defined as the temperature difference above the

FIG. 16 Effect of superheating on the solder wetting characteristics of some lead-free alloys in comparison to the eutectic Sn–Pb benchmark alloy.

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FIG. 17

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The effect of oxygen concentration and superheating on the wetting time of Sn–Zn solder.

melting point of the solder. For example, Hunt and Lea [12] showed that solder wetting characteristics of Sn–Pb, Sn–Ag, Sn–Cu, and Sn–Ag–Cu are almost identical with the same amount of superheating (Fig. 16). In terms of fundamental solderability performance and wetting properties, only Ni–Pd stands out as having a potential problem based on exposure to steam age test conditions, but this may be due to the solderability mechanism (dissolution vs. melting). Tin–zinc (Sn–Zn) suffers from a well-known oxidation problem upon exposure to air (drossing) that requires soldering in an inert atmosphere such as nitrogen. The wetting speed of Sn–Zn as a function of oxygen content is shown in Fig. 17. In general, increasing the superheat of the Sn–Zn solder increases wetting.

VI. MANUFACTURABILITY AND COST EFFECTIVENESS The manufacturability of Sn–Pb component lead finishes is well proven, as it is based on a wellestablished and mature technology. The manufacturability and cost effectiveness of the various types of finishes and coatings utilized for PCBs and components are summarized in Table 22.

A. Pure Tin (Sn) From a component manufacturing perspective, the removal of Pb from a Sn–Pb finish to create a pure Sn finish is virtually a drop-in replacement for Sn–Pb plating. Commercial tin-plating chemistries offer a wide operating window. The cost of pure-tin plating processes is equivalent to or less than commercial tin–lead plating processes.

B. Tin–Bismuth (Sn–Bi) Plating processes for Sn–Bi suffer from several deficiencies including a narrow range of operation, limited throughput due to plating speed limitations, and bismuth immersion on tin anodes that results in the need for additional plating bath and equipment maintenance, and control. As discussed in Sections 2 and 3.2.8, immersion deposits occur when a base metal is immersed into a solution containing a metal of a lower standard electrode potential. As shown in Table 11, Bi has a lower standard electrode potential than Sn; therefore Bi in the Sn–Bi plating solution will immerse on the solid tin anodes present in a production plating cell. This in turn interferes with anodic Sn dissolution and creates maintenance problems. The cost of Sn–Bi plating chemistry is moderately higher than Sn–Pb plating chemistry.

C. Tin–Silver (Sn–Ag) Plating processes for Sn–Ag are virtually nonviable in a manufacturing environment: they have an extremely limited range of operation, exhibit solution and metal instability, have immersion

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TABLE 22 Comparison of the Manufacturability and Cost Effectiveness for Several Board and Component Finishes Difficulty of manufacturability Finish

Board/component manufacturer

Assembly

Relative cost to manufacture

L L H H M L/M

L M/H L L M M/H

L L H H M M

L L M/H M H M L

L L M L/M M H M

L L M M M M H

Board finishes Pb–Sn HASL OSPs ENIG ENEPIG Immersion Ag Immersion Sn Component finishes Sn–Pb Sn Sn–Bi Sn–Cu Sn–Ag Sn–Zn Ni–Pd

Key: L=low; M=moderate, H=high.

problems, and composition control issues. That is, small changes in composition of the alloy have a significant effect on the melting point of the alloy as demonstrated in Fig. 18. It is very difficult to control the composition to achieve the desired alloy. The precious metal content of the process combined with its solution instability and frequent need for bath renewal results in a significantly increased cost compared to tin–lead plating, and one that is difficult to practice.

D. Tin–Copper (Sn–Cu) There are lead finishing problems associated with the Sn–Cu system similar to Sn–Ag: solution instability, composition control issues, and immersion coatings.

E. Tin–Zinc (Sn–Zn) Plating solutions for the Sn–Zn system are relatively uncommon, but appear to offer manufacturers a viable option provided the oxidation and corrosion issues associated with Sn–Zn can be dealt with. The plating solution cost is anticipated to be comparable with Sn–Pb baths, but processing costs are likely higher (e.g., need for inert atmosphere) to suppress the tendency for oxidation.

F. Nickel–Palladium (Ni–Pd) The Ni–Pd finish and its derivatives offer a streamlined manufacturing process, and thus significantly reduce costs in that component manufacturers can eliminate an entire manufacturing process step (plating). All lead finishing utilizing this method is performed by the lead-frame supplier. However, the political instability of those areas that are key producers of Pd makes it a difficult choice for many manufacturers. Because of the uncertainty in Pd supply combined with some reliability-related issues, many in the industry do not consider Ni–Pd and its derivatives a safe, reliable alternative to Sn–Pb finishes, although there are some users that have not experienced these problems.

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FIG. 18 Equilibrium phase diagram for the Sn–Ag system.

VII. RELIABILITY Several potentially significant reliability issues are a concern with some Pb-free component finishes. Eutectic Sn–Pb control has no known reliability issues with regard to its material properties and performance as a finish material. Removal of Pb to create a pure Sn component finish has one significant reliability issue: the formation of tin whiskers.

A. Tin Whisker Phenomenon Tin whiskers are thin single crystal filaments that grow from electroplated tin and tin-rich coatings after plating. The propensity of tin or tin-based coatings to spontaneously form whiskers is well documented and has been known for some time [13,14]. Tin whiskers appear in many different physical shapes and sizes, as shown in Fig. 19. Much has been written on this subject as it is of great concern within the electronics industry as tin whiskers have been known to cause shortcircuit field failures in commercial and military electronic products. Pure tin is not the only candidate to possess a tin whisker risk, and in fact all Sn-rich Sn alloys including Sn–Bi, Sn–Ag, Sn–Cu, and especially Sn–Zn pose reliability risks from the threat of tin whiskers. 1. Formation and Growth Mechanism There are several postulated mechanisms for tin whisker growth discussed in detail in Chapter 21, including recrystallization due to strain aging. The main driving force for tin-whisker formation is believed to be stress. The source of the stress can be either internal (i.e., created within the

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FIG. 19 Series of SEM micrographs that show various size and physical shapes of tin whiskers.

deposited film during the plating operation), or externally induced stress which may occur during assembly or due to the application in the field. Factors affecting internal deposit stress may include grain size and shape, crystal orientation, organic occlusions, inorganic impurities, substrate material properties, and processing steps prior to plating such as pretreatment. Examples of externally induced stress include mechanical stress such as forming or stamping, thermally induced stress, stress induced by IMC formation, etc. It is believed, based on some studies, that compressive stress in the deposit is the main driving force for tin whisker growth, generated either by the properties of the deposit itself or, more significantly, by the formation of IMCs when tin is deposited over copper or copper alloys [15]. The proposed mechanism involves the diffusion of copper into tin as the deposit ages at room temperature and the resulting formation of Cu6Sn5 IMC at the grain boundary locations. As there is an excess of material in a fixed volume, the tin grains are subjected to a compressive stress caused by the IMC surrounding them. As the compressive stress in the tin deposit increases, a ‘‘threshold’’ level of compressive stress is reached, and at this point a tin whisker forms to relieve the stress. Although other mechanisms of tin whisker growth may apply, recent quantitative stress analysis of deposits [16] coupled with focused ion beam (FIB) and scanning electron microscopy (SEM) investigations [17] has demonstrated that this explanation has validity as shown in Figs. 20 and 21. Through the use of the FIB, an analytical tool more commonly associated with front-end semiconductor wafer manufacturing, metallurgical cross sections of very fine features are possible. In these figures, a tin whisker and the tin deposit directly beneath the whisker have been sectioned and examined by SEM. The figures clearly show significant penetration of IMC directly surrounding the base of the tin whisker. These figures lend credence to the concept that the penetration resulted in a compressive stress around the tin grains in question, which subsequently caused a tin whisker to emerge from the deposit directly above the location of the IMC penetration.

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FIG. 20 SEM micrograph showing a sectional tin whisker and the region directly below it prepared using a focused ion beam (FIB). Intermetallic compound (Cu6Sn5) is present near the base of the whisker. The IMC is believed to stress the surrounding Sn grains, which has a role in nucleating tin whiskers.

Annealing and/or reflowing the tin deposit reduces the whisker propensity, through stress relaxation and formation of a regular, dense IMC [18]. Studies have shown that the type of tin– copper IMC formation that occurs at room temperature or at slightly elevated temperatures is uneven and readily penetrates the tin grains. 2. Use of a Nickel Barrier It has been useful to apply a nickel layer between the tin deposit and the substrate to reduce the propensity for tin-whisker growth because it creates a barrier between the substrate and tin deposit. The Ni layer effectively blocks IMC formation between the substrate material and the tin. Many references in the literature confirm the benefits of a nickel barrier coatings in reducing whisker growth [19,20]. In addition, studies have indicated that application of a nickel barrier coating produces a tensile stress in the tin deposit [21]. It is theorized that in the case of tin–nickel IMC formation (Sn4Ni3), tin diffuses into copper, contrary to tin–copper IMC formation where the copper diffuses into tin. As tin diffusion into nickel generates a deficit of material in a fixed volume, the tin deposit experiences a tensile stress. This finding is significant because it demonstrates that it is not necessarily the amount of stress in the deposit that is important for tin-whisker growth, but primarily the type of stress that is present. 3. Stress State Many investigators have confirmed the primary rule of tin whisker growth which is: no compressive stress=no tin whisker growth. Conversely, it has now been demonstrated that a tensile stress in the tin deposit is in fact beneficial for reducing the propensity for tin-whisker formation. Plating process developments are focused on producing tensile stressed tin deposits and some have been commercialized [22]. It is likely that with increased understanding of tin-whisker mechanisms and improvements in plating process technology, pure-tin finishes will be among the best options for producing a reliable, robust, cost-effective, Pb-free component finish.

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FIG. 21 SEM micrograph showing a sectional tin whisker and the region directly below it prepared using a focused ion beam (FIB). The indicated features are similar to those noted in Fig. 20.

B. Issues with Various Lead-Free Finishes Tin–bismuth (Sn–Bi) poses us a potential reliability risk owing to bismuth’s incompatibility with Sn–Pb solder. The presence of Bi in sufficient quantities can cause the formation of a low-meltingpoint Sn–Bi–Pb phase. A 52Bi–30Pb–18Sn ternary alloy forms, which has a melting point of 96jC. The presence of the low-melting phase has severe effects on solder joint reliability, as thermal cycling of through-hole devices can result in a phenomenon known as ‘‘fillet lifting’’ (see Chapter 17 for a detailed discussion). There is a decrease in solder joint strength with increasing Bi content in the alloy, as shown in Fig. 22. For these reasons, Bi-containing Pb-free solders and component finishes are not considered ideal long-term Pb-free solutions. Tin–silver (Sn–Ag) has no major reliability concerns in addition to the tin-whisker issue. Tin–copper (Sn–Cu) is one example where the addition of an alloying agent to tin has been demonstrated to increase the risk of tin-whisker formation. Several studies [23,24] indicate that component finishes of Sn–Cu alloys containing low levels of Cu (<3–5%) significantly increase the propensity toward whisker formation. This represents a case where an assessment must be made of the benefits for utilizing a Sn–Cu finish for a particular application. Among these benefits are that copper additions to tin reduce the tendency for copper thieving (dissolution) from circuit features into the solder. This may be acceptable in a consumer product application, but perhaps not so for a high reliability application. Tin–zinc (Sn–Zn) poses a tin whisker risk and exhibits a propensity to oxidation, resulting in the formation of dross during soldering. It is therefore necessary to perform assembly operations in a nitrogen environment which, although technically feasible, adds to the cost. Inerting is discussed in Chapter 15.

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FIG. 22 The effect of bismuth (Bi) content on the melting point and pull strength of tin–bismuth (Sn– Bi) solder alloys.

Both Ni–Pd and Ni–Pd–Au suffer from two primary reliability issues: low ductility and corrosion on Alloy 42. The relatively low ductility of nickel and palladium coatings often results in a deposit cracking during a component trim and form operation. The cracks can expose underlying copper that can then oxidize, causing a significant loss in solderability performance of a component. The corrosion problem associated with Alloy 42, an alloy consisting of Ni, Fe, and Co, occurs as a result of a galvanic cell set-up due to the strong difference in electrochemical potential between Pd and Fe. The potential drives base material Fe ions to the surface where they are oxidized, thereby degrading the solderability. All component finishes listed are compatible with both Sn–Pb and Pb-free solder and form reliable solder joints with these solder materials, with the exception of Sn–Bi. In summary, Sn-whisker formation remains a concern with all Pb-free finishes except Ni–Pd (Au) as all highly Sn-rich coatings are potentially affected. Tin–bismuth (Sn–Bi) poses some concern due to its incompatibility with Sn–Pb solders if the concentration level is too high. Nickel–palladium (Ni–Pd) and its derivative coatings suffer from cracking and corrosion problems that Sn-based lead finishes are immune to.

VIII. SUMMARY None of the candidate finishes is completely without reliability, manufacturing, or cost concerns, so utilization of a particular finish will be based upon application. For example, from the data presented, Sn–Bi can, for all intents and purposes, be eliminated as a high-volume candidate because of its low supply situation, reliability concerns, incompatibility with Sn–Pb solder, and manufacturability issues. Sn–Ag is likely not a high-volume candidate due to cost and manufacturability issues. Sn–Cu, although mentioned as a potential wave solder candidate, suffers from reliability concerns (increased risk of whisker formation) and manufacturability issues. Sn–Zn has solderability and increased costs concerns. Ni–Pd, which does not have tinwhisker concerns, does have potential corrosion and thus reliability concerns and also costrelated issues, and therefore may not be a viable high-volume contender. One might conclude that pure tin is the leading high-volume candidate to replace Sn–Pb as a Pb-free component finish as it is abundant, cost-effective, and fairly easy to plate. However, the key concern in utilizing pure Sn as a Pb-free component finish is tin-whisker growth, as mentioned previously. However, there are efforts to mitigate and hopefully eliminate tin-whisker formation. The subject of tin-whiskers is discussed in more detail in Chapter 21.

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ACKNOWLEDGMENTS The author wishes to thank Dr. Yun Zhang of Cookson Electronics for useful discussions on tin whisker formation.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

Ormerod, D.H. Getting the Lead Out of the Surface Finish—What are the Options and Challenges ? Proc. Lead-Free Soldering and Interconnection Symposium; Binghamton, N.Y. Section 7, Dec. 1999. Houghton, B. What the EMS Provides Wants in a Board Finish, Proceedings of IPC Annual Meeting Technical Conference; New Orleans, LA, 502-2/3 pp. Schetty, R. Pb-Free Component Finishes, Proceedings of IPC Works ’99; Minneapolis, MN; S-02-3-1– S-02-3-8 pp. Brunno, N. Proc. IPC Printed Circuits Expo; 1999. Goosey, M. Implications of Higher Reflow Temperatures on Boards and Components, Proc. Lead-Free Soldering and Interconnection Symposium, Binghamton, NY Section 6. Furnanz, J., et al. Manufacturing and Reliability Evaluation Of Immersion Silver, Proc. IPC Works ’99; Minneapolis, MN, S-04-5-1–S-04-5-7 pp. Cullen, D. Immersion Silver Performance Results, Proc. IPC Works ’99; Minneapolis, MN, October. 1999; S-04-7-1–S-04-7-16 pp. Abbot, D., et al. Palladium as a Lead Finish for Surface Mount Integrated Circuit Packages. IEEE Trans. Components, Hybrids, Manuf. Technol. Sept. 1991, 14 (3), 567. Abbott, D. Lead-Free Component Lead Finishes, Proc. Lead-Free Soldering and Interconnection Symposium; Binghamton, NY; Section 9, Dec. 1999. Schetty, R. Pb-Free Component Finishes, Proceedings of IPC Works Pb-Free Technical Conference; Minneapolis, MN, Oct. 1999; 48 pp. Schetty, R. Pb-Free Component Finishes, Proceedings of AESF SUR/FIN Technical Conference; Chicago, IL, June 2000; 6–7. Hunt, C.; Lea, D. Solderability of Lead-Free Alloys, Proceedings of APEX Technical Conference; San Diego, CA, Jan. 2000; p-MT2/3-2. Hunsicker, H.Y.; Kenspf, L.W. Growth of Whiskers on Tin-Aluminum Bearings. Quart. Trans. SAE 1947, I, 6. Hawkins, D.T. Metal Whiskers, 1945–1975. Bell Labs internal document, a comprehensive collection of 886 references on metal whiskers. Lee, B.Z.; Lee, D.N. Spontaneous Growth Mechanism of Tin Whiskers. Acta Mater. Oct. 1998, 46 (10), 3701. Xu, C., et al. Understanding Whisker Formation—Driving Force for the Tin Whisker Formation, Proceedings of APEX 2002 Technical Conference; Long Beach, CA, Jan. 2002; 506-2/2-4. Baudry, I.; Kerros, G. Focused Ion Beam in Microelectronic Packaging Applications. Soldertec J. September 2001, 3–4. Zhang, Y., et al. Understanding Whisker Phenomenon, Proceedings of APEX 2002 Technical Conference; Long Beach, CA, Jan. 2002; 506-1/7-9. Zhang, Y., et al. Understanding Whisker Phenomenon, Proceedings of APEX 2002 Technical Conference; Long Beach, CA, Jan. 2002; 506-1/4. Schetty, R. Minimization of Tin Whisker Formation for Lead-Free Electronics Finishing, Proceedings of IPC Works Technical Conference, Miami, FL, Sept. 2002; 5-02-3/3-5. Xu, C., et al. Understanding Whisker Formation—Driving Force for the Tin Whisker Formation. Proceedings of APEX 2002 Technical Conference; Long Beach, CA, Jan. 2002; 506-2/4. Schetty, R. Tin Whisker Growth and the Metallurgical Properties of Electrodeposited Tin, Proceedings from IPC/JEDEC Pb-Free Technical Conference; San Jose, CA, May 2002; pp. 137–145. Moon, K.W., et al. The Formation of Whiskers on Electroplated Tin Containing Copper, Proceedings of 4th Pacific Rim Conference on Advanced Materials and Processing; Honolulu, Hawaii, Dec. 2001; 4 pp. Sriyarunya, A., et al. Lead-Free Plating Qualification, Proceedings of IPC/JEDEC Technical Conference; Taipei, Taiwan, Dec. 2002; 1 pp. Jordan, M. Dr. The Deposition of Tin and Its Alloys; Eugen G. Leuze Publishers: Salgan/Wurtt, Germany, 1995; 33 pp. Cullen, D. Going Beneath the Surface of Surface Finishes. CircuiTree Nov. 2002; Vol. 15, No. 11, 51. Hua, F., et al. Eutectic Sn–Bi as an Alternative Pb-Free Solder, Proceedings from IPC Works Technical Conference; Minneapolis, MN, Oct. 1999; 2 pp. O’Brien, G.; Milad, G. Standard Development Efforts of Electroless Nickel Immersion Gold IPC-1552, Proceedings of IPC Annual Meeting; Orlando, FL, Oct. 2001; S05-2-6.

464 29. 30. 31. 32.

SCHETTY Toben, M., et al. A Study of Interfacial Fracture Phenomena of Solder Joints Formed Using the Electroless Nickel/Immersion Gold Surface Finish, Proceedings of IPC Works Technical Conference; Miami, FL, Sept. 2000; S03-2-2. O’Brien, G. What to Look for From Your Board Supplier When Changing to an Alternate Surface Finish to HASL, Proceedings of IPC Annual Meeting; New Orleans, LA, Nov. 2002; S02-1-3/11. Hunt, C., et al. Evaluation of the Comparative Solderability of Some Lead-Free Solders in Nitrogen, Proceedings of IPC Works Technical Conference; Miami, FL, Sept. 2000; S05-1-3. Baggio, T. The Panasonic Mini Disk Player: Turning a New Leaf in a Lead-Free Market, Proceedings of IPC Works Technical Conference; Minneapolis, MN, Oct. 1999; 10 pp.

13 Formation of Intermetallic Compounds at Pb–Sn/Metal and Lead-Free/Metal Interfaces in Solder Joints Eric J. Cotts and Robert Kinyanjui State University of New York at Binghamton, Binghamton, New York, U.S.A.

Richard Chromik Lehigh University, Bethlehem, Pennsylvania, U.S.A.

Anis Zribi GE Global Research, Niskayuna, New York, U.S.A.

P. Borgesen Universal Instruments Corporation, Cleveland, Ohio, U.S.A.

I. INTRODUCTION Solder joints provide electrical continuity and mechanical stability for interconnects in electronic packages. Intermetallic compounds (IMCs) directly influence solder joint properties. An understanding of the mechanisms of the formation of intermetallic compounds in solder joints is critical to understanding the behavior of interconnections. Intermetallic compound formation can profoundly change the microstructure of solder joints, and therefore their mechanical properties. Factors such as grain size, impurity concentration, secondary-phase precipitates, and dislocation density affect the movement of dislocations, as well as grain growth, in solder joints. These processes determine the mechanical behavior of solder joints, since many deformation processes depend on dislocation growth or on grain boundary sliding. Intermetallic compounds can form in the bulk solder and at the solder/ metallization interfaces of solder joints. Intermetallic compound precipitates in bulk solder can affect dislocation movement and grain growth, and thus mechanical properties. Some IMCs are very brittle, causing a region of weakness, which can result in solder joint failure. Adhesion between IMCs can also be poor, causing solder joint failure. Microstructure affects the mechanical properties of solder joints, and IMCs play an important role in solder joint microstructure [1–3]. Although solder joints are found with various geometries, for the sake of discussion a simple geometry consisting of two essentially planar metallizations joined by solder was selected to illustrate the concept. Whereas the metallizations may be only 10 Am in thickness, solder reactions generally do not consume the metallization material (exceptions include thin, passiva-

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FIG. 1 A sketch (not to scale) of a typical solder joint in microelectronics—two planar terminal metallization pads joined by the solder bulk or matrix, with intermetallic compounds formed at the metallization/solder matrix interfaces. The vertical dimension can vary from approximately 100 Am to 1 mm.

tion layers, such as Au). Thus, in many instances reactions at metallization/solder interfaces can be considered without loss of generality. In Fig. 1 the solder/metallization interfaces are labeled, and the bulk solder is referred to as the solder matrix. The continual reduction in the size of microelectronic components (and so too, their interconnections) along with changes in metallurgies makes the formation of functionally adequate solder joints more challenging, even for the well-studied Pb–Sn solder system. The use of surface finishes such as Au or Pd can result in complex situations. Some metallization deposition methods have changed the course of IMC formation. Examination of these effects provides a good background for the changes brought on by the use of Pb-free solders.

A. Lead–Tin System Interactions between Pb–Sn solder and common metallizations such as Cu or Ni have been well characterized [4–13]. Studies of the growth of IMCs have been conducted for several reflow procedures, annealing schedules, and geometries. Qualitative and quantitative models have been constructed. A good understanding has evolved for Pb–Sn solder/Cu or Ni metallization reactions in bulk geometries. A Pb–Sn solder joint is inherently unstable in that Sn in solder and metallization elements combine to lower their free energy. This is an important factor in the evolution of solder joints; a driving force exists for the formation of Sn-based IMCs. The Cu–Sn, Ni–Sn, Au–Sn, and Pd–Sn phase diagrams [14] presented in Figs. 2–5, respectively, provide important examples of these IMCs and their range of stabilities. On the other hand, Pb is stable with respect to compound formation with Ni or Cu. In considering the formation of IMCs in Pb–Sn solder/metal joints, Pb is generally considered to be inert. Interreactions at Pb–Sn/metallization interfaces generally yield one or more binary Sn–metal (M) intermetallic compound(s), with more complex ternary phases forming under certain circumstances. Common metallizations include Cu and Ni, while Au, and sometimes Pd, is used as a surface finish on Ni. Gold dissolves quickly in the solder matrix during reflow. Sn–M compounds form during reflow and continue to grow during annealing. Intermetallic compounds formed at solder/metallization interfaces are important electrical and mechanical links in solder joints. Intermetallic compounds may also be found in bulk of solder joints and are either an inherent part of the solder, or they are formed during soldering. The formation of IMCs occurs at solder/metal interfaces when molten solder contacts a metallization surface and continues, however slowly, throughout the lifetime of a solder joint (assuming a sufficient supply of metals). The initial formation of one or more IMCs at a solder/

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FIG. 2 Equilibrium phase diagram for the Cu–Sn system. (From Ref. 14.)

metal interface during reflow is generally the most prodigious formation process. Besides the initial growth of an IMC being generally the fastest, temperatures are of course higher during reflow, and the solder is in the liquid state, both of which facilitate the reaction process. For instance, a Cu6Sn5 layer can grow at a Pb–Sn/Cu interface during a reflow of approximately 10 sec at a temperature of 220jC to a thickness of approximately 2 Am [15], whereas annealing for an additional 150 hr at a temperature of 150jC only results in the growth of two more micrometers [9].

FIG. 3 The equilibrium phase diagram for the Ni–Sn system. (From Ref. 14.)

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FIG. 4 The equilibrium phase diagram for the Au–Sn system. (From Ref. 14.)

B. This Chapter The advent of Pb-free solders has had a profound effect on the metallurgy of solder interconnections [16–39]. The Pb in Pb–Sn solders has been replaced with relatively small amounts of other constituents (e.g., Cu or Ag) that form IMCs with Sn. These constituents, although present in small concentrations, diffuse rapidly in Sn and react with termination-pad metallizations. This chapter examines the effect of these changes on the formation and evolution of intermetallic compounds in solder joints. The chapter addresses several Pb-free solder systems among the hundreds proposed for use as solders for electronic packages [40]. Many of these alloys are Sn based. Some of them (for instance Sn–Bi and Sn–In alloys) are currently used in applications, while others have essentially

FIG. 5 The equilibrium phase diagram for the Pd–Sn system. (From Ref. 14.)

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never been applied. The focus is on eutectic and near-eutectic compositions of the Sn–Ag–Cu, Sn–Ag, and Sn–Cu systems. The formation of intermetallic compounds in solder joints during both reflow and subsequent annealing below the solidus temperature is also discussed. The classic Pb–Sn/metallization systems serve as a reference point [4–13]. In this context, the case of thermal cycling of solder joints of Cu/Ni/Au/Pb–Sn and the formation of Au–Ni–Sn intermetallic compounds at metallization interfaces is considered in detail [5,41–62]. The fast diffusion of Au controls the formation of Au–Ni–Sn alloys in this system [54–56]. In fact, some fundamental observations of the diffusion characteristics in Sn provide useful generalizations for all Sn-based solder systems, including Pb-free solders. Many Pb-free solders have near-eutectic compositions of Sn and various elements, often in ternary compositions. Those elements are often in relatively low concentrations, so the reactions between pure Sn and various metallizations are discussed as a basis for comparison. The formation of intermetallic compounds at Pb-free solder/metallization interfaces is discussed, and an attempt is made to understand the basic mechanisms of the growth processes and their relation to the reliability of interconnections.

II. FORMATION AND GROWTH OF INTERMEDIATE PHASES A. Solder Joint Structure The formation of solder interconnections in microelectronic packages involves the melting of solder in contact with a metal surface. The joining of the metals is facilitated by the presence of fluxing agents to reduce oxides on the metal surfaces, and the use of controlled (i.e., inert) atmospheres to prevent their formation during reflow operations. Termination pads are often coated with a surface finish. For instance, Ni is coated with approximately 0.1 Am of Au, or Cu is coated with an organic material referred to as an organic solder preservative (OSP). The intermetallic compounds formed in the solid metal/liquid solder diffusion couple also form in the solid state (i.e., at service temperatures below the solidus temperature) because of growth by solid-state diffusion.

B. Kinetics vs. Energetics Kinetics, rather than energetics, tends to dominate phase selection at interfaces in solder interconnections. This is reflected in the fact that the most Sn-rich alloys generally form first in metal–Sn systems [5,52,53]. Energetics, on the other hand, often favors the formation of other alloys. For instance, in Cu/Sn diffusion couples Cu6Sn5 is observed to form first, while in Pd/Sn diffusion couples PdSn4 is found to grow first [57,63]. The Pd–Sn alloy system contains seven intermediate alloys, which can form at room temperature (phase diagram, Fig. 5). In consideration of the enthalpy of formation [64,65] of these intermediate alloys as a function of Sn content (Fig. 6), energetics is seen to favor the formation of Pd-rich alloys while the most Sn-rich alloy, PdSn4, is the least energetically favored, but it is PdSn4 that forms first. The implication is that understanding the kinetics of formation of intermetallic compounds in solder systems enhances the ability to predict phase formation. 1. Effect of Crystal Structure An examination of solute diffusion coefficients in Sn [56] (Fig. 7) provides additional insight into the growth of Sn-rich alloys in some Sn/metal diffusion couples. It also provides an insight into the diffusion of constituents in some Pb-free solders, such as Sn–Ag–Cu [28,34,36]. Anomalously high rates of metal diffusion are observed at temperatures near the melting point of Sn. Similar averaged rates of interdiffusion are found in many Sn-rich alloys at similar temperatures. Notably, the diffusion of Pd in Sn is almost the same as the rate of interdiffusion in PdSn4, while the rate of interdiffusion in less Sn-rich Pd–Sn and Cu–Sn alloys is lower (Fig. 8). Actually, the structure of the PdSn4 alloy is quite similar to that of pure Sn. Fig. 9 illustrates how the PdSn4 structure, viewed along the [010] crystallographic direction, is nearly as open as the Sn structure

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FIG. 6 The effect of Sn content on the enthalpy of formation of Pd–Sn alloys. The most Sn-rich compound has the smallest enthalpy of formation; nevertheless, PdSn4, although not energetically favored, is observed to form first in Pd/Sn diffusion couples. (From Ref. 64.)

FIG. 7 A plot of solute diffusion coefficients in Sn for a number of constituents found in solder interconnections. The solid lines represent the temperature regions where the measurements were made, while the dashed lines are extrapolations. The point at 303 K for diffusion in Pd is an estimate [37,38]. The anomalously large values found near typical reflow temperatures help to explain some of the phenomena observed in solder/metal diffusion couples. (From Ref. 56.)

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FIG. 8 A plot of the logarithm of the averaged interdiffusion coefficient vs. the inverse temperature for a number of different systems. (From Refs. 37 and 38.)

along the [100] direction. The parallel planes of Sn atoms exhibit a similar degree of separation in both the Sn and PdSn4 structures. When the space between these planes is filled with Pd atoms, as in the PdSn2 structure (Fig. 10), the averaged interdiffusion constants are seen to decrease by several orders of magnitude in Fig. 11. Faster interstitial diffusion of metal atoms in Sn-rich environments explains both the higher values of the averaged interdiffusion coefficient in, and faster growth of, the most Sn-rich phase in the Pd–Sn system. It can also be seen from Fig. 7 that Ag and Cu diffuse rapidly in Sn. While these measurements are for single-crystal Sn at tracer diffusion levels, it is to be expected that these constituents in Sn–Ag–Cu Pb-free solder diffuse very rapidly in the solder matrix. For instance, Cu is observed to have a diffusion coefficient of approximately 3
107 cm2/sec at 150jC in the slow direction in single-crystal Sn [56]. If this number is used for the diffusion coefficient in polycrystalline Sn, then Cu atoms would only require approximately 100 sec to diffuse throughout a 50-Am-diameter solder ball. In fact, as reviewed in detail below, large Cu concentrations in Sn–Ag–Cu (SAC) alloys profoundly affect IMC formation.

FIG. 9 Illustrations of the crystal structures of (a) PdSn4 and (b) Sn. Tin atoms are shown as smaller octahedrons, while palladium atoms are shown as larger dodecahedrons.

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FIG. 10 An illustrated comparison of the structure of (a) PdSn2 and (b) PdSn4. It is evident that while PdSn4 contains Sn/Sn parallel planes (tin atoms are shown as small octahedrons, while palladium atoms are shown as dodecahedrons) as observed in Sn, in the PdSn2 structure the Sn/Sn planes are filled with Pd atoms. The parallel planes of Sn in PdSn4 apparently provide an easy diffusion path for Pd atoms in the growing PdSn4 phase, but in the PdSn2 compound such a diffusion path is not available.

As mentioned above, Sn is the major component of Pb-free solders. In addition, in Pb–Sn solders, Sn is the most active component in the formation of IMCs. Therefore, the formation of IMCs in Pb–Sn solder/metallization diffusion couples is considered first, as a basis for examination of Pb-free solder alloys. This treatment is followed by a discussion of the formation of Au0.5Ni0.5Sn4 compound in Pb–Sn solder joints, because the mechanisms for its formation are similar to the formation of ternary IMCs in some Pb-free solder joints.

FIG. 11 A plot of the logarithm of the averaged interdiffusion coefficient vs. inverse temperature times 1000. The two lines and the open symbols are measurements of the averaged interdiffusion coefficients in either PdSn4 or PdSn2, while the dotted lines are extrapolations. (From Ref. 37.)

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III. EUTECTIC SN–PB SOLDER ON STANDARD PACKAGING METALLIZATIONS A. On Copper Metallization The evolution of the microstructure in Pb–Sn solder joints has been examined thoroughly in the past [4–13]. The growth of intermetallic compounds in the Cu/Pb–Sn system is different above and below the melting temperature of the solder. The rate of formation of IMCs (primarily Cu6Sn5) is more than an order of magnitude greater above the liquidus than in the solid state. The growth kinetics is also different. These differences are thought to stem from the variation in the morphology of the growing phase. Below the solidus, Cu6Sn5 is observed to cover the Cu/Pb–Sn interface in a planar fashion, while above the liquidus temperature the alloy grows in a faceted or scalloped manner, with distinct grain boundaries between the facets. These grain boundaries are thought to directly affect the growth kinetics, providing short circuits for diffusion. 1. Growth Below the Solidus Temperature Several investigators examined the formation of Cu–Sn alloys above the liquidus [15,23,66–68]. For instance, during the reflow of Pb–Sn solder on Cu surfaces, Cu6Sn5 was observed to grow at a relatively prodigious rate at the solder/metal interface. At a temperature of 220jC, the mean radius, r, of Cu6Sn5 grains growing at a Pb–Sn/Ni interface was found to increase as r = (0.8t0.33) Am, where t is time in seconds [15]. That is to say, r was found to be 3 Am after approximately 60 sec. Careful investigation indicates that Cu3Sn forms during reflow as well, between the layer of Cu6Sn5 grains and the Cu, as would be expected. This layer is relatively thin, generally submicron in size, but subsequent heat treatment of solder joints below the solidus eventually leads to more evident growth of the Cu3Sn phase. The models for growth of Cu6Sn5 above the Sn liquidus take the morphology of the growing alloy into consideration. For instance, the model of Schaefer et al. [59] simplifies this geometry into one where all Cu6Sn5 grains are equiaxed, with essentially spherical tops and hexagonal bases of uniform size [Fig. 12(a)]. An actual series of Cu6Sn5 scallops formed at a Cu/Pb–Sn solder interface during reflow is shown in Fig. 12(b). By assuming that diffusion in the boundaries

FIG. 12 (a) A sketch of the geometry of the Cu6Sn5 grains and scallops, assumed by Schaeffer et al.’s model. (From Ref. 59.) (b) An electron micrograph that reveals the formation of Cu6Sn5, scallops at a Pb–Sn/Cu interface upon reflow. This SnPb/Cu solder joint was reflowed at 200jC for 10 min, and then etched. (From Ref. 15.)

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between the grains dominates the growth process, and that there is one dominant diffusing species, Cu, Schaefer et al. developed a theoretical expression for the time dependence of the average layer thickness, x, finding x to be proportional to t0.33. The experimental results of Schaefer et al. showed slower time dependence than t0.33. By assuming that the average grainboundary diffusion distance increases with x2, the model was modified to a t0.25 time dependence in agreement with the experimental results [59]. Modeling efforts by Kim and Tu [15] included consideration of grain coarsening and volume diffusion processes. These investigators included a combination of the interfacial reaction between Cu and Sn(L) and scallop ripening, and found a t0.33 time dependence. They also observed good agreement between their experimental data and model predictions, as shown in Fig. 13 [15]. The time dependence for the growth of Cu6Sn5 between Cu and liquid Pb–Sn (Fig. 13) does not agree with the experimental observation of Schaefer et al., but does agree with their simple model [59]. 2. Growth Below the Solidus Temperature In the solid state, the growth of solder alloys is generally diffusion limited. Choi et al. [9] examined the thickness of Cu6Sn5 and Cu3Sn IMCs during growth at different temperatures in the solid state. They found a diffusion-limited growth regime for the period they investigated; that is, the square of the intermetallic compound thickness, x, was linearly proportional to time, t, x2 = kt. The growth coefficients were found to be similar to previous results [69]. As might be expected, growth rates were found to be distinctly lower in the solid state compared to IMCs growing where liquid solder is in contact with solid Cu. For instance, at a temperature near 150jC, the value of k for the growth of Cu6Sn5 was found to be 5
1014 cm2/sec. The activation energy for this growth process was reported to be 111 kJ/mol. Vianco et al. [70] also examined this system and their work also indicated that the growth of the intermetallic layers was diffusion limited, with a value for k for the combined IMCs of 6
1013 cm2/sec at 170jC.

B. Pb–Sn Solder on Ni Metallizations Most reports consistently show that the reflow and subsequent heat treatment of Pb–Sn solder/Ni joints lead to the formation and growth of Ni3Sn4 [7,8,10,12,71–74]. The rate of growth of Ni3Sn4 in Pb–Sn/Ni systems has been reported to be less than half that observed for Cu6Sn5 in Pb–Sn/Cu

FIG. 13 A plot that displays the mean radius of Cu6Sn5 scallops formed at a Pb–Sn/Cu interface vs. the cube root of annealing time at three different reflow temperatures: 200jC, 220jC, and 240jC. (From Ref. 15.)

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systems [72]. While Cu6Sn5 grows at a faster rate than Ni3Sn4, both of these phases grow the most rapidly in their respective systems. In fact, kinetics rather than energetics dominate phase selection in these solder/metal systems. Several studies addressing the formation of solder alloys in Ni/Pb–Sn diffusion couples have been performed at temperatures above the liquidus [7,8,10,71–74]. Early during the reaction, scallops are found at the Ni interface, with grooves between the scallops that extend to the Ni interface. For instance, in diffusion couples with eutectic Pb–Sn and electroplated Ni/Pd metallization, Ni3Sn4 scallops were observed [7,8] to form on the Ni surface and to coarsen over time. Kinetic data for the radial growth of the scallops and for the increase in the average thickness of the Ni3Sn4 layer were computed. It was determined that the data fit well as an empirical relation between scallop radius, r, and time, r = Bt0.33, with B a proportionality factor dependent on temperature. There are some differences in the morphology of the growing Ni3Sn4 alloy compared to the growth of Cu6Sn5 in the Cu/Sn system. After prolonged growth, the Ni3Sn4/Ni interface is generally found to be essentially smooth. A planar layer of Ni3Sn4 between the Ni3Sn4 scallops and the Ni substrate could slow the growth of the IMC. This is based on the consideration that the most salient conclusion of the theories of growth of Cu6Sn5 at Cu/liquid solder interfaces was that grooves down almost to the Cu metallization (with a thin layer of Cu3Sn separating the Cu6Sn5 scallops and the Cu) result in high growth rates. The presence of a planar layer of Ni3Sn4 apparently affects the growth rate at Ni/IMC interfaces. If additional elements are present in the metallization in significant quantities, such as thick Au overlayers, or P incorporated in the Ni during electroless deposition, IMCs may form in addition to Ni3Sn4. For instance, as discussed in detail in Sec. III.C.4, Au0.5Ni0.5Sn4 forms at the Ni3Sn4/solder interface if annealed below the solidus temperature. The phase Ni3P has been observed to form between electroless Ni and Ni3Sn4 layers [12]. Both of these IMCs (Ni3P and Ni3Sn4) affect solder joint reliability.

C. Gold in Pb–Sn Solder Joints 1. Rapid Dissolution The formation of some phases in solder alloys can be the cause of adverse reliability behaviors in solder joints. Interconnections consisting of Pb–Sn soldered to Au/Ni terminal pads provide an important example of how the fast diffusion of metal atoms in Sn results in the growth of intermediate alloys that dramatically affect interconnection reliability. When eutectic Sn–Pb solder is initially reflowed on an Au/Ni surface, the thin (generally submicron) coating of Au rapidly dissolves into the molten Sn–Pb solder, whereas Ni3Sn4 is formed at the Ni/Sn–Pb solder interface (Fig. 14).

FIG. 14 An electron microscope image of a cross section of a Ni/Pb–Sn solder joint after reflow. The Pb–Sn solder is evident on the bottom section of the figure, while the Ni is the dark section at the top of the figure. A thin interfacial layer of Ni3Sn4 is observed between the solder and the Ni. The Au flash layer on the Ni dissolved into the Pb–Sn solder.

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2. Effect of Gold (Au) Concentration In the case of flip chips reflowed on chip carriers with a relatively thick Au layer, (e.g., some electrolytically deposited layers), the dissolution of Au upon reflow may lead to an average volume concentration of 2% to 3% Au in a solder joint, resulting in a significant reduction in its fatigue resistance. In fact, this has been observed to affect the reliability of flip chip assemblies in spite of underfilling. For example, Au–Sn intermetallics have been shown [41 42] to degrade the thermal fatigue life of Pb–Sn solder joints when present in concentrations of 1.5 to 5 at.%. In these investigations, AuSn4 was observed as a precipitate dispersed throughout the solder. It has been generally asserted that Au concentrations of greater than 1.5 at.% in Pb–Sn solder joints causes a severe degradation in fatigue resistance [43]. 3. Stacked Intermediate Compound Layers The present example concerns a failure mechanism due to a ternary Au solder phase that manifests itself at Au concentrations as low as 0.1 at.%. A small concentration of Au has also been reported to cause premature mechanical failures in ball grid array (BGA) Cu/Ni/Au/Pb–Sn solder joints [43–46]. Gold has a high solubility in molten Pb–Sn solder, so it quickly dissolves into the molten solder during reflow [46]. The final solder joint microstructure and composition depend on the initial thickness of Au, the reflow profile, and subsequent thermal aging. Mei et al. observed that AuSn4 nucleated in Pb–Sn solder after reflow [43]. They noted that upon thermal aging at 150jC, AuSn4 migrated from the bulk solder to the interface adjacent to the preexisting Ni3Sn4 intermetallic layer. These investigators reported that the AuSn4 had a detrimental effect on the bending strength of BGA assemblies and exhibited a considerable reduction in failure resistance. Poor adhesion between the Ni3Sn4 and AuSn4 phases was reported to be responsible for the observed deterioration in mechanical integrity of the solder joints. 4. Effect of Thermal Aging Additional studies [34,75–78] of this phenomenon showed that Au atoms migrated to the Ni3Sn4/ Pb–Sn interface to combine with Ni and Sn, forming a Ni0.5Au0.5Sn4 alloy (Figs. 15, 16). In one study [34,76], printed circuit board substrates consisting of Cu features overplated with Ni/Au metallizations were used, with individual Ni pads (islands) that were approximately 12 Am thick and 0.5 mm in diameter. Eutectic Sn–Pb solder spheres were reflowed on the pads at a maximum temperature of 209jC for 45 sec above the melting point of the eutectic solder (183jC). The samples were then annealed in argon at 150jC for various times (0, 0.5, 1, 4, 9, 40, 150, and 450 hr). Optical and electron microscopy were used to examine cross-sectioned samples in which Cu, Ni, Ni3Sn4, and the Sn-rich and Pb-rich phases were identified in all samples by wavelength dispersive spectrometry (WDS) spectra, with maps and line scans acquired at various locations. a. Growth of Au0.5Ni0.5Sn4 is Au Diffusion Controlled. In general, for these BGA packages, the concentration of Au in the bulk solder after reflow was lower than the solubility limit of Au in molten eutectic Sn–Pb solder (approximately 4 at.%). Therefore, the Au completely dissolved into the molten solder. An examination of two electrolytic solder alloys after reflow using WDS spectra and maps indicated that Au remained well dispersed in a solder ball upon cooling from reflow (Fig. 15) [34]. Thus, the formation of the Au0.5Ni0.5Sn4 alloy occurred by solid-state diffusion during the subsequent annealing at 150jC conducted as part of the study (Fig. 15). During thermal aging at 150jC, the ternary phase, Au0.5Ni0.5Sn4 (Fig. 16), was observed to grow at the solder/Ni3Sn4 interface during annealing at a rate dependent upon the Au concentration in the eutectic Sn–Pb solder after reflow. Fig. 17 provides a comparison of the thickness of the Au0.5Ni0.5Sn4 layer during annealing of Sn–Pb/Au/Ni/Cu solder joints at 150jC together with data by Minor and Morris [77]. It is apparent that the growth rate of Au0.5Ni0.5Sn4 observed in the study by Minor and Morris [77] was approximately two times greater than the growth rate measured by Zribi et al. [34]. This difference is due to the rate-controlling mechanism for the Au0.5Ni0.5Sn4 formation. Because the initial Au concentration in the solder joints investigated by Minor et al. (0.2 at.%) is twice that in the Zribi et al. study, the results are consistent with the hypothesis that the Au0.5Ni0.5Sn4 growth is controlled by Au diffusion.

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FIG. 15 A sketch of the evolution of the microstructure of Cu/Ni/Au/Pb–Sn solder joints, focusing on the formation of Au0.5Ni0.5Sn4 at the Pb–Sn/Ni3Sn4 interface during post reflow annealing. (a) Pb– Sn solder and Cu/Ni/Au metallization before reflow. (b) Solder joint after reflow reflecting dissolution of Au into the solder matrix, including formation of AuSn4 upon cooling, and formation of Ni3Sn4 at the Pb–Sn solder/Ni interface. (c) Solder joint after postreflow annealing in the solid state (e.g., 150jC). Au0.5Ni0.5Sn4 has formed at the Pb–Sn/Ni3Sn4 interface.

b. Mechanism of (Ni,Au)Sn4 Formation. The observation of the growth of Au0.5Ni0.5Sn4 at the Ni/Ni3Sn4 interface is consistent with previous observations of Sn-rich phases growing at solder joint interfaces. Considering the rapid diffusion of Au in Sn and Pb, a ready supply of Au is available at the Ni3Sn4/solder interface. Ni0.5Au0.5Sn4 is the most Sn-rich phase in the Ni–Au–Sn alloy system [47]. Furthermore, the structure of the Au0.5Ni0.5Sn4 alloy was found to be similar to PdSn4 [62]. It is known that Ni atoms substitute for Au atoms in the AuSn4 crystal structure to

FIG. 16 Scanning electron microscopy backscattered electron image showing Au0.5Ni0.5Sn4 and Ni3Sn4 at a Pb–Sn solder/Ni interface. The sample was annealed for 150 hr at a temperature of 150jC. (From Ref. 34.)

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FIG. 17 A plot of the measured thickness of the intermetallic alloy, Au0.5Ni0.5Sn4 at the Pb–Sn solder/Ni interface vs. time. (From Ref. 28.)

produce compounds of composition Au1xNixSn4,, where x < 0.5. The structure of these compounds can be considered isotypical with the pure AuSn4 phase (the PtSn4 structure type), with even a symmetry increase from Aba2 to Ccca observed [62]. This was determined via crystal structure refinement from powder data using the Rietveld method. Two alloys were examined, x = 0.25 and 0.50, yielding Au0.73Ni0.27Sn4 (space group Ccca, a = 6.448, b = 11.606, c = 6.441 A˚), and Au0.51Ni0.49Sn4 (space group Ccca, a = 6.424, b = 11.522, c = 6.384 A˚), respectively [62]. In the Zribi et al. study [34,76] most of the Au atoms diffuse distances of the order of the diameter of the solder ball before they could combine with Ni and Sn to form the intermediate alloy (Fig. 16). Still, the rapid rate of diffusion of Au in Sn (Fig. 7) allows this to occur, as does the relatively high rate of interdiffusion in the growing intermediate alloy. c. Growth of Au0.5Ni0.5 Sn4. The indication that the rate of formation of Au0.5Ni0.5Sn4 depends strongly on the Au concentration indicates that Au diffusion is the controlling mechanism of the growth of this intermetallic compound at micron thicknesses in Pb–Sn/Au/ Ni/Cu joints. This phenomenon also provides some insight into the behavior of Pb-free solder joints, where the constituents added to Sn (such as Cu) rapidly diffuse distances corresponding to the dimensions of a solder ball diameter and determine phase selection and kinetics for the solder intermediate alloy formation at the interface. GROWTH MODEL. A simple model was constructed by Zribi and Cotts [28] to estimate the growth thickness of Au0.5Ni0.5Sn4 for different geometries, Au metallization thicknesses, temperatures, and annealing times. The model assumes that Au is homogeneously distributed within a solder joint (hemisphere) upon reflow, and that all Au atoms that diffuse to the interface are immediately consumed to form Au0.5Ni0.5Sn4 (Fig. 16). In a further simplification, the diffusion constant for Au in Pb–Sn solder is assumed to be the same as the diffusion constant for Au in Sn. They generated the model by using the scaled diffusion Eq. (1) and solved for Au concentration in a two-dimensional, hemispherical domain representing the cross section of a solder sphere utilizing a version of Flex PDE [79]. ðD=L2 Þj2 u ¼ s1 BðuÞ=Bt

ð1Þ

where D is the diffusion coefficient of Au in Sn at the annealing temperature T, L is the diameter of a solder sphere measured upon reflow, s is the diffusion time scale, and u is the scaled concentration of Au, the concentration of Au in solder upon reflow. CALCULATED AND MEASURED GROWTH RESULTS. A plot of the calculated dimensionless thickness (the calculated thickness of the Au0.5Ni0.5Sn4 intermetallic alloy divided by the initial

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FIG. 18 A plot of the calculated dimensionless thickness (the calculated thickness of the Au0.5Ni0.5Sn4 intermetallic alloy divided by the initial Au thickness) vs. dimensionless time (time divided by a characteristic time). A curve (solid line) was fitted to the points calculated with the simple model: [(xAu0.5Ni0.5Sn4 /xAu)fit = 9(1  exp( 17.4t/si))]. (From Ref. 28.)

FIG. 19 A plot of the dimensionless thickness (the thickness of the Au0.5Ni0.5Sn4 intermetallic alloy divided by the initial Au thickness) vs. dimensionless time (time divided by a characteristic time). (From Ref. 28.)

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Au thickness) vs. dimensionless time (time divided by a characteristic time (the diffusion time constant)) is provided in Fig. 18. This plot is based on the model whose detailed basis is given in the Appendix. In Fig. 19 the nondimensional experimentally measured thicknesses of the Au0.5Ni0.5Sn4 alloy are plotted utilizing data from three investigators [32,34,77] along with corresponding simulated data. The simulated data presented in Figs. 18 and 19 are obtained directly from the diffusion calculations with no adjusted parameters. The relatively good agreement between experimental and corresponding simulated data again suggests that Au diffusion is the growth-controlling step in the formation of Au0.5Ni0.5Sn4.

IV. GROWTH OF INTERMEDIATE PHASES IN Pb-FREE SOLDER ALLOYS A. Composition and Multielement Compounds Tin is the main constituent for most Pb-free solders of interest (e.g., eutectic Sn–Cu, Sn–Ag, and Sn–Ag–Cu) as replacements for eutectic Sn–Pb solder. In fact, Sn is found at concentrations greater than 90 at.% in all of these eutectic solders. Relatively small amounts of additional elements such as Cu or Ag are added to Sn to reduce the melting point of the solder alloy and improve its mechanical and wetting properties [16–40]. As some of these elements form IMCs with Sn (e.g., Cu6Sn5 or Ag3Sn), the microstructure of the solder matrix is different from eutectic Sn–Pb. In addition to these alloying metals present as part of a Pb-free solder, others are added during processing such as the partial dissolution of metallizations (Cu, Ni, or Au) during soldering, further increasing the complexity of the system. Furthermore, many of the constituents (e.g., Cu, Au, etc.) found in Pb-free solders, such as the Sn–Ag–Cu system, diffuse at anomalously fast rates in Sn [56], as noted previously (Fig. 7). The availability and mobility of all these constituents considerably increases the likelihood of the growth of IMCs. This includes IMCs that contain three or four elements that may grow in Pb-free solder joints, in a fashion similar to the growth of Au0.5Ni0.5Sn4 in Pb–Sn [34].

B. A Model for Sn/M Systems A significant body of work concerning the formation of intermetallic compounds at Sn/M (M=Ni or Cu) interfaces at temperatures above the melting temperature of Sn has been reported in the literature. Bader et al. [80] found that at temperatures between 513 K (240jC) and 603 K (330jC), Cu6Sn5 and Cu3Sn grew simultaneously, with Cu6Sn5,, the more Sn-rich phase, growing at a much faster rate than Cu3Sn. Intermetallic compound formation in Pb-free systems during reflow was found to be similar to Pb–Sn/M solder interconnections, with Cu6Sn5 growing in a scalloped morphology. The kinetics observed is perhaps best described as the average thickness proportional to the cube root of time. Simultaneous grain-coarsening processes and standard diffusion processes occur through the solid Cu6Sn5. Studies show that Cu6Sn5 forms at Cu/Sn interfaces, with Cu3Sn subsequently growing between the Cu6Sn5 phase and Cu metal. In Ni/Sn diffusion couples, Ni3Sn4 has been observed to grow at the Ni/Sn interface. These phase selections are similar to those observed in Pb–Sn/M diffusion couples. Since Pb-free solders consist primarily of Sn, the behavior of Sn/M diffusion couples provides a basis for understanding intermetallic compound formation in Pb-free solder/M solder joints. In a study of the Cu–Sn system using differential scanning calorimetry (DSC) measurements to study the growth of Cu3Sn from Cu/Cu6Sn5 thin films, it was determined that a diffusionlimited growth regime existed after the initial nucleation of the Cu3Sn phase [63]. In another study [81] that used an electron probe microanalysis on quenched bulk samples after isothermally annealing them in an Ar atmosphere, the growth of Cu6Sn5 and Cu3Sn for temperatures between 170j and 220jC was found to be parabolic, as was determined in a similar study by Mei et al. [82]. Vianco et al. [83] also carried out similar experiments with Cu/Sn and Cu/63Sn–37Pb diffusion couples at temperatures between 30j and 205jC and found that most samples followed parabolic or slightly subparabolic growth laws (time exponent = 0.5 or 0.42, respectively).

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Reactions have also been examined in Ni/Sn diffusion couples. For instance, in Ni/Sn diffusion couples with Sn layers on the order of 3 Am thick, whiskers and scallops were observed to form simultaneously with a planar Ni3Sn4 layer. A Ni3Sn4 layer was observed to form first in a study on Ni/Sn(L) diffusion couples [80]. Further growth was in the form of Ni3Sn2 scallops. Gur and Bamberger [60] concluded that a planar layer of Ni3Sn4 forms by solid-state diffusion between Ni3Sn4 scallops and the Ni substrate, ultimately slowing the growth of the IMC.

C. Formation of Intermetallic Compounds in Pb-Free Solder Systems The formation of IMCs in Pb-free solder systems is in many respects reminiscent of those observed in Pb–Sn, with several intriguing added complications. In Pb-free solders such as Sn– Ag–Cu, the Cu and Ag additives form IMCs with Sn. These elements also diffuse rapidly in Sn, thus a high flux of these constituents is available for otherwise favorable reactions, such as the formation of ternary IMCs at metallization interfaces. The evolution of the microstructure of Pbfree solder joints during reflow, cooling, and subsequent annealing has been shown to be a complex subject. As far as intermetallic compound formation and growth at metallization interfaces are concerned, Sn–Ag–Cu solder alloys were found to behave similar to eutectic Sn–Pb solder on Cu substrates. In contrast, distinct differences were observed on Ni surfaces between the behavior of Sn–Ag–Cu and Sn–Pb solder. 1. Cu–Sn Intermetallic Phases in Sn–Ag–Cu Alloys Reflow of Sn–Ag–Cu solder on Cu substrates yielded results quite similar to Sn on Cu, or Pb–Sn on Cu. Fig. 20 is a scanning electron microscopy (SEM) image of Sn–4.7Ag–1.7Cu (Sn–5.1Ag– 3.1Cu in atomic percent) solder/Cu interface. WDS analysis indicated the growth of Cu6Sn5, similar to Pb–Sn/Cu and Sn/Cu interconnection interfaces. In addition, a layer of Cu3Sn had already formed between the Cu6Sn5 layer and Cu substrate during the reflow process. The average, total layer thickness was measured to be 2 Am. The morphology of the Cu6Sn5/Cu3Sn duo layer is similar to that observed in Pb–Sn/Cu joints [84]. Also visible in the image are distinct dark regions on the order of 2 to 10 Am (e.g., A in Fig. 20) in size, identified by WDS as Cu6Sn5, and lighter regions of similar size (e.g., B in Fig. 20)

FIG. 20 Scanning electron microscope backscattered electron image of a cross-sectioned sample made by reflowing Sn–4.7Ag–1.7Cu (Sn–5.1Ag–3.1Cu in atomic percent) on Cu at 230jC for 60 sec. The black phase at the bottom is the Cu substrate, the dark gray phase at the interface is Cu3Sn, the lighter gray phase on top of it is Cu6Sn5, and the brightest upper part of the picture shows the solder with fine Ag3Sn precipitates in the middle and a coarse precipitate on the left side (B). The picture also displays a coarse Cu6Sn5 precipitate (A) close to the interface on the right side. (From Ref. 29.)

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identified as Ag3Sn. Similar regions were found in the solder before reflow and are associated with the relatively high Cu and Ag content of this particular solder, causing the precipitation of relatively large quantities of intermetallic compounds in accordance with equilibrium requirements. The growth in the solid state of Cu–Sn IMCs at SAC/Cu interfaces has been examined by several researchers. In Fig. 21, measurements from one study of the average thickness of the combined Cu6Sn5 and Cu3Sn phases are plotted as a function of time as solid circles, for annealing at a temperature of 150jC [84]. The data correspond reasonably well to diffusionlimited growth behavior. The rate of growth of these combined phases is of similar magnitude to that previously observed in Cu/Sn and Cu/Pb–Sn diffusion couples [81,84]. 2. Ni–Sn Intermetallic Phases in Sn–Ag–Cu Systems Bulk samples of Sn–4.7Ag–1.7Cu solder reflowed on pure Ni substrates were observed by Zribi et al. [36] to exhibit results distinctly different from Pb–Sn/Ni or Sn/Ni solder joints. Recall that in both Pb–Sn/Ni and Sn/Ni solder joints, Ni3Sn4 was observed to grow first. In contrast, Cu–Ni–Sn compounds are observed to form at Sn–Ag–Cu/Ni interfaces. For instance, Fig. 22 is an SEM image of a Sn–4.7Ag–1.7Cu (Sn–5.1Ag–3.1Cu in atomic percent) solder/Ni interface, where WDS analysis identified the composition of the growing phase as CuxNiySn.45, where the Cu concentration x varied between 44 and 53 at.%, and the corresponding Ni concentration y between 10 and 1 at.%, respectively. These compositions are consistent with the stoichiometry of the compound, (Cu,Ni)6Sn5 [36].

D. Intermediate Phases in the Cu–Ni–Sn Ternary System While both the Cu–Sn and Ni–Sn phase diagrams have been well characterized, less information is available for the ternary phase diagram of the Cu–Ni–Sn system. A section of the Cu– Ni–Sn ternary phase diagram at 235jC (based on experimental data and thermodynamic modeling) provides insight into phase formation in SAC/Ni solder joints [85]. The (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 compounds are presented as narrow-composition-range compounds in the diagram, which extends the domain of the respective binary Cu6Sn5 and Ni3Sn4 compounds to

FIG. 21 Growth of intermetallic compounds at interfaces of different samples after thermal annealing at 150jC. n The average thickness of the Ni3Sn4 layer at the Sn/Ni interface. . The average thickness of the Cu6Sn5 layer at the Sn–4.7Ag–1.7Cu (Sn–5.1Ag–3.1Cu in atomic percent)/Cu interface. o The average thickness of the (Cu,Ni)6Sn5 at the Sn–3.5Ag–1.0Cu (Sn–3.8Ag–1.85Cu in atomic percent)/Ni interface. (From Ref. 29.)

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FIG. 22 Scanning electron microscope backscattered electron image of a cross-sectioned sample made by reflowing Sn–4.7Ag–1.7Cu (Sn–5.1Ag–3.1Cu in atomic percent) on Ni at 230jC for 60 sec and etched in 20% nitric acid and 80% ethanol by volume. The black phase at the bottom is the Ni substrate, the dark gray phase at the interface is (Cu,Ni)6Sn5, and the lighter gray phase on top of it is Ag3Sn. (From Ref. 29.)

a ternary domain. The Cu–Ni–Sn phase diagram also displays a large two-phase region between (Cu,Ni)6Sn5 and the Cu–Ni solid solution, and a three-phase region between (Cu,Ni)6Sn5, (Ni,Cu)3Sn4, and the Sn–Cu–Ni solid solution. These observations support the idea of a substitutional mechanism where atoms such as (Cu and Ni) and (Ni and Au) can substitute for each other in their binary compounds with Sn. This mechanism was investigated in a study which focused on the Au–Ni–Sn system, where the Au0.5Ni0.5Sn4 intermetallic compound is formed by the substitution of Au for Ni atoms in the AuSn4 lattice [24,28]. Thus, it was concluded [29] that the phase observed to form at SAC/Ni interfaces does indeed correspond to (Cu,Ni)6Sn5. The rate and extent of growth of (CuNi)6Sn5 was determined to depend upon the thickness of the SAC solder. With a relatively large supply of SAC solder, the growth of this compound is prodigious, with a measured layer thickness of approximately 2 Am after a reflow of 60 sec at a temperature of approximately 230jC. The morphology of the layer was found to be somewhat similar to that observed in Pb–Sn/Cu joints, with evidence of scalloping [36]. The effect of a limited supply of SAC solder is discussed in Sec. IV.E. The thickness of the intermetallic layer was monitored over the annealing time at a temperature of 150jC. The average thickness of the (Cu,Ni)6Sn5 phase is plotted (open circles) in Fig. 21, along with data (closed circles) for the thickness of Cu6Sn5 in the Cu/SAC diffusion couple. As can be seen from the figure, the growth rates of the (Cu,Ni)6Sn5 and Cu6Sn5 phases were found to be similar, in this case where a large supply of SAC solder was available. After 20 hr of annealing the average thickness of the (Cu,Ni)6Sn5 layer was 4.5 Am, indicating a growth rate distinctly higher than the growth rate of Ni3Sn4 in Ni/Sn diffusion couples. Data (closed squares) for the growth of Ni3Sn4 as a function of time at a temperature of 150jC in Ni/Sn diffusion couples are provided in Fig. 21 for comparison. The average thickness of the Ni3Sn4 layer was 1.7 Am after annealing for 17 h [36]. This observation is consistent with the concept that kinetics rather than energetics generally dominates phase selection at solder/metal interfaces, i.e., faster growing phases grow first. Similar to the previous observation that Cu6Sn5 grows faster than Ni3Sn4, it is observed that when a supply of Cu is present, (Cu,Ni)6Sn5 grows instead of Ni3Sn4, at a rate similar to Cu6Sn5. That the rate of growth of (Cu,Ni)6Sn5 is similar to Cu6Sn5 is also an indication that the rate of Cu diffusion through the bulk solder to the interface does not appreciably slow the rate of formation of (Cu,Ni)6Sn5.

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E. Intermediate-Phase Studies in 95.9Sn–3.4Ag–0.7Cu/Metal Pad Systems The formation of intermetallic compounds in small solder joints was also examined [36]. The low-magnification electron microscope image of a cross section in Fig. 23 provides a typical geometry of such joints. A rectangular Si die is located at the top of the image. The Sn–Ag–Cu solder ball [Sn–3.5Ag–1.0Cu (Sn–3.8Ag–1.85Cu in atomic percent) in this case] connects the die to the chip carrier consisting of a Cu trace plated with Ni and a thin layer (about 100 nm) of immersion Au. In one study [29] the formation of intermediate alloys in Sn–3.4Ag–0.7Cu/metal systems was investigated for Ni and Cu metallization (with the same die and Ni–V die metallization). In producing these samples, a Cu–Ag–Sn solder paste was printed onto Ni–V die terminal pads and reflowed. The resulting solder bumps were fluxed and reflowed a second time in order to attach them to an OSP-coated Cu metallization or Ni/Au/Cu metallization on a chip carrier. The chips were reflowed under standard industrial conditions. A second reflow was conducted to attach the solder to either a Ni or a Cu metallization. This reflow consisted of peak temperatures between 234j and 250jC, and times above the liquidus temperature ranging between 50 and 80 sec. 1. Reactions with Cu Pads In the case of Cu pads, Cu6Sn5 was observed to form at the Sn–3.4Ag–0.7Cu/Cu interface. Fig. 24 provides a view of the interface formed during reflow conditions of 238jC peak temperature and a time above the liquidus of approximately 70 sec, which reveals growth of a 2-Am-thick layer of Cu6Sn5. The layer morphology follows the interface morphology quite well, although some broad scalloping is observed. The Cu6Sn5 phase is the same as observed at Sn/Cu and Pb–Sn/Cu interfaces. The thickness of the Cu6Sn5 layer was similar in magnitude to previous measurements in Sn/Cu systems, with an indication that the growth is more prolific in these SAC/Cu diffusion couples. The Cu–Sn intermetallic alloy thicknesses measured by other investigators in systems such as Pb–Sn/Cu (reflowed at a higher temperature of 255jC for similar times) are approximately 1 Am in thickness [70,84,86] as opposed to the 2 Am observed in the Zribi et al. study [29].

FIG. 23 Scanning electron microscope backscattered electron image of a cross-sectioned solder joint made by reflowing Sn–3.5Ag–1.0Cu (Sn–3.8Ag–1.85Cu in atomic percent) solder on a Au/Ni-coated Cu substrate. The black phase at the bottom is the Ni-coated Cu substrate after complete dissolution of Au within the solder, the dark gray phase at the interface is (Cu,Ni)6Sn5, and the lighter gray phase on top of it is solder with white needlelike precipitates of Au0.5Ni0.5Sn4 and a few dark gray Cu6Sn5 precipitates scattered within the joint. Also visible at the top of the picture at the interface, a layer of (Ni,Cu)3Sn4 is observed to grow.(From Ref. 29.)

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FIG. 24 Scanning electron microscope backscattered electron image of the interface of a crosssectioned joint made by reflowing Sn–3.5Ag–1.0Cu (Sn–3.8Ag–1.85Cu in atomic percent) solder on a Cu substrate pad. The black phase at the bottom is the Cu substrate, the dark gray phase at the interface is Cu6Sn5 (D), and the lighter gray phase on top of it is solder (E) with a few dark gray Cu6Sn5 precipitates scattered within the joint. (From Ref. 29.)

The thicker layer of Cu6Sn5 may possibly be attributed to the presence of Cu in the Sn–3.4Ag– 0.7Cu solder, which according to Schaefer et al. increases the intermetallic compound growth rate while correspondingly slowing down the dissolution of Cu at the Cu/Sn interface. 2. Reactions with Ni/Au-Plated Pads Distinctly different results are observed to occur in the case of Sn–3.4Ag–0.7Cu solder bumped die on Ni chip-carrier pads [36]. The phase (Cu,Ni)6Sn5 forms at the Sn–3.4Ag–0.7Cu/Ni interface after reflow (Fig. 25 ). The thickness of this ternary intermediate alloy was observed to be approximately 2 Am after reflow. Chemical compositional analysis of the solder matrix after reflow indicated the Cu concentration to be very low. The formation of (Cu,Ni)6Sn5 is much different from observations in Pb–Sn solders, where Ni3Sn4 is formed during reflow. However, after 16 hr of annealing at 150jC, a (Ni,Cu)3Sn4 layer with an average thickness of 0.6 Am formed at the (Cu,Ni)6Sn5/Ni interface (Fig. 26). After 64 hr, this new phase attained a thickness of approximately 0.9 Am. The combined thickness of the two layers, (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5, was measured at various stages during annealing by optical and electron microscopy (compositional line scans across the interface) (Fig. 27). At later times in this small sample, the overall thickness did not increase significantly, but the (Cu,Ni)6Sn5 phase transformed to the (Ni,Cu)3Sn4 phase. In fact, the thickness of (Ni,Cu)3Sn4 at the Ni surface increased with continued annealing at 150jC. Fig. 28 shows the (Ni,Cu)3Sn4 phase at the interface with a Ni– V die pad metallization. After 261 hr of annealing, it is clear that the Cu is concentrated at both terminal-pad metallization interfaces (Figs. 28 and 29). The depletion of Cu from the solder was observed to result in an increased solder melting point and change in mechanical properties [36]. a. Ni vs. Cu Reaction Differences. A clear difference is observable in comparing the growth of intermetallic compounds at the interfaces of Ni and Cu pad metallurgies. Figs. 30 and 31 are scanning electron microscopy images of joints on Cu/OSP and Cu/Ni/Au chip carrier, respectively, achieved during a 238jC peak temperature and 54-sec dwell time condition. The intermetallic compounds have a dark appearance, and are located at the chip-carrier and die-pad

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FIG. 25 Scanning electron microscope backscattered electron image of the interface of a crosssectioned solder joint made by reflowing Sn–3.5Ag–1.0Cu (Sn–3.8Ag–1.85Cu in atomic percent) solder on a Au/Ni substrate. The black phase at the bottom is the Ni-coated Cu substrate after all the Au dissolved into the solder (H) during reflow, the light gray phase at the interface is (Cu,Ni)6Sn5 (F), and the needlelike light gray phase dispersed inside the solder matrix is Ag3Sn (G). (From Ref. 29.)

FIG. 26 Scanning electron microscope backscattered electron image of the interface of a crosssectioned solder joint made by reflowing Sn–3.5Ag–1.0Cu (Sn–3.8Ag–1.85Cu in atomic percent) solder on an Au/Ni substrate. The sample was annealed for 16 hr at 150jC. The black phase at the bottom is the Ni substrate, the light gray phase at the interface is (Ni,Cu)3Sn4, and the darker gray phase is (Cu,Ni)6Sn5 phase; the white matrix on the top is Sn–3.5Ag–1.0Cu (Sn–3.8Ag–1.85Cu in atomic percent) solder. (From Ref. 29.)

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FIG. 27 Average thicknesses of (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 at the Sn–3.5Ag–1.0Cu (Sn–3.8Ag– 1.85Cu in atomic percent) solder/Ni interface as a function of annealing time at a temperature of 150jC. o The average thickness of (Ni,Cu)3Sn4. n The average thickness of (Cu,Ni)6Sn5. 5 The average combined thickness of (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5 at the Sn–3.5Ag–1.0Cu/Ni interface. For comparison, the measured average thickness of Cu6Sn5 at a Sn–Ag–Cu/Cu interface as a function of annealing time at a temperature of 150jC is also provided, i.e., x the average thickness of Cu6Sn5. (From Ref. 29.)

FIG. 28 Scanning electron microscope backscattered electron image of the interface of a crosssectioned solder joint made by reflowing Sn–3.5Ag–1.0Cu (Sn–3.8Ag–1.85Cu in atomic percent) solder on a Au/Ni substrate. The sample was annealed for 261 hr at 150jC. The phases marked with letters were identified as follows: A (Cu), B (Ni,P), C (Ni,Cu)3Sn4, D (Cu,Ni)6Sn5, E Au0.5Ni0.5Sn4, F (Ni,Cu)3Sn4, and G Sn–Cu–Ag solder depleted in Cu and Ag. (Ref. 29.)

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FIG. 29 Wavelength dispersive spectroscopy (WDS) maps showing the distribution of Cu in a Sn– 3.5Ag–1.85Cu/Au/Ni joint annealed for 261 hr at 150jC. The upper image is a Cu map (white areas) while the bottom picture shows a phase contrast image of the analyzed area. (From Ref. 29.)

interfaces. The solder/Cu interface exhibits a scalloped shape characteristic of Cu6Sn5 growth during reflow. It is evident from these figures that intermetallic compound growth at solder/metal interfaces is more prodigious in the case of Cu metallizations compared to Ni. Fig. 32 provides a side-by-side comparison of two such joints at the same length scale. This observation is consistent with a previous discussion, in which it was surmised that the (Cu,Ni)6Sn5 alloys grow faster than Ni3Sn4, and thus is kinetically selected to grow first at the solder/Ni metallization interfaces. As noted earlier, this growth depletes the solder of Cu, eventually stopping the growth of the (Cu,Ni)6Sn5 intermediate phase. Apparently, in the case of Cu metallizations the growth of

FIG. 30 Scanning electron microscope image of Sn–Ag–Cu solder reflowed for 74 sec on a Cu/OSP substrate. The Sn–Ag–Cu solder was reflowed on a die with a NiV under bump metallization. The bumped die was then reflow attached to a Cu/OSP substrate pad with a time of 74 sec above the liquidus temperature and a peak temperature of 238jC.

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FIG. 31 Scanning electron microscope image of Sn–Ag–Cu solder reflowed for 54 sec on a Ni/Au substrate. The Sn–Ag–Cu solder was reflowed on a die with a NiV under bump metallization. The bumped die was then reflow attached to a Ni/Au substrate pad with a time of 54 sec above the liquidus temperature and a peak temperature of 238jC.

Cu6Sn5 does not deplete the solder of Cu near the interface. In fact, it allows the growth of (Cu,Ni)6Sn5 to continue for longer times at the Ni–V/solder interface as well. As shown in Fig. 33, a 6jC higher peak reflow temperature and 20-sec longer dwell time results in an even greater thickness.

V. SUMMARY Many significant differences exist in the evolution of intermetallic compounds in Pb-free solder joints, such as Sn–Ag–Cu, as compared to Pb–Sn solder joints. Copper and silver, the constituents added to Sn to lower its melting temperature and increase its wettability, form

FIG. 32 Comparison of growth of intermetallic alloys for Sn–Ag–Cu solder (with same die) on different substrates. The Sn–Ag–Cu solder was reflowed on chips with a NiV under bump metallization. Then the bumped chips were reflowed on a substrate with either Ni/Au or Cu/OSP pads or on a substrate (as indicated in the figure) with a time of 54 sec above the liquidus temperature and a peak temperature of 238jC. There is more intermetallic compound growth in the case of the joint with (a) the Cu pad metallization compared to (b) the one with the Ni pad metallization.

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FIG. 33 Comparison of the growth of intermetallic alloys for different reflow times with Sn–Ag–Cu solder. The Sn–Ag–Cu solder was reflowed on chips with NiV under bump metallization. Then the bumped chips were reflowed to substrates with Cu/OSP pads above the liquidus of (a) 545j, and (b) 745j, and a peak temperature of 238jC. An increase in time above the liquidus resulted in an increased intermetallic compound formation.

intermetallic compounds on cooling from the melt (Cu6Sn5 and Ag3Sn). In fact, the morphology and distribution of these intermetallic compounds immediately after reflow affect the mechanical properties of solder joints. Furthermore, the Cu in the Sn–Ag–Cu solder matrix has been shown to react with Ni metallizations. In contrast, Pb does not form intermetallic compounds with Sn, and generally does not react with metallization constituents. As compared to Pb–Sn solder joints, significant differences exist in the growth of intermetallic compounds at Pb-free solder/metallization interfaces during reflow and subsequent solidstate heat treatments. These differences are not huge, generally less than one order of magnitude, but they are distinct. These differences are perhaps best epitomized by the difference in the rates of growth of Ni3Sn4 and Cu6Sn5 at a given temperature, with Cu6Sn5 reported to grow faster than Ni3Sn4. Because phase formation in Sn-based solder joints is dominated by kinetics, rather than energetics, until Cu is depleted from Sn–Ag–Cu solder, Cu allows the growth of (CuNi)6Sn5 at Ni/ Sn–Ag–Cu interfaces. The particularly rapid diffusion of solute metals in Sn and Sn-like structures strongly favors the growth of the most Sn-rich intermetallic compounds. This explains, among other things, the growth of the Au0.5Ni0.5Sn4 phase at the interface of interconnections of eutectic Sn–Pb solder interacting with Ni/Au-coated pads. This may reduce the bonding to the pads, allowing for socalled ‘‘brittle’’ or planar failures, after high-temperature aging. The growth of (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 layers at terminal-pad surfaces depletes copper from the Sn–Ag–Cu bulk solder between two pads each with a Ni metallization. The resulting solder joint, which is essentially Sn– Ag, exhibits different mechanical properties, and perhaps thermomechanical fatigue resistance than standard Sn–Ag–Cu solder joints. It is clear that careful consideration of all constituents in a solder joint, including metallizations, is required in order to understand the evolution of the microstructure of solder joints, and associated mechanical and reliability issues.

APPENDIX The boundary conditions imposed are a zero flux condition at the outermost surface of the solder and a Dirichlet zero condition at the interface solder/metallization (Au sink). The diffusion coefficient for Au in Sn–Pb was determined by other investigators for several annealing temperatures. Comparison of diffusion coefficients of Au in Sn–Pb to those of Au in single crystal Sn (a

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direction) shows little difference between these quantities in the temperature range 80–160jC. The dependence of DAu in Sn in the c direction on temperature is expressed as, DAu

in Sn

¼ 0:16 expð17:7=1:98
103 TÞ

ð2Þ

where T is the annealing temperature in degrees Kelvin and DAu in Sn is the diffusion coefficient of Au in Sn in units of centimeters squared per second at temperature T [56]. The calculations yield the number of moles of Au, which diffuse across the interface (integrated flux). Using the stoichiometry of the ternary compound, Au0.5Ni0.5Sn4, which forms at the interface, the corresponding number of moles of Au0.5Ni0.5Sn4 was calculated, and finally the thickness of the ternary compound was computed. The calculations assume that the thickness of the ternary compound covers the entire surface area of the solder/metallization interface as it grows. Accordingly, the expression for the thickness of Au0.5Ni0.5Sn4 as a function of the integrated flux is:
 ð3Þ XðAu0:5 Ni0:5 ÞSn4 ðtÞ ¼ 0:5FðtÞMðAu0:5 Ni0:5 ÞSn4 =qðAu;0:5 Ni0:5 ÞSn4 where XAu0.5Ni0.5Sn4 is the thickness of the ternary Au0.5Ni0.5Sn4 after t hours of annealing at temperature T, F(t) is the integrated diffusion flux of Au atoms across the interface after t hours of annealing, MAu0.5Ni0.5Sn4 is the molar mass of Au0.5Ni0.5Sn4 and qAu0.5Ni0.5Sn4 is the density of the ternary phase. Such behavior is less pronounced in the Ni/Pb–Sn system, where the Ni3Sn4/Ni interface is found to be planar. Both whiskers and spalling are observed at the Ni3Sn4/solder interface, but any grooving apparently does not extend all the way to the Ni3Sn4/Ni interface. Thus, the growth of Ni3Sn4 is significantly slower than Cu6Sn5 in the case of Cu/Pb–Sn interfaces. Reflow studies of Pb–Sn-based systems generally show the formation of Cu6Sn5 at Cu/Pb–Sn interfaces, while the Au on Ni/Au-coated pads dissolve into the solder matrix and Ni3Sn4 is formed at the pad surface.

ACKNOWLEDGMENTS Innumerable comments and contributions from Lawrence Lehman and Anthony Clark are gratefully acknowledged. Some of the research described in this chapter was supported in part by grants from the National Science Foundation (DMR9902783 and DM0218129) and by the Integrated Electronics Engineering Center (IEEC) at the State University of New York at Binghamton. The IEEC is a New York State Center for Advanced Technology and receives funding from the New York State Office of Science, Technology and Academic Research and a consortium of industrial members. This research also utilized the Cornell Center for Materials Research Shared Experimental Facilities, supported through the National Science Foundation Materials Research Science and Engineering Centers Program (DMR-9632275). We gratefully acknowledge TMS (The Minerals, Metals, and Materials Society) for permission to use figures and text from the article ‘‘The growth of intermetallic compounds at Sn-Ag-Cu solder/Cu and Sn-Ag-Cu solder/Ni interfaces and the associated evolution of the solder microstructure’’ from the Journal of Electronic Materials 2001, 30, 1157.

REFERENCES 1. 2. 3. 4. 5.

Frear, D.R.; Burchett, S.N.; Morgan, H.S.; Lau, J.H. The Mechanics of Solder Alloy Interconnects; Van Nostrand Reinhold: New York, 1994. Yost, F.G.; Hosking, F.M.; Frear, D.R. The Mechanics of Solder Alloy Wetting and Spreading; Van Nostrand Reinhold: New York, 1993. Lau, J.H. Flip Chip Technologies; McGraw Hill: New York, 1996. Goyal, D.; Kinzie, P.; Panichas, C.; Chong, K.M.; Villaloboos, O. Failure mechanisms of brittle solder joint fracture in the presence of electroless nickel immersion gold (ENIG) interface. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002. Kinyanjui, Robert K.; Zribi, Anis; Cotts, Eric J. Effects of reflow conditions on the formation of Au–

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21. 22.

23. 24. 25. 26. 27. 28. 29.

COTTS ET AL. Ni–Sn compounds at the interface of Au–Pb–Sn and Au–Sn solder joints with Ni substrate. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002; 161 pp. Alam, M.O.; Chan, Y.C.; Hung, K.C. Reaction kinetics of Pb–Sn and Sn–Ag solder balls with electroless Ni–P/Cu pad during reflow soldering in microelectronics packaging. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002. Ghosh, G. Interfacial microstructure and the kinetics of interfacial reaction in diffusion couples between Sn–Pb solder and Cu/Ni/Pd metallization. Acta. Mater. 2000, 48, 3719. Ghosh, G. Coarsening kinetics of Ni3Sn4 scallops during interfacial reaction between liquid eutectic solders and Cu/Ni/Pd metallization. J. Appl. Phys. 2000, 88, 6887. Choi, S.; Bieler, T.R.; Lucas, J.P.; Subramanian, K.N. Characterization of the growth of intermetallic interfacial layers of Sn–Ag and Sn–Pb eutectic solders and their composite solders on Cu substrate during isothermal long-term aging. J. Electron. Mater. 1999, 28, 1209. Ho, C.E.; Chen, W.T.; Kao, C.R.; Ho, C.E.; Chen, W.T.; Kao, C.R. Interactions between solder and metallization during long-term aging of advanced microelectronic packages. J. Electron. Mater. 2001, 30 (4), 379. Su, P.; Korhonen, T.; Li, C. Intermetallic phase formation and growth kinetics at the interface between molten solder and Ni-containing under bump metallization. Proceedings Electronic Components and Technology Conference, Las Vegas, NV, May 2002; 1712–1718. Jang, J.W.; Kim, P.G.; Tu, K.N.; Frear, D.R.; Thompson, P. Solder reaction-assisted crystallization of electroless Ni–P under bump metallization in low cost flip chip technology. J. Appl. Phys. 1999, 85, 8456. Ghosh, G. Kinetics of interfacial reaction between eutectic Sn–Pb solder and Cu/Ni/Pd metallizations. J. Electron. Mater. 1999, 28, 1238. Massalski, T.B. Binary Alloy Phase Diagrams; 2nd Ed.; ASM International: Materials Park, OH, 1990. Kim, H.K.; Tu, K.N. Kinetic analysis of the soldering reaction between eutectic SnPb alloy and Cu accompanied by ripening. Phys. Rev. B 1996, 53, 16027. Kang, S.K.; Choi, W.K.; Shih, D.Y.; Lauro, P.; Henderson, D.W.; Gosselin, T.; Leonard, D.N. Interfacial reactions, microstructure, and mechanical properties of Pb-free solder joints in PBGA laminates. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002. IPC Roadmap: A Guide for Assembly of Lead-Free Electronics, Draft IV. IPC: Northbrook, IL, 2000. Tu, K.N.; Zeng, K. Reliability issues of Pb-free solder joints in electronic packaging technology. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002. Kujala, A.; Reinikainen, T.; Ren, W. Transition to Pb-free manufacturing using land grid array packaging technology. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002. Balkan, H.; Patterson, D.; Burgess, G.; Carlson, C.; Elenius, P.; Johnson, M.; Rooney, B.; Sanchez, J.; Stepniak, D.; Wood, J. Flip-chip reliability: Comparative characterization of lead free (Sn/Ag/Cu) and 63Sn/Pb eutectic solder. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002. Zheng, Y.; Hilman, C.; McCluskey, P. Intermetallic growth on PWB’s soldered with Sn3.8Ag0.7Cu. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002. Kang, S.K.; Shih, D.Y.; Fogel, K.; Lauro, P.; Yim, M.J.; Advocate, G.; Griffin, M.; Goldsmith, C.; Henderson, D.W.; Gosselin, T.; King, D.; Konrad, J.; Sarkhel, A.; Puttlitz, K.J. Interfacial reaction studies on lead(Pb)-free solder alloys. Proceedings Electronic Components and Technology Conference, Orlando, FL, May 2001; 448–451. Jang, S.; Wolf, J.; Ehrmann, O.; Gloor, H.; Reichl, H.; Paik, K. Investigation of UBM systems for electroplated Sn/37Pb and Sn/3.5Ag Solder. Proceedings Electronic Components and Technology Conference, Orlando, FL, May 2001; 950–956. Zribi, A.; Zavalij, L.; Borgesen, P.; Primavera, A.; Westby, G.; Cotts, E.J. The kinetics of formation of ternary intermetallic alloys in Pb–Sn and Cu–Ag–Sn Pb-free electronic joints. Proceedings Electronic Components and Technology Conference, Orlando, FL, May 2001. Taguchi, T.; Kato, R.; Akita, S.; Okuno, A.; Suzuki, H.; Okuno, T. Lead free interfacial structures and their relationship to Au plating. Proceedings Electronic Components and Technology Conference, Orlando, FL, May 2001; 687–692. Lin, J.; DeSilva, A.; Frear, D.; Guo, Y.; Jang, J.; Li, L.; Mitchell, D.; Yeung, B.; Zhang, C. Characterization of lead-free solders and under bump metallurgies for flip-chip package. Proceedings Electronic Components and Technology Conference, Orlando, FL, May 2001; 455–462. Zeng, K.; Vuorinen, V.; Kivilahti, J.K. Intermetallic reactions between lead-free Sn–Ag–Cu solder and Ni(P)/Au surface finish on PWBs. Proceedings Electronic Components and Technology Conference, Orlando, FL, May 2001; 693–698. Zribi, A.; Cotts, E.J. Diffusion controlled growth of ternary intermetallic compounds in Pb-based and Pb-free electronic solder joints. Proceedings SMTA International Conference, 2001; 810 pp. Zribi, A.; Clark, A.; Zavalij, L.; Borgesen, P.; Cotts, E.J. The growth of intermetallic compounds at

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Metallurgical and reliability aspects of leadfree mixed technology electronic assembly for mobile communication products. IPC Works, 2000. Zribi, A.; Chromik, R.R.; Presthus, R.; Teed, K.; Zavalij, L.; DeVita, J.; Tova, J.; Cotts, E.J.; Clum, J.; Erich, R.; Primavera, A.; Westby, G.; Coyle, R.J.; Wenger, G.M. Solder metallization interdiffusion in microelectronic interconnects. IEEE CPMT 2000, 23, 383. Chen, S.; Yen, Y. Interfacial reactions in Ag–Sn/Cu Couples. J. Electron. Mater. 1999, 28, 1203. Zribi, Anis; Borgesen, Peter; Zavalij, Lubov; Cotts, Eric J. Growth of Cu–Ni–Sn alloys in Pb free CuSnAg solder/Au–Ni. Metallization Reactions Symposium Y, Proceedings of the Fall Meeting of the Materials Research Society, 1999; Y8.10652. Chromik, R.R.; Cotts, E.J. Thermodynamic and kinetic study of phase transformations in solder/ metal systems. Proc. Mater. Res. Soc. 1997, 69, 445. Chromik, R.R.; Cotts, E.J. A Study of the kinetics and energetics of solid state reactions in Pd/Sn diffusion couples, thermodynamics and kinetics of phase transformations. Proceedings Fall Meeting of the Materials Research Society, Pittsburgh, PA, 1995; 307 pp. Neils, W.K.; Chromik, R.R.; Dreyer, K.F.; Grosman, D.; Cotts, E.J. Calorimetric study of the energetics and kinetics of interdiffusion in Cu/Cu6Sn5 thin film diffusion couples, thermodynamics and kinetics of phase transformations. Fall Meeting of the Materials Research Society, Pittsburgh, PA, 1995; 313 pp. Abtew, M.; Selvaduray, G. Lead-free solders in microelectronics. Mater. Sci. Eng. 2000, 27, 95. Kim, P.G.; Tu, K.N. Fast soldering reactions on Au foils. Mater. Res. Soc. Symp. Proc. 1997, 445, 131–136. Ferguson, M.E.; Fieselman, C.D.; Elkins, M.A. Manufacturing concerns when soldering with Au plated component leads or circuit board pads. IEEE Trans. Compon. Packag. Manuf. Technol. Part C July 1997, 20 (3), 188–193. Mei, Z.; Kaufmann, M.; Eslambolchi, A.; Johnson, P. Brittle interfacial fracture of PBGA packages soldered on electroless nickel/immersion gold. Proceedings 48th Electronic Components and Technology Conference, 1998; 952–961. Darveaux, R.; Banerji, K.; Mawer, A.; Dody, G. Reliability of plastic ball grid array assembly. In Ball Grid Array Technology; Lau, J.H., Ed.; McGraw Hill: New York, 1995; 400–410. Bradley, E.; Banerji, K. Effect of PCB finish on the reliability and wettability of ball grid array packages. Proceedings 45th Electronic Components and Technology Conference, 1995; 1028–1038. Bader, W.G. Dissolution of Au, Ag, Pd, Pt, Cu and Ni in a Molten Sn–Lead Solder. Weld. J. Res. Suppl. 1969, 48 (12), 551s–557s. Anhock, S.; Opperman, H.; Kallmayer, C.; Aschenbrenner, R.; Thomas, L.; Reichl, H. Investigations of Au/Sn alloys on different end-metallizations for high temperature applications. Proceedings IEEE/ CPMT Berlin International Electronics Manufacturing Technology Symposium, 1998; 156–165. Villars, P.; Alan, P.; Okamoto, H. Handbook of Ternary Alloy Phase Diagrams; ASM International: Materials Park, OH, 1995; Vol. 4, 5214–5228. Kulojarvi, K.; Tu, K.N. Effect of dissolution and intermetallic formation on the reliability of FC joints. Microelectron. Int. 1998, 15 (2), 20–24. Tu, K.N. Interdiffusion and reaction in bimetallic Cu–Sn thin films. Acta Metall. Mater. 1973, 21, 3471. Marinkovic, Z.; Simic, V. Comparative analysis of interdiffusion in some thin metal film couples at room temperature. Thin Solid Films 1992, 217, 26. Tu, K.N. Single intermetallic compound formation in Pd–Pb and Pd–Sn thin film couples studied by X-ray diffraction. Mater. Lett. 1982, 1, 6. Tu, K.N.; Rosenberg, R. Room temperature interaction in bimetallic thin film couples. Jpn. J. Appl. Phys. 1974, Suppl. 2 (Pt. 1), 633. Dyson, B.F.; Anthony, T.R.; Turnbull, D. Interstitial diffusion of copper and silver in lead. J. Appl. Phys. 1966, 37, 2370–2374. Dyson, B.F. Diffusion of gold and silver in tin single crystals. J. Appl. Phys. 1966, 37, 2375–2377. Warburton, W.K.; Turnbull, D. Diffusion in Solids: Recent Developments; Nowick, A.S., Burton, J.J., Eds.; Academic Press: New York, 1975; 172–229.

494 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86.

COTTS ET AL. Chromik, R.R.; Cotts, E.J. A study of the kinetics and energetics of solid state reactions in Pd/Sn diffusion couples. In Thermodynamics and Kinetics of Phase Transformations; Im, J.S., Park, B., Greer, A.L., Stephenson, G.B., Eds.; Proc. Mater. Res. Soc. 1996, 398, 307–312 (Pittsburg, PA). Chada, S.; Laub, W.; Fournelle, R.A.; Shangguan, D.J. Electron. Mater. 1998, 23, 1194. Schaefer, M.; Fournelle, R.A.; Liang, J.J. Theory for intermetallic phase growth between Cu and liquid Sn–Pb solder based on grain boundary diffusion control. J. Electron. Mater. 1998, 27, 1167. Gur, D.; Bamberger, M. Reactive isothermal solidification in the Ni–Sn system. Acta Metall. Mater. 1998, 46 (14), 4917–4923. Kulojarvi, K.; Vuorinen, V.; Kivilahti, J. Effect of dissolution and intermetallic formation on the reliability of FC joints. Microelectron. Int. 1998, 15 (2), 20–24. Zavalij, L.; Zribi, A.; Chromik, R.; Pitely, S.; Zavalij, P.Y.; Cotts, E.J. Metallography and crystal structure of Au1xNixSn4 alloys. J. Alloys Compd. 2002, 334. Dreyer, K.F.; Niels, W.K.; Chromik, R.R.; Grosman, D.; Cotts, E.J. Calorimetric study of the energetics and kinetics of interdiffusion in Cu/Cu6Sn5 thin film diffusion couples. Appl. Phys. Lett. 1995, 67, 2795. Bryant, A.W.; Bugden, W.G.; Prat, J.N. Acta Met. 1970, 18, 101. Guadagno, J.R.; Pool, M.J. J. Phys. Chem. 1968, 72, 253. Zuruzi, A.S.; Chiu, C.-H.; Lahiri, S.K.; Chua, K.M. Kinetics of copper and high Pb/High Sn bilayered Pb–Sn solder interactions. J. Electron. Mater. 1999, 28, 1224. Schaefer, Matt; Laub, Werner; Fournelle, R.A.; Liang, J. Evaluation of intermetallic phase formation and concurrent dissolution of intermetallic during reflow soldering. Design and Reliability of Solders and Solder interconnections, The Minerals, Metals and Materials Society, 1997; 247–257. Liu, A.A.; Kim, H.K.; Tu, K.N.; Totta, P.A. Spalling of Cu6Sn5 spheroids in the soldering reaction of eutectic SnPb on Cr/Cu/Au thin films. J. Appl. Phys. 1996, 80, 2774. Wu, Y.; Sees, J.A.; Pouraghabagher, C.; Foster, L.A.; Marshall, J.L.; Jacobs, E.G.; Pinizzotto, R.F. J. Electron. Mater. 1993, 22, 769. Vianco, P.T.; Hlava, P.F.; Kilgo, A.C. Intermetallic compound layer formation between copper and hot-dipped 100In, 50In–50Sn, 100Sn and 63Sn–37Pb coatings. J. Electron. Mater. 1994, 23, 583. Jang, S.; Wolf, J.; Kwon, W.; Paik, K. UBM (under bump metallization) study for Pb-free electroplating bumping: interface reaction and electromigration. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002. Pan, T.; Blair, H.D.; Nicholson, J.M.; Oh, S. Intermetallic compound formation of Sn/Pb, Sn/Ag, and Sn solders on Ni substrate from the molten stage and its growth during aging. EEP Adv. Electron. Packag. 1997, 19 (2), 1347. Kang, S.K.; Rai, R.S.; Purushothaman, S. Interfacial reactions during soldering with lead–tin eutectic and lead (Pb)-free, tin-rich solders. J. Electron. Mater. 1996, 25, 1113. Ho, C.E.; Chen, Y.M.; Kao, C.R. Reaction kinetics of solder-balls with pads in BGA packages during reflow soldering. J. Electron. Mater. 1999, 28, 1231. Zribi, A.; Chromik, R.R.; Presthus, R.; Clum, J.; Zavalij, L.; Cotts, E.J. Effect of Au-intermetallic compounds on mechanical reliability of Sn–Pb/Au–Ni–Cu joints. EEP Adv. Electron. Packag. 1999, 26 (2), 1573. Zribi, A.; Chromik, R.R.; Presthus, R.; Clum, J.; Teed, K.; Zavalij, L.; DeVita, J.; Tova, J.; Cotts, E.J. Solder metallization interdiffusion in microelectronic interconnects. Proceedings Electronic Components and Technology Conference, San Diego, CA, June 1999; 451 pp. Minor, A.M.; Morris, J.W. Jr. Growth of a Au–Ni–Sn intermetallic compound on the solder– substrate interface after aging. Metall. Mater. Trans. 2000, 31A, 798. Ho, C.E.; Zheng, R.; Luo, G.L.; Lin, A.H.; Kao, C.R. Formation and resettlement of (AuxNi1x)Sn4 in solder joints of ball-grid-array packages with the Au/Ni surface finish. J. Electron. Mater. 2000, 29, 1175. FlexPDE version 2.19b, 1996–2000 from PDE solution Inc. URL: http://www.flexpde.com. Bader, S.; Gust, W.; Hieber, H. Rapid formation of intermetallic compounds by interdiffusion in the Cu–Sn and Ni–Sn systems. Acta. Metall. Mater. 1995, 43, 329. Onishi, M.; Fujibuchi, H. Trans. JIM 1975, 16, 539. Mei, Z.; Sunwoo, A.J.; Morris, J.W. Jr. Metall. Trans. A 1992, 23, 857. Vianco, P.T.; Erickson, K.L; Hopkins, P.L. J. Electron. Mater. 1994, 23, 721. Korhonen, T.M.; Su, P.; Hong, S.J.; Korhonen, M.A.; Li, C.-Y. Reactions of lead-free solders with CuNi metallizations. J. Electron. Mater. March 2000, 29 (10), 1194. Dabritz, S.; Bergner, D.; Hoffmann, V. Phase growth between copper and different tin solders. Defect and Diffusion Forum 1997, 143–147, 579–584. Schaefer, M.; Laub, W.; Sabee, J.M.; Fournelle, R.A.; Lee, P.S. A numerical method for predicting intermetallic layer thickness developed during the formation of solder joints. J. Electron. Mater. 1996, 25, 992.

14 Electronics Assembly and the Impact of Lead-Free Materials Anthony A. Primavera Universal Instruments Corporation, Cleveland, Ohio, U.S.A.

I.

INTRODUCTION

While the electronics industry continues to strive towards eliminating lead when assembling electronic devices, it is important to understand the fundamental issues that influence the assembly process, yield and reliability that result. Current standard surface mount devices such as peripheral leaded packages and area array (Ball Grid Array) devices offer robust assembly yields and good long-term solder joint reliability [1–3]. When developing a new assembly process, component technology, or packaging method, it is critical that they are comparable to, or perform better than those that they replace. Not only should a new device/process perform well in assembly, it must demonstrate acceptable package and solder-joint reliability in order to gain acceptance and wide utilization.

A.

Critical Parameters

Typically, current surface mount components (SMC) and area-array assembly processes rely on solder paste deposition through stencil printing, component placement, and mass reflow soldering. In order to ensure high process yields during assembly the process parameters of the various assembly operations must be evaluated. Factors that affect the assembly process yield can be divided into four categories: human factors, equipment and tooling, materials, and process methods. Figure 1 is a so-called fish bone diagram that illustrates the interrelationship among the various factors and their influence on assembly yields. In Insertion Mount Component (IMC) assembly, two general approaches are practiced. The first method is the traditional method, that consists of component insertion, cutting device wires, cinching the lead tail to prevent the part from falling out of the hole, and passing the board through a wave solder operation. The second approach utilizes a mass reflow method, and provides solder by printing or utilizing pre-formed solder arrays. Through-hole components such as headers can alternatively be assembled using the mass reflow approach if the component body base materials can withstand the temperatures experienced in a reflow oven. In a pin-in-paste or alternative mass reflow process, solder paste is printed onto the circuit-board plated through-hole pads corresponding to the pins on the IMC devices. Paste is printed into the plated hole as well to provide sufficient solder to form adequate joints on both sides of the circuit board. After paste application, the IMC components are placed into the plated holes and subjected to reflow temperatures sufficient to melt the solder and form

495

496

PRIMAVERA

FIG. 1 Cause and effect diagram of factors that influence lead-free assembly.

solder joints. This differs from wave soldering in that the IMC component must be capable of withstanding high peak reflow temperatures, since the entire component body is subjected to a thermal excursion while passing through a reflow oven, rather than only the through hole leads which are exposed to the wave. Utilizing a lead (Pb)-free alloy complicates a pin-in-paste assembly since the melting temperature of theses alloys is typically in the range of approximately 220jC to 240jC. This temperature range is significantly higher than eutectic Sn-Pb or neareutectic solders, for example, (60Sn-40Pb, or 62Sn-36Pb-2Ag) whose melting points are approximately 179jC to 185jC.

B.

Solder Joint Perfection

1. Miniaturization With the continual miniaturization of electronic components and overall systems, the interconnection between printed circuit boards (PCB) and components, namely the solder joint, becomes smaller as well. A reduction in the physical size of solder joints places additional demands on solder mechanical properties to ensure adequate robustness. In addition to footprint size reductions, there is an emphasis on an overall reduction in the size of packages that house silicon chips. As surface mount technology (SMT) migrates towards smaller package dimensions, the physical and thermal characteristics of materials used in microelectronic packages becomes increasingly more critical. Significant differences in properties between package layers, or between a chip carrier and die, result in large internal stresses during thermal excursions. When a component is mounted onto a PCB, the solder joints must absorb strains induced by the thermal expansion mismatch between the chip carrier and PCB materials. For traditional SMT components, such as quad flat packages, both the leads and solder joints provide the compliance necessary to compensate for the mismatch in coefficient of thermal expansion (CTE) between the package and PCB. However, in area array devices such as ball grid array (BGA) and chip scale packages (CSP), the joint alone must provide the CTE mismatch compliance [4–6]. 2. Defects As the joint size is reduced, the joint quality becomes increasingly more critical. Physical defects caused by poor solderability, excessive intermetallic formation, and voids negatively impact joint

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robustness and soldering yields. A change to lead (Pb)-free solder can adversely affect one or more of these factors. For example, voids that are cavities and bubbles trapped in molten solder during solidification, typically degrade mechanical, thermal and electrical properties of solder joints [7]. Higher levels of voids have been reported in lead (Pb)-free solder joints compared to their eutectic Sn-Pb counterparts. These observations are likely due to factors associated with solder paste, flux chemistry, and assembly processes.

C.

Lead-Free Alloys

Factors that influence the assembly of components using a lead-free solder are similar to the parameters that influence standard tin-lead (Sn-Pb) solder. Although assembly of SMC and IMC devices differ in the specific steps, equipment, and solder joint shape, the general goal is the same: creation of robust soldered interconnections. The main difference in lead-free soldering is the higher reflow temperatures needed to fully melt the solder alloy. The need for higher process temperatures places greater demands on component and PCB materials.

D.

Fundamental Assembly Issues

In general, the electronic component assembly process and yield are influenced by several main factors including: human factors, machine issues, assembly process, and assembly materials [8– 11], which are discussed in detail in this chapter. While the general concepts hold true for IMC and SMC assembly, they have been shown to apply to area array assembly as well. This chapter describes some fundamental issues associated with the lead (Pb)-free assembly of electronic devices. Specifically, assembly process parameters such as solder paste and wave solder material selection, and circuit board metallurgy, affect the yield when attaching components to organic motherboards. Factors such as the presence of voids, reflow atmosphere, attachment pad metallurgy and assembly parameters have a dramatic affect on the solderability and wetting of lead-free solder alloys. Lead-free assembly procedures are similar to those practiced utilizing eutectic Sn-Pb solder. However, differences in alloy soldering performance, melting point, and physical properties, require that standard assembly procedures be modified. This chapter focuses on the assembly of the three main categories of electronic components: surface mount components (SMC) such as peripheral leaded devices; insertion mount components (IMC) such as through-hole components, and area array components, including ball grid array (BGA) and chip scale packages (CSP).

II.

SOLDER PASTE MATERIALS

The materials used in the surface-mount, area-array assembly process include solder paste, stencils, components, flux, and the PCB. In wave soldering applications, solder paste and paste deposition processes are eliminated, and a cured adhesive is used to retain bottom-side components on a PCB during exposure to a solder wave.

A.

Solder Paste

Solder paste is a homogenous and kinetically-stable mixture of flux that serves as a vehicle, and solder alloy powder. Solder forms the metallic bond between metal terminal pads on a component and mating metal pads on a circuit board. During solder reflow, the flux is thermally activated and wets the metal surfaces. The flux removes the metal oxides formed on the surface of the pads, component leads, and on the solder particles themselves, leaving an oxide-free surface for metallurgical bonding. Once the melting point of the solder alloy is attained, the individual particles in a solder paste coalesce into a uniform liquid mass that reacts with the PCB pad and lead surface forming intermetallic compounds at the interfaces. Solder paste technology encompasses many technologies including; flux chemistry, fluid dynamics of flow, metallurgy of alloying, and rheology of the combined flux, powder and vehicle mixture. In general, lead-free solder pastes

498

PRIMAVERA

have print, roll, slump, and aperture release characteristics similar to eutectic Sn-Pb solder paste. However, lead (Pb)-free pastes are darker in color, tend to form taller printed deposits, roll slightly slower, and have dramatically reduced wetting compared to eutectic Sn-Pb solder.

B.

Formulation

Characteristics of the solder paste that are important from a process perspective include; the solder powder particle size, metallurgy, slump, temperature and humidity sensitivity, solids content, type of flux residue, viscosity, and the propensity for solder balling. Viscosity is an important property of a paste related to metal loading. The ratio of metal particles to flux vehicle changes the flow characteristics of the mixture. A higher percentage of powder in the paste increases the viscosity. Lead-free solders, typically have a slightly lower metal loading (88-89%) compared to Sn-Pb (90-91%) to achieve the same viscosity and print characteristics due to the difference in alloy density. No-clean Sn-Pb (63/37) solder pastes with high metal content, (approximately 90 percent by weight), are widely used to attach SMT, CSP, and BGA packages to PCBs during assembly. Lead-free solder pastes are formulated with a lower density than leadbearing alloys due to the difference in mass of equivalent volumes of solder. The solder paste physical properties such as the viscosity are therefore different for eutectic Sn-Pb and lead-free pastes formulated with the same metal loading percentage. For Sn-Pb pastes at a nominal 90% metal loading by weight, the volume of solidified solder is approximately 50% of as-deposited paste.

C.

Powder Particle Size

The size of the particles in a solder paste determines the print characteristics, amongst other things. The particles are produced by a variety of methods, but they are most commonly made by dispersion of a stream of molten solder onto a rotating disk. The particles fall into a tank filled with an inert atmosphere where they solidify and then are collected at the bottom of the chamber. The collected particles are separated by size utilizing in a series of wire-mesh sieves. The mesh size is typically given in wires or holes per square inch. The Joint Industry Standard, J-STD-005, provides for solder paste particle size classification as listed in Table 1. The choice of solderpowder particle size for SMT applications is based on component pitch, part mix and pad arrangement. For example, the paste particle size required for an area array device is smaller compared to a peripheral-leaded device with the same pitch. A 0.5-mm pitch area array device may have 0.25-mm diameter pads which require a 0.25-mm to 0.3-mm stencil aperture to print a Type IV solder paste with an approximately at 60-80% transfer efficiency. Comparatively, a 0.5–mm pitch quad flat pack device would typically have an 0.2 mm to 0.25mm wide pad, but require a 0.15 to 0.2mm wide stencil aperture to print a Type III paste with approximately a 80–90% efficiency.

TABLE 1 Solder Paste Type I II III IV V VI

Solder Paste Classification with Particle Size Requirements Particle Size Range, Content Required: 80% Minimum (microns) 150–75 75–45 45–25 38–20 25–15 15–5

Maximum Size (Microns)

Minimum Particle Size Content Allowed: Less Than 1% (microns)

Minimum Particle Size Content Allowed: Less Than 10% (microns)

160 80 50 40 30 20

150 75 45 38 25 15

20 20 20 20 15 5

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For the assembly of standard BGAs (1.27-mm), the stencil aperture size is typically 0.55 to 0.65-mm in diameter. The solder-powder particle size could range from 200/+325 mesh size (Type 2) to 400/+500 mesh size (Type IV). As the component pitch decreases, the aperture size must decrease correspondingly. For example, to mount a solder ball component with a pitch in the range of 0.8 to 1.0 mm requires a stencil aperture size about 0.4 to 0.5 mm to apply a Type III solder paste. As the pitch is reduced, a reduced particle size paste, for example Type IV, should be used. When assembling 0.5-mm pitch components, the stencil aperture size may be as small as 0.25 to 0.35 mm.

D.

Printing Efficiencies

The aperture area-to-wall area parameter plays a key role in solder-paste print efficiency. The efficiency is defined as the printed volume divided by the aperture volume. A 100% efficiency means that the amount of paste printed equals the aperture volume, while a 50% efficiency means only half of the volume was deposited. In fine-pitch print experiments [12] it has been demonstrated that the aperture opening area-to-wall area ratio is a critical parameter in achieving a high paste transfer efficiency. For circular apertures, the area ratio may exceed 1.5, which leads to a low print transfer efficiency. However, for long rectangular apertures, the area ratio may be 1.0 or less, depending on the stencil thickness, resulting in a much higher transfer efficiency. The print efficiency for circular and rectangular apertures of different thickness is compared in Figure 2 for eutectic Sn-Pb and Sn-Ag-Cu pastes.

E.

Solder Paste Characteristics

A viscosity test is used to help determine the flow characteristics of a solder paste. Solder paste is usually deposited using a ’stencil printing’ approach. Solder pastes are thixotropic fluids which have the characteristic of reducing viscosity with time when subjected to constant shear conditions. In other words, solder pastes are more fluid when dispensed with a squeegee (applied shear stress), but the paste remains thick when no stress is applied. When shear-thinned, solder pastes are capable of flowing into stencil apertures. When a print stroke is complete, the paste viscosity increases allowing the deposit to maintain its printed geometric shape. If a paste is overly viscous, it results in insufficient paste volume resulting in open joints. However, low viscosity results in excessive volumes, slumping and bridging. 1. Viscosity The rheology of a solder paste affects its ability to be dispensed and deposited, and is affected by its flow and elastic properties. Rheology control is a vital pre-production check that assures that the solder paste can be adequately and reproducibly dispensed. In general, the viscosity of a solder paste depends on flux type, powder size and distribution, and the metal content. The kinematic state of a body is known if the position and velocity of all points in the body are known at some time, t. In the case of flow, the kinematic variables are strain and the strain rate. The dynamic state of a body can be determined when the forces acting on the body are known. The forces that act on a body fall into two categories - body forces and surface tractions. Body forces are those that act throughout the surface; but the body is not in physical contact with the source of the force, such as the gravitational force. Surface tractions arise by having direct contact with a boundary of a body. If a force, F, acts on an arbitrary element of surface area, Sa, which is sufficiently small enough so the force is constant over the surface, then the shear stress is defined as the ratio of the force, F, to the area, Sa, Shear Stress ¼ s ¼ F=Sa where: H - Shear Stress in Pascal (Pa) F  Force in Newton ðNÞ Sa Surface Area in m2 :

ð1Þ

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FIG. 2 The effect of area ratio on the transfer efficiency of various shaped stencil apertures.

Viscosity is the ratio shear stress to shear rate, Viscosity ¼ A ¼ H =S

ð2Þ

where: H - Shear Stress S  Shear Rate: 2. Rheology – Newtonian vs. Non-Newtonian Fluids The flow properties of a fluid vary depending on the method of application and material type. For materials that have Newtonian characteristics, viscosity is independent of shear rates. NonNewtonian solutions, (suspensions), occur when the suspension causes either dilatency or thixotropy. A dilatant suspension is quite fluid until stressed, at which time the suspension becomes resistant to motion. Thixotropy is the opposite, in that the suspension is thick and gelatinous, but when stirred or mixed it flows more readily. While solder pastes typically exhibit similar (but not the same) thixotropic behavior, they only differ in the shear force necessary to provide flow characteristics. The difference among solder paste characteristics is due to the quantity and type of thixotropic agents utilized, which also affects paste rheology.

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For Newtonian fluids such as water, the viscosity A is constant for any practical shear rate S. In other words, viscosity is independent of shear rate. Non-Newtonian Fluids do not have a constant shear-stress to shear-rate ratio, and are grouped into three classes: dilatant, pseudoplastic and plastic. Dilatant fluids increase in viscosity with increasing shear rate, referred to as shear thickening. Pseudo-plastic fluids exhibit the characteristic of decreasing viscosity with increasing shear rate, known as shear thinning. Plastic materials differ from others in that as a shearing force is decreased, there is a threshold at which for all practical purposes, the material stops flowing. This point is called the plastic yield point. Once this point is exceeded, the material may exhibit any of the previously described flow characteristics. Some fluids exhibit a change in viscosity as shearing proceeds at a constant rate. The rheological equation of state includes time as an independent variable. s ¼ fn ðs; tÞ

ð3Þ

3. Thixotropic Fluids For non-Newtonian thixotropic fluids, the changes in viscosity are a result of mechanical interactions of a fluid’s constituent components and not a chemical reaction. For a fixed shear rate, thixotropic fluids decrease in viscosity with time, ultimately achieving a constant value. In a series of tests subjecting a sample to progressively higher shear rates, the result is a family of curves each with successively lower viscosity values. Thus, the decrease in viscosity depends on the time and magnitude of the shear rate. To account for this behavior, an ‘‘internal structure’’ is assumed present that is broken down when subjected to a shearing condition reducing the internal resistance and viscosity. At any given constant shear rate, only a portion of this structure is broken down, finally attaining a corresponding constant viscosity. The shear-thinning behavior of a thixotropic substance is accounted for in the same manner as a pseudo-plastic substance; however, for a thixotropic substance, rebuilding ‘‘the internal’’ structure is not instantaneous. As a result of this, a hysterisis loop can be experimentally observed when the shear rate is ramped up to a maximum shear rate and then reversed back to zero. The area of the thixotropic loop is sometimes used as an indication of the ‘‘degree’’ of thixotropy. The loop area can be interpreted as the energy necessary to break down the thixotropic structure for a particular volume of material. One method of calculating a ‘‘thixotropic index’’, is given as: Ti ¼ log

ðA1 =A2 Þ ðS1 =S2 Þ

ð4Þ

Where A1 and A2 are the viscosity at shear rates S1 and S2, respectively. Solder pastes are thixotropic; the viscosity decreases as the shear stress increases but slowly recovers its original value when the shear is removed. The viscosity of a solder paste depends on the solids content of the flux; on the quantity, size and shape of the solder particles; and on the thickeners added to make the paste thixotropic. Viscosity is a temperature-sensitive property and decreases markedly at higher temperatures. As the metal content decreases, solder pastes become more dependent on the thickening agents for viscosity control, particularly when relatively high viscosity is required. These thickening agents include thixotropic agents, otherwise known as rheological modifiers, which are used to control paste viscosity.

F.

Solder Paste Tests

Solder pastes used for SMT assembly are subjected to a battery of tests which are listed in Table 2 to evaluate screenability, and the ability to maintain feature definition prior to reflow in a furnace [13]. Several tests are specified by the IPC, (for example IPC-SF-818 and IPC-SP-819), while others have been adopted by the electronics assembly industry. 1. Viscosity Test The viscosity of a solder paste can be an indication of the ability of the paste to be printed and transferred properly through apertures of a solder paste stencil. If the viscosity is low, the paste

502 TABLE 2

PRIMAVERA Solder Paste Tests

Test . . . . . . .

Viscosity Solder Balling Slump Wettability Tack Strength Spread Printability

Description Measure of fluidity / flow properties Ability to coalesce into a uniform mass Ability to retain a printed deposit shape Measure of solder coverage on metal surfaces Ability of paste to retain components during assembly Increase in area of print deposit or separation of flux Ability to form well defined printed shapes

tends to not roll properly, stick to the squeegee blade, and the printed deposits do not retain their geometric shapes. Low viscosity pastes often slump excessively, and bleed underneath the stencil and smear onto the surface of the circuit board. However, highly viscous pastes do not roll sufficiently, do not fill the stencil apertures completely, and tend to stick inside the aperture walls rather than release onto the PCB pads. Solder pastes are typically formulated in different viscosities depending on how the paste is to be deposited onto the PCB pads. For stencil printing application, solder paste should have a viscosity in the range of 800,000–1,100,000 centipoise (cps). However, for solder dispensing applications that utilize a dispense pump, the viscosity should be 350,000–500,000 cps, to ensure the material does not clog the pump tip. Two main types of viscosity tests are performed on solder paste. The first method utilizes a T shaped spindle that is spun inside a container of paste in a helical path. The T spindle viscometer is also known as a Brookfield viscometer. The bar is rotated at 5–20 revolutions per minute (RPM) in 5 RPM increments. The viscosity is recorded at each speed for several minutes. The bar moves up and down in the Z axis (perpendicular to the surface of the paste sample jar) as the bar rotates. The helical path ensures the paste is shear thinned excessively by the bar during the viscosity test. The second type of viscometer utilized in solder paste testing is a Malcolm spiral viscometer. The spiral viscometer consists of an inner rod, and an outside grooved shell. Paste is introduced into the viscometer at one end of the spiral shell and is forced up into the space between the inside rod and the outside shell, and exits at the top of the shell. The outside shell rotates and shears the paste against the inside rod. 2. Printability Printability is the relative measure of a paste to be deposited into well formed geometric shapes without slumping, spreading or bridging. Printability includes a variety of paste performance characteristics such as paste roll, aperture filling capability, exhibiting paste curtain (sticking to a squeegee), crisp and sharp feature definition, and transfer efficiency. Other quality-related aspects could include the amount of paste adhering to the stencil wall and number of prints a paste can withstand before the print volume or shape is degraded. In many respects, lead-free pastes behave similar to eutectic Sn-Pb. Figure 3, shows a comparison of eutectic Sn-Pb and Sn-Ag-Cu solder paste print bead color and texture. In addition, the paste roll was found for Sn-Ag-Cu to be visually identical to eutectic Sn-Pb, as shown in Figure 4. The paste curtain resulting from the squeegee lifting away from the stencil following printing of Sn-Ag-Cu and eutectic Sn-Pb pastes is compared in Figure 5, whereas the printed deposits for eutectic Sn-Pb and Sn-Ag-Cu paste exhibit a similar deposition shape and volume as shown in Figure 6. 3. Slump Test Slump is the propensity of the solder paste to spread out after being deposited on PCB pads. Excessive slump can cause bridging of adjacent solder paste deposits. In order to achieve acceptable soldering yields, solder paste slump must be minimized. Slump also depends on the percentage of metal content in a solder paste. Slump tests are performed to determine the ability

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FIG. 3 Solder paste bead coloration of eutectic Sn-Pb and Sn-Ag-Cu pastes.

of a solder paste to maintain its geometric shape both before and during reflow. The necessity for a solder paste to hold its shape increases as the component pitch decreases. When solder paste is subjected to elevated temperatures, the flux spreads rapidly since the viscosity is reduced during heating. Flux spreading must be minimized to prevent contacting solder deposits on adjacent pads. Thixotropic agents added to solder pastes improve the print characteristics, but also help minimize the spread of solder paste deposits during reflow. Typically, the slump tests consist of a stencil with a series of decreasing pitch apertures, and the smallest gap not bridged determines the slump point. Figure 7 illustrates a solder paste slump test stencil and deposit. 4. Spread Tests The spread test provides a quantitative picture of the slump performance of the solder paste by measurement of a printed deposit before and after exposure to simulated factory floor conditions. The spread test measures the change in print deposit area of a circular deposit after a predetermined time period. Typically, a deposit of paste is subjected to aging in an isothermal aging chamber and in a humidity/temperature chamber for up to 8 hours. Typically, the change in printed diameter is measured at 1 hour, 3 hours, 5 hours and 8 hours and compared to the time zero print diameter. 5. Tack Tests Tackiness is the ability of a solder paste to hold surface mount components in place between placement and reflow soldering. The tackiness of a paste is one indicator of whether the paste is still usable. Tack strength provides a measure of a pastes ability of retaining a component in place. Whereas solder paste tack life in an indication of how long a solder paste can hold a

FIG. 4 Solder paste stencil roll of eutectic Sn-Pb and Sn-Ag-Cu pastes.

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FIG. 5 Solder paste separation of eutectic Sn-Pb and Sn-Ag-Cu pastes.

component in place. The tackiness of a solder paste greatly depends upon the choice of solvent since evaporation causes the deposit to dry, and the tack strength and time may be reduced. The size and shape of particles in a solder paste can affect tack since the ratio of surface area to flux increases when the particles sizes decrease. Factors such as viscosity, particle size distribution, metal loading, etc. also affect tack strength but they are interdependent. For example, varying particle size and distribution by adding a significant amount of small ‘‘fines’’ increases the viscosity and in general lowers the tack strength. Tack strength can be measured by several methods including; placing a weight block on printed deposit and then placing the sample in an inclined surface. Other methods include use of a materials test system to measure the load versus displacement curve for a probe placed into and retracted in an axis perpendicular from the surface of a printed PCB. Figure 8 shows a tack test probe for an out of plane retention test. A shear tack test consists of placement of a small block onto a printed PCB and measuring the amount of shear required to push the block from the deposit in an axis parallel to the PCB surface. Tack shear

FIG. 6 Solder paste deposition shape of eutectic Sn_Pb and Sn_Ag_Cu pastes.

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FIG. 7 Solder paste slump test, (a) stencil and (b) depositusing type IV, no-clean, Sn-3.8Ag-0.7Cu solder paste.

FIG. 8 Solder paste tack test probe fixture.

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strength can be related to the size of the placed component since the retention force is equal to the mass of the component times the acceleration imposed on the device. 6. Solderability Solderability is defined as the ability of a surface to be wet by molten solder. In peripheral-leaded mass reflow applications, the ability of the solder to climb and adhere to leads is more important that spreading out on metal terminal pads. In general terms, wettability is a more accurate description of solderability. Solder- wettability is the ability of the molten solder cover all metal surfaces present in the solder joint, namely the component pads or leads, PCB pads, and solder balls or columns on area array devices. The definition of good solderability does not, however, take into account whether alloying of the solder and pad surfaces have been achieved. Solder may flow out onto the pads but not react and bond with the surface. In order to chemically clean metallic surfaces and influence the surface energies of the solder and base metal, the flux must be in intimate contact with both surfaces. A flux must adequately wet the surfaces to be joined by displacing the adsorbed vapor on the metallic surfaces to be soldered [2]. In general, lead-free solders exhibit a much lower degree of wettability compared to eutectic Sn-Pb solder, a trend observed for many component and PCB pad finishes. This is believed due in part to the inability of the flux systems utilized in lead-free solder pastes to properly wet the metal surfaces. Figure 9 shows a comparison of eutectic Sn-Pb wetting on a blanket substrate compared to several leadfree solder alloys. A flux formulation is necessary that takes the metal systems and higher melting point of typical lead-free solder alloys into account. Many fluxes utilized in lead-free solder pastes have been formulated for eutectic Sn-Pb reflow operations. Due to substantially higher reflow temperatures of most lead-free solders, the flux is often lost due to evaporation well below the reflow temperature of lead-free solders resulting in insufficient oxide removal. 7. Solder Ball Test During reflow, the metal particles in a solder paste should coalesce into a single molten solder mass without any extraneous satellites (i.e. small molten solder balls not part of the main mass) remaining on the PCB surface. Solder fines, extremely small solder particles (less than 5 microns), prevent the solder paste from coalescing into a single molten mass. In general, solder fines are highly or completely oxidized which alters the wetting characteristics of a solder and melting point of the solder powder. Consequently, the solder powder melts at a higher temperature or it may not melt at all depending on the level of oxidation. Extraneous solder satellites can cause electrical bridges if they remain adhered to the PCB following reflow. Solder ball exposure tests characterize the ability of a solder paste to coalesce into a single mass. A printed deposit is reflown onto a non-wettable surface such as ceramic and the number of extraneous satellites surrounding the main molten mass are counted. If a paste contains excessive amounts of fines, these fine powder particles may be carried away from the main deposit by the flux and therefore can not coalesce into the molten solder mass. A solder ball test is shown in Figure 10.

FIG. 9 Wetting plot comparing eutectic Sn-Pb with two lead-free solders with several board finishes.

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FIG. 10

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Solder ball reflow test results for several lead-free solders in comparison to eutectic Sn-Pb.

STANDARD PRINTED CIRCUIT BOARDS

Boards or chip carriers made of organic materials are commonly used in surface mount assembly operations. Board-related parameters that influence the assembly yield due to variations in their dimensional parameters include: fiducial deviation, warpage, mask registration, pad size, and pad location deviation (radial deviation). The trends of reduced size and weight for increased portability coupled with ever increasing functionality fuels the development of new, high-density packages with high-density wiring and high I/O counts that must be escaped from the component to the PCB. This trend is anticipated to continue in order to meet higher levels of integration, performance and miniaturization of electronic components, at an affordable cost. If the pin count requirements, along with package size and cost reductions are to meet consumer product expectations in the future, a drastic reduction in the I/O pitch will be required and technologies to accommodate them. Smaller packages and reduced pitches will result in shorter signal traces and improved performance. Simultaneously, improvements in the PCB are also required to meet demands of reduced-pitch components. A detailed description of the various processes associated with the fabrication of printed wiring boards capable of enhanced routing capabilities to wire high pin count components including direct chip attachment is given in the Appendix in the back of this chapter. This is often referred to as PCB build-up technology or high density interconnect HDI circuitry. This section discusses standard PCB technology and the influence various factors, (Table 3), a printed circuit board has on the overall solder joint formation process. Additionally, the components being placed onto the PCB can change the required technology used to fabricate the PCB and thus influence the solder joint formation quality. The ‘‘components’’ sub-group assembly parameters that can influence solder joint formation and subsequent reliability are shown in Table 4.

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TABLE 3 Circuit Board Features and Factors That Influence Solder Joint Formation Features and Factors . . . . . . . . .

A.

Attachment Pad Metallurgy Circuit Board Thickness Attachment Pad Shape and Size Circuit Board Base Materials Technology to Form Via Structures Circuit Routing Methodology (to Fan-out Inside of Pad Arrays) Board Layer Count Solder Mask Technology (Material, Aperture Formation Methods) Pad Geometry Definition (Feature-Defined or Solder-Mask Defined)

Effect of Process Temperatures for Lead-Free Solder

Typical circuit boards consist of multiple layers of dielectric and conductive materials. The dielectric insulating layers selected greatly influence the overall thermal and mechanical properties of a finished board. Pre-formed sheet material with fiber-reinforced epoxy is generally used for the dielectric layers, however, n