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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