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

Although great care has been taken to provide accurate and current information, neither the author(s) nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage, or liability directly or indirectly caused or alleged to be caused by this book. The material contained herein is not intended to provide specific advice or recommendations for any specific situation. Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress. ISBN: 0-8247-4870-0 This book is printed on acid-free paper. Headquarters Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 10016, U.S.A. tel: 212-696-9000; fax: 212-685-4540 Distribution and Customer Service Marcel Dekker, Inc. Cimarron Road, Monticello, New York 12701, U.S.A. tel: 800-228-1160; fax: 845-796-1772 Eastern Hemisphere Distribution Marcel Dekker AG Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41-61-260-6300; fax: 41-61-260-6333 World Wide Web http://www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special Sales/Professional Marketing at the headquarters address above. Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Current printing (last digit): 10 9 8 7 6 5 4 3 2 1 PRINTED IN THE UNITED STATES OF AMERICA

MECHANICAL ENGINEERING A Series of Textbooks and Reference Books

Founding Editor

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

2

PUTTLITZ

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.

4

PUTTLITZ

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

6

PUTTLITZ

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

8

PUTTLITZ

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.)

10

PUTTLITZ

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.

40

PUTTLITZ

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

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

41

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.

42

PUTTLITZ

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

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

43

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

44

PUTTLITZ

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

LEAD-FREE SOLDER ISSUES INCLUDING SELECTION

45

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.

46

PUTTLITZ

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.

47

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

49

50

SMITH

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

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD TABLE 2

51

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

52

SMITH

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.

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

53

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

54

SMITH

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.

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

55

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.

56

SMITH

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

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

57

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.

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

69

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.

70

SMITH

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.

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

71

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.

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

73

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.

74

SMITH

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.

76

SMITH

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,

78

SMITH

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-

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD

79

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.

80

SMITH

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.

HEALTH AND ENVIRONMENTAL EFFECTS OF LEAD 27.

81

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

83

84

TURBINI AND BERNIER

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

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

85

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.)

86

TURBINI AND BERNIER

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.)

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

87

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

88

TURBINI AND BERNIER

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

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

89

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

90

TURBINI AND BERNIER

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,

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

91

(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

92

TURBINI AND BERNIER c)

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.

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

93

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

94

TURBINI AND BERNIER

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

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

95

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

96

TURBINI AND BERNIER

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

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

97

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.

98

TURBINI AND BERNIER

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

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

99

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

100

TURBINI AND BERNIER

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.

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

101

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

102

TURBINI AND BERNIER

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.

103

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

104

TURBINI AND BERNIER

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

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

105

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

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

107

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.

108

TURBINI AND BERNIER

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

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

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

110

TURBINI AND BERNIER

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.

IMPACT OF LEAD AND ALTERNATIVES IN ELECTRONICS

111

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

112

TURBINI AND BERNIER

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.

113

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

115

116

PELZEL ET AL.

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

ENVIRONMENTAL STEWARDSHIP

117

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

118

PELZEL ET AL.

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

ENVIRONMENTAL STEWARDSHIP

119

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

120

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-

ENVIRONMENTAL STEWARDSHIP

121

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

122

PELZEL ET AL.

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

ENVIRONMENTAL STEWARDSHIP

123

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)

124

PELZEL ET AL.

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

ENVIRONMENTAL STEWARDSHIP

125

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

126

PELZEL ET AL.

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,

ENVIRONMENTAL STEWARDSHIP

127

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.

128

PELZEL ET AL.

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

ENVIRONMENTAL STEWARDSHIP

129

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

130

PELZEL ET AL.

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 R