A Guide to Lead-free Solders: Physical Metallurgy and Reliability

211 Pages·2007·2.998 MB·English

by  John W. Evans PhD (auth.), Werner Engelmaier Dipl.-Ing. (eds.)

A Guide to Lead-free Solders
John W. Evans
With contributions by Dongil Kwon and Jillian Y. Evans

A Guide to Lead-free
Solders
Physical Metallurgy and Reliability
Edited by Werner Engelmaier

123
John W. Evans, PhD Werner Engelmaier, Dipl.-Ing.
14193 Wellwood Road Engelmaier Associates
Silver Spring, MD 20905 7 Jasmine Run
USA Ormond Beach, FL 32174
USA

Dongil Kwon, PhD Jillian Y. Evans, PhD
Seoul National University Quentech
School of Materials Science and Engineering 14913 Wellwood Road
56-1 Shinlim-dong Silver Spring, MD 20905
Kwanak-gu, Seoul 151-742 USA
Republic of Korea (contributing author of Chapter 8)
(contributing author of Chapters 2 and 5)
British Library Cataloguing in Publication Data
Evans, John W., 1957
A guide to lead-free solders : physical metallurgy and reliability
1.Solder and soldering 2.Lead-free electronics
manufacturing processes
I.Title II.Engelmaier, Werner, 1939-
671.5'6
ISBN-13: 9781846283093
ISBN-10: 1846283094

Library of Congress Control Number: 2006936884

ISBN 978-1-84628-309-3 e-ISBN 978-1-84628-310-9 Printed on acid-free paper

© Springer-Verlag London Limited 2007

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To the memory of Mr. Jae Bang Shin. He believed hard
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Preface
The transition to lead-free solders seems to be a foregone conclusion with Japan
and Europe leading the way. Indeed some key companies will move to lead free
soon and the WEEE Directive 2002/96/EC on Waste Electrical and Electronic
Equipment will require all companies doing business in the EU to transition to
lead-free. Compliance with forthcoming lead-free regulations will ultimately fall
to individual companies and the engineers responsible for design and produc-
tion of electronic products and they must be prepared with adequate knowledge
of the materials that are leading candidates and they must be prepared to fill the
gaps in the data base for their own products.
Research worldwide over the past 10 years has produced data and direction
for choosing an alloy to substitute for near-eutectic SnPb alloys. This book will
provide a valuable resource for engineers involved in the transition to products.
Basic theory is presented on the physical metallurgy of soldering technology
including elements of assembly, surface finishes and solder-paste technology,
wetting and solidification, microstructural instability and intermetallic com-
pounds and mechanical, creep and fatigue behavior. Techniques for measuring
and testing are discussed and data on SnPb and various lead-free solders are
presented and compared. If lead-free solder data are not available on the rele-
vant topic, information is presented on near-eutectic SnPb, so as to show where
the gaps in knowledge need to be filled.
This book is intended to provide a resource on the physical metallurgy of sol-
dering to assist engineers responsible for design, reliability and assembly in
making a transition to lead-free solders in response to the forthcoming world-
wide regulation of lead content in electronic products.
John W. Evans
2005
Acknowledgments
The authors would like to express their sincere appreciation to all those who
made this project possible. Special acknowledgement must be given to Mr.
Werner Engelmaier for editing the chapters. His input was significant in produc-
ing this manuscript.
We would like to express our appreciation to Dr. Reza Ghaffarian, of the
NASA Jet Propulsion Laboratory of the California Institute of Technology for his
encouragement and for use of his excellent micrographs and to Mr. Jay Brusse of
the NASA Goddard Space Flight Center, QSS Group for his comments and pic-
tures on tin whiskers. We express our appreciation to Mr. John Hagge for use of
his figures appearing in Chapter 1. Special thanks go to the CALCE Center of the
University of Maryland, particularly Dr. Diganta Das for his support and to Dr.
Abhijit Dasgupta for his comments, support and collaborations extending over
many years with Dr. John Evans. Special thanks also go to Dr. Jennie S. Hwang
of H(cid:16)Technologies Group, Inc. for the use of tensile data on Pb-free solders
appearing on her website. In addition, a special acknowledgement must go to
Mr. Jong-jin Kim and other members of the School of Materials Science at the
remarkable campus of Seoul National University.
The authors also want to acknowledge all the excellent researchers in the area
of soldering technology who have published papers and books that serve as the
basis of moving forward in the complex subject of fatigue in solders. We want to
acknowledge the IPC, ASME, IEEE and ASM technical societies for permission
to republish figures from journals and papers, as well as John Wiley, McGraw–
Hill and Marcel–Dekker.
Finally, this book would not have ever been completed without the support,
patience and persistence of Mr. Oliver Jackson of Springer London.
John W. Evans
Jillian Y. Evans
D. Kwon
Contents
1 Introduction to Solder Alloys and Their Properties.....................................1
1.1 Evolution of Packaging Architectures.................................................2
1.2 Solder Alloys..........................................................................................6
1.2.1 SnPb Alloys.................................................................................7
1.2.2 Pb-free Alloys.............................................................................8
1.2.3 Specialized Processing and Microalloying..............................8
1.3 Wetting and Joint Formation...............................................................9
1.4 Monotonic Properties of Solder Alloys.............................................10
1.5 Creep and Stress Relaxation...............................................................15
1.5.1 Cyclic Behavior and Creep Fatigue Interaction....................17
1.5.2 Microstructure and Cyclic Behavior......................................23
References........................................................................................................25
2 Packaging Architecture and Assembly Technology....................................29
2.1 Packaging Architectures.....................................................................29
2.1.1 Leaded Semiconductor Packages...........................................29
2.1.2 Package and Lead Types..........................................................30
2.1.3 Lead Finishes............................................................................32
2.2 Pure Sn Coatings – Tin Pest and Tin Whiskers................................34
2.2.1 Tin Pest – The Allotropic Transformation of Sn..................34
2.2.2 Extrusions – Whisker Growth................................................35
2.3 Ball-grid Arrays and Chip-scale Packages.........................................37
2.3.1 Flip-chip Packaging.................................................................38
2.3.2 Pb-free BGA, CSP and Flip Chips...........................................39
2.4 Assembly Technology.........................................................................40
2.4.1 Solder Paste..............................................................................40
2.4.2 Stenciling..................................................................................44
2.4.3 Solder-paste Performance: Slumping....................................45
2.4.4 Reflow Soldering......................................................................47
2.4.5 Wave Soldering........................................................................48
2.4.6 Cleaning....................................................................................50
References........................................................................................................50
xii Contents
3 Wetting and Joint Formation........................................................................53
3.1 Wetting.................................................................................................55
3.1.1 Basic Wetting Mechanics........................................................55
3.1.2 Reactive Wetting of Sn on Substrates....................................57
3.1.3 Fluxes........................................................................................58
3.1.4 Wetting Characteristics of Binary Eutectic Pb-
free Solder Alloys.....................................................................61
3.1.5 Wetting Characteristics of Ternary SnAgCu Alloys.............63
3.2 Solderability and Solderability Testing.............................................63
3.2.1 Primary Solderability Defects.................................................64
3.2.2 Immersion Testing...................................................................65
3.2.3 The Wetting Balance................................................................66
3.2.4 Importance of Flux Activity in Wetting
and Solderability Testing........................................................68
3.3 Solidification........................................................................................70
3.3.1 Basic Theory of Solidification.................................................70
3.3.2 Solidification and Melting Studies in Lead-Free Solders
by Differential Scanning Calorimetry....................................72
3.4 As-cast Microstructures of Solder Alloys..........................................73
3.4.1 SnPb Euctectic..........................................................................74
3.4.2 SnAg Eutectic Solder...............................................................74
3.4.3 SnAgCu Ternary Solders.........................................................75
3.5 Segregation and Solidification Defects in some Pb-free Alloys......75
References........................................................................................................77
4 Microstructural Instability in Solders..........................................................79
4.1 Effects of Microstructural Coarsening and Aging on Properties
of Solder................................................................................................79
4.2 Sources of Microstructural Instability in Solder..............................82
4.3 Thermodynamics of Microstructural Instability..............................83
4.4 Kinetics of Coarsening Processes in Microstructure........................84
4.4.1 Grain-growth Models..............................................................84
4.4.2 Mechanical Instability.............................................................85
4.4.3 A Simplified Model for Thermal Coarsening........................85
4.4.4 Strain and Thermal Coarsening in SnAgCu..........................88
4.5 Quantifying Alloy Effects on Microstructure....................................91
References........................................................................................................94
5 Intermetallic Formation and Growth...........................................................97
5.1 Characteristics of Intermetallic Compounds....................................97
5.1.1 Cu-Sn.........................................................................................98
5.1.2 Ag-Sn.......................................................................................102
5.1.3 Au-Sn......................................................................................103
5.1.4 Ni-Sn.......................................................................................106
Contents xiii
5.2 Growth Kinetics of IMCs in Layered Morphologies.......................107
5.2.1 Thermodynamic Driving Forces in Intermetallic Systems107
5.2.2 Kinetics and Diffusion: Basic Theory...................................112
5.2.3 Experimental Kinetics for IMC Growth in Solders.............112
5.3 Characterization of IMC Layers and Secondary Phases................116
5.3.1 Microstructural Preparation and Samples..........................116
5.3.2 SEM and EDX.........................................................................117
5.3.3 Hardness and Mechanical Properties..................................117
5.3.4 X-ray Diffraction....................................................................121
References......................................................................................................122
6 Mechanical Properties and Creep Behavior...............................................127
6.1 Monotonic Properties of Solders......................................................127
6.1.1 Basic Monotonic Behavior....................................................127
6.1.2 Pb-free Alloys: Strength vs Ductility....................................129
6.1.3 Temperature and Strain-rate Sensitivity.............................131
6.1.4 Strain-rate Sensitivity: Estimating Fatigue and Creep
Performance from Tensile Properties..................................133
6.2 Creep and Stress Relaxation.............................................................135
6.3 Influence of Microstructure..............................................................139
References......................................................................................................142
7 Thermomechanical Fatigue.........................................................................145
7.1 Mechanisms of Fatigue Cracking in Solder Alloys.........................145
7.2 Cyclic Strain: A Simplified Perspective............................................149
7.3 Constitutive Analysis and Viscoplastic Finite Element
Calculations for Strain Analysis in Solder Alloys...........................153
7.4 Fatigue Models for Solder Alloys.....................................................155
7.5 Statistical Analysis of Fatigue Data..................................................164
7.5.1 Monte Carlo Simulations of Fatigue Life.............................168
7.5.2 Statistical Results for Selected Pb-Free Alloys....................173
7.6 Isothermal Fatigue Curves for Pb-free Solders...............................176
7.6.1 Summarizing SnAgCu Fatigue Behavior.............................177
7.7 Comparative Testing of Alloys: Statistical Approaches.................179
7.7.1 Experimental Results for Alloy Comparative Tests............180
7.7.2 Discussion on Fatigue and Microstructural Test Results
for Alloy Comparison Example............................................182
References......................................................................................................183
8 Product Assurance........................................................................................187
8.1 Environment......................................................................................187
8.2 Design for Reliability.........................................................................189
8.3 Controlled Processes.........................................................................191
xiv Contents
8.4 Accelerated Testing...........................................................................196
8.5 Qualification Testing Development: A Case History for Space
Flight Development Using SnPb Eutectic Solder............................199
References......................................................................................................201
Index .........................................................................................................................203