Lead-free Solders E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Wiley Series in Materials for Electronic & Optoelectronic Applications
• Sprache: Englisch

Seitenzahl: 803

Veröffentlichungsjahr: 2012

Beschreibung

Providing a viable alternative to lead-based solders is a major research thrust for the electrical and electronics industries - whilst mechanically compliant lead-based solders have been widely used in the electronic interconnects, the risks to human health and to the environment are too great to allow continued widescale usage. Lead-free Solders: Materials Reliability for Electronics chronicles the search for reliable drop-in lead-free alternatives and covers:

• Phase diagrams and alloy development
• Effect of minor alloying additions
• Composite approaches including nanoscale reinforcements
• Mechanical issues affecting reliability
• Reliability under impact loading
• Thermomechanical fatigue
• Chemical issues affecting reliability
• Whisker growth
• Electromigration
• Thermomigration

Presenting a comprehensive understanding of the current state of lead-free electronic interconnects research, this book approaches the ongoing research from fundamental, applied and manufacturing perspectives to provide a balanced view of the progress made and the requirements which still have to be met.

Leseprobe

Contents

Cover

Series Page

Wiley Series in Materials for Electronic and Optoelectronic Applications

Series Editors

Published Titles

Forthcoming Titles

Title Page

Copyright

Series Preface

Wiley Series in Materials for Electronic and Optoelectronic Applications

Preface

List of Contributors

Thematic Area I: Introduction

Chapter 1: Reliability of Lead-Free Electronic Solder Interconnects: Roles of Material and Service Parameters

1.1 Material Design for Reliable Lead-Free Electronic Solders Joints

1.2 Imposed Fields and the Solder Joint Responses that Affect Their Reliability

1.3 Mechanical Integrity

1.4 Thermomechanical Fatigue (TMF)

1.5 Whisker Growth

1.6 Electromigration (EM)

1.7 Thermomigration (TM)

1.8 Other Potential Issues

Thematic Area II: Phase Diagrams and Alloying Concepts

Chapter 2: Phase Diagrams and Their Applications in Pb-Free Soldering

2.1 Introduction

2.2 Phase Diagrams of Pb-Free Solder Systems

2.3 Example of Applications

2.4 Conclusions

Acknowledgments

References

Chapter 3: Phase Diagrams and Alloy Development

3.1 Introduction

3.2 Computational Thermodynamics as a Research Tool

3.3 Thermodynamic Databases – the Underlying Basis of the Modelling of Phase Diagrams and Thermodynamic Properties, Databases for Lead-Free Solders

3.4 Application of the SOLDERS Database to Alloy Development

3.5 Conclusions

References

Chapter 4: Interaction of Sn-based Solders with Ni(P) Substrates: Phase Equilibria and Thermochemistry

4.1 Introduction

4.2 Binary Phase Equilibria

4.3 Ternary Phase Equilibria Ni-P-Sn

4.4 Thermochemical Data

4.5 Relevance of the Results and Conclusion

4.6 Acknowledgments

References

Thematic Area III: Microalloying to Improve Reliability

Chapter 5: ‘Effects of Minor Alloying Additions on the Properties and Reliability of Pb-Free Solders and Joints’

5.1 Introduction

5.2 Controlling Ag3Sn Plate Formation

5.3 Controlling the Undercooling of Sn Solidification

5.4 Controlling Interfacial Reactions

5.5 Modifying the Microstructure of SAC

5.6 Improving Mechanical Properties

5.7 Enhancing Electromigration Resistance

5.8 Summary

References

Chapter 6: Development and Characterization of Nano-composite Solder

6.1 Introduction

6.2 Nano-composite Solder Fabrication Process

6.3 Microstructure

6.4 Physical Properties

6.5 Mechanical Properties

6.6 Challenges and Solutions

6.7 Summary

Acknowledgments

References

Thematic Area IV: Chemical Issues Affecting Reliability

Chapter 7: Chemical Changes for Lead-Free Soldering and Their Effect on Reliability

7.1 Introduction

7.2 Soldering Fluxes and Pastes

7.3 Cleaning

7.4 Laminates

7.5 Halogen-Free Laminates

7.6 Conductive Anodic Filament (CAF) Formation

7.7 Summary

References

Thematic Area V: Mechanical Issues Affecting Reliability

Chapter 8: Influence of Microstructure on Creep and High Strain Rate Fracture of Sn-Ag-Based Solder Joints

8.1 Introduction

8.2 Coarsening Kinetics: Quantitative Analysis of Microstructural Evolution

8.3 Creep Behavior of Sn-Ag-Based Solders and the Effect of Aging

8.4 Role of Microstructure on High Strain Rate Fracture

8.5 Summary and Conclusions

Acknowledgments

References

Chapter 9: Microstructure and Thermomechanical Behavior Pb-Free Solders

9.1 Introduction

9.2 Sn-Pb Solder

9.3 Pb-Free Solders

9.4 Summary

References

Chapter 10: Electromechanical Coupling in Sn-Rich Solder Interconnects

10.1 Introduction

10.2 Experimental

10.3 Results

10.4 Discussion

10.5 Conclusions

10.6 Acknowledgments

References

Chapter 11: Effect of Temperature-Dependent Deformation Characteristics on Thermomechanical Fatigue Reliability of Eutectic Sn-Ag Solder Joints

11.1 Introduction

11.2 Experimental Details

11.3 Results and Discussion

11.4 Summary and Conclusions

References

Thematic Area VI: Whisker Growth Issues Affecting Reliability

Chapter 12: Sn Whiskers: Causes, Mechanisms and Mitigation Strategies

12.1 Introduction

12.2 Features of Whisker Formation

12.3 Understanding the Relationship between IMC Growth, Stress and Whisker Formation

12.4 Summary Picture of Whisker Formation

12.5 Strategies to Mitigate Whisker Formation

12.6 Conclusion

Acknowledgments

References

Chapter 13: Tin Whiskers

13.1 Low Melting Point Metals and Whisker Formation

13.2 Room-Temperature Tin Whiskers on Copper Substrate

13.3 Thermal-Cycling Whiskers on 42 Alloy/Ceramics

13.4 Oxidation/Corrosion Whiskers

13.5 Mechanical-Compression Whiskers in Connectors

13.6 Electromigration Whiskers

13.7 Whisker Mitigation

13.8 Future Work

References

Thematic Area VII: Electromigration Issues Affecting Reliability

Chapter 14: Electromigration Reliability of Pb-Free Solder Joints

14.1 Introduction

14.2 Failure Mechanisms of Solder Joints by Forced Atomic Migration

14.3 IMC Growth

14.4 Effect of Sn Grain Structure on EM Reliability

14.5 Summary

Acknowledgments

References

Chapter 15: Electromigration in Pb-Free Solder Joints in Electronic Packaging

15.1 Introduction

15.2 Unique Features for EM in Flip-Chip Pb-Free Solder Joints

15.3 Changes of Physical Properties of Solder Bumps During EM

15.4 Challenges for Understanding EM in Pb-Free Solder Microbumps

15.5 Thermomigration of Cu and Ni in Pb-Free Solder Microbumps

15.6 Summary

Acknowledgments

References

Chapter 16: Effects of Electromigration on Electronic Solder Joints

16.1 Introduction

16.2 Effects of Electromigration on Solders

16.3 Effects of Electromigration on Interfacial Reactions

16.4 Modeling Description of Effects of Electromigration on IMC Growth

16.5 Conclusions

Acknowledgments

References

Thematic Area VIII: Thermomigration Issues Affecting Reliability

Chapter 17: Thermomigration in SnPb and Pb-Free Flip-Chip Solder Joints

17.1 Introduction

17.2 Thermomigration in SnPb Flip-Chip Solder Joints

17.3 Thermomigration in Pb-Free Flip-Chip Solder Joints

17.4 Driving Force of Thermomigration

17.5 Coupling between Thermomigration and Creep

17.6 Coupling between Thermomigration and Electromigration: Thermoelectric Effect on Electromigration

17.7 Summary

Acknowledgments

References

Thematic Area IX: Miniaturization Issues Affecting Reliability

Chapter 18: Influence of Miniaturization on Mechanical Reliability of Lead-Free Solder Interconnects

18.1 Introduction

18.2 Effect of Miniaturization on Static Properties of Solder Joints (Tensile and Shear)

18.3 Creep and Relaxation of Solder Joints

18.4 Summary and Conclusions

Acknowledgments

References

Index

Wiley Series in Materials for Electronic and Optoelectronic Applications

www.wiley.com/go/meoa

Series Editors

Professor Arthur Willoughby, University of Southampton, Southampton, UK

Dr. Peter Capper, SELEX Galileo Infrared Ltd., Southampton, UK

Professor Safa Kasap, University of Saskatchewan, Canada

Published Titles

Bulk Crystal Growth of Electronic, Optical and Optoelectronic Materials, Edited by P. Capper

Properties of Group-IV, III–V and II–VI Semiconductors, S. Adachi

Charge Transport in Disordered Solids with Applications in Electronics, Edited by S. Baranovski

Optical Properties of Condensed Matter and Applications, Edited by J. Singh

Thin Film Solar Cells: Fabrication, Characterization and Applications, Edited by J. Poortmans and V. Arkhipov

Dielectric Films for Advanced Microelectronics, Edited by M. R. Baklanov, M. Green and K. Maex

Liquid Phase Epitaxy of Electronic, Optical and Optoelectronic Materials, Edited by P. Capper and M. Mauk

Molecular Electronics: From Principles to Practice, M. Petty

Luminescent Materials and Applications, Edited by A. Kitai

CVD Diamond for Electronic Devices and Sensors, Edited by Ricardo S. Sussmann

Properties of Semiconductor Alloys: Group-IV, III-V and II-VI Semiconductors, S. Adachi

Mercury Cadmium Telluride: Growth, Properties and Applications, Edited by P. Capper and J. Garland

Zinc Oxide Materials for Electronic and Optoelectronic Device Applications, Edited by Cole W. Litton, Donald C. Reynolds and Thomas C. Collins

Forthcoming Titles

Silicon Photonics: Fundamentals and Devices, M. J. Deen and P. K. Basu

This edition first published 2012

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Library of Congress Cataloging-in-Publication Data

Subramanian, K. N., Ph. D.

Lead-free solders: materials reliability for electronics / edited by K.N. Subramanian.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-470-97182-6 (cloth)

1. Lead-free electronics manufacturing processes. 2. Solder and soldering. I. Title.

TK7836.S825 2012

621.381–dc23                                                      2011038790

A catalogue record for this book is available from the British Library.

Print ISBN: 9780470971826

Series Preface

Wiley Series in Materials for Electronic and Optoelectronic Applications

This book series is devoted to the rapidly developing class of materials used for electronic and optoelectronic applications. It is designed to provide much-needed information on the fundamental scientific principles of these materials, together with how these are employed in technological applications. The books are aimed at (postgraduate) students, researchers and technologists, engaged in research, development and the study of materials in electronics and photonics, and industrial scientists developing new materials, devices and circuits for the electronic, optoelectronic and communications industries.

The development of new electronic and optoelectronic materials depends not only on materials engineering at a practical level, but also on a clear understanding of the properties of materials, and the fundamental science behind these properties. It is the properties of a material that eventually determine its usefulness in an application. The series therefore also includes such titles as electrical conduction in solids, optical properties, thermal properties, and so on, all with applications and examples of materials in electronics and optoelectronics. The characterization of materials is also covered within the series in as much as it is impossible to develop new materials without the proper characterization of their structure and properties. Structure–property relationships have always been fundamentally and intrinsically important to materials science and engineering.

Materials science is well known for being one of the most interdisciplinary sciences. It is the interdisciplinary aspect of materials science that has led to many exciting discoveries, new materials and new applications. It is not unusual to find scientists with a chemical engineering background working on materials projects with applications in electronics. In selecting titles for the series, we have tried to maintain the interdisciplinary aspect of the field, and hence its excitement to researchers in this field.

Arthur WilloughbyPeter CapperSafa Kasap

Preface

Over the last two decades, significant progress has been made to facilitate the replacement of leaded solders in microelectronics. Global pressures to adopt lead-free electronics, brought about by environmental concerns, have made such changes mandatory. This book is intended to update the current state of understanding, and major developments, in electronic lead-free solder interconnects to improve their reliability in service.

At present there are no drop-in lead-free substitutes for leaded solders used in microelectronic packaging. Lead-free solders used in microelectronic packages that use organic polymeric boards need to have low melting points around 200 °C, in addition to having good wettability, thermomechanical fatigue resistance, etc. Among the various possible alloy systems considered, solders with significant amounts of tin have emerged as leading candidates. Sn-Ag-Cu-based solder alloys, known as SAC alloys, are widely adopted in current microelectronic applications based on several suitable attributes like low enough melting point, good wettability, etc. Sn-based solders containing other alloying additions like, Cu, Bi, Zn are also in current use.

At temperatures relevant in microelectronics, Sn exists in body-centered tetragonal structure with a c/a ratio of about 0.5, and is highly anisotropic. In addition, it is not as compliant as lead to imposed service environments. The layered intermetallics that form at the solder/substrate interfaces, and distributed intermetallics that form within the solder coarsen during service, significantly affecting the mechanical reliability of lead-free solder interconnects. Numerous studies that have been undertaken provide significant insight into the issues and mechanisms. Such studies include alloy development, composite solders with intermetallic or inert reinforcements, etc. In spite of all these efforts, at present, no ideal perfect replacement for leaded solders exists.

Miniaturization of microelectronic components is a constantly advancing area. This raises important issues like electromigration of atoms/ions due to high current densities encountered, and thermomigration due to large temperature gradients. As a consequence, the material developments to address the issues of lead elimination in electronic packaging are facing constantly changing targets. Although whisker growth in tin has been known for over half a century there is no clear understanding of the process and there exists no reliable means to prevent such whisker growth. Miniaturization of microelectronic packages makes whisker growth a very important issue since whiskers can cause short circuits affecting the reliability of such packages.

Studies to face these emerging challenges have resulted in a significant number of important findings at a very rapid phase warranting an update of the current status every few years. This book is aimed at addressing such a goal. Researchers known internationally for their important contributions to the field of lead-free electronic solders were invited to contribute chapters in chosen thematic areas. Most of these invitees have been well acquainted with the editor for over fifteen years, and the others were recommended by the ones known to the editor.

This effort provides the current understanding of issues and solutions relevant to improving the reliability of lead-free electronic solder interconnects.

K. N. SubramanianEast Lansing, MichiganJuly 2011

List of Contributors

Pavel Broz, Institute of Chemistry, Masaryk University, Brno, Czech Republic

Seung-Hyun Chae, Microelectronics Research Center, The University of Texas at Austin, TX, USA

Yuan-wie Chang, Department of Materials Science and Engineering, National Chiao Tung University, Hsin-chu, Taiwan, P.R. China

Eric Chason, School of Engineering, Brown University, Providence, Rhode Island, USA

Chih Chen, Department of Materials Science and Engineering, National Chiao Tung University, Hsin-chu, Taiwan, P.R. China

Chih-ming Chen, Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan

Hsiao-Yun Chen, Department of Materials Science and Engineering, National Chiao Tung University, Hsin-chu, Taiwan, P.R. China

Si Chen, SMIT Center & Dept of MicroTechnology and Nanoscience, University of Technology, Göteborg, Sweden and Key Laboratory of New Displays and System Integration, SMIT Center and School of Mechatronics and Mechanical Engineering, Shanghai University, Shanghai, P.R. China

Sinn-wen Chen, Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu, Taiwan

Deep Choudhuri, Department of Chemical Engineering and Materials Science, University of Michigan, East Lansing, MI, USA

Alan Dinsdale, National Physical Laboratory, Teddington, UK

Indranath Dutta, The School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA

Darrel R. Frear, Freescale Semiconductor, Tempe, AZ, USA

Rajesh Ganesan, Departments of Materials Chemistry and Inorganic Chemistry, University of Vienna, Vienna, Austria

Yulai Gao, School of Materials Science and Engineering, Shanghai University, Shanghai, P.R. China

Wojciech Gierlotka, Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li, Taiwan

Jung Kyu Han, Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA, USA

Paul S. Ho, Microelectronics Research Center, The University of Texas at Austin, TX, USA

Hsiang-Yao Hsiao, Department of Materials Science and Engineering, National Chiao Tung University, Hsin-chu, Taiwan, P.R. China

Chia-ming Hsu, Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu, Taiwan

Zhe Huang, The School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA

Herbert Ipser, Departments of Materials Chemistry and Inorganic Chemistry, University of Vienna, Vienna, Austria

Nitin Jadhav, School of Engineering, Brown University, Providence, Rhode Island, USA

Sung K. Kang, IBM T.J. Watson Research Center, Yorktown Heights, NY, USA

Golta Khatibi, Physics of Nanostructured Materials, University of Vienna, Vienna, Austria

Ales Kroupa, Institute of Physics of Materials, AS CR, Brno, Czech Republic

Praveen Kumar, The School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA

Martin Lederer, Physics of Nanostructured Materials, University of Vienna, Vienna, Austria

Andre Lee, Department of Chemical Engineering and Materials Science, University of Michigan, East Lansing, MI, USA

Shih-wei Liang, Department of Materials Science and Engineering, National Chiao Tung University, Hsin-chu, Taiwan, P.R. China

Shih-kang Lin, Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI, USA

H. Y. Liu, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

Johan Liu, SMIT Center & Dept of MicroTechnology and Nanoscience, University of Technology, Göteborg, Sweden and Key Laboratory of New Displays and System Integration, SMIT Center and School of Mechatronics and Mechanical Engineering, Shanghai University, Shanghai, P.R. China

Ravu Mahajan, Assembly Technology Development, Intel Corporation, Chandler, AZ, USA

Clemens Schmetterer, Departments of Materials Chemistry and Inorganic Chemistry, University of Vienna, Vienna, Austria

J. K. Shang, Department of Materials Science and Engineering, University of Illinois, Urbana, IL, USA and Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

Ganesh Subbarayan, School of Mechanical Engineering, Purdue University, West Lafayette, IN, USA

K. N. Subramanian, Department of Chemical Engineering and Materials Science, University of Michigan, East Lansing, MI, USA

Katsuaki Suganuma, Institute of Scientific and Industrial Research, Osaka University, Suita, Osaka, Japan

Tian Tian, Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA, USA

K. N. Tu, Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA, USA

Laura Turbini, Research in Motion Ltd, Cambridge, Ontario, Canada

Jan Vrestal, Institute of Chemistry, Masaryk University, Brno, Czech Republic

Chao-hong Wang, Department of Chemical Engineering, National Chung Cheng University, Chia-Yi, Taiwan

Yiwei Wang, Microelectronics Research Center, The University of Texas at Austin, TX, USA

Z. G. Wang, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

Andy Watson, Institute for Materials Research, SPEME, University of Leeds, UK

Brigitte Weiss, Physics of Nanostructured Materials, University of Vienna, Vienna, Austria

Hsin-jay Wu, Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu, Taiwan

Adela Zemanova, Institute of Physics of Materials, AS CR, Brno, Czech Republic

Q. L. Zeng, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

Qijie Zhai, School of Materials Science and Engineering, Shanghai University, Shanghai, P.R. China

L. Zhang, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

Q. S Zhu, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

Thematic Area I

Introduction

Chapter 1

Reliability of Lead-Free Electronic Solder Interconnects: Roles of Material and Service Parameters

K. N. Subramanian

Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI, 48824-1226, USA

Abstract

This chapter is meant to provide a general overview of the issues affecting the reliability of lead-free electronic solder joints subjected to service environments. It is meant to be an introduction to the various thematic areas that are covered in this book. Hence no attempt to provide references to any of the topics mentioned in this chapter is given at the end of this chapter. Extensive references for each of these topics are cited by the authorities contributing various chapters to this book.

1.1 Material Design for Reliable Lead-Free Electronic Solders Joints

It is important to point out that solder joint is a multicomponent system. Solders used in electronic interconnects are in the joint geometry and its overall response to environmental and in-service parameters are influenced by the constraints present in that configuration. Such a joint has substrates, interface intermetallic compound layers that are necessary to form the necessary bonding, solder matrix with its own individual phases and intermetallic compounds (IMCs). In addition to geometrical issues, processing method used for fabrication of joints and the resultant microstructural features, service parameters encountered, and the response of the solder material to external influences, play significant roles in determining the reliability of the electronic solder joints. The service environments encountered are becoming more severe and the continuous rapid advances in microminiaturization of the electronic packages impose ever-increasing demands on such solder joints. Since solder joints present in modern microelectronics operate at very high homologous temperatures significant microstructural changes such as coarsening of the features present in the joint occur affecting their reliability. Because of the changes in the joint geometry to accomplish the microminiaturization such joints are expected to provide structural integrity in addition to providing electrical pathways. They are also expected to be mechanically compliant to dissipate the stresses that develop during service but be dimensionally stable.

Most of the lead-free solder alloys that are in current use contain significant amounts of tin. Such alloys have the suitable melting temperatures and wetting characteristics for utilization in consumer electronics. Among these, Sn-Ag-Cu alloys have been widely adopted. In order to minimize the deleterious effects of thermally induced coarsening of the phases present within the solder matrix and at the solder/substrate interfaces, and to improve the mechanical properties, several detailed studies on phase stability along with resultant developments have taken place. They have provided strategies involving minor amounts of additional alloying additions, as well as reinforcements to produce composite solders. Some of these approaches also help to improve the reliability of the lead-free solder joints. Since solders have to bond with substrates, the substrate materials and its finish will interact with the solder during the reflow process and during service. Several studies aim at combating reliability issues arising from the coarsening of these reaction products, which are quite often brittle.

The improvements to address problems listed in the last few paragraphs are the main contributions from those studying phase diagrams to develop suitable lead-free solder alloys, alloy additions to improve their service reliability, and composite solders. Such judicial material design to improve the service reliability invariably can take place only with the clear understanding of the service environments in which the electronic packages will be placed and the material response and resultant behaviors that affect their reliability. Joint geometry is an added contributing factor that can aggravate the influence of the service environment. However, such joint design is outside the realm of the material development to meet the challenges. If material design can either alleviate the material processes that affect the joint reliability either completely (or most of it), joint geometry hopefully will have minimal influence on the joint reliability.

The following sections address the material processes that influence the service reliability of lead-free solder joints. Detailed discussions on the developments in these avenues are brought out by world-renowned researchers in these fields in the chapters presented in various thematic areas.

1.2 Imposed Fields and the Solder Joint Responses that Affect Their Reliability

This schematic illustrates damage resulting from multiple fields and their complex interactions. Processes identified in this schematic are EM – electromigration, TM – thermal migration, PF – plastic flow and fracture, JH – Joule heating, CS – current stressing, TMF – thermomechanical fatigue. The scenario presented in this schematic illustrates the complex state of damage accumulation resulting from various fields encountered during service (direct effects: EM, TM, PF), and their mutual interactions (coupled effects: TMF, JH, CS), that affect the reliability of lead-free electronic solder joints.

Even during service these fields are time and position dependent. For example, temperature depends on the environment and Joule heating from the current density that can vary with hills and valleys that form due to electromigration. Similarly, the mechanical stress state will depend on the stresses that develop due to coefficient of thermal expansion (CTE) mismatches between the entities present in the joint, stresses that develop due to atom/ion migration caused by electron wind forces, and externally imposed loads. Among those listed the major damage contributors that affect the reliability of the lead-free electronic solders are (i) mechanical integrity, (ii) thermomechanical fatigue, (iii) whisker growth, (iv) electromigration, and (v) thermomigration. It should be pointed out that this is not an ordered list, and that there are significant mutual interactions between them. Such mutual interactions will become progressively more important with the continued efforts towards microminiaturization of electronic packages. Among the five processes listed above, whisker growth, electromigration, and thermomigration have become reliability concerns mainly due to such miniaturization.

1.3 Mechanical Integrity

An electronic package contains several solder joints and their reliability is what needs to be understood. However, reliability studies carried directly with such complex packages quite often cannot provide the means to evaluate the actual material-related issues that cause the failure, a critical piece of information warranted for material developments. On the other hand, if model system studies are carried out, the model geometries used should be representative of those actually encountered in the electronic packaging. Carrying out studies on bulk solder specimens, without any of the constraints encountered in the joint configuration, will not be of any relevance to what happens in the joints.

Depending on the application the solder joints present in electronic packages may be experiencing different ranges of temperatures. In addition to the heating that results from passage of electric current, ambient conditions encountered during service can play significant roles. The deformation mode of Sn-based solders is highly sensitive to temperature and strain rate. Any reliability modeling should take this issue into account, along with issues of constraints and joint geometry.

Reliability under impact loading is a very important consideration not only for shipping considerations, but also for accidental dropping of a device. Industrial drop tests are carried out to check for the impact reliability during shipping. Charpy-type impact tests where the impact load is delivered to the individual solder ball attached to the substrate are also employed. In a realistic electronic package the impact delivered to some other location is realized by the solder. Hence, such tests cannot provide the necessary information about the detailed stress states, modes of fracture, and so on, that are critical for material design. In addition, there are several scenarios, like in automotive and aerospace applications, where random bumping can cause repeated impact loading.

1.4 Thermomechanical Fatigue (TMF)

Thermal excursions encountered in service cause significant damage to solder joints affecting their service reliability. Several material-related processes occur during heating, cooling, and dwell at temperature extremes. For example, the heating and cooling rates, temperature regime (high/low), temperature difference, dwell times at high- and low-temperature extremes, do significantly affect the integrity of the solder joints. These studies have shown that heating rate is an important contributor affecting the joint reliability. Damage accumulation in solder joints subjected to TMF results from a highly inhomogeneous stress distribution. Such stresses arise from CTE mismatches between various entities present in the joint. Anisotropy of tin could be a major contributing factor for such damage accumulation since the CTE difference between a- and c-directions of body-centered tetragonal β-Sn is almost twice that of the CTE difference between polycrystalline copper and polycrystalline Sn. Manifestation of the damage from TMF occurs only after several hundred TMF cycles, although the residual mechanical and electrical properties deteriorate significantly from the very early stages of TMF. Grain-boundary sliding and decohesion are the predominant damage modes that result from TMF. Although such events occur throughout the solder present in the joint, the predominant surface manifestation of the same is highly localized to the solder regions adjacent to solder/substrate IMC layer. Constraints imposed by the substrate appear to cause strain localization to such regions. During the later stages of TMF, when the residual properties tend to stabilize, the surface damage progresses by joining of the individual distributed cracks and cause the catastrophic failure.

Based on TMF evaluation with realistic temperature profiles, and findings from actual electronic packages placed in service, several new solder compositions with various minor alloying additions to Sn-Ag-Cu (SAC) alloy have been developed. A major hurdle encountered in this approach is the coarsening of the intermetallic compounds that form during service affecting the joint reliability. Studies dealing with dwell-time issues indicate presence of small amounts of Ni in addition to Cu in the solder significantly improves the reliability of the solder joint under situations with longer dwell at the high-temperature extreme. Inert particle reinforcements have not been effective since they do not bond with the solder matrix. An alternate approach to improve service reliability of solder joints is to incorporate compatible nanostructured reinforcements with surface-active radicals to promote bonding with metal, following which they become inert. Such strongly bonded inert reinforcements that do not coarsen during service improve the reliability of lead-free solder joints subjected to TMF during service.

1.5 Whisker Growth

It has long been known that Sn exhibits whisker growth. However, such events did not receive any attention in electronic interconnects till recently. Microminiaturization of electronic components has resulted in close spacing of current carrying lines. Quite often the spacing is of the order of about 100 μm. If whiskers grow to a length of about 50 μm in adjacent lines shorting can occur resulting in electrical failure. Although several models have been proposed for whisker growth from solid substrate, none of them have been proved to be satisfactory. Compressive stresses are believed to make whiskers grow from their base. In Sn-based solder joints such compressive stresses that can arise from the formation of Cu-Sn IMC at Sn grain boundaries present in the solder are believed to cause such whisker growth. Such IMC formation can be facilitated by Cu diffusion from the substrate. For continuous growth of whiskers from the base such compressive stresses need to be present on a continuous basis, and such stresses should not be relaxed. Hence, stresses that are externally applied, or resulting from volume changes involved in formation of IMCs, and those that develop during electromigration, can facilitate whisker growth. There are conflicting views on whisker growth directions and locations from which whiskers grow. If the whisker growth is caused by Cu diffusion from the substrate followed by IMC growth, one should be able to arrive at a solution to this problem. However, no known reliable solution to this problem exists at present. Some of the difficulties in whisker growth investigations are encountered due to uncertainties about when and where will whiskers form and grow. As a consequence evaluation of the effectiveness of attempted mitigation strategies to prevent whisker growth becomes difficult.

1.6 Electromigration (EM)

EM in electronic solders has become an important concern in recent years. Ion migration in the presence of high current density has long been known in computer industry where incorporation of copper atoms at grain boundaries present in aluminum current-carrying lines has provided a solution to such a problem. In electronic interconnects the presence of high current density has not been a significant concern until miniaturization of electronic components and higher service temperatures has caused EM to become a potential reliability issue in electronic interconnects. Although events contributing to this mass movement are due to material-related issues, it can be further aggravated by geometry in which the material is employed. The latter can impose current crowding and associated localized Joule heating, resulting in enhanced mass movement in localized regions. Based on the intended roles, several alloys used in electronics and energy-related applications are multiphase materials. In these alloys the electromigration-induced changes will depend strongly on the atomic species present, solid solubility, morphological features of the microstructural constituents, and phase stability.

Localized Joule heating and current crowding, have been of great concern. Grain growth and reorientation of grains, phase segregation, and interfacial events; contribute to damage accumulation by electromigration, in addition to hill and valley formation. Unlike the case of aluminum lines in computers, solders present in the electronic interconnects operate at very high homologous temperatures. As a consequence lattice diffusion, in addition to grain-boundary diffusion becomes an important consideration

1.7 Thermomigration (TM)

Microstructural coarsening in solder joints can occur due to the high temperatures encountered during service. Segregation and coarsening of the phases can occur due to electromigration. In addition to these effects, small temperature differences in adjacent regions can result from joint geometry and localized Joule heating during electromigration. Even though such temperature differences can be small, they can result in very significant temperature gradients in the miniaturized electronic solder joints. These large temperature gradients can give rise to additional microstructural evolution and damage. This reliability issue has gained attention in recent years.

1.8 Other Potential Issues

Sn exhibits polymorphism. At temperatures above 13 oC the stable form of Sn has body-centered tetragonal structure (β-Sn) and below this temperature the stable crystalline form is diamond cubic (α-Sn). Transformation form one form to the other is extremely sluggish and is very sensitive to the purity of the metal. Hence, Sn present in solder joints under environments encountered in service exists in body-centered tetragonal structure. However, if the solder joints are exposed to extremely low temperatures for significant lengths of time in future applications, β to α phase transformation can occur resulting in a significant volume increase of about 27%. Since α-phase is extremely brittle, such increase in volume causes extensive cracking and spalling. Such an event, known as ‘tin pest’ could potentially become a reliability concern for microelectronics, in applications such as aerospace, and extremely cold locations like in Polar regions.

Synergistic aspects of the various issues that affect the reliability of the solder joint need to be addressed in its entirety. Segmentation to address the individual issues can quite often provide a solution to a particular concern, while totally destroying the integrity of the entity by affecting the other issues. In the current scenario TMF is not the only issue that affects the reliability of a solder joint. EM and whisker growth have become important due to the microminiaturization of the electronic components. Among these TMF is concerned with flow and adaptation of material to stresses that develop from thermal excursion encountered. However, the other two involve atom/ion migration. EM is concerned with formation of valleys and hillocks. Such events can attribute to significant additional Joule heating that once again should affect the conditions encountered during TMF. Such self-perpetuating coupled events cannot be considered as simple additive effects to TMF.

The following block diagram illustrates some such synergistic issues that need to be considered to evaluate the total damage affecting the reliability of lead-free electronic solder joints.

Another major concern is long-term reliability of microelectronic interconnects. Consumer electronics have relatively short life, and reliability evaluation can be carried out with realistic service parameters. In applications like in space or military, lifetime of an electronic component, the expected lifetime could be several decades. Suitable accelerated test methodologies are still to be developed to guarantee reliability for such applications.

Thematic Area II

Phase Diagrams and Alloying Concepts

Chapter 2

Phase Diagrams and Their Applications in Pb-Free Soldering

Sinn-wen Chen1, Wojciech Gierlotka2 Hsin-jay Wu1 and Shih-kang Lin3

1 Department of Chemical Engineering, National Tsing Hua University, #101, Sec. 2, Kuang-Fu Rd., Hsin-Chu 300, Taiwan

2 Department of Chemical Engineering & Materials Science, Yuan Ze University, #135, Yuan-Tung Rd., Chung-Li 32003, Taiwan

3 Department of Materials Science and Engineering, National Cheng Kung University, #1, University Rd., Tainan City 701, Taiwan

Abstract

Pb-free soldering is one of the most important technologies in the electronics industry. A phase diagram is a comprehensive representation of thermodynamic properties of a multicomponent material system. Basic knowledge and information of phase diagrams of Pb-free solder systems are reviewed. In this chapter, several examples of applications of phase diagrams in Pb-free soldering are presented, including general applications of melting, solidification and intermetallic compounds (IMC) formation caused by interfacial reactions. With the aid of phase diagrams, interpretations of some abnormal phenomena in Pb-free soldering such as effective undercooling reduction by Co doping, unexpected IMC growth rate in Sn-Bi/Fe couples, unexpected solder melting in Sn-Sb/Ag joints, special up-hill diffusion in the Cu/Sn-Cu/Ni sandwich structure, and the influence of limited Sn supply upon the interfacial reactions in the Au/Sn/Cu sandwich specimens, and so on, will be given.

2.1 Introduction

Soldering is currently the most important joining process in electronic packaging. Properties of solder joints are crucial to the reliability of electronic products and very often are directly related with their lifetime. As new Pb-free solders are being introduced into the electronics and optoelectronics industries, it is of practical importance to evaluate these emerging materials [1–4]. Although there are several different soldering technologies currently available, they share some common features. During soldering, solders are first melted, followed by wetting and reacting with substrates, and then solidified, and, if necessary, remelted again. Multiple phase transformation steps, such as melting, solidification, and interfacial reactions resulting in the formation of intermetallic compounds (IMC), are involved in the manufacturing processes. Moreover, since solders are usually low melting temperature materials, noticeable interfacial reactions continue in the solid-state solder joints upon Joule heating during device operation. These phases formed via interfacial reaction or solidification during or post soldering determine critically the properties of Pb-free solder joints and thus the reliability of electronic products.