Lead-Free Solder Process Development E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 418

Veröffentlichungsjahr: 2011

Beschreibung

Discusses the growth mechanisms of tin whiskers and the effective mitigation strategies necessary to reduce whisker growth risks This book covers key tin whisker topics, ranging from fundamental science to practical mitigation strategies. The text begins with a review of the characteristic properties of local microstructures around whisker and hillock grains to identify why these particular grains and locations become predisposed to forming whiskers and hillocks. The book discusses the basic properties of tin-based alloy finishes and the effects of various alloying elements on whisker formation, with a focus on potential mechanisms for whisker suppression or enhancement for each element. Tin whisker risk mitigation strategies for each tier of the supply chain for high reliability electronic systems are also described. * Discusses whisker formation factors including surface grain geometry, crystallographic orientation-dependent surface grain boundary structure, and the localization of elastic strain/strain energy density distribution * Examines how whiskers and hillocks evolve in time through real-time studies of whisker growth with the scanning electron microscope/focused ion beaming milling (SEM/FIB) * Covers characterization methods of tin and tin-based alloy finishes such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) * Reviews theories of mechanically-induced tin whiskers with case studies using pure tin and other lead-free finishes shown to evaluate the pressure-induced tin whiskers Mitigating Tin Whisker Risks: Theory and Practice is intended for the broader electronic packaging and manufacturing community including: manufacturing engineers, packaging development engineers, as well as engineers and researchers in high reliability industries.

Leseprobe

Table of Contents

Cover

Table of Contents

IEEE Press Editorial Board

Title page

Copyright page

TECHNICAL REVIEWERS

PREFACE

INTRODUCTION

CONTRIBUTORS

1 REGULATORY AND VOLUNTARY DRIVERS FOR ENVIRONMENTAL IMPROVEMENT: HAZARDOUS SUBSTANCES, LIFE-CYCLE DESIGN, AND END OF LIFE

1.1 INTRODUCTION

1.2 SUBSTANCES OF ENVIRONMENTAL CONCERN

1.3 DESIGN FOR ENVIRONMENT/ENERGY EFFICIENCY

1.4 RECYCLING AND TAKE-BACK

1.5 SUMMARY

2 LEAD-FREE SURFACE MOUNT TECHNOLOGY

2.1 INTRODUCTION

2.2 NO-CLEAN AND WATER-SOLUBLE LEAD-FREE PASTES

2.3 SOLDER PASTE HANDLING

2.4 BOARD AND STENCIL DESIGN

2.5 SCREEN PRINTING AND PRINTABILITY OF LEAD-FREE SOLDER PASTES

2.6 PASTE INSPECTION

2.7 COMPONENT PLACEMENT (PASTE TACKINESS)

2.8 REFLOW SOLDERING AND THE REFLOW PROFILE

2.9 EFFECT OF NITROGEN VERSUS AIR ATMOSPHERE DURING LEAD-FREE REFLOW

2.10 HEAD-IN-PILLOW COMPONENT SOLDERING DEFECT

2.11 VISUAL INSPECTION OF SOLDER JOINT

2.12 AUTOMATED OPTICAL INSPECTION (AOI)

2.13 X-RAY INSPECTION

2.14 ICT/FUNCTIONAL TESTING

2.15 CONCLUSIONS

2.16 FUTURE WORK

ACKNOWLEDGMENTS

3 LEAD-FREE WAVE SOLDERING

3.1 WAVE-SOLDERING PROCESS BOUNDARIES

3.2 SOLDERING TEMPERATURES ON THE CHIP AND MAIN SOLDERING WAVES

3.3 ALLOYS FOR LEAD-FREE WAVE SOLDERING

3.4 FUNCTION OF NITROGEN IN WAVE SOLDERING

3.5 EFFECT OF PCB DESIGN ON WAVE SOLDER JOINT FORMATION

3.6 STANDARDS RELATED TO WAVE SOLDERING

3.7 CONCLUSIONS

3.8 FUTURE WORK

ACKNOWLEDGMENTS

4 LEAD-FREE REWORK

4.1 INTRODUCTION

4.2 SURFACE MOUNT TECHNOLOGY (SMT) HAND SOLDERING/TOUCH-UP

4.3 BGA/CSP REWORK

4.4 BGA SOCKET REWORK

4.5 X-RAYING

4.6 THROUGH-HOLE HAND-SOLDERING REWORK

4.7 THROUGH-HOLE MINI-POT/SOLDER FOUNTAIN REWORK

4.8 BEST PRACTICES AND REWORK EQUIPMENT CALIBRATIONS

4.9 CONCLUSIONS

4.10 FUTURE WORK

5 LEAD-FREE ALLOYS FOR BGA/CSP COMPONENTS

5.1 INTRODUCTION

5.2 OVERVIEW OF NEW LEAD-FREE ALLOYS

5.3 BENEFITS OF NEW ALLOYS FOR BGAS AND CSPS

5.4 TECHNICAL CONCERNS

5.5 MANAGEMENT OF NEW ALLOYS

5.6 FUTURE WORK

5.7 SUMMARY AND CONCLUSIONS

ACKNOWLEDGMENTS

6 GROWTH MECHANISMS AND MITIGATION STRATEGIES OF TIN WHISKER GROWTH

6.1 INTRODUCTION

6.2 ROLE OF STRESS IN WHISKER GROWTH

6.3 UNDERSTANDING STANDARD ACCELERATION TESTS

6.4 PLATING PROCESS OPTIMIZATION AND OTHER MITIGATION STRATEGIES

6.5 WHISKER GROWTH ON BOARD-MOUNTED COMPONENTS

6.6 SUMMARY

7 TESTABILITY OF LEAD-FREE PRINTED CIRCUIT ASSEMBLIES

7.1 INTRODUCTION

7.2 CONTACT REPEATABILITY OF LEAD-FREE BOARDS

7.3 PROBE WEAR AND CONTAMINATION

7.4 BOARD FLEXURE

7.5 CONCLUSIONS

ACKNOWLEDGMENTS

8 BOARD-LEVEL SOLDER JOINT RELIABILITY OF HIGH-PERFORMANCE COMPUTERS UNDER MECHANICAL LOADING

8.1 INTRODUCTION

8.2 ESTABLISHING PWB STRAIN LIMITS FOR MANUFACTURING

8.3 SMT COMPONENT FRACTURE STRENGTH CHARACTERIZATION

8.4 PWB FRACTURE STRENGTH CHARACTERIZATION

8.5 PWB STRAIN CHARACTERIZATION

8.6 SOLDER JOINT FRACTURE PREDICTION—MODELING

8.7 FRACTURE STRENGTH OPTIMIZATION

8.8 CONCLUSIONS

ACKNOWLEDGMENTS

NOTE

9 LEAD-FREE RELIABILITY IN AEROSPACE/MILITARY ENVIRONMENTS

9.1 INTRODUCTION

9.2 AEROSPACE/MILITARY CONSORTIA

9.3 LEAD-FREE CONTROL PLANS FOR AEROSPACE/MILITARY ELECTRONICS

9.4 AEROSPACE/MILITARY LEAD-FREE RELIABILITY CONCERNS

9.5 SUMMARY AND CONCLUSIONS

10 LEAD-FREE RELIABILITY IN AUTOMOTIVE ENVIRONMENTS

10.1 INTRODUCTION TO ELECTRONICS IN AUTOMOTIVE ENVIRONMENTS

10.2 PERFORMANCE RISKS AND ISSUES

10.3 LEGISLATION DRIVING LEAD-FREE AUTOMOTIVE ELECTRONICS

10.4 RELIABILITY REQUIREMENTS FOR AUTOMOTIVE ENVIRONMENTS

10.5 FAILURE MODES OF LEAD-FREE JOINTS

10.6 IMPACT TO LEAD-FREE COMPONENT PROCUREMENT AND MANAGEMENT

10.7 CHANGE VERSUS RISKS

10.8 SUMMARY AND CONCLUSIONS

Index

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IEEE Press Editorial Board

Lajos Hanzo, Editor in Chief

R. AbariM. El-HawaryS. NahavandiJ. AndersonB. M. HammerliW. ReeveF. CanaveroM. LanzerottiT. SamadT. G. CrodaO. MalikG. Zobrist

Kenneth Moore, Director of IEEE Book and Information Services (BIS)

Copyright © 2011 by Institute of Electrical and Electronics Engineers. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey.

Published simultaneously in Canada.

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Library of Congress Cataloging-in-Publication Data:

Henshall, Gregory.

Lead-free solder process development / Gregory Henshall, Jasbir Bath, Carol Handwerker.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-470-41074-5 (cloth); ISBN 978-1-118-10274-9 (ePub)

 1. Lead-free electronics manufacturing processes. 2. Solder and soldering. I. Henshall, Gregory Arthur. II. Handwerker, Carol A. III. Title.

TK7836.B38 2010

621.9'77—dc22

2010028407

TECHNICAL REVIEWERS

The editors would like to acknowledge the following persons who helped to review chapters in this book.

Jennifer Shepherd, Canyon Snow Consulting LLC

Quyen Chu, Jabil

Doug Watson, Flextronics International

Craig Hamilton, Heather McCormick, Joel Trudell, Zenaida Valianu, Celestica

Richard Coyle, Alcatel-Lucent

Linda Woody, Lockheed Martin

Alan McAllister, Intel Corporation

Andy Ganster, Crane Division, Naval Surface Warfare Center (NSWC)

Mark Fulcher, Continental Automotive

PREFACE

The transition to lead-free soldering in electronics started its first main push in the late 1990s in Japan, with accelerated efforts to convert lead-free consumer products that were subject to RoHS legislation of the European Union in the two to three years before 2006. This lead to major evaluations of component temperature requirements, lead-free solder process development, and reliability evaluations, and thus required a significant amount of company resources.

The change to lead-free soldering for consumer products also affected other segments of the industry, such as IT infrastructure, telecom, military and automotive that were not directly subject to the EU RoHS legislation because most of the electronics supply chain (consisting of component, soldering material, and board suppliers) had started to move to lead-free technology.

This supply-chain transition, along with anticipated changes in future regulations, is forcing the telecom, enterprise server, and military/ defense segments of the electronics industry to conduct evaluations on lead-free technology. For example, the anticipated shortage of tin-lead component and board materials has led to intensive investigation of the processes and reliability impacts of “mixed” lead-free/tin-lead interconnect solutions.

Based on the evolution of lead-free technology over the past few years, there is a need to provide an update on the concerns and developments in these high-reliability areas as well as a review of more recent developments in lead-free process assembly and reliability. Many of the authors contributing to this book are involved in these high-reliability segments and have concentrated their efforts on addressing the continuing challenges associated with lead-free electronics. Their contributions make this book a useful resource to those transitioning to lead-free soldering as well as to a wider audience.

The book covers a list of key topics, including legislation, SMT, wave, rework, alloy development, component and solder joint reliability. Each subject area is discussed by those who have conducted work in the field and can provide insight into what are the most important areas to consider. The book gives updates in areas for which research is ongoing, and addresses new topics that are relevant to lead-free soldering. A practicing engineer will find the book of use because it goes into these (and other) topics in sufficient detail to provide a practical guide to address issues of concern in lead-free technology.

As already mentioned, the book is particularly timely as companies that include the IT infrastructure, telecom, defense, and aerospace industries are moving to lead-free implementation. Some of the authors are seeing lead-free issues that have not been reported widely before, so the book provides up-to-date assessments of these issues.

INTRODUCTION

In the past few years major regulatory changes have spurred the development of lead-free soldering. The book covers the evolution of lead-free soldering technology in key areas written by researchers specializing in these areas. The chapters also address issues of concern in lead-free solder technology. The topics listed below give an overview of the issues related to the areas of interest.

Environmental regulatory and voluntary efforts in electronics products

Challenges of lead-free surface mount technology, wave-soldering and rework

Research results on various lead-free alloys for BGA/CSP components

Tin whisker growth and mitigation strategies

Testability of lead-free soldered printed circuit assemblies

Board-level solder joint reliability under mechanical loading

Reliability of lead-free electronics in aerospace and military environments

Reliability of lead-free electronics in automotive environments

The legislation chapter (Chapter 1) looks at some of the regulatory trends and voluntary efforts that are affecting the electronics industry worldwide in the move to make product designs more environmentally friendly. The challenge facing the industry is to ensure that product design, manufacture and end-of-life activities are environmentally sound. The chapter covers substances of environmental concern with a review of EU-RoHS, EU REACH, and other areas that include China-RoHS and Korea-RoHS. It also covers the areas of Design for Environment, Energy Efficiency, Recycling and Take-Back.

Lead-free surface mount technology (SMT) development has been fairly rapid over the last few years with improvements being made in soldering materials, processes and inspection techniques. Chapter 2 reviews the development and testing of halogen-free lead-free solder pastes, solder paste handling techniques, and board and stencil design. It discusses solder paste printing parameters as well as paste inspection techniques. Solder paste reflow profiles are assessed by comparing tin-lead with lead-free with a discussion on using air versus a nitrogen reflow atmosphere. The issues discussed related to the reflow process include the head-in-pillow component soldering defect. Solder joint inspection techniques are also reviewed along with test techniques for the assembled product board.

Various developments in lead-free wave-soldering optimization (Chapter 3) have proved to achieve good wave-soldering results. Areas discussed include wave solder machine conveyor speed, solder wave contact length, flux chemistry and flux application, preheating, solder temperature, solder alloy, and the use of nitrogen. The effect of PCB design on wave soldering is also reviewed with a discussion of optimization that can help reduce solder bridging, solder opens, and solder balls. A review is provided of evaluations used to improve lead-free wave hole-fill especially on thicker product boards. Standards related to lead-free wave soldering are discussed as well.

Various developments in lead-free rework (Chapter 4) from passive and lead-frame component rework to BGA/BGA socket rework are discussed, including through-hole hand and solder fountain rework. Some other areas covered are ECO wiring, solder pad repairs, and temperatures and times used during the lead-free rework processes.

In Chapter 5, the impact of increasing lead-free alloy choice for BGA and CSP components is explored. The drivers to replace the first-generation lead-free alloys (near-eutectic SnAgCu alloys like Sn4Ag0.5Cu and Sn3Ag0.5Cu) are first presented. Chief among these is the improvement in mechanical shock resistance. Next the concerns and risks related to these new alloys, most of which are low-silver SnAgCu, are discussed. These include risks in the reflow process in 100% lead-free and “mixed” lead-free/tin-lead solders, potentially poor thermal fatigue resistance, and a lack of established properties. The chapter concludes with a description of recent industry activity that addresses these concerns, namely by establishing new alloy test standards, and of iNEMI efforts to determine the thermal fatigue properties of the new alloys.

The current understanding of the driving forces and the kinetics of tin whisker growth is described in Chapter 6 in the context of industry standard acceleration tests. The stress reversals and the specific morphologies associated with air-to-air thermal cycling are covered and related to tin whisker formation at ambient and higher humidity, higher temperature isothermal anneals. Whisker growth on board-mounted components after reflow is analyzed in detail along with some of the mitigation techniques to reduce the risk of whisker growth for electronic systems. This chapter shows how the driving force changes with testing conditions and the processing of the surface finish or the solder joint contribute to the tin whisker growth mechanism as well as the relationship between changing temperature and humidity conditions, seen in the field and the industry standard acceleration tests.

The challenges in manufacturing test of lead-free PCAs are considered in Chapter 7. Three areas of concern are reviewed: contact repeatability, wear and contamination of ICT probes, and the impact of transient bend flexure on lead-free PCAs. The chapter shows that because of the increased levels of flux residues remaining on test targets after the lead-free manufacturing process, test probes have a difficult time penetrating and making electrical contact. Improved probe tip geometries, among other solutions, can enable better performance in ICT. Unfortunately, the life of test probes is significantly reduced for lead-free PCAs compared to tin-lead due to the higher levels of contaminants, as well as the high yield strength and stiffness of the SnAgCu solders. Overflexure of the PCA is also considered, since high probe forces can cause partial cracking of the second-level interconnect. Manufacturers and component suppliers will be able to use standards such as IPC/JEDEC 9707 (when published) to determine a maximum strain level for components or PCAs that can be used as guidelines in manufacturing, test, and assembly.

The integrity of solder joints under mechanical loading, as can occur during PCA manufacture or in service, is critical to high-performance computer manufacturers (and to manufacturers of other electronic products). In Chapter 8, the author reviews the issues and solutions relevant to the various mechanical reliability concerns. The setting of appropriate strain limits during manufacturing, and how to measure them, are described. The characterization of fracture strength through a variety of tests, modeling, and fracture strength optimization are described in sufficient detail to provide a good guide to practitioners in this field.

Although aerospace and military electronics are currently exempt from legislation requiring lead-free assemblies, their components, boards, and materials are obtained through the supply chain for commercial electronics. Chapter 9 presents the current understanding of the performance of lead-free components, boards, materials, and assemblies in harsh military and aerospace environments. The military/aerospace industry and university research consortia are described along with their efforts to understand the risks associated with using lead-free solders in these environments and the standards that have been developed or are being developed to assess the performance of lead-free rather than tin-lead in military/aerospace systems. The specific concerns for using lead-free assemblies in military/aerospace systems are presented in the context of the results from the JCAA/J-GPP Lead-Free Solder Project. Based on these concerns, the chapter offers mitigation measures that can minimize the effects of shock and vibration on solder joints and control whisker growth on tin-plated components.

The challenges of manufacturing reliable, lead-free electronics for automobiles are described in Chapter 10. Automotive electronics have high-reliability requirements, long design lives, and operate under harsh conditions of temperature and vibration compared to most consumer and IT electronics. At the same time automotive manufacturers are now driven by legislation that is pushing to set the timing for products being lead-free. The chapter discusses risk management as the overriding factor in the transitioning of automotive electronics to a lead-free design. Managing the transition effectively will require discipline in the procurement portion of the business, with component database management, tracking, and error proofing. Further the design of new electronics will need to be based on new data for lead-free components and circuit board layout requirements. Finally, testing all aspects of an electronic assembly with new lead-free components will be needed to ensure a successful transition.

The authors hope that this book will provide readers with useful information from their experiences in investigating and implementing lead-free soldering.

CONTRIBUTORS

Aileen M. Allen, Hewlett-Packard Company, Palo Alto, California

Denis Barbini, Vitronics-Soltec, Stratham, New Hampshire

Jasbir Bath, Bath Technical Consultancy LLC, Fremont, California

Alan Donaldson, Intel Corporation, Hillsboro, Oregon

Carol A. Handwerker, Purdue University, West Lafayette, Indiana

John Hawley, Palm, Sunnyvale, California

Gregory Henshall, Hewlett Packard Company, Palo Alto, California

Keith Newman, Sun Microsystems, Sunnyvale, California

Jennifer Nguyen, Flextronics International, Milpitas, California

Richard D. Parker, Delphi Electronics and Safety, Kokomo, Indiana

Rosa D. Reinosa, Hewlett-Packard Company, Palo Alto, California

Sundar Sethuraman, Jabil, San Jose, California

Peng Su, Cisco Systems, San Jose, California

Thomas A. Woodrow, The Boeing Company, Seattle, Washington

1

REGULATORY AND VOLUNTARY DRIVERS FOR ENVIRONMENTAL IMPROVEMENT: HAZARDOUS SUBSTANCES, LIFE-CYCLE DESIGN, AND END OF LIFE

John Hawley

(Palm)

1.1 INTRODUCTION

The market for environmentally friendly electronic products is growing rapidly. Growing just as rapidly is the responsibility companies are assuming or are being compelled to assume for products through their entire life cycle and aftermath, including end of life, product recycling, and product take-back. Energy efficiency of electronic products has also become increasingly important because of their profusion and the associated load they impose on national electrical grids.

In the past the electronics industry did not consider the environmental effects of its products through their life cycles. Its primary concerns centered on how manufacturing processes or facilities infrastructure might impact the immediate environment. It also considered hazardous substances used in manufacturing processes that could have detrimental effects in the event of human contact or exposure. One example of the former consideration was the transition to volatile organic compound–free (VOC-free) processes. Freon, a type of chlorofluorocarbon (CFC), for instance, was used to clean electronics, even though it was known to be a highly ozone-depleting chemical. The electronics industry responded by developing “water-clean” or “no-clean” processes that eliminated the need for CFCs. The challenge now facing the industry is to ensure that product design, manufacture, and end-of-life activities are equally environmentally sound.

This chapter examines some of the regulatory trends and voluntary efforts that are transforming the electronics industry worldwide in the drive to design more environmentally friendly products.

1.2 SUBSTANCES OF ENVIRONMENTAL CONCERN

For the last ten years there has been a concentrated effort to address the problem of potentially hazardous substances found in electronics products. Many parties have been involved, including governments, electronics producers, universities, and nongovernmental organizations. Providing impetus to the effort are environmental and public health issues. There are concerns, for instance, about improper disposal of electronic waste containing potentially hazardous substances. In some third world countries low-temperature burning of electronic parts in open pits for metal recovery has had seriously deleterious effects on both the health of the workers and their environment [1]. In developed, as well as developing, countries, leaching of heavy metals such as cadmium, hexavalent chromium, lead, and mercury from landfills containing electronics into the groundwater has created public health concerns. One well-known example is a case involving hexavalent chromium. An electric utility in California had used hexavalent chromium to mitigate corrosion in a cooling tower in the town of Hinkley between 1952 and 1966. The wastewater slowly dissolved the hexavalent chromium and discharged it into unlined ponds. Some of this material leached into the groundwater and eventually entered the town’s drinking water. Over time the contamination resulted in serious health problems [2]. Another well-known case is that of Minamata, Japan, where a chemical company dumped mercury compounds directly into the bay between 1932 and 1968. Three thousand people developed very serious health issues and many died [3]. A third example is in Silicon Valley, where the US EPA (Environmental Protection Agency) Superfund sites were required to clean up groundwater contamination from chemicals linked to birth defects, such as trichloroethane and Freon, from certain semiconductor processing facilities [4].