Dr. Jennie Hwang’s magisterial volume on lead-free manufacturing will certainly not be the last word on this high-interest subject; but it is likely to be the most words. In over 900 pages, Dr. Hwang comprehensively covers every aspect of the subject, from the business, technical, and legal underpinnings, to the multitude of possible solutions and the myriad of unintended consequences, whose social, political, and economic costs have yet to be estimated.

The book might best be reviewed in four sequential parts: the introduction (why are we doing this?), the recipes (how might we do this?), the results (what will happen when we do this?), and the recommendations (what should we do?).

The four introductory chapters set the scene, covering industry trends, legislative obstacles, and the fundamentals and methodologies of lead-free technology. A key point drawn from the industry trends is that, separately from any health issues, the market-driven industry move to increase the complexity and density of electronic circuitry heightens the demands made on solder joint performance. As the later text shows, some lead-free solders may improve that performance.

The legislative chapter quotes enough turgid legalese to show how the various political fiefdoms have created a tangle of conflicting resolutions, requirements, and timetables, depending on the masters and the motives that they serve. Industry observers and experts have attempted to add some flavor of reality to the bureaucratic stew, with limited results. The argument that consumers are making their choices based upon environmental concerns appears to have a strong counter-example in the ascendancy of the environmentally destructive "sport-utility" vehicle in the United States.

The chapter on the fundamentals of lead-free technology is an excellent introduction to the subject. The foundation is a detailing of both the strengths and the limitations of lead-based solders. Based upon these, materials science and practical considerations dictate that the replacement system should be tin-based.

Lead is the Achilles’ heel of eutectic lead-tin solder, with lead-coarsening a root cause of joint cracking. Replacing the lead in a lead-tin system could improve the mechanical strength and the ability of the joint to meet the increased performance demands. Hundreds of tin-based solder formulations are possible, but not all are practical or desirable. Their performance ultimately depends on maintaining, in Dr. Hwang’s words, " a stunningly intricate balance" of the constituents.

The second part of the book presents a wide variety of those tin-based formulations, and details both the intricacy of the balance, and the trade-offs that are required in choosing among them. Thirteen chapters cover more than a hundred formulations, including information on their phase transition temperatures, stress versus strain characteristics, wetting and solderability, alloy behavior, and susceptibility to low-cycle fatigue. Six binary, three ternary, five quaternary, and six pentanary systems are detailed, with multiple variations. Other ternary and quaternary systems are summarized, and a separate chapter is devoted to eight candidate systems for high or low temperature lead-free solder.

I confess that I did not read through all of this part of the book, which could be a mind-numbing experience for anyone but a metallurgist. However, the convenient taxonomy and the handbook approach make this part a most valuable resource. When a consulting client emailed to me a question about an exotic solder formulation, I was able to respond with an intelligent answer within minutes.

The third part of the book is both informative and interesting. Obviously, solder is but one component of a complete interconnection system. A change in solder may (and likely will) require changes in many other parts of the system. To address all of these related questions and issues, Dr. Hwang has brought in a blue-ribbon group of experts, who share their expertise with us in the next ten chapters. The complexities of her "intricate balance" are well substantiated here.

As in any new system, reliability is always a primary concern. The reliability chapter raises a key issue. The most important reliability question for lead-free solders is their thermomechanical fatigue performance. The fatigue performance of eutectic tin-lead solders has been well characterized in many applications, with well-understood failure modes. The failure modes of many lead-free solders are as yet poorly understood, impeding the development of a trustworthy reliability model. Simple extrapolations from properties to performance often have no basis, and may not be supported by later data. Clearly, establishing satisfactory reliability models remains a major task for lead-free implementation.

The chapter on component leads (the connecting structures, not the element Pb) and printed circuit board surface finishes is an example of the inter-related complexities of the intricate balance. Present lead and lead-frame coatings include Sn/Pb, palladium (Ni/Pd, Ni/Pd/Au), and other materials. Each coating must be examined for solderability in the new system. The common Pb/Sn coating also raises concerns of Pb contaminating the lead-free joint, a topic addressed in a later chapter.

Similarly detailed chapters examine reflow compatibility (including fluxes), intermetallics and solder microstructure, and conductive adhesives. Chapters covering potential problems such as tin whiskers, lead (Pb) contamination, and bismuth-related fillet lifting report some experimental results. A Design-of-Experiments (DOE) chapter gives preliminary results, based upon using two solder formulations, applied to differing surface finishes, with differing reflow profiles and environments.

After disposing of, or at least disabling, all of these frightening monsters, we reach what might appear to be the safe haven of the penultimate chapter, offering a selection of industry perspectives and views on lead-free assembly. However, even here the views are sometimes disturbing. They range from corporate puffery to thoughtful and sobering comments, both on the task ahead, and on its supporting premises.

Dr. Hwang’s final chapter is a lifesaver to anyone drowning in data. She applies her solder expertise to filter for us the preceding deluge of data, choices, problems, and concerns. She narrows the solder choices to six candidate systems, selects optimal compositions in each system, compares them to each other and to conventional solders, and gives us her final ranking of them. Telling you her answer here would spoil the suspense; this chapter alone is worth the price of the book!

I recommend this book highly and without reservation, especially for all of the thousands of us whose professional careers and future livelihoods may depend on understanding and implementing lead-free technology. This is the place to start!

REVIEWED BY:
Dr. George Riley, FlipChips Dot Com
Copyright FlipChips Dot Com, 2004
All rights reserved.

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