Environment-friendly Electronics: Lead-free Technology
Environment-friendly Electronics: Lead-free Technology
Jennie S. Hwang Electrochemical PublicationsLimited, IOMPages 879 +xxxiiISBN 0 901150 40 1Price £ 119; US$ 238 + p&pURL: http://www.elchempub.com
The biggest negative criticism I must make about this book is its title,which is somewhat misleading. There is no in-depth treatment of the environmental consequences surrounding the use of lead in electronics, only a repetition of the eco-political regulations, which are often divorced from the real issues of lead in the environment. In addition, there is no discussion on the use of lead, or its elimination, in any part of electronics other than the small proportion used in solder. Nor is the notion of recycling discussed in relation to either lead-containing or lead-free solders. This criticism apart,this book is an excellent and monumental dissertation, essentially on the metallurgy of lead- free solders, and a worthy complement, in its own rights, to Klein-Wassink's Soldering in Electronics.
Physically, this book is the second fattest in the Electrochemical Publications' range, beaten only by a short head by the second volume of William Macleod Ross' A Comprehensive Guide to the Design and Manufacture of Printed Board Assemblies. It is produced in the same style and quality to which we are accustomed. The typesetting seems, subjectively, to be slightly less consistent than usual, in that the line- and letter- spacings appear to be variable from chapter to chapter. The majority of the chapters are written by the author, but there are also a few from other sources. Perhaps this provides a good compromise between the single-author work and the edited compiled work. It has not, however, totally eliminated the repetitions and contradictions that sometimes occur in the latter style. For example, Figures 21.8, 23.6 and 28.7 are all an identical microsection of solder fillet lifting.
On the assumption that lead-free solder in electronics, if not yet a fait accompli, will become so within the decade, the majority of us within the industry are at the start of a difficult learning curve, especially with respect to choosing the right alloy for a given application. Where near-eutectic tin/lead is an ideal compromise for most soldering work, there is no single substitute that can be dropped in to replace it for all jobs. This is where this book is both ideal and timely. It discusses most, if not all, of the candidates from binary to pentanary (sic! Wouldn't quinternary be better and more logical?) and higher alloys and all their technical characteristics that may have an influence on their efficacy as a chosen substitute. Their secondary characteristics, such as the cost and availability of the components of each alloy, are not discussed in much detail, just a little bit at the end. This is important when we come to alloys containing elements with market prices in the order of hundreds of dollars per kilogram, such as gallium or indium. Dr Hwang puts quite a lot of emphasis on alloys containing bismuth, paradoxically ignoring the fact that much of this metal is produced as a by-product of lead mining and smelting. The chemical side of fluxing and cleaning is not discussed in any great detail.
The first three chapters constitute an introduction to the book, including such matters as industry trends and legislation. The fourth chapter is devoted to the basics of the technology. The fifth chapter is remarkable as listing all the existing patents on lead-free solder alloys, covering 51 different ternary to sexternary systems. Chapter 6 covers generally all the major binary systems. In this chapter, and the following ones, all the tables of characteristics of the alloys include a comparison row of 63Sn37Pb, so it is easy to discern the major differences between the “new” alloys and our old faithful one.
Chapters 7 to 14 each discuss a single ternary or quaternary system, covering the most common candidates, whereas the two following chapters discuss the more exotic ternary and quaternary ones, respectively. Chapter 17 is devoted to higher systems. Up to now, the emphasis has been on substitutes for ordinary solder, whereas chapter 18 discourses on lead-free low- and high-temperature alloys.
The nineteenth chapter touches a more 'hands- on' approach, because its subject matter is the finishes required for component leads and printed circuits to maintain both lead-free technology and the ability to solder the components onto the boards to produce a reliable joint. Closely related to this subject is the formation of intermetallic compounds, which is handled in the following chapter, written by experts from the Sandia National Laboratory. Two authors from the Indium Corporation of America discuss the compatibility of lead- free solders with the reflow process in chapter 21. They developed an empirical eqsfor determining the compatibility based on wetting, solder balling, tack time, shelf life and solder appearance. The weightings may be somewhat subjective, but the results are quite interesting. Tables 21.9 to 21.11 summarise the results for 2 profiles, 11 alloys and 10 flux types (developed for tin/lead solders), a remarkable experimental matrix. This may be useful for the beginner to shortlist a number of candidates or, at least, to eliminate the less likely ones, in conjunction with the earlier chapters.
Chapter 22 discusses how the structure of tin- silver alloys evolves, bearing in mind the copper of the intermetallic layer. This is quite a long chapter covering both the theoretical and practical aspects of the subject. The following chapter, written by three US authors from diverse organisations,covers the reliability of lead-free solders. Chapter 24 is perhaps a sop to the title of the book, as it treats a non-solder interconnection material,conductive adhesives, and is written by a representative of Cookson Electronics. Although it is a long chapter (74 pages), it obviously cannot be as comprehensive as Liu's Conductive Adhesives for Electronics Packaging. Notwithstanding, this chapter is a very good introduction to the subject and is perhaps complementary to Liu's book as it has a more 'hands-on' approach.
The 25th chapter must not be ignored as it will give invaluable help to anyone wishing to choose a soldering process for his own requirements, by defining the methodology of the experiments necessary in the selection process. As there were so many variables in the experiment design, I was rather surprised that the author, from the University of Massachusetts Lowell, did not suggest a Taguchi matrix for the first selection. Chapter 26 bears the title Application Considerations. This title does not convey much, but the contents are important. Amongst other things, the formation of whiskers, tin pest and the rôle of gold are discussed. Chapter 27 is also important, bearing in mind that some components cannot be obtained with a surface finish of the connections other than tin/lead. What effect does this have on joints made with lead-free solders?With some alloys, its effect proves somewhat negative. Dr Hwang, in chapter 28,discusses in detail the part that bismuth plays in lead-free alloys. This metal has some unusual properties, on its own, but imparts some significant changes to alloys containing it, some of which may be considered as good and some less so. Like so many things, choosing a bismuth alloy is a matter of compromise.
Chapter 29, written by Bob Willis, is a useful English contribution to the book and is a well illustrated guide to defects, which one hopes will never be seen in the practical use of lead-free alloys. Some of these are quite banal,not to be taken in a pejorative sense, but many are less so. Chapter 30 is unusual: it takes the form of a number of views, of the converted (at least, in spirit), of course. Some of them do, nevertheless, express some practical concerns, such as long-term service of assemblies or recycling issues. The last chapter is a series of diverse recommendations and data.
The chapters are concluded by many references and most of them have an additional bibliography. The book itself ends, of course, with an index in the usual Electrochemical style, but this is preceded by lists of the many figures and tables.
Now, for the nitty-gritty: who needs to buy this book? Well, just about everyone who will be concerned with phasing out lead from PCB fabrication and assembly within the next decade. That means just about every factory in our industry should have at least one copy. Even though I, personally, remain to be convinced that there is any need for eliminating lead in solder on environmental grounds, my recommendation, as a pragmatist realising that it will happen, is buy it.
