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Alloys that undergo solidifi cation over a wide range of temperatures generally exhibit a difference in the contraction behavior of the ensuing solid and liquid phases. Furthermore, dissolution of substrate metals during process refl ow can lead to shifts in phase composition, additional primary phases, and volumetric contraction artifacts. The extent and frequency of surface roughness, shrinkage voids, fi llet lifting, and hot tearing seen in lead-free solders are different than for eutectic tin lead solder. Shrinkage effects have been reported in Sn/Pb, Sn/Pb/Ag, Sn/Ag/Cu, and Sn/Cu/Ni solders for various components, but few studies have examined their impact on solder joint reliability. Nevertheless, they warrant proper identifi cation due to the shift toward lead-free solders. This article is a review of the effects of shrinkage in Sn-Pb and lead-free solders as well as a discussion of some of the factors that contribute to their formation. Visit the site regularly and click on "What's New" to fi nd out the most up-to-date information on meetings, publications, membership activities, and more. www.tms.org ■ CMS-Plus is available to accept abstracts for the 2006 TMS Annual Meeting. http://cmsplus.tms.org ■ The Members Only site has been updated and reorganized. http://members.tms.org/members/mempage.asp ■ The Metal-Matrix Composites database is now available as a member benefi t. http://members.tms.org/MMCDatabase/MMC.asp

Purpose -Historically, tin-lead solder has been a commonly used joining material in electronics manufacturing. Environmental and health concerns, due to the leaching of lead from landfills into ground water, have necessitated legislation that restricts the use of lead in electronics. The transition from tin-lead solder to a lead-free solder composition is imminent. Several alternative solder alloys (and their fluxes) have been researched for electronics assembly in the last few years. The objective of this research was to develop a systematic selection process for choosing a "preferred" lead-free solder paste, based on its print and reflow performance. Design/methodology/approach -After a detailed study of industry preferences, published experimental data, and recommendations of various industrial consortia, a near eutectic tin-silver-copper (SAC) composition was selected as the preferred alloy for evaluation. Commercially available SAC solder pastes with a no-clean chemistry were extensively investigated in a simulated manufacturing environment. A total of nine SAC pastes from seven manufacturers were evaluated in this investigation. A eutectic Sn/Pb solder paste was used as a baseline for comparison. While selecting the best leadfree paste, it was noted that the selected paste has to perform as good as, if not better than, the current tin-lead paste configuration used in electronics manufacturing for a particular application. The quality of the solder pastes was characterized by a series of analytical and assembly process tests consisting of, but not limited to, a printability test, a solder ball test, a slump test, and post reflow characteristics such as the tendency to form voids, self-centring and wetting ability. Findings -Each paste was evaluated for desirable and undesirable properties. The pastes were then scored relative to each other in each individual test. An aggregate of individual test scores determined the best paste. Originality/value -This paper summarizes a systematic approach adopted to evaluate lead-free solder pastes for extreme reflow profiles expected to be observed in reflow soldering lead-free boards.

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