Rapid Solidification of Sn-Cu-Al Alloys for High-Reliability, Lead-Free Solder: Part II. Intermetallic Coarsening Behavi
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ESTABLISHING the effects of CuxAly phases on the nucleation of Cu6Sn5 and b-Sn in Pb-free solder alloys is a subject of continuing interest given the need for Pb-free alloys with better fatigue properties and, hence, with smaller b-Sn grain sizes. Some effects of CuxAly on Cu6Sn5 intermetallic compound (IMC) formation have been well established. Xian et al. determined that a heteroepitaxial relationship exists between Cu6Sn5 and both the Cu33Al17 (d) and the Cu9Al4 (c1) phases.[1] This led to the observation that, even with small additions of KATHLENE N. REEVE, Ph.D. Candidate, and CAROL A. HANDWERKER, Reinhardt Schuhmann Jr. Professor, are with Purdue University, 701 West Stadium Ave., West Lafayette, IN, 47907. Contact e-mail: [email protected] STEPHANIE M. CHOQUETTE, Graduate Student, and IVER E. ANDERSON, Senior Metallurgist, are with Ames Laboratory (USDOE), Iowa State University, Ames, IA, 50011. This manuscript has been authored, in whole or in part, under Contract No. DE-AC02-07CH11358 with the U.S. Department of Energy. The U.S. government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. government purposes. Manuscript submitted March 17, 2016. Article published online October 6, 2016 6526—VOLUME 47A, DECEMBER 2016
Al (0.05 wt pct) to Sn-Cu alloys, and hence a low-volume fraction of CuxAly phase, the number of primary Cu6Sn5 particles after initial solidification increased by a factor of seven, while the length of the extracted Cu6Sn5 particles decreased by a factor of four.[2] After isothermally aging for one week at 523 K (250 C), the Cu33Al17 (d) particles showed little coarsening and segregated to the top of the sample due to IMC’s density relative to liquid-Sn (as shown previously[3]). No results were provided on changes in the Cu6Sn5 particle size or morphology from the isothermal hold.[2] Given the need for multiple solder joint melting and solidification (reflow) cycles during microelectronics manufacturing, microstructural evolution over the course of multiple heating and cooling cycles must be tracked to determine its viability as a commercial alloy. In Part I, it was shown that the size, distribution, and morphology of CuxAly particles could be controlled by rapid solidification processing (RSP) of three different Sn-Al-Cu alloys from homogeneous liquids at 1473 K (1200 C), with the morphologies varying as a function of both composition and cooling rate. In particular, the orientation relationship between CuxAly and Cu6Sn5 (identified by Xian et al. in References 1 and 2) worked in tandem with RSP to reduce the average particle size of both phases into the submicron range.[4] Part II of this study focuses on analysis of the CuxAly and Cu6Sn5 IMC after multiple METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Representative DSC data regarding the melting and solidification of the b-
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