Rapid Solidification of Sn-Cu-Al Alloys for High-Reliability, Lead-Free Solder: Part II. Intermetallic Coarsening Behavior of Rapidly Solidified Solders After Multiple Reflows

Controlling the size, dispersion, and stability of intermetallic compounds in lead-free solder alloys is vital to creating reliable solder joints regardless of how many times the solder joints are melted and resolidified (reflowed) during circuit board assembly. In this article, the coarsening behavior of Cu _x Al _y and Cu₆Sn₅ in two Sn-Cu-Al alloys, a Sn-2.59Cu-0.43Al at. pct alloy produced via drip atomization and a Sn-5.39Cu-1.69Al at. pct alloy produced via melt spinning at a 5-m/s wheel speed, was characterized after multiple (1-5) reflow cycles via differential scanning calorimetry between the temperatures of 293 K and 523 K (20 °C and 250 °C). Little-to-no coarsening of the Cu _x Al _y particles was observed for either composition; however, clustering of Cu _x Al _y particles was observed. For Cu₆Sn₅ particle growth, a bimodal size distribution was observed for the drip atomized alloy, with large, faceted growth of Cu₆Sn₅ observed, while in the melt spun alloy, Cu₆Sn₅ particles displayed no significant increase in the average particle size, with irregularly shaped, nonfaceted Cu₆Sn₅ particles observed after reflow, which is consistent with shapes observed in the as-solidified alloys. The link between original alloy composition, reflow undercooling, and subsequent intermetallic coarsening behavior was discussed by using calculated solidification paths. The reflowed microstructures suggested that the heteroepitaxial relationship previously observed between the Cu _x Al _y and the Cu₆Sn₅ was maintained for both alloys.