Rapid Solidification of Sn-Cu-Al Alloys for High-Reliability, Lead-Free Solder: Part I. Microstructural Characterization of Rapidly Solidified Solders

Particles of Cu _x Al _y in Sn-Cu-Al solders have previously been shown to nucleate the Cu₆Sn₅ phase during solidification. In this study, the number and size of Cu₆Sn₅ nucleation sites were controlled through the particle size refinement of Cu _x Al _y via rapid solidification processing and controlled cooling in a differential scanning calorimeter. Cooling rates spanning eight orders of magnitude were used to refine the average Cu _x Al _y and Cu₆Sn₅ particle sizes down to submicron ranges. The average particle sizes, particle size distributions, and morphologies in the microstructures were analyzed as a function of alloy composition and cooling rate. Deep etching of the samples revealed the three-dimensional microstructures and illuminated the epitaxial and morphological relationships between the Cu _x Al _y and Cu₆Sn₅ phases. Transitions in the Cu₆Sn₅ particle morphologies from faceted rods to nonfaceted, equiaxed particles were observed as a function of both cooling rate and composition. Initial solidification cooling rates within the range of 10³ to 10⁴ °C/s were found to be optimal for realizing particle size refinement and maintaining the Cu _x Al _y /Cu₆Sn₅ nucleant relationship. In addition, little evidence of the formation or decomposition of the ternary-β phase in the solidified alloys was noted. Solidification pathways omitting the formation of the ternary-β phase agreed well with observed room temperature microstructures.