Effect of cooling rate on microstructure of Ag–Cu–Sn solder alloys

Abstract

The effect of cooling rate on the formation of Sn dendrites during freezing was investigated for Pb free solder alloys having compositions near the Sn rich ternary eutectic of the Ag–Cu–Sn system. For Ag and Cu concentrations less than eutectic, the volume fraction of Sn dendrites tended to increase with cooling rate, while for Ag and Cu concentrations greater than eutectic a decrease was observed. For an alloy having the composition 3.8 wt-%Ag, 0.7 wt-%Cu and 95.5 wt-%Sn, the volume fraction of Sn dendrites was found to vary from ∼5 vol.-% at a cooling rate of 1 K s⁻¹ to 65 vol.-% at 100 K s^&minus1, resulting in a corresponding enrichment of Ag and Cu in the eutectic regions, in the form of increased volume fractions of Ag₃Sn and Cu₆Sn₅. The surface contour of frozen samples was observed to depend on the scale of interdendritic shrinkage during freezing, and was especially smooth for eutectic freezing.