Growth mechanism of bulk Ag3Sn intermetallic compounds in Sn-Ag solder during solidification

Thermal analysis was used to clarify the growth mechanism of bulk Ag3Sn IMCs in Sn-Ag lead-free solders and compared with Pandat software calculations and theoretical calculations. In slowly-cooled Sn-Ag lead-free solders, bulk Ag3Sn IMCs formed when Ag3Sn crystal nuclei formed at the onset of the eutectic reaction. The fractions of bulk Ag3Sn IMCs in hypereutectic Sn-4.4 mol%Ag solders, measured by thermal analysis, are larger than those predicted by the equilibrium phase diagram and software calculations. The reasons for this could be attributed to eutectic Ag3Sn phases clinging to the primary Ag3Sn crystals during the eutectic reaction with their matching crystalline orientation relationship. The driving force for the growth of the faceted bulk Ag3Sn IMCs phase is proportional to the degree of undercooling achieved, which fits with the prediction of classical solidification theory. The result explains the formation and rapid growth of bulk Ag3Sn IMCs in slowly-cooled eutectic Sn-Ag solder with a minimal degree of undercooling during solidification.