Formation of bulk Ag3Sn intermetallic compounds in Sn-Ag lead-free solders in solidification

A series of Sn-Ag solders were prepared by arc melting and their phase evolution was investigated as a function of cooling rates. It was found that bulk Ag3Sn intermetallic compounds (IMCS) separated out only in the slowly cooled Sn-4.0Ag solder. This would be explained by the strong kinetic undercooling, arising from the rapid cooling conditions, which leads to the actual eutectic point shifts in the direction of higher Ag concentration. Thus, the eutectic and hypereutectic alloys experience a metastable hypoeutectic solidification route instead. All formed fractions of bulk Ag3Sn IMCs in solders, measured by quantitative microstructural analysis and thermal analysis, are larger than those predicted by the equilibrium phase diagram. The reasoning for this could be attributed to fine Ag3Sn phases, which cling to the primary Ag3Sn crystal during the eutectic reaction for their matching crystalline orientation relationship. Furthermore, the fraction of bulk Ag3Sn IMCs increases gradually with increasing the cooling rate in the slowly cooled Sn-4.0Ag alloy, which fits with the prediction of eutectic solidification theory: the increase of cooling rate would decrease the surface energy of fine Ag3Sn particles and primary Ag3Sn crystal, and make fine Ag3Sn particles cling to primary Ag3Sn crystal easily to form bulk Ag3Sn IMCs.