Growth mechanisms of Cu₃Sn on polycrystalline and single crystalline Cu substrates

A comprehensive transmission electron microscopy (TEM) study was conducted to investigate the growth mechanisms of Cu₃Sn on polycrystalline and single crystalline Cu substrates in solder joints. On single crystalline Cu the solder reflow process resulted in the growth of columnar Cu₃Sn grains aligned in a thin uniform layer perpendicular to the interface, while a thick Cu₃Sn layer formed from fine equiaxed grains on the polycrystalline substrate. In the subsequent solid state aging, columnar growth of Cu₃Sn continued on the single crystalline Cu before it was replaced by nucleation and growth of new triangular Cu₃Sn grains at the triple junction sites of the Cu/Cu₃Sn interface. On the polycrystalline Cu the solid state aging caused much more rapid growth of the Cu₃Sn layer due to nucleation and the growth of new Cu₃Sn grains at both the Cu/Cu₃Sn and Cu₆Sn₅/Cu₃Sn interfaces. These different growth behaviors of Cu₃Sn were related to the diffusive supply of reactive elements.