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.