Enhanced growth of the Cu6Sn5 phase in the Sn/Ag/Cu and Sn/Cu multilayers subjected to applied strain

Thin Ag and Sn layers were deposited on flexible copper clad laminates (FCCLs) to form two types of multilayers, Sn/Ag/Cu and Sn/Cu. The Sn/Ag/Cu and Sn/Cu interfacial reactions subjected to applied strains were investigated by bending the FCCL and then placing the samples in an oven at 200 °C. Experimental results indicated that the growth rate of the Cu6Sn5 phase formed on the FCCL subjected to applied strain was enhanced regardless of the strain type, while the enhanced effect from compressive strain was more significant. Focused-ion-beam analysis indicated that the Cu6Sn5 phase formed on the FCCL subjected to compressive strain exhibited a multi-grain structure with smaller grain size, suggesting that the grain growth in the Cu6Sn5 phase was significantly retarded under compression. This multi-grain structure, however, provided high grain boundary density for atomic diffusion and thereby enhanced the growth rate of the Cu6Sn5 phase.