Suppression effect of Cu and Ag on Cu3Sn layer in solder joints

Abstract

Cu3Sn intermetallic compound (IMC) layer is usually formed in solder joints. Since the formation of Cu3Sn could induce large volume shrinkage, and further cause a lot of reliability issues, many works focused on suppressing the formation or growth of the Cu3Sn layer. This work explored that Cu and Ag alloying elements also have benefit in suppressing the Cu3Sn growth during isothermal aging stage. The Cu6Sn5 IMC layer seems to be much stable in the Sn/Cu solder joint during aged at 150 and 180 °C, its thickness changed little, while the Cu3Sn IMC layer grew much quickly. After about 300 h, the thickness of Cu3Sn layer exceeds that of Cu6Sn5 layer. For the Sn-3.5Ag/Cu and Sn-0.7Cu/Cu solder joints, the thickness of Cu3Sn layer is near half of that of Cu6Sn5 layer. According to the relation between interface location and aging time, the reaction generated at the Cu6Sn5/Cu3Sn interface, which is governed by atom fluxes, controls the growth of Cu3Sn IMC layer. Since Ag and Cu alloying elements suppress the coarsening of Cu6Sn5 IMC grains, the diffusion paths for Cu atoms toward the solder are more for Ag or Cu containing solder joints. Therefore, the growth of the Cu3Sn layer by consuming Cu6Sn5 layer is slower in the SnAg/Cu and SnCu/Cu solder joints than that in the Sn/Cu joints.