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

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.