A small amount of Zn addition was found to be beneficial in depressing the growth of intermetallic compound (IMC) layer at Sn–3.5wt% Ag/Cu interface as aged isothermally. The results show that Cu 6 Sn 5 was still the original interfacial product, regardless of the concentration of Zn addition in the eutectic solder. While the formation of Cu 3 Sn was retarded as aged at 150°C, instead, a continuous or non-continuous Cu 5 Zn 8 IMC layer, which was determined by the Zn concentration, was formed. Since the diffusion rate of Cu in Cu 5 Zn 8 was much lower than that in Cu 6 Sn 5 , the growth rate of IMC layer at the interface was slowed down. Furthermore, the evolution and morphology of the IMC layer were correlated with the concentration of Zn in the solder.