The Pb-free Sn-Zn-Al solder was dip coated on Cu contact to investigate the interaction between them under aging at 150°C for up to 1000 hours. The results of EMPA analysis indicate that Al gathers at the Cu/solder interface and inhibits the interdiffusion between Cu and Sn. This solder itself provides diffusion barrier for Cu contact and thus is termed Inherent Barrier Solder. Cu was found to be able to diffuse to solder at long term heat treatment but not Sn. Nevertheless, Cu was not found to form any compund with Sn in this study. The as dipped solder reacts with Cu to form Al4.2Cu3.2Zn0.7 compound. The high temperature heat treatment transforms the compound to Cu5Zn8, Al4Cu9 in heating duration up to 400 hours. The final product is Cu5Zn8 after 600 hours of aging.