Interfacial reactions between Sn – Ag – Cu lead-free solders and Au were investigated by the use of SAC/Au solid/solid reaction couples which were annealed at 150, 180 and 200 °C for various reaction times. Five intermetallic compounds, AuSn, AuSn 2 , AuSn 4 , (Au, Cu)Sn, and (Cu, Au) 5 Sn 6 , were formed at a higher temperature and lower Cu content. As the reaction temperature was dropped and Cu content was increased, only three intermetallic layers, AuSn, (Au, Cu)Sn, and (Cu, Au) 5 Sn 6 , could be observed. Meanwhile, when the Cu content in SAC alloys increased, the thickness of total reaction layers decreased, but that of (Au, Cu)Sn/(Cu, Au) 6 Sn 5 layers increased. It likely resulted from (Au, Cu)Sn/(Cu, Au) 6 Sn 5 layers acting as a diffusion barrier to resist Sn diffusing to Au – Sn intermetallic compounds and decrease the growth rate of Au – Sn intermetallic compounds. The growth and formation of intermetallic compounds are sensitive to the reaction temperature and Cu content in the SAC/Au couple systems.