The work presented here focuses on the role of 0.5 wt % Cu in the interfacial reaction between the molten Sn−3.5%Ag solder and the Au/Ni metallization for ball grid array (BGA) applications. The Sn−3.5%Ag−0.5%Cu solder forms the ternary intermetallic compound (TIMC) of (Cu,Ni)6Sn5, while the Sn−3.5%Ag forms the binary intermetallic compound (BIMC) of Ni3Sn4. The growth rate of the (Cu,Ni)6Sn5 was found to be much higher than that of the Ni3Sn4. However, the formation of this TIMC ceases when the Cu content in the BGA solder ball goes below 0.2 wt %. A new TIMC, (Ni,Cu)3Sn4, starts to form between the Ni and the (Cu,Ni)6Sn5. The figure to the right shows the variation of the total IMC thickness with the reaction time.