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 Ni3-Sn4. The growth rate of the (Cu, Ni)6Sn5 was found to be much higher than that of the Ni3-Sn4. 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.