Effect of Cu Thickness on the Evolution of the Reaction Products at the Sn-9wt.%Zn Solder/Cu Interface During Reflow

Interfacial reactions between Sn-9wt.%Zn solder and Cu substrates at 230°C were investigated. The substrate thickness was found to have noticeable effects on the evolution of the reaction products formed at the solder/Cu interface. The CuZn 5 and Cu 5 Zn 8 phases were formed at the early stage of reflow, regardless of the Cu thickness, while, with increasing reflow time, the two phases displayed different growth behaviors on the Cu substrates with various thicknesses. For the thicker Cu substrates with a thickness of 6 μm, 10 μm, and 0.5 mm, CuZn 5 disappeared but Cu 5 Zn 8 kept on growing after a longer reflow time. In contrast, for the thinner Cu substrates with a thickness less than 3 μm, Cu 5 Zn 8 shrank with increasing reflow time but CuZn 5 grew dominantly. A different evolution of the grain morphology of CuZn 5 was also observed between the thicker and thinner Cu substrates. When the reflow time was increased, the CuZn 5 grains retained a rounded shape on the thinner Cu substrates; however, the grain structure became faceted on the thicker Cu substrates.