Effect of Zn concentration on the interfacial reactions between Sn–3.0Ag–0.5Cu solder and electroplated Cu–xZn wetting layers (x = 0–43 wt%)

Cu–xZn wetting layers (x = 0, 20, 30, and 43 wt%) were electroplated in a non-cyanide solution and the interfacial reaction with Sn–3.0Ag–0.5Cu (SAC) solder was investigated after reflow and isothermal aging. After reflow, a Cu₆Sn₅ intermetallic compound (IMC) formed at the SAC/Cu interface and a small amount of Zn atoms dissolved into the Cu₆Sn₅ at the SAC/Cu–Zn interface. After aging, Cu₃Sn and microvoids were observed between the Cu₆Sn₅ and Cu layer, whereas such voids were not formed at the SAC/Cu–Zn interface. A dominant reaction product during aging was Cu₆(Sn, Zn)₅ in the Cu–20Zn and Cu–30Zn samples, which was changed from Cu₆(Sn, Zn)₅ to Cu(Zn, Sn) in the Cu–43Zn sample. In addition, a thin CuZn IMC was observed beneath the Cu(Zn, Sn) in both the SAC/Cu–30Zn and SAC/Cu–43Zn joints after aging for a long time. X-ray diffraction showed that α-CuZn and β-CuZn peaks were obtained from the Cu–30Zn and Cu–43Zn wetting layers. The crystal structure of Cu(Zn, Sn) and/or CuZn IMCs was a β′-CuZn (ordered phase) with a superlattice peak. As the Zn concentration in the Cu–xZn wetting layers increased to 30 wt%, the IMCs growth rate was gradually reduced, but was increased in the Cu–43Zn sample. The excessive IMC growth of the Cu–43Zn sample was attributed to the fast growth of Cu(Zn, Sn) IMC.