Microstructure and adhesion strength of Sn–9Zn–1.5Ag–xBi (x = 0 wt% and 2 wt%)/Cu after electrochemical polarization in a 3.5 wt% NaCl solution

The microstructure and adhesion strength of the Sn–9Zn–1.5Ag–xBi (x = 0 wt% and 2 wt%)/Cu interface after electrochemical polarization have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and pull-off testing. The equilibrium potentials of Sn–9Zn–1.5Ag/Cu and Sn–9Zn–1.5Ag–2Bi/Cu are −1.31 Vsce and −1.22 Vsce, respectively, indicating that Sn–9Zn–1.5Ag–2Bi/Cu has a better corrosion resistance than that of Sn–9Zn–1.5Ag/Cu. The intermetallic compounds of Cu6Sn5, Cu5Zn8 and Ag3Sn are formed at the soldered interface between the Sn–9Zn–1.5Ag–xBi solder alloy and the Cu substrate. The scallop-shaped Cu6Sn5 is close to the Cu substrate and the scallop-shaped Cu5Zn8 is found at the interface in the solder matrix after soldering at 250 °C for 10 s. The corrosion products are ZnCl2, SnCl2 and ZnO. On the other hand, pits are also formed on the surface of both solder alloys. The interfacial adhesion strength of the Sn–9Zn–1.5Ag/Cu and Sn–9Zn–1.5Ag–2Bi/Cu decreases from 8.27 ± 0.56 MPa and 12.67 ± 0.45 MPa to 4.78 ± 0.45 MPa and 8.14 ± 0.38 MPa, respectively, after electrochemical polarization in a 3.5 wt% NaCl solution. The fracture path of the Sn–9Zn–1.5Ag–2Bi/Cu is along the solder alloy/ZnO and solder/Cu6Sn5 interfaces.