Electrochemical corrosion and oxidation behaviors of high temperature Pb-free solders

This study aimed to investigate the corrosion resistance in 3.5%NaCl solution and high temperature oxidation behavior of Pb-5Sn solder and its alternatives under development, Bi-11Ag and Zn-40Sn. Compared with Pb-5Sn and Zn-40Sn, Bi-11Ag exhibited higher corrosion potential and relatively low corrosion current density. The ductile Ag-rich phase which dispersed in the Bi matrix was able to accommodate the stress arising from the formation of a passive layer and contributed to the two-stage passivation. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results confirm that the corrosion products comprised BiOCl, Bi₂O₃ and AgCl. Among the die-attach solders investigated, the Zn-Sn alloys exhibit a superior oxidation resistance. Pb and Bi based alloys possess inferior protection function in the liquid state. The additions of Ag can significantly reduce the oxidation of Bi and thus Bi-llAg has a comparable performance with Pb-5Sn. The thin layer of the oxides of Zn protects the liquid solder from oxidation, because ZnO is thermodynamically stable and tends to slow down further oxidation as indicated by the kinetics data (the rate constant and activation energy).