Oxidation behavior of Sn–Zn solders under high-temperature and high-humidity conditions

The oxidation behavior of Sn–9Zn and Sn–8Zn–xBi (x = 1, 3, 6) solders under high-temperature and high-humidity conditions has been investigated as a function of exposure time. The evolution of surface and cross-sectional microstructures during exposure has been examined, and an oxidation model for the Sn–Zn alloys was proposed. The poor oxidation resistance of Sn–Zn alloys is attributed to the oxidation of element Zn, which diffuses to the grain boundaries of Sn matrix, forming ZnO along those boundaries. It was also found that Bi forms a solid solution in Sn with a solubility limit of 4.4 wt.%, which causes the Sn matrix to become liable to form cracks, as well as to have a high distortion energy. The solid solution of Sn(Bi) is considered to be one of the reasons for the rapid oxidation of Bi added alloys.