Oxidation behavior of Sn-9Zn-3Bi-xCr solders under high-temperature exposure

The oxidation behavior of Sn-9Zn-3Bi-xCr(x=0, 0.1, 0.3, 0.5) solders under 250°C has been investigated as a function of exposure time. The evolution of surface and cross-sectional microstructure has been examined through back-scattered electron detector (BSE). Energy-dispersive x-ray spectroscopy (EDX) and x-ray diffractometer analysis (XRD) were also carried out to check the overall composition of the different samples. An oxidation model for Sn-9Zn-3Bi-xCr solders was proposed. The poor oxidation resistance of the solders is attributed to the oxidation of Zn-rich phase and other Zn atoms which diffused to the β-Sn matrix grain boundaries, forming ZnO. Two kinds of Sn-Zn-Cr phases, along the grain boundaries of β-Sn matrix and across the Zn-rich phase, were detected in Cr-bearing solder alloys, which kept the oxygen from diffusing into the bulk of the solder. Adding a small amount of Cr can improve the oxidation resistance of the solders, and the Sn-9Zn-3Bi-0.3Cr alloy had the best oxidation resistance. After 25h of aging under 250°C, the both the two kinds of Sn-Zn-Cr phases re-crystallize.