Effect mechanism of Ag and Bi elements on the corrosion process in Sn–9Zn–1Ag and Sn–9Zn–3Bi solders

Abstract

Sn–9Zn lead–free solder is widely used in low melting temperature welding field due to its low melting point (198.5 °C), good connection reliability, and low cost. However, its poor wettability and corrosion resistance weaken its performance. According to previous studies, addition of 1% Ag and 3% Bi to Sn–9Zn alloy, respectively, can optimize the wettability and mechanical properties of solder. Nonetheless, the effect of added elements on corrosion process has not been determined. In this study, the effect mechanism of Ag and Bi elements on the corrosion process in Sn–9Zn–1Ag and Sn–9Zn–3Bi solders was analyzed. The changes of the surface state of each solder in the corrosion process were detected by electrochemical polarization experiment. The microstructural compositions of solders in different corrosion stages were compared through the immersion test. The results indicated that compared to Sn–9Zn solder, Sn–9Zn–1Ag solder exhibited the existence of a finer Zn-rich phase, which improved the pitting corrosion resistance of solder by blocking the entry of corrosive solution. In passivation film forming process, Ag–Zn compounds were formed on the surface of Sn–9Zn–1Ag solder, which reduced the content of Zn-rich phase, leading to the decrease of the overall corrosion rate. After the formation of passivation film, Ag compounds were precipitated on the surface of passivation film of solder, which prevented further destruction of the passivation film. However, Sn–9Zn–3Bi solder produced a large number of loose structural corrosion products on the surface of the corroded sample, thus the substrate could not be effectively protected. Furthermore, Sn–9Zn–3Bi solder showed a larger Zn-rich phase, thus it was easier for the corrosive solution to enter the sample, resulting in severe corrosion in the interior of sample. In this study, the effect mechanism of Ag and Bi addition on the corrosion process of Sn–9Zn–1Ag and Sn–9Zn–3Bi solders was revealed, which provides a theoretical basis for the study on improving the corrosion resistance of Sn–9Zn solder.