The Dissolution and Supersaturation of Zn in the Sn9Zn Solder Under Current Stressing

The Sn-Zn phase diagram indicates that Sn and Zn exhibit nearly zero mutual solubility. However, this study found that Zn shows certain solubility in the Sn matrix of Sn9Zn solder under current stressing with 6000–8000 A/cm². The dissolution of the Zn crystal induced by electric current was evidenced by the diminishing of the x-ray diffraction orientation of Zn(002) and the cavity formation on the Zn crystal. The Zn concentration reaches supersaturation of 4–5 wt.% at 8000 A/cm². The dissolution behavior was further investigated with scanning electron microscopy and a focus ion beam of the specimen quenched with liquid nitrogen after current stressing. The morphologies of the cavities on the Zn crystals were examined and three dissolution mechanisms were identified.