Microstructural, wetting, and mechanical characteristics of Sn-57.6Bi-0.4Ag alloys doped with metal-organic compounds

The metallurgical and mechanical properties of the commercial low-temperature solder alloy, Sn-57.6Bi-0.4Ag (wt. %), were altered by doping with each of Pd, Co, Zn, and Ni, through reactive reflow processing by using the appropriate metal-organic compound. The use of metal-acetates resulted in appropriate doping concentrations, while the use of metal-acetylacetonates and -stearates resulted in insufficient doping concentrations. This indicates that the degree of doping is strongly dependent on the nature of the metal-organic compound used in the reactive reflow process. Notably, a concurrent decrease in the melting point and the degree of undercooling were observed only in the case of the Pd-doped alloy. In addition, the Pd-doped alloy exhibited an increase in the fraction of the primary β-Sn phase in its microstructure, and greater wettability as tested on a Cu plate. Meanwhile, the Co-doped alloy exhibited a notable increase in the size and spacing of its lamellar structure, and the Ni-doped alloy showed a refinement of its lamellar structure. Accordingly, doping with Pd and Co mitigated the brittleness of the parent Sn-57.6Bi-0.4Ag alloy, which thereby showed a pronounced increase in its plastic displacement during shear tests. Considering the increase in wettability and reduction in brittleness of the original alloy, Pd is considered to be the most suitable dopant, among all the different doping elements analyzed in this study.