Microstructure and mechanical properties of Sn-Bi, Sn-Ag and Sn-Zn lead-free solder alloys

The aim of this paper is to develop a comparative evaluation of mechanical properties of as-cast Sn-Bi, Sn-Ag and Sn-Zn alloys as a function of the resulting microstructural arrays with a view to application as solder materials. The resulting microstructures, ultimate and yield tensile strengths and elongations of Sn-40 wt.% Bi, Sn-3.5 wt.% Ag and Sn-9 wt.% Zn alloys were examined and compared with the corresponding results of the traditional Sn-40 wt.% Pb solder alloy. Tensile tests were carried out using specimens selected from specific positions along the length of the directionally solidified castings and Hall-Petch type correlations have been proposed relating the resulting tensile properties of each alloy to a parametric scale of the microstructure. It was found that all Pb-free alloys examined comply with a compromise between compatible mechanical strength and environmental protection. The Sn-Bi alloy has the highest ultimate tensile strength of all alloys examined, while both Sn-Ag and Sn-Zn alloys are lighter and exhibited higher ductility than the Sn-Pb and Sn-Bi alloys.