The Effects of Gallium Additions on Microstructures and Thermal and Mechanical Properties of Sn-9Zn Solder Alloys

The effects of gallium additions on microstructures and thermal and mechanical properties of the Sn-9Zn solder alloys are investigated in this study. The results show that the melting temperature of the alloys decreases with the increase in the Ga concentration, while the pasty ranges of the alloys are simultaneously enlarged. By adding a 0.25–0.5？wt.% Ga element, the Sn-matrix region is slightly increased and the Zn-rich phase becomes slightly coarser; however, the overall microstructure is still very similar to that of the Sn-9Zn alloy. It is found that, when the Ga concentration is less than 0.50？wt.%, the ultimate tensile strength and elongation are maintained at the same values. The addition of a 0.25–0.50？wt.% Ga to the Sn-9Zn alloy also leads to small cup and cone fracture surfaces which exhibit near-complete ductile fracturing. With the addition being increased to 0.75？wt.%, larger cup and cone fractures are observed. The 1.00？wt.% Ga alloy has lower strength and ductility due to the coarser and nonuniform microstructures. However, the fracture surfaces of the 1.00？wt.% Ga alloy show partial cleavage and a partially dimpled fracture. 1. Introduction Conventional Sn-Pb solders have commonly been used as the interconnection materials for soldering electronic components and devices. However, the use of Pb is restricted due to health and environmental issues. On the other hand, an alloy of Sn-Ag-Cu has been recognized as a potential lead-free solder even though Sn-Ag alloy systems have higher melting points (say, 216 to 221°C), as compared to an eutectic Sn-Pb alloy [1, 2]. A high melting point is accompanied by high soldering temperatures, which may give rise to substrate instability problems. Recently, the Sn-9Zn alloy system has received increased interest since it features low cost, great mechanical properties, and a low eutectic temperature (198°C), close to that of the Sn-Pb alloy [2, 3]. The eutectic structure of the Sn-9Zn alloy system consists of two phases: a body centered tetragonal Sn matrix phase and a secondary phase of hexagonal Zn containing less than 0.039？at.% Sn in solid solution [3, 4]. However, the tendency of oxidation and poor wetting ability of this alloy system limits its application [5, 6]. In recent years, to overcome the shortfalls in the Sn-9Zn alloy, some authors have tried to add a third element, such as In [7], Ga [8], Bi [9–13], Al [14–18], Ag [19–22], Cr [23], Cu [24], and Ce/La [25–27], to the Sn-Zn binary system to improve the melting temperature, wettability, oxidation resistance, corrosion, and mechanical