The potentials of the newly designed Zn-xSn (x ¼ 40, 30, and 20 mass%) alloys as high temperature lead-free solders and their interface properties on Cu substrate were investigated, focusing on the interface microstructure and mechanical properties. Hypereutecic alloys show two endothermic peaks in differential scanning calorimetry (DSC), one appears at 200 C and the other varies from 365 to 383 C with decreasing Sn content. These peaks are well associated with the eutectic and liquidus temperatures of binary Zn-Sn alloys, and little undercooling were observed on cooling. Two Cu-Zn compound layers are formed at the Zn-Sn alloys/Cu interface. The reaction phases are identified as -Cu 5 Zn 8 and "-CuZn 5 phases from the Cu side, and no Cu-Sn compound was identified. The thickness of the reaction layers and the joining strength increased with decreasing Sn content. Each joint shows a different fracture pattern, which gradually changes from transgranular in Zn-Sn alloys near the interface to the at "-CuZn 5 /-Cu 5 Zn 8 reaction layers with decreasing Sn content.

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The potential of newly-designed Zn-xSn (x ¼ 40, 30, and 20 mass%) and Zn-30 mass%In alloys as high temperature lead-free solders was evaluated, with particular focus on the fundamental thermal properties and phase stability during thermal and humidity exposure. From DSC results, the melting temperature of Zn-Sn alloys increased with decreasing Sn content, and the final undercooling was about 3 C. The liquid fraction of the alloys calculated using Scheil's model is lower than that of the alloys calculated according to the phase diagram by approximately 10 mass% at the eutectic temperature and 250 C. The coefficients of thermal expansion (CTE) of Zn-Sn alloys increased with decreasing Sn content, i.e. 29:2 Â 10 À6 ÁK À1 to 33:2 Â 10 À6 ÁK À1 in the temperature range of À50 C to 200 C for Zn-Sn alloys and 31:3 Â 10 À6 ÁK À1 in the temperature range of À50 C to 140 C for Zn-30In alloy. With increasing temperature above eutectic temperature, all alloys began to deform, indicating the formation of a liquid phase. The thermal deformation of Zn-Sn alloys decreased with increasing Sn content. The ultimate tensile strength (UTS) and 0.2% proof stress of the as-cast Zn-Sn alloys were almost the same, but the elongation of the as-cast Zn-Sn alloys decreased with increasing Sn content. After thermal and humidity exposure for 1000 h (85 C/85% Relative Humidity), only the outer surface of Zn-Sn alloys oxidized. However, Zn-30In alloy rusted quite seriously resulting in Zn oxidation after 1000 h. The UTS and 0.2% proof stress of Zn-Sn alloy slightly decreased with increasing exposure time. The elongation of Zn-Sn alloys decreased with decreasing Sn content for 100 h exposure. However, the elongation of Zn-Sn alloys showed no further degradation beyond 100 h exposure.

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