Thermal Properties and Phase Stability of Zn-Sn and Zn-In Alloys as High Temperature Lead-Free Solder

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 � 10 � 6 � K � 1 to 33:2 � 10 � 6 � K � 1 in the temperature range of � 50 � C to 200 � C for Zn-Sn alloys and 31:3 � 10 � 6 � K � 1 in the temperature range of � 50 � 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. [doi:10.2320/matertrans.48.584]