Interfacial Properties of Zn–Sn Alloys as High Temperature Lead-Free Solder on Cu Substrate

Abstract

The potentials of the newly designed Zn–xSn (x=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 γ-Cu₅Zn₈ and ε-CuZn₅ 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 ε-CuZn₅/γ-Cu₅Zn₈ reaction layers with decreasing Sn content.