Oxidation resistance and joining properties of Cr-doped Zn bonding for SiC die-attachment

Pure Zn exhibits excellent properties for high temperature lead-free solder such as the high melting point (419.5°C), surpassing tensile strength, high thermal and electrical conductivity, in addition to the reasonable cost. However, the brittleness and oxidation of pure Zn can potentially cause a reliability problem, when it is employed at high operation temperature expected for SiC power devices. In this study, the effects of minor Cr-doping in pure Zn die-attach is investigated, particularly on the enhanced reliability due to the improved joining microstructure and anti-oxidation at high temperature. As-cast Cr-doped Zn alloy indicates finer grain size in the microstructure than as-cast pure Zn, implying higher oxidation resistance as well. To evaluate the high temperature reliability of the Cr-doped Zn solder, the growth of intermetallic compound (IMC) at the interface with Cu substrate has been investigated at soldering and also during subsequent thermal aging. The aging test at 250 °C for 100 h the joining sample has proved no crack developed at Zn-Cr alloy/Cu interface, while a considerable number of cracks have observed inside the γ-Cu₅Zn₈ phase grown at pure Zn/Cu interface. The minor Cr addition thus remarkably suppresses IMC growth at the interface, and significantly improves the reliability of pure Zn die-attach method.