On the Microstructure of Off-Eutectic Au-Ge Joints: A High-Temperature Joint

Joining delicate electronic components for high-temperature applications is challenging. Regular soldering with lead-free or lead-based materials is typically not suitable for high-temperature applications due to their low melting points. Using off-eutectic compounds for joints offer an easy and gentle process creating joints that can be formed at a lower process temperature than the final operation temperature. Microstructural evolution near the eutectic melting point is key to be able to form reliable joints. A layered Au/eutectic Au-Ge/Au structure was used to form Au-rich off-eutectic Au-Ge joints. Columnar-like structures of primary \( \alpha \)-phase (Au) protruded through a Ge-rich off-eutectic Au-Ge mixture at the center of the joint. These structures connect the joined pieces with a single solid phase with a melting point of ca. 1064 °C. The microstructure coarsened when exposed to temperatures between 300 °C and 380 °C, i.e., near the eutectic melting point at 361 °C. Ge diffused and accumulated along grain boundaries between Au grains. Annealing above the eutectic melting point, Ge rapidly diffused and formed larger colonies of pure Ge surrounded by a Au matrix. This accords well with our previously published results demonstrating shear strength capacity of similar joints at temperatures well above the eutectic temperature.