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 a-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 degC. The microstructure coarsened when exposed to temperatures between 300 degC and 380 degC, i.e., near the eutectic melting point at 361 degC. 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.