Abstract: This study reveals a novel degradation mechanism in Ag-Au sintered joints under high-temperature conditions (> 250 °C). Unlike Ag-Ag joints, which exhibit monotonically improving strength with sintering temperature, Ag-Au joints show peak strength at 275 °C/1 h followed by significant degradation. We attribute this anomaly to the formation of abnormally large (up to 1.8 μm) Kirkendall-like voids at the interface. Through multiscale characterization, a three-stage grain boundary diffusion process is identified: (1) grain boundary widening with stacking faults and defects, (2) crystallization into strip-like Ag-rich alloys, and (3) lateral growth consuming Au grains via stacking faults. In addition, a substantial lattice contraction (5.9 %-8.8 %) is quantified in adjacent Au crystals, which is attributed to stress fields from Ag accumulation in grain boundaries (GBs). These findings provide new insights into establishing practical guidelines for pad material selection in high-power packaging.

Key words: Power electronics packaging, Nano paste, Kirkendall effect, Lattice contraction, Pore evolution