The evolution of the transient liquid-phase sintered (TLPS) Cu–Sn skeleton microstructure during thermal aging was evaluated to clarify the thermal reliability for die-attach applications. The Cu–Sn skeleton microstructure, which consists of Cu particles connected with Cu–Sn intermetallic compounds partially filled with polyimide resin, was obtained by the pressure-less TLP sintering process at 250 °C for 1 min using a novel Cu-solder-resin composite as a bonding material in a nitrogen atmosphere. Experimental results indicate that the TLPS joints were mainly composed of Cu, Cu6Sn5, and Cu3Sn in the as-bonded state, where submicron voids were observed at the interface between Cu3Sn and Cu particles. After thermal aging at 150, 175, and 200 °C for 1000 h, the Cu6Sn5 phase fully transformed into Cu3Sn except at the chip-side interface, where the number of the submicron voids appeared to increase. The averaged shear strengths were found to be 22.1 (reference), 22.8 (+3%), 24.0 (+9%), and 19.0 MPa (−14%) for the as-bonded state and specimens aged at 150, 175, and 200 °C for 1000 h, respectively. The TLPS joints maintained a shear strength over 19 MPa after thermal aging at 200 °C for 1000 h because of both the positive and negative impacts of the thermal aging, which include the transformation of Cu6Sn5 into Cu3Sn and the formation of submicron voids at the interface, respectively. These results indicate an excellent thermal reliability of the TLPS Cu–Sn skeleton microstructure.

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Sintered silver bonding processes are expected to offer bonding solutions with high heat endurance for power modules using wide bandgap semiconductors. This study reports the die-attach reliability of the bonding process under thermal cycling tests, focusing on the metallurgical and mechanical properties of sintered silver joints. A nanocrystalline (NC) structure with 150-nm-sized grains was observed in the as-sintered state, while a coarsened structure with microsized grains and pore coalescence was observed after annealing at 350°C for 1 h. In addition, the increase of bonding pressure reduced the number of coarse pores. Transmission electron microscope observations showed favorable crystalline structure along the grain boundaries. Tensile tests at room and high temperature revealed that the sintered silver materials showed the inherent mechanical properties of NC metals. Thermal cycling tests of die-attached specimens demonstrated the temperature dependence of crack resistance at constant amplitude. Furthermore, coalescence of pores and coarsening of grains reduced bonding reliability. It can be inferred from the results that NC structure and minute pore dispersion improves bonding reliability.