Low temperature and low pressure bonding of plateless Cu–Cu substrates by Ag-based transient liquid phase sintering

Bonding silicon carbide/gallium nitride (SiC/GaN) based power modules, particularly epoxy-molded modules to heat-substrate and/or heat sink, requires low processing temperature preferably lower than 250 °C, and low pressure as low as 0.1 MPa to prevent damage to the modules. In addition, due to the impracticality of depositing metal-plating to the epoxy-molded module, bonding of plateless Cu-substrates is in great demand. Furthermore, post-processing residual flux cleaning, which is costly and unfavorable to industry need to be avoided as possible by opting out the usage of flux. Up to authors’ knowledge, our study is the first to fulfill all requirements stated above. Transient liquid phase sintering (TLPS) of mixed fluxless Sn–Bi (tin–bismuth) eutectic alloy and Ag (silver) particles was applied to bond plateless Cu–Cu substrates. Sintering temperature of 250 °C, sintering pressure of 0.02 MPa, and reducing environment were applied during processing. The effects of addition amount of Sn–Bi and sintering holding time to the shear strength and microstructure were investigated. The remelting temperature after sintering was also examined. Shear strength of 30 wt% added Sn–Bi was over than 20 MPa, which qualify the requirement of MIL-STD-883K, and larger than conventional Pb-based solder. Formation of intermetallic compounds are thought to strengthen the interface and matrix. Remelting temperature shifted from eutectic temperature of Sn–Bi to approximately 262 °C, allowing the application of modules at higher operating temperature than the processing temperature.