Low temperature and low pressure fluxless Cu-Cu bonding by Ag-based transient liquid phase sintering for high temperature application

This work is the first to achieve a low temperature (≤250 °C) and low pressure (≤0.1 MPa) fluxless bonding of plateless Cu-Cu substrates by transient liquid phase sintering (TLPS) of Ag nanoparticles and Sn-Bi eutectic powder. Sintering was conducted under formic environment to assist the wetting of Sn-Bi and to suppress the oxidation of Cu substrate. Effect of mixture composition to the shear strength, microstructure, and remelting temperature was investigated. The effect of formic gas environment to the shear strength and microstructure was also examined. It was found that shear strength of 30wt% added Sn-Bi (Ag-30SnBi) that was sintered at 250 °C was higher than 20 MPa, which is comparable to the conventional Pb-based solder. The determined remelting temperature by differential scanning calorimetry (DSC) test was found to shift to higher temperature compared to the Sn-Bi eutectic temperature, implying the possibility of high temperature operation. Formic gas environment was observed by SEM to assist the densification of the sintering microstructure by the reduction of the oxide layer on the sintering structure and subsequently promoting the formation of sintering necks.