Low temperature Cu–Cu bonding by transient liquid phase sintering of mixed Cu nanoparticles and Sn–Bi eutectic powders

Fluxless bonding of plateless Cu–Cu substrates at processing temperature lower than 250 °C and low pressure of 0.1 MPa was achieved by transient liquid phase sintering (TLPS) of mixed Cu nanoparticles and Sn–Bi eutectic powders. The effects of mixture composition, and sintering temperature on the shear strength, microstructure, and remelting temperature were investigated. Lowering the sintering temperature of Cu mixed with 65 weight percentage of Sn–Bi (Cu–65SnBi) resulted in decreased shear strength, however, at 200 °C sintering temperature, the obtained highest shear strength was more than 20 MPa. It was found that it is essential to use Cu nanoparticles to accelerate the consumption so that no initial Sn–Bi phases remained after processing. The liquid phase generated at approximately 196 °C during sintering from the reaction between newly formed Cu₆Sn₅ and Bi-phase was expected to facilitate the densification and strengthening of the joints. Although this newly generated liquid phase was known to solidify as hypereutectic Sn–Bi, by controlling the sintering temperature at 200 °C, the remelting event at 139 °C was not observed by differential scanning calorimetry. It is assumed that the proportion of solidified Sn–Bi eutectic phases in Cu–65SnBi that was sintered at 200 °C were significantly small, hence, when reheated at 150 °C, the obtained shear strength was equivalent to that at room temperature.