Effects of size distributions of copper nanoparticles on the pressureless bonding performance of the copper paste for die attachment

Cu nanoparticles (NPs) with two different size distributions were synthesized by a chemical reduction method, which are polydispersed Cu NPs with size in the range of 10 nm to 100 nm and bimodal-sized Cu NPs with large irregular-shaped particles (125 nm) surrounded by small NPs (9 nm). Cu pastes prepared by mixing the Cu NPs with the solvent were used to bond dummy die (i.e., copper plate with silver coating) and bare copper substrate. The results demonstrate that the bimodal-sized Cu NP based paste performs better than the other one in mechanical property of joints. Besides, the shear strength of joints fabricated by such paste can reach 52.6 MPa after pressureless sintering at 280 °C for 10 min in the atmosphere of nitrogen. The copper paste capable of pressureless sintering and making joints with high strength is a promising material for die attachment of IGBT.