A comprehensive and in-depth study of pressureless and low-temperature in-air sintering performance of bimodal-sized Cu nanoparticle pastes for die attachment and the joint strengthening mechanism

Abstract

Recent years have seen the rapid development of third-generation semiconductors, which has driven the demand for high-performance, low-cost die-attachment materials. As a continuation of a series of exploratory research on preparation of Cu particles and Cu pastes as well as their sinterability as die-attachment materials performed by the authors, this paper presents systematic studies on utilizing an efficient method for preparing bimodal-sized Cu nanoparticles (NPs) and corresponding Cu NP pastes formulated with four types of reducing solvents, and characterizing the air-sintering performance of the Cu pastes subjected to varying process parameters of sintering pressure, temperature and time. Taguchi method analysis shows the shear strength of joints is most influenced by sintering pressure, followed by solvent composition, temperature and time. The novel Cu paste composed of bimodal-sized NPs and glycerol exhibits excellent antioxidant properties, enabling pressureless in-air sintering and bonding. Increase in sintering pressure, temperature and time brings about significantly reduced porosity in the sintered Cu matrix and greatly enhanced shear strength of the joints. After pressureless sintering in air at 280 °C for 10 min, the porosity is 24.3% and shear strength reaches 22.3 MPa; comparatively, increasing pressure to 5 MPa leads to porosity decreasing to 4.1% and strength rising to 70.1 MPa. Increasing pressure and temperature, and prolonging sintering time promote Cu bulk growth and atomic diffusion, thus strengthening the matrix and forming robust interfaces. The above results indicate the great potential of the bimodal-sized Cu NP paste for die attachment in high-power electronic devices under mild sintering conditions.