Low-Temperature Sintering of Nanoscale Silver Paste for Power Chip Attachment

European electronic manufacturers, particularly those in power electronics, are aggressively implementing a silver sintering technology for interconnecting semiconductor chips. It has been shown that power modules with sintered chip-attachment have 3x better performance, 5x better reliability, and higher chip junction temperature up to 175°C. However, because of the use of commercial thick-film silver pastes in the process, a serious drawback of the sintering technology is the need of a high quasi-static pressure (~ 40 MPa) to lower the sintering temperature to about 250°C. Using the large pressure complicates the manufacturing process and places critical demands on substrate flatness and chip thickness. In this paper, we describe a strategy of using nanoparticles of silver to lower its sintering temperature without any applied pressure. From the science of sintering, driving force for densification of a particle compact increases with decreasing particle size, thus the densification rate-a product of thermodynamic driving force and kinetics-could still be high at low temperatures even with low atomic diffusion rate. We formulated a uniform silver paste containing 30 to 50 nm silver particles. The sintered attachment at temperatures below 275°C without applying any pressure had a density of about 80% and a uniform distribution of micron-sized pores. The sintered chip-attachment had die-shear strength over 20 MPa, and it showed little change after aging at 300oC for 400 hours. Temperature-cycling tests have been performed and results are presented in this paper. Based on the results, we believe that the nanoscale silver paste can enable a quick adaptation of the low-temperature silver-joining technology for manufacturing electronics products.