Pressureless, Low Temperature Sintering of Micro-scale Silver Paste for Die Attach for 300°C Applications

Low temperature Ag sintering technology provides a lead-free die attachment compatible with high temperature (300°C) applications. Previous work with Ag sintering has required pressure during the sintering process or been limited to small area die. In this paper, the pressureless sintering of micro-scale silver paste is examined for larger (8mm × 8mm) area die. Experimental combinations included: Si die metallization (Ag and Au); thick film substrate metallization (Au, Ag and PdAg); and sintering temperature. For Au metallization (die and/or substrate), the initial shear strength results were good with 8mm × 8mm die sintered at lower temperatures (200°C). The shear strength was beyond the limit of the shear test machine (100 kg), corresponding to >15.3 MPa. However, after aging for 24 hours at 300°C, the shear strength dropped significantly. An SEM was used to characterize cross sections of as-built and aged sample. The decrease in die shear strength with high temperature sintering (250°C and 300°C) or high temperature aging is attributed to surface diffusion of Ag along the Au surface resulting in a dense Ag layer adjacent to the Au surface and a depletion layer within the die attach adjacent to the dense Ag layer. Shear failures occurred through the depleted region. For Ag metallization (die and substrate), no decrease in shear strength was observed with 300°C aging. Shear strength of 8 mm × 8 mm dies was >100 kg ( >15.3 MPa) as-built. The shear strength remained >15.3MPa after 3000 hours of 300°C aging.