Micron-sized Ag flake particles direct die bonding on electroless Ni–P-finished DBC substrate: low-temperature pressure-free sintering, bonding mechanism and high-temperature aging reliability

Abstract

Adoption of wide-bandgap semiconductors in power electronic systems enables these devices to operate at high switching speeds and at high temperatures (above 200 °C). Sinter Ag joining as an attractive die attach material received more and more attention. This study introduced a robust bonding of sinter Ag joining direct on an electroless Ni(P) surface-finished DBC (direct bonded copper) substrate with a low-cost micron-sized Ag flake particle. Die shear strength of sinter Ag joining increased from sintering temperature of 180 °C to 300 °C and achieved 43.6 MPa at 300 °C in air and without pressure during sintering. The shear strength decreased at the sintering temperature of 350 °C due to mass Ni oxide and dense voids generation. The bonding mechanism of sinter Ag joining direct on the Ni-plated structure was discussed via SEM, EDX, XRD and TEM analysis. There is not clear inter-diffusion between Ni and Ag element at the bonding interface. TEM observation reveals that many Ag nanoparticles with the size < 10 nm were self-generated and touched on the thin Ni oxide layer to form a robust interface bonding. In addition, the high-temperature aging test also was investigated for the sinter Ag joining at the aging temperature 250 °C for 1000 h. The die shear strength keeps 30 MPa after 500 h aging and down to 17.9 MPa after 1000 h, may mainly induce by the interface connection ratio decrease with the Ag grain coarsening during high-temperature aging. This work provided experimental and theoretical studies for high-strength connections between sinter Ag joining and Ni-finished substrate, which impacts the application of sinter Ag joining for die attach modules in high temperature.