Is Pressureless Sintering Ready for Power Electronic Modules?

IGBTs, power MOSFETs and diodes used in power modules operated at high temperatures of 175deg C and above are increasingly pressure sintered instead of soldered. Pressureless sintering methods are not widely used for power modules as the mechanical robustness in shear tests is not comparable to those of pressure sintered dies. Furthermore, the die size is limited to about 10 mm x 10 mm in pressureless sintering processes. This paper investigated the properties of recently proposed pressureless silver and copper sinter pastes with high mechanical robustness. Shear tests of 2.3 x 2.3 mm2 large test chips sintered by copper pastes onto copper substrates reach up to 78 MPa. Extraordinarily high shear values of up to 110 MPa using pressureless silver sinter paste on Electroless Nickel, Electroless Palladium, Immersion Gold (ENEPIG) clad DBC substrates have been measured. Due to this high mechanical robustness the pastes are evaluated for sintering larger dies up to a size of 4.0 x 4.0 mm2. Additionally, copper bond buffers have been sintered without pressure on the top side of electrochemically copper-plated IGBTs measuring 4.0 x 4.0 mm2 to increase their operational robustness. It has been found, that especially for sintering on copper substrates the surface quality strongly influences the shear values.