Microstructural and mechanical evolution of silver sintering die attach for SiC power devices during high temperature applications

Silver sintering is a promising die attach technology to ensure high thermal reliability. The long-term reliability of SiC device sintered by nano-Ag paste has been evaluated by the high temperature storage (HTS) process at 350 °C in air and vacuum, respectively. Although the SiC chip and direct bonding copper (DBC) substrate could be bonded firmly by the nano-Ag paste after sintering at low temperature, the microstructure and shear strength of sintered die attachment experienced the huge evolution during HTS process. The bondline of die attachment became compact, and some pores grow up and pore distribution became nonuniform after HTS in air. While the densification of bondline was significantly delayed because residual organics inhibited the growth and migration of pores during HTS in vacuum. The shear strength of die attachment first increased then decreased slowly with the increasing of storage time. The fracture surface showed that the Ni(P) layer was oxidized, and the formed NiO layer provided the failure location. The results indicated that the electroless nickel/immersion gold (ENIG) surface of DBC substrate was not the ideal metallization when the sintered die attachment applied at the long-term high temperature.