Thermal Shock Reliability of Silver-Sintered Bonding of Metal-Plated Aluminum Surfaces

Silver (Ag) sintering is becoming more critical for future wide bandgap (WBG) power module substrate attachments. However, sintered Ag joints with plated aluminum (Al) heatsinks and directly bonded aluminum (DBA) substrates presently suffer from poor reliability. To resolve this problem, this work studies the reliability of plated (Nickel) Ni/Ag metallization on Al for sintered Ag joints and proposes a new plated Ni/Copper (Cu)/Ag metallization stack for improved reliability. The shear strength and thermal shock (TS) reliability of the sintered Ag joints for different metallization layers are studied, and microstructural and elemental analyses were performed to analyze the failure modes. The results show that the reliability of the sintered Ag joints by the traditional Ni/Ag metallization is rather limited because of poor adhesion between Ni and Ag. In contrast, the shear strength of the new Ni/Cu/Ag metallized sintered Ag joints is consistently above 40 MPa up to 500 TS cycles, with the dominant failure modes formed by Al/Ni delamination and cohesive failure. Preparing sintered Ag joints with the Ni/Cu/Ag metallization with longer sintering times removed the unwanted delamination failure mode and only left the preferred cohesive failure mode; moreover, the shear strength improved significantly, with values reaching 130 MPa. Furthermore, a new failure mode appears in the sintered Ag joint of the Ni/Cu/Ag stack, implying that the Al/Ni metallization weakness there is less of a limiting factor. This proves that our new metallization stack resolves present delamination issues in Ag sintered joints with Al heatsinks and DBA substrates and thereby supports exploiting the full potential of sintered Ag joints.