Effect of substrate surface finish on bonding strength of pressure-less sintered silver die-attach

The quality of die-attach by sintered-silver bonding depends on the bond-line cohesion from powder sintering and interfacial adhesion from atomic inter-diffusion. The latter depends on the surface finish, its chemistry and microstructure, on the device and substrate. Reports of sintered-silver die-attach on different substrate surface finishes can be found in the literature, but by separate research groups focused on bonding to one or two surface finishes. In this study, we evaluated sintered-silver die-attach on DBC substrates with five different surface finishes: electro-plated Ag, electroless-plated Ni(P)/Ag, electro- and electroless-plated Ni(P)/Au, and native Cu. All of the bonding runs were done pressure-less in air using a nanosilver paste at sintering temperatures ranging from 220°C to 300°C. The shear strengths of sintered-silver bonding on the two Ag-finishes and native Cu increased with sintering temperature to values above 45 MPa. Cross-sectional microstructural and chemistry analyses of the bond-lines on Cu showed Cu oxidation at bonding interface, but the shear strength was still above 45 MPa in the presence of Cu oxidation. The shear strengths of sintered-silver bonding on the two Au-finishes increased at first and then decreased with temperature. Kirkendall voiding at the Au/Ag interfaces was found responsible for the weaker bonding at higher sintering temperatures.