How to determine surface roughness of copper substrate for robust pressureless sintered silver in air

It is significant to attach power devices on low-cost copper (Cu) substrates by pressureless silver-sintering in air. The Cu surface in direct-bond-copper (DBC) substrates could have a wide range of surface roughness due to the various processing techniques. It is crucial to find out how the surface roughness influences the bonding strength of the sintered-silver (Ag) die-attachment. In this study, we did die-attach on Cu substrates with surface roughness Ra  = 0.15-8.41 µm and Rz = 1.22-50.97 µm by pressureless sintering of nanosilver paste in air. When the surface roughness (Ra of 0.65 μm) is slightly larger than the silver nanoparticles (diameter of 0.6 μm), the high shear strength in excess of 50 MPa was achieved, likely due to both strong metallic bonds and mechanical interlock. This work could help to prepare DBC substrates with optimized surface roughness to accommodate the viscosity, the particle size, and the sintering shrinkage of the silver paste for well using the sintered-Ag die-attach technology.