Pressureless sintering bonding using hybrid microscale Cu particle paste on ENIG, pure Cu and pre-oxidized Cu substrate by an oxidation–reduction process

To investigate the properties of the oxidation–reduction bonding (ORB) process with different kinds of microscale Cu particles, we characterized the microstructure and shear strength of ORB-produced joints using microscale flake-shaped Cu particles, spherical Cu particles, and their hybrid particle pastes. The Cu–Cu joints were bonded with the ORB process at 300 °C for 60 min without pressure, and 31.2 MPa shear strength was achieved by using the hybrid particle paste comprising 75 mass.% flake-shaped Cu particles and 25 mass.% spherical Cu particles. Moreover, ORB joints were also fabricated with pre-oxidized Cu and electroless nickel/immersion gold (ENIG)-finished Cu substrates. The bonded ENIG-finished Cu joint has a shear strength that is similar to that of the bonded pure Cu joint. On the other hand, the bonded pre-oxidized Cu joint shows a shear strength of 28.1 MPa, only 10% less than the shear strength of the bonded pure Cu joint. This result reveals that the shear strength of an ORB joint is insensitive to the surface condition of the substrate, which makes the ORB process acceptable in practical large-scale production.