A novel multiscale silver paste for die bonding on bare copper by low-temperature pressure-free sintering in air

Nanosilver sintering is expected to overcome the limitation of relatively high production cost and become widely available for the die bonding of power electronics. A potential application of nanosilver sintering is bare copper bonding, where replacing substrates with auxiliary silver or other plating that can damage bonding would be advantageous. Here, we introduce a novel multiscale silver paste containing both nanoparticles (20–100 nm) and microparticles (1–5 μm) for the bonding of high-power chips on a bare copper substrate by pressure-free sintering in air. The energy potential difference generated in the surface force field was critical in the formation of sintering necks between the nano and microparticles, which, together with other microparticles, formed the high-density sintered structure. Despite the development of a copper oxide film, the interfacial bonding was comparable to or higher than the sintering force due to the high surface energy of porous sintered structure and easy diffusion of nanoparticles occurred. A processing temperature of 265 °C was considered optimal for bare copper joint (shear strength: 53 MPa, transient thermal impedance: 0.132 °C/W) considering the trade-off between achieving excellent mechanical and thermal properties while minimizing oxidation.