Low-temperature and pressureless sinter joining of Cu with micron/submicron Ag particle paste in air

Sinter joining with Ag particle paste has emerged as a promising lead-free die-attachment solution for power devices owing to its low-temperature and pressureless processing characteristics. Using this process, the achieved performance is superior to that of traditional solders. In this work, a robust, direct Cu bonding was realized using a low-cost, hybrid Ag paste. The bonding strength of the bare Cu joint structure achieves approximately 30 MPa when bonded at 250 °C without assisted pressure in air. To understand this excellent result, the bonding microstructure was investigated via SEM, EDS, XPS and TEM. This research revealed that the direct Cu bonding is closely related with the Cu2O nanoparticles formed on the bare Cu surface under the effect of solvent and self-generate Ag nanoparticles during the sintering. These two nanoparticles can form an intact bond, which allows a preferable bonding strength at pressureless, low-temperature and atmospheric sintering condition. Also, a possible mechanism has been proposed to interpret the formation of this successful direct Cu bonding.