Brass bonding using copper nanoparticles and zinc powder for packaging of power electronic devices

Zn powder was mixed with Cu nanoparticle joints by applying a very low additional pressure of 20 kPa to improve the bonding strength. Cu-Zn alloying throughout the bonding layer was achieved by firing at 623 K for 5 min. The phase composition of the bonding layer corresponded approximately with the weight ratio of Zn/(Zn + Cu) (Zn mixing ratio). As the Zn mixing ratio increased, the low-density regions in the parent phase and cracks in the bonding layer decreased, whereas voids in the bonding layer increased. The bonding strength of the SiC/direct bonded copper joints increased with the Zn mixing ratio up to 30 wt%, and the maximum bonding strength measured was 65.0 MPa, which is approximately 1.8 times greater than that of the conventional Cu nanoparticle joint. Alloying in the bonding layer containing 30 wt% of Zn progressed with the firing temperature. Simultaneously, the bonding strength increased with the firing temperature up to 623 K. However, the joint fired at 673 K exhibited a lower bonding strength than that fired at 623 K, because of cracks and voids in its bonding layer. The results indicate that Zn powder mixing can enhance Cu nanoparticle joints without additional high-pressure processing. •Mixing of Zn powder with Cu nanoparticle joint at very low pressure was examined.•Cu-Zn alloying occurs throughout the bonding layer.•The bonding strength depends on the Zn mixing ratio and firing temperature.•Optimum Zn mixing greatly improves the bonding strength of Cu nanoparticle joints.