Effects of Fine Cu Particle Size on Sinter-Bondability in Die Bonding Using Cu Paste Possessing Effective Reducing Formulation

The application of wide-bandgap semiconductors in next-generation power modules requires cost-effective Cu particles and a reduced bonding time in the die attachment process to enable efficient industrial-scale manufacturing. Therefore, this study aimed to analyze the effect of Cu particle size variation on pressure-assisted sinter-bondability and bond line shear strength. Cu particles were synthesized through a simple wet-chemical process, in which pH variation was employed to obtain submicrometer-sized Cu particles with average diameters of 500, 300, and 150 nm. The synthesized particles exhibited pure Cu composition, forming only a native oxide layer on their surfaces. In pastes containing these Cu particles, smaller particle sizes led to the delayed evaporation of the reducing solvent, which in turn delayed the exothermic reactions associated with particle sintering and oxidation. However, the sintering-induced exothermic peak became more pronounced as the particle size decreased, confirming that smaller particles improved sinterability. Pressure-assisted sinter bonding performed in air at 300 °C indicated that a decreased particle size contributed to the densification of the bond line structure and an increase in shear strength. Specifically, the paste containing 150 nm Cu particles achieved a highly dense microstructure and an exceptional shear strength of 36.7 MPa within just 30 s of sinter bonding. These findings demonstrate that reducing the particle size is essential for enhancing the sinter-bondability of cost-effective Cu particle-based sinter-bonding pastes.