In Situ Reduction Behavior and Sinter-Bonding Characteristics of CuO-Containing Cu Flake Composite Paste in Air

CuO-containing Cu flake composite pastes were developed for pressure-assisted Cu-Cu sinter bonding in air by utilizing the in situ reduction of CuO in a glycol-based reducing solvent. Composite pastes with Cu:CuO mass ratios ranging from 10:0 to 7:3 were prepared and characterized with respect to thermal behavior, bond-line microstructure, and shear strength. TG-DTA and XRD analyses revealed that the glycol-based solvent suppressed oxidation of the Cu flakes upon heating and promoted the in situ reduction of CuO. Cross-sectional observations of the bond-line showed that the resulting Cu nanoparticles filled the interparticle voids between Cu flakes, thereby enhancing bond-line densification. As the CuO content increased, the amount of reduced Cu nanoparticles also increased, which markedly improved the sinterability of the paste. However, excessive CuO addition at a Cu:CuO ratio of 7:3 caused incomplete reduction of CuO, resulting in residual oxide phases within the bond-line and consequently degrading sinter-bondability. Among the compositions investigated, the paste with a Cu:CuO ratio of 8:2 exhibited the most effective sinter-bonding behavior. Under bonding conditions of 300 ℃ and 10 MPa in air, this optimized composition formed a relatively dense bond-line and achieved a shear strength of 25.1 MPa after 5 min. These results demonstrate that solvent-assisted in situ reduction of CuO is a promising approach for developing Cu-based die-attach pastes for power module applications.