In situ nanostructured Cu surfaces for low-temperature joining: interfacial behavior and bonding mechanism

Reliable Cu–Cu bonding at low temperatures is widely regarded as a promising approach for electronic packaging. This study proposes the in-situ growth of Cu nanoparticles (NPs) for direct low-temperature Cu–Cu bonding via oxidizing the Cu surface and then reducing, without introducing additional Cu NPs or Cu-containing reagent. A high shear strength of 67.2 MPa is obtained at 150 °C/5 min/20 MPa in air, and the strength further increases to 98.4 MPa when the bonding temperature is raised to 225 °C. In addition, after aging in air at 250 °C for 500 h, the joint prepared at 150 °C/5 min/20 MPa retains a shear strength of 51.8 MPa, indicating excellent high-temperature reliability. The in-situ synthesized Cu NPs by oxidation and reduction show a well-bonded interface with the substrate, improving the driving force for the subsequent sintering. The grain orientation, dislocations and stacking faults within the synthesized Cu NPs are also beneficial to the sintering, thereby promoting rapid densification and enabling high-strength Cu–Cu bonding under low-temperature conditions.