Cu–Cu low-temperature diffusion bonding by spark plasma sintering: Void closure mechanism and mechanical properties

Abstract

• Combined process of SPDT and SPS diffusion bonding can realize high-strength Cu-Cu bonding of 271 MPa at 202°C. • A new mechanism is proposed that the pulsed current significantly promotes the closure of interfacial micro-voids during the SPS diffusion process. • A new “evaporation-deposition” mechanism for the closure of interfacial nano-voids during SPS diffusion bonding is proposed. • Molecular dynamics simulations show that energetic atoms promote the void closure during SPS diffusion bonding. In this study, combining the single point diamond turning (SPDT) and spark plasma sintering (SPS), we achieved high-strength diffusion bonding of copper at an ultra-low temperature of 202°C (0.35 T m , T m : absolute temperature of the melting point). Additionally, the closure mechanism of interfacial micro- and nano-voids during the Cu–Cu SPS diffusion bonding is systematically revealed for the first time. For micro-voids, the pulsed current is found to induce additional diffusion flux and plastic deformation, thereby facilitating the void closure. Molecular dynamics (MD) simulation revealed that at the atomic scale, high-energy Cu atoms induced by the pulsed current can significantly promote the diffusion of low-energy atoms in their vicinity and accelerate the void closure. This study also proposes a novel “evaporation–deposition” nano-void closure mechanism for the previously unstudied nano-void closure process. The results show that the synergistic effect of the pulsed current and nanoscale surface roughness can significantly improve joint strength. At a low temperature of 405°C (0.5 T m ), on combining the computerized numerical control (CNC) turning and SPS diffusion bonding, the joint strength can reach 212 MPa, while that for the joint obtained by traditional hot pressing diffusion bonding at the same temperature is only 47 MPa. We obtained an ultra-high joint strength of 271 MPa using the combined process of SPDT and SPS diffusion bonding at an ultra-low temperature of 202°C (0.35 T m ), which is approximately 600°C lower than the traditional diffusion bonding process temperature of 800°C (0.79 T m ). To sum up, this study provides a novel method and theoretical support for realizing low-temperature high-strength diffusion bonding.