Effect of Various Parameters on the Shear Strength of Solid-State Nanoporous Cu Bonding in Cu–Cu Disks for Power Device Packaging

Nanoparticle sintering is considered a promising alternative bonding method to Pb-based soldering for the attachment of components in high-temperature electronic devices. However, the technology still poses certain challenges, such as the difficulty in controlling the joint thickness and the generation of voids owing to solvent evaporation. In this study, a solid-state (solvent-free), nanoporous-Cu (NPC) bonding method was examined. The purpose of this study was to find the effect of the bonding temperature (200–400°C) and atmosphere (N2 or formic acid) on the shear strength of the joints formed between the NPC sheets and bare Cu disks for the replacement of the Pb–5Sn solder joint for high-temperature applications (> 300°C). The NPC/Cu joints were investigated by scanning electron microscopy, x-ray diffraction, and transmission electron microscopy. It was shown that the bondability of NPC under a N2 atmosphere is closely related to the oxide layer formed on the NPC surface that impairs the diffusion of Cu atoms between the NPC and Cu substrate. Furthermore, the densification of the NPC microstructure under a formic acid atmosphere at ≥ 350°C owing to the rapid diffusion of Cu atoms and accompanying plastic deformation induced by surface stress enhances the shear strength of the resulting NPC/Cu joint. The shear strength of the NPC/Cu joints formed under a formic acid atmosphere increased from 14.1 MPa to 35.9 MPa with an increasing bonding temperature. These results suggest that the NPC/Cu joints are a good alternative to Pb–5Sn solder joints. Based on the results of the investigation, a mechanism was proposed to explain the superiority of the Cu–Cu joints achieved using this method.