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 difficulty controlling 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 effect of bonding temperatures (200–400°C) and atmospheres (N2 or formic acid) on the shear strength of joints formed between NPC sheets and bare Cu disks were investigated by scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. It was shown that the bondability of NPC under an N2 atmosphere is closely related to the oxide layer formed on its surface that impairs the diffusion of Cu atoms between the NPC and Cu substrate. Furthermore, the coarsening 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 NPC/Cu joints formed under a formic acid atmosphere increased from 14.1 to 35.9 MPa with increasing bonding temperature. Based on the results of the investigation, a mechanism was proposed to explain the superiority of the Cu–Cu joints achieved using this method.