<p>Cu sinter bonding has emerged as a promising technology for highly reliable electronic packaging due to its low cost and excellent electrical and thermal conductivity. However, Cu particles are prone to oxidation, and their sinterability decreases significantly at lower temperatures, requiring high-pressure or controlled atmospheric conditions at elevated temperatures. This study investigated reducing the bonding temperature of Cu sinter bonding under low bonding pressure in ambient air by utilizing the in-situ oxidation and subsequent reduction of the Cu surface, leading to the formation of Cu nanoparticles. Slightly oxidized Cu particles were prepared through a plasma flash evaporation process under an Ar–H₂ plasma, followed by vacuum refrigeration. These particles were used to facilitate surface reactions with polyethylene glycol (PEG400) during the bonding process. We found that the prepared particles contributed to lowering the reduction temperature through the temporary formation of Cu₂O nanoparticles. These nanoparticles, precipitating above 160 °C from Cu complexed with PEG400, were subsequently reduced to Cu nanoparticles. The generated Cu nanoparticles acted as bridges among Cu fine particles, enhancing sinterability. Consequently, Cu–Cu sinter bonding was achieved at 220 °C under 0.3 MPa in ambient air, forming a sufficiently sintered Cu layer. This approach offers a pathway for low-temperature, low-pressure bonding suitable for industrial-scale applications.</p>