A pressureless sintering process of copper nanoparticle based on the copper formate pyrolysis reaction was developed. The aim was to address challenges faced by the existing copper sintering technologies, such as copper oxidation and the need for additional pressure. Copper nanoparticles (Cu NPs) were treated with formic acid solution to generate dense copper formate films, which, upon sintering, formed Cu-Cu joints. Quality tests confirmed that a formic acid reaction time of 10 min, PEG solvent, substrate surface polishing with 0.048 mm sandpaper, and a heating rate of 5 ℃/min were the optimal sintering conditions. This approach achieved pressureless sintering of copper nanoparticles, yielding joints with a shear strength of 16.18 MPa and low resistivity of 570 μΩ/m. Even after a 200 h high-temperature aging test, the joints maintained a shear strength of 9.38 MPa, which verified the reliability of the sintering process. Therefore, this method presents a novel approach for realizing reliable interconnections in third-generation semiconductor chips.