Abstract: Bimodal-sized Cu nanoparticles (NPs) with a conformation of NPs (~9 nm) capping on large ones (~160 nm) are synthesized through a one-step chemical reduction method, which are then used to prepare the Cu paste capable of pressureless sintering at low temperature. The lactic acid based capping agents show both effective oxidation protection and sintering enhancing effects for Cu NPs. Cu paste joints after pressureless sintering at 280℃ have high shear strength up to 65 MPa, which is underpinned by the strengthening effect of bulk Cu particles and the formation of nearly defect-free and dense interfaces joined metallurgically between sintered Cu paste matrix and substrates. The sintered Cu paste joint is formed by means of two nano-joining processes, i.e., nano-joining between Cu NPs and Cu NPs via bridging under the driving force from their high specific surface energy, as well as nano-joining between Cu NPs and substrates through formation of continuous yet dense diffusion layers at interfaces of sintered particles and substrates due to the enhanced self- and inter-diffusion of Cu NPs produced from decomposition of Cu lactate. Despite formation of some voids at the interface of sintered Cu paste/Cu substrate after high temperature storage at 250℃ for 1 000 h, Cu paste joints still show relatively high strength (>56 MPa).

Key words: Cu nanoparticle paste, bimodal distribution, pressureless sintering, nano-joining, reliability