Die Bonding Performance Using Bimodal Cu Particle Paste Under Different Sintering Atmospheres

A one-step polyol method was employed to synthesize bimodal Cu particles with average diameters around 200 nm and 1000 nm, respectively. The bimodal Cu particles were mixed with a reductive solvent of polyethylene glycol (PEG) to form a paste. The Cu paste was used as die bonding material to prepare Cu joints under N-2 or vacuum sintering atmosphere. The results showed that the strength of the Cu joints in N-2 atmosphere was always higher than that in vacuum. The shear strength of a Cu joint processed at 350A degrees C under only 0.4 MPa bonding pressure in N-2 was above 40 MPa, which was far higher than that obtained using single-sized nano-Cu particle paste. It is related to the dense packing of the bimodal Cu particles and slow decomposition behavior of the reductive PEG solvent. The reductive PEG solvent in the Cu paste, which effectively removed oxides on the surface of the Cu particles, was necessary for easy-oxidized Cu pastes. These results suggested that Cu pastes with suitable particle sizes, reducing solvent and sintering atmosphere could be a proper candidate for low-temperature and low-pressure bonding process.