Extraordinarily enhanced sintering performance of pressureless sinterable Cu nanoparticle paste for achieving robust die-attach bonding by using reducing hybrid solvent

Hybrid solvents with reducing property, composed of diethylene glycol (DEG) and glycerol (G), are developed for use as carrier solvent of the Cu nanoparticle paste, which has been employed successfully to achieve die-attach bonding through a pressureless low-temperature sintering process, with sintered joints having shear strength of 72 MPa, the highest reported thus far. The amount of residual solvent in high temperature range (150-250 °C) determined by boiling point of carrier solvents is directly related to the ratio of unbonded areas in the sintered Cu paste matrix. Solvents with the reducing property, which is crucial to decomposition of Cu lactate during sintering process, are indispensable for forming highly dense Cu bulks as the reinforcing phase in the sintered matrix. Finite element simulations of the influence of unbonded areas (defects) in the sintered Cu paste matrix on thermal cycling performance of the joints show that equivalent total strain increases with unbonded area, but shear stress behaves in the opposite way. For sintered joints using DEG+G formulated Cu paste, the dwell time of high temperature storage (HTS) at 250°C has limited effect on joints' strength. The Cu NP paste stored in the fridge at -5°C and -40°C for 6 months still retains the capability of pressureless low-temperature sintering, which meets the requirement of the industry application, concerning with transportation, storage and handling of Cu paste.