Improvement of oxidation resistance and bonding strength of Cu nanoparticles solder joints of Cu-Cu bonding by phosphating the nanoparticle

Under an optimal phosphating time of 30 min, the nanoparticles can be effectively protected from oxidation at temperatures below 300 degrees C and stored for months in air. The shear strength of the phosphated joint was higher than that of the untreated one while the joint had a better performance in storing and aging tests. The shear strength decreased 23% after storing for 90 days and 17% after aging at 80 degrees C for 12 h (the figures for the untreated joint were 60% and 44%, respectively). The major oxidation product formed on the surfaces of the nanoparticles was Cu2O and the formation of voids in the sintering structure due to oxidation reduced the shear strength of the joints. A joint with a bonding strength of about 20 MPa was achieved even at 250 degrees C and under a pressure of 8 MPa.