Microstructure Evolution and Shear Strength of the Cu/Au80Sn20/Cu Solder Joints with Multiple Reflow Temperatures

In order to present the multiple reflow process during electronic packaging, the influence of the different short-time reheating temperatures on the microstructure and shear strength of the Cu/Au80Sn20/Cu solder joints was studied and discussed. The results showed that high-quality Cu/Au80Sn20/Cu solder joints were obtained with 30 degrees C for 3 min. The joints were mainly composed of the zeta-(Au,Cu)(5)Sn intermetallic compound (IMC) with an average thickness of 8 mu m between Cu and solder matrix, and (zeta-(Au,Cu)(5)Sn +delta-(Au,Cu)Sn) eutectic structure in the solder matrix. With an increase in the multiple reflow temperature from 180 degrees C to 250 degrees C, the microstructure of the joint interface showed little change due to the barrier effect of the formed zeta IMC layer and the limitation of short-time reheating on the element diffusion. The eutectic structures in the solder matrix were coarsened and transformed from lamellar to the bulk morphology. The shear strength of the as-welded joint reached 31.5 MPa. The joint shear strength decreased slightly with reheating temperatures lower than 200 degrees C, while it decreased significantly (by about 10%) with reheating temperatures above 250 degrees C compared to the as-welded joint. The shear strength of the joints was determined by the brittle solder matrix, showing that the joint strength decreased with the coarsening of the delta phase in the eutectic structure.