Influence of Silver Addition and Reflow Cooling Rate on Microstructure and Strength of Tin-Lead System Solder Ball Bonding

Sn-37 mass%Pb and Sn-36 mass%Pb-2 mass%Ag solder balls have been bonded on Cu pads at the cooling rates of reflow. The microstructure and the mechanical properties of the ball bonding have been investigated. The eutectic lamellar structure (Sn phase/Pb phase) of both ball was finer. The lamellar spacing of Sn-36 mass%Pb-2 mass%Ag solder ball was much smaller than that of Sn-37 mass%Pb one at the faster cooling rates. The lamellar structure became finer since Ag may interrupt the diffusion of Sn and Pb. The ball hardness depended on the cooling rate and increased with the smaller lamellar spacing at the faster cooling rate. Since the eutectic Ag3Sn intermetallic compound formed in Sn-36 mass%Pb-2 mass%Ag solder ball, the hardness of this alloy ball was higher than that of Sn-37 mass%Pb one. The shear strength of both ball bonding dropped remarkably at 10 K/min since Ni3Sn4 reaction layer at the bonding interface was thicker. The shear strength of Sn-36 mass%Pb-2 mass%Ag ball was lower than that of Sn-37 mass%Pb one at each cooling rate. The primary needle-shape Ag3Sn was observed in Sn-36 mass%Pb-2 mass%Ag ball and near the bonding interface. The lower shear strength of Sn-36 mass%Pb-2 mass%Ag ball bonding was due to the primary needle-shape Ag3Sn near the bonding interface and the brittle Ni3Sn4 reaction layer. Since the fracture face of the Sn-36 mass%Pb-2 mass%Ag ball was flatter than that of the Sn-37 mass%Pb ball, the brittle fracture occurred at the Sn-36 mass%Pb-2 mass%Ag ball bonding interface.