Joint strength and interfacial microstructure between Sn–Ag–Cu and Sn–Zn–Bi solders and Cu substrate

Abstract

In the present research, in order to fundamentally investigate the strength and interfacial microstructure of the joints between Sn–3.5Ag–0.7Cu (SAC) and Sn–8Zn–3Bi (SZB) solders and Cu, examinations using the standardized solder joint specimen for measuring the shear strength were performed. The effects of the addition of Ag, Bi and Pb to the solders, which are the ingredients of possible lead plating materials of quad plat package, solder layer thickness and aging treatments ranging from 358 to 423 K were investigated. The SAC base solder joints, whose strength hardly degraded after aging, were more reliable than the SZB base solder joints. Since no detrimental effect of the Bi or Pb addition was recognized, the lead plating materials are found to have no significant effect on the strength of the SAC solder joints. Although the SZB base solder joints had higher strength than the SAC base solder joints in as-soldered condition, a significant degradation of the joint strength occurred after aging. In the case of thinner solder layer of 20 μm, the Cu6Sn5 layer that formed at both the solder/Cu5Zn8 and Cu5Zn8/Cu interfaces after aging caused the fracture of the solder joints and resulted in the degradation of the joint strength. In the case of thicker solder layer of 100 μm, the Cu5Zn8 interfacial layer continuously grew during aging, which caused the fracture of the solder joints and resulted in the decrease in the joint strength. As for the effect of the addition elements, while Pb addition caused a marked degradation of the joint strength, Ag addition and Bi addition had no detrimental effect on the strength of the solder joints. In particular, the joint with the Ag addition solder had the better strength in the case of thinner solder layer. Thus, as for lead plating for the SZB solder, Sn–Pb should be avoided and Sn–Bi or Sn–Ag as Pb-free plating has no detrimental effect on the joint strength. In particular, Sn–Ag plating is preferable in the viewpoint of the joint strength.