Effect of aging on the microstructure and shear strength of SnPbAg/Ni-P/Cu and SnAg/Ni-P/Cu solder joints

The effect of aging on the microstructure and shear strength of 62Sn36Pb2Ag/ Ni-P/Cu and SnAg/Ni-P/Cu surface mount solder joints was investigated. An intermetallic (IMC) layer of Ni,Sn, forms at the interface between both solders and the Ni-P barrier layer and it thickens with aging time, with a decrease in the thickness of remaining Ni-P layer. The SnAg solder joint initially has a greater shear force than that of SnPbAg, but it drops dramatically after 250 h aging, and fracture occurs at the Ni-P/Cu interface afterwards, although it initiates in the solder in the initial stage of aging. The fracture in SnAg solder joint may arise from the excessive depletion of Ni characterized by a rapid accumulation of P in the remaining Ni-P layer, which results in a poor adhesion between the Ni-P layer and the Cu substrate. However, for the SnPbAg solder joint, the shear force initially decreases rapidly then asymptotically approaches a minimum, and fracture occurs from inside solder toward the solder/Ni-P interface. SnPbAg solder joint keeps relatively higher shear strength compared to SnAg solder joint after long term aging even though it decreases with aging time.