Effect of aging on the microstructure and shear strength of SnPbAg/Ni-P/Cuand 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 so lders and the Ni-P barrier layer and it thickens with aging time, with a de crease in the thickness of remaining Ni-P layer. The SnAg solder joint init ially has a greater shear force than that of SnPbAg, but it drops dramatica lly after 250 h aging, and fracture occurs at the Ni-P/Cu interface afterwa rds, 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, wh ich 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 r elatively higher shear strength compared to SnAg solder joint after long te rm aging even though it decreases with aging time.