High Temperatures Of Lead-free Solder Surface Mount Solder Joint Reliability

In this thesis, the reliability of several kinds of lead-free solderjoints (Sn96.5Ag3.5, Sn95.5Ag3.8Cu3.5 and Sn95Sb) for high temperature application in automotive electronic assemblies were investigated by using accelerate tests including aging, thermal cycling and high temperature & high humidity The main results are summarized as follows: 1. The reliability of SnAg solder joint with Cu pad was investigated. It is found that the reaction rate between SnAg and Cu is smaller compared with SnPbAg, and the shear strength of SnAg/Cu solder joint is higher and decreases at a smaller rate during aging at 1 500C. The fracture surface analysis shows that as the aging time increases, the fracture takes place along the solder/Cu6Sn5 interface with an extension toward the Sn-Cu intermetallic layer. During thermal cycling, cracks developed inside the solder joint and shear strength of the solder joints decreased with cycling number. The strength lose of SnAg solder joint is smaller than that of SnPbAg solder joint after same cycles. High temperature & high humidity process has little influence on the microstructure and shear strength of both SnAg/Cu and SnPbAg/Cu solder joints. 2. The reliability of SnAg solder joint with Au/Ni-P/Cu pad was investigated. An intermetallic layer of Ni3 Sn4 forms at the interface between both solders and the Ni-P barrier layer. The thickness of the Ni3Sn4 layer increases, meanwhile the thickness of the remaining Ni-P layer decreases during aging at 1500C. The SnAg solder joint initially has a greater shear force than that of SnPbAg, but it drops dramatically after 250h aging, and fracture occurs at the Ni-P/Cu interface afterwards, although it occurs 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, fractures at the Ni-P/Cu interface do not occur in SnPbAg solder joint even after a long time of aging and SnPbAg solder joint keeps a higher shear strength than that of SnAg solder joint. During thermal cycling, the shear strength of SnAg solder joint decrease at a larger rate than that of SnPbAg solder joint, and fracture at Ni- P/Cu interface appears again in SnAg solder joint. The microstructure and shear strength of both SnAg and SnPbAg with Au/Ni-P/Cu solder joints are influenced slightly by High temperature & high humidity process. 3. The joint shape, microstructure and shear strength of 95Sn5Sb solder joints with different terminal metallizations (AgPd and Ni/AgPd) of the component were investigated with comparison to 62Sn36Pb2Ag solderjoints. The results show that the drastic reaction between SnSb solder and AgPd leads to the solder not spreading entirely on the Cu pad but concentrating on the terminal of component. The shear strength of SnSb/AgPd solderjoint is very low and the fracture occurs at the original interface of AgPd/ceramic. The Ni layer in SnSb/Ni/AgPd solder joint effectively avoids the leaching of AgPd to the solder so that a high strength solder joint with perfect shape is achieved. The terminal metallizations of the components have little effect on the shape and the shear strength for SnPbAg solder joint. Fracture occurs inside the SnPbAg solder after shear test. 4. The interdiffusion be...