Comparison of interfacial reactions and isothermal aging of cone Ni-P and flat Ni-P with Sn3.5Ag solders

Electroless Ni-P plating has good prospects for application as a diffusion barrier layer for interfacial reactions. In this study, the interfacial reaction between the microcone-structured Ni-3.5%P coating with high crystallinity and the Sn3.5Ag solder was investigated and compared with Sn3.5Ag/flat-Ni-(2%, 5%, 10%)P couples. Wetta-bility experiments of Sn3.5Ag on four types of Ni-P coatings were carried out. Reflow at 250 degrees C for various minutes was performed, as well as isothermal aging experiments at 175 degrees C for various durations. The results showed that the Sn3.5Ag achieved the best wettability on cone Ni-3.5%P due to the pyramidal morphology that provides an additional driving force for wetting. During isothermal aging, the growth of intermetallics (IMCs) in the Sn3.5Ag/cone-Ni-3.5%P couple was attributed to the contribution of boundary diffusion mechanism to the rate control process, while that in the remaining three couples was a grain boundary diffusion-limited process. After reflow and isothermal aging, fewer microvoids and no harmful Ni-Sn-P layers were detected in the interface of Sn3.5Ag/cone-Ni-3.5%P couples, while they were detected in the other three couples, indicating that the cone Ni-3.5%P couple could inhibit the formation of Ni-Sn-P layer and microvoids. As a result, Sn3.5Ag/cone Ni-3.5% couples exhibited better shear resistance.