The wetting interaction between electroless NiP deposit/Cu substrate and SnAg solder

Electroless NiP deposit has been frequently mentioned as the barrier laer for Cu substrate or metallization for the soldering process. The NiP deposit is solderable with many solders at appropriate temperature and operation condition. The present study attempted to investigate the wetting behavior of the Sn3Ag solder on the electroless NiP with wetting balance at 250°C and 270°C. The cross section of the wetting specimen was further investigated for the interaction and the interfacial microstructure between the solder and the NiP/Cu substrate. The interface was composed of Ni₃Sn₄ and Ni₃P compound layers. A Ni-Sn-P layer was detected between these two compound layers. The thickness of these layers was analyzed for the growth kinetics. The growth of these layers were found to follow an empirical power law log h(thickness) = log k(constant) + n log t(time). The variation in n values was discussed in relating to the growth mechanism of these two layers.