Stability of Ni3P and its effect on the interfacial reaction between electroless Ni-P and molten tin

The stability of Ni3P and its effect on the reaction with molten tin were evaluated in the present study by comparing the reaction behaviour of Ni-P coatings in the as-plated and heat-treated conditions. It was found that after the solder reaction a dark layer of Ni3P was formed at the interface on Ni-P coatings in both conditions, but its thickness was thinner on heat-treated coatings than on as-plated coatings. From the compositional analysis on the cross-sections, it was found that not all the P ejected from the interfacial reaction between Sn and Ni-P was consumed by the formation of Ni3P, as P was detected in a second layer of interfacial phase which also contained Ni and Sn. The formation of Ni3P was found to be characteristic of a diffusion process, whilst the thickness of Ni-P coating consumed during the reaction was found to be linearly proportional to the reaction time. It is concluded that the Ni3P phase formed during the solder reaction is not stable and therefore cannot act as an effective barrier to reactions with liquid solders.