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.