A comparative study of the reaction characteristics between molten tin and both as-plated and heat-treated Ni-P coatings was carried out, with a specific focus on the stability of the Ni 3 P intermetallic layer and its effects on the subsequent reaction. It was found that a continuous layer of Ni 3 P may be formed on both types of Ni-P during the interfacial reaction, despite the fact that heat-treated Ni-P is a two-phase mixture of Ni 3 P and Ni. The Ni 3 P formed on the heat-treated Ni-P was thinner than that on as-plated Ni-P. A mass conservation analysis of P revealed that no or limited P was lost into the molten tin when the Ni 3 P layer was thin, whereas a significant loss of P took place as the Ni 3 P thickness increased. It is proposed that the Ni 3 P phase is stable and may not undergo chemical decomposition during the interfacial reaction. The loss of P to the molten tin observed in the present study is most likely due to the crumbling of Ni 3 P particles into the liquid phase, as a result of the enhanced mass transport due to use of thin copper wire substrates rather than a planar surface. Finally, the results show that the Ni 3 P phase cannot act as an effective barrier layer to the attack of molten tin toward the substrate. Defects in the Ni 3 P were found to allow localized penetration of molten tin.