A comparative study of the interfacial reaction between electroless Ni–P coatings and molten tin

Abstract

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 3P intermetallic layer and its effects on the subsequent reaction. It was found that a continuous layer of Ni 3P 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 3P and Ni. The Ni 3P 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 3P layer was thin, whereas a significant loss of P took place as the Ni 3P thickness increased. It is proposed that the Ni 3P 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 3P 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 3P phase cannot act as an effective barrier layer to the attack of molten tin toward the substrate. Defects in the Ni 3P were found to allow localized penetration of molten tin.