Study of the reaction mechanism between electroless Ni–P and Sn and its effect on the crystallization of Ni–P

The reaction mechanism between electroless Ni–P and Sn was investigated to understand the effects of Sn on solder reaction-assisted crystallization at low temperatures as well as self-crystallization of Ni–P at high temperatures. Ni₃Sn₄ starts to form in a solid-state reaction well before Sn melts. Heat of reaction for Ni₃Sn₄ was measured during the Ni–P and Sn reaction (241.2 J/g). It was found that the solder reaction not only promotes crystallization at low temperatures by forming Ni₃P in the P-rich layer but also facilitates self-crystallization of Ni–P by reducing the transformation temperature and heat of crystallization. The presence of Sn reduces the self-crystallization temperature of Ni–P by about 10 °C. The heat of crystallization also decreases with an increased Sn thickness.