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.