Correlation between Ni₃Sn₄ intermetallics and Ni₃P due to solder reaction-assisted crystallization of electroless Ni–P metallization in advanced packages

Ni₃Sn₄ intermetallic was formed by the depletion of Ni from electroless Ni–P, and a Ni₃P layer was formed simultaneously due to solder reaction-assisted crystallization during solder reflow. Both Ni₃Sn₄ and Ni₃P grew rapidly due to the solder reaction-assisted crystallization and their growth was diffusion controlled during the first 15 min of annealing at 220 °C. After that, the growth rate of Ni₃Sn₄ was greatly reduced and the crystallization of electroless Ni–P to Ni₃P was no longer induced. Based on kinetic data and scanning electron microscope morphology observations, underlying mechanisms causing this specific phenomenon are proposed. This finding is indeed very crucial since we may control the growth of Ni–Sn intermetallics by monitoring the solder reaction-assisted crystallization of electroless Ni–P.