Reaction kinetics of Ni/Sn soldering reaction

Soldering between solid Ni and liquid Sn is studied, including late stages of reflow. Only during the very early stages of the process, intermetallic grains grow with a 1/3 power dependence on time. Later, kinetics clearly changes to parabolic growth, but remarkably in two subsequent regimes distinguished by different rate constants. The observed kinetics is discussed with respect to recent flux-driven ripening theory. This theory is only valid, if at all, for short reflows up to about 4 min. Transmission electron microscopy reveals the predicted scallop-like microstructure only at the very beginning. A sponge-like structure of equiaxed grains then develops, triggered by permanent nucleation of new grains at the Ni/Ni3Sn4 interface. Wetting of grain boundaries appears only up to a certain depth in the reaction zone. This remarkable behavior is explained by thermodynamic arguments.