Scallop formation and dissolution of Cu–Sn intermetallic compound during solder reflow

In an effort to elucidate the mechanism of scallop formation during reflow of solder/Cu joints in flip-chip and ball-grid array technologies, a planar intermetallic compound layer (mostly Cu6Sn5) was initially formed by solid-state annealing of SnPb/Cu joints at 150 °C for ten days. Upon subsequent reflow, dissolution of the intermetallics and formation of scallops were observed. Detailed investigation has indicated that a thermal grooving process due to dissolution of the intermetallic compound during the initial reflow cycles contributes to the formation of scalloped structures, the morphology of which is dictated by the force equilibrium condition. Upon subjecting the solder joints to several reflows, the average intermetallic layer thickness was found to first decrease and then increase, which is shown to be in good agreement with the prediction made by a model based on simultaneous dissolution and growth of the intermetallic compound during reflow. The kinetics of dissolution-growth process during reflow has also been shown to depend on the initial intermetallic thickness and the shape of the solder cap.