Nucleation rates of Sn in undercooled Sn-Ag-Cu flip-chip solder joints

The nucleation of Sn from the melt in commercial SnAgCu flip chip solder joints was monitored at a number of different temperatures. Nucleation rates were estimated from measurements of nucleation times for 440 solder balls after one reflow and were found to be well epitomized by the expression I = 2 × 10⁹ exp[(-1.6 × 10⁵)/(T × (ΔT) ²)] m^-3 s^-1, as per classical nucleation theory. After an additional reflow, the nucleation rates of the same 440 samples were observed to increase to I = 2 × 10⁹ exp[(-8.9 × 10⁴)/(T × (ΔT)²)] m^-3 s ^-1. Thus it was shown that the expressions of classical nucleation theory well characterize nucleation kinetics for this system. These changes in nucleation kinetics were correlated with continued dissolution of Al and Ni in to the SnAgCu melt. Such increases in nucleation rates meant increases in the average solidification temperatures of the solder balls after reflow. Variations in the Sn grain morphology of the solder joints were correlated with these changes in solidification temperature, with larger Sn grains (beach ball Sn grain morphology) observed at higher solidification temperatures.