Nucleation Kinetics and Solidification Temperatures of SnAgCu Interconnections During Reflow Process

The nucleation kinetics of beta-Sn in liquid SnAgCu interconnections was simulated on the basis of nucleation theory. After evaluations of the compositions of liquid solder interconnections during reflow soldering, the free energy of nucleation can be calculated thermodynamically. Since homogeneous nucleation rate is very low, the mechanism that initiates solidification of interconnections is the heterogeneous nucleation at the Liq/Cu₆Sn₅ interface and the corresponding nucleation rates in interconnections are therefore simulated. Additional simulation of the nucleation rate in a tin droplet was also done to determine a critical range of nucleation rate. The contact angle for the heterogeneous nucleation was evaluated with the help of recent differential scanning calorimetry (DSC) measurements. The simulations allow us to evaluate the actual solidification temperature of interconnections, which is a fundamental parameter in studying the formation of solidified microstructure in solder interconnections. The variation of the solidification temperature with cooling rate, interconnection size, and morphology of intermetallic compounds is also discussed. Even though the actual solidification temperatures of interconnections depend on Ag-content, the supercooling range is shown to be fairly constant (18-20degC), which offers a possibility to predict the actual solidification temperature of interconnections via phase equilibria information