Combined thermal, thermodynamic and kinetic simulations of the solidification of SnAgCu interconnections

Combined thermodynamic, kinetic and thermal modeling and simulation tools have been employed for investigating the solidification of lead-free solder interconnections during reflow soldering of CSP components on printed circuits boards. Thermodynamic calculations have been used for evaluating the compositions of liquid interconnections and the free energy of nucleation. The kinetic simulations were performed to predict the nucleation rate of the primary β-Sn formation and to evaluate the actual solidification temperatures. The thermal models at reflow oven, component and interconnection levels were established hierarchically for evaluating the temperature fields around interconnections during cooling. It was shown that the solidification of solder interconnections is initiated by the heterogeneous nucleation of the primary β-Sn at the Liq/Cu₆Sn₅ interfaces. Because the temperature gradients over the interconnections were predicted to be insignificant, it is likely that the solidification is initiated by almost simultaneous formation of β-Sn at both components and boards interfaces. Even though the actual solidification temperatures of interconnections depend on Ag-content, the supercooling range is fairly constant (18-20°C). It enables us to predict important parameters for the solidification of lead-free solder interconnections, which are necessary in studying the formation of cells, dendrites, colonies as well as other microstructural units of solder interconnections.