Phase-field simulations of intermetallic compound evolution in Cu/Sn solder joints under electromigration

In this work, we present a preliminary model based on the multiphase-field formalism that is used to investigate the evolution of intermetallic compound (IMC) layers in the Cu/Sn solder system under electromigration conditions. The simulation considers the concurrent evolution of the Cu3Sn and Cu6Sn5 IMC layers under electrochemical driving forces. We use CALPHAD descriptions for the thermodynamics of the bulk phases (Cu, Sn and both IMCs) and incorporate electrochemical contributions to the total free energy of the microstructure in order to simulate the evolution of the interfacial microstructure due to mass-current couplings. The simulation considers grain-boundary diffusion as the dominant mechanism for chemical-potential-driven mass flux. The simulation considers the effect of current density and polarity on the growth of IMCs. The simulations are in turn compared to experimental measurements of growth rates, morphology of IMC grains and evolution of interface roughness. The analysis of experiments and simulations suggest that back-stress resulting from the non-equilibrium vacancy generation due to diffusive flux imbalance among the individual diffusants (Cu and Sn) plays a fundamental role in the evolution of the IMC layers. Overall, the simulations and experiments show good qualitative agreement.