3-D Numerical Multiphysics Model for Cu-Al Wire Bond Corrosion

Copper is a new low-cost material used for wire bonding because of its excellent electrical and mechanical properties. However, copper is easier to oxidize in comparison with gold and susceptible to corrosion in the presence of moisture. In the presence of chlorine-ions in EMC, the Cu-Al wire bond interface has a high propensity for fracture resulting from corrosion processes. Multiphysics methods are needed to model the diffusion and corrosion processes in plastic encapsulated electronics. In this paper, a novel 3D multiphysics model for the corrosion of copper aluminum ball bonds has been developed. The model can be used to simulate the diffusion mechanism and kinetics under extreme environmental conditions. Corrosion-kinetics of the ball bonds has been modeled with the Butler-Volmer equation and the transportation of the chlorine ions in the EMC electrolyte has been modeled with the Nernst-Planck equation. Chlorine-ions act as catalyst for the corrosion process. The deformation of the corrosion cell geometry after the initiation of the corrosion process and the following change of electric field of potential and current density in the domain has been captured in the model. Damage initiates with the initiation of a crack which would propagate in the wirebonds. Since the transportation of chlorine ions in EMC is governed by both electric potential gradient and concentration gradient, both the Butler-Volmer Equation and the Nernst-Planck Equations are coupled. The time-dependent 3D Multiphysics model has been implemented in COMSOL Multiphysics.