Computer modeling of contact degradation by intermetallic growth

A single-spot finite-element contact model has been used to study the degradation process of a Copper to Aluminium contact. The computer software has a multi-physics approach, i.e. it simultaneously solves the electrical, thermal and mechanical equations. In addition, non-linear and transient phenomena can be treated. The model also includes the thermodynamics and growth kinetics of intermetallic layers. When intermetallic layers are formed in the contact spot, the temperature maximum is shifted from the central parts of the spot to the rim. The spot temperatures and the growth rate of the intermetallics layer increases non-linearly with time. This results in a self-accelerating degradation by diffusion that will eventually transform to electromigration when critical levels of temperature and current density are reached. Even at extremely high current densities the temperature level for electromigration is reached after several hours. This indicates that temporary overcurrents do not cause contact degradation by growth of intermetallics.