Computer simulation of the influence of intermetallics on the stability of electrical contacts

The influence of the thicknesses of intermetallic layers on the stability of Aluminium to Copper contacts, has been simulated in a computer by means of finite element calculations. A two-dimensional axi-symmetric model with a simplified layer structure was used for two cases: single a-spot and conductive interface. The model included coupled electrical and thermal equations. The temperature profiles of the two cases were calculated. The following findings were made: Both the total thickness of the intermetallic layers and the formation of a-spots have a significant effect on the thermal stability of the joint. When the current density in the a-spots has reached a critical level that results in a high enough local temperature, the prerequisites for thermally induced inter diffusion between Al and Cu starts. Growth of the Cu/Al intermetallics layer will cause the a-spot equilibrium temperature and current density to exceed the limits for electromigration, which in turn gives rise to significant increases in the intermetallics growth rate. According to our computational model, the conditions for current induced, rapid growth of intermetallic layers between Al and Cu can only be fulfilled for the case of constriction of the current to a-spots.