Kirkendall voids in the intermetallic layers of solder joints in MEMS

Microelectronic systems consist of the functional chip unit itself and a surrounding package which includes several electro-mechanical connections, e.g., solder joints between different metal layers. Experimental observation shows that aging of the solder alloy as well as the formation and growth of so-called Kirkendall voids significantly contributes to the degradation of the joining capability. Starting with a phenomenological explanation of the Kirkendall effect, we present in this contribution a constitutive model for void nucleation and growth. The model accounts for the effects of vacancy diffusion, surface tension and rate-dependent plastic deformation on ensembles of spherical voids. It can be applied to predict the temporal development of voids in solder joints. Numerical simulations on the material point level provide insight into the (not yet completely understood) mechanisms of failure by formation and growth of Kirkendall voids and show the potential of the model for the failure analysis of joints.