Product failures: Power-law or exponential voltage dependence?

The product failures voltage acceleration has traditionally been modelled with exponential voltage dependence. However with voltage scaling, the voltage acceleration parameter (VAP) in an exponential model has increased as V^-1 - as expected for dielectric breakdown in either back-end or front-end. This suggests an exponential model is probably quite conservative and a power-law model may be more appropriate for 90 nm and beyond. Even if an exponential model continues to be used, this understanding can help assess the amount of conservatism built in such a model.