Nanometric SRAMs are more vulnerable to experiencing particle induced soft error due to lower operating voltages and higher packing density coupled with increased process variations. In this paper, we present a compact model for the critical charge of a 6T SRAM cell for estimating the effects of process variations on its soft error susceptibility. The model is based on the dynamic behavior of the cell and a simple decoupling technique for the non-linearly coupled storage nodes. The model describes the critical charge in terms of transistor parameters, cell supply voltage, and injected current parameters. Consequently, it enables investigating the spread of critical charge due to process induced variations in these parameters and to manufacturing defects, such as, resistive contacts or vias. In addition, the model can estimate the improvement in critical charge when MIM capacitors are added to the cell in order to improve the soft error robustness. The critical charge calculated by the model is in good agreement with SPICE simulations for a commercial 90nm CMOS process with a maximum discrepancy of less than 5%.