The SRAM cells that form the configuration memory of an SRAM-based FPGA make such FPGAs particularly vulnerable to soft errors. A soft error occurs when ionizing radiation corrupts the data stored in a circuit. The error persists until new data is written. Soft errors have long been recognized as a potential problem as radiation can come from a variety of sources. This paper presents an FPGA fault model focusing on routing aspects. A graph model of SRAM nodes behavior in case of fault, starting from netlist description of well known FPGA models, is presented. It is also performed a classification of possible logical effects of a soft error in the configuration bit controlling, providing statistics on the possible numbers of faults. Finally it is reported the definition of fault metrics computed on a set of complex benchmarks proving the effectiveness of our approach.