As the feature size shrinks to the nanometer scale, SRAM-based FPGAs will become increasingly vulnerable to soft errors. Existing reliability-oriented placement and routing approaches primarily focus on reducing the fault occurrence probability (node error rate) of soft errors. However, our analysis shows that, besides the fault occurrence probability, the propagation probability (error propagation probability) plays an important role and should be taken into consideration. In this paper, we first propose a cube-based analysis algorithm to efficiently and accurately estimate the error propagation probability. Based on such a model, we propose a novel reliability-oriented placement and routing algorithm that combines both the fault occurrence probability and the error propagation probability together to enhance system-level robustness against soft errors. Experimental results show that, compared with the baseline versatile place and route technique, the proposed scheme can reduce the failure rate by 20.73%, and increase the mean time between failures by 39.44%.