As stated, two big variables are clock rate and feature size, which both effect mean time between failures (MTBF). Being more conservative increases this metri

As stated, two big variables are clock rate and feature size, which both effect mean time between failures (MTBF). Being more conservative increases this metric. I know from working in a fab that there are many electrical inspection steps along the process, so failures are caught during the manufacturing process (reducing the chance that you see them in the final product). Once the chip is packaged, and assuming that it is operated in a nominal environment, then failures are not that common.