Effect of Circuit Design on System Reliability

It is shown that circuit drift failures may be eliminated by worst-case design procedures but that a considerable price is paid for this immunity in the form of increased system complexity, increased component stresses and increased power demand. Consideration of the entire problem leads to the conclusion that decreasing the probability of circuit drift failures (by increasing the tolerance margin of the circuit) tends to increase the probability of component catastrophic failures and that consequently an optimum component tolerance design point exists for maximum system reliability. The optimum tolerance margin depends upon the specific system and generally varies inversely with the number of components comprising the system. Thus, to maintain a specified system reliability in the face of increasing system complexity, it is necessary to assume a decreasing component parameter spread (tighter tolerances) and a decreasing component catastrophic failure rate. Both of these requirements may be relaxed if some form of redundancy is introduced to overcome the inevitable occurrence of catastrophic failures.