Reliability vs component tolerances in microelectronic circuits

It is well known that the reliability of a circuit can be increased by designing it for worst-case conditions so that, even if component characteristics drift, the circuit will still operate satisfactorily. However, it is shown in this paper that extreme worst-case design can lead to increased operating temperature and, therefore, again reduced reliability. A method, illustrated by two practical examples, is indicated to find the compromise in component and circuit design tolerances leading to maximum reliability at any specified time or over any specified time interval.