Which defects are most critical? optimizing test sets to minimize failures due to test escapes

Traditionally, test set quality has been estimated through fault coverage. However, even with 100% fault coverage, some defects may escape the testing process - making defect level a more accurate estimate of the quality of test. However, even the defect level may not truly capture the reliability experience of the customer. Specifically, different undetected defects will produce different failure rates in the user's environment. Depending on the user's application, this may mean the difference between a part that is acceptable and never fails, one that rarely fails and can be tolerated, and one that experiences frequent, catastrophic failures. This paper explores the effect of circuit functionality and a user's application on the field failure rates that result from various faults and surrogate defects. We then propose a simple optimization procedure to create test sets that significantly reduce the field failure rates of test escapes without significantly increasing (and sometimes even decreasing) the final defect level.