Causes and Costs of No Fault Found Events
No-Fault-Found (NFF) events occur when a system level test,such as built-in test (BIT),indicates a failure but no such failure is found during repair. With more electronics continuously monitored by BIT,it is more likely that an intermittent glitch will trigger a call for a maintenance action resulting in NFF. NFFs are often confused with false alarm (FA),cannot duplicate (CNDs) or retest OK (RTOK) events. NFFs are caused by FAs,CNDs,RTOKs as well as a number of other complications. Attempting to repair NFFs waste precious resources,compromise confidence in the product,create customer dissatisfaction,and the repair quality remains a mystery. The problem is compounded by previous work showing that most failure indications calling for repair action are invalid. NFFs can be caused by real failures or may be a result of false alarms. Understanding the cause of the problem may help us distinguish between units under test (UUTs) that we can repair and those that we cannot. In calculating the true cost of repair we must account for wasted effort in attempting to repair unrepairable UUTs. This paper will shed some light on this trade-off. Finally,we will explore approaches for dealing with the NFF issue in a cost effective manner.