ASIC jeopardy-diagnosing without a FAB

Proceed 1089-35 IC diagnosis is the process of finding the defect causing a failure at IC test. But what do you do when you have a bad ASIC that does not fail IC test? When you are a systems manufacturer, this puts the quality of your products in jeopardy. Here is the all too common scenario. A board or system test fails, and an ASIC has been diagnosed as being faulty. It has been sent back to the vendor, has passed, been inserted in another board, and failed again. We call these parts NTFs, for No Trouble Found. The goal of IC diagnosis is to resolve the problem to improve yields. Our goal is different. We wish to either add a test as early as possible to screen out bad parts, or to eliminate the root cause of the failure, if possible. Since we are fabless, we are not concerned with yield improvement, though we would like it to happen so that we will get fewer escapes through our imperfect IC tests. The most obvious cause of this problem is low fault coverage. Since we use full-scan, and get very high fault coverage vectors, improving fault coverage of the logic is rarely the answer.