Delay fault coverage: a realistic metric and an estimation technique for distributed path delay faults

In this paper, we propose a new and realistic definition of delay fault coverage, based on the percentage of fabricated faulty chips which can be detected as faulty by a given test set. This metric takes into account the probability distribution of delay fault sizes caused by fabrication process effects, as opposed to previously defined metrics which have been based primarily on the percentage of faults tested. In addition to proposing a realistic delay fault coverage metric, we also present a computationally viable scheme for using this metric to estimate the coverage of any given test set for a class of path delay faults caused by distributed fabrication process variations. We use the results for the ISCAS'89 benchmark circuits to demonstrate wide discrepancies between distributed path delay fault coverage estimates for robust test sets obtained using our realistic definition, and the ones obtained by using the traditional notion of coverage as the percentage of paths tested.