Exploiting distribution of unknown values in test responses to optimize test output compactors

Test output compactors can effectively reduce the data volume of test responses without scarifying fault coverage. However, when there are unknown values (X-bits) in the test output, the fault coverage can be severely comprised. Many compaction schemes that can handle X-bits have been developed. However, existing test response compaction schemes are designed without considering the locations of errors and X-bits. This design methodology essentially assumes that observable errors as well as X-bits are randomly distributed among all scan cells. Recent studies show that X-bits may not be randomly distributed; some scan cells could capture much more X-bits than others. In this paper, we propose to exploit the nonuniform distribution of X-bits to optimize test response compactors such that a higher compression rate is achieved with lower hardware overhead. The proposed design method is applicable to various test output compaction schemes that can handle X-bits in the test responses, including X-blocking, X-masking, and X-tolerant circuits. Experimental results show that, in the presence of X-bits, the compression results will be significantly improved with the help of the proposed method.