Improving Output Compaction Efficiency with High Observability Scan Chains

Output selection is recently proposed for test response compaction. This scheme achieves zero aliasing, full X-tolerance, and high diagnosability, at the cost of inflated test set and non-trivial hardware overhead. The time/space penalty in test output compaction is mainly attributed to the loss of observability. In previous methods, it was in general assumed that erroneous responses are uniformly distributed among all scan chains, and the output compactors are designed accordingly. In this paper, we present three techniques to improve the performance of output selection based test response compaction. (1) The uneven distribution of erroneous test responses is exploited to optimize compactor design. (2) A test dynamic compaction algorithm is provided to deal with the test set inflation problem. (3) A low-cost test response compactor is presented. Experimental results indicate that the proposed techniques can achieve better compaction results with lower hardware overhead.