Hybrid delay scan: a low hardware overhead scan-based delay test technique for high fault coverage and compact test sets

A novel scan-based delay test approach, referred as the hybrid delay scan, is proposed in this paper. The proposed scan-based delay testing method combines advantages of the skewed-load and broad-side approaches. Unlike the skewed-load approach whose design requirement is often too costly to meet due to the fast switching scan enable signal, the hybrid delay scan does not require a strong buffer or buffer tree to drive the fast switching scan enable signal. Hardware overhead added to standard scan designs to implement the hybrid approach is negligible. Since the fast scan enable signal is internally generated, no external pin is required. Transition delay fault coverage achieved by the hybrid approach is equal to or higher than that achieved by the broad-side load for all ISCAS 89 benchmark circuits. On an average, about 4.5% improvement in fault coverage is obtained by the hybrid approach over the broad-side approach.