Compaction of a Transparent-Scan Sequence to Reduce the Fail Data Volume for Scan Chain Faults

Abstract

Scan chain faults produce large volumes of fail data that prevent the entire fail data from being collected for the purpose of defect diagnosis. Even with a single scan chain fault, the fail data volume is magnified by scan shift cycles that propagate faulty values into larger numbers of flip-flops, thus increasing the fail data volume. An approach referred to as transparent-scan allows smaller numbers of scan shift cycles to be used compared to a conventional scan-based test set, thus allowing the fail data volume to be reduced. Motivated by these observations, this paper describes a procedure that reduces the volume of fail data that scan chain faults produce under a transparent-scan sequence by omitting scan shift cycles from the sequence, or replacing them with functional capture cycles. The procedure is applied as a postprocessing step to test generation, similar to a static test compaction procedure. This is the first procedure that reduces the fail data volume for scan chain faults by modifying tests, and it is unique in that it achieves this goal by reducing the number of scan shift cycles, which is possible under transparent-scan. Experimental results for benchmark circuits demonstrate that the reduction in the fail data volume is in many cases larger than the reduction in the length of the sequence.