Fine-Grained Steepening of the Fault Coverage Curve of a Pool of Functional Test Sequences

Functional test sequences can complement scan-based tests by enhancing the detection of defects that can affect the correct functional operation of a design. A large pool of functional test sequences may already be available for the design. When selecting a subset of the functional test sequences for test application, test compaction procedures select subsequences from the pool to reach the fault coverage of the entire pool using as few clock cycles as possible. However, a large number of clock cycles may be needed for applying the entire pool even after test compaction, exceeding the numbers of clock cycles that can be accommodated during short in-field test periods. Motivated by this observation, the article addresses the following problem: given a bound on the number of clock cycles for test application, which subsequences should be applied to achieve the highest possible fault coverage. The solution suggested in this article takes a fine-grained view of steepening the fault coverage curve of the pool. Compared with a coarse-grained view, experimental results for benchmark circuits show that a higher fault coverage can be achieved with the same number of clock cycles.