X-alignment techniques for improving the observability of response compactors

Despite the advantages of performing response compaction in Integrated-Circuit (IC) testing, unknown response bits (x's) inevitably reflect into loss in test quality. The distribution of these x's within the captured response, which varies for each test pattern, directly impacts the number of scan cells observed through the response compactor. In this work, we propose a two-dimensional X-alignment technique in order to judiciously manipulate the distribution of x's in the test response prior to its compaction. The controlled response manipulation is performed on a per pattern basis, in the form of scan chain delay and intra-slice rotate operations, and with the objective that x's are aligned within as few scan slices and chains as possible. Consequently, a larger number of scan cells are observed after compaction for any test pattern. The computation of the control data, i.e., rotate and delay bits, is formulated as a MAX-SAT problem, and efficient heuristics are provided. The proposed technique is test set independent, leading to a generic, simple, and cost-effective hardware implementation. The X-alignment technique can be utilized with any response compactor to manipulate the x-distribution in favor of the compactor, thus improving the test quality.