Timing Evaluation Tests for Scan Enable Signals with Application to TDF Testing

Scan based transition delay fault (TDF) tests are generally applied in the launch-on-capture (LOC) mode because the scan enable control signal broadcast to all flip-flops on the die is expensive to implement as a fast switching signal needed to support at-speed launch-on-shift (LOS) tests. However, there is mounting evidence that even when applied at much slower speeds, LOS tests often detect a significant fraction of the timing defects, including many unique failures that are missed by LOC test. This suggests the use of combined LOC and LOS test to increase the TDF coverage beyond that attainable by LOC tests alone. However, to maximize the detection of real delay defects, the LOS tests must be applied at the fastest possible speed (up to the functional clock rate) within the timing limitations of the scan enable. To support such testing, in this paper we present the first test technique to reliably evaluate the switching speed of the scan enable signal. This can vary significantly for individually manufactured instances of the same design due to normal process variations at advanced technology nodes, amplified by near quadratic delays in the long interconnect lengths of this broadcast signal. Once the scan enable speed is determined, LOS tests can be applied at the fastest, most effective speed. Furthermore, the availability of this effective low cost LOS test capability also facilitates the partitioning of the scan flip flops through the use of multiple scan enable signals that apply the delay test in mixed LOS and LOC modes in the different partitions. We show that the use of only two scan enable signals in such an approach can significantly increase TDF test coverage, and raise it very close to the highest possible that is achievable only by enhanced scan.