Effective scan chain failure analysis method
In order to improve the testability of a digital design, most internal flip-flops are changed into scan cells that are connected together to build shift registers called scan chains. Scan chain failures remain the most challenging sources of difficulty in fault localization. This is complementary to fault localization of products with working scan chains, in which the fault diagnosis tools are delivering quite satisfying results. However, if the scan chain itself is broken, either the entire chain or a larger segment out of the chain is reported as a fail. Only in some best case scenario can the diagnosis identify the failing flip flop. This is evenmore challenging if the scan chain and related ATPG structures operate only in compressed mode. In any case, even if a defect is found in scan chain diagnosis, the tools would report only the failing flip flop and do not provide any additional information for fault localization. Here we review the various diagnosis methods applied specifically to the broken scan chain and show additional physical fault isolation methods when the result of such diagnosis is not sufficient.