Using Multiple Expansion Ratios and Dependency Analysis to Improve Test Compression

A methodology is presented for improving the amount of compression achieved by continuous-flow decompressors by using multiple ratios of scan chains to tester channels (i.e., expansion ratios). The idea is to start with a higher expansion ratio than normal and then progressively reduce the expansion ratio to detect any faults that remain undetected. By detecting faults at the highest expansion ratio possible, the amount of compression can be significantly improved compared with conventional approaches. The expansion ratio is progressively reduced by concatenating scan chains together using MUXes to make fewer and longer scan chains. Selecting which scan chains to concatenate is done by using a dependency analysis procedure that takes into account structural dependencies among the scan chains as well as free-variable dependencies in the logic driving the scan chains to improve the probability of detecting faults. Results for applying the proposed approach to industrial designs using various types of decompressors indicate significant improvements in compression are possible.