Multiple scan tree design with test vector modification

In this paper, we propose a method of test compression for multiple scan designs. Instead of the conventional serial scan chains, the proposed method constructs scan trees in which scan flip-flops are placed and routed in a tree structure. Inputs of the scan trees drive several scan trees of different lengths (height). Since test data volume and test application time are dominated by the scan tree with the maximum height among the constructed scan trees, the proposed method distributes the scan flip-flops to the scan trees so as to minimize the maximum height of the scan trees. In addition, the proposed method modifies the given test vectors to maximize the reduction in test application time. Experimental results for ISCAS-89 benchmark circuits show that the proposed method could reduce, on the average, test data volume by 77% compared with the conventional multiple scan design. The scan tree construction enlarges the number of scan outputs required. However test data volume could be reduced by 66% even if the number of scan outputs is limited.