Improving test quality of scan-based BIST by scan chain partitioning

Test effectiveness of a test-per-scan BIST scheme is highly dependent on the length and the number of the scan chains. Fewer cycles arc adopted to capture test responses when the length of the scan chains increases. On the other hand, the number of test inputs should be increased when the number of the scan chains increases. Another important feature of the test-per-scan BIST scheme is that test responses of the circuit at the inputs of the scan flip-flops are unobservable during the shift cycles. A new scan architecture is proposed to make a scan-based circuit more observable. The scan chain is partitioned into multiple segments, according to which multiple capture cycles can be inserted to receive test responses during the shift cycles based on the test-per-scan test scheme. This scheme directly makes the circuit more observable and testable. Unlike other BIST schemes using multiple capture cycles after the shift cycles, our method inserts multiple capture cycles inside the shift cycles, but not after the shift cycles. Sufficient experimental results arc presented to demonstrate the effectiveness of the method.