Cost Efficient Methods to Improve Performance of Broadcast Scan

This paper presents techniques to improve compressions by improving fault coverage that can be achieved by broadcast scan. Due to reconvergent gates that artificially occur in broadcast scan, broadcast scan fault coverage is often much lower than standard serial scan fault coverage. The proposed scan chain reordering technique improves broadcast scan fault coverage by minimizing the number of reconvergent gates and hence no or very small number of test patterns are required to be applied by standard serial scan to detect faults undetected by broadcast scan. This increases the overall compression ratio. To eliminate or minimize increase in routing overhead, the distance that each scan cell can be relocated by the scan chain reordering procedure is limited. Test points are inserted to further reduce correlation among outputs of scancells. The proposed scan chain reordering technique improved broadcast scan fault converge by up to 8.5%. Large fault coverage improvement was achieved by the proposed method, especially for circuits that suffer low broadcast scan fault coverage. Broadcast scan fault coverage for the largest two industrial designs was even higher than standard serial scan fault coverage.