A study of undetectable non-feedback shorts for the purpose of physical-DFT

Undetectable shorts may decrease the long term reliability of a circuit, cause intermittent failures, add noise or delay, or increase test pattern generation costs. This paper describes the undetectable nonfeedback shorts that are likely to occur in standard cell implementations of the ISCAS´85 combinational test circuits. For ten layouts of each circuit, all shorts between adjacent wires were extracted and the undetectable ones analyzed. We found that approximately 0.4% are undetectable and that nearly half of these can be easily predicted before the physical layout of the circuit is generated. Since only a small percentage of the shorts are undetectable, and many of the undetectables are easily identifiable, it appears that it is possible to reduce the likelihood, or completely eliminate, the occurrence of a large portion of these shorts by incorporating design-for-test strategies into routing software