Test strategies described in previous chapters relied on two concepts: controllability and observability (C/O). Good controllability makes it easier to drive a circuit into a desired state, thus making it easier to sensitize a targeted fault. Good observability makes it easier to monitor the effects of a fault. Solutions for solving C/O problems include scan path and various ad-hoc methods. Scan path reduces C/O to a combinational logic problem which, as explained in Chapter 4, is a solved problem (theoretically, at least).
IDDQ monitoring is another approach that provides complete observability. Current drain in a properly functioning, fully static CMOS IC is negligible when the clock is inactive. However, when the IC is defective, due to the presence of leakage in the circuit, or possibly even to an open, current flow usually becomes excessive. This rise in current flow can be detected by monitoring the current supplied by the tester. How effective is this technique for spotting defective ICs? In one study, it was shown that IDDQ testing with a test program that provided 60% coverage of stuck-at faults provided the same AQL as a test program with 90% stuck-at coverage without IDDQ.1
The stuck-at fault model that we have been dealing with up to this point is not intended to address qualitative issues; its primary target is solid defects manifested ...