Limitation of structural scan delay test

Beyond the traditional stuck-at fault model, there are transition fault model and path delay model, followed by many, many other fault models that have appeared over the years. Transition model essentially define a node that is slow to rise or slow to fall and that this slow transition make its way to a primary output. Meanwhile path delay fault is more precise and define that a slow event be propagated along specific paths to a particular primary output. Due to their name (transition/delay) and nature, these fault models would appear to detect structural delay faults (whether it is process induced or delay defect induced). There is a general belief in the industry that achieving good coverage with these models would guarantee high product quality.