How can I model a component when all the manufacturer will tell me is that BIT covers 98% of the component's failures?
This is a common situation when working with COTS components. When the manufacturer refuses to provide any information about what is and isn't covered by BIT, the Analyst is forced to play it conservatively.As it turns out, this situation is relatively simple to model in eXpress using failure modes.
In order to model this, the following changes are made to the component:
Define a single failure mode for the BIT, whose percentage is set to 98%. The remaining 2% does not require a failure mode to be created.
What happens is that upon running the BIT failure mode test, 98% of the component failure rate will become proven or detected. However, this allows nearly all ambiguity to be removed by the BIT test, resulting in a much lower occurance of higher fault groups. Should the remaining ambiguity need to be resolved, additional functional tests can be added.
The following premise should be challenged prior to adopting this approach:
"If I prove all the functionality of a part that is used within the design is working properly, do I care if something else in the part isn't?"
In other words, the vendor can always add more than you need, just as long as what you need is working. If this premise rings true, then a failure mode test that covers 98% is a highly efficient detection test, that provides the ability to detect failures at a rate equivalent to the data that the vendor provided.
