One of the ways we dealt with "how far do you go?" in a
pfmea is to use the occurrences levels.
-- using occurrence levels [of defect counts, ppms, etc.]:
1. If you take all the defects produced on an entire processing line, your occurrence might end up being an 8 or a 9.
2. If you break that down into processing areas 1, 2, and 3, your occurrences might fall to 4, 5, and 6.
3. If you break down the processing area down into sub-processes a, b, and c, your occurrence might then fall to 2, 3, and 4.
When you reach an acceptable occurrence range, you have probably gone far enough. Once you get to a 1 or 2 occurrence there is very little risk.
We started with the flow and hit all the major items; then we went to inputs to each major item; and if needed, we went to the inputs of the inputs.
The more detail an
fmea shows, the easier it is to demonstrate that you have tried to examine all of the risks.