DFMEA vs PFMEA
Both are Failure Mode and Effects Analysis (FMEA) tables. They differ in what they analyze.
| DFMEA | PFMEA | |
|---|---|---|
| Full name | Design FMEA | Process FMEA |
| Focus | Product design: geometry, materials, interfaces, tolerances | Manufacturing and assembly process: steps, fixtures, tooling, operators |
| Failure modes | How the design could fail to meet its function or requirement | How the process could produce a nonconforming part |
| Severity | Effect on the end user or system function | Effect on the next operation, the plant, or the end customer |
| Prevention controls | Design changes, tolerance tightening, material selection | Process controls (poka-yoke, fixtures, trained operators) |
| Detection controls | Design verification testing (prototype tests, simulations) | In-process inspection, SPC charts, automated vision |
| Owner | Design engineering team | Manufacturing/process engineering team |
| APQP phase | Phase 2 — product design | Phase 3 — process design |
Worked example: a press-fit pin assembly
DFMEA failure mode: Pin diameter too small — fit loosens in service. Cause: tolerance stack-up not analyzed. Effect: loss of electrical contact. Design action: tighten pin tolerance, run tolerance stackup study.
PFMEA failure mode: Pin pressed in at wrong depth — protrudes too far. Cause: press stop not set correctly after shift change. Effect: assembly fails clearance check, requires rework. Process action: add poka-yoke to press (depth sensor with automatic stop).
Common exam traps
- The DFMEA does not analyze operator error. How an operator might assemble a part incorrectly belongs in the PFMEA, not the DFMEA. The DFMEA asks: could this design fail even if manufactured perfectly?
- Severity is fixed in DFMEA; controls reduce occurrence and detection. You cannot design away severity (the effect on the customer is what it is). Actions reduce the chance the failure mode occurs or goes undetected.
- Both FMEAs are live documents. They are not completed once and filed; they are updated when design or process changes occur.
- AIAG-VDA (2019) replaced standalone RPN with AP (Action Priority). AP uses S, O, D in a lookup table rather than multiplying them. Both systems appear on the exam—know both.
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