PFMEA Practice Clarification (Part 2) | If Defects Are Found in Final Inspection, Why Are Failure Modes Listed in the Welding Process? —— The "Occurrence Process" and "Detection Process" of Failure Modes

1. Reviewing the Scene: Failure Modes Misplaced

Flipping through a PFMEA, the page for the final inspection process is filled with failure modes: "Poor Welding," "Missing Material," "Scratches," "Dimensional Deviation" —— almost all the defects from the entire workshop are piled under the final inspection. The review expert asks, "Are these defects generated in the final inspection?" The engineer is taken aback: "No, they are detected in the final inspection." The expert asks again, "Then where are they generated, and why aren't they listed in the PFMEA for that process?" The meeting room falls silent.

This is another common mistake in PFMEA preparation: listing failure modes based on where they are detected, rather than where they occur. Processes like final inspection, assembly, and testing, which are primarily for detecting failures, often become the "shelters" for failure modes due to their comprehensive inspection records. Conversely, the PFMEA for the processes that actually generate the failures — such as welding, injection molding, and painting — often remains blank. The principle for assigning failure modes is straightforward: they should follow the process that generates them, not the process that detects them.

2. Standard Logic: Failure Modes Are the Failures "Output" by the Process

PFMEA is a tool for analyzing each process step. The AIAG-VDA manual refers to each process step as a "focus element," and defines a failure mode (FM) as the failure output by this focus element — that is, which characteristic of the product or semi-finished product processed by this step does not meet the requirements.

This definition determines the归属 of failure modes: FM belongs to the process that produces it. The output of the welding process is the "weld seam," and a poor weld seam is a failure mode of the welding process. The output of the injection molding process is the "molded part," and a missing material is a failure mode of the injection molding process. The output of the painting process is the "coating," and a scratched coating is a failure mode of the painting process. What is the output of the final inspection process? It is the "inspection conclusion" — the output of the final inspection cannot be a poor weld, missing material, or scratch; it merely detects these defects.

Therefore, to determine which process a failure mode should be listed under, the question is not "where was it seen," but "which process produced this state." When you see "poor welding," trace it back to the welding process; when you see "missing material," trace it back to the injection molding process. The process that detects it does not produce it; it is merely a "witness."

3. Separate Accounts for Occurrence and Detection

Separating "occurrence" and "detection" aligns with two distinct columns in the PFMEA:

A failure can travel through multiple processes from its occurrence to its detection: welding process produces poor welding → assembly process detects a fracture under stress → final inspection confirms the defect through non-destructive testing. The division of labor in the PFMEA for these three processes is:

• Welding Process: List the failure mode "poor weld seam," the failure cause (current drift, welder technique, incoming weld wire quality), the occurrence O, and preventive actions.
• Assembly/Final Inspection Process: List "non-destructive testing" and "fracture detection during assembly" as detection controls in the detection column of the welding process row, and evaluate D — the earlier and more reliably it is detected, the lower the D score.

Note that while detection controls are listed in the row of the welding process, they describe "where and when the failure can be detected" — the detection point can be the current process, downstream processes, or even at the customer's end. This is the meaning of one of the three elements of detection assessment in the AIAG-VDA manual, "detection opportunity": the shorter the chain from occurrence to detection, the better the detection opportunity, and the lower the D score. Detection opportunity is about "where it is detected," which belongs to D; failure mode is about "where it occurs," which belongs to FM. Two separate accounts, clearly defined.

4. The Cost of Misplacing Failure Modes

Listing failure modes under the detection process may seem like just a misplaced row, but it can cause the entire analysis chain to be misaligned:

First, the risk analysis of the occurrence process becomes a vacuum. If the PFMEA for the welding process does not list "poor welding," it means the actual risks of welding have never been analyzed — the causes have not been identified, the occurrence has not been evaluated, and preventive actions have not been proposed. The risk does not disappear just because it is registered in a different place; it merely becomes invisible in the risk ledger.

Second, the cause analysis cannot be conducted. Failure causes (4M) act on the occurrence process: current drift occurs in the welding equipment, and missing material occurs in the injection molding machine. Listing the failure under the final inspection process means the cause column will be empty — the final inspection process has neither welding current nor injection molding parameters, so the cause can only be listed as "upstream material defect," pushing the responsibility to the previous process and breaking the analysis chain.

Third, all measures are misaligned. Preventive measures for poor welding (poka-yoke for welding parameters, regular calibration of equipment, incoming inspection of weld wire) should be listed under the welding process. If they are listed under the final inspection process, they become "strengthen final inspection" — using detection to replace prevention, defects will still occur, just with an additional interception. This is the structural reason why many companies update their PFMEAs annually but see no reduction in defect rates.

Fourth, the control plan and special characteristics become disordered. The control plan inherits control measures from the PFMEA. If the PFMEA lists the failure under the wrong process, the inspection station settings, poka-yoke layout, and special characteristic control points in the control plan will all be misaligned. During customer audits, this will be immediately exposed when compared to the process flow diagram.

Fifth, occurrence and detection scores are contaminated. If the failure is listed under the detection process, the occurrence O can only be evaluated in the detection process — the score reflects the "detection frequency" rather than the "occurrence frequency." The detection D score is also unclear. All three scores (S/O/D) become distorted, and the risk ranking loses its meaning.

5. Why It's Always Wrong: Three Common Misconceptions

Misconception One: Copying the detection process directly from the defect report. Inspection records and defect statistics are usually registered by inspection station (detection point). Teams preparing PFMEAs often take the easy way out by copying the failure modes from the defect report directly into the detection process table. Solution: Defect reports only provide "what the failure is," not "where it occurs" — the occurrence location must be traced, not copied.

Misconception Two: Mistaking the "detection point" for the "occurrence point" during reverse analysis. Many companies use reverse FMEA (tracing risks from occurred defects) to update their PFMEAs. They stop at the inspection record and mistakenly assume "detected in final inspection = produced in final inspection." Solution: Reverse tracing should follow the process flow diagram upwards until the process that generates the failure is identified.

Misconception Three: Organizing analysis by inspection station rather than by process. Teams are accustomed to dividing analysis objects by "inspection stations" because these stations have readily available data. However, the analysis unit for PFMEA is the process (each step in the process flow diagram), not the inspection station. Solution: Use the process flow diagram (PFD) as the framework, analyze each process step, and treat inspection stations as nodes on the flow diagram.

6. The Correct Approach: Process Flow Diagram Framework + Reverse Tracing

Step One: Use the process flow diagram (PFD) as the framework. Each row in the PFMEA corresponds to a process step on the flow diagram. The flow diagram must be complete before the PFMEA can be prepared — an incomplete flow diagram will result in a misaligned PFMEA.

Step Two: Ask "Will the output be nonconforming?" for each process. For each process step, ask what problems might occur with its output (product or semi-finished product state), and list the failure modes under the "process that produces it."

Step Three: Trace back to the occurrence process when a failure is detected. For any detected failure, first ask "which process produced this state," and do not list it until it is traced — before listing it in the PFMEA, the occurrence process must be confirmed.

Step Four: List detection information in the detection column. Write "where and when it can be detected" in the detection control column for the failure mode, and evaluate D based on this. The same failure mode can list multiple detection points (current process inspection, downstream inspection, final inspection), and the best detection opportunity should be used to evaluate D.

On-site verification questions during review: Randomly point to a failure mode and ask three questions — "Which process produced it?" "Where was it detected?" "Are these two processes the same?" If the answers to the last two questions are different, but the failure mode is listed under the detection process, it is misaligned.

Let's walk through an example with an injection molded part. A defect of missing material was detected in the assembly process and registered under "assembly process" in the defect report.

• Step One (Flow Diagram Framework): List each process step according to the process flow diagram — injection molding, de-gating, assembly, final inspection.
• Step Two (Process Output Analysis): The output of the injection molding process is the "injection molded part," which may have missing material, shrinkage, or flash. The output of the assembly process is the "assembly," which may have missing parts or incorrect assembly — list the failure modes for each process separately.
• Step Three (Trace Back to Occurrence Process): The occurrence process for the missing material defect is injection molding (material temperature, holding pressure, material shortage in the barrel), not assembly — move "missing material (shrinkage)" from the assembly process to the injection molding process.
• Step Four (Detection Information in Detection Column): The detection of missing material in the assembly process is listed as a detection control in the injection molding process row: "visual inspection at the assembly station can detect missing material," and evaluate D accordingly. At the same time, the injection molding process should list the failure causes (low material temperature, insufficient holding pressure, abnormal material level in the barrel) and occurrence O, along with preventive actions.

After the changes, the injection molding process, which was previously blank, now has a complete analysis: "missing material failure mode + four causes + preventive actions + detection methods." The PFMEA for the assembly process retains only the failure modes it produces. The same defect data, organized in two different ways, results in vastly different risk analysis quality.

7. Common Misconceptions and Self-Check List

8. Conclusion

Failure modes are the "outputs" of processes, not the "witness statements" of processes. Poor welding belongs to the welding process, missing material belongs to the injection molding process, and scratches belong to the painting process — even if they are all detected in the final inspection. The place where they are detected is the domain of detection controls, evaluated for detection (D); the place where they are produced is the domain of failure modes, evaluated for occurrence (O), analyzed for causes, and improved for processes. Remember this: failure modes follow the production process, detection controls follow the detection points; each to their own, and risks will not be overlooked.

Failure modes are listed under the occurrence process, detection controls under the detection points; two separate accounts, each in their own place.

Knowledge code: 8.3.1

Version: v20260808

Author: Quality Think Tank The Quality Think Tank is dedicated to providing systematic professional knowledge, methodologies, and practical tools for quality management practitioners, helping companies continuously improve their quality capabilities.