Most manufacturing engineers have experienced some version of the same problem: A production process successfully completes validation. The production part approval process (PPAP) is approved. Yield is stable, customer requirements are being met, and everyone assumes the process is under control. Then one day, defects begin to appear.

When that happens, the natural reaction is usually to look at the manufacturing process first. Engineers review machine settings, adjust process parameters, introduce additional inspections, and launch experiments.

Sometimes those actions appear to work. The defect disappears temporarily, everyone breathes a sigh of relief, and production returns to normal.

Then the defect comes back.

Over time, I’ve learned that one of the most expensive mistakes manufacturing engineers can make is assuming that every production problem originates in the manufacturing process itself. Sometimes the production line is performing exactly as designed. The real problem might be hidden in supplier materials, poor process discipline, environmental conditions, or manufacturing changes that occurred long before the defect was discovered.

The two investigations described here fundamentally changed how I approach root cause analysis. Although they involved different industries and different products, they reinforced the same lesson:

Before changing a validated manufacturing process, ensure that the manufacturing process is actually the problem.

The first investigation involved laminated photovoltaic modules where bubble defects suddenly appeared after a long period of stable production. What initially appeared to be a manufacturing process failure ultimately turned out to be something entirely different.

Case 1: The manufacturing process wasn’t the problem

During routine production of laminated photovoltaic modules, bubble defects suddenly began to appear after the manufacturing process had remained stable for an extended period. Because no engineering changes had been implemented, the manufacturing process immediately became the primary focus of the investigation.

Investigation

Once the defect was confirmed, a cross-functional investigation team was formed to determine whether the bubble defects originated from the manufacturing process or another source. Because no engineering changes or process deviations had been documented, the investigation naturally focused on the manufacturing process. Production records, machine parameters, and process conditions were reviewed, while the affected modules underwent reliability testing to determine whether the defects could affect long-term product performance. Despite several rounds of process verification, no abnormalities were identified. The manufacturing process remained stable, and the available evidence didn’t support a process-related failure. With the manufacturing process effectively eliminated as the primary suspect, the investigation shifted to the incoming supplier material.

Investigation continued

During the investigation, one production operator mentioned that the suspect material felt noticeably rougher than the previously approved material. Although this observation wasn’t technical evidence, it prompted the team to investigate whether the supplier material, rather than the manufacturing process, could be responsible for the defects.

To test that possibility, the team conducted a controlled design of experiments (DOE) using both the previously approved material and the suspect material while keeping all manufacturing conditions unchanged.

The results were consistent. Modules produced with the previously approved material completed the lamination process without bubble defects, whereas modules produced with the suspect material repeatedly developed bubbles under identical production conditions.

Several process parameter adjustments were evaluated, but none consistently eliminated the defects. Regardless of the process settings, the defects continued to appear whenever the suspect material was used.

At that point, the investigation shifted away from the manufacturing process and focused on the incoming supplier material. Samples from both materials were submitted for independent chemical analysis, which identified measurable differences between the suspect material and the previously approved material. This was the first objective evidence that the investigation was moving in the right direction.

Findings

The laboratory analysis confirmed that the suspect material had a chemical composition that was different from the previously approved material.

The investigation team then reviewed the supplier’s material specifications, engineering change history, and change management records. The review confirmed that the supplier had introduced a material formulation change without completing the required customer approval process.

That discovery changed the entire direction of the investigation.

Up to that point, every effort had been focused on the manufacturing process. Machine parameters had been reviewed, multiple DOE trials had been completed, and process adjustments had been evaluated. However, the manufacturing process itself had remained stable throughout the investigation.

The repeated bubble defects were ultimately traced to an uncontrolled supplier material change rather than a manufacturing process failure. Once the supplier material was identified as the root cause, corrective actions could be directed toward the supplier instead of continuing unnecessary process modifications on the production line.

More important, this investigation prevented the engineering team from making permanent process changes to compensate for a problem that didn’t originate in manufacturing. Instead, the corrective action addressed the true source of the defect while preserving a validated production process.

Practical lessons

Looking back, this investigation taught me that the first answer isn’t always the correct one. Like many engineering teams, our initial instinct was to focus on the manufacturing process because the defects first appeared on the production line. That assumption nearly sent the investigation in the wrong direction.

The experience reinforced three practical lessons that I continue to apply.

Don’t change a validated process without evidence. A stable manufacturing process shouldn’t become the primary suspect simply because a defect appears during production. Objective evidence should always determine the direction of an investigation.

Design investigations that eliminate possibilities instead of confirming assumptions. The controlled DOE played a critical role because it isolated one variable at a time. Once the same defect appeared repeatedly under identical manufacturing conditions, the investigation could confidently shift away from the production process.

Never underestimate supplier change management. Small material changes introduced without proper approval can create failures that closely resemble manufacturing instability. Before changing machine settings, adding inspections, or modifying process parameters, engineers should verify that incoming materials remain consistent with the originally approved specification.

Looking back, the laboratory analysis identified the technical root cause. But the real success of the investigation was preventing unnecessary changes to a manufacturing process that had never been the problem.  

Case 2: A validated process failed 2 years later

Several years after a quarter-glass assembly had successfully completed validation and entered mass production, an automotive OEM began receiving field reports that quarter-glass assemblies were detaching from vehicles during normal operation.

The issue was particularly concerning because the product had been in production for several years without any significant quality concerns. Initial inspections, production records, and customer acceptance data all indicated that the manufacturing process had been stable.

As additional field failures were reported, the issue quickly escalated into a vehicle recall, requiring an immediate investigation involving the OEM, the glass manufacturer, and the encapsulation supplier responsible for the secondary bonding process.

Then I joined the investigation as the quality engineer responsible for supporting supplier qualification, process verification, and PPAP revalidation activities.

Investigation

Once the field failures were confirmed, a cross-functional investigation team was established to determine why assemblies that had performed successfully for years were suddenly failing in service.

The investigation initially focused on the glass manufacturing process because the detached quarter-glass assemblies were supplied by the original glass manufacturer. Production records, historical quality data, customer complaints, and manufacturing documentation were reviewed to determine whether any process changes or quality issues had occurred during glass production.

At the same time, previously approved PPAP documentation, validation records, and customer specifications were reviewed to determine whether the original manufacturing process had been properly validated before mass production.

Despite the severity of the field failures, no abnormalities were identified. The manufacturing process had remained unchanged, historical quality performance was stable, and all available production records indicated that the process continued to operate within its approved conditions.

With the original glass manufacturing process effectively eliminated as the primary suspect, the investigation expanded to include the encapsulation supplier responsible for the secondary bonding process.

Investigation continued

The investigation team conducted an onsite audit at the encapsulation supplier to verify compliance with the approved manufacturing process.

The audit included operator interviews, direct observation of production activities, review of work instructions, verification of equipment settings, and evaluation of material preparation practices.

While the documented process met the approved manufacturing requirements, several production activities were found to differ from the documented procedure.

The gap between documented requirements and actual manufacturing practices became a key focus of the investigation.

Findings

The supplier audit ultimately revealed that the issue didn’t originate from the glass manufacturing process but from the secondary bonding process performed by the encapsulation supplier.

The approved manufacturing procedure required both the glass activator and primer to be continuously agitated before application. During the audit, it was confirmed that this critical preparation step had been omitted, and both materials were applied directly to the glass.

To determine whether this deviation could explain the field failures, the team performed a controlled comparison test using identical glass, adhesive, and production conditions. The only difference was whether the approved material preparation procedure was followed.

After accelerated environmental aging, the bonded assemblies underwent a quick knife test to evaluate bond durability. Samples prepared according to the approved procedure met the acceptance criteria, while those produced without the required material preparation consistently failed.

These results confirmed that the deviation significantly reduced long-term bond performance and validated the root cause identified during the supplier audit.

Corrective actions

Once the root cause had been confirmed, the focus shifted from investigation to preventing recurrence.

As the supplier quality engineer supporting this project, my responsibility was to ensure that the supplier could consistently execute the approved manufacturing process before production resumed.

An onsite supplier audit confirmed that the documented manufacturing procedure was fully implemented. Work instructions were updated, operator training was reinforced, and additional process verification was introduced to ensure the critical surface preparation steps were consistently followed.

The supplier successfully completed PPAP revalidation, demonstrating that the revised process could consistently meet customer requirements. Production resumed only after the supplier demonstrated stable and repeatable manufacturing capability.

A quick look at investigating the root cause for both cases.

Conclusion

Both investigations reached the same conclusion, even though they involved different industries and different products.

In one case, engineers almost changed a manufacturing process that had never been the problem.

In the other, a validated manufacturing process gradually failed because a supplier no longer followed the approved procedure.

Neither investigation was solved by making additional process adjustments. Both were solved by following the evidence.

For me, that has become the most important lesson in supplier quality. Validation proves that a process is capable at one point in time. Sustaining manufacturing capability requires engineers to continuously verify supplier materials, process discipline, and execution throughout the entire product life cycle.