A measurement can fall inside the product specification while signaling a change in the manufacturing process. It can also come from a consistent process that routinely misses the specification. Control limits and specification limits answer different questions, so label and use them separately.
NIST’s variables-control-chart discussion distinguishes process consistency from product specification requirements. A chart boundary calculated from production data should not become a new product acceptance limit without the required engineering decision.
Identify the source of each boundary
Record the specification revision that defines acceptable product behavior or dimensions. Separately record how process control limits were established, including the baseline data, subgroup method, and chart type. Make the distinction visible in chart legends and exported reports. Label the source of each line so an engineer reviewing a detached screenshot can still identify its purpose.
Check that all plotted values refer to the same characteristic and measurement procedure. Combining different PCB constructions or test recipes can make a process chart difficult to interpret even if their nominal specification limits happen to match.
Define the response to each signal
| Condition | Question for disposition |
|---|---|
| Inside specification, process signal present | What changed, and which material needs review? |
| Outside specification, process appears stable | Why does the established process miss the requirement? |
| Both process and specification signal | What containment and investigation are required? |
| No signal in either view | What monitoring and acceptance requirements still apply? |
A signal is a prompt for the predefined response plan, not a complete cause analysis. Preserve the affected time interval, equipment, material, and measurement records so the team can investigate a plausible change.
Control recalculation of limits
- Retain the approved baseline and chart configuration.
- Annotate process changes and investigation findings.
- Evaluate whether a new stable operating condition has been established.
- Review any proposed control-limit update.
- Keep earlier chart segments interpretable after the update.
Do not widen limits automatically whenever a point exceeds them. That can normalize a developing problem and remove the signal the chart was intended to reveal. Likewise, do not tighten specification limits solely because one short run showed low variation.
When presenting results to a supplier or customer, identify whether the conclusion concerns process stability, product conformance, or capability relative to a requirement. Preserve the data needed to support each statement rather than using a single green indicator for all three.
The capability-report guide explains the next step of reviewing how a defined process distribution relates to its product specification.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist