Process Qualification Explained: IQ, OQ & PQ

What process qualification is

Process qualification is the structured proof that a manufacturing process will consistently produce a conforming product before it's released for routine production. Rather than one broad check, it's conventionally broken into three stages that each answer a genuinely different question, and skipping or compressing any of them tends to be exactly where "it worked in qualification but not in production" problems come from.

This structure is standard across FDA process validation guidance and most quality frameworks that govern regulated manufacturing, including ISO 13485. It applies to more than final assembly, reflow processes, potting or encapsulation steps, conformal coating, and any process where output quality depends on equipment and parameters staying within a proven range.

The three stages

Stage	Question it answers	What it involves
IQ Installation Qualification	Is the equipment installed correctly, as specified?	Verify equipment matches purchase and design specs, is calibrated, and is installed per the manufacturer's requirements, documented as a formal as-built record.
OQ Operational Qualification	Does the process perform as intended across its full operating range, not just at one ideal setting?	Challenge the process at the edges of its intended operating window, not just the nominal setpoint, and confirm it still produces conforming output.
PQ Performance Qualification	Does the process hold up under real, sustained production conditions?	Run production-representative lots, typically across multiple operators and shifts, and confirm the process remains in statistical control, not just correct on a single monitored run.

The qualification process, step by step

Define the protocolBefore any qualification runs happen, write down what's being qualified, the acceptance criteria for each stage, and the sample sizes and statistical basis for PQ. Writing the protocol after the data exists is a common shortcut that undermines the whole exercise.
Execute IQConfirm and document equipment identity, calibration status, installation conditions, and utilities (power, air, temperature control) against the manufacturer's and your own specifications.
Execute OQRun the process across its intended operating range, not just at nominal, deliberately including worst-case combinations of parameters that the process is expected to tolerate.
Execute PQRun enough production-representative lots, across realistic variation in operators, shifts, and incoming material lots, to demonstrate the process is statistically capable, not just correct on a best-case run.
Analyze and documentCompile the results against the pre-defined acceptance criteria, including any process capability analysis (see our note on Cp/Cpk), and formally release the process for production, with a clear record of what was and wasn't covered.

Worked example: qualifying a reflow oven for a new board

IQ	Confirm the oven's zone count, conveyor speed range, and thermocouple calibration certificates are current, and that the installation matches the equipment's utility and clearance requirements.
OQ	Run the target reflow profile at the nominal setpoint, then again at the edges of the process window: fastest and slowest allowable conveyor speed, highest and lowest allowable zone temperatures, confirming peak temperature and time-above-liquidus stay within the paste manufacturer's specification at every combination tested.
PQ	Run three separate production lots, on different shifts, using incoming solder paste from two different lot numbers, and confirm solder joint quality (visual, X-ray sampling, and any in-line AOI results) remains within acceptance criteria across all three, not just the best-performing one.
Common shortcut to avoid	Running only a single well-monitored "golden" lot and calling it PQ. A single lot, however clean, doesn't demonstrate the process holds up across the real variation production will actually see.

Common mistakes

Jumping straight to PQ. Without a real IQ and OQ behind it, a clean PQ run only proves the process works under the specific conditions of that run, not that it's actually robust.
Testing only nominal conditions in OQ. The point of OQ is to find the edges of the process window, not to confirm the setpoint everyone already expected to work.
Undersized PQ sample sizes. A statistically meaningless sample size can pass qualification and still leave a process that isn't actually in control at production volume.
Writing the protocol after the fact. Deciding acceptance criteria once you've already seen the data invites, even unintentionally, criteria that fit the results rather than results that meet the criteria.

Frequently asked questions

Do IQ, OQ, and PQ always have to be separate documented events?

For regulated manufacturing, yes, each stage typically has its own protocol, execution record, and acceptance criteria, even if they happen back-to-back in the same qualification campaign.

How does process qualification relate to process capability (Cp/Cpk)?

Capability analysis is often a core part of PQ's acceptance criteria: demonstrating the process isn't just producing conforming output on average, but is statistically capable of doing so consistently. See our note on Cp and Cpk for how that's calculated.

Does a process need to be requalified after a change?

Generally yes, a new equipment setting, a new material lot source, a relocated line, or a significant volume change should trigger at least a partial requalification, scoped to what actually changed.

WRYGT builds qualification protocols as part of process development, not as a document written to satisfy an audit after the fact. If a new process needs to be qualified before ramp, talk to us about engineering services.

Process qualification: IQ, OQ, and PQ, in plain terms