Coil FAI and PPAP

What a coil first article has to prove

A first article inspection answers a simple question: does the part made on the production process match the drawing and the specification. For a wound part, the answer has to cover the winding, the electrical behaviour and the materials, because each can fail independently.

A dimensional report on the outside diameter and the pin positions is necessary but not sufficient. A coil can be the right size and have the wrong turns, the wrong wire, a shorted layer or a weak joint. Each of those needs its own evidence.

The first article also proves the process. The winding program, the tension, the termination method, the impregnation cycle and the test limits all have to be the ones that will run in production. A hand built sample proves very little.

Balloon the drawing and list the characteristics

Ballooning assigns a number to every characteristic that has a tolerance, a note or a test requirement. The numbers become the rows of the inspection report, so nothing on the drawing is left unmeasured by accident.

For a coil, the ballooned list usually includes the wire diameter, the turns count, the winding direction, the resistance, the inductance, the outside diameter, the pin positions, the lead length and the termination pull value.

Add the notes that are easy to miss: the insulation class, the varnish type, the marking, the date code and the cleanliness requirement. A note that is not ballooned tends to be the one that is missed on the first shipment.

The ballooned drawing should be the current revision. A first article against a superseded drawing is a common and expensive mistake, and it is easy to prevent by recording the revision on the report.

Measuring a wound part

Some coil characteristics are direct measurements and some are functional. The wire diameter is a direct measurement on the incoming wire, supported by the material certificate. The outside diameter is a direct measurement on the finished coil, taken at the agreed points.

Turns count is rarely measured directly on a finished coil. It is proved by the winding record, the resistance and the inductance. A resistance outside the expected band can mean a wrong wire, a wrong length or a bad joint. Inductance can catch a missing or extra turn, or a wrong core.

The electrical tests need a stable fixture. A four wire resistance measurement removes the lead resistance. An inductance measurement needs the stated frequency and bias, because the value changes with both. A high potential test needs the stated voltage, ramp and trip current.

The tests should be done at a recorded temperature, or corrected to a reference. Copper resistance changes with temperature, and an uncorrected reading can fail a good coil or pass a bad one.

Material and process certificates

A coil contains several materials that each need evidence. The magnet wire certificate should show the grade, the diameter, the insulation build and the thermal class. The bobbin certificate should show the material and the flammability rating where one is required.

The varnish or encapsulant needs a certificate or a data sheet that shows the thermal class and the compatibility with the other materials. The lead wire, the sleeving and the tape are often bought to a lower class than the winding, so they deserve a check.

Process records cover the winding program and the tension, the termination parameters, the impregnation cycle and the cure. A certificate that says the part was impregnated is weaker than a record that shows the vacuum, the pressure and the oven profile.

Traceability links the finished coil to the wire batch, the bobbin batch and the resin batch. Without that link, a material problem cannot be contained to a shipment.

What PPAP adds

PPAP is a production part approval process. It includes the first article evidence and adds the process documents that show the part can be made consistently: the process flow, the control plan, the failure mode analysis, the measurement system study and the capability results.

For a coil, the control plan should name the winding, the termination, the impregnation and the test steps, with the characteristic, the method, the sample size and the reaction plan for each. A control plan that lists only the final test does not control the process.

Capability is difficult on a low volume coil, and a single sample cannot prove it. Where the volume is low, the submission may rely on the first article, the process records and a justified inspection level instead of a capability index. The customer should agree that approach in advance.

The PPAP level also matters. A full submission with all elements is not always required. A new supplier or a new process usually warrants more evidence than a minor change to an approved part.

FAI and PPAP side by side

The two submissions overlap, and the same data should feed both. The difference is scope and ongoing commitment.

Records, deviations and the first shipment

Every result should be traceable to a part, a batch and a test. A report with a single pass value and no raw data is hard to defend if a question arises later. Keep the raw readings, the equipment identification and the calibration status.

Deviations should be raised before the submission, not buried in it. If a characteristic is outside the drawing but functionally acceptable, the customer needs to see the data and approve the concession. A supplier that adjusts the report to fit the tolerance is a bigger problem than the deviation.

The first shipment should carry the approved pack, the material certificates for the batches used and the test results for the lot. The date code and the marking should match the report.

Keeping the evidence current

A first article is a snapshot. A change of wire supplier, bobbin material, winding machine, varnish or test fixture can change the part, and the submission should be updated when it does. The change control procedure should name which changes trigger a new submission.

The control plan should be reviewed against the actual line. If a test was added or a sample size changed, the plan and the FAI should agree. An audit that finds a different process from the approved one undermines the whole pack.

For a long programme, keep a rolling record of the electrical results and the process parameters. That data is more useful than a single first article when a drift appears, and it supports the annual revalidation that many customers expect.