PCB Assembly Functional Test: Who Writes the Pass/Fail Criteria

What "FCT PASS" actually tells you

A functional test powers the assembled board, applies inputs and checks that outputs fall inside limits. It is the only test in a typical PCB assembly flow that asks whether the board does its job, rather than whether parts are present and solder joints are sound.

That makes the word "PASS" on a traveler or a test log only as meaningful as the procedure behind it. If nobody wrote down which steps to run, which points to measure and what limits apply, a pass means "the operator saw nothing wrong". Two assemblers can both report 100% FCT pass on the same design and be running different tests.

The fix is a written functional test procedure, controlled by revision, that both sides sign before production.

Who writes it, who signs it

The split that works for most contract assembly jobs:

The buyer owns the "what" and the limits, because only the design owner knows what a correct output is. The assembler owns the "how" on its own station. The procedure is the document where the two meet. Who pays for the fixture and keeps it is a separate contract point, covered in our test fixture ownership guide.

If the buyer sends only "please do FCT" with no limits, the assembler has to invent them. Expect either a quote with a long list of assumptions or a test that checks power-up and little else.

The fields a procedure needs before "PASS" means anything

A procedure can be short. It cannot skip any of these:

1. Document control. Procedure number, revision, the board part number and assembly revision it applies to. A procedure for Rev B used on Rev C boards is a silent failure.
2. Unit under test setup. Power rails and the order they come up, current limit on the supply, connectors used, jumpers or switches set, any load attached.
3. Software under test. The firmware or test image file name, version and checksum. Who loads it, and whether production firmware is loaded after the test.
4. Step table. One row per check: what is applied, where it is measured, lower limit, upper limit, unit. See the template below.
5. Equipment. The type of instrument for each measurement and the requirement that it is in calibration. Name the type, not a brand, unless a specific instrument matters.
6. Pass rule. A board passes only if every step passes, in order, in one run. State it, because "passed on the second try" is otherwise a pass.
7. Failure handling. What happens to a failed board, how many retests are allowed, and who is told. Covered below.
8. Records. What is logged per board and how long it is kept.
9. Approval. Signatures from the buyer and the assembler, with date, on this revision.

A step table you can copy

Limits below are placeholders. Your design defines the real values.

Three rules keep the table useful. Every measured value gets both limits, or a clear statement that one side is open. Every test point has a name that appears on the assembly drawing. Pass/fail steps with no number (an LED is lit, a string matches) state the exact expected result.

Firmware is part of the test

Most disputes about functional test come down to software, not hardware. The board passed with one image and failed in the field with another, or the assembler loaded a test image and shipped it.

Put these in the procedure:

• File name, version and checksum of every image loaded at the station.
• Whether the test image is replaced by production firmware before shipping, and how that is verified.
• What happens when the buyer releases a new image: the procedure revision changes, and boards tested on the old image are listed.

Treat a firmware change like a procedure change. Our first article inspection guide explains how to tie a revision change to a fresh release.

Failure handling and retest rules

A procedure without failure rules invites "retest until it passes". Write down:

• Retest limit. How many times a failed board may be retested without diagnosis. Many buyers allow one retest to rule out a fixture contact problem, then require diagnosis.
• Repair and retest. After rework, the board reruns the full procedure, not only the failed step.
• Failure log. Each failure is recorded with the step number and measured value, even if the board later passes.
• Escalation. A trigger for stopping the line and telling the buyer, such as the same step failing on several boards in a lot. You set the trigger; the procedure states it.
• Disposition. Who decides scrap, return or use-as-is for boards that cannot be repaired.

Solder and workmanship defects found during diagnosis follow the acceptance class and disposition rules in your solder joint acceptance terms, not the FCT procedure.

Records to ask for

"FCT 100% passed" on a delivery note is a summary. Ask for data you can check:

• Per-board record: serial number, procedure revision, firmware version, date, station ID, result, and measured values for every numeric step.
• Failure records, including boards that passed after retest.
• Lot summary: boards tested, first-pass count, retest count, failed count.
• Calibration status of the station instruments at test date.

With measured values on file, you can see drift toward a limit before boards start failing. A pass/fail flag alone cannot show that.

What to send with the RFQ

If you want functional test quoted correctly, the quote package should include:

• The functional test procedure at its current revision, or a draft with functions and limits filled in.
• Test firmware or a statement of when it will be supplied.
• Test point locations on the assembly drawing.
• Whether you supply the fixture, or want the assembler to build one.
• Record format and retention you expect.

Without these, the assembler can only quote functional test as an assumption. Put the procedure next to the other test and inspection items in your inspection requirement definition, then send the package through the quote page.

FAQ

Who should write the functional test procedure in contract assembly?

The buyer defines the functions and the limits. The assembler's test engineer writes the steps for its own station. Both sign the same revision before production.

Can the assembler write the limits if I don't have them?

It can propose limits from a known-good board, but you must approve them. Limits taken from one golden sample can pass a marginal design, so check them against your design values.

Is "power-on test" the same as functional test?

No. A power-on check confirms the board starts and draws plausible current. A functional test checks each function you list against limits. State which one you are buying.

Does functional test replace AOI or X-ray?

No. Functional test checks behavior. AOI and X-ray check assembly quality, and they catch defects that still pass a functional test, such as a weak joint.

What changes require a new procedure revision?

A board or assembly revision, a new firmware image, a changed limit, a new test step or a new fixture. Boards tested under the old revision should be identifiable from the records.

Do I need measured values, or is a pass/fail result enough?

Ask for measured values on numeric steps. They let you check a disputed board and spot drift across lots.

Related links

• Assembled green PCBA with a large square IC, the kind of board a written functional test procedure must define steps, limits and sign-off for