Short answer: A sound PCB assembly test strategy layers five methods: automated optical inspection (AOI) on every board for placement and solder defects; X-ray for hidden joints under BGAs and QFNs; flying-probe test for electrical faults on prototypes and runs under about 500 pieces; in-circuit test (ICT) with a bed-of-nails fixture for series above that; and functional test against your specification as the final gate. AOI plus functional test is the minimum for industrial products; add X-ray for area-array packages and ICT when the volume justifies the fixture.
The five methods compared
| Method | What it finds | Coverage | Cost profile | Best for |
|---|---|---|---|---|
| AOI (automated optical inspection) | Missing, shifted, rotated, wrong-polarity parts; bridges; insufficient solder | Visible joints, all boards | Per-board seconds, no fixture | Every board, every volume |
| X-ray (2D/3D) | Voids, bridges and opens under BGA, QFN, LGA | Hidden joints | Per-board minutes, no fixture | Area-array packages, class 3, first articles |
| Flying probe | Opens, shorts, wrong values, missing parts (electrical) | Nets with accessible pads | No fixture, slower per board | Prototypes, runs under ~500 pcs, frequent revisions |
| ICT (in-circuit test) | Same as flying probe, faster, plus powered tests | Nets with test points | Fixture €3,000–€15,000, seconds per board | Series above ~500 pcs, stable design |
| Functional test | Does the product work: power, interfaces, firmware, calibration | Whole product | Fixture and software per product | Final gate for every product |
How to choose the mix
- Prototype (1–20 pcs): AOI + X-ray on BGAs + flying probe + functional test with your firmware.
- Low volume (20–500 pcs/year): AOI + flying probe or functional test, X-ray on samples.
- Series (500+ pcs/year): AOI + ICT or well-covered functional test; X-ray sampling; fixture cost amortised.
- Safety-critical / IPC class 3: all of the above, 100 % X-ray on area arrays, records per serial number.
Designing for test
Test coverage is decided in layout: give every net a test point or an accessible pad (1 mm pads on a 2.54 mm grid for ICT), keep a programming and debug connector, plan how the board is powered on the tester, and write the functional test specification before the first prototype. Comtec Labs raises these in the DFM review and builds the test fixtures and software in-house.
Key facts
- AOI catches the majority of assembly defects at a cost of seconds per board and is run on 100 % of boards at Comtec Labs.
- A flying-probe programme is generated from CAD data in hours; an ICT fixture takes 2–4 weeks and €3,000–€15,000.
- X-ray is the only way to inspect solder joints under BGA and QFN packages.
- Functional test finds what electrical test cannot: firmware, calibration and interface faults.
What you receive
Test results per board or per serial number, first-article inspection reports, X-ray images for area-array parts on request and a yield summary per batch. When a test fails, the board is analysed, repaired to IPC-7711/7721 and retested, and the root cause is fed back into the process and, when relevant, into your design.
Frequently asked questions
Is AOI enough on its own?
For simple boards with no hidden joints and a proven design, AOI plus a functional check can be enough. Any BGA or QFN needs X-ray at least on samples, and any product with firmware needs a functional test.
When is an ICT fixture worth it?
When the annual volume is above roughly 500 boards and the design is stable, the fixture pays back through seconds-per-board test time and high fault coverage.
Can you run our functional test software?
Yes. We integrate customer test software and fixtures, or design and build the test system for you.
What test does Comtec Labs run by default?
AOI on every board, X-ray on area-array packages, and electrical or functional test according to the agreed specification.
Ready to reduce PCB surprises?
Comtec Labs offers a full suite of services to streamline your workflow:
PCB design service
PCB prototyping service
PCB component sourcing
PCB component assembly
PCB testing service
PCB repair and modifications
Printed circuit board production
PCB mass production