AOI, X-ray, ICT, and FCT: Which Tests Does Your PCB Assembly Need?
Each assembly test finds a different kind of defect, and none of them finds everything. Here is what each one catches, what it misses, and how to pick a combination that fits your board and volume.
No single test proves that an assembled board is good. Each method looks at the board in a different way and finds a different class of defect. Choosing tests means deciding which defects you can afford to find later.
This guide covers the tests you can select on an assembly request, what each one finds, and what it leaves for the next stage.
At a glance
| Test | Looks at | Finds | Needs from you |
|---|---|---|---|
| AOI | Visible surfaces | Missing, shifted, or reversed parts; visible solder defects | Nothing extra |
| X-ray | Hidden solder joints | Shorts, voids, and opens under BGAs and QFNs | Nothing extra |
| ICT | Electrical connections | Shorts, opens, wrong component values | Test points in the layout; a fixture |
| Flying probe | Electrical connections | The same as ICT, at lower speed | Accessible pads or test points |
| FCT | The powered board | Whether the board does its job | A test procedure, and usually firmware |
AOI: automated optical inspection
A camera system photographs the assembled board and compares it with a reference.
It finds: missing parts, parts placed in the wrong position or the wrong way round, tombstoned passives, solder bridges, and joints with too little solder.
It misses: anything it cannot see. Joints under a BGA or a QFN are hidden. It also cannot tell whether a part with the correct marking is electrically faulty.
AOI adds no fixture cost and little time, which makes it the first test to add for almost any surface-mount build.
X-ray inspection
X-rays pass through the component body and show the solder underneath.
It finds: bridged or missing balls under BGAs, voids in the solder, and poor wetting on the hidden pads of QFNs and other leadless packages.
It misses: electrical faults. A joint can look well formed and still be open.
If your board has BGAs or bottom-terminated parts, X-ray is the only way to inspect those joints without destroying them. If it has none, X-ray adds little.
ICT: in-circuit test
The board is pressed onto a fixture of spring-loaded pins, often called a bed of nails. Each pin touches one test point, and the tester measures the circuit node by node.
It finds: shorts, opens, missing parts, and wrong values of resistors and capacitors. It tests each board in seconds.
It misses: faults that only appear when the circuit runs, such as timing problems or a wrong firmware image.
The cost: the fixture is built for one board design. It takes time to make and has to be changed if the layout changes. This is why ICT suits stable designs built in volume. It also needs test points, which have to be designed into the layout. Adding them afterwards means a new board revision.
Flying probe
A flying probe tester makes the same kind of electrical measurements as ICT, using a few moving probes in place of a fixture.
The trade: no fixture cost and no fixture lead time, but each board takes much longer to test. This suits prototypes and small batches, where the design may still change.
FCT: functional test
The board is powered up and exercised as it will be in the product: inputs applied, outputs measured, interfaces checked.
It finds: faults that only show in operation, and it confirms that the board works as a whole.
It misses: marginal joints that work today and fail after vibration or thermal cycling. That is the job of the inspection steps above.
What you need to supply: a functional test cannot be invented by the assembler. Provide a written test procedure with pass and fail limits, the firmware to load, and any special equipment or a description of it. Without a procedure there is nothing to test against.
How to choose
| Your build | A reasonable starting point |
|---|---|
| Prototype, no hidden joints | AOI |
| Prototype with BGAs or QFNs | AOI and X-ray |
| Small batch, no fixture budget | AOI and flying probe; add X-ray if there are hidden joints |
| Volume production, stable design | AOI, ICT, and FCT; add X-ray if there are hidden joints |
| Any build where a field failure is expensive | Add FCT |
Two questions decide most cases:
- Are there joints nobody can see? If so, X-ray.
- What does it cost you when a faulty board reaches your customer? The higher the answer, the more electrical and functional coverage is worth paying for.
Tests and workmanship class are separate choices
IPC-A-610 Class 2 or Class 3 sets the acceptance criteria for workmanship: how a solder joint must look, how far a part may be offset. Tests are how defects are found. Choosing Class 3 does not add tests on its own, and adding tests does not change the class. You select both on your request.
Plan for testing during layout
The cheapest time to make a board testable is before the layout is finished:
- Add test points on power rails and key signals if you plan to use ICT or flying probe
- Keep test points on one side of the board where you can
- Bring out a programming header or pads for loading firmware
For the rest of the layout checks, see our DFM checklist.
Selecting tests on your request
On the assembly request form you choose the tests you want, and your quotation confirms which tests are included and any additional charges. Fixture-based tests take time to prepare, so mention them when you first ask for a quote, not after the boards are built. Our quality page describes the inspection flow in more detail.
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