PCB electrical test and AOI: how boards are checked
What a flying-probe electrical test and automated optical inspection look for, how they differ and where their limits are.
Why testing matters
Faults inside a printed circuit board cannot be seen with the eye: a trace that is interrupted or a short circuit between two nets. If such a board is assembled, the cost of the faulty board multiplies by the cost of the components and the assembly. Testing the bare board is the cheapest point in the chain to find the fault.
Two checks that complement each other
- AOI (automated optical inspection). Cameras scan a layer and compare it with the design. AOI finds visible defects: traces that are too narrow or nicked, copper residues, missing or damaged features. It is most useful on the inner layers and on the copper pattern before the solder mask covers it.
- Electrical test. Probes touch the pads and measure every net: continuity (no open circuit inside a net) and isolation (no short circuit between different nets), compared with the Gerber data or netlist.
Flying probe or fixture test
A flying-probe tester moves needles over the board from point to point. It needs no test fixture, so it suits prototypes and small series without extra tooling cost. A fixture test (bed of nails) is faster per board, but the fixture pays off only in large volumes. At PCB-ELECTRA every board is tested with a flying probe.
What is tested at PCB-ELECTRA
- 100 % electrical test (short-circuit and open-circuit, flying probe) against the Gerber data and netlist on every board, at no extra charge.
- AOI (automated optical inspection, including the inner layers of multilayer boards and the copper pattern before the solder mask is applied) on every board, at no extra charge.
- Aluminium (IMS) boards: in addition a dielectric withstand test (hi-pot), which confirms that the insulating layer holds its rated voltage without breaking through to the metal base.
What these tests do not cover
- Component function: a bare-board test says nothing about parts; that is the job of the test of the assembled board.
- Signal quality: the test checks connections, not impedance or high-speed behaviour. Controlled impedance is not available yet; contact us if you need it.
- Solderability over time: the finish (HASL lead-free) is applied for good solderability, see HASL and RoHS.
How you help the test
Send a complete, consistent Gerber set — the test is compared against it (see the Gerber guide). For assembled boards, plan test points in your own design. Certified quality systems back the process: ISO 9001:2015 and ISO 14001:2015, see capabilities. For an offer, use the price calculator.
Frequently asked questions
Are all boards tested?
Yes. Every board goes through a 100% electrical test (short circuit and open circuit, flying probe) and AOI, at no extra charge.
Is AOI done on every board?
Yes. Every board gets a 100% electrical test (short circuit and open circuit) and AOI, at no extra charge. On multilayer boards AOI also covers the inner layers, and on every board it checks the copper pattern before the solder mask is applied.
What does a flying-probe test check?
Movable probes check each net for continuity and for shorts between nets, according to the data of your design.
What does AOI find?
Visible defects in the copper pattern, such as breaks, shorts, missing copper and wrong widths, before the board is finished.
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