What AOI catches and what it does not
Automated optical inspection catches placement defects (missing component, wrong component, tombstoned), solder defects (insufficient paste, bridging, cold joint), and silkscreen issues. It runs fast, on every board, and is cheap to run.
What AOI cannot catch:
- Firmware boot failures
- Sensor calibration drift
- RF transmit power below spec
- Analog gain out of range
- Communication protocol bugs that only manifest under load
- Power consumption above spec
- Thermal behavior under stress
These are functional defects. AOI reports the board looks fine, but the board does not work.
What FCT catches
Functional test exercises the unit's actual behavior. Boot to known service state. Read sensor inputs against reference values. Drive outputs to known states and verify response. Transmit RF and measure power. Pull current and check against spec.
Each test is a vector in the production test plan. Each unit cycles through the full vector list before it gets a serial and a label.
Field-failure cost analysis
A defect caught at AOI costs $1 to scrap or rework.
A defect caught at FCT costs $5 to $10 (more time, possibly already programmed).
A defect caught at integrator (next stage in supply chain) costs $50 to $100. The integrator stops the line, returns the unit, and bills hours.
A defect caught by the end user costs $500 to $5000. Returns logistics, warranty, sometimes a service truck.
The rule of 10x escalation is approximate but holds across most product classes. Spending a few dollars on FCT pays back many times over against the cost of one field failure.
Fixture NRE economics
A custom bed-of-nails or edge-connector test fixture costs roughly EUR 5000 to 15000 NRE for a connected board with moderate complexity. Lead time 4 to 8 weeks.
Modular fixtures (cartridge-style) cut per-SKU NRE by 30 to 50 percent after the first SKU.
The NRE pays back at the volume where field-failure prevention savings exceed the fixture cost. For most connected products, that is well below 1000 units total volume.
When FCT pays off
Always, for any product where firmware is on the board.
For bare PCBA with no firmware, FCT may be optional if AOI plus a downstream integrator test catches the same defects at acceptable cost. For programmed and serialized production, FCT is non-negotiable.
Sources
- IPC industry data on defect detection and field-failure costs
- FixturFab and similar fixture vendor public pricing
- Connected-product industry case studies on field-failure economics