What is AOI Inspection? PCB Optical Testing Explained
AOI (Automated Optical Inspection) is a camera-based check that scans every solder joint after reflow to flag missing parts, wrong polarity, lifted leads, and solder defects. It is the workhorse quality gate on every modern SMT line — including the DDM Novastar line at Energetika-VDS in Strumica.
This article covers what AOI does, what it misses, the Class 2 vs Class 3 sampling difference, and when to add X-ray or in-circuit / functional test on top.
How AOI actually works
An AOI machine sits inline after the reflow oven. As each board passes through, an array of high-resolution cameras (typically 18-25 MP) captures the top of every solder joint from multiple angles under structured LED lighting (red, green, blue, side, top). Software compares the image against a golden reference and flags deviations.
A typical AOI cycle on a 200 x 150 mm board with 600 components takes 25-40 seconds. That fits into a 60-second line takt without bottlenecking the SMT line.
What AOI catches (and how well)
| Defect type | AOI detection rate | Notes |
|---|---|---|
| Missing component | 99%+ | Trivial — silhouette mismatch |
| Wrong component | 95-98% | Reads laser-etched IDs and body markings |
| Reversed polarity | 98%+ | Detects polarity dots, cathode bands |
| Lifted lead | 92-96% | Hard on fine-pitch QFP < 0.5 mm |
| Solder bridge | 95-98% | Visible bridges only |
| Insufficient solder | 85-92% | Side-camera angle helps |
| Tombstoning | 99%+ | Obvious silhouette change |
| Cold joint (visual) | 70-80% | X-ray is more reliable here |
| BGA solder balls | 0% | Hidden under package — needs X-ray |
| QFN thermal pad voids | 0% | Hidden — needs X-ray |
The honest read: AOI catches ~95% of surface defects but cannot see anything hidden under a body. For BGA, QFN, LGA, or stacked-die packages, you either accept the risk, add 2D/3D X-ray, or fall back on functional test at the end of the line.
Class 2 vs Class 3 — the sampling difference
IPC-A-610 defines three classes of electronic assembly. The AOI strategy differs sharply between them.
| Class | Typical product | AOI coverage | Operator review |
|---|---|---|---|
| Class 1 | Consumer (toys, prototypes) | Sampling 5-10% | Spot checks |
| Class 2 | Industrial, commercial, IoT, automotive interior | Sampling 10-25% or 100% on critical nets | Engineer reviews flagged boards |
| Class 3 | Medical, aerospace, life-safety, military | 100% AOI every panel, dual operator sign-off | Every flag manually verified |
At Energetika-VDS we default to 100% AOI on every panel regardless of class — the line is already paid for, the marginal cost per board is roughly 0.18 EUR, and it removes a whole category of escape defects. Class 3 customers additionally get dual-operator review and documented traceability via our quality and traceability program.
AOI vs X-ray vs ICT vs FCT — where each belongs
These are not interchangeable. A real quality plan layers them.
| Test | What it finds | Where in flow | In-house at Energetika-VDS? |
|---|---|---|---|
| SPI (solder paste inspection) | Paste volume, area, height before placement | After printer, before pick-and-place | Yes |
| AOI | Surface solder, presence, polarity | After reflow | Yes |
| X-ray (2D / 3D) | Hidden joints under BGA, QFN, voids | After reflow, before depanel | Sourced via partner lab |
| ICT (in-circuit test) | Shorts, opens, component values | Bare board test fixture | Sourced |
| FCT (functional test) | End-to-end behavior, firmware | After assembly, customer-specific jig | Sourced — we run customer-supplied jigs |
A typical Class 2 IoT board ships with SPI + AOI + FCT. A Class 3 medical board adds X-ray and ICT. Cost layering for a 200-board lot is roughly: AOI 0.18 EUR/board, X-ray 3-8 EUR/board, ICT setup 800-2000 EUR + 0.4 EUR/board, FCT depends entirely on cycle time.
For context, see our cost guide and the low-volume guide for which tests actually pay off below 500 units.
Sampling vs 100% — when to argue with your EMS
If your EMS proposes AOI sampling (e.g. "every 5th board"), ask why. The honest reasons:
- Line speed mismatch (AOI is the bottleneck — rare on modern equipment)
- Customer chose sampling to save money (valid for Class 1)
- The AOI program is poorly tuned and false-call rate is wasting operator time
The bad reason: "We always sample." That is a process gap. At our SMT line AOI runs 100% on every board because the alternative — finding the defect at FCT or, worse, in the field — costs 10-100x more.
How AOI fits into the Energetika-VDS process
Our manufacturing process layers inspection at four points: SPI after solder paste, AOI after reflow on the top side, AOI after reflow on the bottom side (for double-sided assemblies), and visual + electrical at the end of line. With three-shift capacity of 14.4M placements per year and single-shift baseline of 4.8M, AOI throughput is matched to placement throughput so no inspection backlog forms.
For component sourcing and BOM with active or high-risk parts, we offer X-ray sampling on the first article and AOI 100% thereafter — a practical compromise for medical and automotive customers.
Get a quote estimate or request a formal quote with your BOM and Gerbers to see AOI coverage on your specific board.
Frequently asked questions
What is AOI?
AOI stands for Automated Optical Inspection. It is a camera-based machine that photographs every solder joint on a PCB after reflow soldering and compares those images against a golden reference to flag defects like missing parts, reversed polarity, or solder bridges.
AOI vs X-ray — what is the difference?
AOI uses visible-light cameras and can only inspect what the camera can see from the top. X-ray penetrates packages and inspects hidden joints under BGA, QFN, LGA, and other bottom-terminated components. The two are complementary, not alternatives — Class 3 boards use both.
Does AOI catch BGA defects?
No. BGA solder balls are hidden under the package body, so optical inspection cannot see them. To verify BGA joints you need 2D or 3D X-ray inspection. AOI will only confirm that the BGA package itself is present and correctly oriented.
Should I demand 100% AOI or is sampling enough?
For Class 1 consumer products, sampling 10-25% is industry standard. For Class 2 and above, request 100% AOI — the marginal cost is under 0.20 EUR per board and the defect escape risk drops by an order of magnitude. At Energetika-VDS, 100% AOI is the default on every panel regardless of class.
How accurate is modern AOI?
Modern AOI machines achieve ~95% detection rate on surface defects with a false-call rate of 0.5-2%. The remaining 5% of escapes are typically very-fine-pitch lifted leads, partial wetting issues, and any joint hidden under a component body — those require X-ray or electrical test to find.