What is AOI? Automated optical inspection and the review station

What AOI (automated optical inspection) is

Automated optical inspection is a machine that looks at assembled boards so people don't have to look at every one. A camera system images the board under controlled lighting, software compares what it sees with what the board should look like, and every difference beyond a set limit becomes a call: a location on the board, a component reference and the reason it was flagged. AOI doesn't repair anything and doesn't make the final decision on most calls. Its job is to find every place that might be wrong, so that people and repair stations can focus on those places.

On an electronics line AOI is one of the main visual quality gates. It checks large numbers of components on every board at line speed, which is something a person with a magnifier can't do consistently over a shift. What it can't do on its own is judge each call the way an experienced inspector would — and that is why AOI comes with a review step.

Where AOI sits on an SMT line

A surface-mount line builds a board in a few steps, and different inspections watch different steps. Line layouts vary, but the typical order looks like this:

• SPI (solder paste inspection), before placement. After solder paste is printed onto the pads, an SPI machine checks the paste deposits — whether each pad has paste, and whether the deposit looks right. It inspects paste, not components.
• AOI before reflow (pre-reflow), on some lines. After placement and before the oven, AOI can check that parts are present and placed on their pads, while a misplaced part can still be corrected without rework of a soldered joint.
• AOI after reflow (post-reflow). A common position. After the oven, AOI checks the finished solder joints and the final position and orientation of each part.
• AXI (automated X-ray inspection), where joints are hidden. Optical inspection can only see what is visible from above. Joints under a component body, such as the balls under a BGA, need X-ray; AXI is used where those joints matter.
• Electrical tests later. In-circuit test (ICT) and functional test check whether the board works electrically. They catch problems AOI can't see, but they come later in the process, when more boards may already have been built.

How AOI works

Under the hood, an AOI machine combines three things: images, an inspection program and limits.

• Imaging and lighting. The board is imaged with cameras and lighting from several angles, so solder fillets, component bodies and markings show up clearly. Some machines also measure height, which helps with lifted leads and solder volume.
• The inspection program and library. For each board type, a program tells the machine where every part is and what it should look like. Parts are described in a library: body size and color, leads, markings, the expected shape of a good joint.
• Thresholds. Every check has limits. A measurement inside the limits passes; outside them, the location is flagged. Where the limits are set decides how many real defects are caught and how many acceptable joints are flagged along with them.

Typical checks include whether a part is present, whether it sits on its pads, whether it is rotated or reversed (polarity), whether each solder joint has an acceptable fillet, and whether solder bridges two pins that should be separate. What counts as acceptable is set by the manufacturer's own workmanship criteria; the common reference for electronic assemblies is IPC-A-610 (Acceptability of Electronic Assemblies). The defects AOI is asked to find — solder bridges, insufficient solder, tombstoning, missing or shifted parts, reversed polarity — are described in more detail in our guide to AOI false calls and PCB defects.

How post-AOI review is done manually today

AOI is designed to flag on the safe side, so not every call is a defect. Every call therefore goes to a person at the review station (sometimes called the verification or repair-buffer station) before the board moves on.

1. 01

   The board arrives with its calls

   The review station shows the operator the board's flagged locations, usually as saved images of each location next to a reference image of a good one.

2. 02

   The operator looks at each call

   For each location the operator compares the image with the reference — and, where needed, looks at the board itself, sometimes under a magnifier or microscope.

3. 03

   A decision per call

   A real defect is confirmed and the board goes to repair. A call that turns out to be acceptable is marked as a false call, and the board continues.

4. 04

   Feedback to the program

   When the same false call keeps appearing, the AOI programmer adjusts the program or library for that part or location.

This works while the queue is short. The method strains as the number of calls grows:

• False calls fill the queue. Tight limits, shiny solder, shadows from tall parts and second-source components can all flag acceptable joints and parts. Each one costs the operator a look.
• Fatigue. Confirming call after call is repetitive. Over a shift, attention drops and the decision can become a habit rather than a look.
• Real defects waved through. When most calls in a row turn out to be false, a real bridge or a reversed part can be accepted along with them. The defect was flagged — and still escaped.
• The tuning cycle never ends. Opening the limits reduces false calls and lets more real defects pass unflagged; tightening them brings the queue back. Every new product, part source or process change can start the cycle again.
• Sampling and later tests come late. A sample audit or an electrical test can find what review missed, but by then more boards with the same defect may have been built.

AOI is good at finding every place that might be wrong. The hard part is the next step: deciding, for every call on every shift, whether an experienced inspector would accept what was flagged.

What to look for in an AOI review process

Whatever equipment and software you run, these are fair questions to ask about how AOI calls are reviewed on your line:

• Are acceptance criteria written down and shared? Operators on different shifts should judge the same joint the same way, against your workmanship standard and your customers' requirements.
• Is every decision recorded? A record of which calls were confirmed and which were passed, by board type, line and shift, is what lets engineers see patterns.
• Do false calls feed back to programming? Repeating false calls on the same part or position are a signal for the AOI programmer, not just extra work for review.
• Is review load watched? When the queue grows — a new product, a new part source — is there a way to see it before attention suffers?
• Are escapes traced back? When a defect is found at test or in the field, can you check whether AOI flagged it and how the call was decided?
• If you add software support, is it explainable and proven on your boards? Any tool that helps decide calls should give a reason a person can check, and should be piloted on your own products against criteria agreed in advance.

From manual review to an AI second look

Primarch builds on and enhances vision-language models (VLMs): models that read images and language together. (For a plain explanation of what a VLM is, see what is a vision-language model.) Applied to AOI, a VLM is designed to take a second look at each call the way an experienced inspector would: it reads the flagged part with its pads, joints and neighbors, takes into account what the AOI flagged it for, and writes its reading in plain language next to the image — likely false call, or real defect and why.

The AOI keeps its limits on the safe side, and the operator still has the final say; the second look adds a reason to every call. How that works in practice, and the defects behind the calls, are covered in our guide to AOI false calls and an AI second look.

How Primarch's Custom VLM Solutions help

This section is about our product. Custom VLM Solutions put your domain knowledge on top of the Primarch platform core, for problems that can be seen but are not in a catalog. For AOI review:

• Takes a second look at AOI calls. A detailed PCB inspection runs as a post-filter on the locations the AOI machine flagged, separating likely false calls from real defects such as a solder bridge.
• Works from the images the AOI machine already saves for each call, so no extra camera is needed at the review station.
• Writes each finding in plain language, next to the image it was read from, so the operator and the engineer see the same thing.
• Uses your acceptance criteria. Your criteria and your customers' requirements are part of the domain knowledge the expert is configured with.

Putting it to work

1. 01

   Discovery

   We define the problem and the success criteria together: which boards and calls, which defect types, which decision the second look supports.

2. 02

   Configuration

   An expert profile is built from a domain knowledge base, rule templates and thresholds; targeted fine-tuning if needed.

3. 03

   Pilot

   Validation on real images from your own line, tested together against measurable criteria.

4. 04

   Deployment

   On-premise or cloud rollout, team training and a continuous improvement loop. With on-premise deployment, your board images and data never leave your facility.