AOI (Automated Optical Inspection)

Overview

AOI (automated optical inspection) is the use of cameras, lighting and image analysis software to check manufactured parts for visible defects without touching them. On electronics assembly lines it checks boards after solder printing, component placement and reflow; in semiconductor packaging, similar optical systems inspect bumps, wafers and package surfaces. AOI finds what can be seen from above or from the side, so hidden joints such as those under a ball grid array need X-ray or electrical test instead.

How it works

An AOI system moves the part, or its cameras, through a series of fields of view under controlled light. Software compares each image with what should be there, either against rules (a minimum line width, a required polarity mark) or against the design data for that board or part. Anything outside the limits is flagged, and an operator or a classification step decides whether it is a real defect or a false call.

Older systems work from flat 2D images. Many current systems also measure height, building a 3D profile of components, solder joints and foreign material, which helps with shiny packages and shadows from tall neighbouring parts. Side-view cameras extend coverage to leadless packages and connectors. On assembly lines the most common position is after reflow, where one machine can catch most defect types, though systems are also placed after paste printing and before reflow.

In advanced packaging the task is harder. Defects can be a few nanometres deep, warpage across a die can exceed the depth of focus of many optical systems, and higher sensitivity also picks up far more harmless signals. Inspection tools therefore rely increasingly on machine learning to sort real defects from nuisance ones, while infrared, acoustic and other methods cover what visible light cannot reach.

Examples

• A post-reflow AOI machine on a surface-mount line flagging a tombstoned resistor and a solder bridge between two leads.
• A 3D AOI system measuring solder joint height and component lift instead of relying on a flat image.
• Optical inspection of bumped wafers before flip chip assembly, looking for missing or misshapen bumps.
• A machine learning classifier that reviews thousands of AOI hits per wafer and passes only likely real defects to an engineer.
• AOI on a board line being followed by X-ray inspection for ball grid array joints that cameras cannot see.

Why it matters

AOI catches assembly defects while they are still cheap to fix, and its data shows which process step is drifting. Inspection engineers own the programmes and thresholds, process engineers act on the trends, and quality teams rely on it to keep escapes low without inspecting every part by hand.

In the chip lifecycle, AOI appears in assembly and packaging, both for semiconductor packages at the OSAT and for boards that carry the finished chips, and it feeds yield and failure analysis during production ramp and volume manufacture.

It matters most to: AOI and X-Ray Inspection Engineer, Defect Inspection Algorithm Engineer, Package Inspection and Metrology Engineer, PCBA Process Engineer, SMT Process Engineer

Common confusions

Not the same as X-ray inspection

AOI sees only surfaces in its line of sight. Automated X-ray inspection looks through the part, which is needed for joints hidden under packages such as ball grid arrays.

Not the same as solder paste inspection

Solder paste inspection measures the printed paste deposits before parts are placed. AOI usually refers to inspection of placed components and finished joints, although both use similar optics.

Not the same as in-circuit test

In-circuit test checks electrical values through probes on the board. AOI is non-contact and finds visible defects, so a board can pass one and fail the other.

Sources

1. How Advanced Packaging Is Reshaping Inspection (Gregory Haley, 10 July 2025) (opens in a new tab), Semiconductor Engineering, 2026-10-22
2. Automated Optical Inspection Technology (opens in a new tab), Koh Young, 2026-10-22
3. Automated optical inspection (Wikipedia) (opens in a new tab), Wikimedia Foundation, 2026-10-22

Related terms

• SMT
• X-ray inspection

Where it appears on the Hub

Used on 10 pages of the Hub. In each group, the pages that mention it most come first.