# AOI Machine Guide: How Automated Optical Inspection Works

An AOI machine uses cameras, controlled lighting and algorithms to evaluate visible PCB features. It can detect missing, shifted, rotated, lifted or incorrect components and visible solder anomalies. AOI does not prove electrical function and cannot fully inspect joints hidden beneath packages.

2D, 3D and Defect Coverage

2D AOI analyses color, contrast, edges, text and shape. 3D adds height or surface information for lifted parts, coplanarity and solder geometry. Many systems combine both. JUKI describes inspection using 3D and 2D imagery for component and solder defects; see JUKI overview.

False Calls and Escapes

A false call rejects a good feature; an escape accepts a defect. Tight limits may overwhelm review while loose limits hide risk. Use defect standards, known-defect challenges and controlled disposition. Hidden BGA joints generally need X-ray or another suitable method.

Selection

Compare coverage, resolution, 2D/3D method, board range, side views, cycle time, programming, review, traceability, data export and service. Test dense, reflective and tall-component areas plus real known defects.

FAQ

Can AOI replace electrical test?

No. AOI evaluates visible physical evidence; electrical tests evaluate circuit behavior.

Where is AOI installed?

Common positions are after placement and after reflow, selected according to defect risk and correction cost.