Automated optical inspection (AOI) is an automated visual inspection of printed circuit board (PCB) (or LCD, transistor) manufacture where a camera autonomously scans the device under test for both catastrophic failure (e.g. missing component) and quality defects (e.g. fillet size or shape or component skew). It is commonly used in the manufacturing process because it is a non-contact test method. It is implemented at many stages through the manufacturing process including bare board inspection, solder paste inspection (SPI), pre-reflow and post-reflow as well as other stages. .(Wikipedia)

AOIs for a PCB board with components may inspect the following features:

Area defects
Component offset
Component polarity
Component presence or absence
Component Skew
Excessive Solder Joints
Flipped component
Height Defects
Insufficient Paste around Leads
Insufficient Solder Joints
Lifted Leads
No Population tests
Paste Registration
Severely Damaged Components
Solder Bridges
Volume Defects
Wrong Part
AOI can be used in the following locations in the SMT lines: post paste, pre-reflow, post-reflow, or wave areas.
AOI for a bare PCB board inspection may detect these features:
Line width violations.
Spacing violation.
Excess copper.
Missing pad – a feature that should be on the board is missing.
Shorts circuits
Hole breakage – a drilled hole (via) is outside of its landing pad.
The triggering of a defects report may be either rule based (e.g. no lines on the board should be smaller than 50μ) or CAD based in which the board is locally compared with the intended design.
This inspection is much more reliable and repeatable than manual visual inspection.
In many cases, smaller circuit board designs are driving up the demand for AOI vs in-circuit testing. (Wikipedia)

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