What Is Automated Optical Inspection (AOI)?
Also known as: AOI, AOI inspection, automatic optical inspection
Definition
Automated Optical Inspection (AOI) uses cameras and image-processing software to inspect printed circuit board assemblies for defects such as missing, misaligned or wrong components and poor solder joints, without a human checking each board.
Automated Optical Inspection (AOI) Explained
AOI machines capture high-resolution images of each board from one or more angles and compare them against a golden board or a rules library built from the CAD data. They check presence, polarity, position, component value markings and solder fillet shape. AOI can sit after paste printing, after placement (pre-reflow) or after reflow, and many lines have it at the post-reflow stage.
AOI cannot see hidden joints under parts like BGAs, so it is paired with X-ray inspection, in-circuit test (ICT), flying probe and functional test. It also produces false calls, where good boards are flagged, so operators review flagged boards at a verification station and classify them as true defect or false call.
The valuable output is data. Each inspection result carries serial number, board side, reference designator and defect code. Aggregated, that points to a specific feeder, nozzle, stencil aperture or oven zone. Sending counts and defect codes to the MES and ERP supports first pass yield reporting, scrap and rework transactions against the work order.
In regulated work, AOI results become part of the build record linked to the serial number.
Why It Matters
- AOI catches defects before functional test where they are far cheaper to fix.
- Defect data by reference designator exposes repeat machine or material problems early in a lot.
- False call rate affects labor cost and line throughput, so tuning AOI programs matters.
- AOI records provide evidence for customer quality audits and corrective action reports.
In Practice
A line builds 300 boards. AOI flags 24 for a missing 0402 capacitor at C37. The pattern on one machine suggests a worn nozzle. The operator replaces it, the remaining boards pass, and the 24 boards are reworked by hand. The ERP work order logs 24 rework hours and the defect code feeds the first pass yield report.
Frequently Asked Questions
What does AOI inspect on a PCB?
Component presence, position, polarity, markings and solder joint appearance, based on camera images compared to a reference.
Can AOI inspect BGA solder joints?
No. Hidden joints under a BGA need X-ray inspection or electrical test.
Where is AOI placed in an SMT line?
Commonly after solder paste printing, after placement before reflow, and after reflow. Post-reflow is the most common.
Related Terms
Surface Mount Technology (SMT)
Surface Mount Technology (SMT) is the method of placing and soldering electronic components directly onto the surface of a printed circuit board, instead of inserting leads through holes. It is the dominant way modern electronics are assembled.
First Pass Yield
First pass yield is the percentage of units that complete a process correctly the first time, without rework, repair, or retest. Units that eventually pass after rework are excluded, so the metric captures hidden factory loss.
Rework
Rework is action taken on nonconforming product to bring it into full conformance with the original specification. It differs from repair, which makes product acceptable for use without meeting the original requirement.
Scrap Rate
Scrap rate is the percentage of production output discarded as unusable because it cannot be economically reworked to specification, calculated as scrapped units divided by total units produced over a defined period.
SPC (Statistical Process Control)
SPC (Statistical Process Control) is the use of control charts and statistical rules to monitor a process in real time, distinguishing normal common-cause variation from special-cause variation that signals a genuine change requiring action.
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