What is Automated Optical Inspection (AOI)?
Automated Optical Inspection (AOI) is a machine-based technique used to test and evaluate PCB boards for potential faults like surface defects, dimensional defects, and component placement defects. As circuits become more complex, it has become almost impossible to manually inspect the boards and thus AOI plays an important role in checking printed circuit boards after assembly. It helps to attain the desired product quality in the production line by keeping the cost as low as possible (as it is an automated process).
The inspection starts with visual scanning of the board followed by a comparison of a captured image of the board and comparing it to the ideal board image that is input in to the AOI machine.
The board is placed under highly illuminated light sources such as fluorescent, LED/ infra-red or ultra-violet lighting with one or more high definition cameras. This is how the AOI machine captures the picture of the PCB board. The AOI machine then matches the captured picture with already installed information through processing software like what the board should look like. After this comparison, AOI highlights the defects.
The AOI is programmed and uses different algorithms for acceptable PCB board standards.
What types of defects can be identified during AOI?
Surface defects like nodules, scratches and dimensional defects such as open circuits, shorts and thinning of the solder can be detected by AOI. Detection of wrong components, missing components, and incorrectly placed components can also be detected.
For a PCB board with components (SMT inspection): Area defects, Billboarding, 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, Tombstoning, Volume Defects, Wrong Part, and Solder Bridging.
For PCB boards: Line width violations, Spacing violation, Excess copper, Missing pad, Short circuits, Cuts, Hole breakage (whether via is going outside of its pad), and incorrect mounting of components
Limitations of AOI:
AOI can only defect surface issues so if there is an internal issue in a multi-layer PCB then AOI will not be able to detect this.