AOI Detection Working Principle

#1 What is AOI Detection?

Automated Optical Inspection (AOI) is a technology that uses optical principles and image processing techniques to inspect component solder joints on PCBA boards. It can quickly identify subtle defects in solder joints, effectively overcoming the limitations of manual visual inspection.

(Image source: Internet)

#2 AOI Detection Working Principle

AOI detection equipment mainly relies on the principle of light reflection to identify and inspect component solder joints on PCBA boards. The image information obtained during the inspection process is converted into data and compared with qualified parameters in the AOI database to determine whether the inspected solder joint has defects.

(Image source: Internet)

Based on the AOI detection workflow, it can be divided into the following four stages:

Image Acquisition Stage

Responsible for scanning the inspected object and collecting images.

The AOI light source is composed of a combination of high-angle and low-angle RGB ring lights (red, green, blue). By illuminating solder joints on different layers with various angles and colors, the reflected light from the solder joints is collected into the computer.

(Image source: Internet)

The camera is positioned vertically above the PCBA board to capture images of the component solder joints. According to the principle that the angle of incidence equals the angle of reflection, light information reflected at different angles and layers of the solder joints is captured by the camera in different colors.

(AOI Image Acquisition Principle)

Data Processing Stage

Responsible for classifying and converting the collected image data.

The AOI equipment's computer processes the collected image features using algorithms, such as noise removal, correction, and enhancement, to improve the image quality for subsequent analysis.

(Image after data processing)

Image Analysis Stage

Responsible for extracting features from the processed image and comparing them with qualified parameters in the AOI database using methods like template matching and color extraction to identify defects in the image.

Color Extraction Comparison: Generally, the red area on the solder joint is the reflection from the flat plane of the solder joint, green area represents a smaller solder climbing angle on the component pin, and blue area represents a larger solder climbing angle. By extracting the proportion of each color in the image and comparing it with the proportion in the qualified template, the board is qualified if it falls within the threshold range; otherwise, it generates a defect report based on pre-defined defect types.

(AOI Color Extraction Comparison: Within threshold = Qualified board)

Template Comparison: Typically, qualified parameters in the AOI database are based on imported component package data as templates. During AOI detection, the image of the PCBA board being inspected is compared with the qualified parameters in the AOI database to determine if the solder joint has defects.

(AOI Template Comparison: Solder bridging)

Reporting Stage

Responsible for reporting the inspection results.

After image analysis, PCBA boards that do not match the qualified parameters in the AOI database are labeled as NG boards, and the defects are automatically marked for further inspection and repair by staff. Boards that match the qualified parameters are labeled

as OK boards and are automatically transferred to the receiving machine for the next process.

#3 Differences Between 2D AOI and 3D AOI

2D AOI Detection

Primarily uses RGB planar lighting to capture 2D images of components from a single angle. By processing the light reflections from the solder joints, the color proportion is analyzed in the image analysis phase to detect defects.

(2D AOI Imaging)

Advantages: 2D AOI equipment has a wide application range, relatively low cost, and can quickly detect soldering defects such as missing components, misalignment, exposed copper, and solder bridges on PCBA boards.

(Image source: Internet)

Disadvantages: 2D AOI cannot inspect defects like lifted or floating IC pins from multiple angles, and edge measurement is significantly affected by changes in illumination, limiting its capabilities.

3D AOI Detection

Building on 2D AOI, 3D AOI adds structured light imaging. By using gratings to restore height information absent in 2D, it enables detection of soldering conditions on component pins from multiple angles, addressing limitations such as lifted or floating IC pins in 2D AOI.

(3D AOI Imaging)

Disadvantages: The technical complexity of 3D AOI is higher, requiring professional maintenance, and its cost is higher compared to 2D AOI.

Since AOI equipment captures images based on the surface of objects, solder joint defects beneath components with large grounding pads cannot be detected through AOI and require X-Ray inspection instead.