Comprehensive Guide to AOI Technology in PCBA

After SMT assembly, ensuring product quality is crucial. Traditional manual inspection is slow and prone to human error. Automated Optical Inspection (AOI) emerged as a solution, leveraging high-speed imaging and intelligent processing to detect PCB defects with precision. This article explores how AOI validates the soldering reliability of PCBA.

How Does AOI Technology Work?

AOI uses optical imaging and image processing to identify defects on PCB assemblies. RGB lighting illuminates the solder joints from multiple angles, and a high-speed camera captures reflected light to generate image data. Algorithms then compare the data with reference templates to determine soldering quality.

Optical Imaging

AOI systems utilize ring-shaped RGB lighting to enhance the contrast of solder joints through combination of light angles and light colors. Cameras capture reflections based on refraction principles, producing high-precision images.

Data Analysis and Defect Detection

AOI systems apply advanced image processing techniques, including noise reduction, correction, and contrast enhancement. Defect detection relies on:

• Color Extraction Analysis: AOI breaks down the solder joint into red, green, and blue regions, analyzing their proportions to assess quality.
• Template Matching: The captured image is compared to a reference template to detect deviations in component placement and shape.

Key AOI Inspection Workflow

AOI inspection follows four key stages:

Image Acquisition

Proper lighting design and camera positioning are critical for accurate detection. RGB lighting ensures multi-angle illumination for comprehensive component imaging.

Data Processing

The captured images undergo noise reduction, enhancement, and correction to generate high-quality data for analysis.

Image Analysis

The image analysis stage of the AOI system extracts pre-processed image features and combines colour extraction comparison, and template comparison technologies to accurately identify and classify solder joint defects. AOI identifies solder joint defects using two major techniques:

• Color Extraction Analysis:

During colour comparison, the AOI system quickly determines the quality of solder joints by analysing the colour distribution of solder joint images. Typically, the colour characteristics of the solder joint surface reflect the soldering condition:

• • Red regions indicate light reflection on the solder joint.
  • Green regions represent small-angle solder flow onto component leads.
  • Blue regions signify large-angle solder climb.

By extracting the percentage values of each colour in the image, the system compares the measured image with the corresponding percentages in the qualified template. If the percentage values of each colour are within the set threshold range, the solder joint is deemed qualified. If they exceed the threshold, a defect report of the non-conforming type is generated, such as solder bridging, cold solder joints, or insufficient solder.

• Template Matching:

AOI systems store a library of standard component templates. During inspection, the actual PCBA image is compared to these templates, evaluating solder joint shape, size, and alignment. Any inconsistencies indicate potential defects.

Defect Report Generation

The system automatically highlights defect locations and generates a detailed report for operator review and rework.

2D AOI vs. 3D AOI: What’s the Difference?

AOI Type	Working Principle	Advantages	Limitations	Best Use Cases
2D AOI	Captures flat images using RGB lighting and analyzes surface defects based on color variations.	– Cost-effective – Wide range of applications – Fast detection of missing components, misalignment, and solder bridging	– Inability to capture solder joint height information – Difficulty in detecting three-dimensional defects such as pin warpage, float, etc. – Edge measurements are affected by lighting variations	Detecting surface defects like missing components, misalignment, and short circuits
3D AOI	Uses structured light and grid projection to measure solder height and volume.	– Detects lifted leads and insufficient solder – Provides full 3D analysis	– Higher cost – Requires expert calibration	Inspecting IC pin soldering, solder joint volume, and lead height