What is Automated Optical Inspection

An effective technology for inspecting electronic components is optical inspection, which enables the detection of internal circuits as well as the assessment of assembly and soldering quality. In the early stages, this process relied heavily on manual inspection, where technicians identified defects either with the naked eye or with the assistance of a microscope. However, this approach posed two challenges: a reliance on subjective judgment and difficulty in detecting intricate issues. As a solution, Automated Optical Inspection emerged.

Introduction to Automated Optical Inspection

aoi pcb

Automated Optical Inspection, abbreviated as AOI, is a core system for visual inspection on the production line. Its primary task is to examine potential defects, surface feature errors, and quality issues that may arise during the manufacturing process. Through the collaborative efforts of optical devices, cameras, and image processing systems, AOI inspection systems achieve high-speed, accurate, and consistent inspection tasks, making significant contributions to the manufacturing standards in modern electronics across safety-critical sectors such as automotive, medical, and aerospace.

In the realm of circuit boards, AOI is virtually present at every stage of the manufacturing process. From inner layers to outer layers, from SMT to THT, AOI can detect aspects including but not limited to open circuits, short circuits, component positioning, and orientation. The use of high-definition images and comparative graphics allows for clear visualization of any defects.

Necessity of Integrated AOI Systems

AOI systems can exist in electronic manufacturing as standalone or integrated solutions:

Standalone: Operates independently, not collaboratively with other systems. This flexibility makes AOI inspection suitable for various manufacturing scenarios, particularly effective for prototype manufacturing projects.

Integrated: Integrated with other equipment through a conveyor belt, where products awaiting inspection are transferred to AOI after production and, once inspected, move on to the next stage of production.

In the modern electronics manufacturing industry, integrated AOI inspection systems are the mainstream trend and are almost considered standard for manufacturers pursuing quality. While more advanced equipment exists, many manufacturers still use 2D AOI devices due to their significantly lower cost compared to other inspection devices. As a long-term investment, integrating AOI systems into the production line is virtually beneficial for everyone involved – customers, manufacturers, and consumers alike.

For customers, manufacturers using integrated AOI systems can avoid production delays due to rework. Additionally, since testing is seamlessly integrated into the production line, there are no extra costs involved, resulting in improved product quality and turnaround time without increasing expenses.

For manufacturers, timely detection at various stages of production can increase the success rate of final inspections. Identifying defects promptly allows for adjustments to avoid entire batches of products being faulty.

For consumers, this translates to enjoying higher-quality electronics and a better overall experience.

Benefits of AOI Testing for PCB

Low Cost, High Returns

Investing in AOI equipment is considered a long-term commitment, but its performance in reducing failure rates and improving production efficiency creates more profit opportunities for manufacturers. Through AOI testing, manufacturers can effectively reduce the risk of faults in manufacturing and assembly, gaining the trust of customers, which will further lead to more business and market share. For customers, AOI testing as part of quality control provides assurance without additional costs.

Precise and Comprehensive Detection

As circuit board designs become more intricate, manual inspection cannot meet the demands of complex circuits. AOI equipment, with its built-in image capture and analysis capabilities, along with high sensitivity to tiny positional deviations, ensures reliable detection even of 01005 defects.

Compatibility with Diverse Light Sources

Light sources are at the core of visual applications, and AOI systems demonstrate flexibility in adapting to the ever-changing electronic manufacturing environment. AOI systems have the capability to work with various light sources, including fluorescent lamps, LED lighting, infrared, or ultraviolet. This diversity of light sources enhances image quality, making defects more prominent, thereby reducing the time required for subsequent processing and improving detection speed and accuracy.

Powerful Networking Features

AOI systems generate a significant amount of data through optical inspection, including text outputs, image collections, and database information. These data records critical information for each detection cycle on the production line. The powerful networking features enable the system to efficiently retrieve, analyze, report, and trace this information. Whether extracting data for a specific time period from the database, filtering defects with specific attributes, or obtaining detection results for a particular product batch, the AOI system can accomplish these tasks quickly and accurately.

How to Detect PCB Defects Using AOI

Insufficient Solder

After reflow soldering, solder paste solidifies on the PCB pad, forming a smooth curvature surrounding the component pins. Place the PCB under an illumination system, and the illuminated surface of the curvature will show two bright spots with the surroundings relatively darker. With an AOI inspection system, use the frame selection function to choose these two bright spots and apply a contrast algorithm to assess the degree of reflection. Adequate solder creates a clear reflection with high contrast, while less solder leads to reduced contrast.

Component Upright

When welding time or temperature is insufficient, components on the PCB may stand upright, causing soldering defects. Use a vertical camera and illuminate the light source from directly above. Normally, the melted solder should appear dark, and the area becomes brighter when there is an issue. Employ the DARK algorithm to detect this defect through image processing techniques.

Missing Components

To observe whether components are missing, different strategies can be used based on factors such as component contrast and the presence of a silkscreen layer on the PCB. Here is a common method:

Utilize a vertical camera and outer circle ring light. If the detection area initially lacks components, it appears relatively dark. If a component is missing, a smooth curved reflection is present. To further confirm the accuracy of the detection, use the position where the reflection is blocked in the absence of a missing component, set a detection window, and employ the NO CONTRAST algorithm. When the component is present, there is no contrast on the component surface, while in the absence of a component, the solder pad exhibits brightness, forming a clear contrast.


Check for solder bridging between pins by using an angled camera to view the pins from above, illuminating the light source from the camera direction. When solder bridging is present, a horizontal bright line forms between the pins. Set a window between the pins, choose the NO CONTRAST algorithm, observe the change in contrast, and determine if there is bridging between the pins.


The polarity detection method depends on whether the component provides clear polarity markings. Apply the PERCENT WHITE or PERCENT BLACK algorithm to the text on the component to quickly determine if the component’s polarity is correct.

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