What is the meaning of AOI testing?

Today we would like to reply these questions.What is the meaning of AOI testing? What does an AOI do? How does AOI work?

As components become increasingly smaller in size and BGA, CSP, and other components are widely used, AOI has become a commonly used testing technology in the modern SMT field.

What is AOI?
AOI is the shorten form of Automatic Optic Inspection.
AOI equipment can be divided into two main categories based on their inspection principles: laser AOI and CCD camera-based AOI.The advantages of laser AOI include its ability to accurately inspect heights, such as solder paste thickness, component height, and solder height. However, its disadvantages lie in the technical complexity of processing, extremely complex programming, and relatively slow inspection speeds. Additionally, due to the deformation of PCB caused by heating during reflow soldering, this situation often leads to misjudgment for laser AOI, resulting in its rare usage nowadays.Most AOI systems use CCD camera-based methods. The main reasons for this choice are low noise, high sensitivity, and the ability to perform digital binarization processing on the images sampled by CCD, making the technical processing relatively simple compared to laser methods.
From an application perspective, AOI can be classified into two types: desktop and inline. Desktop AOI systems are mostly semi-automatic, requiring manual placement of boards and manual initiation of inspections. Inline systems are fully automatic and can perform testing either in-line or offline. They are generally not used for multi-variety and small-batch production.

What are basic principles of AOI?
Humans observe objects by judging the amount of light reflected back, where more reflection appears bright, and less reflection appears dark. AOI operates on a similar principle to human judgment. The basic principle of AOI inspection involves using artificial LED light sources to replace natural light and utilizing optical lenses and CCDs to compare, analyze, and assess the amount of light reflected back from the object against programmed standards.

AOI inspection consists of two main parts: the optical section and the image processing section.

1.Optical Section
the optical section consists of an image acquisition module and a light source module. The quality of the image acquisition module directly impacts the quality of the images. By employing telecentric lenses and high-resolution cameras, the workload of adjusting inspection programs can be significantly reduced, while also decreasing false results. For inspecting large-sized ICs and densely spaced solder joints, telecentric lenses can provide higher image resolution and smaller distortion under more flexible lighting and configuration conditions. To meet the speed requirements of modern production without compromising image quality, a larger field of view (FOV) is necessary. Industrial-grade 4-megapixel cameras can cover an area of nearly 20cm² in a single high-resolution photo.

In AOI, the selection and control of the light source are crucial. The brightness of the light source is essential, as brighter illumination generally leads to higher defect detection rates. Flexible lighting is necessary for inspecting a wide range of electronic components and identifying various defects. In recent years, LEDs of various shapes and colors have been widely used in AOI systems and have become standard configurations.

Light sources with multiple directions, angles, and configurations are suitable for inspecting solder short circuits, reducing false positives. They can also display laser markings with high contrast, which is essential for effective use of OCR (Optical Character Recognition) functionality and polarity inspection. For inspecting solder joints around the bottom of components such as PLCC and SOJ, camera systems equipped with light sources at certain angles of incidence are necessary.

Intelligent control of lighting variations is crucial. Lighting is a key factor in image recognition for AOI systems. However, light sources are influenced by factors such as ambient temperature and internal temperature rise within AOI equipment, leading to fluctuations in illumination. Therefore, intelligent control of lighting variations, such as automatic tracking of light transmission rates, is necessary. This may involve regular calibration of light intensity to maintain consistent illumination levels.

2.Image Processing Section
The image processing section is the key technology in AOI. It can be likened to the brain of the AOI system. The images acquired through the optical section need to be analyzed, processed, and judged through the image processing section. This part of the AOI system requires robust software support because different defects require different computational methods for analysis and judgment. Some AOI software packages offer dozens of calculation methods, including black/white analysis, ratio of black to white, color analysis, synthesis, averaging, summation, difference calculation, plane fitting, edge detection, and more.

What are AOI applications and what improvement can AOI be made?
AOI applications:
Placed after printing: Inspection of solder paste quality; detection of excessive or insufficient solder paste, offset solder paste shapes, bridging or solder balls between solder paste shapes, and missing solder paste.
Placed after placement machine and before soldering: Inspection of component misplacement, component offset, component reverse mounting (e.g., polarity reversal), component tilt, missing components, polarity errors, and bridging between solder paste shapes caused by excessive mounting pressure.
Placed after reflow soldering: Inspection of soldering quality; detection of component misplacement, component offset, component reverse mounting (e.g., resistor reverse mounting), missing components, polarity errors, wetting of solder joints, excessive or insufficient solder, solder bridging, solder balls (between pin), component lifting (tombstoning), and other soldering defects.

The improvement on AOI in future:
AOI can only perform visual inspection and cannot fully replace In-Circuit Testing (ICT).
Unable to inspect invisible solder joints such as BGA, CSP, and flip-chip components.
Accurate inspection of PLCC requires the use of side-view CCD cameras.
Some AOI systems with low resolutions may not support OCR (Optical Character Recognition) character recognition inspection.

How to programme in AOI?
Before programming AOI, a fully soldered board is needed as a reference. This standard board is placed in the AOI system, and PCB parameters are set. Then, a scan is performed, and mark points are set. Next, all components and solder joints on the standard board are programmed (test data is compiled), and finally, the lens is optimized. Once programming is complete, the program for the standard board is saved in the program library under a specific filename. At this point, 20 to 30 fully soldered boards are used to debug and learn from the programmed data.During normal inspection, the machine automatically compares the scanned boards with the standard board program and marks or prints any discrepancies. Throughout the testing process, false positives may occur due to differences in component batches or variations in component appearance compared to the taught components. Therefore, when replacing components, a simple adjustment to the standard board program may be necessary.AOI programming can easily input PCB, component coordinates, material codes, and other information into the software through CAD conversion. AOI systems have powerful databases, allowing for various data to be compiled through library operations or custom methods. There are two programming methods: online and offline. Online programming requires inputting component positions and material codes directly into the AOI system. Component positions can be entered using X and Y coordinates and angles, similar to teaching programming on placement machines. Then, various defect inspection data for each component is compiled, and thresholds, upper limits, lower limits, and other information for defects are inputted. Online programming occupies AOI machine time, affecting inspection efficiency. Offline programming, on the other hand, utilizes dedicated software on a computer, improving AOI utilization.

Overall, AOI plays a crucial role in enhancing product quality, increasing manufacturing efficiency, and reducing production costs in various industries, including electronics, automotive, aerospace, and medical devices. That is why we use the AOI in and after production to check the PCBA to make sure stable quality and reliability offered to the customers all over the world.

0 replies

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply

Your email address will not be published. Required fields are marked *