Automated Optical Inspection During PCB Assembly: How It Works and Its Importance

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Automated optical inspection brings mechanics, optics, electronics, and software on a single platform to make one single autonomous system to replace the human eye. The inspection is done through a scanning camera for the high-quality manufacturing of the printed circuit board (PCB).

For the manufacturing purpose of almost every product, there is a need for an automated optical inspection (AOI) system. But for the use of PCB, a specialized AOI system is developed. It is specifically designed to manufacture PCB objects. It involves visually scanning the quality and any missing parts of the cricket board.

AOI is a single big platform to analyze end-to-end failures with failure analysis tools along with real-time monitoring of the devices that use PCB. It is one of the significant inspection systems that make PCB competitive worldwide.

Potential of Automated optical inspection

AOI is potentially able to detect any kind of damage in the production unit and its products. The damages could be in the form of scratches, missing components, incorrect items, items without polarity matching, and many others.

At the very first step, AOI involves a detailed inspection of all the devices and components of the PCB. After that, it analyzes the assembly of all the printed circuit board (PCBA) components for the competitive environment.

Objects for inspection

Mainly there are two major objects of inspection while PCB assembly. AOI involves inspecting both printed circuit boards (PCBs) and printed circuit board assembly (PCBA). The former object involves scanning.

  • Soldering shorts
  • Soldering opens
  • Soldiers that are not sufficient for PCB

The later object involves the inspection of;

  • Polarity matching
  • Quality values
  • Component soldering

Reliability & modularity

AOI is a very flexible scanning system that modulates the PCB assembly according to the consumer's requirements. It can change the item or device at any stage of its manufacturing according to the company or customer's need.

It is a cost-effective inspection system because it has the potential to mold the product during the manufacturing process. It could remove any inefficient component or function of the final product while processing.

Therefore, its flexibility produces a correct PCB object for sufficient functionalized PCB assembly. Hence this capability of AOI avoids any kind of object rejection while testing its functionality and final testing.

Better than automated X-ray inspection

It is much better than automated X-ray inspection because it has great potential to reflow the manufacturing process according to the consumer's needs.

It makes it a low-cost assembly process while joining the manufactured items for PCBA.

PCB assembly

Need of the AOI for PCB assembly

AOI system is featured to have a card requirement to connect with autonomous tools such as robotic arms and CCD cameras. AOI system needs more than one kind of card to produce an inspection system with the potential of co-planarity and analyzing the data volumetrically.

The cards will not only increase the efficiency of the AOI system by improving the speed of its production unit.

How AOI works for PCB Assembly

AOI is featured differently for PCB inspection and assembly with the following working principle.

PCB illumination under different lights

At the very first step, AOI testing involves illuminating the PCB by using infrared light or ultraviolet light or by fluorescent or LEDs lighting. AOI uses a specialized autonomous camera to take pictures of the PCB object under different light sources.

The illumination facilitates the AOI to inspect the surface of the optics or PCB to analyze any surface damage or scratches.

Along with the source of lightning, the position of the light source also matters while taking pictures of the inspected surface.

Image analysis by processing program

The processing program is the element of the AOI machine that analyze the captured images of the PCB after illumination. For this purpose, high-quality images by more than one camera are captured, and for better efficiency, software-guiding moving cameras could be used.

After taking images from one or more cameras, they are compared to select high-resolution images with a wider field of view. It will generate large data based on high quality and rate of scanning.

Images comparison with board parameters

Now the high-resolution images will be compared with real board parameters. For this comparison,

  • The board parameters are added to the AOI machine in advance.
  • The processing program in the AOI machine will compare the images with the board parameters, i.e., a comparison under software guidance.
  • The software will analyze whether the PCB is perfectly designed or have an inaccuracy in its parameters.
  • Processing software having board parameters in advance facilitates depicting any manufacturing issue at any spot of the manufacturing line.

This will provide real-time monitoring of a PCB device to not even detect a failure but also to correct it.

Kingford PCB

Application of algorithms

After comparing the board features and the images of the PCB, the inaccuracies will be accurate by applying some kind of algorithms by the AOI system. A simple inspection system applies an algorithm of pixel-counting under the following methods;

  • Area-based method/correlations

Under this method, a template containing PCB features is compared with the selected high-quality image to check the correlation between them.

The applied algorithm will use the pixel-counting method to detect the possible correlations between the image and the template.

The high value of normal correlations will depict higher matching of the template with the inspected image.

  • Object view method

The applied algorithm under this method will recognize the object by comparing the camera image with a real image of board parameters.

Image separation from the background with the Vectorial method

A blob analysis is performed to separate the object from the background in-camera image. Then the pixels of the object are grouped to make a blob to specifically test the problems of the object without the interruption of the background.

After that, a Vectoral method is applied to the generated blob to make the PCB and PCBA testing system highly effective and efficient. The vector method is a geometric way of imaging that contributes to the progress of inspection technologies of AOI machines.

It is an efficient method that reduces the inaccuracies and failures of the object due to background elimination. Also, colors and shapes do not have any impact on this technology, that's why it does not affect the color of the image.

Inspecting AOI from PCB cart by processing software

The final step to inspecting PCB before its assembly is analyzing it by processing software present in the PCB cart. PCB cart is a full fledge software based element that contains;

A "Golden board" is involved in selecting a better product by scanning for the AOI system. It is designed to observe the different aspects of the targeted product to generate the variance data for the processing software.

A programming unit based on algorithms uses the data generated by the "Golden board" to generate the profile of the product.

In this way, an autonomous inspecting system will be generated basis on the processing software.

Hence the PCB cart analyzes the final issues and failures of bare PCBs autonomously by using the software and programming units before PCB assembly. This is the way AOI inspect the PCBs before their assembly (PCBA).

Importance of AOI during PCBA

PCB has a dense manufacturing line, so it is very important to apply automated optical inspection on PCB to make it a better production unit with high competency over the entire world.

Therefore, AOI is important;

  • Potential to test both bare and assembled PCB

AOI is such an effective system of monitoring the PCB objects separately by generating a blob along with the assembled PCB too.

  • The amalgam of different techniques in a single platform

It combines different techniques of mechanics, electronics, optics, and software to provide PCB testing at a single wider platform. Most importantly, it is highly able to arrange all the techniques properly to make an algorithm.

  • Camera scanning integrated with software processing.

It is an inspection system that greatly integrates the scanning of bare PCBs with software processing. It will automatically read different aspects of PCB and generates its profile.

  • Reduce faulty PCBs

AOI is capable of real-time monitoring, so it reduces faulty PCBs generated by removing all defects during PCB production at any stage of processing. So it is very important to ensure accurate PCB manufacturing and assembling.

  • Reduce cost

Real-time monitoring makes the AOI able to remove the inaccuracies side by side during PCB production. This will avoid generating incorrect objects, so an accurate final product is formed, which reduces the overall cost.

Hence, it is a highly significant low-cost inspection system with faster speed.

Conclusion

An automated optical inspection machine is highly valuable for not only testing the manufacturing process of PCBs but also for assembling them. The system is specifically featured to monitor the PCB from all aspects before its assembly.

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