Significance of PCB AOI test

Significance of PCB AOI test

Table of Contents

What is the PCB AOI test?

PCB AOI test means that electronic printed circuit boards, or PCBs, are manufactured and tested using a key technique known as automatic or automated optical inspection, or AOI. PCB AOI test makes it possible to quickly and accurately evaluate electronics assemblies, particularly PCBs, to make sure the goods are correctly constructed and have no manufacturing flaws before they leave the production line.

What is a PCB AOI test

Automatic optical inspection (AOI) systems check printed circuit boards for flaws visually. They can identify a wide range of dimensional flaws, including open circuits, shorts, and solder thinning, as well as surface feature defects like nodules, scratches, and stains. They are also capable of identifying misplaced, missing, and poorly assembled parts. As a result, they can carry out all visual tests that manual operators previously carried out, but much more quickly and precisely.

How is AOI working?

The PCB board’s surface is scanned during the AOI examination. The HD color cameras on the AOI machine scan the surface at high magnification, clearly displaying on a larger monitor the intricacies of each PCB piece and each component within it. The camera uses a light source, such as fluorescent or LED lights, PCBs, and UV or infrared light, to take pictures of the PCB surface.

After that, the image is compared to the pre-programmed information and supplied into the PCB AOI test processing system’s memory. The process is instantly stopped if the gap or Normalized Cross Correlation (NCC) is determined to be greater than the tolerance level. The difference between actual photos and pre-programmed (template) pictures gets smaller the higher the NCC. Various algorithms running on the AOI machine based on pixel counts and NCC values determine if the board will ultimately be deemed defective and the process will be stopped.

The AOI machine operator can manually halt the process as needed by visually inspecting the images the AOI machine has captured on the screen. By differentiating the object image from the backdrop, the “blobs or pixels” of the pixels-based image are integrated or grouped, and errors are then found.

Common PCB defects the AOI test detects

PCB defects

The PCB AOI test is a remarkable technique for automated PCB inspection on a big production line. The AOI may check for open and short circuits, incorrect component polarity, misplaced or missing components, incorrect IC package orientation, stains, cuts, and scratches, as well as for tombstoning, lifted leads, lifted leads, solder balls, package height (3D AOI), billboarding, and coplanarity.

Comparing AOI with other methods of PCB inspection

In-circuit testing

In-circuit testing

ICT has many benefits, including quick test turnaround and rapid defect detection. Users that require large quantities of products are more likely to use them because of their quick features and convenience. On the downside, because bed nails need to be replaced frequently and it could be challenging to find flaws in complicated circuits with higher density, businesses that require low volume and a variety of products might not value ICT.

Automated X-ray inspection

Automated X-ray inspection

Solder joints that are invisible to the naked eye can be examined by AXI. It has a 97% production fault rate. On the negative side, AXI does not test for flaws in electrical performance. Both AXI and AOI take pictures for examination, making them both part of the structural test and inspection category. However, the main distinction is that AOI machines rely on the light source, but AXI machines depend on X-rays to take images. But X-ray inspection is more expensive, so test AOI is a more budget-friendly option for inspection.

Manual visual inspection

Manual visual inspection

A more trustworthy method than a human visual inspection is the PCB AOI test. A tiny PCB fault might be easily overlooked by a person. But the PCB AOI test can detect even the smallest mistakes. It can scan the board in greater detail than the human eye can see, and it is dependably trustworthy throughout all phases of the production process.

What are the advantages of the PCB AOI test?

  • Low cost: The AOI test can increase PCB pass rates, lowering PCB costs.
  • Multiple inspection targets: The PCB AOI test is effective for PCB assembly as well as PCBs. In terms of PCB, it examines flaws including shorts, openings, and insufficient solder.
  • Positional accuracy: As component sizes get smaller, it becomes more crucial for manufacturing equipment and inspection equipment to be accurate in their positions. AOI system has sub-pixel accuracy as a rule-of-thumb, able to detect the small deviations in position that can result in a 01005 defect.

What are the disadvantages PCB AOI test?

Similar to the human eye, the PCB AOI test can only really investigate the surface of an object, therefore as long as the shape can be seen on the object’s surface, it may be verified as correct. However, the PCB AOI test could not have adequate strength for areas that are concealed beneath or along the edge of a portion. A lot of AOIs can already improve the camera angle of some hidden components and enhance their ability to detect IC pin tilt. However, the results are never perfect, and obtaining a test pass rate of 100% is challenging.

How much does Automated Optical Inspection cost?

Price variation results from the costs of the custom implementation service, the program,  software and the hardware together. The average price for an AOI solution is $30K.

What precautions should be taken when conducting PCB AOI testing?

Following precautions must be kept in view while conducting PCB AOI test:

precautions must be kept in view while conducting PCB AOI test

  • Decrease condensation:

Condensation may form on your board surface if the temperature differential between it and its surroundings is too high, which might cause testing to be inaccurate. By managing the temperature and providing an appropriately ventilated testing environment, this may be prevented.

  • Use tools with caution:

When dealing with circuit boards, caution should always be used; however, since smaller boards have a higher risk of damage, utilizing the right equipment is essential.

  • Improper PCB handling must be avoided:

When prototyping your idea, failure to do so may result in delays in the development process, such as the requirement to re-clean or bake boards to eliminate moisture. Additionally, contaminated boards might exhibit inconsistent behavior, making precise testing challenging.

  • Use correct technology:

Make sure you are using the correct technology and type of lightning system when conducting the PCB AOI test.


In conclusion, we can state that the PCB AOI Test is the most crucial step in the PCB manufacturing and PCB assembly. It can be used at any stage of the process depending on the needs or requirements of the user. It is strongly advised to run AOI for the bulk manufacture of complex SMT-based PCBs.

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