AOI, also known as Automated Optical Inspection or Automatic Optical Inspection, is a key technique used for quality control in terms of both PCB fabrication and PCB Assembly. It can inspect the electronic assemblies fast and accurately, special to ensure the PCBs’ quality.
Inspection is the most important step of the entire manufacturing process of PCB. It is the last line of defense before the product is shipped to the customer. So today, let’s introduce an important inspection system -AOI.
Why AOI Test is Necessary?
Currently, all the electronic devices cannot work without a circuit board. The modern circuits are far more complicated than the printed circuit board a few years ago. Furthermore, the introduction of Surface Mount Technology (SMT) leads circuit boards to develop towards miniaturization and high density. Nowadays, even an ordinary printed circuit board contains lots of solder joints and layers, and these are where the majority of problems are found.
For PCBs and PCBAs, manual inspection did have sense for a period of time, but with the increase in the complexity of boards means that manual inspection is not a viable option these days. Because the inspectors will feel tried soon, poor and incorrect construction will be easily missed. Now the market require high volume, high quality PCB products to be brought to market very quickly. So the reliable and fast methods are needed to ensure the products’ high quality. That’s why AOI becomes so significantly important for the entire process. In addition, AOI test can reduce the cost and detect faults early in the production line.
AOI, Automatic Optical Inspection Test Property and Function
AOI use visual methods to monitor printed circuit boards for defects, such as scratches, nodules, stains, opens, shorts, insufficient or excessive solder, incorrect components, missing components, incorrect polarity of components etc. They are able to perform all the visual checks performed previously by manual operators, and far more swiftly and accurately.
AOI test is suitable for both PCBs and PCBAs. For PCB, it can check some defects such as shorts, opens and insufficient solder. For PCBA, it can inspect component soldering, polarity and values.
Flexibility is another important factor that can reduce the cost, because it is an in-process test and as soon as issues are discovered, manufacturing or assembly parameters can be instantly modified.
Compared with Automated/Automatic X-ray Inspection (AXI), post-reflow AOI is less expensive when it comes to solder joint defects inspection including bridges, broken joints, dry joints etc.
Automated Optical Inspection has good accuracy and reliability.
Basic Work Principle of AOI Test
AOI test achieves its inspection process by scanning printed circuit board surface. AOI test achieves its inspection process by scanning board surface. Depending on one or more high-definition cameras, this equipment can capture images of PCB surface with the help of numerous light sources including fluorescent lighting, LED lighting or infra-red or ultra-violet lighting. Then, comparison will be made between the captured image and board parameters that have been input into computer in advance so that differences, abnormalities or even errors can be clearly indicated by its built-in processing software. The whole process can be monitored at any second. (Sourcing from pcbcart https://www.pcbcart.com/assembly-capability/aoi.html )
AOI uses a number of techniques to provide the analysis of whether a board is satisfactory or has any defects:
* Template matching: Using this form of process the AOI, automated optical inspection system compares the image obtained with the image from a “golden board”. Normalized cross correlation (NCC) will be calculated between the template and inspection image. The larger the value of NCC is, the higher matching between them will be.
* Pattern matching: Using this techniques the AOI system stores information of both good and bad PCB assemblies, matching the obtained image to these.
* Statistical pattern matching: This approach is very similar tot hat above, except that it uses a statistically based method of addressing problems. By storing the results of several boards and several types of failure, it is able to accommodate minor acceptable deviations without flagging errors.
Why Use Automated Optical Inspection
* Quick Inspection Speed
AOI machines are quick. After initial programming and tuning, an entire panel of electronic circuit boards that may have taken an hour to inspect visually could potentially only take a minute to run through the machine.
* Inspection Accuracy
Even the inspection speed is quick, the cameras in AOI machines are extremely precise. With an experienced machine operator supporting the machine, the programs can be tuned to catch any little defect that might rear its head.
* Quick Catches
These errors being caught could even be stopped as they start. As the machine finds errors, the AOI operator can alert the SMT line to any problems that arise, and can be fixed before the entire process finishing. This can improve the first-pass-yield from SMT, and less time spent on rework for that process.
* Inspection Consistency
With an AOI machine, the images taken are in the exact same place with every single panel that goes through. The camera won’t miss inspecting a part on accident, and it could catch some things that an inspector may miss.
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