AOI (Auto Optical Inspection) is an optical identification system. It is now widely used in the circuit board assembly line of the electronics industry to replace the previous manual visual inspection operations. It uses imaging technology to determine whether there is a difference between the object tested and the standard image, then judge if the test object meets the standard. In the early days, it was mostly used to detect defects in the surface printing after IC packaging. With the evolution of technology, it is now used to detect the quality of PCB assembly on the SMT assembly line, or check whether the solder paste meets the standard after printing.
The biggest advantage of AOI is that it can replace the previous manual visual inspection work before and after the SMT reflow furnace, and it can judge the shortcomings of SMT parts and assembly quality more accurately than the human eye. But just like the human eye, AOI can basically only perform surface inspection of the object, so as long as it is a shape that can be seen on the surface of the object, it can be checked correctly, but for the solder joints hidden under the part or on the edge of the part, may be a bit weak. Of course, there are many AOI that can do multi-angle photography to increase their ability to detect IC pin deform, and increase the camera angle for some hidden components. Although it can provide more detection rate, but the effect is always not ideal, and it is difficult to achieve 100% coverage.
In fact, the biggest shortcoming of AOI is the points that some grayscales or shadows behind big parts, it is easier to bring misjudgment. These may be distinguished by different colors of lights, but the most troublesome ones are components covered by other parts and solder joints located under the components, because traditional AOI can only detect the places that can be reached by direct light, for shielding frame ribs or components under the edges, AOI cannot detect them.
Therefore, the general circuit board assembly line rarely only uses AOI to ensure its assembly quality. It usually has to go through ICT and functional testing. Some production lines will add an extra AXI and use X-Ray to check the soldering quality of points (such as BGA) on the line.
As far as we know so far, the defects AOI can detect are: missing parts, shifting, tombstone, wrong parts, reverse polarity, leads warping, tin bridge, little tin, pseudo welding and cold welding. Most of these shortcomings can be checked by manual visual inspection if without mistakes.
In short, although AOI is easy to use, it does have some inherent limitations. However, it is used for real-time SMT preliminary quality analysis, and immediately feedback the quality of SMT, and improve it, which can indeed effectively increase the yield of SMT. Generally, it is already 24 hours after the problem is caught by the ICT test machine and then responded to the SMT. At that time, the SMT status has usually changed, and even the products on SMT line has been changed. Therefore, from the perspective of quality control, AOI does have its existence necessary.