Circuit Boards Assembly Defects That Can Be Detected by AOI

AOI (Auto Optical Inspection) is an optical identification system. It has now been 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 to be tested and the standard image. Whether the test object meets the standard. In the early days, it was mostly used to detect defects in 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 furnace, and it can judge the shortcomings of SMT parts and assembly more accurately than the human eye. But just like the human eye, AOI can basically only perform a 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 hidden under the part or the edge of the part The solder joints may be a bit weak. Of course, many AOIs can already perform multi-angle photography to increase their ability to detect IC pin tilt and increase the camera angle of some masked components.

Provides 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 that some grayscales or shadows are not very obvious, and it is more prone to misjudgment. These may be distinguished by different colors of light, but the most troublesome is those that 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, such as the ribs of the shielding frame or the components under the edges, often because 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 additional AXI and use X-Ray to check the soldering of the components on the line. The quality of points (such as BGA). As far as the current AOI knows, the defects of the parts that should be able to be fully checked are missing parts, skew, tombstone, wrong key, reverse polarity, foot warping, tin bridge, little tin, false welding, and cold welding.

Most of these shortcomings can be detected without errors during manual visual inspection. 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 immediate feedback on 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 line has been changed. So from the point of view of quality control, AOI does have its existence.