AOI (Auto Optical Inspection) is an automatic optical identification system, which has been widely used in the appearance inspection of circuit board assembly lines in the electronics industry and replaced the previous manual visual inspection. The basic principle of AOI is to use imaging technology to compare whether the object to be tested and the standard image are too different to judge whether the object to be tested meets the standard, so the quality of AOI basically depends on its image resolution and imaging ability. and image analysis technology. In the early days, AOI was mostly used to detect.
In the early days, AOI was mostly used to detect whether the surface printing of IC (integrated circuit) packaging was defective. With the evolution of technology, it is now used to detect components on circuit boards on SMT assembly lines. Solder assembly ( PCB Assembly), 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 before and after the SMT furnace, and it can judge the SMT patch defects more accurately than the human eye. Just like the human eye, AOI can basically only perform surface inspection of objects, so as long as it is the shape that can be seen on the surface of the object, it can be checked out correctly, but for objects hidden under the part or on the edge of the part. Soldering joints is still quite difficult. Of course, there are many AOIs that can achieve multi-angle photography to increase their ability to detect IC foot warping, and increase the photography angle of some shielded components to provide more. The detection rate is not always satisfactory, and it is difficult to achieve 100% test coverage.
In fact, the biggest disadvantage of AOI is that some gray levels or shadows are not very obvious, which is more prone to false rejection. These may be judged by using different colors of light, but the most troublesome. It is also those components that are covered by other parts and the solder joints under the components, because traditional AOI can only detect the places where the direct light can reach, such as the ribs of the shielding frame or the components under its edges, often because of AOI is not detected and missed.
Therefore, in general circuit board assembly production lines, in addition to using AOI to ensure its assembly quality, it is usually necessary to add ICT (In-Circuit Test) and functional test (FVT) testing, and some production lines will add an additional AXI (Automatic X- ray Inspection), using X-Ray to check the quality of the solder joints (such as BGA) under the components to ensure that the circuit board can reach 100% test coverage.
Can AOI detect those shortcomings of assembled circuit boards?
Based on the knowledge of Xinling Industry, the current AOI should be able to completely check out the following defects:
Missing
Skew
Tombstone
In addition, since optical inspection is subject to factors such as light, angle, resolution, etc., the following shortcomings can only be detected under certain conditions, but it is difficult to achieve a 100% detection rate, that is to say :
Wrong component ：
If it is a wrong part with a different shape, or a part with a different printing on the surface, AOI should also be able to check it out. However, if the appearance is not significantly different, and there is no surface printing, such as resistors and capacitors below the size of 0402, these are difficult to detect using AOI.
Wrong polarity:
This must also be performed depending on whether the part itself has a symbol indicating the polarity of the part, or the difference in appearance and shape.
Lead lift , lead defective：
Severe warped feet can be judged by the difference in light and shade reflected by light, but slight warped feet may be difficult. Severe foot deformation can also be detected by AOI, and slight foot warping needs to be determined according to the situation, which usually depends on the strictness of parameter adjustment, but also depends on the experience value of the engineer or operator.
Solder bridge ：
Generally speaking, tin bridges are easy to check, but if it is a tin bridge hidden under the part, there is no way. For example, the solder bridge of some connectors occurs at the bottom of the component body. At this time, there is no way to detect it using AOI.
Insufficient solder ：
When the amount of tin is seriously insufficient, of course, AOI can be used to easily judge, but there will always be some errors in the amount of solder paste printed. At this time, it is necessary to collect a certain number of products for judgment and research.
Fake welding:
This is the most annoying problem, because the appearance is usually difficult to check the welding condition. Even if it can be judged by its appearance and shape, the difference is really very small. If the parameters are adjusted too strictly, it is easy to misjudge. Problems like this always require a period of tuning to get the best parameters.
In short, although AOI is easy to use, it does have some limitations, but it can be used for real-time SMT preliminary quality analysis, and immediately feedback the quality of SMT, so that SMT process operations can be improved, which can indeed effectively improve the yield of SMT.
Generally, the ICT test machine is used to catch the problem and then feed it back to the SMT for correction. There is usually a time difference of more than 24 hours. At that time, the SMT situation has usually changed, and even the line has been changed. Therefore, from the perspective of quality control, AOI does have its existence.
In addition, with the development of 3D technology and the improvement of MCU computing power, many equipment manufacturers have begun to develop stereoscopic AOI technology. At present, the technology is relatively complete mainly for imported brands. The imaging technology of stereoscopic AOI is indeed better than the original two-dimensional AOI technology. The image is more realistic, and it is easier to compare the problem point.
