In the early days, AOI was mostly used to detect whether the surface printing of IC (integrated circuit) was packaged for defects. With the evolution of technology, it is now used to detect solder assembly of parts on circuit boards on SMT assembly lines (PCB Assembly factory), or check whether the solder paste meets the standard after printing.
The basic principle of AOI
The advantages of AOI
The biggest advantage of AOI is that it can replace the manual visual inspection before and after the SMT furnace, and it can check the defects of SMT production line and more accurately than human eyes. 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 out correctly, but it’s hard to detect hidden parts or the edge part.
Of course, many AOIs can take multi-angle photography to increase their ability to detect IC feet, and increase the photography angle of some covered components to provide more. But the effect is always not ideal, and it is difficult to achieve 100% test coverage.
The disadvantages of AOI
One of the biggest drawbacks of AOI is that it is often false reject where the grayscale or the shade is not very strong, which can be used as a reject, but its parts that are hidden by other parts and the solder joints underneath them that are the most troublesome. Because traditional AOI can only detect the places that direct light can reach, such as the rib of the screen or the components under its edge, often because AOI can not detect and miss.
Therefore, AOI is rarely used to ensure the assembly quality in PCBA production lines. In general, in-circuit test (ICT) and functional test (FVT) are also required. In some production lines, an automatic X-ray inspection (AXI) is added. X-Ray is used to check the quality of the solder joints (e.g. BGA) under the components along the line to ensure that the circuit board can achieve 100% test coverage.
What defects can AOI detect in assembling the PCB
Defects can not be fully detected
In addition, since optical inspection is subject to light, angle, resolution, therefore, the following defects can only be detected under certain conditions, but it is difficult to achieve a 100% detection rate.
AOI should also be able to check out wrong parts with different shapes or parts with different printing on the surface. However, if the appearance is not significantly different, and there is no surface printing, such as resistance and capacitance below 0402 size, these will be difficult to use AOI to detect.
This must also depend on the part itself has a symbol indicating the polarity of the part, or the appearance of the shape of the difference can be implemented.
Lead lift and lead defective
Severe foot warping can be determined by the difference in light reflection, but mild foot warping can be more difficult. Severe foot deformation can also be easily detected using AOI. Slight foot warping or closing depends on the situation, which usually depends on whether the parameter adjustment is strict or not, but also depends on the experience of the engineer or operator.
Generally, the tin bridge is easy to detect, but if it is hidden under the parts of the tin bridge can not be detected. Some tin Bridges, such as connectors, occur at the bottom of the component body and cannot be detected using AOI.
AOI can be used to judge the quantity of tin seriously, but there will always be some error in the quantity of tin paste printing, at this time, we need to collect a certain number of products to judge the quantity of tin
Fake welding and cold welding
This is the most annoying problem, because the light is usually difficult to check the appearance of fake, cold welding, even if you can use its appearance shape to judge, but the difference is really very small, the parameters are too strict and easy to misjudge. Problems like this always require a period of tuning to get the best parameters.
In a word, although AOI is useful, it does have some inherent limitations. However, it can be used in the real-time preliminary quality analysis of SMT, and feedback the quality status of SMT immediately, so as to improve the SMT process operation. It can indeed effectively improve the yield of SMT.
Generally, the ICT test machine is used to catch the problem and then respond to the SMT correction. There is usually a time lag of more than 24 hours. By then, the SMT condition has usually changed, or even the line has been changed. So from the perspective of quality control, AOI does have its necessity.
In addition, with the development of 3D technology and the improvement of MCU computing capacity, there are now many equipment manufacturers began to develop three-dimensional AOI technology, in addition to the purpose of differentiation, three-dimensional AOI image technology is actually more real than the original two-dimensional image, but also easier to compare the problem points.
he biggest advantage of AOI is that it can replace the manual visual inspection before and after the SMT furnace, and it can check the defects of SMT assembly and assembly more accurately than human eyes. 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 out correctly, but it’s hard to detect hidden parts or the edge part. Of course, many AOIs can take multi-angle photography to increase their ability to detect IC feet, and increase the photography angle of some covered components to provide more. But the effect is always not ideal, and it is difficult to achieve 100% test coverage.