1. Basic principles of AOI technology
With the need for thinner PCB conductor patterns, miniaturization of SMT devices and high-density development of SMT components, automatic optical inspection (AOI) technology has developed rapidly and has been widely used in SMT inspection technology. The principle of AOI is the same as the principle of the vision system used by the placement machine and the printing machine, usually two methods of design rule checking (DRC) and pattern recognition are used. The DRC method checks the circuit pattern according to some given rules (such as all connections should be based on solder joints, the width of all pins should not be less than 0.127um, and the spacing between all pins should not be less than 0.102mm, etc.). This method can algorithmically ensure the correctness of the circuit to be tested, and has the characteristics of easy manufacturing, easy to achieve high-speed processing of algorithm logic, small amount of program editing, and small data occupation. For this reason, this method is used more frequently. However, this method has poor ability to determine the boundary. Commonly used pin inspection algorithms determine the boundary position based on the average value of the pin, and determine the gray level according to the design. The pattern recognition method compares the stored digitized image with the actual image. The inspection is carried out according to a good PCB or inspection file established based on the model, or according to the inspection program compiled in the computer-aided design. The accuracy depends on the resolution and the inspection procedure used, and is generally the same as the electronic test system, but the amount of collected data is large, and the real-time data processing requirements are high. However, because the pattern recognition method replaces the established design principles in DRC with actual design data, it has obvious advantages.

2. AOI technology detection function
AOI has functions such as component inspection, PCB light board inspection, and post-weld component inspection. The general procedure of the AOI monitoring system for component inspection is: automatically count the PCBs with installed components and start the inspection; check that the PCB has a pin side to ensure that the pin ends are arranged and bent properly; check whether there are any missing components on the front of the PCB. Wrong components, damaged components, improper component mounting direction, etc.; check the type, direction and position of the discrete components of the IC machine to be connected; check the quality of the markings on the IC components, etc. Once AOI finds defective components, the system sends a signal to the operator or triggers the actuator to automatically remove the defective components. The system analyzes the defects, provides the type and frequency of defects to the host computer, and makes necessary adjustments to the manufacturing process. The inspection efficiency and reliability of AOI depend on the completeness of the software used. AOI also has the advantages of convenient use, easy adjustment, and no need to program the vision system algorithm. Table 9-3 shows typical AOI equipment and its detectable content.

3. AOI system composition and principle
Take the Japanese PI-2000 testing equipment as an example to describe the AOI system structure, which is based on the AOI design rules and adds a comparative inspection function. Two camera lenses are used, and the inspection system is shown in Figure 9-1. The inspection subsystem uses a one-dimensional image sensor to capture the image of the printed line, and the resulting image signal is corrected, processed by high-speed A-D conversion and sent to the control subsystem. The control subsystem judges the defects and makes the inspection table move back and forth linearly for scanning, so that the one-dimensional image sensor can obtain the two-dimensional (planar) graphic output signal. The inspection result is real-time and synchronously with scanning. The ink marks the trapped areas of the PCB. The defective areas can also be enlarged and displayed on the monitor, which can be checked visually. The system is operated in the form of dialogue through the CRT display. The output subsystem consists of a digital image monitor, a physical image monitor, a printer and a synchronous oscilloscope. Display the digital color image and physical image of the defect location on the monitor respectively, and print the defect and a number at the same time. With a synchronous oscilloscope, you can observe graphics such as image signals and digital limiter levels, and at the same time make it easier to change settings and adjustments such as illumination and digital limiter levels. The highest inspection speed of PI-2000 machine is 1.24m/min, the resolution is high, the minimum pixel size is 10um, there are two gears of high resolution/low resolution, which can be switched. The size of the printer is 500mm*650mm*2 pieces. The minimum line width/line spacing is 160/80um. There is no restriction on the shape of the checked graphics.