The AOI system includes multi-source lighting, high-speed digital cameras, high-speed linear motors, precision mechanical transmission structures, and graphics processing software. During the inspection, the AOI equipment automatically scans the PCB through the camera, and captures the components on the PCB (including the printed solder paste, the state of the chip components, the solder joint shape and defects, etc.), through the comprehensive comparison between software processing and qualified parameters in the database, judge whether the components and their characteristics are qualified, and then draw the inspection conclusion, such as the components are missing, bridge or solder joint quality and other problems.
The principle of AOI is the same as that of the vision system used by the SMT machine and the printing press. Usually, Design Rule Checking (DRC) and graphic recognition are used. The DRC method checks the circuit pattern according to some given rules (such as all connected wires should be terminated with solder joints, all lead widths are not less than 0.127 mm, and the spacing between all leads is not less than 0.102 mm, etc.). This method can guarantee the correctness of the circuit to be tested from the algorithm, and has the characteristics of easy manufacture, easy algorithm logic, fast processing speed, small program editing amount, small data occupation space, etc., and this test method is adopted often. However, this method has a poor ability to determine boundaries.