Automatic Optical Inspection (AOI) is a high-end product that integrates multi-disciplinary technologies such as optics, computer graphics recognition and automatic control, the core of which lies in the function of its analysis software. Taking PCB inspection as an example, the system internally stores hundreds of board defects in the drawing characteristics. At work, the operator first fixes the PCB to be inspected on the machine, and then AOI utilizes a laser positioner to precisely locate the CCD lens and scan the entire board surface. By abstracting the pattern obtained and comparing it with the defective pattern, as a way to determine whether there is a problem with the line production of the PCB. Common problems such as line gaps, short circuits, broken circuits, and incomplete etching can be detected by AOI. In addition, AOI is able to indicate the type of problem and its exact location on the board. This type of equipment was earlier regarded as a symbol of strict quality control, because the introduction of AOI can effectively improve the yield rate, so now AOI equipment has become a necessary device for PCB manufacturing and electronic machine assembly companies.

Working Principle

The working principle of AOI is the same as that of the optical vision system used in mounters and solder paste printers, and there are generally two methods: design rule detection (DRC) and graphic recognition.

The DRC method is a method that utilizes given design rules to inspect circuit graphics. It can algorithmically ensure the correctness of the inspected circuits, standardize the judging criteria, and help the manufacturing process to control the quality. In addition, the DRC method is characterized by high-speed data processing and low programming effort, but it is poor at determining boundary conditions.

Another commonly used method is the graphic recognition method, which performs inspection by comparing the digitized design graphics with those of the actual product in accordance with an intact circuit prototype or the inspection program prepared during computer-aided design (CAD). Its inspection accuracy depends on the resolution of the optical system and the parameters set by the inspection program. Compared with the DRC method, the graphical recognition method uses design data instead of predetermined design principles, which has obvious advantages, but it has a larger amount of data acquisition and requires a higher level of real-time system reflectivity.

The AOI system uses visible light (laser) or invisible light (X-ray) as the inspection light source, optically collects part of the circuit board graphics that need to be inspected, and then processes, analyzes and judges the data through image processing software. It can not only check the quality of circuit boards and components from the appearance, but also check the quality of solder joints after the patch welding process.

Application of AOI in different process positions

In printed circuit board manufacturing plants, AOI can be used to compare with the design drawings and check whether there are defects in the basic printed wires. In addition, AOI can be arranged at different locations during the board assembly process to fulfill different functions:

1. placing the AOI system after the solder paste printer can be used to inspect the shape, area and thickness of the solder paste print.

2. placing the AOI system after a high-speed placement machine, it can be used to detect component placement defects, wrong type, shape damage, wrong polarity direction, including the relative position of the pins (solder ends) to the solder paste on the pads.

3. After placing the AOI system in the multifunction bonder, it can check the placement error of narrow-pitch and multi-pin ICs, and even recognize the correct variety of ICs from the markings printed on the surface of the components.

4. Place the AOI system after the reflow soldering to check the solder quality and find defective solder joints.

However, it is impractical to have AOI at each of the above stations. the most common location for AOI is after reflow.

Advantages and disadvantages of AOI systems

The AOI system allows normal products to pass through and, upon detecting a defect in the soldering of a circuit board assembly, records the type and characteristics of the defect and sends a signal to the operator or triggers an actuator to automatically remove the defective part and send it back to the rework system. In addition, the AOI system can also analyze the defects and statistics for the adjustment of process parameters of the manufacturing process to provide a basis for the AOI system is easy to use, easy to adjust. AOI systems sold in the market today usually include inspection items such as component missing inspection, component identification, SMD direction inspection, solder joint inspection, lead inspection, reverse connection inspection, and so on.

The shortcomings of the AOI system are that it can only carry out graphical visual inspection, and its detection effect depends on the resolution of the optical system, which is unable to detect invisible solder joints and components, and it is unable to quantitatively test from the electrical properties. In addition, AOI inspection equipment is expensive and costly. Therefore, electronic product manufacturers are more equipped with in-circuit testing (ICT) equipment.