In SMT, AOI technology has functions such as PCB light board inspection, solder paste printing inspection, component inspection, post-soldering component inspection, etc. The emphasis is also different when testing different links;

PCB inspection
In the early PCB production, inspection was mainly done by manual visual inspection and electrical inspection. With the development of electronic technology, PCB wiring density has been increasing, the difficulty of manual visual inspection has increased, the rate of misjudgment has increased, and the detection of Health damage is greater, electrical inspection programming is more cumbersome, more costly, and certain types of defects cannot be detected. Therefore, AOI automatic optical inspectors are increasingly used in PCB manufacturing.
PCB defects can be roughly divided into short circuits (including base copper short circuit, fine wire short circuit, electroplating open circuit, dust short circuit, pit short circuit, repetitive short circuit, stain short circuit, dry film short circuit, insufficient etching short circuit, excessive plating short circuit, scratch Short circuit, wrinkle short circuit, etc.), open circuit (including repetitive open circuit, scratch open circuit, vacuum open circuit, gap open circuit, etc.) and other defects that may lead to PCB scrap (including over-etching, electroplating burnt, pinhole), in PCB production In the process, some defects may occur in the production of the substrate and the copper coating, but the main defects are generated after the etching. AOI is generally inspected after the etching process, mainly to find the missing and redundant parts on it.
In PCB inspection, image comparison algorithms are widely used, and 2D inspection is the main one, which mainly includes data processing (preliminary processing of input data, filtering small pinholes and residual copper, and holes that do not need to be inspected, etc.) , Measurement category (feature extraction of the input data, recorded feature code, size and position and comparison with standard data) and topology category (used to detect added or missing features), Figure 1 is a schematic diagram of the feature extraction method, ( a) is the binary diagram of the standard version and the tested board, (b) is the characteristic diagram after mathematical morphological analysis.
AOI can generally find most of the defects, and there are a small number of missed detection problems, but the main problem that affects its reliability is the false detection problem. Dust, contamination and poor reflectivity of some materials in the PCB processing process may cause false alarms. Therefore, after the defects are detected by AOI, manual verification must be carried out.

Solder paste printing inspection
Solder paste printing is the initial link of SMT and the source of most defects. About 60%-70% of defects appear in the printing stage. If defects are eliminated in the initial link of the production line, losses and costs can be minimized. Therefore, , Many SMT production lines are equipped with AOI testing for the printing process.
There are many types of printing defects, which can be roughly divided into insufficient solder paste on the pad, too much solder paste; solder paste scratching in the middle of the large pad, solder paste sharpening at the edge of the small pad; printing offset, bridging, and Contamination, etc., the reasons for these defects include poor rheology of solder paste, improper processing of template thickness and hole wall, unreasonable setting of printer parameters, low precision, improper selection of squeegee material and accuracy, and poor PCB processing, etc., through AOI It can effectively monitor the quality of solder paste printing and analyze the number and types of defects to improve the printing process.
Figure 2 is a schematic diagram of a solder paste inspection system. The main components of the system are a camera and an optical fiber xy workbench system. The camera is installed on the xy desktop, and the ring-shaped optical fiber moves in the xy direction to collect the overall image of the PCB. Detection, the use of ring-shaped optical fiber and ring-shaped reflector to irradiate the oblique light onto the solder paste, the camera takes a picture from the square of the ring-shaped optical fiber, and measures the edge of the solder paste to calculate the height of the solder paste. A detection method that transforms the shape into a change in light to determine. In the case of normal printing, there will be some bulges in the edge part, which can strongly reflect the light projected from the inclined surface. This detection method uses the width of the light reflected from the edge of the solder paste to determine the solder paste bridging and solder paste ringing, and the PCB surface irradiated by the inclined surface will show a dim image.
With 3D inspection, the shape and thickness of the solder paste can be evaluated to check whether the amount of solder paste is reasonable, whether there are scratches and sharpenings. These defects appear more frequently when using screens and squeegees.
Stainless steel mesh and metal squeegee are used all the time. The thickness of the solder paste is relatively stable, generally not too much, and the scratching phenomenon is also very slight. The focus is to pay attention to the lack of printing (too little solder paste), offset, contamination and bridging And other defects. Using 2D inspection can effectively find these defects, image comparison method and design rule inspection method can be used, the inspection time is short, the equipment price is lower than 3D inspection, and if there is AOI automatic in the subsequent processes such as placement and reflow For optical detectors, 2D inspection can also be used in the printing process considering the cost.

Backflow detection

AOI can be simply divided into two types: preventive problems and discover problems. After printing and patching, the inspection is classified and the prevention of problems. The inspection after reflow is classified as a development problem. In the back-end inspection of reflow, the inspection system can be Check the missing, offset and skew of components, as well as all polarity defects. It is also necessary to check the correctness of solder joints, insufficient solder paste, solder short circuits, and lifted feet. The back-end inspection of reflow soldering is current The most popular choice of AOI. All assembly errors can be found at this location, providing a high degree of safety. Figure 4 shows the inspection image of a certain type of AOI on the PCB after reflow soldering. It uses 3 different lighting modes, focusing on the solder joints. , Collection of parts and laser printing text images. Figure 5 shows different types of defects identified by AOI after reflow soldering.

Placement inspection
The component placement link requires high equipment accuracy, and the common defects include missed placement, wrong placement, offset skew, and reverse polarity. AOI inspection can monitor the above-mentioned defects, and at the same time, it can also check the solder paste on the bonding pads of the close pitch and BGA components. Figure 3 shows the images collected by a certain type of AOI for PCB inspection after patching.
Since the reflow soldering process is immediately followed by the placement process, the inspection after the placement is sometimes referred to as the reflow soldering front end inspection. From the point of view of quality assurance, the reflow soldering front end inspection cannot be detected due to problems that occur in the reflow soldering furnace. It seems meaningless. In the reflow oven, the solder has a self-correcting displacement after melting, so the mounting displacement and solder printing status cannot be detected on the substrate after soldering, but in fact, the front-end inspection of the reflow soldering is the focus of quality assurance. Information that cannot be detected after reflow soldering, such as the component mounting status of each part before soldering, can be seen at a glance. At this time, there are no irregularities on the substrate, which is most suitable for image processing, and the pass rate is very high, and the misjudgment caused by excessively harsh detection is also greatly reduced.
After AOI detects a problem, an alarm will be issued, and the operator will visually confirm the board. The accidental problem report of missing parts can be corrected by repairing tweezers. In this process, when the operator visually inspects the same problem point repeatedly, he will ask the person in charge of each production equipment to reconfirm whether the machine is set Reasonable, the feedback of this information is very helpful to the improvement of production quality, and can achieve a leap in production quality in a short time.