In the early PCB production, testing was mainly completed by manual visual inspection combined with electrical inspection. With the development of electronic technology, the density of PCB wiring continues to increase, the difficulty of manual visual inspection increases, the rate of false positives increases, and the health damage to the inspector is greater. The development of electrical inspection programs is more cumbersome, the cost is higher, and certain types of defects cannot be detected. Therefore, AOI automatic optical detectors are increasingly applied in PCB manufacturing.
PCB defects can be roughly divided into short circuits, open circuits, and other defects that may lead to PCB scrapping. In the PCB production process, the production and copper coating of the substrate may produce some defects, but the main defects are generated after etching. AOI is generally tested after the etching process, mainly to discover the missing and excess parts on it. AOI can generally detect most defects with a small number of missed detections, but the main impact on its reliability is still the issue of false detections. Dust, contamination, and poor reflectivity of some materials during PCB processing can all cause false alarms. Therefore, manual verification is necessary after using AOI to detect defects.
Solder paste printing inspection
Solder paste printing is the initial stage of SMT and the root cause of most defects, with approximately 60% -70% of defects occurring during the printing stage. If defects are eliminated in the initial stage of the production line, it can minimize losses and reduce costs. Therefore, many SMT production lines are equipped with AOI testing for the printing process. There are many types of printing defects, which can be generally divided into insufficient solder paste on the solder pad and excessive solder paste; The middle part of the large solder pad is scraped with solder paste, and the edge part of the small solder pad is pointed with solder paste; Printing offset, bridging, and contamination are the causes of these defects, including poor rheological properties of solder paste, improper processing of template thickness and hole walls, unreasonable setting of printing machine parameters, low accuracy, improper selection of scraper material and accuracy, and poor PCB processing. AOI can effectively monitor the quality of solder paste printing and analyze the number and types of defects, thereby improving the printing process.
The component installation process requires high equipment accuracy, and common defects include missing or incorrect placement, deviation, and opposite polarity. AOI detection can detect the above defects, and also check the solder paste on the pads connecting the dense spacing and semiconductor components. After AOI detects the problem, an alarm will be issued and the operator will visually confirm the substrate. Reports of unexpected issues with missing parts can be corrected through repair tweezers. During this process, when visual operators repeatedly repair the same problem point, they will request the responsible personnel of each production equipment to reconfirm whether the machine settings are reasonable. Feedback on this information is very helpful for improving production quality, and can achieve a leap in production quality in a short period of time.
AOI can be simply divided into two types: preventing problems and discovering problems. Testing after printing and SMT is classified as preventing problems, while testing after reflow soldering is classified as developing problems. In the back-end testing of reflow soldering, the detection system can check for missing, offset, and skewed components, as well as all polarity defects. It is also necessary to detect the correctness of solder joints, as well as defects such as insufficient solder paste, welding short circuits, and warped feet, Reflow soldering backend detection is currently the most popular choice for AOI, where all assembly errors can be detected, providing a high level of safety. Figure 4 shows the detection image of a certain type of AOI for PCB after reflow soldering, using three different lighting modes, focusing on the collection of text images for solder joints, parts, and laser printing brushes.