How is the AOI automatic optical inspection system applied in the production of printed circuits

How is the AOI automatic optical inspection system applied in the production of printed circuits

After nearly 112 years of efforts, the automatic optical inspection system (AOI) has finally been successfully applied to the printed circuit board (PCB) production line. During this period, the number of AOI suppliers has increased sharply, and various AOI technologies have also made considerable progress. At present, from simple camera systems to complex 3-D X-ray inspection systems, many suppliers have almost been able to provide AOI equipment that can be applied to all automatic production lines.

Over the past decade, the performance of solder paste printers and SMT placement machines has been improved, which has enhanced the speed, accuracy and reliability of product assembly. The yield rate of large manufacturers has thus been improved. The increasing number of SMT-packaged components provided by component manufacturers has also driven the development of automation in printed circuit board assembly lines. The automatic placement of SMT components can almost completely eliminate the errors that may occur during manual assembly on the production line.

In the PCB manufacturing industry, the miniaturization and denaturization of components have always been the development trend. This has prompted manufacturers to install AOI equipment on their production lines. Because it is no longer possible to conduct reliable and consistent detection of densely distributed components and keep accurate detection records by relying on manual labor. AOI, on the other hand, can conduct repeated and precise inspections, and the storage and release of inspection results can also be digitized.

In many cases, the inspection and adjustment of the solder paste printer and assembly process by process engineers can ensure that the solder paste contamination rate (spatter rate) on the production line is only a few parts per million (ppm). For a high-output/low-mixing production line, the typical solder paste contamination rate is between 20 parts per million and 150 parts per million. Practical experience has shown that it is difficult to detect the contamination of each and every type of solder paste merely by sampling and testing printed circuit board samples. Only by conducting 100% inspection on all circuit boards can a greater inspection coverage be guaranteed, thereby achieving statistical process control (SPC).

To a large extent, only a very small portion of specific types of solder paste contamination actually exist, and the generation of these solder paste contaminants can be linked to certain specific production equipment. In many cases, you can also attribute the occurrence of a solder paste contamination to a specific device. However, for some variables, such as component offset (due to the self-correction effect during the reflow process), it is impossible to trace back to a specific production step. Therefore, in order to detect all solder paste contamination, it is necessary to conduct 100% inspection on each production step on the production line. However, in reality, due to economic considerations, PCB manufacturers cannot test each circuit board after each process is completed. Therefore, process engineers and quality control managers must carefully consider how to strike the best balance between investment in inspection and the benefits brought by increased production.

Generally speaking, as shown in Figure 1, you can effectively apply AOI after any one of the four production steps in a production line. The following paragraphs will respectively introduce the application of AOI after four different production steps on the SMT PCB production line. We can roughly divide AOI into two categories: problem prevention and problem detection. In the following description, the inspection after solder paste printing, (surface mount) device placement and component placement can be classified as problem prevention, while the last step - inspection after reflow soldering - can be classified as problem detection, because inspection at this step cannot prevent the occurrence of defects.

◆ After solder paste printing: To a large extent, defective soldering stems from defective solder paste printing. At this stage, you can easily and economically remove the soldering defects on the PCB. Most 2-D detection systems can monitor solder paste offset and skew, insufficient solder paste areas, as well as solder splashes and short circuits. The 3-D system can also measure the amount of solder.

After the placement of (chip) devices: The detection at this stage can detect missing components, displacement, skew of (chip) devices and directional faults of (chip) devices. This detection system can also check the solder paste on the pads used to connect close-pitch and ball grid array (BGA) components.
◆ After component mounting: After the equipment mounts components onto the PCB, the detection system can check for missing, offset and skewed components on the PCB, and also detect errors in component polarity.

After reflow soldering: At the end of the production line, the detection system can check for missing, offset and skew of components, as well as defects in all polarity aspects. The system must also detect the correctness of solder joints as well as defects such as insufficient solder paste, short circuits during soldering and lifted feet.

If necessary, you can also add the optical Character Recognition (OCR) and optical Character verification (OCV) methods for detection in steps 2, 3 and 4.

Engineers and manufacturers' discussions on the pros and cons of different detection methods are always endless. In fact, the main criteria for selection should focus on the type of components and processes, the fault spectrum, and the requirements for product reliability. If many BGA, chip-scale packaging (CSP), or flip-chip components are used, the detection system needs to be applied to the first and second steps to maximize its effectiveness. In addition, conducting inspections after the fourth stage can effectively identify defects in low-end consumer goods. For PCBS used in aerospace, medical and safety products (automotive airbags), due to the extremely strict quality requirements, it may be necessary to conduct inspections in many places on the production line, especially after the second and fourth steps. For this type of PCB, X-rays can be selected for inspection.

If the AOI used on the production line is to be evaluated, it is necessary to distinguish between systems that can only perform detection and those that can perform measurement.

Detection systems that can only look for defects such as missing components and incorrect placement cannot provide tools for process control, so they cannot be used to improve the production process of PCBS. Engineers still have to adjust the production process manually. However, these detection systems are both fast and inexpensive.

On the other hand, the measurement system can provide accurate data for each component, which is of great significance for measuring production process parameters. These systems are more expensive than detection systems, but when you integrate them with SPC software, the measurement system can provide the information necessary to improve the production process.

Overall, it is not comprehensive for people to evaluate the quality of a detection system merely based on its accuracy rate of error reporting, that is, the ratio of true errors (accurate error reporting) to false alarms (false error reporting). If a measurement system is to be evaluated, it is also necessary to rely on the assessment results of the accuracy of the measurement system within a smaller tolerance range. Statistical Process control
Finally, if you want to effectively use the data from the AOI system to help you control the production process, thereby enabling your company to achieve higher output and greater profits, you must master the following information:
Accurate measurement data
Reproducible and repeatable measurement
◆ Close to the measurement of events in time and space
As well as the real-time measurement process and all the information related to the production process
Installing an AOI system during the printing or mounting process can help you eliminate other process variables accumulated during the production process. Assuming that you measure whether the components have shifted positions after reflow soldering, the data you collect cannot reflect the accuracy of the mounting process. You should measure the results both after mounting and after reflow soldering. But this information is almost useless for controlling the device mounting. Given the trend of monitoring development, installing an AOI system near the process you must monitor can quickly correct a parameter that is about to enter the next step. At the same time, close-range detection can also reduce the number of non-compliant PCBS before the detection process.
Although most AOI users in the electronics industry still only focus on post-soldering inspection, the future trend of miniaturization of components and PCBS will require more effective closed-loop process control. AOI systems that can provide effective detection and measurement solutions will attract more and more users, and engineers will also consider the investment in such systems to be more worthwhile. For all customers, AOI will continue to play an important role in improving product production lines and increasing the yield of finished products.