Printed Circuit Board Automatic Optical Inspection Technology Strategy

After nearly 10 years of hard work, the Automatic Optical Inspection System (AOI) was finally successfully used in the PCB production line. During this period of time, the number of AOI suppliers has increased dramatically, and various AOI technologies have also developed rapidly. At present, from the simple camera system to the complicated 3-DX optical inspection system, many suppliers have almost provided AOI equipment that can be applied to all automatic production lines.

Application Strategy and Tips

In the electronics assembly industry, the miniaturization and densification of components has been a trend for a long time. This prompted manufacturers to install AOI equipment for their production lines. Because it has been impossible to rely on artificially reliable and consistent detection of well-distributed components, accurate inspection records are preserved. The AOI can be repeatedly and accurately detected, and the storage and distribution of test results can also be realized electronically.

AOI has become an effective process control tool. It can help manufacturers improve the pass rate of online test (ICT) or functional test, reduce the labor cost of visual inspection and ICT, avoid ICT becoming a production bottleneck, shorten the cycle of new product production capacity, and pass Statistical Process Control (SPC) improves yield.

The position of the detection device can achieve or hinder the inspection target, and different positions can generate corresponding different program control information. Generally speaking, the goal of implementing AOI is to improve the overall quality of the final product, so putting the equipment in front of the process may not be of great value. Putting the equipment to the front is because the cost of repairing defects early in the process is much lower than the cost of repairs before shipping. However, many of the defects occurred late in the production process, meaning that no matter how many defects were previously discovered, a complete visual inspection was required before shipment.

In general, as shown in Fig. 1, AOI can be effectively used after any one of four production steps in a production line. The following will introduce the application of AOI after four different production steps on the SMT PCB production line. The AOI can be roughly divided into two types: prevention problems and discovery problems. In the following description, the inspection after the solder paste printing, after the placement of the (chip) device, and after the component placement can be classified as a preventive problem, and the final step (detection after reflow soldering) can be attributed to the discovery. The problem is one of the categories, because detection at this step does not prevent the generation of defects.

After solder paste printing: The largely defective solder is derived from defective solder paste printing. At this stage, the soldering defects on the PCB can be removed easily and economically. Most 2-D inspection systems can monitor solder paste shifts and skew, insufficient solder paste areas, and sputter tin and short circuits. The 3-D system can also measure the amount of solder.

After placement of the chip components: This phase of the inspection can detect missing components, displacements, skews, and directional faults of (chip) devices. This inspection system also checks solder paste on pads used to connect fine pitch and ball grid array (BGA) components.

After component placement: After the device has mounted components on the PCB, the inspection system can check missing, offset, and skewed components on the PCB, as well as detect component polarity errors.

After reflow: At the end of the line, the inspection system can check for missing, offset, and skewed components, as well as all polarities. The system must also check the correctness of the solder joints as well as defects such as insufficient solder paste, shorts in soldering, and curling feet. If necessary, optical character recognition (OCR) and optical character verification (OCV) methods can also be added at steps 2, 3 and 4 for detection.

Discussions between the engineers and manufacturers on the advantages and disadvantages of different detection methods are always endless, in fact, the main criteria for selection should focus on the types of components and processes, fault spectrum and the requirements for product reliability. If you use many BGA, chip scale package (CSP) or flip chip components, you need to apply the inspection system to the first and second steps in order to maximize its effectiveness. In addition, testing after the fourth stage can effectively find the defects of low-grade consumer goods. For PCBs used in aerospace, medical, and safety products (automobile airbags), due to the stringent quality requirements, testing may be required in many places on the production line, especially in the second and fourth. After the step. For this type of PCB, X-rays can be used for inspection.

Avoid the limitations of AOI testing

The significance of introducing AOI equipment is to overcome the limitations of manual inspection. At present, the inspection of product production mainly depends on manual inspection. Due to the subjective nature of manual inspection, the inspection results are not very convincing. Moreover, for high-density and complex surface mount circuit boards, manual inspection is not reliable either. Economics, but for small components, such as 0603, 0402, etc., artificial visual inspection has actually lost its significance.

As AOI technology is a new technology, there are still some imperfections, so it cannot be blindly introduced. For companies that are considering the introduction of this new technology, it is recommended to first introduce a suitable AOI device for offline use based on careful investigation of various models of AOI equipment, which is mainly used for the sampling inspection of all SMT production line products in the factory. In addition, if a special problem arises in the production line, a special inspection of the particular position of the line (eg, after reflow soldering) can be performed according to the situation. This not only takes into account the utilization of the equipment, but also takes into consideration the monitoring of all SMT production lines in the plant. The requirements and save on production investment. If it is in good use, consider re-introducing several AOI devices for online use to change the current status of relying mainly on artificial visual inspection to increase the factory's automated production level.

Nothing can be perfect. AOI technology also has its shortcomings. For example, the AOI system cannot detect circuit errors; it cannot do anything to detect invisible solder joints (such as BGA); the reliability and accuracy of AOI testing equipment cannot be guaranteed. People fully trust; SPC applications are not mature enough. It should be pointed out that due to the above limitation, AOI cannot completely replace ICT, X-ray testing and other technologies. It should be used together with other technologies to achieve the best test results.

The advantages of AOI technology

As an economical and reliable complement to traditional testing methods, AOI is used as a process monitoring tool for measuring the performance of presses or component placement machines. Its advantages include:

1) Check and correct PCB defects. The cost incurred during process monitoring is much lower than the cost after final testing and inspection.
2) Recurring errors can be detected as early as possible, such as placement displacement or incorrect tray installation.
3) Provide SPC data for process technicians.
4) Can meet the requirements for further increase of PCB assembly density.
5) The test program is generated very quickly.
6) Can keep up with the beat of the SMT production line.
7) The reliability of detection is high.