What Is an Automated Optical Inspection?

Automated Optical Inspection ( AOI ) is a high-speed and high-precision optical image inspection system that uses machine vision as the detection standard technology. It can improve the shortcomings of traditional manual inspection using optical instruments. The application level includes: R & D and manufacturing of high-tech industries, as well as defense, people's livelihood, medical treatment, environmental protection, electricity ...

AOI (Automated Optical Inspection) is an effective inspection method for industrial automation. It uses machine vision as the detection standard technology. It is widely used in LCD / TFT, transistor, and PCB industrial processes, and can be extended to security systems for people's livelihood purposes. . Automatic optical inspection is a typical method commonly used in industrial processes. It uses optical methods to obtain the surface state of the finished product, and uses image processing to detect defects such as foreign objects or pattern abnormalities. Because it is a non-contact inspection, semi-finished products can be inspected in the middle process.

AOI is a new type of testing technology that has only recently emerged, but it is developing rapidly. Many manufacturers have launched AOI testing equipment. Take the SMT test as an example. When the test is performed automatically, the machine automatically scans the PCB through the camera and collects the image. The tested solder joints are compared with the qualified parameters in the database.

Using high-speed and high-precision visual processing technology to automatically detect various mounting errors and soldering defects on PCB boards. PCB boards can range from fine-pitch high-density boards to low-density large-size boards, and can provide online inspection solutions to improve production efficiency and welding quality.

By using AOI as a tool to reduce defects, find and eliminate errors early in the assembly process to achieve good process control. Detecting defects early will avoid sending bad boards to subsequent assembly stages, and AOI will reduce repair costs and avoid scrapping non-repairable circuit boards.

PCB inspection was the earliest method of manual visual inspection. With the requirements of high-density circuit wiring and high output, manual visual inspection cannot meet the requirements of reliability. The best solution for repeated, monotonous and strict inspection tasks is to use an automatic inspection system. And AOI can satisfy the comprehensive inspection of PCB on the production line. The AOI system can detect the following errors: missing components, incorrect polarity of tantalum capacitors, incorrectly positioned or skewed solder pins, bent or folded pins, excessive or insufficient solder, solder bridges, or false soldering. Not only can AOI check defects that cannot be detected by manual visual inspection, AOI can detect components and solder joints that cannot be touched by the needle bed in online testing, improving defect coverage. AOI can also collect the work quality of each process in the production process and the types of defects that occur, and feed it back for analysis and management by the process control personnel to reduce the PCB scrap rate.

In the detection of objects, the light source plays a decisive role. Common light sources are: incandescent, tungsten halogen, gas discharge, sodium, metal halide, xenon, pulse, LED, and laser.

In terms of cost-effectiveness, incandescent light source AOI is a better choice for general enterprises. If the parameters are edited properly, false defect false alarms will be greatly reduced, and the rate of missed detection is also very low. Selecting high-brightness LEDs as the light source has the following advantages: The light has good monochromaticity, which is convenient for improving the measurement accuracy, and the installation space is small. The number of LEDs can be easily increased and decreased according to the actual need for the intensity of the lighting. As for the cold light source, it is a new type of illumination light source developed in the near future. It uses optical fiber to transmit the light beam (referred to as optical cable) to transmit the light beam to the place of illumination. Its output of visible light has the advantages of basically no heat, high intensity, no shadow, no vibration, multi-level dimmable and so on. The laser light source is the light source used in the current AXI inspection system. The laser light source can detect the internal defects of the BGA package.

Compare the current choices of lenses by some famous AOI manufacturers. Omron, MVP and Agilent's AOI all use a single lens. Among them, the Omron lens camera uses the patented CHS (Color Highlight System) technology; MVP's company AOI uses high-sensitivity and performance lenses; Agilent SJ50 uses a single high-resolution lens with SSM technology; and Teradyne's AOI usually has five Camera, four tilted and one vertical, because its AOI scans 5 times at the same position, which greatly improves the test coverage; CyberOptics' AOI uses one or two groups of lenses, which can be as many as 9 to 18 Each.

AOI is applied after four different production steps on the SMT PCB production line. (1) After solder paste printing, mainly check the printing of solder paste; (2) After the chip components are placed, to check the correctness of the patch, errors can be detected earlier and the cost can be reduced; (3) Chip paste After being placed, the inspection system can check for missing, offset, skewed chips and chip polarity errors on the PCB; (4) After reflow soldering, at the end of the production line. The inspection system can detect missing, offset and skewed components and component polarity defects. Among the four inspection positions of AOI, the inspection after solder paste printing, (chip) device placement and component placement are aimed at preventing problems. Inspection at these locations can prevent defects from occurring; after reflow soldering, Detection is aimed at finding problems. The problem AOI was placed in front of the furnace, and the problem AOI was found after the furnace. At present, the AOI before the furnace is more important than the AOI after the furnace, but the inspection after the furnace, especially the inspection of bad solder joints, is the difficulty of the AOI inspection. Teradyne's Optima7000 system focuses on the inspection after the furnace. More AOIs have great flexibility. Agilent's SJ50 and SP50 (three-dimensional) solder paste inspection systems use a unified platform, which can interchange the lighting head. Both Omron and Teradyne's AOI can be moved to other test locations.

The development of AOI application software is the key to show its effect after users use it. Most AOI detection systems use PC and Windows operating systems, while MVP chose LINUX and UNIX operating systems to enhance the stability of their software systems.

In terms of software, in recent years, many circuit board image detection algorithms have been used. These algorithms can be roughly divided into three categories: reference comparison algorithms, no-reference verification methods, and hybrid algorithms. Reference comparison algorithms are divided into two categories, image comparison methods and model comparison methods. This kind of method has simple algorithm and easy implementation, but it is not easy to detect defects such as line width and line spacing violations. The non-reference verification method does not require any reference image. It determines whether the PCB image to be inspected is defective according to the pre-defined PCB design rules. If it does not meet the design rules, it is considered defective, so it is also called design rules. Check method. Although this method can get good results when measuring line width and line distance violations, its algorithm is complex and computationally intensive, and it is easy to miss large flaws such as missing lines and missing pads. The hybrid method is a combination of the reference comparison algorithm and the non-reference verification method, which overcomes the shortcomings of the first two methods to a certain extent, and thus exerts their respective advantages. For example, the template matching method is used in combination with the mathematical morphology method, or the linked list method is used in combination with the mathematical morphology method. However, the current method is not mature enough, its algorithm is complex, it cannot meet the requirements of real-time detection, and it is not adaptive enough, and the system has poor scalability. At present, the image processing of the AOI detection system basically uses a reference algorithm, and most imported brands use image matching and rule discriminating methods.

The software of the AOI detection system has a statistical analysis function and provides SPC (Statistic Process Control) data for process technicians. The AOI inspection system monitors the quality of the process flow, and combines it with the SPC process management technology to provide timely feedback information, so as to continuously optimize various parameters in the production process, make the SMT production process more complete, and the yield of PCB assembly has been significantly improved. With the expansion of modern manufacturing industry, the control of production is more and more important, and the demand for SPC data is also increasing.

Agilent's latest SJ50 Series II AOI equipment uses three-dimensional stereo shape model (SSM) technology, which uses multi-angle imaging to generate realistic three-dimensional images. Due to the use of SSM technology, the accuracy of defect alarms has been greatly improved; the resolution is always maintained at 20-25 microns / pixel with a single, high-resolution digital camera and high-speed precision bracket, taking into account speed And resolution performance.

One of the characteristics of Cyberoptics' AOI is that its analysis software uses only one software programming method, regardless of whether it is before or after welding, the same equipment and software can be used without the need for lens or sensor replacement; another feature is the mechanical design Very simple, because the lens is fixed, it can reduce the deviation or calibration caused by too many moving parts.

MVP's fifth-generation AOI platform, 1820 ULTRA, uses the latest digital camera technology. 1820 ULTRA inherits the unique single lens of MVP's previous AOI system, which continuously captures images and simple mechanical structure. MVP's AOI operating system uses Unix or Linux, which has high stability and reliability.

The main feature of Omron products is that it uses the patented technology of CHS (ColorHighlight System), which is the only AOI that uses color light source, color CCD camera and color image processing at the same time.The collected image contains three-dimensional information as detection judgment Standard, which makes it possible to inspect 0201 components and lead-free solder.

Orbotech's VT-9000E series of online optical inspection systems can be used in various inspection stages of electronic assembly. It has both 2D and 3D technologies. Orbotech also has integrated software tools for automated and intelligent process control supporting its AOI products, providing process engineers and managers with more information to enable their AOI products to prevent errors from occurring in SMT production lines.

Teradyne's Optima7300 system is characterized by a large detection area, high density, and fast speed. An Optima7300 can inspect 250 circuit boards per hour. Aiming at the problem of plate shaking during the test, Teradyne's AOI achieved better compensation or sampling compensation through laser calibration and software tracking.

The first domestic AOI inspection system brand Aleader's ALD-H-350 was successfully developed in May 2004 in Dongguan, Guangdong Province, and has been put on the market in 2005 after field use. This AOI system uses a single camera combined with a special ring RGB LED light source, applying color image comparison methods and statistical modeling techniques. It can accurately detect the following defects at high speed: (1) solder joint defects: more tin, less tin, and even tin Etc .; (2) solder paste printing defects: presence, skewness, less tin and more tin, circuit breakage, pollution, etc .; (3) parts defects: missing parts, offsets, skews, monuments, side standings, flips, poles, etc. Sexuality, wrong parts, damage, etc. It can detect a large detection range of 450 × 350mm, the minimum high-speed detection component size is 0402, and the test speed exceeds 60 components per second. Thanks to Chinese, fast learning; Graphical programming interface, simple programming. And can be guaranteed for life. Aleader's ALD-H-350 is the first automatic optical inspection equipment for electronic placement production line in China.

In addition, Beijing Xinghe Company has developed an AOI prototype, which is being tested and improved. Xiamen Fuxin Optoelectronic Integration Co., Ltd. also said that its company's AOI system development project has completed the development of control software, and the first prototype has been successfully produced. ^[1]

The AOI technology field is very broad. The broad AOI equipment is a combination of optical sensing systems, signal processing systems and analysis software. The application level can include space exploration, aviation, satellite telemetry, biomedicine, industrial production quality detection, fingerprint comparison, robot control ,multimedia technology.

The narrow sense of AOI equipment refers to the urgent need for industrial use. In the IC and general electronics industry, machine tools / automation machinery, electrical machinery / electronic industry, metal and steel industry, food processing / packaging industry, textile and leather industry, automotive industry, building materials, security / Monitoring system, etc. ^[2]