AOI technology is used in many different manufacturing fields. Since the development of electronic imaging technology, it has been used by various people in different application fields. The most familiar digital cameras and digital cameras are one of the most commonly used equipment in everyone’s life, and these technologies are also widely used in the production of industrial products. The difference is that in addition to image generation, the AOI system also has more comparisons, Judgment and other extra functions.
The electronics industry is accelerating towards the development of smaller, denser and faster advanced structures. The early reliance on manual inspection of products has been unable to meet the needs. It has become an inevitable trend for a large number of manufacturers to use AOI technology to assist in production work. For example, electronic assembly uses solder paste printing, missing parts inspection, contact status judgment, etc., which are familiar to everyone. Many printing and flat image products also use this technology to check the production process, cost, and semi-finished products. We try to sort out the application status of these technologies and hope to provide you with reference.
2. Typical AOI inspection technology used in circuit board manufacturing
Typical AOI inspection technologies currently used in circuit board manufacturing include: film inspection, drill inspection, hole location inspection, circuit inspection, blind hole inspection, bump inspection, visual inspection, etc. Different inspection applications have different technical connotations, and the AOI auxiliary equipment used is also different.
At present, the most used AOI inspection technology in the circuit board manufacturing industry is the inspection of the appearance of the circuit board. The application of this type of technology has been developed in the industry for more than two decades. Although the size of the circuit characteristics of the circuit board has been reduced by more than ten times, the optical system and the matching hardware and software systems have also made great progress. Most circuit board products can still rely on the existing AOI technology for circuit inspection, but some high-end structure board products are faced with the difficulty of inspection.
The inspection method used in the printed circuit board by AOI , which has been briefly described in the foregoing content. As for other AOI applications, it will be introduced in the subsequent content according to the order of its manufacturing process.
3. AOI film inspection
In the production of circuit boards, film has always been a very important image transfer tool. If the film has flaws, the Repetitive shortcomings cannot be tolerated by the manufacturer, so how to prevent the various shortcomings of the film has become an important issue for production plants.
The optical automatic film inspection machine can use optical sensing and image data comparison to screen out defects and sizes in the film to ensure the quality stability of the manufacturing process. More rigorous factories will also conduct intermediate inspections after each use of film and the number of exposures in a fixed period to prevent quality variations.
In the face of the ever-increasing circuit board circuit density and the unprecedented rate of decline in unit prices, how to improve the yield of products has become another competitive focus of the industry. Many circuit board manufacturers have made a lot of changes in the image transfer process due to the pressure of quality.
For example, because the circuit is gradually popularized, it is no longer to draw negatives through the design center and then send the film, but to use the mode of direct production in the production process. In addition, some manufacturers add a film inspection machine in the manufacturing process. After a fixed number of exposures, the operator is required to remove the film for defect inspection. Certain manufacturers will also install the AOI line inspection system into the manufacturing process, and perform the inspection of the first product when each new set of film starts to produce a circuit board. These changes have improved the yield of circuit board circuit production.
At present, the relevant testing equipment is designed with different inspection modes. Regardless of the inspection speed, missed detection rate, price, etc., there are considerable differences. The industry should make a careful comparison before adopting.
4. AOI’s drill inspection
The production of through-holes in circuit boards and the quality of drill pins are one of the key factors. New drills do not require 100% inspection. The validated and valid drill’s inspection method is to adopt an acceptable quality level (AQL) for drill inspection (for example: according to the MIL-STP-105 standard).
This type of sampling plan can help users determine the percentage of inspection used to assess the quality level of the entire batch of drills. If a certain amount of bad drills are found using this method, all the drills should be returned to the supplier. If the sampled drills pass the inspection and pass the specifications, the entire batch of drills should be acceptable.
The inspection criteria should include the geometric defects of sharp points, damage (debris), diameter (drill and shank), length of chip drain, as well as the correctness of the installation of the collar and the indication of the size (must be the actual diameter and packaging The label on the box matches), etc. In the general manual inspection and processing method, a magnifying glass of 20 to 40 times is used for all inspections in the drill pin manufacturing plant, and the judgment of each allowable value is measured by the tool magnifying glass. In order to confirm the feed of the drill and the quality condition after grinding, it is necessary to use the drill inspection machine to check the end surface of the drill. At present, most manufacturers still control the quality of these drills through manual sampling. However, due to the development of optical systems, there are already semi-automatic inspection management systems available and gradually popularized.
With the advancement of electronic imaging technology in recent years, it is quite easy to convert the image to the screen for semi-automatic or automatic inspection. At the same time, the data can be stored and analyzed and managed in real time,which has a certain help for the stabilization of drill bit quality. The drilling hole diameter of the board is gradually reduced, the impact of the drill bit quality on the mechanical drilling process is increasing, and the impact of the handling, holding, and operation of the drill pin is also more obvious. In order to improve the overall quality control capabilities, the industry has developed a system of automatic upper and lower rings, drill arranging, and automatic inspection, which minimizes the steps required for manual operations, and the image processing and inspection system is also designed to be more simple and convenient.
5. AOI hole inspection
The quality of general mechanical drilling, the main quality inspection items are mainly three parts: the size of the hole, whether it is leaking, and the condition of the hole wall. In the past, the traditional inspection method used manual comparison between the hole needle and the working film to confirm the overall drilling quality.
Thanks to the advancement of electronic and optical technology, there are now so-called “hole readers” that can assist in the inspection of drilling quality. As long as the digital drilling data is read into the machine, the machine will compare the data with the actual reading light spot through optical interpretation to find the shortcomings. Because the speed is fast and the error rate is relatively low, many manufacturers currently use this type of technology to replace the traditional inspection method.
6. Inspection of AOI’s small holes and blind holes
Since 1995, various micro-hole technology products have entered the circuit board field. The introduction of these technologies has changed the design of the circuit board. It has begun to emphasize the use of small hole technology to replace the traditional small through-hole design, especially in high-density applications. Aspects such as: mobile phones, notebook computers, adapter card modules, IC structure loading boards, etc. High aspect ratio> 8: 1 and through holes with diameter less than 0.3mm have also become more, especially in the server board, back board, workstation board. The introduction of these technologies has led to a reduction in the production yield of circuit boards. Many research reports indicate that the defect rate of HDI products is clearly higher, and the through-hole needs a larger hole ring to avoid the problem of hole ring breakage. The situation is more serious in terms of blind holes. The yield and alignment tolerances of multi-layer stacked blind hole structures are more difficult to handle. There is no regional difference in this phenomenon.
This development requires some new process control projects:
l Optimizing drilling parameters
l Confirm the impact of cleanliness
l Overall performance of nearby equipment
l Confirmation of measurement equipment capabilities
l Confirm the defect status before proceeding to the next step
These control tasks can help reduce costs and increase yield, and can also improve drilling process results.
6.1. Different micropore making procedures
In the past, optical hole forming and plasma hole forming were relatively used for manufacturing high-hole-count circuit boards. However, due to the advancement of laser hole-forming technology, most of the current high-density circuit board products have been produced using this technology. Although there are still a small number of microholes that are made with UV lasers, the CO2 drilling process is still the mainstream in the industry so far.
Requirements for drilling quality inspection
No matter which kind of micropore technology produces micropores, the typical defect detection items during inspection are:
l Leaky drilling
l Hole plug
l Semi-closed hole
l Excessive hole processing / too small hole diameter
l Poor hole alignment
l Aperture size check
l There are protrusions on the edge of the hole
l resin dents and scars
The more common detection capability of micropores is over 2mil in diameter, and the detection of the plate-breaking degree is more commonly 12um.
6.2 Defects of small and micro holes
Small and micro-hole defects may be caused by environmental changes, changes in temperature and humidity, or improper working methods for gadolinium drilling, cleaning, and electroplating. Perhaps the operation equipment is set incorrectly, the old program is installed, the wrong drill is used, the wrong drill is inserted, the circuit board is not placed correctly, the circuit board is operated incorrectly, or the residue is not removed before the operation, etc. Pneumatic bearing damage, laser galvanometer (Galvanometer) or work surface needs to be corrected, the pressure foot is abnormal, the operating parameters are improper, the dust collection system is blocked, etc. It is also possible that the material is out of tolerance, such as: drill damage, excessive resin thickness, dirty spots on the bottom surface, deformation of the substrate, etc. These problems can be categorized into five major groups, and problems can be divided into three categories: abnormal hole formation, hole position deviation, and abnormal hole diameter or shape. These defects can be detected for product X-Y position deviation, roundness, size or quality defects.
Micro-hole defects may also be caused by the misalignment of the negative film of the copper window, or due to the deviation of the mechanical drilling and the outer image. To effectively check the increasing micropore structure on the mounting plate, it is necessary to have a system that can selectively define various pore size specifications. When micropores have slight imperfections, such as: suboptimal aperture circles, they should be identified separately, and other key shortcomings may need to be dealt with in a similar manner.
6.3. Techniques for checking blind holes
The circuit board industry has developed some advanced inspection and measurement technology to check the existing problems. The circuit image part adopts basic acceptance rules in parallel and uses computers and multiple sets of sensors to inspect. The circuit board is sent to the room controlled by temperature and humidity in batches, and then sent to the AOI system for inspection by manual or automatic equipment, and then verified and repaired by a relatively low-cost verification workstation. If facing high-precision products, use precision coordinate measuring equipment to inspect the first and small tolerance products.
The two main technologies have been fully established in most circuit board manufacturers, but these systems are quite inefficient in inspecting micropore structures, and the relative cost, output, and inspection capabilities are also poor, because they are not Designed for such inspection purposes. Afterwards, equipment manufacturers have developed some built-in sensor solutions for mechanical and laser drilling, hoping to help control the drilling process and monitor the process changes or the characteristics of the samples. These tools provide timely information on the drilling process, but the disadvantage is that additional special accessories are required and must be installed on each drill shaft, which not only increases costs but may also reduce production efficiency. These solutions seem to be useful, but they are still insufficient to control the quality of HDI products.
Therefore, other visual devices have been developed and used in the industry of semiconductor construction board and electronic assembly. The tolerance requirements of these industries are relatively tight, and the control method is to adopt an online monitoring mode, and the scope of application is mainly based on key control points. The process will use statistical process control tools to maintain the process status, so that priority-based process improvement can be adopted, and the process capability of the overall equipment can also be verified. For these developments, the main applications are still mainly in the field of HDI products.
The method of inspection and measurement is mainly based on the actual situation. The measurement results will produce variated data, which can be used as the basis for the preventive treatment of process quality. AOI’s imaging tools generally use regular definitions to classify defects and do not provide any measurement data. Measurement systems require different design or procurement strategies, including motors to consider, coding systems, better light sources and optical mechanisms, and calibration tools that can be tracked. Most of these devices are intended for use in the general location of the factory, and are not necessarily installed in clean rooms. It is hoped that the inspection can be carried out in the position closest to the defect, so the operation must be carried out in the environment of the manufacturing process. Such as: drilling area or plating area. They are all fully automated operations rather than supporting batch operations manually. Operators generally only need to appear when setting up, running out of adjustments, and processing new products that can be repaired.
A colorful LED light source enters from different angles, uses photoelectric elements and measuring lenses to generate images, and uses a high-resolution monochrome digital camera to capture images. The alignment point and drilling program can be used to verify the position and diameter of the hole, and the mechanical vision technology can be used to measure the position accuracy, diameter, and roundness of each hole.
The intelligent visual computing system can find defects and classify them, and the characteristics of the defects can also be measured. Because different micropore technologies may produce different characteristics, the rules for defect classification should be determined separately according to the process method, and of course the location of the inspection in the process should also be taken into consideration.
So far, the local shortcomings of circuit boards are that they cannot be checked out in every manufacturing process. For example, for the CO2 laser drilling process, it is difficult to determine whether the burnt debris in the hole and the local shielding condition are good or bad. If the cleaning step is performed, this inspection will be more stable and reliable. It is also very difficult to determine the ratio of high aspect ratio micropores to greater than 1: 1, because neither the light source nor the optical system is easily accessible to the detection point, and in fact its light reflection ability is also quite limited.
6.4 Types of materials
Although the micro-defects of various process types are quite similar, the materials and images to be inspected are different. Regular inspection procedures are required to test rather than rely on basic rules. The use of specially strengthened laser detection technology can improve the detection capability of the shortcomings of the microholes, so that the microhole production process has better performance.
When combining reflection and fluorescence to obtain images, it is possible to compare the resin and copper metal to detect critical shortcomings, and the reflection mode can be used to analyze the thickness of the resin at the same time. It is necessary to distinguish the shiny copper metal inside to detect the key shortcomings, and the reflection mode can be used to analyze the thickness of the resin at the same time in the same scanning process. It is indeed a challenge to distinguish between the shiny copper liner inside and the copper skin surrounding the periphery. When special software is used for calculation and judgment, it is possible to use the visual system to penetrate the laser hole for inspection. The use of both reflection and fluorescence images for inspection is necessary to identify the inner liner.
7. AOI bump inspection
In recent years, various SMT parts have begun to quote a lot of the structure of array contacts. The most important structural element is the solder bump for soldering. The height of the bump will affect the ability of the part to withstand pressure, and the volume of the solder will also affect the strength of the connection and assembly. For the carrier board and electronic structure manufacturers who need to make the bump, how to confirm that the bump really exists and it is necessary The volume and correct coplanarity become an important inspection item in the appearance quality of the product. When the metal undergoes liquid reflow treatment, its appearance will be affected by the naturally occurring surface tension and become spherical. Some electronic structures use spherical bumps on the wafer ends, and some wafers use flat metal processing. Due to the difference in wafer contact, the opposite structure loading plate will be processed to flatten the bumps or spherical bumps. Regardless of which treatment is used, the coplanarity and volume of the bumps are important characteristics. Different inspection equipment will use their own methods to check the appearance characteristics of the bumps.
The bumps after the soldering process will not show a perfect circle, and it will be slightly irregular due to the influence of the edge of the edge paint. At this time, the image obtained by the image system can be simulated to simulate a virtual circle, and then the area occupied by the bump can be calculated using the formula of area = [π * (diameter) 2/4]. Get the image simulation status.
As for the volume and height of the bump, the system uses another mode to detect it; the system uses the finite element analysis method to cut the bump into many small blocks for three-dimensional integration processing, so that a volume estimate can be made. If the bump itself is flat, it is of course quite simple to handle, but if there is a spherical bump structure, the calculation process will be different due to the selected block size. If the resolution of the image system is insufficient, the smallest block that can be selected may not reach the ideal level. The measured volume will be quite different from the actual situation, and the measurement result may also be unstable. This measurement method is called VPA-Ball Volume Pixel Acquisition.
8. AOI characteristic size inspection
In the process of etching the inner and outer circuit boards, a line width and line distance measuring instrument can be used to detect the line width and line width and the upper and lower line widths, and then confirm whether the parameters meet the etching standards. The biggest advantage of using the line width and line distance measuring instrument is that the use of a visual system instead of the human eye with a microscope can effectively capture the upper and lower line width and line distance values, and make a report output to analyze and compare the ability of the etching process With pros and cons.
9. AOI’s product appearance inspection
The inspection of the circuit board before shipment is very challenging. The finished circuit board is quite similar to the general bare board, but the acquisition and analysis of its image becomes more complicated due to the addition of a layer of solder resist paint. Up to now, a considerable proportion of the final inspections relies on manual inspections by operators. With the addition of new automatic visual inspection technology, more manufacturers and equipment have entered the production line. Most finished circuit boards differ greatly from the area of the circuit boards in production, and the differences between products are also quite large. The visual inspection equipment required in such a situation is different from the characteristics of the inspection equipment required by the bare board.
This visual inspection system should have the following characteristics:
l It must be able to provide different automatic visual inspection capabilities, and it must be able to adapt to a wide range of product types; the set time must be short, because the batch of product replacement is very frequent, and the production line cannot wait for a long time to replace the test product
9.1 Types of defects
In addition to the defects that can be detected by AOI (inner bare board and outer board), there are two main types of defects in structural loading boards that must be detected during final inspection:
l Defects on the surface: discoloration, uneven surface, foreign objects on or in the solder stop paint;
l Alignment problems of solder stop paint: the liner is over-shielded, the hole or line that should be covered is exposed, the area where the pad or hole should be exposed, and the area that should be protected by complete solder stop paint is incomplete.
9.2 、 Import color CCD function
Due to the difficulty of detecting the changeable appearance, one of the more feasible solutions is to introduce a color CCD image system to the detection process. The traditional inspection is to use a monochrome or gray-scale image system to obtain the image for judgment. The monochrome system can only provide two-dimensional color for image judgment.
Considering the types of materials that need to be detected: pure substrate, combination of substrate and solder stop paint, combination of copper metal and solder stop paint, different metal treatment on the liner, etc. These conditions should be provided without doubt Only a better recognition ability system can be effective. Because of the introduction of the color separation method, the image will not only have the difference between brightness and reflection, so the problem of insufficient material contrast can be significantly reduced. Using color to capture images, even with relatively low contrast and the presence of multiple metal treatments, there is a chance to detect the surface condition of the carrier board. However, the amount of color image data is relatively large, so the data processing capability of the device must be moderately improved.
After research, some manufacturers found that the gray-scale discrimination system is more capable of judging the surface defects of the circuit board than the color CCD system. Therefore, they tried to convert the color system to the gray-scale system again. . 9.3 Strip and sheet inspection mode.
The functionality of IC’s and I/O have become more and more complex. Circuit board manufacturers must produce higher-density connected products at a reasonable cost, and at the same time deliver stable and high-quality products in a shorter time. Coupled with the introduction of circuit board technology and micropore technology, these complex needs can be met. This trend has become the mainstream of the circuit board industry and advanced electronic mounting board technology, completely changing the industry’s ecology.
Because the contact points configured for each inch of the carrier board have increased significantly, the cost of missing a defect will be greater than the previous loss. Therefore, circuit board manufacturers are seeking automated inspection solutions as an important monitoring method for quality control.
The structure loading board method for advanced structure includes: BGA (Ball Grid Arrays), CSP (Chip Scale packaging), flip chip BGA (FC-BGA-Flip chip BGA), COB (Chip on Board), etc. Because the production of the circuit board is carried out in a full board manner, and the product is configured in a repeating circuit layout, a considerable number of units will be arranged on a circuit board. This feature makes the production batch relatively small. Some flexible roll-to-roll loading boards also require special AOI equipment to check. For the construction board or module board manufacturers, the strip configuration is often used for production and shipment. For example: a CSP structure loading board product has four consecutive pieces, each piece contains 25 units of CSP. Assuming that the industry stipulates that as long as the number of defective units is less than 20%, even if it passes the test, the system should deal with this demand. At this time, we must pay attention to confirm the defect status of the product unit, rather than only calculating the number of defects.
When it is found that the number of defects exceeds the specified amount of failure, it should be further confirmed whether this product should be scrapped. At this time, the automatic visual inspection system needs to have the following two capabilities:
Able to confirm the number of defects of each unit and not just the number of defects on the entire product (two fault problems appearing on the same unit product should be identified as a single product with defects, and the data required for quality control can be additionally calculated)
l The system should have the ability to screen the entire product, not just detect the number of defects.
10. AOI molding size measuring machine
The use of size measurement in the PCB manufacturing process can effectively improve the yield of products in the manufacturing process. Especially in the forming stage, due to the irreversible forming process, the defects caused by wear during the use of the milling cutter can be eliminated if they can be detected in time. In addition, because of the use of automatic optical detection as the medium, it can also avoid the inconsistency of the results of the detection before and after or different people.