Estimated reading time: 9 minutes
Today’s boards are extremely intricate. The Printed circuit board (PCB) circuit lines are becoming increasingly thinner. Electronic parts are getting considerably smaller. Surface Mount Devices (SMDs) are positioned closer together by workers. This indicates that there are a lot of parts on the boards. A quality inspection needs to be extremely precise.
The production workshop needs new methods.
Previously, people used their eyes to check boards. Visual inspection was the term for this. However, tiny lines are invisible to the human eye. They are slow to notice minor flaws. This outdated method of evaluating quality is no longer effective. The high quality requirements of contemporary SMT assembly are not met by it. Errors are too likely. It’s moving too slowly.
The industry needs a new approach. This new method must be fast and extremely accurate. To this end, automated optical inspection (AOI) technology emerged. AOI uses light and cameras to evaluate the functionality of solder joints and inspect the entire assembly process. Today, AOI is widely used by many companies. Furthermore, AOI systems are becoming increasingly intelligent. AOI has become a key method for evaluating the quality of SMT production lines, helping to ensure that every product functions correctly.
How AOI Systems Work: Steps and Methods
Every AOI system adheres to a straightforward set of procedures. The system must first take a picture of the target. For this, optical components are used. It snaps a photo of the area it needs to inspect. The image is then captured by the computer. The image’s data is processed by it. The computer then examines the information. It evaluates the quality.
The AOI computer analyzes the pictures using two primary techniques. These techniques are the Computer-Aided Design (CAD) data comparison method and the Design Rule Check (DRC) method. Both are critical to the system’s operation.
A. The Design Rule Check (DRC) Method
This DRC approach makes use of basic math. It employs unique algorithms. The validity of the pattern is ensured by these algorithms. DRC offers a lot of advantages. First, establishing the rules is quite simple. Secondly, it processes the images very quickly. Third, it’s a small computer program. It doesn’t require a lot of data storage. Today, the DRC approach is widely used in the majority of AOI systems. For general checks, it is easy and quick.
However, the DRC approach has a significant flaw. It has trouble identifying precise boundaries. It is necessary to locate the edge of a line or pad exactly. To locate these places correctly, DRC requires extra steps. For general compliance, it is preferable to precise measurement.
B. The CAD Data Comparison Method
Image comparison is how the CAD data method operates. A perfect image is stored in the memory of the AOI machine. The PCB’s design file (CAD data) is the source of this digital image. The machine takes a picture when it checks a board. It contrasts the recent photo it took with the ideal image stored within. The comparison reveals any differences between the boards.
This approach can provide extremely accurate results. The quality of the camera determines the outcome. The specifics of the check program also play a role. This technique is excellent at identifying minor flaws when used correctly.
But there’s a problem with this approach. The camera captures a lot of images. It gathers a lot of information. All of this data must be processed quickly by the computer. For real-time checking, a powerful computer is required. Because of this, the system is more complicated than DRC. However, in many situations, the CAD data method produces better results than DRC. A lot of people believe that the CAD data comparison method offers more advantages in general.
Full Inspection Coverage in SMT Assembly
Throughout the assembly line, AOI technology checks the board numerous times. It examines the PCB that is bare. It looks at the print of the paste. The components are examined. It examines the final solder joints.
A. Examination of Bare PCBs
Checking the bare board is the initial use. Before any parts are placed, this check is made. This stage guarantees that the board is in good condition before assembly time is spent. A separate AOI machine is often used for bare PCB inspection. On the assembly line, this check does not occur immediately.
Numerous flaws are found in the bare PCB checks. It looks for open circuits, which indicate a line break. It looks for scratches and line flaws. It searches for tiny holes in the copper called pinholes. The line spacing is checked. It determines whether the lines’ edges are jagged. It searches for extensive flaws. This is a fundamental step for quality.
B. Checks for Solder Paste and Components
Component checks and solder paste are crucial. AOI machines that interact with other equipment are typically used for these checks. Frequently, they are instant checks. They provide feedback right away.
After the printer applies paste to the board, a solder paste printing check is performed. The paste printer is compatible with the AOI machine. It measures the thickness of the paste print. The paste edge appears to be accurate. The final solder joint will be poor if the paste is either too high or too low. A ring light is used by the system. The paste is exposed to this light at an angle. The image is captured by a camera from the upper center. This configuration aids in measuring the thickness and edge of the paste. By observing how the paste shape alters the light, the system makes decisions.
After the components are placed on the board by the placement machine, a component placement check is performed. The placement machine is in conjunction with the AOI machine. It determines whether any parts are lacking. It determines whether the components are positioned correctly. It determines the component’s direction. To be certain, it also counts the number of components. The operator is immediately notified by the system if a defective part is discovered. The malfunctioning component can then be replaced by the operator. This prevents the error from becoming worse later on. It averts a major issue later on.
C. Inspection of the Final Solder Joint
The finished product is examined. This check is done after the board has been taken out of the soldering oven. The solder joint AOI systems use cameras to capture a three-dimensional image of the solder joint. The image shows the height and shape of the solder. The computer processes this data. An illustration of a flawless solder joint is used to contrast the data. This comparison makes it clear what kind of flaw it is and where it is.
Many topics are covered by the solder joint checks. They look for bent pins. They search for components that are missing. They check for incorrect component placement. They examine the solder joint’s final quality. This last inspection is the last opportunity to identify and address any lingering flaws.
Problems AOI Faces and Future Technology
AOI is a great technology. But, it still has issues on the factory floor. These issues must be solved for better quality.
A. High Rates of False Alarms
The main issue is that AOI frequently reports a flaw when none exists. This is referred to as a false call or false positive. This occurs frequently. Light is the issue. Solder surfaces reflect light in different ways. The machine may become confused by slight variations in component color or light angle. According to the AOI system, the board is flawed. Employees must halt their work. They have to visually inspect each false alarm. This is a huge time waster. It slows down production. One of the primary objectives of new AOI systems is to decrease these false calls.
B. Blind Spots and 2D Limitations
Another issue is that AOI cannot see everywhere. AOI uses light. Light cannot go through parts. Some modern components have solder joints hidden under the body. These are Ball Grid Arrays (BGA) or Quad Flat No-leads (QFN). AOI cannot check these hidden joints. For these parts, the factory must use an X-ray machine. This X-ray machine checks the inside of the board. It works with AOI. The limits of 2D AOI systems were big. They could not check the height or volume of the solder joint correctly. This led to the creation of 3D AOI. 3D AOI uses special light to measure the height of every part. It gives real volume data. This 3D data helps check defects that 2D could not see, like weak solder volume or lifted pins.
C. The Challenge of Programming
It takes a lot of work to set up AOI for a new product. Every day, new PCB designs are released. Every new board requires a new program for the AOI system. It takes a lot of time to write these programs. It’s complicated. New product launches are slowed down as a result. The system needs to change quickly. It needs to pick up the new board layout fast. The factory loses time and money if programming is too slow.
D. AI’s Application in AOI
Artificial Intelligence (AI) is AOI’s future. AI is the ability of a computer system to learn. AI is now used in new AOI machines. Numerous images of flaws are used to train AI algorithms. The AI gains an understanding of what constitutes a good part. It also learns the appearance of a bad part. Teaching the AI to disregard innocuous variations is the aim. This AI application will significantly reduce the false call issue. It will increase the reliability of the inspection. New parts will be checked more quickly by the system.
Conclusions
High quality checks will become more and more necessary. The size of circuit boards will decrease. They will become increasingly intricate. One important tool for maintaining high quality is AOI technology. From basic camera work to intelligent AI systems, it evolved. It resolves issues that the human eye is unable to resolve. To ensure that every electronic product functions properly, AOI is essential. Time is saved. Money is saved. It improves the assembly process as a whole. A key component of smart manufacturing’s future is AOI.
