- Bare board fabrication
- Solder paste application
- Assembly of SMT components
- Post soldering of SMT components
- Automated Laser Test Systems
- X-ray Fluoroscopic Test Systems
- X-ray Lamination Test Systems
With the Printed Circuit Board or PCB being the equivalent of a central processing unit of any electronic product, it is essential that it keeps functioning effectively. As technology advances, PCBs are tending to be smaller, more compact, and more complicated. With the introduction of Surface Mount Components or SMTs, the average board now has several thousand soldered joints, and this is where a large majority of the trouble starts.
As production volumes increase, rigorous manual inspection starts to become impractical. It would require technicians to examine each component in detail, evaluate their orientation, and qualify their solder joints on the board. This would substantially increase the risk of missing many imperfections.
On the other hand is the increasing demand for the manufacture and delivery of high-volumes of products to the market in shorter and shorter time frames. Original Equipment Manufacturers or OEMs need better and more reliable testing and inspection methods for ensuring the quality of their products, and these must be adequately fast so as to not slow down the production process. This is where Automated Optical Inspection or AOI methods come in.
What is Automated Optical Inspection?
Simplistically speaking, AOI is the visual inspection of a printed circuit board in different stages of PCB fabrication process, of bare PCB as well as assembled PCB with electronic components.
PCB manufacturers conduct a detailed scan of the board using a high-definition camera for identifying missing or skewed components, solder defects, short circuits, and other quality defects. The system may illuminate the PCB board with LED, infrared, ultraviolet, and fluorescent lights to capture multiple images. There are two major methods to program an AOI system: Using a “Golden Board”: Using a good board, the images form a golden standard against which to compare all subsequent boards.
In subsequent production runs, the AOI system scans and compares each PCB board image against the golden standard, flagging any differences it finds. Technicians then examine these differences in magnified views and determine if they are defects and the effective ways of rectifying them.
Algorithm Based Programming: The system accepts PCB data and uses it to generate its own profile for a board.
Benefits of Automated Optical Inspection
With AOI, it is possible to indicate process problems in different stages of manufacturing, namely:
The feedback from the AOI stations at each of the above stages helps in address the defects by formulating a rapid response to their occurrence.
Locating defects early on in the manufacturing process helps to substantially reduce the costs of corrections later on. It also helps avoid the issue of the same defects recurring during a high-throughput production run, as this could easily become expensive and a time waster.
As AOI is a real-time process, the resultant data it generates helps to improve the manufacturing process and its quality control. It also helps to quickly rectify and reduce any potential problem to zero occurrence, thereby improving the throughput of a large-scale production run. With effective algorithms, an AOI system can track and document defective PCBs and good assemblies. This enhances the efficiency of process management, operational integrity, and traceability.
AOI equipment is easily tunable, and this allows manufacturers and assemblers to tune the system to effectively pass acceptable PCB boards, while blocking defective ones. Apart from eliminating surface defects, AOI systems also help expose leading defects before electrical testing. This is beneficial to accumulation of data for Statistical Process Control.
Considerations for Automated Optical Inspection
Although there are several benefits accruing due to use of AOI, it is necessary to maintain a high pass-rate to parameters ratio. A high pass-rate can be taken to mean the passed products are more likely to be more reliable in their operation. However, a high pass-rate is not always be an indication of a product’s guaranteed performance.
In the same way, a low pass-rate might not always imply poor build quality. The low pass-rate could be an indicator of specified standards that the manufacturer has put in place.
Therefore, the OEM must look for a high-performance finished product, rather than a high pass-rate.
As a rational approach, the manufacturer must concentrate on the inspection parameters. This is important, as the automated system simply responds to the instructions given to it by the operator. The AOI system only examines what it is asked to, ignoring those the operator has not instructed it to. All in all, using AOI during PCB manufacturing stages does ensure consistent and repeatable output.
Types of Automated Optical Inspection Methods
PCB manufacturers use multiple structural inspection technologies that cater to different costs, performance, and defects. These inspection technologies range from the simplest visual inspection methods to the most involved X-ray inspections. In the industry, it is typical to classify PCB board assembly inspection technologies into two broad categories—manual visual inspection methods and automated optical test inspection methods.
Manual Visual Inspection Methods: There are many steps in PCB board assembly that benefit from visual inspection. Depending on the stage of the assembly, assemblers select the equipment for visual inspection. For instance, immediately after a solder paste printing stage, a manual visual inspection can identify poor alignment, partial printing, excessive paste in small areas, solder paste inside plated though holes, and contaminated solder paste. However, as manual inspection can check about 5 joints per second, this method is useful only for low and medium quantity production runs.
Automated Optical Inspection Methods: As production volumes increase, the importance of accuracy and speed also increases. In many cases, it is also necessary to inspect hidden solder joints such as those under BGAs, J-lead devices, and surface array flip chips. Being high-density packages, these require inspection based on uniform rules established specifically.
The industry uses digital equipment for automated and real-time inspection and analysis of defects. Digital equipment dramatically improve the capability and repeatability of these AOI systems.
Most AOI systems depend on some form of light emission—visible, lasers, and X-rays. The equipment acquires data by processing the image in real-time, thereby measuring and locating defects related to components and their soldered joints. Such automated optical inspection systems do not contact the circuit board physically. However, they are sophisticated enough to offer high repeatability, while effectively eliminating subjectivity from the inspection process.
AOI systems make use of multiple visible light sources. They use video cameras, programmable LEDs, and computerized systems while inspecting soldered joints. The intensity and graphics of the reflected light from soldered joints offers information on the curvature of soldered joints. Analysis of acquired data helps to determine solder joint completeness, solder quantity, solder bridging, wetting, displaced, and missing components.
AOI systems are suitable for inspecting 30-50 solder joints per second, for devices with pitch larger than 20 MIL.
Automated Optical Inspection systems also use other methods in the industry, namely:
The method of inspection primarily depends on the inspection speed, defects type, and costs involved.
Automated Laser Test (ALT) Systems: With ALT or automated laser test systems it is possible to measure the shape and height of solder paste deposition. The system focuses a laser beam on the solder surface for measuring its reflectivity and height, and it does this by measuring the reflection angle on multiple position sensing detectors.
ALT systems help collect information in real-time from solder paste printing that includes alignment, viscosity, cleanliness, and other attributes.
X-ray Fluoroscopic System: By passing a beam of X-rays vertically through the PCB board, the X-ray fluoroscopic test system produces a density image. This helps to accurately inspect solder joints, assess their thickness, and their internal integrity. Digitizing the image assists in inspecting any defects in the solder joints, like misalignment, presence of voids, bridging, inadequate amounts of solder, and cracks.
X-ray Inspection Systems: The X-ray lamination system generates images of local planes of the horizontal section of a PCB board. Usually, these sectional images are with surface thickness of 8-16 MIL, and allow the inspection of different sections of the same solder joint. The X-ray lamination test system is suitable for inspection of PTH or Plated Through Holes and solder joints under BGA ICs.
Conclusion
The PCB board manufacturing industry uses the most appropriate automated inspection methods for fabrication and assembly, depending on the circuit complexity, speed of inspection necessary, and the production volume. As a rule of thumb, increase of complexity, volume, and speed of the printed circuit board requires a higher cost of inspection.
