AOI Systems-Automatic optical inspection
Automatic or automated optical inspection, AOI, is a key technique used in the manufacture and test of electronics printed circuit boards, PCBA, Automatic optical inspection, AOI enables fast and accurate inspection of electronics assemblies and in particular PCBs to ensure that the quality of product leaving the production line is high and the items are built correctly and without manufacturing faults.
Need for AOI, automatic optical inspection
Despite the major improvements that have been made, modern circuits are far more complicated than boards were even a few years ago. The introduction of surface mount technology, and the subsequent further reductions in size mean that boards are particularly compact. Even relatively average boards have thousands of soldered joints, and these are where the majority of problems are found.
This increase in the complexity of boards also means that manual inspection is not a viable option these days. Even when it was an accepted approach, it was realised that it was not particularly effective as inspectors soon tired and poor and incorrect construction was easily missed.
With the marketplace now requiring high volume, high quality products to be brought to market very quickly very reliable and fast methods are needed to ensure that product quality remains high. AOI, automatic optical inspection is an essential tool in an integrated electronics test strategy that ensure costs are kept as low as possible by detecting faults early in the production line.
One of the solutions to this is to use automated or automatic optical inspection systems. Automated optical inspection systems can be placed into the production line just after the solder process. In this way they can be used to catch problems early in the production process. This has a number of advantages.
With faults costing more to fix the further along the production process they are found, this is obviously the optimum place to find faults. Additionally process problems in the solder and assembly area can be seen early in the production process and information used to feedback quickly to earlier stages. In this way a rapid response can ensure that problems are recognised quickly and rectified before too many boards are built with the same problem.
AOI, automatic optical inspection basics
AOI, automatic optical inspection systems use visual methods to monitor printed circuit boards for defects. They are able to detect a variety of surface feature defects such as nodules, scratches and stains as well as the more familiar dimensional defects such as open circuits, shorts and thinning of the solder. They can also detect incorrect components, missing components and incorrectly placed components. As such they are able to perform all the visual checks performed previously by manual operators, and far more swiftly and accurately.
They achieve this by visually scanning the surface of the board. The board is light by several light sources and one or more high definition cameras are used. In this way the AOI machine is able to build up a picture of the board.
The automated optical inspection, AOI system uses the captured image which is processed and then compared with the knowledge the machine has of what the board should look like. Using this comparison the AOI system is able to detect and highlight any defects or suspect areas.
AOI uses a number of techniques to provide the analysis of whether a board is satisfactory or has any defects:
Template matching: Using this form of process the AOI, automated optical inspection system compares the image obtained with the image from a “golden board”.
Pattern matching: Using this techniques the AOI system stores information of both good and bad PCB assemblies, matching the obtained image to these.
Statistical pattern matching: This approach is very similar tot hat above, except that it uses a statistically based method of addressing problems. By storing the results of several boards and several types of failure, it is able to accommodate minor acceptable deviations without flagging errors.
It is essential that any printed circuit board manufacturing area is able to check the quality of the boards coming off the end of the line. Only in this way are they able to monitor quality and when problems are detected to rectify the process so that further boards are not affected by the same problems.