Optical inspection methods have existed ever since electrical assemblies were tested. They are used to check the visible quality features of an assembly, or in other words: was an assembly correctly assembled and soldered. Originally, and still used today,Manual Optical Inspection (MOI) was used for visual inspection. In this process, the test object is inspected with the naked eye or with the aid of a microscope and, if necessary, compared with a "golden board", i.e. a reference sample of the assembly to be produced. The limitations of this inspection method are obvious, the assessment of a quality feature depends heavily on the condition abilities of the inspector. In addition, people tend to follow trends and make their assessment dependent on the most recently tested assemblies.
The biggest disadvantage, however, is the slow speed and rapid fatigue during testing.
Automatic Optical Inspection (AOI) starts at this point. A machine takes over the task of quality assessment by digitally capturing an image of the assembly to be inspected. These images are then evaluated via software algorithms. All this happens at high speed with reliable and reproducible results.
AOI systems are basically similar in the way they work. A system consisting of one or more cameras is moved in the X-Y direction over the assembly while scanning or using the so-called stop-and-go method. The camera is positioned precisely at one point. When the axis stops, images of the scene are taken with the help of lighting systems. We distinguish between orthogonal and angled-view optics, but also combinations of cameras are often used. The processing and evaluation of the captured images is carried out in the system software of an AOI system. This software relies on numerous complex image processing algorithms which have the task of evaluating defined inspection features as "good" or "bad" as realistically as possible.
With the growing complexity of assemblies, increasing miniaturisation of components and growing demands for lower pseudo defect rates, 3D AOI has become the standard in recent years. The aim is to make even more reliable statements regarding the reality of the quality of solder joints, components, etc. Whereas with a pure 2D image only a colour value is determined, 3D provides valuable height information in addition to X and Y values. This allows the interpretation of physical parameters. A combination of 2D, 3D and e.g. angled view additionally increases the reliability of the evaluation of inspection characteristics.
AOI systems usually exist as stand alone or inline systems. Stand-alone or offline systems are operated away from the production line as a so-called "island solution". Due to the manual feeding of the assemblies, these flexible systems are particularly suitable for small and medium production quantities. However, inline AOI systems are the most widely used, as they can be seamlessly inserted as a fixed component in electronics production lines and production lines. The PCB is fed via a conveyor module in a smooth transition to upstream and downstream automatic machines of the SMD process. Inline AOI systems also have interfaces to communicate with other systems and higher-level process control systems (MES).
Today, AOI systems are standard in almost every electronics production. The production quality of an assembly can be determined directly without adapters or specific attachments. User-friendly software also makes the systems easy to operate and program. All test characteristics are tested with consistent reliability. By logging measured values and test results, a well-founded statistical analysis is also possible as a basis for process optimisation. With this option, electronics manufacturers can continuously reduce the number of defectively produced assemblies.