Everything in view with VERSAEYE

Basically, the AOI is a camera system, which should reliably distinguish good from bad solder joints on the basis of predefined test samples. It is important to protect the system as best as possible from external influences. Ideally, no light enters the camera system from the outside – or at least only constant light. It is obvious that a place close to windows should be avoided. After all, the light conditions at sunrise are quite different from those at night, for example.

The reliability of the system also includes regular maintenance. A brief glance by the technical staff at the camera system allows a statement to be made about possible contamination that would impair the inspection programme. Before commissioning and program creation, the mechanical function test of the x-, y- and z-axis must be performed. For THT inspection, it is usually sufficient to inspect in a fixed x/y-level, as all solder joints are located on the bottom side of the PCB. However, it may happen that printed circuit boards are produced in a frame carrier, for example, or that a polarity has to be checked at a different z-height. Therefore, it is possible to position the entire camera head with an accuracy of tenths of a millimeter. The optical visual inspection checks the telecentric lens, the three illumination rings and the eight side cameras. Everything is protected from dirt by a glass pane.

With AOI, it is important to ensure a literally crystal clear view. However, the heart of the AOI, the telecentric lens, can only be cleaned with compressed air so that the special surface coating is not damaged. After cleaning with pure alcohol, the glass pane is subjected to a visual inspection to ensure that no solder balls, flux residues, dust or paper fibres are left behind.

The telecentric lens is an outstanding feature of the AOI. It ensures that no parallaxes occur, i.e. an apparent change in the position of the component on the circuit board. This parallax effect increases the further away the component is from the center towards the edge. The image section, in this case called “Field of View” (FoV), can be fully exploited thanks to the telecentric optics without the described shifts occurring. The VERSAEYE operator benefits from this because as many components as possible can be inspected with one FoV on the basis of this information. The image size of a single FoV is then approximately 38.5 x 38.5 mm. The first time a program is created with the focus level defined, it is often the case that different operators enter a different z-value for the focus point. This is due to the different visual acuity of the respective persons. It therefore makes sense to have the focus plane determined by an autofocus of the camera system. To get a new operator enthusiastic about VERSAEYE at this point – it is programmed completely without coordinates. Of course, it is also possible to quickly create a program with the CAD import option. However, this is only recommended for an inspection program with SMT components, since information such as rotation angle and mirroring have no significance for a THT solder joint. Information such as pad and hole size is important here, but does not appear in the coordinates of the CAD file.

For this reason, there is a simple procedure for program creation. Two different methods of image processing are possible for the inspection. The first is “Pattern Matching”. A positive example of an image is taught in here, and the system then searches for exactly this image on every further printed circuit board. If the taught-in image is found, the inspection result is “good”. This works reliably with SMD assemblies, but has a serious disadvantage with THT solder joints: SMD components always have the same geometry and dimensions of the same body. With a THT solder joint, on the other hand, the pin can move freely depending on the hole diameter and is normally not centered. Therefore, there is the second possibility of image processing – the so-called “histogram”, in which colored pixels are searched for in an inspection area determined by the operator. The tolerance limits for image processing can be narrowly set for the histogram and are therefore not based on taught-in images, but on a condition-oriented decision. If there are enough pixels in the selected tolerance range in a defined inspection area, the result is saved as “good”. Irrespective of the direction in which the pin is located. A remarkable feature of the histogram is the 24-bit color depth, which reproduces a true-to-detail image and thus enables an extremely differentiated inspection.