In case of larger inspection areas, the systems are mounted with special drives which can move
the camera system or the inspected part. The movements of these drives have to be synchron‐
ized with the process of illumination and image acquisition. When the system contains small
number of cameras and the illumination devices are also built-in, then the module itself should
be the moving section. When there are even more cameras, each with its separate illumination
(matrix arrangement), then the PWB should lie on a positioning table.
Two big groups of drive systems are commonly used for this purpose. The first is the conven‐
tional electromechanical drive. It is used for some 2D paste inspection machines. Here the
velocity of the camera can be constant, while in most cases it contains line-CCD sensor. The
other type of motion system is the linear drive which is more accurate and faster and therefore
in more frequent use.
The directional route of the moving part highly depends on a second factor, that of speed and
the properties of image acquisition system. Here also, three main parts can be singled out:
• optics / lenses
• camera / sensor type
• illumination module / lighting source
The system has to get the necessary amount of information and resolution from even the
smallest components. In the SMT field this means zooming down to a 10μm pixel resolution.
To ensure the constant magnification at all points of the entire Field of View (FOV) the use of
telecentric optics is essential. This criterion enables the system to make the required size-
measurements. On an image seen through traditional lenses, the apparent shape of compo‐
nents changes with the distance from the centre of the FOV, therefore sometimes making shape
recognition a hard task.
But it is not just the permanency of magnification that is important, so too is the need to select
the correct level. On one hand, the larger detection area of the image sensor can help solve this
task, but it also increases the computational resources needed. On the other hand, higher
magnification levels give a better resolution but at the expense of reducing the field of view.
The best scenario is if the system is capable of optional magnification. Generally, a relative
large FOV, between 10-25 cm2, could be used and only in certain cases should dedicated Field
of Interests (FOI) should be zoomed out.
In most AOI applications, the LED based lighting is used for illumination purposes. But
independent of the type of illumination source used, the amount of illumination should be
only as much as is required. The optimum depends on the application. For example, a 2D paste
or a through-hole-technology (THT) components solder-joint inspection system needs only
just a small amount of illumination. As the number of failure types / inspection tasks increase
so too the number of illumination modes also increase. The programmable illumination
module is a good tool to develop lighting requirements for dedicated purposes, but it also
carries the risk of inhomogeneous and reduced FOV.
Automatic Optical Inspection of Soldering
http://dx.doi.org/10.5772/51699
417
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