The validity of the relation
can be demonstrated, where: S is the intensity of reflected light, C is constant, I0 is the intensity of emitted light, c is the tracer concentration, α is the attenuation, and t is the conformal coating thickness.
Most of these parameters are unknown, therefore they need to be calibrated. The idea is to use an assessed measurement method of the thickness, for a calibration aimed to obtain a matching curve of intensity-thickness.
The Dragonfly Next> series systems offered by Seica uses this geometry and, thanks to the combination of a software and a scanning technology, with advanced dedicated capabilities, allows to efficiently and easily measure the conformal coating thickness on the whole PCB surface.
The scanning technology deploys a specific optical system which allows for a telecentric image acquisition where all the components on the printed circuit board are imaged in a perpendicular fashion showing only their top sides (see figure 4).
The optical system is mounted on a slide with a RGB line camera, while the area is illuminated by LED stripes, which are mounted on the slide as well. The slide is moved over the PCBA which is positioned on a conveyor belt below the scanner. The biggest advantage of this system is the comparatively low acquisition time in comparison to a complex full area camera system which has to be moved over the scene (see figure 5).