Optical inspection for conformal coating: Page 3 of 6

September 03, 2018 //By Francesco Argentiero
Optical inspection for conformal coating
One of the main challenges in the production of printed circuit boards (PCB) in the electronic industry is the protection against harsh environments with high humidity, strong temperature changes and dust or chemical contamination. Since the ‘60s, the conformal coating process has been proposed as a solution for these problems.

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.

Fig.4: Principle of operation of the CCI system.

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).

Fig. 5: Illumination characteristics of LED stripes.

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).

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