Automated optical inspection for conformal coating
One possibility to measure large areas of PCBs in a timely manner is automated optical inspection (AOI) or respectively conformal coating inspection (CCI). Unfortunately, classical approaches using visible light sources and classical 3D camera systems do not work well for thickness measurements, as the conformal film is most of the time transparent and shows only very little structure, which is needed to solve the correspondence problem of stereo reconstruction. Laser line triangulation suffers from too low height resolution. To surmount this issue, many conformal coating lacquers contain ultraviolet fluorescent tracers, which can be easily measured by cameras (see figure 2).
Since the intensity of this light depends on the amount of tracer molecules at each position, it can be inferred that it is directly correlated to the thickness of the conformal film at each position.
Thickness measurement by fluorescence
Let us understand how an AOI system equipped with UV lights works. The basic assumption of thickness measurements with UV fluorescence is that the fluorescence tracers are nearly evenly distributed and show a constant fluorescence behavior. Equal thick films emit light with same intensity if they are excited with the same UV intensity. The light is captured by a camera which measures its intensity using a measurement model based on the geometry depicted in Figure 3.