Conformal coating is a protective coating widely used in printed circuit boards (PCBs), protecting PCBs from harsh environmental conditions. Its curing extent and thickness are the key factors, determining its protection performance. At present, the traditional method to evaluate the curing extent is metallographic section, which cuts PCB and images its cross-section under a microscope. However, it is destructive. In this study, we proposed to use optical coherence tomography (OCT) to evaluate the coating curing extent. Note that Brownian motion inside the conformal coating gets slower during curing process, leading to a smaller OCT intensity variation over time. Therefore, speckle variance (SV) of OCT imaging, which actually measures the OCT intensity variation, is expected to become smaller during the curing process and can be used to evaluate the curing extent over the whole imaging depth. To demonstrate the capability of SVOCT in detecting the curing extent of conformal coating, multiple OCT images were acquired at each curing status for SV calculation. The results show that the speckle variance of OCT image will gradually decrease during the curing process of conformal coating and eventually stabilize after the coating cures completely. This can be utilized to assess the curing extent of the conformal coating.
Optical coherence tomography (OCT) is a non-destructive and non-contact sensing tool for imaging optical scattering media with microscopic spatial resolution. According to its imaging mechanism, this technology is very suitable for imaging and thickness measurement of multilayer structures. Thus, OCT has been widely used for medical diagnostics and non-destructive inspection in industries. However, due to the limited imaging depth, OCT can only be used for non-opaque materials. In this study, we developed a novel technique based on OCT imaging for thickness measurement of opaque materials. To demonstrate the ability of the technique, we obtained a double side view by establishing two symmetrical sample beams based on a home-built 1060nm swept source OCT system. Using the OCT system we developed, we can collect two surface contour information for non-transparent materials, and eventually calculate the thickness of the nontransparent material. The results show that the developed system keeps the imaging capability of OCT and further extend for opaque material thickness measurement.