Non-Destructive Inspection Of Conformal Coating On PCB By Optical Coherence Tomography

Conformal coating is a protective coating widely used in printed circuit board(PCB).When the conformal coating is sprayed on a PCB,a coating layer with a thickness of several tens to several hundreds of micrometers is formed after being cured to protect PCBs from harsh environments.However,if the conformal coating is not fully cured or the thickness is below the requirement,the PCB will not be effectively protected.Therefore,the two aspects inspection of conformal coating is highly important before PCB assembling.At present,there is no way to implement online no-destructive testing in the two aspects.Therefore,it is necessary to develop a fast,non-destructive coating inspection method.Optical coherence tomography(OCT)is a non-invasive,low-coherence,three-dimensional optical interferometric imaging technique.In this paper,we study thickness measurement and curing state evaluation of the conformal coating based on spectral domain OCT(SD-OCT),the main works include the following two parts:(1)Research on thickness detection of conformal coating based on SD-OCT:firstly,the ultra-high resolution OCT is used to image the PCB coating layer;Secondly,according to the gray-scale characteristics of OCT image of the conformal coating,two image boundary detection algorithm is designed.Then,the two boundary detection algorithms are used to detect the upper and bottom boundaries of the conformal coating in OCT images to achieve thickness measurement of the conformal coating.Finally,the accuracy of the proposed method is verified by means of metallographic sectioning.(2)Research on curing state detection of conformal coating based on SD-OCT:firstly,during the curing process,the change of Brownian motion will cause the speckle to change.In this paper,the curing degree of conformal coating is evaluated by the variance of the speckle.Then,the curing state of conformal coating is evaluated by the change of speckle variance at different depths position during the curing process through the experiment.Finally,the time to complete curing of conformal coating is accurately determined by the time when the speckle variance tends to be stable.This method is simple,easy to implement and has a good practical application potential.In summary,this paper demonstrates the potential of OCT to detect the thickness and curing state of conformal coating.Compared with the traditional method,the method in this paper is fast,accurate and more non-destructive,which provides the possibility for online detection of conformal coating on PCB.