Nondestructive measurement of conformal coating thickness on printed circuit board with ultra-high resolution optical coherence tomography

Conformal coating (CC) is widely used to protect printed circuit board from corrosion, mold growth, and electrical failures. To ensure effective protection, the thickness of the CC layer needs to be well controlled. However, to date, the coating thickness is usually measured in a destructive way under microscopes. In this paper, we proposed to use optical coherence tomography (OCT) to measure the CC thickness nondestructively. Specifically, to obtain a good accuracy in thickness measurement, we constructed a spectral domain OCT with the ultra-high axial resolution to image the CC layer in three dimensions and developed an image segmentation algorithm to detect the CC layer from the OCT images. Finally, we evaluated the effectiveness of the proposed method by comparing it with the conventional method, and the results demonstrate that the measurement by our method is consistent with that by the microscope. This also indicates that OCT with high axial resolution can potentially be used to measure the CC thickness accurately and nondestructively.