Spectral-domain optical coherence tomography for conformal coating thickness measurement on printed circuit board

Conformal coating is a thin film used for protecting printed circuit boards (PCBs) from harsh environmental conditions, which reduces the failure rate of PCBs. The thickness of conformal coating is one of the key factors determining the protection efficacy on PCB. Therefore, the thickness measurement is highly desired to qualify the conformal coating. In this study, we propose to employ high-resolution spectral-domain optical coherence tomography (SD-OCT) for measuring the conformal coating thickness. An SD-OCT with axial resolution of 1.72 μm is developed. The system can provide cross-sectional imaging of the conformal coating layer. Then a boundary detection algorithm is developed to identify the coating layer from the OCT image and eventually calculate the thickness of the coating layer. Our proposed method is evaluated through comparing with metallographic slicing method, which cuts PCB into cross-section and measure conformal coating thickness under a microscope. The results demonstrate that our method produces a very consistent measurement results as compared to metallographic slicing method. In addition to the good accuracy, our algorithm's computation load is low (about one hundred milliseconds per B-scan), indicating the potential to achieve on-line inspection of coating thickness.