三防漆是一种广泛应用在印刷电路板（PCB）的保护性涂层, 可以有效保护PCB使其免受恶劣环境的损害。 三防漆的厚度是评价三防漆涂层质量的关键指标, 因此需要对三防漆涂层进行厚度检测。 提出了将谱域光学相干断层扫描成像技术（spectral domain optical coherence tomography, SD-OCT）与图像分割算法相结合, 实现对三防漆涂层厚度快速无损测量。 为了提高测量精度, 选用了宽带SLD光源（带宽: 180 nm）来设计SD-OCT系统, 系统轴向分辨率达到1.72 μm。 同时设计了基于边缘跟踪的涂层分割算法来实现三防漆涂层的快速准确分割。 为了评估所设计方法的准确性, 利用传统的金相切片方法进行了三防漆的厚度测量, 将测量结果与该方法测量结果进行比较, 分析了两种方法检测到的涂层上下边界吻合程度以及厚度差异。 此外, 还将所提出的涂层分割算法与我们组之前研究的基于图像梯度的边缘检测算法进行对比, 分析了两种方法在测量结果的准确性和运行效率方面的差异, 以此来评估该方法的优劣势。 结果表明, 所设计的三防漆厚度测量方法与传统的金相切片方法的测量结果具有很好的一致性, 可以准确地实现三防漆的厚度测量; 基于SD-OCT系统的三维成像能力可以直观地看到三防漆厚度地形图, 克服了传统的金相切片方法无法进行区域性的三防漆厚度测量的缺陷; 相比之前提出的基于图像梯度的边缘检测算法, 此方法测量结果更加准确, 效率显著提升, 具备实时测量的潜力。

Conformal coating is a protective coating widely used for protecting printed circuit board (PCB) from harsh environmental conditions. The conformal coating’s thickness is one of the key parameters for evaluating the quality of conformal coating, and thus the thickness needs to be measured before PCB assembling. In this paper, we proposed to utilize spectral domain optical coherence tomography (SD-OCT) combined with an image segmentation algorithm to achieve a rapid and non-destructive thickness measurement on conformal coating. The spectral domain OCT system was built with a broadband SLD light source (bandwidth: 180 nm) so that an axial resolution of 1.72 μm was obtained. Meanwhile, an image segmentation algorithm based on boundary tracing was designed to segment the conformal coating in OCT image for rapid and accurate thickness measurement. To evaluate the proposed method, the measurement results in this paper were compared with the traditional metallographic examination, analyzing the consistency of the detected boundaries as well as the measured thickness between these two methods. In addition, we also compared the method proposed in this paper with the edge detection algorithm based on image gradient previously reported by our group in terms of the accuracy and efficiency, so as to justify the advantages of this newly developed method. The results show that the measurement method designed in this paper is in good agreement with the traditional metallographic method, and the thickness of the coating can be accurately measured. Based on the three-dimensional imaging capability of the system, the thickness topographic map of the conformal coating can be visualized clearly, which cannot be realized by the metallographic section method. Compared with the method proposed by our group previously, this method can provide more accurate and efficient measurement, paving the way for real-time and inline thickness measurement on conformal coating.