Journal of Electronic Systems Bangshu Xiong, Yanan Zhang, Lei Yu, Chengli Sun 5 4 2015 http://www.dline.info/jes/fulltext/v5n4/v5n4_1.pdf An accurate pad parameters measurement method for printed circuit board (PCB) is proposed in this paper. Firstly, the measured pad is selected by drawing a circle around it, then the processing image is de-noised and segmentation. After that, the teardrop in connected area of the pad and line is excluded. Secondly, all the possible edge points around the pad are detected by Sobel operator and Zernike moment is used for localization of the edge points on a sub-pixel level. Finally, the detected circle edge of the pad is fitted by a least-square algorithm and the pad diameter and center position are derived. Experimental results reveal that the proposed method is precise, quick and stable, and it has already been applied in our selfdeveloped “automatic optical measurement instrument (AOM1.1)”system.