Development of an automatic stencil inspection system using modified hough transform and fuzzy logic

In this paper, the authors present a system to inspect metal stencil that is used to print solder paste on pads of surface-mounted device on printed circuit board. The developed inspection system is composed of a moderately precise X - Y robot and a vision system. To correct a position error caused by the X - Y robot, the authors define position error vector and apply modified Hough transform to determine the dominant position error vector. Using this extracted dominant position error vector, the reference image is modified. This transformed reference image is compared with the camera image. Fuzzy logic is utilized to judge the correctness of the holes on the stencil. The input variables are the ratio of the overlapped area of two holes and the distance between the centroid of them. The output variable is the grade of the identity of the hole. These methods are verified by a simulation and applied to the inspection system.