Automatic visual measurement of surface-mount device placement

Abstract

An experimental system for automatic visual measurement of surface-mount device placement on printed circuit boards has been developed. Infrared illumination is used to aid in segmentation of component lead and solder pad images from the background. Best fit lines through centroids of the lead and soldering pad images are calculated from which the displacement and angular orientation of the component relative to the pads is computed. Measurements are derived from a combination of slope and mean centroids from opposite sides of a device to reduce the effects of sensor resolution, missing pads, or missing leads. Since leads and pads are viewed simultaneously, the accuracy of the system is not dependent on precise positioning of the board within the inspection station. The system accuracy has been tested with 50 commercially manufactured printed circuit boards containing 500 samples of J-leaded plastic leaded chip carriers and small-outline integrated circuits of various sizes. The positions of the components were measured manually and are compared with measurements determined by the system. >