The fast and highly reliable method reported here uses two techniques to detect all types of defects, such as unsoldered leads, solder bridges, and misalignes leads in the minute solder joints of high density mounted devices. One technique uses external force applied by an air jet that vibrates or shifts unsoldered leads. The vibration and shift is detected as a change in the speckle pattern produced by laser illumination of the solder joints. The other technique uses fluorescence generated by short-wavelength laser illumination. The fluorescence from a printed circuit board produces a silhouette of the solder joint and this image is processed to detect defects. Experimental results show that this inspection method detects all kinds of defects accurately and with a very low false alarm rate.

In this paper we describe a new method of position detection for a mesh-backed printing screen that utilizes an alignment mark overlapped by mesh paterns. A circular hole is selected as an alignment mark so that at least a portion of its contour can be observed through the windows of the mesh wires. To extract the true cntour from a binary image acquired with transmitted illumination, the smallest circle enclosing binary patterns are detected. Then the mark position is calculated by approximating the extracted contour to a circle with the least squares fitting. To speed up the procedure, a convex hull and a farthest-point Voronoi diagran are generated prior to the smallest enclosing circle detection. According to the experimental results, an accuracy of 0.2 pixels and a processing time of 1.0 seconds have been achieved.