A method for automatic inspection of printed circuit boards

Abstract

An algorithm for automatic inspection of printed circuits is discussed which includes elements of the reference-comparison and the design-rule methods. First a directed graph, called the segment graph, is created from the image of the PCB. Construction of this graph involves comparison of adjacent rows, and hence each pixel is examined only twice in this construction. Feature points, including wire intersections, wire ends, and circuit defects, which uniquely represent the PCB image are extracted from the segment graph. In this way a very compact representation of the image is extracted. The feature points of the board being tested are then matched against the feature points from a reference board. Simultaneous with feature point extraction, the design-rule checking algorithm performs wire width and spacing inspection, and marks the unsatisfactory regions. Results of the algorithm implementation are demonstrated.