Patent US 5,455,870

An automated electronic module inspection system having illumination sources that produce digitized gray scale images that are then analyzed by a computer. The inspection system illuminates the module including solder joints, components and printed circuit board using a sequence of illumination that produce visual images which are used to identify defects. The inspection system inspects for all defects as specified in MIL-STD-2000A (Section 4.4) by using both surface reflectance properties as well as topological properties of objects. The system comprises two video cameras. A positioning table aligns a module-under-test with the optical axis of each camera thereby enabling the same field-of-view to be observed under different lighting conditions. A dark field illumination associated with a first camera preferentially illuminates non-flat surfaces, but not vertical surfaces. A bright field illumination associated with a second camera preferentially illuminates relatively flat surfaces. The flat-surface illumination light source is collimated with the optical axis of the camera using a conventional partially reflecting angled mirror that reflects the light along the camera's optical axis onto a circuit board of the module. The angled surfaces are illuminated using four ring-lights concentrically aligned with the optical axis of the second camera. The images observed by the camera are then analyzed by inspection programs for discrepancies between image model attributes and the images, and such programs provide compensation for nonuniformities of the light sources and cameras and compensation of shadows resulting from the dark field illumination light source.