Method and apparatus for electro-optically testing circuits

Abstract

Electro-optical in-circuit testing, especially of large circuits such as those assembled on printed circuit boards, is achieved in an automatic test system by disposing an electro-optical sensor in proximity to the circuit conductors, applying test signals to the circuit under test, and measuring an optical property of the sensor at selected regions thereof corresponding to internal nodes, i.e. test points, of the circuit. The sensor may be an optical probe, having a lens and a layer of electro-optical material which is adapted to be applied to the circuit. The electro-optical material may be either a polymer film or a crystal, the latter requiring a flexible coupling medium on the face applied to the circuit under test. The electro-optical material is provided with a reflective coating on one surface to facilitate a polarimetric measurement made transverse to the plane of the material. Another type of optical probe has a layer of electro-optical material with electrodes on the surface opposite the surface applied to the circuit under test. A polarimetric measurement is made by suitably biasing the electrodes and detecting light that passes through the material parallel to its surfaces. In another type of optical probe, the electro-optical material is coated on both surfaces, one coating being highly reflective and the other being semi-transparent. An interferometric measurement is provided by this type of sensor. Moreover, the polymer film may be applied directly to the printed circuit board during manufacture, in which case the lens is brought into contact with the electro-optical material during circuit testing.