Computer-Controlled Optical Testing of High-Density Printed-Circuit Boards

The increased densities of multi-layer printed-circuit boards have required development of unique approaches to product testing. An optical automatic inspection system developed to test interplanes of the printed-circuit board used in the IBM 3081 processor (TCM board) is described. This system scans the features of the product and locates surface defects of 25.4 µm (1 mil) or larger through changes in reflectivity. It is capable of finding over 90% of the errors on subtractively plated printed-circuit interplanes, as well as shorts and opens of 50 µm or more on glass masters of printed-circuit boards. Alternative approaches to the inspection problem are discussed, together with the technical trade-offs which were made that led to the final system configuration. The tester theory and some hardware/software trade-offs are also covered.