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Drill Hole Optical Inspection Tool

IP.com Disclosure Number: IPCOM000045345D
Original Publication Date: 1983-Mar-01
Included in the Prior Art Database: 2005-Feb-06
Document File: 2 page(s) / 43K

Publishing Venue

IBM

Related People

Johnson, M: AUTHOR [+2]

Abstract

Inside walls of drilled holes, such as vias in multilayer ceramic chip carriers, may be inspected with a fiber-optic reflectometer.

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Drill Hole Optical Inspection Tool

Inside walls of drilled holes, such as vias in multilayer ceramic chip carriers, may be inspected with a fiber-optic reflectometer.

Increasing use of complex multilayer circuit boards, such as multilayer ceramic chip carriers, has increased the need for automatic inspection of drilled or punched holes. Typical defects which must be detected are bridged dielectrics and dielectric-coated conductors. Electrical techniques have significant disadvantages, since connections to unavailable conductor tracks often are desired and cannot be made. The subject technique completely circumvents such difficulties, and may be used at all stages of board construction, both before and after hole plating operations.

The reflectometer (Fig. 1) comprises a light source 10, a beam splitter 12, a coupler 14 and a length of fiber 16. Light coupled into the fiber is emitted, in one embodiment, from a 45-degree polished and mirrored fiber end 18, which is positioned within a hole 20 in circuit board 21 by the same data-file and coordinate table used for drilling the hole. Light reflected at greater or lesser intensity from the walls 22 of the hole recouples into the fiber, travels back along it, and is diverted by the beamsplitter to a photodetector 23. Freshly machined copper tracks 24 and glass-epoxy 26 may be distinguished (for example) because of greatly different reflectivities. By correctly choosing the wavelength(s) for the source, the signal to n...