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Optical Scanning Device for Imaging Interior Surfaces

IP.com Disclosure Number: IPCOM000046113D
Original Publication Date: 1983-Jun-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 83K

Publishing Venue

IBM

Related People

DiStefano, TH: AUTHOR [+2]

Abstract

Detection of smears of copper in repair holes drilled in LSI (large- scale integration) chip multilayer circuit boards to reduce interplane short circuits is accomplished by a focussed optical scanning system which images and detects the smear of copper.

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Optical Scanning Device for Imaging Interior Surfaces

Detection of smears of copper in repair holes drilled in LSI (large- scale integration) chip multilayer circuit boards to reduce interplane short circuits is accomplished by a focussed optical scanning system which images and detects the smear of copper.

A significant reliability problem with buried wiring, in multilayer LSI circuit packaging boards results from damage caused during repair of defects by the use of insertable connectors. Holes are drilled through the board into which the connectors are inserted.

The holes may cut through buried copper conductors, exposing the interior surface of the hole to unwanted circuits. A plastic sleeve insulates the insertable connector from these conductors. However, copper smeared from one conductor to another may cause a short circuit to be produced. If short circuits are caused between parallel power conducting planes in the multiple layer boards the results can be quite serious. Thus, inspection of the boards after insertion to find a copper smear during the repair operation is an important function.

An optical system for scanning the interior wall 9 of the drilled hole is shown in Figs. lA and lB. The optical scan is performed with a rotating optical fiber l0 (Fig. lA) containing an integral optical focussing element and a mirrored beveled facet ll which reflects the focussed beam onto the sidewall 9 of the hole. Light reflected from the focussed spot retraces the optical path through the fiber 10, including the integral focussing elements, back to a beam splitter 12 (Fig. 2) where it is directed through lens 13 onto a detector 14. The output of detector 14 (Fig. 3) is amplified (15), filtered (16), and electronically stored (17) at a location determined by the axial and the azimuthal position of the fiber 10. This stored image is used to form an image on a CRT screen 18, for example. For a fiber rotational velocity of 1800 rpm, a 1200 line image can be formed in less than one minute. The effective NA required of the fiber 10 in order to obtain 0.2 mil resolution (l200 lines in 0.240 in) is about 0.16; this can be accomplished with a graded index fiber...