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Ultrasonic Chemical Method of Cleaning Blind Holes in a Printed Circuit Board

IP.com Disclosure Number: IPCOM000088822D
Original Publication Date: 1977-Aug-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 1 page(s) / 11K

Publishing Venue

IBM

Related People

Lo, JC: AUTHOR [+2]

Abstract

Blind via holes in printed-circuit boards (and especially those created by laser drilling) can contain a significant amount of residual, charred material, with a particularly large accumulation being found at the bottom portion of the blind via. This material may be chemically inert, thus making the task of removing it a difficult one. In addition, debris is often melted on the surface of a copper pad which is located at the bottom of the blind hole and, if not removed, prevents plating of a satisfactory connection to the pad area.

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Ultrasonic Chemical Method of Cleaning Blind Holes in a Printed Circuit Board

Blind via holes in printed-circuit boards (and especially those created by laser drilling) can contain a significant amount of residual, charred material, with a particularly large accumulation being found at the bottom portion of the blind via. This material may be chemically inert, thus making the task of removing it a difficult one. In addition, debris is often melted on the surface of a copper pad which is located at the bottom of the blind hole and, if not removed, prevents plating of a satisfactory connection to the pad area.

It has been found that this charred organic material in the printed-circuit board reacts with chromic acid very rapidly under the influence of an ultrasonically induced liquid jet. The reaction is as follows:

(Image Omitted)

Epoxy + 2H(2)CrO(4) ultrasonic Cr(2)O(3) + MCO(2) + nH(2)O.

In a preferred usage, as in the case of cleaning blind via holes in epoxy glass circuit boards, a solution of 100 to 1000 grams per liter of chromic acid with an ultrasonic intensity of 50 to several hundred watts per square inch has produced an effective removal of debris. The required exposure time varies between 0.5 and 10 seconds of direct ultrasonic liquid jet. A typical ultrasonic frequency would be 20 KHz with a suitable diameter probe.

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