Damage-Free Pad Removal -- A proven method of circuit removal on modern base material.

A steady proliferation of high-frequency, radio frequency (RF) and microwave printed circuits, generally used in the communications, automotive and aerospace industries, is underway. Anyone familiar with these products understands the sensitive design considerations each requires. Occasionally, operators may encounter the need to reduce a circuit pattern size, or remove pad material, when a pad or connection is violating space requirements with an adjacent circuit. Modification is an extremely delicate operation requiring not only precise and steady manual dexterity, but the use of customized tools modified specifically for the application.

Recently, company operators were presented with an RF challenge. The operators were asked to remove a percentage of a solderable pad to provide the specified separation of a land from a circuit running close by. The job would require cutting or scoring through the pad's tin or surface metallization. Then, the material to be removed would be gently shaved off. When performed properly, the difference between the printed circuit board (PCB) laminate area adjacent to the cleared area, and the cleared area itself, should not be visible. The laminate would still be sealed and maintain physical integrity, and the pad material would be cleanly removed without compromising the adhesion of the rest of the pad to remain attached.

Modification options included using a precision drill system, to cut the circuit with an end mill or a hand-held micro drill or to cut the circuit with a ball mill. Any of these methods might accomplish the foil separation, but could possibly create other problems. Also, the chosen method would most likely create some level of impression in the base material underlying the circuit. Depending on the depth, an impression may require a minor repair or a touch-up step.

Using a hand-held precision knife blade to cut, and then lift the circuit material is another option. The underlying base material would likely be impacted and the inconsistency of the technician's touch would become a factor. Some ingenious rework technicians might use a hot soldering iron tip to achieve the delamination of the target area. Others might score the circuit material with a knife blade and then use the heat of the iron concurrently, with the lifting blade, to delaminate the excess circuit material. Such an approach is not preferable, due to the potential for burning of the board or contamination with solder from the iron's tip.

As a solution, special tooling was created to improve operator efficiency and eliminate cutting into the board's base material. A special fixture for scoring the pad surface, and to support the board, was developed. A modified hand-held knife blade was clamped to a press to allow the blade to score the circuit pad (in a precise position) while controlling the depth of the cut to prevent base material disturbance. Some would expect that the initial scoring or cutting should penetrate the entire depth of pad metallization-not so! In doing so, one can cut too deeply, causing a "crazing" of the base board material, and, thus, damage. Even the slightest penetration of this brittle material, beyond the depth of the copper foil, will cause damage. Conversely, cutting three-fourths of the way through the foil still allows the target material to be cleared away easily-without damaging the adhesion of the pad to be kept in place.

With a second modified knife blade, shaped to the size of the removed circuit, an operator could then peel the scored pad material cleanly off the board surface. The use of an unmodified blade, overhanging uninvolved board material, could scratch adjacent mask and cause board defects. In the case described here, the modified blade was a similar blade with a reduced cutting edge (Figure 1). Modifying blades is not difficult and can be accomplished using small metal files available from any number of tool and craft suppliers.

Following the removal of the material, depending on the customer's specifications, the base material surface may be sealed with epoxy, although doing so is rarely necessary.

Jeff Ferry is CEO of Circuit Technology Center, Haverhill, MA; [phone]; www.circuit techctr.com.

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