Trimming Conductors in Very Tight Places

Anyone familiar with high-frequency and microwave-printed circuits understands the sensitive design considerations each requires. Occasionally, you may need to reduce a circuit or conductor width when a pad or conductor violates space requirements.

This type of modification is an extremely delicate operation requiring not only the hands of a surgeon but may require the use of customized tools modified specifically for the application.

Recently, we were presented with such a challenge. We were asked to remove a pad section to provide the specified separation of the pad from a conductor running close by.

We normally would make such a cut with a standard carbide cutter and milling machine, but controlling the depth of the milling would have been difficult on this particular project.

Using a common precision knife to cut and remove the section was the method we used but with a twist. We modified the knife blade and placed it into a milling machine. (See Figure 1).

A special fixture to support the board was developed. The modified blade was placed in the knife, and the knife was installed into the collet of the milling machine. 

The milling machine was never turned on; it was used to precisely control the knife's movement. You might expect we would cut completely through the entire pad depth on the first pass - not so! If we did, we might have cut too deep, causing a "crazing" of the base board material and, thus, damage. Even the slightest penetration of the base material beyond the depth of the pad may have caused damage.

Thus the milling system was set to cut three-fourths of the way through the pad. The final pad material was removed by hand, allowing for a more controlled removal without damaging the adhesion of the remaining pad. Modifying standard knife blades is not difficult and can be accomplished using grinders and files.

This is a special modification calling for special tools and fixtures. A milling machine or similar precision coordinate systems is also needed.