Leave nothing floating in your product design

Article By : John Dunn

This simple precaution to be taken in the design of any product might not affect anything that happens during the product's lifetime, but could save future headaches or heartache.

Sometimes really long and large requirement documents can be boiled down to a couple of key ideas. Consider NASA Handbook 4002a, where we find the following diagram (on page 30).

NASA diagram about internal chargingFigure 1 This internal charging diagram can be found in NASA Handbook 4002a.

The first point is that any metal that has no return path to “ground” can build up a static charge, which might discharge to something else nearby and cause damage. The second point is that very low leakage dielectric materials can also accumulate static charge, which may result in unwanted and damaging electrostatic discharge.

Adding some annotation might make this a little easier to see.

annotated internal charging diagram for clarityFigure 2 This annotated diagram makes the key points easier to see.

Taking the simplest view of this, you the designer must not leave any metal item floating and you must cover as much of the dielectric of your circuit board as possible with ground plane.

Of course, you can’t have the entire dielectric area covered in ground plane, but how much you can leave open is illustrated for FR-4 dielectic material as follows.

graph of permissible area vs depth to ground planeFigure 3 Since you can’t cover the entire dielectric area this diagram shows how much you can leave open.

Of course, all of this is written with regard to spacecraft. However, my thinking is that what we find in the referenced handbook makes sense for ordinary use products as well because not every environment here on planet Earth is always benign.

We do get thunderstorms.

This article was originally published on EDN.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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