Does Copper Pour on a Signal Layer Decrease Signal-To-Signal Isolation?

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Does putting a ground pour on PCB signal layers make the isolation better or worse? It can go either way, but with the proper knowledge and application, this technique will improve your designs.

In this article, I’ll discuss how to simulate trace-to-trace isolation with true electromagnetic simulation software. We’ll also cover a variety of rules of thumb that can help you stay away from trouble.

Fact or Fiction?
Recently an acquaintance told me, “I have heard that putting a copper pour on a signal layer between traces actually makes the isolation between the traces worse.” I grabbed one of my RF boards and said, “If that is so, then how do all these RF boards that I have done with co-planar waveguide over ground manage to function? They all have copper pours on the signal layer, and they work to very high frequencies.”

Since co-planar waveguide over ground (CPWG), which is essentially “pouring copper on a signal layer,” is used for a lot of RF work, and is proven to work for very high-performance RF circuits, how did this contradictory opinion catch on in the industry?

To investigate this, I used a one-inch section of 50-ohm microstrip consisting of an aggressor trace from ports 1 to 2 and a victim trace running in parallel from ports 3 to 4. I used typical values for the dimensions as might be on a real PCB. The trace width is 20 mils, with a spacing of 60 mils from center to center, over an FR-4 substrate, 9.5 mils thick, with a modeled Er of 4.4.

To read this entire article, which appeared in the March 2022 issue of Design007 Magazine, click here.


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