Electronic – Placement of Vias to Connect Ground Planes


I've been wondering a lot about grounding practices on PCB layouts. My first question about it deals with vias. I've noticed that on a simple 2 layer PCB with ground planes on both sides, there will typically be a few or several vias spaced out to connect them with minimal impedance between the two copper pours.

However, on an RF board the via placement looks much more deliberate and I'm wondering about the theory behind this. The vias connecting the ground planes often border the RF trace. See this differential coplanar waveguide example:
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I also have a second question about grounding on PCBs. When is it appropriate to "isolate" ground planes from each other? And how does having the ground planes on one layer (let's say top) isolated from each other help when both of those ground planes are connected to the same ground plane on the bottom through vias. When we have these isolated ground planes, does the via placement differ from either of the above cases?

Note: I'm aware of the possible duplicate here but I'm not satisfied with the answers and think my question asks for more detail.

Thank you for the information.

Best Answer

The layout you showed looks like what's called copper-backed coplanar waveguide (CBCPW). That means the ground return for the waveguide is not just in the coplanar grounds (the ground fills on the same layer as the signal traces) but also in the plane layer immediately "below" the signal layer. This structure is fairly esoteric, in the sense that I've only seen it used in digital systems when data rates exceed 20 Gb/s.

I found what looks like a reasonable discussion on the differences between CBCPW and microstrip in a Microwave Journal article by Rogers Corp engineers.

This article shows that the CBCPW has lower loss than microstrip at frequencies where radiation loss becomes important in the microstrip, roughly from 25 GHz and up, which explains why CBCPW is not widely used at lower frequencies.

Addressing your question, the article points out some special requirements for grounding vias in CBCPW structures:

For proper grounding, CBCPW circuits employ vias to connect the top-layer coplanar ground planes and the bottom-layer ground plane. The placement of these vias can be critical for achieving the desired impedance and loss characteristics, as well as for suppressing parasitic wave modes.

This basically means that without frequent stitching vias between the coplanar ground and the backing ground, power could be transferred to undesired propagation modes, which would cause either excess insertion loss or strong dispersion in the transmission line characteristics.