I'm designing a 4-layer 6GHz RF PCB. The top layer is a signal layer and has the RF microstrip traces and the 2nd layer is an unbroken ground plane. I'm leaning against filling the remainder of the top layer with a ground plane stitched to the 2nd layer ground plane after reading a number of posts on this site indicating that it (1) isn't very useful when a full ground plane is used and (2) can create unintentional antennas. Additionally, since I'm using microstrips, I would need to keep the coplanar ground pour sufficiently far away which means that the total size of the copper pour would be significantly reduced.
However, I'd still like to use a via fence to prevent my microstrips acting like antennas and either disturbing nearby circuits or being disturbed by them. Are there any issues with using a via fence where the vias are not tied to a ground plane on the same layer as the microstrip? The vias would still, of course, be connected to the 2nd layer ground plane. I can't think of any reason why this would be an issue, and based on my understanding should be an equally effective EMI shield. Is this correct?
In a similar vein, is there any benefit to still keeping stitching vias in a \$\lambda/20\$ grid? The idea wouldn't be to stitch ground planes, but instead to act like a sort of general-purpose EM wave short for radiation from non-microstrip traces, or for any signals that managed to get through the microstrip via fence.
Best Answer
First off, depending on the plating you use and the size of your microstrip lines, it could be that you want to pour the top metal anyways. For consistent plating, you need to have even, constant density over the PCB. Usually when you have a PCB that has very low copper density on the outer layers, the manufacturer will either ask you to increase density, or they will use a peel-off copper layer that they place before plating and remove after.
The nature of the microstrip trace is that the current and magnetic fields are usually well-confined around the trace in question. It is hard to say that a via wall is going to help you here.
Usually you do need copper on the top where the via is, because the electroplating process requires a seed to start going. This is in part why you have an annular ring requirement. How far away from your microstrip will these vias be?