# RF design Stitching vias

Having no past experience on RF design i have recently taught that placing stitching vias it is a good practice. Reading the "definition" of stitching vias i am not sure that i can not distinguish stitching vias from common vias. The definition i have read is from Altium:

Via stitching is a technique used to tie together larger copper areas on different layers, in effect creating a strong vertical connection through the board structure, helping maintain a low impedance and short return loops. Via stitching can also be used to tie areas of copper that might otherwise be isolated from their net, to that net.

So my question is if there is a difference on functionality between the two types of vias?

--------EDIT---------

Thank you both for the replies. You are very helpful. My design is based on RN2483 package which includes a MCU and a RF IC. If you check RN's bottom layer it is obvious (or i make a guess) where the RF IC is located because there are stitching vias.

On my PCB should i place similar vias? Here is my current layout:

Now all the vias are connected to GND on top layer. Ignore the one that is overlapping the RFH trace. Now i will add a ground plane to bottom layer as well.

I am open to suggestions/corrections because as i stated is my first RF design and i lack of knowledge.

• The vias in your layout are not stitching vias because they don't connect to the copper fill around them. They're just floating copper that is likely to cause other RF problems. I don't use the same tool as you so I don't know how to fix it, but assigning these vias to be connected to the GND net is probably the first step. – The Photon Jun 20 '18 at 15:39
• You could continue with another question once you have what you believe to be a good start for your layout; reference this question and ask for critiques of the layout. We are happy to help those learning new things and making the effort. – Peter Smith Jun 20 '18 at 16:20
• The vias used for stitching should be connected to the net GND. Vias not connected to any net will be isolated from GND and from each other. For these vias no thermal pads should be used because they are not be soldered to part pins. Just as simple circular pad within a ground plane and nothing more. – Uwe Jun 20 '18 at 20:26

Perhaps pictures might help. The images below are from a design I recently completed.

The schematic:

This is a RF module designed to operate at around 870MHz, and the schematic shows functional connections, but as this is RF, the layout needs a bit more care:

Although the actual vias are the same size as the rest of them in this area, the ones highlighted by black lines are stitching vias and are not listed as functional connections in the schematic, but are really necessary here.

My rule of thumb for spacing is 1/10 of a rise / fall time for high speed digital and 1/10 of the fastest rate for a sinusoidal signal.

The track to the SMA at the bottom is a co-planar waveguide, where the spacing to the surface plane needs to be maintained, which is why stitching is really a necessity here.

• Have you built this PCB? Might you evaluate its RFI, its susceptibility to external fields? Might you have vias scattered around, every cm, and then progressively drill them out and re-evaluate the susceptibility performance? – analogsystemsrf Jun 23 '18 at 4:27
• I have built many like this, and because of the nature of this PCB (enclosed within a case - RF cable to antenna) the situation is relatively benign. Were it to be in a more harsh environment, then susceptibility would be a major concern. – Peter Smith Jun 23 '18 at 18:36

So my question is if there is a difference on functionality between the two types of vias?

No there isn't. If you would ignore all kinds of effects regarding RF, current capability and reliability. Sure, you could use one via to connect one copper area or layer to another layer.

However, this would be a bad idea as one via isn't really a very good and/or solid connection. OK, it is good enough for a signal connection but to properly ground a large copper plate using many vias is preferred. That means "via stitching", similar to how pieces of cloth are stitched together to form clothes, you can use vias to stitch together copper layers.