RF PCB ground plane surface finish

Question

I am designing two boards that are supposed to operate at frequencies 2.45GHz and 5.8GHz.

In my design, I use simple microstrip lines (not coplanar) on FR4 substrate.

There are some quarter-wave transformers in my circuit and I was wondering how to optimize the surface finish.

I decided to remove the silkscreen from the top layer to cover it entirely in ENIG gold finish as the area of traces is quite small. However, the bottom ground plane is almost solid and covering it in ENIG is going to increase my prototype production costs.

Should I cover it with a solder mask (there will be probably some additional losses) or bear the costs? The manufacturer does not allow raw copper finish, only soldermask, ENIG or lead/tin.

Board:

Answer 1

The important side of the ground plane is the one facing up toward the microstrip lines.

Coating the bottom with solder mask will have no effect at all on your RF performance. So the simple solution is just to do that.

Answer 2

Good for you for thinking about every detail!

With ENIG, the gold finish will be so thin that the nickel content dominates. Nickel is ferrous and quite lossy and thicker than skin depth I suspect. Just don't sweat the finishes.

There are low roughness foils you can order to smooth the underside, which can help losses but it's typically adds a good bit of cost and has a modest impact at 5GHz.

Unless you are REALLY focused on slight losses or reflections, just define the impedance of the lines and let the board house manage hitting impedances by ordering with impedance tolerance.

There are too many variables that you can't predict - metal thickness, etch profile, resin content, dielectric thickness, glass weave, etc

For loss, unless you have reason to be chasing a dB, the standard manufacturing flow will be fine.