Distributed element filters/RF stuff solder mask

[darthtony]

Why do distributed element filters, and generallly RF stuff on a PCB have some sort of gold plating and no solder mask on them? https://en.wikipedia.org/wiki/Distributed_element_filter

[jeremy]

I can only guess, but the soldermask will change the average dielectric constant of the region, and thus will affect the trace characteristic impedance at high frequencies.

From my quick googling, see:

http://www.icd.com.au/articles/Stackup_Planning_AN2011_2.pdf
http://www.sigcon.com/Pubs/edn/PassivationandSolderMask.htm

Perhaps it is more difficult to control the thickness of the soldermask layer than it is the shape of the traces.

[darthtony]

That sounds reasonable, thanks!

[tggzzz]

Firstly it will be lossy, which also translates into reduced Q in filters.

Secondly the material will be variable, and the thickness will be variable, which translates in to frequency deviations.

[nctnico]

The soldermask isn't always left off. IMHO there is not much use in not having a solder mask because the etching tolerance will be as worse as the variation in the solder mask layer.

[KJDS]

The only reason to lose it is loss. I've never measured any other difference.

[Dago]

It increases loss and is not as controllable thus affecting the characteristics of the filter in a hard to predict way. At high GHz stuff it can also act as a waveguide and cause rather interesting problems.

[LukeW]

In addition to the facts posted above, it looks cool