Michael,
We have experienced unacceptable loss at 3GHz for 50 ohm
microstrip transmission lines with the standard Ni Au finish on
1/2 oz and 1oz copper using Rogers and Arlon PTFE based
substrates.
What we found OK was a cheat, we applied the standard Ni Au
finish to all the exposed pads, ie solder, test, contact, etc.
and we left the rest of the track bare Cu with the normal, thin
Probama solder resist coating. The solder resist had to cover
part of the Ni Au to ensure sufficient environmental protection.
The solder also adds to the attenuation budget so be sparing with
it. The higher the frequency the worse it gets.
As far as I recall, Ni also does some of its attenuation via a
lossy magnetic interaction with the EM wave.
This gave the microwave guys the performance they were after.
The detail I do not have but I recall that the losses went from
many tenths of a dB to just about 1/4 dB for about 450mm long 50
ohm track
Regards,
                Barry Bates

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