Dumb question about PCB surface finishes

[DenzilPenberthy]

This seems like a really beginner level question but I cant find an obvious answer...

Typical PCB surface finishes like ENIG, Immersion silver, Electrolytic Ni/Au etc etc... are they usually applied to the whole board before solder resist is applied or are they applied to just the pads after solder resist.  So are the traces under the solder resist always bare copper or would they have the Ni/Au/Ag/whatever on them?

I'm dealing with an issue related to high frequency boards which need to be gold wire-bondable so I'm wondering about the pros and cons of having nickel all over the place...

Maybe there is no 'usual' and some will plate before solder resist, and some after?

Cheers!

[Siwastaja]

Just pads, after solder resist.

In the past (80's and before), sometimes HASL tinning was applied before the soldermask, but this often resulted in cracked soldermask either due to uneven tinning to begin with, or during wave solder when the tin layer melted below the soldermask.

ENIG, on the other hand, is quite expensive, so pads only makes more sense anyway.

[In Vacuo Veritas]

Yeah, there is a process called SMOBC, Solder Mask Over Bare Copper. You'll find that under the mask it's just bare copper.

And as for "high frequency", as usual, one man's "high frequency" is another man's rounding error. What frequency? You'll find that the foil roughness UNDERNEATH can be more of a problem than the surface finish.

See:

https://www.rogerscorp.com/documents/749/acs/Copper-Foils-for-High-Frequency-Circuit-Materials.pdf

[DenzilPenberthy]

Thanks!  In the grand scheme of things, not very high. Up to 1GHz probably.  I think the compatibility with the gold wire bonds is going to be our main thing to consider.

[In Vacuo Veritas]

Thanks!  In the grand scheme of things, not very high. Up to 1GHz probably.  I think the compatibility with the gold wire bonds is going to be our main thing to consider.

Like I said: DC. 

[JS]

Thanks!  In the grand scheme of things, not very high. Up to 1GHz probably.  I think the compatibility with the gold wire bonds is going to be our main thing to consider.

Like I said: DC. 

For me it's µwaves 

JS