Silver is a poor choice, because of several reasons:
- The conductivity is not much better, so you're almost always wasting your money. Unless you're extremely space-constrained, it's cheaper to simply use more copper.
- When it does matter, it's usually concerning the underside, where current flow is predominant (i.e., GHz+ range RF losses), where you can't plate anyway. You'll save much more going from FR-4 to Rogers laminate in that case, too.
- Over time, silver diffuses into the copper, leaving a porous, less-conductive alloy on the surface.
- Silver tarnishes, so not only is its advantage lost over time, but the solderability fades quickly.
If a silver surface were required, it would be best achieved with a nickel layer first, which prevents the diffusion reaction. This would be worse for RF applications, because nickel is very lossy, even if it has a modest layer of silver on top.
ENIG is, literally, the gold standard in PCB fabrication. The electroless nickel (EN..) layer prevents diffusion, while the immersion gold (..IG) provides corrosion resistance and excellent solderability (boards retain solderability for... years?), and good contact (particularly for "hard gold" plating on edge connectors).
Solder dip (HASL) is probably the second best, since although the tin (or alloy) layer can oxidize, the relatively thick layer of clean metal beneath the surface allows plenty of opportunity for fluxes and solder to work, even on relatively oxidized boards. It's also easy to apply to hand made PCBs, tinning with the soldering iron.
Tim
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