[TCML] plating

[Jim Lux <jimlux@xxxxxxxxxxxxx>]

These days, where you can get thin metal films evaporated on fairly 
cheaply, the cost of the metal is not a big driver, so you can choose 
for appearance..

Conventional plating is tricky on copper.  For instance, surface prep is 
really, really important, and you can't plate silver or gold directly 
onto copper, unless you want it to diffuse into the copper.  Typically, 
they do a "nickel flash" first, and plate over the nickle.

Nickle shines up nice, but does discolor with time.

Gold looks nice, and doesn't tarnish.  Silver is popular in RF 
performance applications, because silver is a great conductor AND the 
tarnish products don't have bad properties (AgO is a decent conductor, 
for instance).  But you'll never keep silver looking good, even if you 
lacquer it.. (the lacquer is permeable to ions to a certain extent, so 
eventually, it tarnishes under the lacquer).

OTOH, I'm always running across gold plated junk (spare chassis for 
equipment, boxes, etc.) at work (JPL) from 20-30 years ago, and it looks 
nice and yellow.  (We plate things with gold for spaceflight because it 
doesn't tarnish, it's solder-to-able, it has good thermal properties 
(emissivity/absorption), and it doesn't form a surface layer like 
aluminum, copper, or silver... ).  There's not a lot of gold content 
there, although it does lead to all those questions about "gold plated 
electronics".



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