On 23/10/2010 11:57, RussellMc wrote:
> Towards the other extreme is "polyurethane varnish" having the main
> advantages of being low cost, readily available, low cost, easy enough to
> apply and low cost.
>
In theory should allow slight flex or expansion as it's really an =20
elastomer?

The brush on much cheaper than spray. Aerosol might leave pin holes?

I've used polished copper + varnish as alternative to silver plating for=20
RF. Gold plate should be avoided for RF, esp. on PCBs as apart from=20
inferior conductivity to Silver or polished copper that's coated, they=20
often plate a barrier layer on the copper PCB first. Often this has=20
undesirable magnetic properties for RF. Tin Plate is bad for RF  also=20
(leaded or not).


Note that modern high speed logic is really wide band RF and thus gold=20
plating also might be very much poorer than coated copper, esp. if there=20
is a barrier layer. IMO Gold plate on PCB should only be used for mating=20
with gold plated connectors, elastomer LCD connector strips and direct=20
push buttons?

I found with Aluminium that ANY coating is a bad idea if it's fresh=20
exposed/just polished. Anodised is OK and it's OK if exposed (left out=20
in Irish rain for a couple of days). Otherwise polished raw aluminium=20
appears to "scab" at any pin holes or flaws in the coating with "flakes"=20
of oxide rather than the normal invisible film. Raw untreated (i.e. not=20
anodised or exposed then coated) aluminium outdoor grows "snow" and pits=20
badly, with rods and tubes easily cracking eventually.

Coatings are very useful. But metal needs to be treated appropriately.

What kind of varnish or lacquer was the WWII wireless sets=20
"tropicalised" with? It seems to look good many decades later.


--=20
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