On 25 Jul 2012 at 20:37, Tothwolf wrote:
.> With wirewrapped connections, the contact area between the wire
and
the pin isn't the flat surface area of the square
pins, it's the
corners of the square pins. Properly wrapped connections will embed
the corners of those pins into the wrapped wire. Those contact points
are gas-tight and while the outer surfaces may tarnish, the
connections are still very, very good.
Take a good look at machine-pin sockets--the contact area is a round
hole that contacts the edges of the rectangular IC pin. So the
connection is gas tight. I doubt that the gold enters into the
equation much, as we're talking about a few microns of plating over a
very hard beryllium copper alloy over a very small contact area.
The problem is with tin plated contact surfaces
though. Gold and
silver contacts are generally compatible, and while silver will
tarnish, silver oxide is still conductive, so silver plated contacts
are generally not a problem anyway. Even with machined pin sockets,
those contain tiny wiping contacts down inside the barrels. Machined
pin sockets are still much better made than the cheap poorly made leaf
contact sockets, but at least with the better quality leaf contact
sockets, both surfaces are tin and the surface of any tin oxide will
be broken when plugging the IC into the socket.
Oxidation of silver happens very slowly and is generally not an issue-
-that ugly black stuff that forms on your cherished flatware is
silver sulfide (Ag2S). That's why silver tarnishes so much faster
when there's air pollution around. Silver sulfide is between 8 and
10 orders of magnitude more resistive electrically than pure silver.
With wiping contacts, such as a SIMM or DIMM, or PCB edge connector,
you do have problems with dissimilar surfaces. But I've yet to see a
gold-plated machine-pin socket fail.
--Chuck
.