25 Jul
2012
25 Jul
'12
6:37 p.m.
On Wed, 25 Jul 2012, Chuck Guzis wrote:
On 25 Jul 2012 at 18:36, 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.
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.
Again, where you really tend to run into problems is when tin contacts are
mixed with gold contacts since the gold just isn't a hard enough surface
to break through the tin oxide. You can pre-clean tin contact surfaces and
then treat the connection points with something like Stabilant 22A, which
will help quite a lot (I've done this and it can work very very well), but
mixing gold and tin contacts is still just asking for trouble.
With those sockets, you have gold contacts
against tin plated IC leads.
This can cause all sorts of problems where the gold isn't hard enough
to break through the tin oxide. New tin oxide will also begin to
transfer to the surface of the gold contacts. We saw this a lot back
with 30-pin and 72-pin SIMMs when people would put modules with
gold-plated contacts in sockets with tin contacts.
Given that these are machine-pin sockets, I don't think that will be a
problem. Generally, you get a gas-tight connection with these. After
all, silver oxidizes/sulfides like crazy, yet there are acres of old
wire-wrap (silver plating over copper wire wrapped around gold- plated
pins) that are still working just fine.
Wiping-contact sockets, SIMMs, edge connectors, etc., I could see a
problem.