13 Mar
2000
13 Mar
'00
4:35 a.m.
Please see embedded comments below.
Dick
-----Original Message-----
From: allisonp(a)world.std.com <allisonp(a)world.std.com>
To: classiccmp(a)classiccmp.org <classiccmp(a)classiccmp.org>
Date: Monday, March 13, 2000 11:12 AM
Subject: Re: Red Erasors
> PCB's were gold-plated. Since the eraser
always left a clean gold plated
> edge connector, I quickly concluded it was dirt and not corrosion that
was
> accumulating at the interface between PCB and
backplane. I found much
less
> of this occurring in clean environments where dust
didn't accumulate in
the
Actually it can be corrosion. Good gold plated fingers are gold over
nickel over copper and have excellent resistance to wear, corrosion and
metal migration. Many however cheap out and do gold over copper, very
bad. This gold over copper tends to have problems with the copper ions
migrating to the gold surface and turning a bluish-green.
Being "chromatically challenged" (can't tell either blue or green from grey
or pink) to such extent that the corrosion or dirt was just a dark smudge, I
never considered that it might result form low quality boards.
Thanks for explaining this.
Attacking that
with abrasives or solvents are temprorary solutions at best ans the
copper ions will continue to migrate. This is why the layer of nickel is
needed to keep the gold clean. This also depends on the connector being
gold/nickle/copper as well or the metal migration happens from the
otherside (connector). Adding to that boards with tin(solder) plated
connectors and you can effectively posion the connector system. My altair
and later replacement WAMCO backplanes suffered this fate from the mixed
plated connectors and boards. It showed up after about two years in the
slightly salty humid LongIsland air, symptoms were boards must be pulled
and plugged back in before the system would run if powered off for more
than a few days. I would have to use goldwipes and M50 solvent every
few weeks to make it only somewhat stable. Even a film of silicone oil
only helped somewhat.
Oh, add oil vapor (machine shops), tobacco smoke or other pollutants and
the reliability and surface accumulations can be far worse.
Allison