On 07/10/2016 12:07 PM, Paul Koning wrote:
On Jul 10, 2016, at 9:07 AM, Tothwolf <tothw...@concentric.net> wrote:
On Sun, 10 Jul 2016, Paul Birkel wrote:
Stated Tothwolf tothw...@concentric.net:
"Both contact surfaces must also be the same material or tin oxide will form on the
surface of the gold plating and cause a major headache. This was a serious problem with
486 and earlier Pentium PCs with 30 and 72 pin SIMMs and it led to a number of
lawsuits."
Almost every DEC System Unit ("backplane") that I've ever seen uses
tinned-contacts, yet the Modules all use gold-plated fingers.
I'm not familiar with them used in DEC systems in that way, but the problems
with mixing tin and gold plated connectors is well documented. Even the
connector manufacturers warn against mixing different platings.
While "don't mix contact surfaces" is sufficient, it isn't necessary. What matters is the
"anodic index" of the metal, or rather, the difference between those two values for the two metals
in contact. If that difference is large, you have a problem; if it's small enough, you do not. "Small
enough" depends on the environment; aboard an oceangoing ship the number has to be smaller than in an
office setting. I remember looking into this topic for an investigation of what types of contact platings
are acceptable for lithium coin cell battery holders in IT equipment.
This applies to bolted contact for structural things. Gold
connectors usually have light contact pressure to preserve
the soft gold plating. Tin contacts usually have higher
contact force to scrape the oxide off the tin surface. When
they are mixed, the tin can wipe onto the gold and then
allow oxides to form due to the lower contact force. Tin
contacts are supposed to provide enough pressure to form
gas-tight contact areas.
And, of course, when exposed to salty air, then everything
goes downhill REAL fast, corrosion galore.
In a salt environment, I'd use semi-hermetically sealed
connectors, and still expect lots of problems.
The Navy probably knows a LOT about these things.
Jon
