10 Jul
2016
10 Jul
'16
7:07 p.m.
On 07/10/2016 12:07 PM, Paul Koning wrote:
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
On Jul 10,
2016, at 9:07 AM, Tothwolf <tothwolf at concentric.net> wrote:
On Sun, 10 Jul 2016, Paul Birkel wrote:
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.
Stated Tothwolf tothwolf at concentric.net:
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. "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.