Connectors: Both contact surfaces must also be the same material?

[Jon Elson]

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