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

[Paul Koning]

> 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.

https://en.wikipedia.org/wiki/Galvanic_corrosion has some information.


	paul