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

[Tothwolf]

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.

Another problem that can occur is because a gold surface is softer than 
tin oxide, if the header pin is tarnished, mating the gold plated contact 
with the header will not break the surface of the tin oxide. Tin oxide is 
not a good conductor.

> How does that situation jibe with the SIMM issue/experience?

Somewhere around here I still have some 4MB 30 pin SIMMs with gold plated 
contacts that were used in tin plated sockets for several years. The 
damage/contamination of the gold plated SIMM contacts is significant.