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

[Tothwolf]

On Sun, 10 Jul 2016, Jon Elson wrote:
> 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.

Another thing to keep in mind here is that electrical current is being 
passed through the junction. Mixed metals greatly increases the potential 
for electromigration.

> 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.

Even in a reasonably good atmospheric environment weird issues can crop 
up. I once evaluated an air handler controller which had worked perfectly 
in product testing, but once field deployed, had a very high failure rate. 
It was made up of two pc boards with a pair of .100" pin and socket board 
to board interconnects. The two boards were physically held together with 
4 nylon snap-in standoffs. The lower board contained terminal blocks, 
modular connectors, and the power supply circuitry and the upper board 
contained the microcontroller, network circuitry, etc.

The cause of the failures turned out to be fretting corrosion of the board 
to board connectors caused by vibration. Another contributing factor was 
that many installers were not installing all 4 mounting screws when 
mounting the controller inside the unit (these were field retrofitted 
controllers) but were instead only installing 2 screws in opposite 
corners.

The fix was to replace all of the existing board to board interconnects, 
both the header and socket with parts that had 30 microinches of hard gold 
over nickel (the original parts had 15 microinches of gold) and to use a 
contact lubricant during assembly. Repaired boards were also to be 
installed using all 4 mounting screws. The vendor later redesigned the 
controller so it was all on a single board (while still admitting no 
fault, of course).