Silver plated wire

[WattsThat]

Over in another thread, langwadt posted a link (#35) to a NASA workmanship webpage on discrete wiring.

https://workmanship.nasa.gov/lib/insp/2%20books/links/sections/302_jumper%20wires.html

In several places, the text refers to bodge wires in the following way:

Jumper wires are usually solid, insulated copper conductor with tin/lead plating (i.e.: wire wrap wire), although jumpers less than 25mm (0.984 in.) may be uninsulated, provided the jumper is not liable to short between lands or component leads. Silver-plated and/or stranded wire shall not be used.

I find that statement to be contradictory or at best confusing because I have always believed that proper wire wrap wire is silver plated. Any reference I can find online, Wikipedia included (yeah, I know), states that wire wrapping wire is silver plated. Why does NASA specifically prohibit silver plated wire in this application but then confuse things by calling tin plated copper wire wrapping wire?

I doubt it’s solderability but obviously there’s an issue. Is it a concern over the red plague a.k.a. cupros oxide? If that’s true I would expect silver plating wasn’t allowed anywhere. All of the TFE insulated wire I’ve ever encountered as been silver plated. Did NASA have their own exceptions to wire standards? Call me puzzled and confused!

[TimFox]

I don't know why silver-plated solid copper wire is not allowed for this purpose.  They also proscribe stranded wire.
Teflon-insulated stranded wire is never tin-plated, and usually silver plated, since the temperatures involved in applying the Teflon insulation would melt the tin or solder plating, and render the stranded wire rigid.

[penfold]

Does the NASA doc refer to wire-wrap wire as being silver plated? Its best not to mix standards, the NASA stuff is build around a whole internal eco-system of standards and processes which will have been validated. 90% of the limitations they apply to practices that would be perfectly normal outside of NASA are probably just to avoid unnecessary paperwork in process validation and raising concessions against rework jobs.

With the previous in mind, I would imagine the use of tin-lead plated wires is to prevent contamination of a pre-made solder joint with another metal, not that adding a little silver to one of the joints would be a bad thing per say, it would just invalidate the process validation for the soldering job that's already been done.

The further annoyance is that they're probably just using the term wire-wrap wire to suggest the typical kind of wire gauge, possibly something to do with purity and hardness... that NASA doc doesn't state insulation requirements does it? So it may not be TFE... and consider the heritage of these standards, its probably referring to a wire standard that was much more common back then and its simply easier for them to have batches of it custom made to their own spec than rework the standard to not only include a common 'modern' equivalent but also still apply to all pre-existing work done to said standard. (I've never heard it said NASA were cost-conscious!)

[WattsThat]

I don't know why silver-plated solid copper wire is not allowed for this purpose.  They also proscribe stranded wire.
Teflon-insulated stranded wire is never tin-plated, and usually silver plated, since the temperatures involved in applying the Teflon insulation would melt the tin or solder plating, and render the stranded wire rigid.

Thank you Tim, this makes sense on TFE as it is ram extrusion, not a purely thermal process as used for thermoplastics.

I didn’t mean to imply that wire wrap wire was TFE, I was thinking and writing at the same time about the silver plated wire I’ve ever encountered, wire wrap and TFE insulated is the only place I’ve ever seen it.

Since MIL spec wiring is commonly stranded TFE, it doesn’t make sense the restriction is due to the concern of red plague since the same corrosive environment requirements would apply that equipment. Curious.

[JohnnyMalaria]

This long and detailed paper may provide answers:

https://elsmar.com/pdf_files/Corrosion%20of%20Silver-Plated%20Copper%20Conductors.pdf

A couple of things that caught my eye:

a. Poor plating and ingress of water into the wires causes a potential red-plague
hazard.
b. The silver surface may react with substances such as glycol to produce a potential
flammability hazard (recent studies have also shown this reaction to occur with
nickel and tin conductors).

[WattsThat]

Wow, the mystery continues. From the introduction:

These wire and cable materials are those most commonly selected for use in ESA's satellite and manned spacecraft projects. Silver-plated copper wire is also used extensively for military and aerospace applications in both Europe and the United States.

That was great reading, thanks. But, I'm still curious what NASA’s beef is with silver plate, at least as it applies to bodge wires. Gotta be an explanation somewhere. Maybe written in blood, as are most rules in aviation. Manned spaceflight no doubt adds some possibilities.

[CatalinaWOW]

I can't speak for NASA's concern, but in my defense industry job silver plated wire was eliminated some time in the 1990s-2000s.  Reason was red plague.  Red plague can totally eliminate electrical connection.  As I understand it the silver is not consumed in the reaction so it proceeds until all the copper is gone, converted to a non-conducting oxide with no mechanical properties to speak of.  No one at the time knew how to eliminate the potential.  Careful handling and inspection were the general precautions recommended, but neither process is leak proof.  In defense and aerospace failures are written in blood as a previous poster noted so even though the demonstrated failure rate was low the chance was eliminated.  Same reason those industries haven't gone lead free for fear of tin whiskers (or at least hadn't when I retired a decade ago).  In the space world cost of putting something in orbit requires the same conservative approach. 

I don't see why red plague wouldn't be an issue for wire wrap wire used as a jumper (at least until the thing gets on orbit and no longer has oxygen to fuel the red plague reaction) so I am not surprised at the NASA position.

[TimFox]

I had not heard of “red plague” before.  What circumstances are required for it to happen?
Silver-plated wire crimped or wire-wrapped to copper or copper alloy?
Silver-plated wire soldered to copper or tinned copper with tin-lead solder?
Silver-plated wire soldered to copper or tinned copper with silver-tin-lead solder (as in Tektronix vacuum-tube ‘scopes)?
Other?

[CatalinaWOW]

From Wikipedia

Red plague is an accelerated corrosion of copper when plated with silver. After storage or use in high-humidity environment, cuprous oxide forms on the surface of the parts. ... It is an electrochemical corrosion—a copper-silver galvanic cell forms and the copper acts as sacrificial anode.

So any nick in the silver plating along with humidity and presumably some ionic contamination. 

Nicks from wire stripping are an obvious problem, and one reason NASA and other industries rejected mechanical strippers.  Or in your examples silver plated copper wire mechanically connected to copper fittings.   

[Benta]

I suspect the reference to wire-wrap stems from the fact that a 1:1 alloy match (lead/tin wire and lead/tin pins) is desirable, as the wire-wrap connection is not just mechanical pressure, but in fact a cold weld between the wire and the sharp edges of the wire-wrap square pins.

[David Hess]

I didn’t mean to imply that wire wrap wire was TFE, I was thinking and writing at the same time about the silver plated wire I’ve ever encountered, wire wrap and TFE insulated is the only place I’ve ever seen it.

I have also always seen silver plating used with wire wrap wire and Teflon insulated wire.  Wire wrap wire commonly uses Kynar (polyvinylidene fluoride) insulation.