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SPC vs. solid silver - Page 2

post #16 of 23

Some Facts About SPC Wire That Might Interest You

 

Teflon (Polytetrafluoroethylene or PTFE) insulated Silver plated copper wire was originally used by NASA in the 1960's and 70's for the space program. It was thought that it would be more suitable for carrying the high frequency/high current analog signals, some up to 450MHz, common to the avionics of that era. The wire had to be manufactured to very high tolerance with a minimum 2 to 4 micrometer thickness of the silver plating. Additionally, the copper wire had to be inspected microscopically to assure its smoothness and lack of surface defects prior to silver plating and after it was plated prior to the insulation stage, it had to be polished to assure that there were no external defects. The entire process had to be done in an environmentally controlled area where the relative humidity did not exceed 5% at any time. As in all MIL-SPEC projects, rigorous testing of samples of the product were conducted to insure compliance with standards. Yet, the failure rate of these SPC wires manufactured to exacting tolerances was unusually high. To make matters worse, the commercial aviation industry in both the USA and Europe followed NASA's lead and also began to extensively utilize PTFE insulated SPC wire in aircraft designed in the 1960's and 70's.

 

With numerous failures of the wiring system noted by NASA, an extensive investigation into the cause was initiated and the results were surprising. It was found that galvanic micro-corrosion of the wire was occurring at the copper/silver interface greatly increasing electrical resistance which lead to premature failure. Because the corrosion started below the surface of the wire at the Ag/Cu interface, it was not readily discernible upon visual inspection and hence had been missed during routine inspections. Concurrently, in the 1980's an increase in accidents and incidents prompted the FAA to more thoroughly inspect wiring harnesses in all aircraft carrying 30 or more passengers and they too noted "red corrosion" in many of the SPC wires. The solution for NASA was to discontinue the use of SPC wire and to switch instead to pure silver stranded wire in circuits that required long runs that had to carry high current/high frequency (>100MHz) signals, for the rest of the wiring pure PTFE insulated copper was used.

 

My father-in law was one of many  contractors for NASA and he produced some the wire at a plant in NJ, I got a lot of information from him. It is amazing all the stuff people save. He was able to show me the actual specs and even had spools of wire! He also gave me a spool of the Teflon insulated pure silver wire that NASA had them make for the Shuttle program. I doubt that the SPC wire currently being mass produced in China comes anywhere near the high quality that the U.S. companies produced in very limited quantities back in the 60's.

 

BTW, if you look up the skin effect of copper, you will discover that in order for the signal to get out to the 2 micrometer level, the point where the signal might actually be carried in the ultra thin silver coated layer of an SPC wire, it has to passing a signal at or in excess of 100 MHz. At audio signal frequencies the skin effect of copper is measured in millimeters so the 1 or 2 micrometer coating of silver would make no discernible difference, electrically or audibly and might even be detrimental if it was not produced under ideal conditions. For reference purposes, a human hair is 60 micrometers thick so the coating if it is anywhere near the 2 micrometer thickness that NASA demanded for their wire would be 1/30th the thickness of a hair!

post #17 of 23

That was an interesting and informative post.  However, I'm not sure we have a concern in our culture unless we start exposing our equipment to moisture in harsh or oxygen-rich environments.  Granted, the history appears to show that there were some panic incidents with NASA contractors, but I don't think our use of it is comparable.

 

The latest substantive material I've found is the RPCP - the Red Plague Control Plan, by R.M. Cooke of the Johnson Space Center (JSC) in 2010:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100009723_2010011529.pdf

Note that it certainly does not imply the banning of SPC wire in spaceflight.

 

The original, definitive document appears to be this paper published by P.L. Anthony and O.M. Brown in the March, 1965 journal of "Materials Protection:" http://nepp.nasa.gov/docuploads/16F59745-C53E-49B2-AFCDF883CFE54D64/RED%20PLAGUE%20mtrls%20protection%20pub.pdf

In this paper, the authors make it quite clear that even though the process is essentially galvanic (caused by breaks in the silver plating, the presence of moisture and oxygen are required to start that process.

 

Beyond that, it appears that the next equally rigorous attempt at investigating the Red Plague causality was done by B.D. Dunn, A. de Rooij, and D.S. Collins of the European Space Agency (ESA) in 1984:

http://nepp.nasa.gov/docuploads/97164203-A4FB-479A-A075C918D51DD81E/ESA%20Red%20Plague.pdf

I haven't read the entire article, but it's obvious that they are not discussing the banning of SPC wire in spaceflight, either.  In fact, they go further to discuss methods of properly controlling the quality of SPC wire from manufacturers and they discuss a rigorous testing procedure that immerses the subject wire in an environment of 100% humidity and a saturated oxygen atmosphere.

 

It should be noted that silver was originally selected as a coating for copper using PTFE insulation because the silver coating protected the copper from degradation during the elevated temperature process of the PTFE insulation.  Further, the silver offered increased conductivity even in the face of "normal" silver tarnishing, which still happens to be conductive.  NASA, the ESA, and military specs focus a great deal on the silver thickness of the coating.  This is because defects in the coating allow the galvanic corrosion process to take hold under the proper conditions of high moisture and oxygen - something not infrequent with spaceflight/missile environment regimes.

 

Interestingly, the Teflon itself is used (among other materials) because traditional PVC insulated wire outgasses so much in a vacuum that insulation weight loss is significant.  For that reason, traditional PVC wire is not even allowed in space flight applications.  Withstanding temperature extremes are important, but it seems the outgassing resistance in a vacuum predominates.

 

Anyway - as I said, it's an interesting discussion.  It appears from the articles that it has little applicability in home hi-fi.  Admittedly, that's my own opinion - but I'm certainly not going to hesitate to continue to use my PTFE-insulated SPC wire for hookup in all my projects. :)

.

post #18 of 23

I think you're missing my point, many of the problems associated with SPC wire were related to how it was manufactured, the thickness of the coating, the smoothness of the underlying  copper wire, moisture present at the time of manufacture, oxygen present at the time of manufacture just to name a few. Micro-corrosion increases the resistance of the wire at the Ag/Cu interface long before there is visible corrosion at the surface. That means that unbeknownst to you, your cables might be slowly degrading due to increased internal resistance within the copper cancelling out any perceived advantage of silver's lower resistance which BTW is 0.13 x10-8 Ωs or 0.00000013 Ω  lower than copper! While silver has a resistance of 1.59 x10-8 Ω, solder is 15 x 10-8Ω , and lead free 3% silver/tin solder is between 7-10 x10-8Ω, something to think about when you consider that the total resistance is calculated by all the resistance in the circuit including the soldered connections at each end of a cable.

 

Prior to using silver plating, they used tin and nickel to "protect" the copper however, that practice preceded the use of more robust insulating compounds such as PTFE. The article which you sited (and I was familiar with previously) stated, and I am quoting portions below:

 

"All problems were in the past and caused by poor wire manufacturing process controls.

 

Limited Life Article - 10 years from date of manufacture.

 

0.5 micron (-20 micro-inches): Easily damaged during manufacturing and ... 40 micro-inch silver is commonly used in spaceflight applications (equivalent to 1 micron). (The specification for the SPC produced in N.J. was 2 micron minimum or 80 micro-inch).

 

Low skin effect losses in high speed data and RF transmission applications (Again, skin effect becomes a factor when the frequency of the signal traversing the wire is in the Megahertz range)

 

 No tin whiskers was one of the main benefits touted by NASA when silver replaced tin plating. 

.

WHAT MECHANISMS ARE REQUIRED TO INITIATE RED PLAGUE?

 

Mechanical damage resulting in exposure of the copper-silver interface.

 

• Wire manufacturing (i.e.- drawing, stranding, application of

insulation jackets, etc.)

 

INADEQUATE COATING THICKNESS

 

Porous, discontinuous, and thin silver coatings are more likely to develop Red Plague since a

greater number of sites for galvanic cells to form are possible.

 

• 0.5 micron (-20 micro-inches): Easily damaged during manufacturing and handling

• 1 micron (— 40 micro-inches): Good flight history, provided procurement and environmental controls used.

• 2 micron (— 80 micro-inches): Improved resistance to handling /environmental damage and corrosion.

 

Promoted by the presence of moisture (H 20) and oxygen (02) at an exposed copper-silver

interface. (usually placed there during the manufacturing process or at the ends of the wire).

 

— Exposed conductor end (crimp terminations)

— Poor plating quality control (pin-hole, porosity, thin coating)

— Mechanical damage during stranding or handling (scratches, nicks, abrasion)

— Corrosion (chemical, atomic oxygen, silver migration)

— Wicking of moisture, oxygen, flux residue, solvents getting under the Teflon jacket"

 

Given the inherent problems associated with SPC especially the rigorous manufacturing precautions required to do it right coupled with the fact that the ultra thin coating doesn't even come into play when the frequencies carried are in the audible range 20 - 20KHz with a ceiling extending to at least 1 MHz, why use it? Does the cheap SPC wire sold on eBay sourced from China go through all the steps required to assure that the underlying copper is defect free and microscopically smooth prior to electroplating. Are there multiple sample tests done to insure uniformity of the coating and is there a guaranteed minimum thickness certification provided. Is it at least 1 micron? If you want the benefits of silver, buy silver wire. Moon Audio sells Silver Dragon V2 interconnect cable (4 conductor with shield) for $18.00 a foot, it is extremely well made in the USA, flexible, easy to work with and sounds wonderful. IMHO.

Thanks for your comment and the links to the articles. Knowledge is power.

Art

post #19 of 23

It's quite simple why to use it: it works and the insulation doesn't draw back upon soldering.  Whether it has any usefulness for headphone cabling, I don't care - that's someone else's problem.

 

Neither NASA or the ESA have discontinued use of the product and again, I reiterate - even quotes about "life of the product" have to do with the environment within which it's spec'd.  10 years in a spacecraft environment could be a lifetime for us.

 

China vs. US manufacture is also another story altogether and that's not exclusive to wiring.


Edited by tomb - 11/17/13 at 12:57pm
post #20 of 23

I probably shouldn't have said "someone else's problem."  If someone uses it for headphone cabling and they think it improves things, then fine.  Worrying about corrosion in our environment is simply not worth the effort, IMHO.

post #21 of 23

NASA did replace a majority of the wire with pure silver stranded wire. How do I know, because Alpha Wire who was one of the main suppliers and that is where my relative worked as a supervisor discontinued making SPC for NASA entirely and instead provided thousands of miles of pure silver, yes, miles not feet.

 

Again, the majority of problems associated with the wire had to do with it's manufacturing not it's being used in space where by the way there is no humidity. According to NASA and the FAA, if it just sat in a spool on a shelf under ideal storage conditions of controlled temp and humidity, it is unfit for use after ten years, that's why they date every spool and that is for product produced in the US under ideal conditions to exacting MIL-SPEC. The insulation doesn't draw back due to the insulation being  a synthetic fluoropolymer with a melting point in excess of 600ºF and has nothing to do with the silver plating below. If it makes you happy to use it and you think it sounds better due to it's lower resistance than copper .00000013 Ω, by all means enjoy it. My purpose was to inform.

post #22 of 23
Quote:
Originally Posted by Art inTampa View Post
 

NASA did replace a majority of the wire with pure silver stranded wire. How do I know, because Alpha Wire who was one of the main suppliers and that is where my relative worked as a supervisor discontinued making SPC for NASA entirely and instead provided thousands of miles of pure silver, yes, miles not feet.

 

Again, the majority of problems associated with the wire had to do with it's manufacturing not it's being used in space where by the way there is no humidity. According to NASA and the FAA, if it just sat in a spool on a shelf under ideal storage conditions of controlled temp and humidity, it is unfit for use after ten years, that's why they date every spool and that is for product produced in the US under ideal conditions to exacting MIL-SPEC. The insulation doesn't draw back due to the insulation being  a synthetic fluoropolymer with a melting point in excess of 600ºF and has nothing to do with the silver plating below. If it makes you happy to use it and you think it sounds better due to it's lower resistance than copper .00000013 Ω, by all means enjoy it. My purpose was to inform.

Yes - a warehouse full of un-used wire was mentioned in one of my references.  It's only a guess, but I suspect NASA's first reaction to the problem, when noticed, was to suspend use of the wire from certain contractor(s).  That's the way it works - it doesn't mean SPC wire was banned altogether, it may only mean that pure silver wire was all that was acceptable from that contractor.  Or, it could be as you say, that the SPC wire that contractor provided was found unsuitable for a very specific application.  The references confirm, however, that SPC wire is in continuous use at NASA and the ESA - despite your saying that it was banned outright.

 

There is plenty of humidity in space vehicles - especially in oxygenated environments.  That doesn't even count the LOX in the rockets and condensation while it's sitting on the ground.  There's plenty of wiring throughout the vehicle - from the ground up.  Just the rings connecting stages and boosters are packed with wiring harnesses and electronics.

 

This is unproductive, though, so I'm not going to persist in this.  Anyone can now read the posts and references and come to their own conclusion.  I simply disagreed with your "sky is falling" attitude concerning PTFE Teflon wire - especially in home high-fi. :)

post #23 of 23

Thank you for your comments.

 

To set the record straight, I never in any of my posts indicated that I had any issues with PTFE Teflon wire.

 

And no I don't think the sky is falling, that is your interpretation of my post, I merely laid out the alleged benefits of SPC wire in audio applications citing basic physics vs the risks.

Increased resistance in an audio cable is unlikely blow up your headphones your amp or your house, it would probably just degrade the sound quality making it's use of questionable efficacy. :)

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