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!