# What is the metallic coating on these stranded copper wires?

The 5-colored wires's strands looks like aluminium, but it is basically copper wire at centre, with a thin conductive coating (my teacher shown it to me) of silvery-color, so that the copper doesn't get weathered .

If the rubbery insulator removed, and the metallic strands Scrapped with a knife or a shaving-blade, the silvery-colored layer removed, and the red, copper-interior comes out.

Now, i want to know, what substance(s) used in this coating?

These wires sold as 5-color electronic wire (for low voltages). I've given a scale also. Since nothing is printed on the insulator, and i buy in retail amount (1 or 2 yard) from a large spool, i've also no clue to the datasheet right now. (however, these wires are not very costly (around Rs. 10 per yard)).

The upper strands (with respect to photo) of the upper, yellow wire is scrapped with a blade and it shows red, coppery colour inside. the blue-one shown as reference or control.

• In which coating? Are you going to provide a picture or just leave it up to the community to imagine what kind of coating it could be? Please be specific provide links and pictures. – Voltage Spike Jun 3 '16 at 19:19
• Most metals have a silvery-gray color, so we'd be guessing. And if the wire conductor has been "tinned" then it will be coated with the solder alloy. The outer jacket of the wire insulation may have some manufacturer's markings that could lead towards finding a datasheet. Yes, there are datasheets for wire. – MarkU Jun 3 '16 at 19:25
• This question is useless wihtout a photo or a MUCH better description. – Richard Crowley Jun 3 '16 at 19:25
• WE have not the slightest clue what you are talking about. Without a photo, your question is completely dead and likely to be voted down. I have seen headphone and earbud very small cable that is made with enamel-coated "magnet wire" which has a plastic coating (vs. an extruded plastic insulation). Magnet wire was extensively discussed in your other question. – Richard Crowley Jun 3 '16 at 19:25
• @AlwaysConfused Then you should have asked that question tomorrow – fscheidl Jun 3 '16 at 20:48

What is it?

The coating is most likely tin plating.

The wire is similar to that used in Amphenol's Spectrastrip.

Figure 1. Amphenol's Spectrastrip.

Figure 2. The Spectrastrip datasheet lists the conductors as "tinned copper".

Why is it?

Figure 3. Despite severe oxidation her popularity remains untarnished.

Copper is a great conductor but is prone to corrosion - see the Statue of Liberty. Tin plating brings the following benefits:

• Corrosion resistance, including marine environments.
• At high temperatures (> 100°C) the corrosion resistance of copper declines.
• Soldering is easier as tin is a primary component in solder.
• Tin plating strengthens the copper wire underneath.

But ...

Steve Lampen, Belden, in his blog post In Defense of Tinned Copper makes some very interesting observations.

• Tin coating prevents copper from tarnishing. The green copper oxide is a semiconductor and is generally to be avoided in electrical connections.
• At high frequencies when skin effect comes into play the tin layer, if used, becomes more prominent. Tin has a resistivity 6.5 times that of copper ($1.1 \times 10^{-7}$ and $1.7 \times 10^{-8} ~\Omega m$ respectively) so having the signal predominately in the tin layer results in higher impedance. High frequency cables are not tin plated for this reason.
• Certain formulations of Teflon (PTFE) are very caustic and can cause oxidation of copper during extrusion. Having a tinned copper conductor reduces this effect. (Silver is an expensive alternative.)

In summary ...

• For low frequencies use tinned conductors for ease of soldering and resistance to corrosion.
• For high frequencies use bare-copper.
• Though only an inorganic-chemical test would be the right answer, common sense tells it is either tin or tin-lead solder. because some other substance, say aluminium or zinc would decrease the wire's solderability , and silver would make the wire too costly. see also en.wikipedia.org/wiki/Solderability – Always Confused Jun 5 '16 at 4:03
• Is Kynar similarly caustic? – Ignacio Vazquez-Abrams Jul 7 '16 at 20:48
• Never heard of it. What is it? – Transistor Jul 7 '16 at 20:54
• It's chemically similar to Teflon, and very commonly used for wire-wrap wire. – Ignacio Vazquez-Abrams Jul 7 '16 at 21:05

1) Because of the (somewhat new) RoHS standards, many wires are now nickel plated and cannot be soldered. You must use crimp connectors and headers for these wires.

2) The Reduction of Hazardous Substances act has changed the manufacturing process in almost every plant that uses wires in their products.

3) It has been a very expensive conversion process costing thousands of dollars, but it has settled in as the 'norm'. Those that must use tin/lead solder or a silver mix must declare it on the documents for that product, and labeled on the product.

4) Lead car batteries would be one example. Doppler radar front-end boards would be another. Pure silver plating is mostly used by the military along with Teflon insulation for higher currents in small gauge wires. I forget the mil-spec number for that but it exist. You can still buy tin/lead solder and tin-plated wire as a hobbyist for personnel use, or for in-house test equipment.

5) Be careful of taking cables out of old PC's and appliances for general use. If they are nickel plated you cannot solder them. Use must use crimps or acidic fluxes.

I found this link and PDF about nickel plated wires. Lots of details.

Nickel plated wires:
It is estimated that over 10 000 tones of copper wire are plated worldwide per year with silver or nickel. These plated wires are used principally for stranded conductors in high performance electric cable for the aerospace, airframe, defense, computer, telecommunication and professional electronics industrial sectors.

In addition, plated wire is used for high temperature cable, spark ignition leads and fuses. Nickel plated copper wires can resist temperatures up to 750°C. They are corrosion resistant and weld easily. Stranded conductors in this material are coated with suitable temperature resistant materials for cables. This coating process is requiring high temperatures, makes it unsuitable for silver plated wires which would oxidize.

A drawback with nickel plated copper is its reluctance to solder easily without special fluxes and the need to plate the nickel under carefully controlled conditions in order to give a pore - free and suitably ductile deposit for drawing.

It's almost always tin. Silver is plated on Teflon insulated wire and some others (rare), and silver has a little different look than tin. The Chinese have been known to use aluminum for shielding braid for things like USB cables.
I have never seen aluminum as a plating on wire, but it could be done. Lead-tin plating was once common on stranded wire for easy soldering back in the day when TVs were made in this country.

It could be aluminium, tin or perhaps even silver. If your teacher said that the purpose is to protect the copper core it could be aluminium as it conducts electricity really well but doesn't corrode because it forms a protective oxide layer. There's no definite answer with such little information though.

• seemingly not aluminium, because these wires are very well- solderable. aluminium would decrease solderability. en.wikipedia.org/wiki/Solderability. Silver would make high price. so it may be tin or tin-solder. – Always Confused Jul 7 '16 at 18:07

Adding a bounty to a bad question should not make it into a good question.

The coating can be different thing depending on the process/manufacturer (as others have noted). That being said there can only be one correct answer to this question, RTFDS (Read the Data Sheet).

(Outside of metallurgical testing, which is most likely outside of the scope of most of us.)

• Although I fundamentally agree with you, I'm not sure it was worth making an answer out of that. Or: "Making an answer from what would have been a relevant comment should not make it into a good answer". – dim lost faith in SE Jul 8 '16 at 12:46