In Defense of Tinned Copper

Oxidized-Copper-Statue-of-LibertyI spend a lot of time in the fancy cable section of Belden. These are cables for high-frequency applications, such as digital video coax, or 10-gigabit data cables. These cables all use bare copper conductors. On the other hand, if you look at the Belden catalog from the 50,000 foot view, you will see a whole lot of cables we make use tinned copper. If you’re an ancient engineer or installer (like me) you came from a time when tinned copper conductors were pretty much everything. Products like 8451 and 9451 and almost any analog audio cables are all tinned copper. Go back to the coax cables from long ago, and many of them are tinned copper. So what happened? And why is Lampen bringing up this issue now? Very simple. Belden has brought out a line of cables that are all tinned copper.

Tinned copper is an easy and effective way of keeping a copper conductor from tarnishing, from becoming oxidized. You’ve probably seen copper when it is oxidized. It turns green. The Statue of Liberty is a famous example of copper oxidation. The real problem is the fact that this green copper oxide is a semiconductor. This is bad news on top of a really good conductor like copper. There are two ways to prevent copper oxide. One is to put something over the metal to prevent air from reaching it. No air, no oxide, no corrosion. However, if you strip off this layer (usually plastic) that leaves the conductor in contact with air and in danger of oxidizing. Then you might have to use a connector that also seals out air. Soldering wires helps seal in the copper. Other connectors, such as many data connections, are “insulation displacement”. This only makes contact in one specific place on the wire and the wire on either side is still covered with plastic insulation. The other way to prevent copper oxide is to put a layer of tin on the conductor. Then, even with the insulation stripped away, the air is much less likely to get to the copper and cause oxide to form.

The problem with the tin layer begins to show up as the frequencies on the wire get higher and higher. There is something called “skin effect” where the signal does not use the entire conductor. At DC (no frequency) the entire conductor is used. Then a tin layer is only a tiny percentage of the conductor and is a good trade-off to protect that conductor. As frequencies go higher and higher, less and less of a conductor is used, and so the tin layer, if there is one, becomes more prominent. You can easily calculate the “skin depth” of a signal. Here is the formula:

formulaD is the depth of the signal in inches and F is the frequency of the signal in Hertz. Just remember, you have to compare the depth of the signal to the size of a conductor. A small conductor runs out of area much sooner than a large wire. At low frequencies, like audio or slow speed data, most of a conductor is used to carry that signal, so tinned copper might be a good compromise to preserve the conductor. As we get higher and higher in frequencies, the skin depth gets smaller and smaller. By the time we’re at a Megahertz (1,000,000 Hertz), the skin depth is 0.00261 inches, or less than 3/1000ths of an inch. That means the signal is entirely in the tin layer, if one is present. Not a good thing, as tin is not a very good conductor. Tin has about seven times the resistance of copper). So anything at 1MHz or higher should not have a tin layer. It needs to be bare copper. That means video cable, digital audio cable, RF coax or Category data cables all need to be bare copper. No tinned copper. And that’s fine because we make bare copper cables for all these high-frequency applications. Thus, you now realize that all our tinned copper cables are for low frequency applications. They will actually last longer than many of the bare-copper versions, so it’s really a question of what kind of signal, and what frequency, is running down the cable.

There is one other place you might see tinned copper, and that is when we make plenum cables. Certain formulations of Teflon® are very caustic and can create copper oxide just by being extruded onto a copper wire. Having a tinned copper conductor reduces this effect. One other way we can do this is silver coated copper. When silver oxidizes, that silver oxide is exactly the same conductivity as normal silver. Using silver as that protective layer is therefore fantastic except for one problem: it is expensive. So, if you want to double or triple the cost, just say “Silver-coated copper, please.” We would be happy to comply. Until you get that rich, your choice is tinned copper or bare copper. And now you know which to ask for.


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