Tech Brief - Tin Vs. Bare

There seems to have always been differences of opinion when it comes to certain topics, be it politics, sports and even which drive-through has the best coffee.

Another age old argument that we face as a high-end data cable producer is whether or not a data grade copper conductor should be plated or left in its bare copper form.  A practical discussion on the topic with a focus on the real world rather than the lab bench is long overdue. 

To begin with, copper has to be specified properly for the application.  Asking for bare or tinned copper does not in itself guarantee any particular performance level.  There is bad tinned copper as well as good bare copper and vice versa.  For the sake of this discussion; let's stipulate that the copper we are talking about in both the bare and tinned variety has been specified correctly for the application, since it will eliminate many variables. 

On the lab bench and under ideal circumstances, the bare copper is going to outperform the `tinned' variety.  Too bad it isn't quite that easy.  If it was, we would be done, end of discussion.  However, you can't have an "age old argument" if the situation is that straightforward.  Why the argument then?  Well simple, the real world practitioners have experienced different results than the lab testing would suggest.  Our own testing has proven both types of copper can be right if you add in the missing variable...time.  Fresh, new, bare copper conductor works great.  Unfortunately, we have found that it will oxidize much more quickly than the tinned variety which leads to a degradation in electrical performance.

When you think of rust, old cars, front gates or even bikes and bird feeders may come to mind.  Usually, not cable.  Rust is scientifically known as oxidation which occurs when oxygen comes into long term contact with certain metals.  Over time the oxygen will combine with the metal at an atomic level, forming a new compound called an oxide.  When the metal forms an oxide it will weaken the bonds of the metal itself.  If the base metal is iron, you get iron oxide, if it's copper, you get copper oxide.  The main catalyst for the process of oxidation is water.  The metal doesn't need to be submerged in water, moisture in the air is often enough to get the reaction underway.  To the naked eye, a copper conductor may appear to be smooth and uniform, however microscopic pits and cracks still exist and moisture can easily penetrate these imperfections.  The hydrogen atoms present in the water vapor will combine with other elements to form acids which eventually cause even more metal to be exposed by enlarging the imperfections in the base metal.  It gets even worse when sodium is present as in the case of salt water.  Sodium accelerates the chemical reaction that forms the destructive oxide compound.  As more of these atoms combine they weaken the integrity of the original metal making it brittle and crumbly, a process commonly known as corrosion. 

Some pieces of metal are thick enough to maintain their integrity even when rust forms on the surface, but remember we're talking about data cable which is typically in the 24 to 26 AWG range.  The thinner the metal, the better the chance that oxidation will impact the intended application.  Oxidation is a chemical reaction that generates heat as a byproduct.  Eventually, the underlying metallic bonds of the original metal will be destroyed by the heat and the entire item will disintegrate.  That of course is the extreme and we probably won't ever see an indoor cable in use that is about to disintegrate.  However, when metal starts to oxidize the materials properties (conductive, tensile, thermal, etc.) will begin to change.  As we said above, if the metal is thick enough and it just has a little surface rust, it will maintain its structural integrity.  But we aren't talking about structural integrity when it comes to passing an electrical signal; we're talking about the metals conductivity.  When restoring the bridge in the center of town a little surface rust may not be a big deal, just sand it off and re-paint.  Conversely, a quality, non oxidized finish on the surface of a copper conductor is paramount to good electrical performance.  One of the most threatening enemy's of a high performance copper conductor is oxidation on the surface.  We will discuss why next, but before we leave the topic of oxidation, remember, once oxidation starts it is very difficult to stop.  The best way to avoid the process of oxidation is to treat the metal to resist the onset of oxidation or at least slow it down.

In the previous section the comment was made that a quality, non-oxidized finish on the surface of a copper conductor was paramount to good electrical performance.  Why?  Well, there is a concept called the "skin effect" that explains the way electrons move through a wire.  What the `skin' terminology describes is the tendency of the majority of a signals current flow to be heavily concentrated closest to the outside of the conductor.  The higher the frequency of the signal the more the current concentration will favor the surface of the conductor, the lower the frequency, the deeper it will flow in a conductor.  In Ethernet transmission, the frequencies are generally in the higher spectrum and the faster we communicate i.e.: 1 gig or even 10 gig, the higher the transmission frequency and the more oxidation will affect performance.  Obviously, if the conductor surface is oxidized and oxidation degrades properties like conductivity, the conductor will have degraded electrical performance.  In layman's terms, if the high frequency Ethernet signal is supposed to travel on the `surface' of the conductor and that conductor's surface is oxidized it will not conduct the signal as well.  With imperfections on the conductor surface that impede the transmission, the electrons (signal) will be forced deeper into the conductor which causes more friction and heat, also known as, resistance.

In stranded wires the charge on an individual strand may cross from strand to strand.  Since the strands are thin, but still in electrical contact with neighboring strands, we can expect the effect of internal impedance to be similar to that of a solid wire of a comparable diameter to the bundle of strands.  Hence the current will tend to flow near the `skin' of the bundle of wires, just as it does with a single conductor of like diameter.

Now that we understand what oxidation can do to a copper conductor, what do we do about it?  Well we already said that once oxidation starts, it's hard to stop.  The solution is to keep oxidation from getting a foothold on our copper.  The generally accepted practice is to coat the copper with another conductive metal.  Tin, right?  Well, yes usually it is tin, but it could be other metals like gold or silver.  Let's touch on them briefly.  Silver is the most conductive metal.  However, it has its own oxidation problems and it's expensive.  So, just like at the dinner table, silver is for special occasions.  Gold, in comparison to silver is less conductive, but it doesn't tarnish.  Gold would be great except it's pricey and doesn't work that much better than tin when you consider the delta in cost.  Why use gold at all?  In all but the most unique circumstances gold isn't necessary, but it sure does sell well to retail consumers who want to impress their buddies by showing off bling on the back of their stereo system! That leaves us with tin which is a perfect combination of stability and electrical performance.  Tin is the 49th most abundant element, and has the largest number of stable isotopes (10) in the periodic table making it extremely stable.  This means cost is kept low while resistance to corrosion is kept high.  These factors make tin a fantastic choice for coating other metals to prevent oxidation.

That covers most of the academic principles regarding the coating of copper conductors.  What real world problems does the use of tinned copper conductors solve?  For starters, shelf life of the un-insulated conductor is improved.  This may sound like a manufacturing concern and not your problem, right?  Wrong!  Oxidation is like a cancer of metal that starts very small and grows.  We've already established that once oxidation starts, it's very difficult to stop the process.  Copper may check out fine at the time of manufacture, but the oxidation seed may already be planted.  It is nearly impossible to determine when the bare copper will become unacceptable.  Our studies have shown that copper conductor, even from the same lot, doesn't necessarily age at the same rate.  Yet, tinning the copper produces a consistent and predictable end performance that can be plotted and relied on.  Furthermore, stranded cable (used for high performance patch applications) is impacted even faster by oxidation than solid conductor.  Some of the reasoning for this is proprietary, but let's just simplify things by saying there is more exposed surface area available to oxidize in a stranded cable.  Moreover, the oxidation affects not only the surface of the bundle, but the continuity between strands within the bundle.  Over a period of 12 to 13 years we have seen more, bare copper patch cable fail insertion loss than tinned cable.  Why is this?  Let's talk about the assemblies.    

Insulation-displacement connectors are designed to be connected to the conductor of an insulated wire by a connection process which forces a blade or blades through the insulation, removing the need to strip the wire before connecting. These connections are usually seen in low current applications such as telecom and networking.  (Think RJ style/modular connectors)  When properly terminated, the connector blade will cold weld to the wire, making a highly reliable gas-tight connection.

What is cold welding?  Cold welding or metallic bonding was first officially recognized as a general materials phenomenon during the 1940s. It was discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum or a vacuum like environment.  Cold welding is a process in which joining takes place without fusion at the interface of the two parts to be welded. Unlike in the fusion-welding processes, no molten phase is present or required to make the bond thus the term "cold" welding.  Instead the metals bond by sharing electrons with each other.  In a metal, groups of atoms readily `lose' electrons to form positive ions.  These ions are surrounded by and share the `lost' electrons, which are responsible for conductivity. The resulting attachment of ions produced is held in place by electrostatic interactions between the ions as well as the electron cloud which is called a metallic bond.  Metallic bonding is the electromagnetic interaction between delocalized (or `loose') electrons, called conduction electrons which are gathered in an "electron sea" around the metallic nuclei within the metal.  Commonly understood as the sharing of "free" electrons among a `lattice' of positively-charged ions, the conduction electrons divide their density equally over all atoms that function as neutral (non-charged) entities. Metallic bonding accounts for many physical properties of metals, such as strength, malleability, ductility, thermal and electrical conductivity.  Metallic bonding is mostly non-polar, because even in alloys there is little difference among the electronegativities of the atoms participating in the bonding interaction (and in pure elemental metals, none at all). Thus metallic bonding is an extremely delocalized communal form of covalent bonding.  A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds.  Covalency is greatest between atoms of similar electronegativities, but covalent bonding does not necessarily require the two atoms be of the same elements, only that they are of comparable electronegativity.

Now that we have discussed cold welds (metallic bonding), let's get back to the insulation displacement connectors that started the conversation.  As we have already covered, an insulation displacement connector drives a blade through the insulation of the wire and makes contact with the copper conductor inside.  If things go right, we get a cold weld or more scientifically, metallic bonding.  Guess what interferes with the metallic bonding?  You guessed it, oxidation.  If the wire is oxidized, it could stop the metals from bonding.  If the oxidation is present on the conductor but not pronounced, the metal may bond.  However, in time as the hard to stop oxidation process marches on; the bonds will deteriorate as the material turns from copper to copper oxide dust.  This is usually on the microscopic level, but it has an impact on conductivity.  Feedback received from associates who manufacture modular plugs has shown it is much more likely the connector's connection will corrode with bare copper.  Consequently, the bare copper assembly will eventually fail the contact resistance test after aging.  Many people ignore this test, but it is just as much a part of the TIA and IEC standards as any of the other parameters and it should be taken seriously.  What we are talking about here is insertion loss.  In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a channel.  Insertion loss is usually introduced into a channel by connectors and splices.  Usually expressed as a ratio in db relative to the transmitted signal power, insertion loss can also be referred to as attenuation.  We've all heard those terms and we already know we need to watch out for them.  Peace of mind is available at just a few extra cents per foot...use tinned conductors, and have a nice stress free day!