Not exactly space-grade

I still remember that one meeting at the cable company which was going to do the assembly for the cable of the Zaunkoenig M2K. The cable company CEO took the M2K cable, held it up in the air and said: «Mr. Schmalzried, do not get me wrong. This is a nice cable and it is a tricky cable. But it is not exactly space-grade. We will be able to work with this no problem.»

Back then that was what I wanted to hear. After having made a few hundred M2K cables myself I knew first-hand how time-consuming that cable was to work with. Having found a cable assembly company that thought the M2K cable would not pose a problem meant that Zaunkoenig could outsource the M2K cable assembly to free up valuable time.

Time we could instead put into our carbon fiber production. Or into the design of the cable for the extreme edition of the M2K: the M2S.

Exactly space-grade

I started thinking about which company to contact about making the raw cable for the M2S (what I mean by raw: just the cable, on a spool; soldering connectors to that raw cable would be the job of the above-mentioned cable assembly company). And I just could not get that «not exactly space-grade» comment out of my head. So I decided to straight-up go to a company that does space-grade cables for a living: New England Wire Technologies in New Hampshire, USA. New England Wire Technologies could be the worlds most vertically integrated cable company (meaning: they do a crazy amount of stuff in-house, instead of just buying it from other companies, like many cable companies do).

In the past New England Wire Technologies has made cables for the Apollo program, for example. As is true for gaming mice, low weight is of paramount importance for space flight, so my assumption was I would not have to explain the importance of a very small and light cable to the company that did cables for the moon landing.

While there is an obvious overlap between gaming mouse cables and cables for rockets, there also are some differences.

The biggest two differences probably are these two: with rockets you need cables that can withstand extremely low as well as extremely high temperatures, whereas with gaming mice everything that survives room temperature is good to go.

The second big difference is resistance to radiation: important for everything that leaves earths atmosphere; not so much for something that mostly is sitting on your desktop.

The heart and soul of a cable

One of the first decisions you have to make when you design a custom cable is the design of the conductors (usually several copper wires bundled together) plus the insulation (usually plastic) encapsulating those conductors.

The conductors and their insulation are the heart and soul of any cable. You literally have to design the rest of the cable around them.

In the M2K cable each conductor had a diameter of 0.2 mm (or, measured in «American Wire Gauge» (AWG): 32 AWG) with an insulation around it that had a wall thickness of 0.15 mm. One insulated conductor thus had a total diameter of 0.5 mm (0.15 + 0.2 + 0.15 = 0.5).

We decided to not change the size of the conductors, since they were already very hard to work with. Instead we decided to reduce the wall thickness of the insulation from 0.15 mm down to 0.1 mm. That may not sound like much, but you have to consider that this change reduces the total diameter of each insulated conductor by 0.1 mm; and remember that there are four conductors present in a gaming mouse cable. That is a big difference for a small gaming mouse cable (total diameter of the M2K cable, for reference: 2.5 mm).

Obviously there is a huge drawback to a thinner insulation. Insulation is not only there to prevent electrical shorts; it also has to protect the conductors from mechanical forces. And with an insulation that is just 0.1 mm thin (human hair has a thickness of between 0.05 and 0.12 mm, for comparison) we were a bit worried that the insulation might not offer enough protection.

And this is where it is handy when you work with a cable company that makes space-grade cables for a living: they had the perfect solution at hand without even breaking a sweat.

Proprietary high strength copper alloy meets Allianz Arena

In order to counteract the loss in strength New England Wire Technologies suggested a two-pronged attack.

Their first suggestion was to use a recently developed proprietary high-strength copper alloy to make the copper conductors significantly stronger as well as giving them much more flex-life (flex-life describes how well a cable handles repeated bending).

I did ask them by the way how proprietary that high-strength copper alloy was, to which they answered «Very.»

We also used these high strength copper alloy wires in the electromagnetic shielding surrounding the four conductors. Where the M2K cable used 0.08 mm thick copper wires for the shield, we went with 0.05 mm thick copper alloy wires for the M2S, resulting in an overall cable diameter reduction of 0.06 mm.

The second suggestion of the rocket cable company was to use a stronger plastic for the insulation of the four conductors.

In the M2K cable we used a plastic called «high density polyethylene» (HDPE).

HDPE is an okayish insulation material: it is quite slick and has an attractively low density of just 0.95 g/cm³ (that is very low; even for a plastic). When it comes to strength and flex-life however HDPE is nothing to write home about; especially at a wall thickness of just 0.1 mm. Which is why we needed to replace it. The rocket cable company recommended to go with a plastic called «ethylene tetrafluoroethylene» (ETFE).

Originally developed by DuPont in the 1970s as an insulation material for the aerospace industry, these days ETFE is also used in high-tech buildings, like the Allianz Arena in Munich, Germany, for example.

The Allianz Arena has a roof made from 2,784 inflated ETFE pillows; together they result in the worlds biggest membrane shell. Because these ETFE pillows have a wall thickness of just 0.2 mm they can easily be illuminated from within, which gives the Allianz Arena a unique look even at night.

But back to cables.

ETFE truly is an interesting plastic. Being a fluoropolymer it belongs to one of the most extreme families of plastics there is. But even among the fluoropolymers ETFE is special due to its unique combination of exceptional strength, flex-life, slickness, temperature resistance and low density.

Where PTFE, also known as the worlds heaviest plastic, has a density of a whopping 2.2 g/cm³ (aluminum is not much heavier at 2.7 g/cm³), ETFE comes in at just 1.7 g/cm³.

Compared to the very low density of HDPE (0.95 g/cm³) ETFE might seem heavy, but because ETFE is so strong you can use it with much lower wall thicknesses; and that is how you end up with a lighter cable regardless of the relatively high density.

This is how one of the four conductors inside the M2S cable looks like:

Lifesavers and cable jackets

In the middle of the M2K cable there are Kevlar fibers that protect the conductors from strong pulls. In the event of a strong pull these Kevlar fibers can literally be a lifesaver and they barely weigh anything nor do they increase the overall diameter of the cable (with four conductors you always have a little empty space in the middle). That is why for the M2S cable they were a given from the get-go.

The cable jacket of the M2K cable (the jacket of a cable is its outmost layer) is made from a medical grade polyurethane (PUR). PUR offers an extremely attractive combination of flexibility, slickness, low density (1.15 g/cm³ in the case of the PUR used in the M2K cable jacket), strength, resistance to abrasion and cuts (cats can still chew through it though; be warned); this is why we wanted to use PUR for the M2S cable jacket as well.

For the M2S cable jacket we decided to use significantly less wall thickness though. In the M2K cable the PUR jacket had a wall thickness of 0.5 mm. For the M2S we dialed that down to 0.32 mm, a reduction of 36 percent, greatly increasing flexibility and reducing weight of the M2S cable.

Partly that reduction in wall thickness was made possible by having a thinner insulation around the conductors as well as having a thinner electromagnetic shield (in general: the larger the diameter of a cable, the more wall thickness you need for your cable jacket). Partly however the cable jacket of the M2K cable was just over-engineered (meaning: it was thicker than it needed to be for a gaming mouse cable).

A little bit of silver sprinkled on top

Pretty much the smallest design change in the M2S cable was the choice of the copper wire plating material.

Since pure copper would oxidize pretty badly you typically plate copper wires with a protective layer. There are many different forms of copper plating, like tin, nickel or silver for example.

The wires for the conductors as well as the shield of the M2K had a tin plating. For the M2S we opted for a silver plating, to increase resistance to oxidation as well as improve crimpability and solderability.

Note that silver does not make the signals magically fly faster or something silly like that; it just gives the cable a small boost in reliability.

The following picture shows a bunch of silver plated copper wires that the M2S cable is made from:

Putting it all together

All these changes together resulted in a cable that compared to the M2K cable was much more flexible, 20 percent smaller in diameter (2.0 mm instead of 2.5 mm) as well as 17 percent lighter (6.4 grams per meter instead of 8.8 grams per meter).

Most surprisingly however it still was extremely strong. I tried pulling it apart, but gave up doing so after ten seconds; even though I was wearing gloves, the pain of the cable cutting into my hands was too much. This is good news for future Zaunkoenig cables by the way: we can go even smaller.

At this point we were quite happy with ourselves. Mission: accomplished. Or so we thought.

No problem?

Remember that CEO from the company that did the assembly of the M2K cable who called the M2K cable «not exactly space-grade» and «no problem» to work with? It turned out that after his company had done almost two thousand M2K cables he had changed his verdict of the M2K cable from «not a problem» to «a problem». The tiny conductors inside the M2K cable had pushed his company to its limit.

Unsurprisingly he was not too happy with the M2S cable. In fact, the M2S cable was so difficult for them to work with, that they could not even do prototype cables for us, without renting a new machine.

The thing that makes the M2S cable even harder to work on than the M2K cable is the ETFE insulation. Not only because it is so thin. But also because ETFE itself is a tricky material: it is strong as well as elastic. And this combination, thin plus strong plus elastic, causes issues when you try to remove the insulation (this is called wire stripping, by the way). And you need to strip the insulation if you want to work with the cable; USB A plugs do not attach themselves to cables magically.

Traditional wire stripping machines have two blades cutting into the insulation. But with very thin ETFE insulations these two blades are at the end of their wits: too often they fail to remove the insulation because they do not cut deep enough. The result: the ETFE insulation either stretches instead of breaking away, or the ETFE insulation does not break away with a clean cutting edge. Both cases are bad if you want to put crimps on those conductors; crimps need to a clean cutting edge so they have enough material to bite into.

You can kind of make traditional wire stripping machines work if you let the two blades cut deeper; but that literally is a double edged sword, as you now run the risk of accidentally nicking the wires.

This is why cables made for aircrafts or spacecrafts, where cables have to be ultra reliable, typically are stripped with rotary wire strippers. In rotary wire strippers four ultra sharp blades cut into the insulation in a circular fashion, enabling a clean cutting edge with no nicked wires. The following picture shows such a rotary wire stripper:

Needless to say that these kind of rotary wire strippers come with a significant price tag. A price tag so significant, that buying one for Zaunkoenig as the only customer would have been a stretch. And thus the cable assembly company decided to not do the assembly for the M2S cable.

So there we were, in December of 2022. Three hundred meters of raw and almost space-grade cable on our hands; but with no-one willing to work with it.

From outsourcing to insourcing

Now we were confronted with a tough decision: we could either search for another cable assembly company; one with experience in doing cables for aerospace applications; or we could source cable production back in.

The thing with aerospace cable assembly companies is that they charge mad amounts of money, because usually their customers are not very price-sensitive.

So we decided for the alternative: sourcing cable assembly back in. We already had done hundreds of cables in the past and we now had that new hire who is really good with his hands. And if you buy a hand-held rotary wire stripper, instead of a big machine, you will not break the bank; you will not be able to produce more than a few thousand cables per year (hand-held wire strippers are very slow compared to machines), but as of now a few thousand cables is all we need.

This kind of sounds like the beginning of a happy end. Sadly it is not. Up to this point this article was mostly about the mechanical aspects of the M2S cable. And I am confident in writing that we nailed those. However cables also are supposed to carry electric signals. And whether or not a cable can do that depends on its electromagnetic properties.

Electromagnetic fire

In March of 2021 we were trying to put out a fire. An electromagnetic one.

We had just launched the M2K when we found out there was an issue with 8,000 hertz. The M2K was our first gaming mouse that did driverless 8,000 hertz. Or so we thought: on some PCs, on some USB ports, some of time, the M2K would not get recognized by the PC at all. Zero hertz, so to say.

We dug deeper and arrived at the underlying cause: poor electromagnetic shielding.

In my 2019 article about theoptimal gaming mouse cable I listed four criteria that together made a good gaming mouse cable: flexibility, slickness, low weight and durability.

Turns out that at least for 8,000 hertz gaming mouse cables there is a fifth criterion: signal integrity.

Signals in a 8,000 hertz gaming mouse fly fast. Way faster than in a 1,000 hertz mouse. So fast in fact, that the electromagnetic shielding of the M2K was insufficient.

Our quick fix in April of 2021 was was to take hundreds of finished M2K cables and manually shorten them from 200 cm down to 165 cm. This kind of worked most of the time, but there still were some PCs out there on which some of the USB ports were not working.

All in all it was far from an optimal solution, but it was the best we could do, considering that the cable itself could not be modified.

Then we tested the M2S cable at 8,000 hertz. And what we quickly learned was that the M2S cable had even more problems with 8,000 hertz than the M2K cable.

What now?

Death begins with compromise

We soon found out that the root issue of our 8,000 hertz problems was the specific kind of electromagnetic shielding we were using on the M2K and M2S cable: a spiral shield (helically wrapped wires around the four insulated conductors). The following picture shows the spiral shield of the M2S cable:

A spiral shield provides adequate shielding for a 1,000 hertz gaming mouse cable and it is a very flexible type of shielding (which is the reason we were using it in the first place).

But at 8.000 hertz a spiral shield really does not cut it any more. What you would need instead is a so-called braided shield (many wires woven together). A braided shield provides excellent shielding, even at high frequencies. The downside? A slightly thicker and more rigid cable as well as higher cost.

The ideal solution for our problem would have been to exchange the spiral shield with a braided one. Sadly, once a cable is produced, you cannot simply exchange its shielding.

The alternatives?

We tried making printed circuit boards (PCBs) with high fidelity shielding, to maybe counteract the poor shielding of the cable, but to no avail.

We could have tried the same trick we tried on the M2K: making shorter cables. However: since the M2S shield was even worse than the M2K shield, that would have resulted in rather short cables; 140 cm or so short. For anyone not gaming on a laptop that would have been a stretch.

Sure; there are extension cords. But when you have a weak signal because of insufficient shielding an extension cord will degrade that signal even more. So you would need an active USB hub; one capable of boosting the USB signal quality.

At that point however we would be well into «compromise territory»: a 650 Euro gaming mouse that a significant portion of customers would have to use with an active USB hub?

No.

Zaunkoenig is about the relentless pursuit of performance; not compromises.

M3K cable to the rescue?

In December of 2022 we started the design of the M3K cable. We took everything we had learned from the 8,000 hertz issues of the M2K cable, and we took everything we learned by making the M2S cable smaller and lighter.

When the M3K cable arrived four months later we were relieved: not only did the M3K cable handle 8,000 hertz like a chad. It was almost as flexible and light as the M2S cable.

Hence we were tempted to just use the M3K cable for the M2S and call it a day.

However: the raison d’être of Zaunkoenig S models is to boldly go where no gaming mouse has gone before. To be trailblazers for the K models. Not the other way round.

That is why we decided to not use the M3K cable for the M2S and to design a new cable for the M2S instead. One that will go where no gaming mouse cable has gone before.

So the bad news is that the M2S has to be pushed back till 2024 (making a custom cable from scratch takes several months). The good news is that when the M2S launches it will have an even better cable than originally planned.

More good news: the M3K cable has no issues. In fact it is pretty awesome; but more on that in the next blog article.