Litz wire termination in high vibration environment

[uer166]

I've been pondering on how do people crimp Litz on say an ETD54 core and terminate it to a PCBA? I've only ever seen some combination of solder+crimp or crimp+solder in a way that ensures that all strands are linked, but I've been in an argument over this and would like opinions/ideas. This is an automotive application so needs to be representative of what people already do in the field.

[coppercone2]

you want to look at crimp conductor classes, and that will be valid if the ends are stripped properly, so chemical stripping + class 5 crimp or something.

Imitating cheap china power bank litz coil for automotive use is probably a bad idea.

[T3sl4co1l]

If it were like aerospace, I'd suppose chemical strip, crimp, plus potting would be the solution.

Automotive, just stake it down well enough it doesn't shake.  Some goop around the base of the wire should be fine. Heat shrink for strain relief. That sort of thing.  Or potting anyway, if it's a module that needs it.

And, I take it, it happens to be too big to tie off on the bobbin?  Eliminating slack obviously helps the most.  But if you don't have enough pins, or ampacity through them, then yeah.

Edit: I don't actually see anything about litz cable termination from NASA.  I'd love to see references if you can find any!

Tim

[coppercone2]

how about a micromachined crimp that has like 20 holes in it for small bundles of wire. Not even sure if that would help that much, thin wire might be its own thing

Miller electric in the welders shield braid they just clamp it down super hard under a screw

[TimFox]

Assuming you mean high-frequency Litz wire, where each strand is individually insulated, you must strip the insulation first.
When last I used it (for RF purposes), I purchased Litz wire that used heat-strippable insulation (which I believe was a polyurethane material).
After removing the "serve" (a protective textile layer), we dipped the wire end carefully into a solder pot filled with molten solder;  a typical temperature spec is 390^o C to strip the insulation.
The result was suitable for soldering to the rest of the circuit, but probably not suitable for crimping.
(I asked about your Litz wire, since the literal translation of the German is "stranded" wire, but "real" "Litz wire" has the insulated strands.)

[uer166]

Assuming you mean high-frequency Litz wire, where each strand is individually insulated, you must strip the insulation first.

Yeah, 600-1200 strand count, ~3.2mm OD individually isolated. It's definitely a challenge and the solution seems to be obvious to my counter-part, but not to me.

[JohnG]

Assuming you mean high-frequency Litz wire, where each strand is individually insulated, you must strip the insulation first.
When last I used it (for RF purposes), I purchased Litz wire that used heat-strippable insulation (which I believe was a polyurethane material).
After removing the "serve" (a protective textile layer), we dipped the wire end carefully into a solder pot filled with molten solder;  a typical temperature spec is 390^o C to strip the insulation.

Having made quite a few transformers and inductors with Litz of this type, this is the only way I have done it, or seen it done.  The insulation between strands must be removed or you will get a poor and possibly unreliable connection due to the insulation. You will also get more loss because you have a fairly high chance that you have poor contact to a substantial number of strands.

I would be very wary of chemical stripping since the solvents will tend to wick up the strands quite a ways, and that will be very difficult to control.

John

[coppercone2]

yeah you would need to spread it out and do alot of tests to see what works and monitor it. If its the kind of insulation that comes off easily with solder then its better I guess. For fine wire you want Cu bearing solder.

[JohnG]

I forgot to mention that I have seen some commercial/industrial applications and they used a solder pot. Where I saw this used was in LLC converter transformers and resonant inductors for 400VDC to 12 or 48 V telecom/data center PFC rectifiers. So, not consumer volume, but still significant volume. They used Litz because the AC losses were significant and they were after an efficiency in the upper 90s.

Also, if the equivalent gauge is not crazy, you can do this on the bench with a hefty soldering iron. The trick is to use a big tip to heat the Litz from the side, but right near the cut end. Then, feed the solder into the cut end. Once it wets the cut tips of the strands, it starts to flow and burns off the insulation on the wire. When this starts to happen, you can move the iron to contact some of the molten solder and it goes pretty quickly from there. I've done this a fair number of times and it gets pretty easy after the first few.

For heavier gauge Litz, I have seen techs just burn off the insulation with a small butane torch, and then use some extra flux and solder. I have not done this myself, but I have seen it done and it looked straightforward.

John

[T3sl4co1l]

Can confirm, we did the crimp and solder-pot method at a PPoE.  This included stuff up to *ahem* appendage size -- several-ought AWG equivalent.  Induction heating, 100s kW range stuff.

I don't think burning it off beforehand is going to be all that productive... if it's not fluxed and displaced by solder I'm pretty sure it's going to turn to some impenetrable charred varnishy crud.  That said, I have some ancient enameled wire that's literal enamel, not any poly-thing, and not only is it unsolderable, it just goes from dark brown to ever-darker shades of black, until it's so hot that the carbon evaporates... or the copper melts.  I have enough of it that I can (grudgingly) use it for smaller litz builds; I solve the problem by treating it with an appropriately(?) aggressive chemical: molten potassium chlorate.  Which oxidizes the FUCK out of anything the least bit organic, while the molten salt provides a flux cover for the copper, keeping it bright.  It is... not something I can recommend for general use, needless to say!


Anyway, for smaller stuff, and for insertion into board, crimp terminals probably aren't important.  Binding it up with a dip in the solder pot (be careful not to fray any strands on the way in) makes it solderable as any other loose wire/pin.  You can use crimp wire holder thingys, this sort of thing:
https://www.digikey.com/en/products/detail/molex/0002092105/403217
though if it's tinned before insertion, the holder / spring clip feature probably won't work; and if not, then you need a long enough wave soldering cycle to ensure the cable makes connection, which may be more than you'd otherwise expect for the assembly.

And if you're tinning beforehand, you're basically using it as a ferrule, which I mean... sure, why not?  So, those too, just regular ferrules.

Again, goop can be used to hold the wire in place, which might even be doable before soldering so it doesn't shift at all during wave.  Or if this is hand soldering, yeh doesn't even matter just do it however works alright.

Tim

[SeanB]

For home use use a rosin flux, or for wire that is not going to respond to flux use an aspirin tablet with the wire held to the tablet with a large hot soldering iron, with the tip loaded with solder, and feed in more as it smokes. Will strip it, but for large amounts use some thick resin, dip wire end and then dip into solder pot till it tins.

For terminals you need to wash with some solvent to get the flux off, let dry then use some form of encapsulant on the joint, either pot the whole thing, or use a flexible acetoxy free copper and solder safe electronics grade silicone, or some other flexible adhesive. Glue the whole core assembly to the board as well, do not rely on the solder pins alone, so either conformal coat on that area, or the whole board.

[JohnG]

I solve the problem by treating it with an appropriately(?) aggressive chemical: molten potassium chlorate.  Which oxidizes the FUCK out of anything the least bit organic, while the molten salt provides a flux cover for the copper, keeping it bright.  It is... not something I can recommend for general use, needless to say!

Ok, that seems absolutely certifiable nuts to me! Potassium (and sodium) chlorates used to be my favorite oxidizers for experimenting with pyrotechnics as a youth many years ago. They would turn just about anything combustible, and quite a few things not normally thought of as combustible, into something quite dangerous. It didn't matter whether they were organic or not, and ignition points were always low.

John

[coppercone2]

I stripped it before using molten sodium hydroxide.

But if you do it, do NOT drill a 'shaft' into something and think you can melt it in there. It likes to bubble and expand if you over heat it, and thats how you get splatter. You need to melt it in a basin, like a small bowl. I think a larger quantity of molten hydroxide in a basin is much less dangerous then a small amount in a shaft. I was real careful but it still 'bubbled over'. Maybe melt it in a small metal crucible and dip wire in there. I thought that the design parameter should be smallest amount of molten caustic, but the correct one is least likely to splatter. When you dip wire in there, I recommend attaching it to a rod with a fixed axis that moves only towards the chemical strip pot (like a plunger), so it cant 'swing' or anything like that. So attach wire to a shaft that is put inside of a tube that submerges it into the pot at 45 degrees with a range of motion thats like 2 inches with mechanical stops. That way you can only dip and undip the wire, and even if you get a twitch or something you wont fling hydroxide around. Then you let it cool down and solidify then detatch the wire from the shaft and wash it good in ultrasonic or boiling water and neutralize the bath after washing the wire good with hot water thats poured over it to get any residual chemicals off.

[TimFox]

For non-soldermelting Litz wire, the old books recommended having an alcohol lamp or bunsen burner sitting next to an open jar of alcohol on the bench:  first heat the wire end and then plunge it into alcohol.
For safety, you should not be smoking when you do this.

[jonpaul]

Bonjour
We avoid litz, usually bunched wire or foil cheaper better availability and much easily termination

Every transformer bobbin has PCB pins so the winding is terminated to the pins.

If we are obligated to resort to Litz, our proceedure....

Litz is available in Solderease Nylease insulation that burns off in solder pot or Metcal  iron.

The residue of  burned insulation is cleaned with an exacto knife blade.

Finally, careful soldering  to the bobbins pins, with solder pot or Metcal iron.

After decades of manufacturing and field use, we had no failures.

Bon courage

Jon