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When using Litz wire, the accepted method of stripping the wire is by dipping it in a solder bath, which effectively turns the end of it into solid-core wire, so my question is thus:

How good is the contact between a tightly-screwed-down screw-type terminal block and a solid-core, slightly squishy due to solder, wire? Should I expect significant contact resistance, as the round wire makes only a small amount of contact with the flat screw body? Or am I overthinking things?

This is part of a research project. It will not be exposed to any sort of high-vibration environment (there are no motors or anything nearby) and environmental conditions will be pretty much STP (with the possibility of occasional high humidity, if that matters any). It will be operated indoors, with air conditioning.

I don't have a specific spec for contact resistance because I simply don't know what is reasonable. Contact resistance has never previously been something I need to worry about; this is my first time working on a high-current device.

Hearth
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  • Do not use solid core wire with clamp down devices of any kind. The connection is not reliable. – DKNguyen Mar 19 '19 at 20:56
  • @Toor - why not? The standard method of terminating a solid-core "Romex" wire to a circuit breaker in an electrical panel is a screw-down clamp on the breaker. Pretty much the same when you connect that same wire to an outlet or switch. – brhans Mar 19 '19 at 20:58
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    Especially do not depend on a clamp termination to a wire end that has been dipped in solder. This is very prone to crushing over time and working loose. – Garrett Mace Mar 19 '19 at 20:59
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    I do however agree with Garret - a solder-dipped stranded wire is not anywhere near the same as a solid copper wire. Do not clamp one of those and expect it to be reliable. – brhans Mar 19 '19 at 21:00
  • Is it? That's not my understanding. Maybe my thing only applies to assemblies where movement or vibration is expected (where you shouldn't be using solid wire anyways). – DKNguyen Mar 19 '19 at 21:01
  • Thanks @brhans, I'll go for crimped-on ring terminals for the litz wire, then. – Hearth Mar 19 '19 at 21:01
  • I don't know Litz wire practices, but generally you're not supposed to crimp solid-wire or soldered wire either, but it may be different with Litz wire since it seems you would be kind of hosed if you could not. Now that I think about it, if you aren't allowed to crimp or clamp down on solid wire, then you could almost never get it connected without soldering! So maybe what I said earlier is just for vibration sensitive assemblies. – DKNguyen Mar 19 '19 at 21:04
  • @Toor Considering the wire gauges involved, we would practically need a blowtorch to solder them! I expect we might be using tools for soldering copper plumbing. – Hearth Mar 19 '19 at 21:06
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    The problem with clamping on to solder-dipped stranded copper (or the wrong alloy of solid/stranded aluminum wire too) is that the softer solder/aluminum tends to deform and 'migrate' away from the point of pressure over time, which results in a loose connection. I *think* (although I'm not sure) that you may have a similar issue with crimp terminals on solder-dipped stranded wires unless you re-flow the solder such that the wire is actually soldered to the terminal after you crimp it. – brhans Mar 19 '19 at 21:08
  • Are you not able to cleanly strip the litz wire in something like an acid bath? Instead of using heat solider? – DKNguyen Mar 19 '19 at 21:09
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    This is turning into a discussion of how to terminate litz wire now, so I think I'll be making this question specifically about that and splitting off the bit about screw and ring terminals into another question. – Hearth Mar 19 '19 at 21:09
  • What is your spec for contact resistance and max environmental stress? **one should not ask a question like this without specs.** *( I know I keep harping on this)* – Tony Stewart EE75 Mar 19 '19 at 21:15
  • @SunnyskyguyEE75 I suppose that's part of the problem: I don't know. This won't be a high vibration environment, I can tell you that much, but the only contact resistance spec that I have is the unhelpful "as low as possible". I honestly don't know what would be an acceptable amount, as I've never had to worry about contact resistance before. – Hearth Mar 19 '19 at 21:17
  • @Toor the plastic coating is tolerant to acid, but there are other solvents for plastic. – Tony Stewart EE75 Mar 19 '19 at 21:17
  • @Hearth I have dealt with cable strain relief issues or a rotary VCM going at 30Hz for 1.5cm that failed in 50k cycles and a better design that lasted 1e6 cycles and vibration levels for instruments at 15g sine and 100g shock.... again you need to define specs. Strain Relief must be done right – Tony Stewart EE75 Mar 19 '19 at 21:20
  • @SunnyskyguyEE75 Strain relief is not a (major) concern here. I've added environmental conditions to the original question. – Hearth Mar 19 '19 at 21:22
  • Can you at least define the wire interface options? stud bolt? screw hole? flat plated steel? SMPS Busbar? The best option is threaded bolt with spade lug as the thread area can match the lug <<0.1mOhm and very low ESL – Tony Stewart EE75 Mar 19 '19 at 21:24
  • @SunnyskyguyEE75 Threaded bolt with spade lug (well, ring terminal, but they're about the same thing) is absolutely an option; see my other question asking about how to make good contact with screw and ring terminals. – Hearth Mar 19 '19 at 21:28
  • I once designed a box for Lucent Avaya and the ground lug went to a chassis ground stud threaded bolt with a paint-masked plated steel bottom. Then the sheet metal guy ask me for cost reductions by removing the paint-masking operation. I said sure and came up with a simple ring terminal torqued between two nuts and measure 10mV with 10A flowing thru it when it only had to be 0.1 Ohm to satisfy the UL guy. They approved it quickly with test data. You just need a bigger washer lug surface area with automotive torque rated hardware ( letter number codes on them) for high torque. – Tony Stewart EE75 Mar 19 '19 at 21:37
  • 5% of your cable resistance is reasonable 1% is possible . What is your wire diam. and length? 12mm x 1m =320 uOhm per pair/m, shorter is better. What are your cable specs? and total ESR,ESL specs? – Tony Stewart EE75 Mar 19 '19 at 21:50
  • Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/91274/discussion-between-sunnyskyguy-ee75-and-hearth). – Tony Stewart EE75 Mar 19 '19 at 21:52
  • Hmm... Clamp type terminals are fine on solid wire(tinning makes no difference as the softness and conductivity are similar), so long as they are designed for the purpose. It's actually preferable for the wire to deform a bit when tightened to correct torque, and this is why flat lugs often have a ridge to help bite into the copper. For example, taking a stranded wire, stripping it to the correct length for the correctly rated clamp/lug, finger twisting it as tight as you can, and tinning it if you want to get fancy(or not), you should be able to achieve a good permanent connection. – K H Mar 19 '19 at 22:20
  • Other methods can be far better, but a competent person putting in a sufficient effort with the correct tools(Knife, strippers, screwdrivers, linesmen plus torque tools for large wire or aluminum) can make reliable flat clamp connections with stranded or solid wire. My comments about tinning are made with the assumption you haven't left the wire with a gob of hardened solder btw. – K H Mar 19 '19 at 22:27
  • @Hearth, I'm just piling-up references: [Litz wire termination guide (write-up)](https://www.newenglandwire.com/litz-wire-termination-guide/) and [Terminating litz wire – how to (video)](https://youtu.be/S0sfTD3DJKs). – Nick Alexeev Apr 04 '19 at 23:39

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