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When tinning a wire with solder, is it better to allow the solder to go under the insulation (so long as it doesn't bulge or burn the insulation)? I've heard arguments for both, but never really got a final answer.

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  • \$\begingroup\$ Are you asking about wicking (title) or soldering (body of question)? \$\endgroup\$ – user207421 Oct 4 '16 at 18:48
  • \$\begingroup\$ I heard (from manufactures of connectors/ terminal blocks) that it is better not to tin the wire before putting into a screw terminal... if that is what you are asking about.) \$\endgroup\$ – George Herold Oct 4 '16 at 18:55
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    \$\begingroup\$ How is the tinned wire going to be used? \$\endgroup\$ – Andrew Morton Oct 4 '16 at 19:36
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    \$\begingroup\$ A vital point, commented on by George, and largely not mentioned by others - In one special case you must not tin any of the wire except perhaps the very tip. That is: If the wire is to be used in a connector with a screw that screws down onto the wire or, perhaps safer (perhaps) with a screw down clamp, then tinning of the wire will lead to joint failure. This is because the initial solid solder + wire mass will initially be clamped tightly and will then "flow" / creep under pressure and the joint will loosen. This is a very real affect and well covered by regulations in power situations. \$\endgroup\$ – Russell McMahon Oct 12 '16 at 1:20
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NASA, in the NASA Training Program Student Workbook for Hand Soldering, page 9 et seq., say:

... adding solder to the wire until the tinning has reached no closer than 0.5 mm (0.020 in.) to insulation.

I think NASA is a reasonably authoritative source for that.

At a guess, I would say that one of the reasons is to avoid embrittlement of, or other undesirable effects on, the insulation. Also, allow the tinning to flow all the way to the insulation could push flux inside - flux should be cleaned off after the soldering operation and that can't be done if it is stuck inside. My suspicion of wire embrittlement through heating looks like it could be unfounded, as thermal strippers can be used to melt insulation to remove it without damaging the wire.

It is desirable to have no stress or strain on the tinned portion of the wire (because tinned wire is more brittle). That can be achieved by clamping the insulation to ~something~. If it is in an environment where there is high vibration and/or thermal cycling, it may or may not be desirable to leave a curve/loop in the wire, but that would require an analysis of the operating environment.

I refer you again to the aforementioned NASA document where they repeatedly refer to "Proper insulation clearance" after making a soldered connection with tinned wire.

If you want insulation over the tinned portion of the wire, you can use heat-shrink tubing. It is available with an adhesive inner coating which is activated when the heat shrink tubing is heat-shrunk, and with the adhesive can provide a watertight seal.

However, Asmyldof points out that:

The point where the insulation stops over the soldered wire, be it PP, PVC or heatshrink, will become the "focal point" of bending stress. A soldered strand of wire breaks easily. If you leave some length of stranded wire, that length will be the most bendable point in the wire, thus taking the risk somewhat away from the solder. Adding heatshrink to the soldered wire only moves the problem down.

[I changed "room" to "length" in the quote.]

So heatshrink tubing is not a substitute for proper support of the wire.

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    \$\begingroup\$ Your information is right, your conclusion (and H.S. advice) less so. Solder to under the insulation means the soldered wire becomes the most bendable part in normal insulation types (only exception is soft silicone and the like), which by solder application becomes a single stranded wire, which is a very weak thing indeed. \$\endgroup\$ – Asmyldof Oct 4 '16 at 19:25
  • \$\begingroup\$ @Asmyldof So I can edit my answer to be more accurate, can you clarify "the soldered wire becomes the most bendable part" for me? I have always found that solder-impregnated wire is less bendable. And doesn't the H.S. increase the resistance to bending, thus taking some strain off the soldered portion? \$\endgroup\$ – Andrew Morton Oct 4 '16 at 19:31
  • \$\begingroup\$ The point where the insulation stops over the soldered wire, be it PP, PVC or heatshrink, will become the "focal point" of bending stress. A soldered strand of wire breaks easily. If you leave some room of stranded wire that will be the most bendable point in the wire, thus taking the risk somewhat away from the solder. Adding heatshrink to the soldered wire only moves the problem down. \$\endgroup\$ – Asmyldof Oct 4 '16 at 19:37
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I must take this chance to advertise my absolute favourite soldering video: Basic Soldering Lessons 1-9, made in 1980 by PACE. Some of it may be slightly dated, but the basics are still very relevant, and the intro music is awesome. These are training videos for soon-to-be professional repair and manufacturing engineers.

Lesson 2 deals with tinning wires, and explains that you should not let the tin creep under the insulation:

enter image description here

When done this way, no solder will be pushed up under the insulation. Remember: There should always be a gap left between the end of the insulation, and the beginning of the tinning.

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For screw/spring terminals: Use of a decent quality ferrule, properly crimped is by very far the best way to connect.

All other cases make sure there's no notable stress on the soldered wire. Like in the NASA advice of 0.5mm clearance, though I'd suggest at least double the copper conductor's radius (2*r is of course d, but I always think and calculate in r). Or by clamping-down (properly) the insulated part of the wire with something rigidly connected to the other side of the solder joint. Like happens in most quality circular connectors with solder-on pins.

If soldered wire is allowed/likely to bend, it will break in the near future.

EDIT:
To clarify: If the solder goes into the insulation and the insulation is a standard mildly damage-resistant type like PVC, PP, etc, the weakest point will be the soldered wire. Meaning bending stress will be put more onto the soldered wire than the stranded wire inside the insulation. Which is the wrong way around.

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  • \$\begingroup\$ You really want to avoid places that concentrate stress like that completely... a soldered joint should be immobilized as best as possible, and then strain relief used to spread out any unavoidable movement/bending. \$\endgroup\$ – Daniel Oct 4 '16 at 19:47
  • \$\begingroup\$ @Daniel any connection should be immobilised as best as is possible in that situation. There are more than a few situations in which it is hard or hardly possible, in which case you practise mitigation of risk. No one rule covers all. \$\endgroup\$ – Asmyldof Oct 4 '16 at 20:46
  • \$\begingroup\$ Well, having an unsupported solder joint is pretty much guaranteed to cause a problem. \$\endgroup\$ – Daniel Oct 4 '16 at 20:59
  • \$\begingroup\$ That is, in any sort of vehicle. If it's in a piece of music gear, you might get away with it for a long time. \$\endgroup\$ – Daniel Oct 4 '16 at 21:00

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