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I am searching for a reliable wire-to-board connection. The wire is 0.1 mm enamelled wire. The wire is solderable, but the polymer is very temperature resistant and needs to be mechanically removed before the soldering process. Currently we are mechanically removing the polymer and then soldering the wire to the board. However, some of the solder joints fail later on, although visually they look fine and also measuring the the resistance of the critical joints does not show any hint of an unreliable joint. We even started doing a HF measurement to maybe catch suspicious joints (looks promising), but it is a lot of effort for every system. Some joints fail during the next production steps, some fail over a period of 3 months.

So I am searching for alternatives - either to improve the soldering process or to have a completely different connection method (crimping?). Any ideas?

Conrad
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  • Just to clarify - you're *not* seeing a broken wire at the joint and it still appears to be mechanically connected? – brhans Sep 22 '21 at 13:38
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    What sort of strain relief are you providing for the wire? If the polymer is that tough, you may have a break in the copper that is being held together by the polymer. – Dave Tweed Sep 22 '21 at 13:51
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    If the resistance measurement shows no problem then how do you know they have failed? – user253751 Sep 22 '21 at 14:06
  • Is this FFC/FPC or something else? Because I'd say that's the obvious solution. If they are "reliable", well it depends on what you compare with. – Lundin Sep 22 '21 at 14:33
  • Question: How do you know you're removing enough coating??? – Kyle B Sep 22 '21 at 15:04
  • You could have a failed joint X-rayed or cross-sectioned pretty cheap. Might shed some light on it for you. Microscopes are your friend here too... If you're doing this w/o any magnification, I think you'll be amazed at what you find when you're into it at 60x zoom – Kyle B Sep 22 '21 at 15:06
  • Could try a small induction heater to remove the enamel. Commercial units exist. – rdtsc Sep 22 '21 at 15:32
  • Suggestion: wire strippers like [this](https://www.eraser.com/products/wire-cable-strippers/wheel-strippers/rt2s-magnet-wire-stripper-2/), [that](https://www.eraser.com/products/wire-cable-strippers/blade-strippers/dcf1-wire-stripper-for-magnet-and-enamel-wire/#vtabs-5), [here](https://www.schleuniger.com/en-us/products/strip/wire-stripping/?gclid=EAIaIQobChMI-vLWj_OS8wIVA4bICh0a9gLsEAAYASAAEgJ7A_D_BwE), and there. Wrap on a post a few times, then solder. – jay Sep 22 '21 at 15:40
  • What's the exact insulation material and what is the temperature classification? If you use polyurethane-nylon 155 insulation, the insulation melts/strips while soldering. – qrk Sep 22 '21 at 17:47
  • A solder pot at suitable temperature may allow pretinning. Also suitable flux. – Russell McMahon Sep 23 '21 at 06:40
  • Thanks for the detailed questions, let me try to answer them. I actually do not "see" a broken wire or solder joint as the whole assembly is filled with epoxide during the production process. So it could actually be that the copper is being held together by the insulation. But by doing an HF measurement we seem to see an effect right after soldering, so this is our strongest guess at the moment. But there are many good ideas that I will need to think about and possibly try out. – Conrad Sep 23 '21 at 06:58

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They sell terminals for magnet wire just for that purpose. The IDC in the terminal cuts through the magnet wire's enamel and makes a permanent and reliable connection. Then, the terminal is soldered to the PCB. For example, this one.

In other cases, the terminal is fist affixed to the PCB and later the magnet wire is inserted in the terminal.

Davide Andrea
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    Note that 0.1 mm corresponds to AWG38. Usually such fine wires are soldered to a post that is fixed to the component for strain relief, and then the post is soldered to a PCB. – Dave Tweed Sep 22 '21 at 13:49
  • @DaveTweed Could you elaborate? Sounds like soldering to a post just changes the point that is stressed. – DKNguyen Sep 22 '21 at 14:13
  • The terminal avoids the issues of the poor solder joint to the magnet wire, and the labor to prepare the wire, which is what OP complained about. No, it doesn't address the stress point of the wire. – Davide Andrea Sep 22 '21 at 14:15
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    @DKNguyen: Usually such fine wires are associated with inductors, transformers or certain kinds of transducers. The post or terminal is fixed to the component, eliminating any movement between the wire and the post. – Dave Tweed Sep 22 '21 at 14:27
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    I will try the wire post approach next and see how that works. – Conrad Sep 23 '21 at 06:59
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I would recommend against mechanical removal of the enamel on a thin wire like this one. Your blade may easily create a small scribe on the surface where the copper may break later. (Learned the hard way).

The way an older colleague taught me (in the past century) was:

  • Have a piece of old PVC. Not every PVC works well - an old reddish PVC pipes seemed to be the best.
  • Use older tip on your soldering iron you do not use anymore for fine work.
  • Heat up the soldering iron a bit more.
  • Cover the tip in solder and press against wire supported on the plastic. The plastic releases some chlorine (The "C" in PVC stands for Chloride) and you may be surprised how nicely it works. (Don't breathe the fumes!)
  • Then - when all wires are tinned - switch back to your normal tip and solder your circuit.
Martin Vana
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  • We tried that already. The insulation is a PU/PA combination and it will just burn, but not really "vanish" from the wire. We used temperatures up to 500 °C for that. There was still a mechanical step needed afterwards to remove the burned polymer. – Conrad Sep 23 '21 at 07:35