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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?

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  • \$\begingroup\$ Just to clarify - you're not seeing a broken wire at the joint and it still appears to be mechanically connected? \$\endgroup\$
    – brhans
    Commented Sep 22, 2021 at 13:38
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    \$\begingroup\$ 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. \$\endgroup\$
    – Dave Tweed
    Commented Sep 22, 2021 at 13:51
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    \$\begingroup\$ If the resistance measurement shows no problem then how do you know they have failed? \$\endgroup\$ Commented Sep 22, 2021 at 14:06
  • \$\begingroup\$ 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. \$\endgroup\$
    – Lundin
    Commented Sep 22, 2021 at 14:33
  • \$\begingroup\$ Question: How do you know you're removing enough coating??? \$\endgroup\$
    – Kyle B
    Commented Sep 22, 2021 at 15:04

2 Answers 2

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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.

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    \$\begingroup\$ 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. \$\endgroup\$
    – Dave Tweed
    Commented Sep 22, 2021 at 13:49
  • \$\begingroup\$ @DaveTweed Could you elaborate? Sounds like soldering to a post just changes the point that is stressed. \$\endgroup\$
    – DKNguyen
    Commented Sep 22, 2021 at 14:13
  • \$\begingroup\$ 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. \$\endgroup\$ Commented Sep 22, 2021 at 14:15
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    \$\begingroup\$ @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. \$\endgroup\$
    – Dave Tweed
    Commented Sep 22, 2021 at 14:27
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    \$\begingroup\$ I will try the wire post approach next and see how that works. \$\endgroup\$
    – Conrad
    Commented Sep 23, 2021 at 6:59
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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.
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  • \$\begingroup\$ 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. \$\endgroup\$
    – Conrad
    Commented Sep 23, 2021 at 7:35

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