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I cant seem to properly solder a stranded wire to that cathode terminal. Checked the answer https://electronics.stackexchange.com/a/90605/35737 - fluxing it did not help and solder does not adhere to it. Perhaps I am not applying flux correctly. I just put on some tacky flux I have got, then touch the fluxed cathode with the hot iron and immediately touch the iron + cathode terminal with solder. it barely sticks to it. Afterwards I bring that copper knot over it and apply the iron + more solder to the assembly. It just comes off in small balls. Any hints ?

enter image description here

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  • \$\begingroup\$ What is that metal part you're trying to solder to, exactly? \$\endgroup\$
    – Finbarr
    Commented Dec 5, 2017 at 17:20
  • \$\begingroup\$ It's the spring part of a car 12v lighter socket plug. \$\endgroup\$
    – Passerby
    Commented Dec 5, 2017 at 22:44
  • \$\begingroup\$ Ah, then cathode is a rather curious term for it! \$\endgroup\$
    – Finbarr
    Commented Dec 6, 2017 at 11:15

2 Answers 2

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Most likely, not enough heat. Both the terminal and the wire need to be heated to the melting point of solder for the solder to flow properly. But that spring terminal is acting as a very effective heat sink.

The only real answer is a more powerful soldering iron, or an alternative method of making the joint, such as spot welding.

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  • \$\begingroup\$ That was it, just switched the tip to a larger one and it did the trick! \$\endgroup\$
    – kellogs
    Commented Dec 5, 2017 at 18:25
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The electrical contact seems to be a nickel-plated spring, in which case normal electrical solder (tin/silver or tin/lead) will adhere, but some electrical solder fluxes (rosin, RMA) may not work. If you can find an acid flux (normal for steel, stainless steel, or nickel alloys), use that to prepare the surface. After soldering to the nickel, rinse excess flux off, then you can solder to that tinned surface using electrical solder and flux.

Alpha Metals 51022 flux is one suitable product for this task.

Either acid flux and solder, or welding, are suitable for connecting wire to nickel, but 'electrical solder' with rosin flux is not.

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