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I am doing a hobby project where I am weaving stranded wire through vias on the PCB. There is a matching copper trace and solder mask under the stranded wire. The stranded wire is soldered onto the exposed copper trace. I do this to allow high amperage on the board which is only 1 oz copper weight. (Can't use another copper weight because of the PCB manufactorer)

I will also be adding conformal coating to this board, so then the question becomes:

Which type of flux should I be using?

Because flux residue should be cleaned when adding conformal coating, I was first considering a water soluble flux. But the data sheet says that stranded wire should not be used, which makes sense: Flux residue could be trapped inside the stranded wire.

So now I am considering to use a no-clean flux instead, because this flux residue should be OK to leave inside the stranded wire. It still needs to be cleaned on the surface however, to allow the conformal coating to bind. The downside is that cleanining it will be harder and nastier than a water soluble one.

What is my best option in this case?

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https://www.chemtronics.com/essential-guide-to-flux-for-soldering-electronics

Don't use fluxes with additives that are corrosive if it is hard to clean.

If not corrosive consider a dab of PU around the base for stress relief and also as a conformal coat. It is found used in all power supplies to bond heavy parts from vibration solder cracks.

I suggest ROL0 flux with low viscosity for a wire dip and manual solder process.

RO stands for rosin, the L is for low activity and the number 0 is for no detectable halides. a halide-free, rosin-based solder paste with low activity is categorized as ROL0.

Hopefully, you used Saturn PCB to compute the required track widths for current use. Add some margin as the conformal coating acts as thermal insulation as well.

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