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I've designed a board that I ordered from JLCPCB. I have traces with 7 mm lowest spacing between them and with voltage of at most 400V AC, 525V peak between them.

The traces couldn't be wider due to ensure proper spacing between them, so to increase the current carrying capability, I exposed the copper and added solder on top.

However, I noticed that due to the large amount of solder, there was also a large amount of yellow flux residue on the board. Can this residue decrease the breakdown voltage between the traces for the voltages and trace widths mentioned to problematic values?

The solder I'm using is lead solder and has these labels: Sn60Pb SW32 2,2%

If this residue is problematic, should I rather use a different solder (if yes, which one please) or clean the residue of this solder? Or is there any better solution?

Also, does wave soldering also leave problematic (although barely visible) residue?

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  • \$\begingroup\$ A picture of the board would be helpful. It sounds as if you have a through-hole PCBA? \$\endgroup\$
    – Lundin
    Commented Nov 15 at 7:41
  • \$\begingroup\$ The standard "no clean" flux is supposedly not very conductive. It can be cleaned with isopropyl and in some cases water. A small ESD brush plus isopropyl is probably the best. Now if you are concerned about creepage, the same situation could arise when there is dust and dirt covering the PCB. So maybe considering milled air spaces in between the traces. \$\endgroup\$
    – Lundin
    Commented Nov 15 at 7:43

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You could have specified that the boards should be cleaned for a nominal cost adder.

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All types of soldering leave flux residue. Wave soldering more so since the flux has to be spread over the entire board. Whether it is cleaned off depends on the type of flux and the board requirements. Rosin flux is fairly benign, some other types less so.

I don't know what standards you have to meet, but you could also have a slot milled between the traces to increase the creepage distance to 8mm or more.

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  • \$\begingroup\$ That is for my last question. But the most important is about the solder I'm adding because I don't know of any PCB assembly service that adds solder on top of copper-exposed traces. So I've added the solder myself, after the boards arrived, that is what I'm asking about. I don't HAVE to meet standards since it is a DIY project, but I would like to meet as much as possible for safety reasons. \$\endgroup\$
    – Ohmnivore
    Commented Nov 14 at 19:19
  • \$\begingroup\$ You are adding solder yourself so you presumably know exactly what flux core you have. RMA (Rosin mildly activated) fluxes such as Kester 44 do not really have to be removed in most applications, but you can remove them with alcohol or other solvents and it helps inspect the board. If you don't know what flux you have, assuming it's electronic grade solder, remove it thoroughly with a brush and solvent. If it's not electronic grade, you might have other issues. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Nov 14 at 19:30
  • \$\begingroup\$ The flux of the solder I'm using is SW32. The solder is Sn60Pb. Do I need to clean this and if yes how? What solder would you suggest that would perhaps be ok and has a flux that would not decrease the breakdown voltage? \$\endgroup\$
    – Ohmnivore
    Commented Nov 14 at 19:38
  • \$\begingroup\$ I don't like no-clean flux much, it leaks to a significant extent on some kinds of analog PCBs (where a 100MΩ makes a difference) and the residue it leaves is almost impossible to remove using ordinary solvents without vigorous physical scrubbing. In your case, it's probably okay to just leave it, assuming it's not a safety issue. The makers recommend proprietary cleaners such as Zestron and Vigon if cleaning is deemed necessary. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Nov 14 at 20:01
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    \$\begingroup\$ To that last point, solder has about 9 times the resistivity of copper, so you need a layer of solder that's 9 times a thick as the copper just to cut the total resistance in half. \$\endgroup\$
    – Dave Tweed
    Commented Nov 14 at 22:37

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