I am soldering a 5-12V 5-8amp max electronic to a pcb board, without a schematics. I just want to ask how far should each joint in PCB board? what I mean is for example a cathode and anode when soldering a led emitting diode how far should each pin be? I am worried that a arc electric may travel because of close contact to one another, that its impossible to extend for other electronic pin like mosfet and jb connector. 
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7\$\begingroup\$ At these low voltages, if your eyes can see a space, you are fine. There won't be an arc. \$\endgroup\$– DKNguyenCommented Apr 20, 2021 at 5:05
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2\$\begingroup\$ For low voltage, 0.1mm is enough. \$\endgroup\$– user57037Commented Apr 20, 2021 at 5:49
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1\$\begingroup\$ A bigger concern is that most of these joints are cold and won't stand oxidation nor physical force very well. I've done more sloppy solder jobs than I can count myself - out of experience from my own mistakes in the past I'd strongly recommend to learn how to do it proper from the start. It will save you lots of time troubleshooting broken prototype boards. \$\endgroup\$– LundinCommented Apr 20, 2021 at 14:35
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\$\begingroup\$ @Lundin I am using lead free solder would be the case maybe? Anyway I still newbie to soldering but would flux help? \$\endgroup\$– Norm OnlyCommented Apr 21, 2021 at 19:24
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\$\begingroup\$ @NormOnly This isn't caused by lead free solder. My boards looked exactly like this when I was a newbie too. The thing is, we shouldn't treat solder as some fancy hot glue :) The purpose isn't just to hold things together mechanically, but also connect them electrically and that the joint will endure over time. Simply put, melting the solder isn't sufficient, it should wet across the two metal surfaces you are connecting. The majority of the joints in the picture have not done this. \$\endgroup\$– LundinCommented Apr 22, 2021 at 7:12
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1 Answer
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The minimum breakdown voltage for a gap of any size in air at 1 bar is about 360V. That minimum occurs with a gap of about 8um. Image from 2006 paper "Electrical breakdown across micron scale gaps in MEMS structures"
0.008mm is a very small gap so basically you don't need to worry about it. Of course if conductive debris or liquid falls on the board it will be more likely to cause problems with a narrow gap.
Mains voltage is another matter, and it's good to maintain gaps in the >8mm range, perhaps more, depending on various factors.
answered Apr 20, 2021 at 5:22
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\$\begingroup\$ You still can't get arcs in air from mains voltage, right? You just want large gaps in case of spills, accidental shorts, dust contamination, etc? \$\endgroup\$ Commented Apr 20, 2021 at 9:40
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\$\begingroup\$ @user253751 Engineering for what should happen is easy, engineering for what does happen only sometimes is not so obvious. You can't get arcs in air from 240 V rms, or 340 V peak. However, mains will frequently have 1500 V spikes on it, and these can jump significant gaps. \$\endgroup\$– Neil_UKCommented Apr 20, 2021 at 10:00
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\$\begingroup\$ extending your plot, 1500V should be able to jump about 0.3 mm or less. That doesn't really explain the remaining 7.7mm. But thanks for pointing out that power surges happen. \$\endgroup\$ Commented Apr 20, 2021 at 10:06