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.
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.