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You can easy find some SMD components which are pretty small for such a high voltage (500V @ 0805 package for example - crazy?!). 0805 case would have 1.3 mm distance between the contacts, so I doubt that this voltage would be normal under real world circumstances.

So my questions are:

  1. Would you pick such component for your circuit? I'd better place two or even three components in series to make the voltage across 0805 case not more than 100-150 Volts or 150-200 for 1206. But this estimation based on my sensation (I never experienced any failures or never researched the limits)

  2. Probably such components should be used inside sealed cases or under a layer of a good insulating varnish (which is rare for me)? This could explain their existence.

  3. Do any of you guys have any experience with these components? What is the real-world V/mm limitation?

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  • \$\begingroup\$ It is up to the board layout engineer to make sure that the PCB is up to handling such voltages. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 4 '18 at 10:24
  • \$\begingroup\$ You're right, @IgnacioVazquez-Abrams. But the component has it's own contacts which is not under PCB engineer's control. Moreover - you will need some minimum space between pads to make the component solderable. \$\endgroup\$ – Roman Matveev Jan 4 '18 at 10:27
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    \$\begingroup\$ conformal coating can mitigate creepage problems. or perhaps the 500V is transient. \$\endgroup\$ – Jasen Jan 4 '18 at 10:30
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When deciding the voltage rating of a 'component on a board' system, you need to define what sort of rating you mean. Safety critical is much more stringent than simply working.

For instance, the withstand voltage on a clean dry board will be much higher than the rated personal safety voltage on a dirty board in a damp salt-laden atmosphere.

If the component is rated at 500v, then you can assume that the component itself, under conditions that the manufacturer specifies, will withstand 500v.

Would an 0805 footprint meet the creepage clearance regulations for separation between mains carrying and low voltage tracks, for personal safety? No. However, 'mains' is expected to have transients of at least 1500v on it, which is why capacitors for mains connection are specifically rated for such, see X and Y rated capacitors. For a regulated 500v, in a dry environment, when personal safety is not an issue, then there's no problem supporting 500v across an 0805 footprint.

If you are intending your PCB+components system to be used in a dirty damp environment, then you may well need a conformal coating to mitigate board surface contamination problems, even if you don't have to meet saftey rated creepage distances.

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  • \$\begingroup\$ Good answer Neil_UK. Regarding dirt: In some applications (i.e. trains), it is common to varnish any PCB holding SMT components. The varnishing is done to better withstand the effects of lightly conductive dust. The varnishing is done after all soldering and assembly. Prior to the varnishing connectors and other surfaces are temporarily covered to exclude these from being varnished. A more extreme method is potting en.wikipedia.org/wiki/Potting_(electronics) where parts or the whole PCB is covered in a compound, i.e. epoxy. \$\endgroup\$ – boink Jan 9 '18 at 19:14

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