The first thing to recognise with this question is that optoelectronic isolators may be used in a variety of applications where inputs and outputs need to be maintained at different potentials. However, many of these components are able to withstand considerable voltages: 2–3 kV is a typical specification, but many will function with voltage differences of over 10 kV.
At low voltages, the main problem will be the surface resistivity of the material, but, as the voltage increases, so does the potential for breakdown, either through the surface, or across the surface.
Breakdown through the thickness of the material is unlikely to
be much of a problem, unless the planned board is very thin.
In terms of the specification of the material, we have to be careful
to procure material of sufficient quality as defects, voids and
foreign particles in the board will lower the dielectric strength.
Breakdown is more likely across the surface, due to corona discharge. All sharp points must be avoided, and in particular sufficient gaps left between conductors with a high potential difference. Something to look out for in your later reading is the possibility of applying both solder resist and a conformal coating in order to improve the performance of the circuit, especially under conditions of elevated humidity.
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