I would say that each different resin material has different breakdown
characteristics and little has been done to determine maximum voltage
gradients for each type. AFAIK, the original MIL spec (220???), later
incorporated into IPC specs was derived using G-10 material, which would
be mighty hard to find today. I know FR-4 of 2-3 decades ago was more
prone to breakdown than G-10, which had generally better electrical
characteristics, but these were sacrificed for lower flame propagation.
As for modern FR-4s and other resins, I haven't a clue. Maybe the time
has come to determine anew safe voltage gradients for each type of
material. This is not an easy task because there are many little-known
factors which can influence the priming of flashovers, including ion
mobility within the polymers, ionic heating, humidity gradients
throughout the polymer, cure levels, etc. I would guess there is enough
meat in the subject for a master's or even a doctor's thesis, so that
the industry can start to understand the implications.
My empirical rule of thumb for FR-4 has always been a peak-to-peak
low-frequency or DC gradient of 100 V/mm max, but even this causes ionic
migration.
Brian
Hechenberger Philipp wrote:
> Hello!
>
> does any one have any specifications or guidlines regarding conductor spacing in dependance of the PCB-material used? I couldn't find any information about this in the IPC-2220 series.
> We observed a difference between FR-2 boards and CEM-1. On a FR-2 board flashovers seem to happen more likely than on CEM-1 boards at the same conductor spacing and voltage. We are not sure if it is a matter of process material (e.g. flux) which reacts differently with the these two PCB materials or if it is a general design issue which needs to be considered for each material individually. Are there any design guidlines for the proper spacing of conductors at a certain voltage difference in dependance of the PCB material?
> I would be grateful for any answers and inputs! Thank you!
> Best regards
>
> Philipp Hechenberger
> Quality Engineer
>
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