There are several criteria to consider and it also depends on the type of resin
involved. Taking an ordinary FR-4, this contains some molecules of sodium chloride
which, under humid conditions, can easily ionise and slowly migrate under low voltage
gradients, even as low as 10 V/mm. This can create rogue charges for high impedance
circuits. Then, the old MIL-P-275 suggested a conductor spacing for unprotected outer
layers at low altitudes conditions approximating 150 V/mm (and this was in the days of
G-10: FR-4 is slightly more susceptible). Personally, I think that anything over 100
V/mm for high impedance inner or outer layers (>1E9 ohms) is asking for trouble. For
low impedance or power circuits, you can possibly double this figure for inner layers.
On some SIR meter circuits I designed, I had all the hi-Z tracks on inner layers and I
kept a 5 mm spacing around them, for 100 V max. Similar considerations should apply to
inter-layer spacings, except that no-one wants the PCB to be a centimetre thick :-(
The risk is punch-through if the voltage gradient is too high under humid conditions or
curing is poor or there are minute inclusions. Personally, I think a gradient of 2
kV/mm, such as you quote, is most certainly not overkill and more likely to be
underkill if your circuitry is Hi-Z, to be used under humid conditions or cannot be
guaranteed to be perfectly manufactured. IMHO, FWIW
Brian
Goran Matijasevic wrote:
> Hello, Technetters!
>
> I was wondering about the rules for conductor spacing in PWB for
> electrical clearance if there is up to 500 V (low current) in the
> conductors. IPC-2221 (table 6-1) states that innerlayer conductors
> should be 10 mils apart. What is the reasoning behind this and is this
> "overkill"? It states that this is for x-y and z-axis. Does this vary
> with different material sets? I assume someone did testing some time
> ago and came up with this table. Anybody has experience with departing
> from these rules? Do OEMs stick to them?
>
> Thanks in advance,
>
> Goran
> ___________________________________________
> Goran Matijasevic
> Ormet Corporation
> 2236 Rutherford Road, Suite 109
> Carlsbad, CA 92008
> Tel: (760) 931-7099 Fax: (760) 431-6971
> E-Mail: [log in to unmask]
>
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