Your surmise may be partially true if you had FR-4 between the
conductors, but you don't, you have air, and often humid air, at that.
When I was a student, I was given a rule of thumb that humid air broke
down (i.e. ionised) at a gradient of 15,000 V/inch (600 V/mm). Does that
tell you anything? Of course, this rule of thumb is not a scientific
absolute as there are many more variables, such as air pressure
(altitude), temperature, relative humidity and, in the case of
semi-planar devices such as PCBs, surface contamination.
However, it doesn't even end there. FR-4 contains molecules of sodium
chloride which "attract" humidity and can therefore ionise under the
influence of a voltage gradient, and migrate within the resin matrix.
This is measurable at gradients as low as 5 V/mm and can cause all sorts
of funny effects at very high impedances, and it can "remember" its
history over hours, because the fields between the positive and negative
ions (after the causal gradient is removed) are so low that the reverse
migration is very slow. I described this phenomenon in papers published
in the second half of the 1980s and I also adjudicated the dissertation
of a student at the Swiss Federal Institute of Technology who studied
this phenomenon. Because of this, I recommended a max gradient of 100
V/mm for critical tracks on circuits working up to 10 megohms impedance,
down to 5 V/mm at the order of a teraohm.
Brian
- Bogert wrote:
> February 2, 2005
>
> Table 6-1 defines minimum electrical spacing requirements between traces on a PWB. What is the technical basis for the Table? Is there a formula associated with the values in the table? For example, if I have a PWB operating at 30 volts DC, the minimum spacing required between internal traces is 0.05 mm. What if a PWB operates at 30 VDC but only has 0.04 mm spacing, is there a technical concern? Based on what formula or technical consideration?
>
> The minimum spacing in the Table are much less than the electric strength requirements for FR4. For example, my understanding is that FR4 is about 1250 volts/mil. Therefore, at 1 mil of spacing the PWB should be able to withstand 1250 volts between conductors.
>
> The values in the table are also different than those included in UL 840. Why?
>
> Any help you can offer on this point would be appreciated.
>
>
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