Ron,

You are not quite correct in your assumptions. First, the "A" side of
the table is for component lead terminations, not lands and conductors.
The lands, as well as conductors, fall on the "B" side of the table.
That said, the B2 value would be higher than the A6 value since there
are always traces of contamination on the surface of the board which
will affect the isolation characteristics. Whereas the component leads
face air as an insulator and will not see the full effects of surface
contamination.

Hope this clears it up some.

Regards,

  Gary Ferrari
Ferrari Technical Services
860-350-9300

Ron O wrote:
> Can anyone help interpret the IPC2221 Electrical Conductor Spacing chart
> 6-1 (pg 39)?
> I have a pcb requirement of 500V spacing between nets. Board is FR4
> 2layer, soldermask both sides, no conformal coating. Based on the chart,
> I need a B4 spacing (0.8mm) for the soldermasked conductors and an A6
> spacing (1.5mm) for the exposed pad areas (I'm assuming this is pad to
> pad spacing not pad to coated trace spacing).
>
> Is my interpretation correct so far?
>
> Next question is the column B2, which applies to uncoated (no SM or
> conformal) and has a spacing requirement of 2.5mm. So this would be bare
> trace to bare trace. Why would this spacing need to be greater than bare
> pad to bare pad (the A6 rating)?
>
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