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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