[sdiy] guard ring Q

[Ian Fritz]

Hello PCB experts --

I'm looking at including a guard ring on a S/H board.  I know how guarding 
works electrically and how to lay out and connect the ring.  My question is 
about the role of the soldermask layer.  If the coating covers the ground 
ring, then the ring cannot intercept surface leakage current, so it seems 
to me that leakage currents can flow right over the coating from pad to pad 
on the board.  OTOH, the ring will still reduce the potential gradients 
around the guarded pins, so there should still be reasonably effective 
guarding just from electrostatic screening.

How is this question usually approached?

Thanks!

   Ian