[sdiy] guard ring Q

[Magnus Danielson]

Ian,

Ian Fritz wrote:
> 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!

You want metal on the surface, possibly on both sides of the board. As 
board gets contaminated you want to drain any creep currents into the 
guard-ring. No silkscreen and no coating over the guard-ring.

Best regards,
Magnus