[sdiy] guard ring Q

[Roy J. Tellason]

On Saturday 02 August 2008 11:08, Magnus Danielson wrote:
> 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.

Agreed.  I know ARP told me when I was there that they'd originally started 
out with a soldermask coating but ran into trouble in just this area (S/H 
circuits,  specifically) and had to do away with it because of contaminants 
creating leakage.

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