[sdiy] guard ring Q

[cheater cheater]

I understand some people also have the guard ring covered in solder -
sometimes they don't if there's an option, because the solder oxidizes
and the surface becomes less conductive therefore making the guard
ring less effective - but that's experience with Pb solder. What about
the new solder? Does it oxidize as readily as the old solder?
I understand HF circuits have gold-plated guard rings around them...

Cheers

On Sun, Aug 3, 2008 at 4:49 AM, Roy J. Tellason <rtellason at verizon.net> wrote:
> 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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