[sdiy] guard ring Q

[Ian Fritz]

At 09:08 AM 8/2/2008, Magnus Danielson wrote:

>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.

Thanks for your input.

I agree that that will work and is a good method.  BUT ...  you can only 
have "creep" currents if there is a potential difference driving 
them.  Having a guard ring -- even if it is buried -- should largely 
eliminate any electric fields in the vicinity of the pins being guarded, so 
there doesn't seem to be an effective mechanism to actually support the 
creep current flowing into the pins being guarded.

Perhaps there could be some surface charge buildup because the guard ring 
can't  effectively drain away extra leakage charge all along its length, 
but at some point the guard circuit *is* exposed to the surface, and 
drainage can occur at that point.


   Ian