Xiaofan Chen wrote:

>>> Yesterday an engineer showed me a strange thing. It is an isolated DC
>>> input I/O with relay output. The isolation specification is low. So
>>> every input is going through an opto-coupler and the circuits are
>>> quite simple. Strange thing is that there is a long trace underneath
>>> all the optos and it is connecting to the common shield connection.
>>> Apparently this is not for safety reasons as it actually reduce
>>> clearance/creepage distance (still enough).
>> 
>> I would imagine that is a flashover guard. Spurious high voltage on an
>> input pin should arc to the protective guard before reaching the logic
>> side. 
> 
> I was thinking the same. Last time we did something similar but I
> remember then the bare copper traces needed to be exposed. Am I wrong?

If it's for taking down high frequencies, the capacitive coupling through
the solder mask may be enough.

Gerhard

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