Re: Repost ---> PCBs and Moisture?

On Apr 7, 11:25 am, EdV <ed_vo...@xxxxxxxxxxx> wrote:
"EdV" <ed_vo...@xxxxxxxxxxx> wrote

We got a batch of boards built and two of them show a curious
tendency(consitent) to shut down the crystal oscillator at -20 C.  

The design engineers have swapped all of the clocking components from
two boards that work properly through temperature with two boards that
don't.  The "temperature intolerance" follows the PWBs

Do you have a guard ring around the sensitive input?

On-chip oscillators usually self-bias with a high impedance
in feedback to the oscillator 'input' pin from the output pin,
and a small amount of leakage current (either due to surface
cleaning or buried layers) is not unexpected. A printed
guard ring (even if it's only a broken ring, I.E. a C) will handle
the surface leakage, and you can replicate the guard in
any buried layers easily enough.

It isn't the board that's changing at low temperature, it's the
self-bias source (which is often just like a leakage current).
.

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