Your comment about how you have days without any shorts, then start
observing them might lead you to look at your etchant process, chemical
condition, line speed, etc etc...

...something to  consider, could those days you begin seeing shorts possibly
correlate to just before the etchant was replenished? replaced? or otherwise
have maintenance performed?

Aside from etch related conditions, drill or layer mis-registration in my
experience, is the second most likely cause of such unusual occuring shorts

Franklin

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