[time-nuts] OCXO and fluctuations after EFC adjustment

[Ben Bradley]

Despite my interest in the precision high-end of electronic design
(and thus being a subscriber to this list), I have very little
experience with such high-fallutin' designs. Still, I've done and seen
some "interesting" things in my career. It's amazing how the minutiae
of even jelly bean components can cause product failures.

A few decades ago (many of my jobs were engineering positions at
companies that made equipment for the POTS phone line), the company I
was with had some rather large surface-mount ceramic capacitors that
were failing short in a new product that used them across the phone
line (they were RATED for this application right there on the data
sheet).  The concern was our product would some day short out the
phone line and someone with another phone on the line would have an
emergency and wouldn't be able to call 911 to get help. The possible
legal liability to the company was obvious. In a discussion with other
engineers and managers, someone wondered if it was the way the board
was manufactured, maybe the solder process caused some caps to crack,
or whether the failures were strictly the fault of the capacitor
manufacturer. I volunteered to test the caps in an environmental
chamber that wasn't being used at the time, putting them near the edge
of, but still within, specs. I made a jig that put a bias voltage on
many in parallel (using a mechanical spring connection, not soldered)
and left them for a few days/weeks. Out of 100 to 150 devices, about 5
or 10 became unacceptably leaky. I wrote up my report and emailed it
to the other engineers and managers involved, and didn't hear back
anything more about it. The good news (!) is the product wasn't in
production for very long for several reasons, more recently of course
that the POTS phone line is no longer the most common form of
telecommunications. The bad news is it's no longer helpful to have
POTS product design on my resume.

More recently, I saw this Kemet presentation on Digikey about tantalum
capacitors. Certainly for aluminum electrolytic capacitors, the rated
voltage is "the rated voltage" and as long as the capacitor never goes
ABOVE that voltage (and has no overcurrent that would heat it up,
etc.), the cap is good for its combination of temperature and lifetime
rating. I (and as far as I know, everone I've known) assumed this was
the same for tantalums, but it appears that's not the case (this
presentation mentions several failure causes and shows how they are
multiplicative). As you go from 1/2 rated voltage to full rated
voltage, the chances of a tantalum failing goes up substantially. The
implied rule seems to be for maximum reliability, don't operate a
tantalum above HALF the rated voltage. I'd heard a lot of anecdotal
things about tantalums suddenly shorting out for this or that reason,
but hadn't heard of this, and here it is straight from the
manufacturer.
https://www.digikey.com/en/ptm/k/kemet/derating-guidelines-for-surface-mount-tantalum-capacitors/tutorial