A few years ago I had a similar problem with potting compound causing
oscillator problems. My fix was to place a glob of RTV (clear bathtub
caulk) around the crystal and oscillator circuit, and allow it to cure fully
before you pot the circuit with other stuff.

One other possible problem is that while the 27pF caps can TAKE 80C,
they might lose their capacitance badly at 80C.

--Bob

On 6/27/08, Scott Touchton <STouchton@petstop.com> wrote:
> I had almost this exact problem several years ago.  Turns out the
> increased permeativity of the potting compound caused too much parasitic
> capacitance.  The low power oscillator design was marginal at best, and
> this extra capacitance pushed it over the edge.
>
> Removed the potting and all was well.  You might want to try that.
>
>
> Also, I have had problems with Epson crystals damaged during reflow (in
> a Quad oven running the correct profile).  Seems some crystals were
> marginal and would not sustain the profile spec'd by Epson.  Changed to
> another manufacturer and that problem went away.
>
> Scott F. Touchton
>
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Leslie, Shawn J
> Sent: Friday, June 27, 2008 1:13 PM
> To: 'piclist@mit.edu'
> Subject: [PIC] Timer 1 Oscillator Failure at high temperature
>
> I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator as
> an RTC. I am driving the oscillator with a Epson FC-135 series 32.768KHz
> 12.5pF crystal good to 85C. I am hanging 27pF caps off the crystal.
> After 2 years of successful builds we are suddenly having this circuit
> fail at high temp. The oscillator fails at 60-70C and a few even do not
> work at room temperature. I am having 40% fallout at temp. If you lower
> the temperature to <40C it starts to work while power is applied. The
> problem is made more difficult since the units are potted. We cannot get
> at the components to troubleshoot. We are trying to test unpotted units
> over temperature. In the meantime I have tried to find a threshold of
> failure on a working unpotted unit by increasing and decreasing the 27pF
> caps - amazingly the circuit works at room temperature with 47pF caps on
> the high end to 6pF on the low end. With 6pF I can make it fail if I
> raise the temperature to 70C, but as it cools it begins to wor!
>  k. I have wrote Epson for crystal ideas and sent them my layout - any
> ideas from you guys? What have you all used? Maybe someone has a
> successful crystal oscillator design I could use.
>
> Cheers,
> Shawn
>
>
>
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