Re: Repost ---> PCBs and Moisture?
- From: "PN2222A" <pn2222a@xxxxxxxxxxxxx>
- Date: Mon, 07 Apr 2008 16:09:24 GMT
"EdV" <ed_vogel@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. We
also notice that the capacitance of the xtal caps changes more on the
"bad" PCBs at -20C than the "good" boards.
1. Moisture in PCB Layers that didn't get baked out?
2. Uncured epoxy betweem layers?
3. Residual flux?
Why do you suspect the PCB?
First, rule out failures due to the capacitors.
If the caps used by your vendor have a different tempco than you
specified,
then they may cause the oscillator to stop.
Measure the caps individually, outside of the PCB / oscillator
environment.
If that doesn't answer your question, measure the characteristics of the
crystal over temperature (good ones and bad ones).
Also determine the gain of your amplifier stage over temperature.
regards
PN2222A
NPN (Is = 14.34f Xti = 3 Eg = 1.11 Vaf = 74.03 Bf = 255.9 Ne = 1.307 Ise
= 14.34 Ikf = .2847 Xtb = 1.5 Br = 6.092 Isc = 0
Ikr = 0 Rc = 1 Cjc = 7.306p Mjc = .3416 Vjc = .75 Fc = .5 Cje = 22.01p
Mje = .377 Vje = .75 Tr = 46.91n Tf = 411.1p Itf = .6
Vtf = 1.7 Xtf = 3 Rb = 10)
.
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