Repost ---> PCBs and Moisture?

"EdV" wrote

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)

=3D 22.01p

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 not the components. This is also the first time we have used this board vendor(Malaysia). The previous builds(here in town) did not show this problem.

Ed V.

CustomPCB by any chance?

Maybe. Have you used them before? I did a "Reply to aauthor" earlier and forgot my "deja"/google id mail account is not reachable. Sorry for the misdirection.

Thanks, Ed V.

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).

"EdV" wrote in

OK, look for over-etched traces or fractures within the PCB. Measure the resistance of the specific traces around the clock oscillator. The copper has a higher coefficient of expansion than the glass/epoxy. it _could be_ that a trace is going open as the copper shrinks.

I'd be watching the suspect PCBs alone as they're chilled.

Good luck!

regards PN2222A

=A0

I checked the layout and there are guard traces leading from the xtal ground plane to the uC xtal pins. It is also laid in accordance with the uC manufacturers recommendation. The manufacturer is also pretty baffled by this one.

All of the components from a "bad" PWB; uC, xtal and caps work on a "good" board and all the same components from a "good" board don't work on a bad board. (at -20C it works at room fine)

Their technician also said that the capacitance at the xtal caps increases more on a bad board than a good board. Sorry I didn't get the numbers.

Thanks for the suggestions,

Ed V.

Place the clock circuitry on a small daughterboard, encapsulate it, and attach to main circuit by way of pins.

Some caps could have hygroscopic surfaces, and going below dew point makes the surface accessible to water. Also, many SMD caps are attached to the PCB via a spot of glue. Said glue spot typically does not displace all the space under an SMD part, therefore, you are left with another place for accumulations to occur. Another point for vacuum encapsulation, which displaces all voids allowing for little change.

Both pre and post assembly. You also have to be careful that your fab house is laminating your boards with very little moisture around.

Shouldn't be. You tried different PCB strata types... have you tried a different fab house?

That depends on both your assembly process, and your choice of soldering system, and most importantly, your post assembly cleaning process.

If it is a small board, as opposed to making the daughterboard, one supposes that one could encapsulate your entire assembly. If it a larger board, then the daughterboard suggestion is the best I could come up with for temperatures that low.

Yes, I was going to say that our (entirely unsatisfactory) experience with them was unrelated to your problem until I saw PN2222A's post. One of the problems we experienced was severe over-etching to the point of losing continuity on 12 thou traces on both straight sections and 45deg bends.

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Thanks for the info.

Thanks!