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Repost ---> PCBs and Moisture?

Discussion in 'Electronic Design' started by EdV, Apr 7, 2008.

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  1. EdV

    EdV Guest

    sorry to repost but I thought mine may have got lost in the sea of
    spam:

    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?


    #3 I could address with vapor degreasing or other cleaning. #1 and 2
    would mean baking at 100C or so which I guess is ok to see if it has
    an effect.


    Any thoughts, insights or expereinces as always are welcome.


    Ed V.
     
  2. PN2222A

    PN2222A Guest

    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)
     
  3. EdV

    EdV Guest

    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.
     
  4. EdV

    EdV Guest

    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.
     
  5. whit3rd

    whit3rd Guest

    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).
     
  6. PN2222A

    PN2222A Guest

    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
     
  7. EdV

    EdV Guest

    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.
     
  8. 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.
    100 C bake for an hour is a good thing. You can also place it in a
    vacuum for a while to release any trapped moisture in the PCB strata as
    well.
    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.
     
  9. EdV

    EdV Guest

    Thanks for the info.
     
  10. EdV

    EdV Guest

    Thanks!
     
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