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conformal coating

Discussion in 'Electronic Design' started by John Larkin, Nov 8, 2004.

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  1. John Larkin

    John Larkin Guest

    Hi,

    can anybody suggest a good conformal coating? I have a PCB that is
    moisture sensitive; apparently the PCB itself absorbs enough moisture
    to change the laminate dielectric constant enough to cause me trouble.
    We coated the critical oscillator section with household polyurethane
    varnish, and it helped a lot; absorption after 48 hours at 100%
    humidity is down by about 3:1, and the recovery time constant is much
    better (shallower moisture penetration?). But this stuff is thin, and
    we'd like something gloppier that we can apply a bit thicker. We'll
    brush it on by hand, to keep it exactly where we want it.

    Any suggestions? We'd prefer something that sticks well, goes on
    fairly thick (high viscoscity) and is reasonably easy to rework. Who
    makes good stuff these days?

    John
     
  2. Here's a good overview of the types and manufacturers of conformal
    coatings:

    http://www.thermospray.com/ccfaq.html

    Most assembly houses should be able to do the coating for you. The
    ones I've seen have separate rooms, with their own ventilation and a
    robotic sprayer or something like that.


    Best regards,
    Spehro Pefhany
     
  3. There are some clear serviceable potting compounds out there that
    resemble silicon that could be considered for dipping. The stuff can
    be picked out in chunks for servicing then re-sealed. I'll ask my QC
    director for the MFGR's name tomorrow.

    The thin conformal coatings will likely create more nightmares than if
    left untreated. Nothing like leaking in moisture then sealing it in
    over time!
     
  4. Greg Neff

    Greg Neff Guest

    Type AR (acrylic) is the easiest to work with by hand, and the easiest
    to rework. In high volume type UR (urethane) is prevalent. For small
    jobs you will probably need something that is solvent cured. You will
    need good ventilation to work with it though. UV cure is best, since
    you don't have the solvent to evaporate. We have UV curing equipment
    but this is expensive. You might want to get something with a UV
    tracer in it for inspection. Look for MIL-I-46058C compliant
    coatings.

    For AR hand-work we use Humiseal 1B73 in spray cans.

    http://www.humiseal.com/protect/acrylic.htm

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  5. I've often wondered about this.

    Surely you must accept that you will *never* seal it against *some* form of
    moisture ingress.

    If that's the case then surely, once equilibrium with the outside world is
    reached, conformal coating will hold the mositure *in* as well as it will hold
    it *out*.

    What have I missed?

    Gibbo
     
  6. Generally, if it's that critical they'll bake them.

    Cheers!
    Rich
     
  7. John Larkin

    John Larkin Guest


    Well, if I can get the diffusion time constant up to 30 years or so,
    that would be OK.

    John
     
  8. Robert Baer

    Robert Baer Guest

    How about changing the PCB type?
    Try Megtron 5 from MEM: http://www.mem-or.com/
    I have some sample material if you have need for immediate delivery,
    but i would have to make a box for shipping and you would have to
    specify the sheet size and thickness (20 mil and 40 mill, up to 18x22).
     
  9. Fred Bloggs

    Fred Bloggs Guest

    Good link- the process is *all* in the application- an inexperienced
    user unaware of all the pitfalls can make the best product in the world
    fail.
     
  10. Ken Smith

    Ken Smith Guest

    If it was me, I'd put the whole thing in a sealed housing. The coating
    can increase the time until the board's vapor pressure matches the
    external vapor pressure, from hours to weeks but not to years.
     
  11. On Tue, 9 Nov 2004 18:47:13 +0000 (UTC), the renowned
    Can always put some dessicant (eg. Silica gel) in there. Silica gel is
    the RH equivalent of a capacitor. ;-)


    Best regards,
    Spehro Pefhany
     
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