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Conformal coating for highimpedence design?

Discussion in 'Electronic Design' started by Jim Miller, Sep 23, 2004.

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  1. Jim Miller

    Jim Miller Guest

    I'm beginning a design which will have a front end working in the subnanoamp
    region. Fortunately that part of the circuitry is a pretty small area of the
    design. Is there an affordable conformal coating which I could apply to keep
    leakage currents due to contamination to a minimum once I get it clean and

  2. John Larkin

    John Larkin Guest

    Polyurethane varnish, from the hardware store.

  3. Do you think Future (acrylic) would work?

    Best regards,
    Spehro Pefhany
  4. Guy Macon

    Guy Macon Guest

    Look here:
    Acrylic Lacquer Conformal Coating 419B

    Avoid any hardware store products. You don't know how well they adhere
    and whether they attack anything on the board, and they don't have a
    PWB-safe stripper.

    Also, strongly consider using grounded guard rings.
  5. Ken Smith

    Ken Smith Guest

    The simple answer is "no; put it in a sealed box".

    If this is a production unit, you need to study the conformal coating
    issue before you layout the PCB. In my experience, a sealed box is less
    costly in the long run.


    Don't use "no-clean flux" and be careful in selecting water washed fluxes.

    It looks like some lead free solder actually works so if CE will be an
    issue you can try them. I think they replace the lead with an mixture of
    plutonium and unobtainium.

    Dry the board before coating or placing in the box. This is usually done
    by placing them in the hot box over night or for a few days. In the long
    run the coated board will have the same vapor pressure inside as out but
    at least you get a clean start on it.

    Never apply power to the board until the coating has aged to the point
    where the strong smell is gone. Some coatings contain ions that you can
    move around when they are wet. If it still smells wet, its still "wet".

    Don't believe anything the maker of the coatings tells you. If they say
    the sky is blue, check. This is from sad experience. The people that
    apply the stuff are much more likely to know the right answers.

    Mil-spec coatings are just ones that have made it past the approval
    process. They worked well on tube based radios. If you aren't having to
    meet some mil-spec. do your own picking based on performance.
  6. Ken Smith

    Ken Smith Guest

    Do you think Future (acrylic) would work?[/QUOTE]

    Melted crayons?
  7. Ken Smith

    Ken Smith Guest

    Better yet, use driven guard rings. This will raise both the DC and AC
  8. I read in that Ken Smith
    'An' mixture? What a dreadful grammatical error! (;-) It's not
    plutonium, it's goofium.
  9. Most any of the commercially available UV viewable coatings will work
    but your biggest concern will be just how clean is clean. If you use a
    typical water soluable flux used in wave soldering you must be very,
    very certain that the boards are absolutely clean. If you coat over
    just the slightest amount of flux residue in an application like that
    you have identified you will have nothing but trouble when that flux
    becomes conductive in the presense of just the slightest humidity.

    Conformal coating is only as good as the surface it protects,
    especially in a subnanoamp environment. If your circuit can withstand
    hipot testing you may consider probing the board to verify that the
    flux is completely removed.

    I would clean and verify then pre-heat the assembly to around 130 deg
    F to flash off any local moisture and coat immediately after or even
    during the application of heat. If this is a short run you can use an
    enviromental chamber to preheat the board(s) then coat them. Once the
    coating is applied verify consistancy with a UV source. If all is well
    return the boards to the chamber for curing according to the coating
    mfgr's procedures.
  10. Robert Baer

    Robert Baer Guest

    You also might consider replacing FR-4 with a better material, such as
    Megtron 5 made by Matsushita Electronics Materials, also available thru
  11. Tony

    Tony Guest

    Guard rings are often a must (grounded if that's appropriate).

    FWIW I'd also like to share an experience with a sensitive inverting
    amplifier with switched feedback which worked over a 130dB range
    starting at around 10fA. The first production batch worked fine, but
    the next batch had some subtle drifts. As far as we could determine,
    the smt parts in the feedback networks had, more than before, bonded
    closely with the solder mask underneath and any flux that started out
    underneath couldn't be cleaned out, and caused leakage. The only way
    we could guarantee no problems was to route a few 1mm wide x 2mm long
    slots under the most sensitive few parts.

    Tony (remove the "_" to reply by email)
  12. John Larkin

    John Larkin Guest

    Don't know. You've got to be careful that the stuff doesn't attack any
    parts, and that it doesn't absorb moisture, as some varnishes do. The
    polyurethane varnish I used to use (when I was poor, and did this all
    myself) was nice except on polystyrene caps, and you could solder
    right through it for rework. My production people do this now, and I
    think they buy proper conformal coating material, probably the same as
    my old varnish but 5x the price.

  13. John Larkin

    John Larkin Guest

    Now, John, be nice; English is not our native language over here.

  14. John Larkin

    John Larkin Guest

    Second that. Ionic contamination from *anything* water-soluble can be
    a nightmare.

  15. Roy McCammon

    Roy McCammon Guest

    So, John
    How is the lead free initiative working out?
  16. I read in that John Larkin <[email protected]> wrote (in <[email protected]>) about 'Conformal coating for highimpedence design?', on Thu,
    23 Sep 2004:
    I'll believe that when you post in Choctaw. Or, preferably, Lockjaw.
  17. I read in that Roy McCammon <>
    I'm not following it in detail, because no-one is paying me to do so,
    and people ARE paying me to do many other things.

    I believe that many of the consumer products manufacturer have switched
    to lead-free, and there has lately been some sort of agreement on which
    alloys to use. Initially, there were compatibility problems between the
    alloys used by manufacturers and those available to service technicians.

    I still have a large reel of 60/40, and I'm not proposing to throw it
    away, even if I could do that legally. (;-)

    I've been brought up on lead; lead water pipes, heavy lead paint, 40/60
    solder initially, then 60/40. Any damage has already been done. That's
    probably why I'm not as clever as you guys.
  18. John Larkin

    John Larkin Guest

    My native language is Yat [1]. We conjugate...

    I We
    You Y'all
    He, She, It They


    [1] The spoken language of New Orleans, named for the Aloha-like
    universal greeting, "Where y'at?"
  19. Jim Thompson

    Jim Thompson Guest

    On Thu, 23 Sep 2004 13:23:13 -0700, John Larkin

    Aren't the correct forms "We'uns" and "Them" ?:)

    I called back to WV the other day to make hotel reservations for my
    wife's 45th high school reunion. I almost died from the slow talking

    ...Jim Thompson
  20. John Larkin

    John Larkin Guest

    No, you're thinking about the South. New Orleans is its own private
    island of weirdness. The native accent resembles, if anything,

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