Applying conformal coating

I have read about conformal coatings, but it applies to non-heat dissipating devices. Could I get some advice on applying an acrylic coating to the myriad of electronics that contain
  • transistors with the metal tabs held down to heat sinks
  • 3 watt or greater ceramic resistors
  • PCB mounted potentiometers
  • electrolytic capacitors

I tried coating an audio amplifier containing MOSFETs screwed to an anodized heat sink with white grease. The white grease ran when I sprayed hexanes for the initial wash. This prevented adhesion of the acrylic spray where the grease ran. I also found the coating ran underneath the masking tape.

Conformal coating seems like cheap corrosion insurance. $15 for the acrylic spray and $15 for the mixed hexanes spray wash. I also hear, please correct me, that acrylic can reduce tin whiskers on new electronics that do not have leaded solder.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Re: Applying conformal coating

    The few times I used conformal coating in a production environment, it was to hold down rework wires. Sort of nice in a high vibration environment.

    In general, any board conformal coated is almost unrepairable. For (large) expensive boards, we wanted to be able to repair them.

    Regarding preventing failures... For multilayer printed circuit boards, there can be copper dendrite growths too that short out in between layers (conformal coating will not prevent--This is usually a PCB mfg. problem).

    The general solution for circuit boards exposed to weather... Put them in a (best as you can) sealed metal/plastic box. Install the box where it will be used, and drill a small weep hole at the lowest corner of the box.

    It is almost impossible to (cost effectively) seal an electronics enclosure that is exposed to weather--Best bet it a small weep hole to let water/condensation drain out (and small enough to keep most insects out).

    -Bill "my 2 cents" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    Re: Applying conformal coating

    How does conformal coating compare to full potting? Seems like potting would be even more durable (and completely unrepairable). On the other hand, I did manage to get the soft silicon gel out of an engine control computer.

    I am available for custom hardware/firmware development

  • mike95490mike95490 Solar Expert Posts: 9,274 ✭✭✭✭✭
    Re: Applying conformal coating
    KC-watts wrote: »
    I also hear, please correct me, that acrylic can reduce tin whiskers on new electronics that do not have leaded solder.

    Myth. A thin layer of plastic will not stop a metal crystal from growing through it. It might give you a couple extra days. 5% lead in the solder is the fix, but that's been "banned" except for spacecraft and medical electronics. "lead free" solder, is NOT really solder, it's hot melt tin glue.
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  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    Re: Applying conformal coating
    KC-watts wrote: »
    I also hear, please correct me, that acrylic can reduce tin whiskers on new electronics that do not have leaded solder.

    Other coatings will do that. See 2010-Panashchenko-IPC-Tin-Whisker

    I am available for custom hardware/firmware development

  • SulfurSulfur Solar Expert Posts: 62 ✭✭
    Re: Applying conformal coating

    I have found computer hard drives to be the most problematic of circuit boards where I work, which has small amount of H2S in the air. I have sprayed the exposed circuit boards of hard drives with acrylic spray and it does stop them from corroding where coated, but the other side or other areas still corrode and eventually kills the drive. Hitachi seems to have more recently been coating their spin drives or using lead since most don't corrode, I also have had good luck with all SSD drives in this H2S environment. Old non ROHS drives all don't corrode of course. I did seem to kill one drive coating it with acrylic fairly thick, or else it was DOA, not sure which.
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Re: Applying conformal coating

    Coatings can add "capacitance" to read channel/timing circuits and could (I believe) cause analog specs to drift out of range, impedance issues, and crosstalk.

    Is that what happened with your drives--I do not have a clue.

    Disk drives also include breather/vents (for the "sealed" disk drive housing)--Don't plug those up with sprays.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SulfurSulfur Solar Expert Posts: 62 ✭✭
    Re: Applying conformal coating

    I am not sure Bill what happened to that drive, but either of your suggestions sound plausible. I don't coat them anymore since I have found certain drives to be somewhat resistant to the corrosion, I assume they are coating the board? Spin drives are lasting a couple years now rather than a few months. I think the corrosion conducted electricity and created shorts between circuits on the board. The corrosion would wipe right off with my finger and is black, sometimes wiping it off would even make the drive work again briefly.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Applying conformal coating
    Sulfur wrote: »
    I am not sure Bill what happened to that drive, but either of your suggestions sound plausible. I don't coat them anymore since I have found certain drives to be somewhat resistant to the corrosion, I assume they are coating the board? Spin drives are lasting a couple years now rather than a few months. I think the corrosion conducted electricity and created shorts between circuits on the board. The corrosion would wipe right off with my finger and is black, sometimes wiping it off would even make the drive work again briefly.

    yikes, that sounds like mold. you do indeed have a humidity problem.
  • SulfurSulfur Solar Expert Posts: 62 ✭✭
    Re: Applying conformal coating
    niel wrote: »
    yikes, that sounds like mold. you do indeed have a humidity problem.

    I assume that was a joke, no it is not mold. It is Hydrogen Sulfide attacking solder that does not have lead. Very low humidity here.
  • KC-wattsKC-watts Solar Expert Posts: 37
    Re: Applying conformal coating

    Sulfur, have you considered doing some accelerated environment testing? Occasionally I'll toss scrap electronics in a box with a hydrogen chloride generator (tray of muriatic acid and Damp Rid). I want to discriminate between ocean salt buildup from chloride corroded metals in my boat. That, and I also like to torture failed electronics.

    Look at this pdf link for electronics in corrosive gasses.
    http://www.purafil.com/Literature/Control_of_Corrosive_Gases_to_Avoid_Electrical_Equipment_Failure.pdf
    It mentioned low levels of sulfur dioxide passivate reactive metals, retarding corrosion. There must be a reason I didn't find a company that markets pre-tarnished PCBs.
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