FET mounting issue

Discussion in 'General Electronics Chat' started by seagull369, Feb 18, 2016.

  1. seagull369

    Thread Starter New Member

    Jan 23, 2016
    Running into some trouble re-mounting some FETs onto a heat sink in a power inverter. Originally the FETs were secured onto a heat sink using thermal conductive tape in between. When I reassembled, I did away with the tape and just used thermal compound. When I put everything back together, and connected the power up, the inverter overload buzzer went off and unit failed to operate. I took it apart and removed the FETs from the heat sink reconnected the inverter with it open and the inverter worked just fine. To double check, I remounted the FETs again as before, and the buzzer went off again.

    Is it normal for transistors of this type to require isolation like this when being mounted? I thought the manufacturer used the tape perhaps cause it was a less messy option than the compound, but it seems I'm wrong.
  2. DickCappels


    Aug 21, 2008
    We can't give you a conclusive answer without more information such as how the transistors were arranged in the circuit and what kind of packages they are in.

    Many high current MOSFETs come in plastic cases with metal heatsink tabs. These tabs are almost always connected to the drain. It might be that you are shorting the drains together, which is probably bad, or that you are shorting them to ground, which is also bad.

    Usually a thermally conductive insulator is used, such as Mica or silicone impregnated fiberglass. In more exotic cases, a ceramic insulator.

    Most likely if you use thermally conductive insulators your problem will be solved. Thermal grease such as Wakefield Compound should be applied thinly and evenly to both sides of the insulator.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    The paste is not intended to be "structural", it is just intended to fill in the air gaps between the tab on the transistor an insulator (tape, film, chip, whatever). Anything liquid is going to flow when it is tightened down and allow metal-metal contact if you don't have a rigid insulator between.
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    As a small follow-up to Dick Cappels' comment, how were the fets attached to the heat sink? Were screws used? Screws can be isolated electrically by using nylon or other insulator "hat" washers. Were such washers present? Or, was the attachment by pressure on the plastic part only? If screws were used, then you probably need to find the insulating washers. In other words, using an electrically insulating heat pad won't cure the condition if non-insulated screws are used.

    DickCappels likes this.
  5. seagull369

    Thread Starter New Member

    Jan 23, 2016
    Thank you all again for the great, speedy replies.

    The FETs. do have a metal heat sink tabs on them, as you described, DickCappels

    To answer your question jpanhalt, screws are used but they are on the PCB board the transistors are soldered to and don't actually make any electrical contact with them directly or indirectly.

    So, looks like I'm going going to be picking up some thermal tape. Would there happen to be a tape out there that would preclude the need for using the compound. If so, I guess my next question would be is it as effective in cooling as a tape and thermal compound combination?
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Have fun.. all can be purchased from your favorite electrical supplier.. Digikey/Mouser/Newark/Jameco,etc...
    The "tape" you have is thermally conductive and electrically insulating type

    No need to use tape and compound (the thermal performance will just suffer).. The tape more than likely isn't doing anything as far as mechanical attachment either.. Just conducting heat and blocking the flow of electricity..

    The datasheet for the FET will tell you if its tab needs to be insulated or not.. (clearly it does)
  7. crutschow


    Mar 14, 2008
    I would think a very thin coating of thermal compound on both sides of the tape would help the thermal conductivity, but it's probably not needed if the original had no compound on the tape.
    GopherT likes this.
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Thermal compound is ONLY used to fill microscopic gaps between "hard" surfaces (aluminum/copper/plastic,etc..) to avoid small air pockets when no other thermal interface material is being used..

    Its not used with thermal tape/pads because thats what they already do..

    Adding more material is only going to increase the thermal resistance..
  9. Lestraveled

    Well-Known Member

    May 19, 2014

    Sekisui makes a great tape for electronic use and I think that, or something like it, was used to attach the Mosfet to the heat sink. I have a roll of it and it is very handy stuff.


    3M makes a double sided tape called "VHB" tape. (Very High Bond). You could probably find some locally. Don't get the foam type because it would be a thermal insulator. (Modern Marvels included VHB tape in their show on adhesives.)

    I suggest you put it back to the way it originally was, clean the compound off and use double sided tape.
  10. seagull369

    Thread Starter New Member

    Jan 23, 2016
    In the original setup, compound was actually used on the side on the tape that faced the heat sink but not on the side of the FETs.

    I'll buy that tape you mentioned, Lestraveled. The original tape (or pad) that was used in the inverter was the foam-type.

    Thanks again for all the help!