Thermal compound becomes liquid when hot : normal ?

Discussion in 'Solid State' started by Bert 1100, Mar 2, 2017.

  1. Bert 1100

    Bert 1100 Super Member

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    I noticed yesterday that the thermal compound I use becomes very runny once it is heated up.
    When cold it is very thick, but once at maybe 80 degrees centigrade it is runny and the extra paste starts flowing and running off.
    I noticed this yesterday when I did a differential pair of transistors with the paste in between them and I was heating the shrink sleeve I had put around them.
    Then this morning I did a voltage regulator that gets quite hot, soon after switch on the fresh paste becomes liquid...

    So.. is it normal or did I buy junk ? It is a tube, supposedly made in Germany, bought from ebay...:confused:

    Thank you !
     

     

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  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Never had that problem, the current tube I am using is 'Dow Corning 340' - I forget where I got it from, but it definitely wasn't ebay.
     
  3. Bert 1100

    Bert 1100 Super Member

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    Thanks, I think I'll get some Dow Corning 340 from a reputable source.
    Thermal compound is not something to play with :eek:
     
  4. elnaldo

    elnaldo Addicted Member

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    I think the viscosity at different temperatures is detailed in the datasheets. That will give you an idea if it's normal or not.

    Remember to apply a very thin coat of it.
     
  5. whoaru99

    whoaru99 Epic Member

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    I might be more concerned with the 80C first? :idea:
     
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  6. ConradH

    ConradH Addicted Member

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    You might have gotten lucky. Read about phase change thermal compounds. Aavid is the one I know, but no doubt there are others. They liquefy when they get hot, resulting in a smaller gap and better performance. They're expensive. https://www.aavid.com/product-group/interface/phase-change

    Or, you might have something weird!
     

     

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  7. Blue Shadow

    Blue Shadow Waiting for Vintage Gear from this century Subscriber

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    47 grams of that good stuff cost about what a pound of the regular grease is.
     
  8. Bert 1100

    Bert 1100 Super Member

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    Well, I noticed that quality when I was doing the heatshrink on two diff pair transistors, so the temperature was applied. Of course this diff pair does not get hot.
    Now, that voltage regulator to-220 does get quite hot, maybe 60 deg. C.
    In fact my paste gets liquidy at maybe 50 deg. C, and that can't be good...or can it ?
    I definitely don't have the aavid stuff.
     
  9. ConradH

    ConradH Addicted Member

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    Is there any description of what you've got? Part number or something? Most silicone based thermal greases don't change viscosity much when heated. The Aavid product is a lot like very soft white candle wax in the way it liquefies.
     
  10. Bert 1100

    Bert 1100 Super Member

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    Thats what it looks like.
    I just saw on a german forum somebody mentioning that the "non toxic" stuff is often based on wax, thats what it feels like actually....
    A little heat, it becomes liquid, let it cool down and it will be thick again.

    So what do the experts say ? use it ? or get some dow corning 340 (or other ? suggestions welcome!)
    I'm about to place an order with mouser, so if anybody has a reference from them....
    Thank you !

    IMG_2489.jpg

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    IMG_2487.jpg
     
    Last edited: Mar 2, 2017
  11. roger2

    roger2 . Subscriber

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    Don't use it, that should be obvious I think :)

    If you are ordering from mouser get what Hyperion suggested if they have it. Or get this: http://www.digikey.com/product-detail/en/120-2/345-1007-ND/340305
    which is what EW has recommended in the past. The Wakefield also comes in small 4gram size which is more than enough for many applications IF spread very thin as should be done.


    EDIT: link corrected
     
    Last edited: Mar 2, 2017

     

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  12. hnash53

    hnash53 AK Subscriber Subscriber

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    Maybe it froze and its molecular behavior has changed???
     
  13. whoaru99

    whoaru99 Epic Member

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    I have a small jar of Wakefield Type 120 that I use. I've observed no unusual behavior of it.
     
  14. ConradH

    ConradH Addicted Member

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    That's exactly how the Aavid stuff works. The trick is you use very little of it, so capillary action keeps it in place, or wipe around the device with a rag while it's warm. I like it because I understand how it works and know it's better, if different than what you're used to. Others hate it, so do what you like. Remember that devices and heat sinks should be close to the same temperature. If you put your finger on a device and it's a lot hotter than the heat sink close by, the thermal connection isn't what it should be. The thermal compound needs to be present, but it needs to be as thin as possible- most people slather it on too heavily. The phase change (it melts) stuff solves that problem.
     
  15. Bert 1100

    Bert 1100 Super Member

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    Interesting answers !
    Thanks all !
     
  16. Jim Marantz

    Jim Marantz Active Member

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    I use Super Lube's heat-sink compound which works the same way. Very thin after it heats up. It seems to work very well just like the rest of their products. I've never had a problem.
     

     

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  17. markn2wae

    markn2wae Mark T N2WAE

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    Passing note- NEVER use Arctic Silver (the one that is silver based) on ENERGIZED devices (those that have Voltage applied to the device housing or tab)

    Mark T. :music:
     
  18. Bert 1100

    Bert 1100 Super Member

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    Yes, that's very clear, thanks for the warning anyway!
    I've got some wakefield-vette 120-2 from mouser on the way, we'll see how that behaves...
    In the meanwhile I'm not worried anymore that I've been using some sub-standard stuff, thank you AK :)
     

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