Connect with us

Arcing in Thermal Adhesive

Discussion in 'Electronic Design' started by D from BC, Mar 5, 2007.

Scroll to continue with content
  1. John Larkin

    John Larkin Guest


    You can use a thermally conductive epoxy that has very small fill
    particles, and add just a little dab of glass beads to enforce spacing
    until the epoxy sets. The thermal properties of the beads doesn't
    matter.

    John
     
  2. D from BC

    D from BC Guest

    On Tue, 06 Mar 2007 16:09:05 -0800, John Larkin

    [snip]
    I think thermal epoxies have better thermal conduction than glass. I
    might shoot for minimal glass beads.
    Also, too much glass and the bond strength probably gets worse.
    Too few glass beads and the part shorts out if it's glued on crooked.
    I'll guess...maybe 10 parts glass beads to 1 part epoxy....
    Somewhat of a probability problem.

    Dabbing sounds better than math.. :)
    D from BC
     
  3. D from BC

    D from BC Guest

    oops ..Correction..
    Not 10 parts glass beads to 1 part epoxy..
    Should be
    10 parts epoxy per glass bead...
    D from BC
     
  4. John  Larkin

    John Larkin Guest


    No, 100 parts epoxy to 1 part beads.

    John
     
  5. D from BC

    D from BC Guest

    Let's say.. the hot part has a 5mm x 5mm contact area with the heat
    sink.
    Assume the glass beads are 0.1mm in diameter.
    There are 50 x 50 places for glass beads.
    Using a 100:1 epoxy to glass bead ratio yields 25 glass beads over a
    5mm x 5mm contact area.
    Or 1 bead per sq mm..

    Kinda like holding up a table with 25 legs... :)
    100:1 looks good.
    I just guessed at the 10:1 ratio..
    Thanks
    D from BC
     
  6. GregS

    GregS Guest


    Diamonds.

    I have used very small dust just for insurance. The micron sized
    particles a a bit small, but clumps of the diamonds work for me.
    Diamond is something like 20 times the conductance of copper.
    Cheaper than you might think. I got three vials for $50, probably at
    least 20-25 teaspoons worth.15-25u 100 cts.
    greg
     
  7. GregS

    GregS Guest

    That was just one vial, I think I ordered 250 cts.

    greg
     
  8. John  Larkin

    John Larkin Guest

    Diamond grit is not super expensive, but I doubt it's worth it as a
    thermal filler. The thermal conductivity of a filled grease or epoxy
    is dominated not by the filler but by the epoxy or grease itself.
    There's not very good contact between the filler particles, so even if
    they were thermal superconductors the net result wouldn't be much
    better than some standard ceramic.

    Diamond particles as epoxy spacers are really no better than glass
    beads, since the density is low. The most important thing about spacer
    beads is that the gap be accurately controlled, and most grits or
    sandy things don't do that well. Cataphote does sell "spacer grade"
    beads, sorted for diameter, but they're very expensive.

    The key to good thermal conductivity is to have very flat, smooth
    surfaces and, if you need insulation, the absolute minimal insulating
    gap. A thin wafer of BeO or AlN, with silicone grease, under moderate
    pressure, is about as good as you can reasonably do. No spacered
    filled epoxy will be that good. Hard anodize and grease is very good,
    too.

    John
     
  9. Rich Grise

    Rich Grise Guest

    Just out of curiosity, where do you get these things, and how big
    should they be? I googled for "glass beads" and got 30,000 pages
    on how to make a necklace. ;-)

    Thanks,
    Rich
     
  10. John  Larkin

    John Larkin Guest

    Cataphote. Or your nearest beach, maybe. The Pacific stuff is kind of
    coarse and non-uniform; I'd guess that Pensacola has the best domestic
    sand.

    The Cataphote stuff is sold mostly as retroreflectors, but they do
    sell "spacer grade" beads.

    John
     
  11. GregS

    GregS Guest

    They do sell flat polished diamond specifically for his purpose.
    Some new semiconductors use diamond technology.

    greg

    greg
     
  12. D from BC

    D from BC Guest

    On Wed, 07 Mar 2007 13:48:37 GMT, (GregS) wrote:

    [snip]
    Probably ok for low breakdown voltage but the glass beads can set a
    precise predictable bond thickness.
    The more bond thickness, the higher the breakdown voltage.
    D from BC
     
  13. D from BC

    D from BC Guest

    BeO or AlN wafers?? I haven't seen that yet.
    Hopefully available at Digikey :)
    I'll have to google that someday. ...
    D from BC
     
  14. GregS

    GregS Guest

     
  15. g

    g Guest

    When I was epoxying LEDs, I set up a multimeter to measure ohms as I
    was adjusting spacing
    to make sure there was no low ohm shorts. I don't know how the
    Lumileds are spaced
    when they are assembled to make stars, but it seems epoxy is the only
    spacer and insulator.
    greg
     
  16. John  Larkin

    John Larkin Guest

    That's tiny. If you need serious cooling, solder or thermal epoxy the
    hot gadget to a copper heat spreader and insulate *that* from the
    grounded heatsink. More footprint, less theta.

    John
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-