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Recommend potting compound for my application?

Discussion in 'Electronic Design' started by [email protected], Mar 8, 2005.

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  1. Guest

    Hello,

    I have a small circuit board that is just several "high power" SMT
    LED's which require reasonably large copper pads on the board to
    dissipate the heat. This small LED board is about 1.5" long and 1"
    wide and fits inside a similarly shaped recess in the back of a plastic
    lens. It will be placed LED side down into the plastic lens and then I
    need to expoxy/pot the PCB into the plastic lens housing. The epoxy
    should be thick enough so that it doesn't run down the size of the PCB
    and interfere with the LED's. It should also be clear so that any drops
    do not show up as discolorations inside the clear lens. It also
    (ideally) have good thermal properties and help to dissipate the heat
    generated by the LED's. Of course it needs to be non-conductive and
    should hold up well to outdoor temperatures and be unaffected by
    contact with water.

    Can anyone recommend such a beast? Or can anyone recommend a company I
    can contact who has a line of expoxies/potting compounds and see which
    one would work for me?

    Thanks!
    Mike
     
  2. Guest

    Mike,

    I was in a similar situation with my product. Unfortunately I think
    you're going to have a hard time finding something to suit all of your
    criteria. In my experience potting compound is not very good at
    thermal transfer. The stuff that has the lowest thermal resistance is
    not clear. A lot of it is toxic and needs special handling precautions
    (something to be concerned about if you go into production). That, and
    try to perform failure analysis on a dead product through potting
    material. You have a number of headaches in store for you.

    Contact Dow Automotive and/or Loctite Corporation as a start. They
    both sell potting materials. Maybe you'll get lucky and find something
    right for you.

    On a side note, you might want to try conformal coating the PCB and
    sealing it off from the elements inside your housing with gaskets.
    Better heat dissipation and cheaper process. Also, I am currently
    looking into low temperature injection molding to replace our potting
    process. Look into this too. It appears to be a reasonable
    alternative.

    Good luck,

    Gerb
     
  3. John Larkin

    John Larkin Guest

    Tough requirement. Transparent and thermally conductive don't go
    together. Consider gaskets or o-rings, maybe; much less messy.

    Master Bond is maybe the best supplier for custom epoxies.

    http://www.masterbond.com/


    John
     
  4. Electrical conductivity and transparent don't go together, but for
    thermal conductivity, diamond is VERY good (the best known, AFAIK) as
    well as being transparent over a wide range of wavelengths.


    Best regards,
    Spehro Pefhany
     
  5. P.S., I don't think it's available as a potting compound.


    Best regards,
    Spehro Pefhany
     
  6. Rich Grise

    Rich Grise Guest

    These guys might have something:
    http://www.dowcorning.com/applicati...h.aspx?searchfor=potting compound&country=USA

    Good Luck!
    Rich
     
  7. Rich Grise

    Rich Grise Guest

    Just fill your box with coal and have Superman squish it. ;-)

    Cheers!
    Rich
     
  8. John Larkin

    John Larkin Guest

    Well, the crappy diamond is about 2x better than copper. But the
    isotopically pure version is 50x.

    John
     
  9. John Larkin

    John Larkin Guest

    You can buy diamond-filled epoxy. But the thermal conductivity is
    still dominated by the epoxy between the grains, so it's not a whole
    lot better than epoxy filled with AlN or something.

    John
     
  10. I read in sci.electronics.design that wrote (in
    I suggest a re-think. Once you get a whole series of special
    requirements applied to a part or technique, it's probably the wrong way
    to go.

    If you just want to secure the board in place, consider one or more
    spring clips.
     
  11. Al

    Al Guest

    First the rant. Packaging is an engineering discipline by itself. Don't
    try this at home ;-)

    One answer:http://www.epotek.com/

    Al
     
  12. Oh, that must make a good high-security potting compound. We used some
    carburundum (IIRC) filled epoxy on the crypto key storage unit I worked
    on in the early '90s.
     
  13. How would the diamond, carbide or whatever particles affect
    decapsulation in the usual manner? I'm not sure you'd even notice the
    fill.

    It might be effective for filling a hollow bar for a lock though.
    Apparently cheap cordless cutoff saws with a diamond wheel can deal
    with hardened metal locks pretty effectively. Or maybe just fill it
    with something that gums up a diamond saw.


    Best regards,
    Spehro Pefhany
     
  14. John Larkin

    John Larkin Guest

    The lock on our gate froze up (lots of salt air here, eats locks) and
    I cut off the hardened hasp with a Dremel and a fiber cutoff wheel in
    under a minute. So much for security.

    We once used some glass-filled epoxy for potting. If you tried to mill
    open a brick, it would destroy a couple of end mills per block. I
    can't imaging the tool life you'd get machining carburundum or
    diamond-filled stuff. Sounds grim.

    John
     
  15. Jim Thompson

    Jim Thompson Guest

    On Wed, 09 Mar 2005 13:16:11 -0800, John Larkin

    [snip]
    What's a "lock" ?:) "eats locks"? My dogs eat interlopers ;-)

    [snip]

    ...Jim Thompson
     
  16. Heh. And that's a relatively wimpy tool.
    Why even try? Just use the usual array of chemicals to dissolve the
    binder away and the diamond particles get flushed away with everything
    else.

    Were the end mills solid carbide or some kind of HSS or HSS with TiN
    coating? Of course PCB material eats HSS drills for just that reason,
    so carbide is the way to go. The thin ones break if you look at them
    sideways too hard, but you can't have everything.


    Best regards,
    Spehro Pefhany
     
  17. John Larkin

    John Larkin Guest

    I guess, but that would keep going and eat the parts, too.
    Just the regular steel ones, I think. And right, I've never had any
    luck drilling a pcb with a carbide drill. Unless you have a 100K RPM
    air-bearing excelon drill with the right entry/exit materials, they
    snap off after a couple of holes. Better to use a steel drill and run
    it until it gets dull. Better yet to buy the danged PCBs.

    Potting always sounds cool, but in practise it's a messy nightmare.

    John
     
  18. Right. The crypto key storage unit had the PCB mounted in a plastic
    (can't remember the particular plastic) case, then wound with four
    insulated wires connected in a bridge configuration. Any two wires
    shorting or any cut wire causes an alarm to the key storage circuitry
    and the keys are zapped. The intention of the filler was to make a
    mess of any tools trying to surgically remove the potting, thus greatly
    increase the chance of cutting/shorting wire(s). The insulation on the
    wire was designed to dissolve in any chemical that would attack the
    potting, also causing an intrusion alarm.
    Let someone else break their bits?
    It is cool, if it's someone else's messy nightmare. ;-) I was just
    the (lead) design and physical security engineer and on the project.
    We had the materials people pick the potting and yucky manufacturing
    processes.
     
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