Recommend potting compound for my application?

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

Reply to
fastturbovette
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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

Reply to
gerbermultit00l

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.

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John

Reply to
John Larkin

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

John

Reply to
John Larkin

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

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Reply to
Spehro Pefhany

P.S., I don't think it's available as a potting compound.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

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

Reply to
John Larkin

I read in sci.electronics.design that snipped-for-privacy@yahoo.com wrote (in ) about 'Recommend potting compound for my application?', on Tue, 8 Mar 2005:

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.

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Reply to
John Woodgate

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

Reply to
John Larkin

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

Reply to
John Larkin
[snip]

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

[snip]

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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.

--
  Keith
Reply to
Keith Williams

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

One answer:

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Al

Reply to
Al

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

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Reply to
Spehro Pefhany

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

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"it's the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

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.

--
  Keith
Reply to
Keith Williams

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