I would like to heatsink it via the PCB groundplane. Also it needs to be fixed very solidly to minimise movement under vibration. I could solder it down but would be worried about temperature.
I don't need to isolate the case, but I don't want to use that electrically conductive silver paste either since will likely short out the anode lead.
Any suggestions?
The "thermal" epoxy compounds I found seem to be designed with filler particles that ensure insulation, but would limit heat transfer. I guess normal epoxy would work, or maybe cyanoacrylate (low viscosity so get thinner layer)?
I've used a couple of 3M types of thermal adhesive. TC2810 is one of them (iirc).
From what I remember about their thermal conductivity numbers, they're not much better than regular epoxy in the net thermal resistance that you can achieve. For very thin layers of glue, I doubt you'd see any real difference between official "thermal epoxy" and regular off-the-shelf epoxy. I'm not sure about cyanoacrylate.
Bob
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Suspiciously low storage temperature range-- only to 100°C.
I don't suppose you'd consider pushing it through a grounded ~4.7 mm plated-through hole in the PCB and bending the leads 'round back?
Best regards, Spehro Pefhany
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Solder conducts heat pretty good. Better than epoxy, and NO, you won't be getting it so hot it reflows in use, so what is the worry? The only worry with solder is mic-crystalline fracture over time, and solder creep.
That would depend on how you place the epoxy, and what steps you take to keep the lead isolated. Silver paste is a no go, but silver filled epoxy IS the right animal.
The silver epoxy is best. Mainly because it doesn't flow into nooks and crannies. It stays right where you put it, so a big bead outside your metallic housing will not creep up under the shroud and invade conduction paths if you are careful when you apply it.
Such applications are very labor intensive, so I hope you are not talking about a production environment.
EpoTek makes very good thermal bonding epoxies, and they are used by the entire industry. Nearly all the heatsink capped chips in any of your circuits use it. It is conductive enough to actually make traces out of as well.
It requires an 80C 4 hour cure cycle though. I think it is the H20E product.
Fillers are used when the epoxy has a poor thermal performance as the fillers are typically better thermal conductors, so 'fillers' in thermal epoxies AID in thermal transfer.
Your ideal direct contact model means nothing when you are talking about gap filling or thermal conduction across media. The gap filling in a near zero gap interface is a totally different animal than is the gap fill between a wide space and a case wall or thermal block, and further different than full fill thermal media physics.
Stycast cures HARD. The black stuff is a fairly poor thermal conductor. Their blue, thermally conductive stuff, however, is a very good thermal conductor.
Glass fibers can be added for strength. Silica powders can be added for thermal benefit.
Abs max power dissipation is 150mW so it is not like its going to cook anyway.
Have you actually got one in your hands? The shoulder on the pin welded to the case and/or epoxy/glass seal on the other pin tends to make such devices not sit flat on a PCB anyway. A pad with a big enough hole to take the pin all the way to the 'hilt' may end up shorting out on the case so you may need a spacer. The data sheet says something about not having a 'step piece' but doesn't give enough information to know if there is or isn't a problem.
If it does sit flat then you just have a tiny air gap from mismatch in flatness of the case/PCB, and you don't have much power so I don't see you need anything specially heat conductive. I would use a dab of ordinary silicon rubber.
Stupid grasp of the paradigm, John. The key to low theta on a directly interfaced flat surface against another is DIRECT, FULL contact with a NON-epoxy gap fill that is as thin as can be. Filed epoxies are NOT meant for these interfaces. Attachment of chip dies does not count, because the particle size on that silver filled epoxy is VERY small.
Direct mated interfaces are meant to have THIN gap filling pastes used on them. Pastes that typically have no granularity to them. FILLED epoxies are NOT meant to be used in closely clamped interfaces, idiot. You need to learn where a given media is meant to be applied. You have always had a problem with application though.
Filled epoxies DO work well compared to straight epoxies, because in the job such mixes are for, which is in no way meant for gap filling on clamped interfaces, they work BETTER than straight epoxies do in thick cross sections, which is where you got lost. Silica powder is the best fill because it does have a small particle size. The diamond powder thing is an exotic application that has likely been copied by some lame ass that want kids to think they have the best stuff out there for their PC CPU. The REAL diamond filled media used by NASA and the like DO have excellent characteristics.
I wouldn't expect a guess as you go asshole like you to get it though.
That's what I said: the key to low theta is to keep the interface layer thin. If the OP needs epoxy to hold things in place, as he says he does, a thin layer of epoxy is best, and filled epoxy won't help much. Since the surfaces here are not going to optically flat and ringed together, a gap filler (like silicone grease or epoxy) will indeed help a little. A filled epoxy may actually increase gap distance and make things worse, depending on the particle size.
Is there an echo in here? That's what I said.
Did you actually READ the op's post? He wants to epoxy a TO-18 can to a pc board. No clamps. Neither surface will be especially flat.
Depends a lot on the preparation, and especially on the bake temperature. Fillers tend to be in the 40 to 200 W/m/K range, vs. about
0.1 for plastic, so it matters a lot whether the filler particles are touching each other in the cured material. It's certainly true that getting the manufacturer's specified theta value generally requires extreme care (and an unusually honest or conservative manufacturer).
My impression is that the fill particles don't generally contact all that well, so most of the conduction is still through plastic. If the particles are too big, they can open up gaps and make things worse. I suppose the ideal filler would be flakes that squish flat... but that wouldn't fill small pit-type gaps very well. The best filled gap is no gap at all: so, very flat surfaces and thin, unfilled epoxy, clamped well until cured.
I've tried some diamond filled epoxy, but the stuff I got had really big particles, so was worse than unfilled.
Where can I find one of those unusually honest manufacturers?
We need to run it close to the maximum temperature and current (at least at the upper end of our products temperature range). Even a small case temperature affects how we have to derate it. We have confirmed with the supplier that the published derating applies to a non-heatsinked LED mounted on its legs. So it should be possible to run it at a higher current than would otherwise be possible, provided we keep the case temperature below what it "would have been" at the derated current, non-heatsinked.
No, it's not a problem. There is a step but it is on the cathode, which we ground anyway.
Remember we are also trying to make it immune to vibration.
Nice idea - I should have thought of that earlier. I bet I could solder the base to a surrounding "pad" that way too (without frying the led too badly). Or better some strategically placed bolt heads could clamp solidly it in position. The case is the cathode and is grounded anyway, so there would only be one lead to worry about.
For other reasons I think that is too big a change to do right now (it changes the mechanical arrangement too much), but I will keep it in mind for a future version.
WRONG. If the user needs an epoxy to hold things in place, thermal transfer interfaces are NOT where the epoxy goes. All of that management has to be in place. Epoxy is for fixturing or potting, not for attaching a heat source to a heat sink. And NO, that is NOT what you said.
Then, he also does NOT need a thermal epoxy, idiot. If the device is getting hot, the circuit design has issues that go beyond a canned device attachment.
Epotek provides to ALL the chip makers, and chip dissipation is a science, so the folks that buy these epoxies don't f*ck around with guessing, and epotek doesn't futz around with their numbers the way you do with application discussions.
The particle size in many filled epoxies is so small that the 'problems' you cite only serve to indicate your lack of knowledge of the industry and the advances they have made.
If that device is getting hot, his design has issues. Even at full bore, they are meant to work cool. They are 100% duty cycle devices without sinking.
You could attach the can pad that gets sold in the industry. It is a plastic standoff that keeps it off the PCB. Then, you can backfill the standoff area with an epoxy that is non-conductive and has a modulus of elasticity (some give) after curing. THEN, you can apply the silver filled epoxy around the can, and tie it thermally to the pcb. You get a vibration immune device that also has heat transfer. Still labor intensive.
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