Choice betwen substandard thermal interfaces

I am building a headlamp for my father for a trek we are going on monday. Basically it is going to be a Q3L emitter or possibly a high Vf R2H star on an old pc alu heatsink with some thrashy plastic reflector/lens on top, and a cord to a 3AA (direct drive or a pot) pack carried in a pocket.

Since I don't have any "approved" thermal interfaces between the luxeon and the sink, I'm asking you fellows to advice me in the choice between A: rapid superglue, B: 10 min. Araldit (two-part epoxy glue) C: Hermetite (fast set two-part epoxy adhesive), D: contact glue (this is the one that has to dry for 10 minutes on both sides before you press them together), and E: the pad that was between the CPU and heatsink.
Any suggestions?

If you have any heatsink compound laying around, mixing this with epoxy (50-50) is meant to give good results.
Then, when you get back, order some Arctic Alumina.

kakster is right! the "best" thermal epoxy currently going around is Arctic Alumina Adhesive/Epoxy. The thinner you can get it in your application, the better the waste heat will flow.

Larry

I don't have no compound of any thermal sort, and no time to buy it. I think I'll just "stick" with the superglue gel since it probably will allow the thinnest layer. Alternately I'll use the star R2H with the thermal pad, it will maybe allow an exchange later. I wonder if plain old veggie oil or toothpaste could be used, temporarily?

AilSnail, the key to thermal transfer from metal-to-metal is the "thinness" of the interface layer. Super glues/cyanoacrylates can "kick/polymerize/set-up" at as much as .005+" The thermal transfer equations are liniear with respect to thickness- thinner is better. IMO any slower-setting compound which lets you minimize said thickness will improve the results.

Larry

ca does work well.
i have mounted several 1w parts and they seem to do well.
ex- 1w bare emitter to chipset heatsink from an old mobo.
drove it over 1 amp without trouble.probably not as good as as epoxy but ca(krazy glue)can be gotten at any stop n rob.

Thanks alot, didn't know that. That leaves me to the slower curing rapid-epoxies (10min), and the "contact glue", but the latter I think is too viscous to allow a thin layer.

how much better is the alumina than the silver filled epoxy?

[ QUOTE ]
tvodrd said:
kakster is right! the "best" thermal epoxy currently going around is Arctic Alumina Adhesive/Epoxy. The thinner you can get it in your application, the better the waste heat will flow.

Larry

[/ QUOTE ]

edit: deleted, too stupid

Snakebite : Im not sure how much better it is at heat conductance than the silver epoxy, but Alumina's main advantage is that it is non-condctive electrically. Silver epoxy is said to have a slight capacitance, so you risk shorting something, especially if you're gluing the emitter itself.

My money's on the heatsink pad from the CPU, that's what it's designed to do (unlike any of the adhesives mentioned), conduct heat.

Failing that I'd go with oil of either mineral ('motor oil') or vegetable (cooking oil). You only need a drop, it's sure to be at least as thin as anything else, and a better conductor of heat than the alternatives. Cleans up easier, too (you can wash the LED in soap and water).

In general, things that conduct heat well also conduct electricity well. All your glues are excellent (electrical) insulators......

Doug Owen

Thanks, Doug. I would have gone with the pad if I was going to use a star. I was afraid it wouldn't be a good enough heat spreader for just an emitter, just an intuitive guess. It seemed to work very well on the star I tried. However, I have destroyed the pad, and will probably go with your advice on the cooking oil for now. That way I can exchange later to something more permanent, like AA/AS for instance.
I'm not afraid of the electrical connection between the slug and the sink, as long as the sink is grounded with the emitter lead. Doug_S has made some experiments that seemed to conclude this was safe practice, and it is also well tried out by other experienced folks here.

Thanks again for the help.

Sigurd

Note: If i remember correctly, grounding via the heatsink is safe with the blue/white/green LS, but if using Red or red/orange or amber, the slug MUST be isolated from the power source.

The slug is connected to the negative lead from the emitter on the blues, but MUST be isolated for reds.

Hope that helps.

The need to isolate the stud on blue/green/white LSs seems like a non issue to me, there is after all a couple of tenths of a mm of sapphire there, right? Good insulation, that....

Doug Owen