Luxeon newbie

Just received an assortment of the Luxeon white LED's (stars and emitters) and had some basic questions.

I want to use a couple of the emitters on a PCB and was wondering if someone can give me a brand name and part number on a suitable epoxy to use.

But first of all, let me understand....

Are the slugs supposed to be electrically isolated from not only the heatsink but from each other as well? (Some of the data gives me that impression but I can't understand why....unless the cathode ties to the slug or something).

What I'd like to do is make a simple "sandwich" of regular FR4 PCB material on top and some 090 aluminum sheet on the back....maybe a 1" by 5" arrangement or so.

Why can't I just mount the emitters on the top layer of the pcb with all the traces and circuitry on top and have a .375" hole behind the emitter. (Of course, completely isolating the cathode and anode in the process).

Then I could simply put the epoxy in the .375" hole and mount my aluminum to the backside of the pcb....I could use machine screws as well to tighten my "sandwich".

Is there an epoxy someone can suggest? (Is there anything that can simply air cure?) I'll probably run these at slighty lower than rated maximum.

Thanks very much.

"Are the slugs supposed to be electrically isolated..."

The Luxeon slug is connected via a zener to the anode (for ESD protection) and therefore NOT electrically neutral as discussed here previously ad nausium, and also in the Lumiled's application notes. So yes, they have to be isolated. The only exception is if you're running a single LED and there is absolutely no chance of the heatsink touching any of the circuit connections. Even then, IIWY, I would isolate the bugger just to be safe.

"Why can't I just mount ..."

This should work. In fact if you look at a Lux star closely, this is exactly how it's made, except the slug is NOT isolated. Having worked a fair amount with high-power LEDs, I can tell you that heat management will be your biggest challenge, so make sure the heat sink can handle it. For the testing, phase I suggest you get a temperature probe with a very small remote sensor and attach the sensor as close to the slug as possible and make sure you're not running over the operating temperature range. I use one of those Oregon Scientific indoor/outdoor thermometers available at Target and elsewhere which comes with a sensor probe and a 5' cord and it does the job, however it has a 150 degree F upper limit so it's not perfect but it does the job.

"Then I could simply put the epoxy ..."

I would not count on the epoxy alone to isolate the slug, unless you lay down a very thin coat on the heatsink (or the slug) first, let it dry and test to make sure there are no gaps or holes. What I did on one of my projects is get some mica isolators (the kind you see on the bottom of power transistors) cut it into shape and epoxy it to the heatsink. This ia a fool-proof way to isolate things. Those mica sheets usually come in two or three layers that you can easily separate to get them even thinner and therefore reduce the thermal resistance even further.

"Is there an epoxy..."

Use Arctic Silver Thermal Adhesive: http://www.sidewindercomputers.com/arsiltherad.html or make your own: http://www.overclockers.com/tips683

Thanks kindly for your response....I appreciate it.

I like your idea of using a mica insulator, in fact, I was wondering would some sort of non-conductive grease be better than using an epoxy? And are they making mica insulators now that don't break down like they used to?

Like I say, I intend to solder the emitter on the top side of the PCB and if I use thin enough pcb, I can push it through to where, when I mount my aluminum backpiece it will rest nicely on the slug.

(I'm going to use some 4-40 hardware to actually secure the heatsink say a half an inch to each side of the emitter).

I ordered some arctic alumina...thanks for the link to the DIY stuff.