Mounting bare emmiter to heatsink?

[LED4LYF]

I am in the process of making an xr-e r2 thrower. I am going to push the xre pretty hard.(1.5-2 amps). I have a few questions.

1. will an xre on a star be good heat transfer? Or just the bare emmiter?

2. If I just buy the bare emmiter How do you mount it to a heat sink?

3. What do you have to do to isolate the back of the emitter from the heatsink?

4. And last, where can I purchase an xr-e r2, with our without board?

 

[tx101]

A quick use of the SEARCH function and I found this thread

https://www.candlepowerforums.com/threads/178772


The most difficult thing about using bare emitters is soldering
wires onto the tiny contact strips

Good luck

 

[Justin Case]

I assume that this is the sort of bare XR-E that you are talking about: Cree XR-E Q5 bare emitter

You would use something like Arctic Alumina thermal adhesive (epoxy) to glue the bottom of the ceramic base board to the heat sink (I've used both AA epoxy and Arctic Silver epoxy successfully).

If your heat sink is anodized, you don't have to worry about electrical isolation. Otherwise, AA epoxy is the safest approach, while AS epoxy has the higher thermal conductivity.

DX has XR-E R2s on a star. Digikey sells bare XR-E R2s.

In theory, the bare emitter should have a better thermal path. You are removing one thermal resistance element from the path -- the star board. But the star makes it easier to make the electrical connections. The star has convenient solder pads that make it simple to solder your driver's hookup wires to power the LED. With a bare XR-E, you should thermally glue the LED to your heat sink first to protect the LED from overheating from the soldering operation. Use some solder flux on the skinny contacts on the XR-E's substrate board and then tin those contacts. Also tin your hookup wires. Then solder the wires to the LED contacts.

Here is an example:

 

[LED4LYF]

I will be using AA epoxy, so a thin layer of epoxy is enough to keep the emitter from shorting out on the heatsink? That does not seem like it would be very safe.

 

[Justin Case]

Why is a thin layer of AA not safe? It's probably thicker than a layer of anodizing on a heat sink, and that works just fine to electrically isolate a slug that is electrically connected, such as for a Seoul P4. I've glued many P4s to unanodized aluminum heat sinks using AA epoxy with no shorting problems. You get your DMM out and check for electrical isolation before you power up the LED.

In the photo I posted above, that's seven Cree XR-Es glued to a hard anodized heat sink, using Arctic Silver epoxy. There is zero electrical shorting.

If you are unconvinced and still concerned, then get a star-mounted XR-E. But you might check this link regarding the thermal transfer properties of some star boards.

Or you can sand the bottom corners of the XR-E substrate board (or clip the corners) to remove the electrical connection between the vias and the contact pads on the bottom of the board. The XR-E's thermal pad is electrically isolated. So another approach could be to put a thin layer of liquid electrical tape onto the power contacts to isolate them. Then use any thermal epoxy of your choice for the thermal pad. Or even reflow solder the pad to your heat sink.

 

[LED4LYF]

That is amazing the difference in temperature with and without the star. I suppose an anodizing layer is thinner that epoxy so I guess I won't worry about it and check with a dmm before I power up.