Cree heatsinking and soldering

[wakibaki]

Cree have very kindly sent me a sample XR-E emitter. Unfortunately there is no indication of the binning or any other identifying mark anywhere on the device or packaging. They didn't include a star. Now I have to figure out some way to mount and heatsink it.

Firstly, does anyone know where I can get a star intended for this device?

Secondly, has anyone any experience of attempting to heatsink this device other than with a star?

Thirdly, has anyone any experience of soldering this device manually, i.e. in the absence of a reflow facility?

The heatsink pad is electrically neutral. We have plenty of 1/8" and heavier sheet copper and brass here which we mainly use for bus bars. I thought I might try a disc of this with some cutouts for the electrodes, or I could get a custom slug made since we have a CNC mill. There's no shortage of aluminium either, but soldering to that is problematic.

With a brass or copper slug the problem is getting the solder hot enough to melt without leaving enough heat in the sink to cook the device before it cools.

Any input would be greatly appreciated.

w

 

[Bikecop]

Some guys just use arctic alumina to epoxy the emitters to the heatsink, and some clip the corners of the emitter for electrical isolation first. The bottom of the bare emitter MUST be electically isolated, as it does have both + and - pads on it
. Check out some of the threads in the elektrolumens portion of the dealer section. He cautions to solder your wires to the emitter first, as it is difficult to heat up the pads once on the heatsink. Check out post #29 in this thread: http://candlepowerforums.com/vb/showthread.php?p=1753799&page=2&pp=30


One source that I know of for MCPCB's as well as optics, lLED'setc is here:
http://ledlightingsupply.com/ledlightingsupply/Howtoorder.htm

They require a $50 minimum order; however one of us will soon be putting together a "mini group buy".

Hope this helps!

 

[Quantumstate]

As to soldering, you can do home-made reflow. You'll need a dental pick or two, a pair of Coil 14x stereo glasses, a butane microtorch (an Ultratorch UT-50, for example), a flux pen, and some soldering paste.

Flux pen at Fry's, and soldering paste maybe an engineering sample from Kester. Paste goes bad fast though, so keep it well frozen. The rest at eBay, or elsewhere online. Dental picks at gun shows.

Don't try to solder to the bus bar; you may fry the emitter. Your part should have come in a bag or container with a part number. Bin is part of the part number. I don't see any use in getting the emitter without a star.

To solder, make sure your pads are clean. Use a pencil eraser or other non-depositing eraser to clean off any oxide. Use the flux pen to put flux only on the soldered surfaces, both emitter and star; not too much to smear. Apply the solder paste to the star. The right amount is a layer thick enough only to cover, and over about 50%-75% of the pad's total area. A grid pattern is best, but maybe unattainable manually. Too much paste, and the emitter will be floating on a blob of underconductive solder; too little, and poor contact. Fire up your microtorch, and apply heat, waving the torch to spread heat over each side of the emitter. In a few secs you'll see the paste blob to solder,and the emitter may move itself to self-center on the pads. If you're sure both sides reflowed, remove heat and let cool.

With the arctic silver epoxy, mash the star with a weight to make it as thin as possible. The less AS that heat has to go through, the better. Consider non-electrically conductive AS, for safety.

 

[Mr_Light]

If it's not already on a star just clip or drill the corners to isolate the base and use thermal epoxy to glue directly to your heat sink. No reason to ad another thermal boundary.

 

[Julian Holtz]

Yes, I would advise to using thermal epoxy, too.

NewBie from this forum did a test and he found out that a soldered and a glued cree do hardly noticable differ in temperatire, while a star mounted cree was hotter because of the isolation between the LED and the star.

So, as soldering does not offer benefits over glueing, I would do the latter.

Simply grind off the metal contact layer at the edges of the bottom, and use thermal epoxy.

Cheers,

Julez

 

[wakibaki]

OK this seems to clear everything up in favour of adhesive and remove the vias in the corners, thanks a lot.

I guess in that case I could locate it in a milled recess. That would solve a repeatability problem.

w

 

[wakibaki]

I found the bin number on a printed slip in the padded bag. Good job I hadn't binned it... XR7090WT-U1-WG-P3-0-0001. Thank you Cree.

w

 

[yellow]

problem with removing the corners is that the contacts now get even closer to the led (maybe interfering with that damn original optic holder or some reflector)

another way: drill into the connecting holes (from up or down, whatever You want) just to remove the connection, or scrape away the contact material at the bottom, or (as You mentionned) drill 2 "lines" into the heatsink.

As to glueing/soldering, as usual:
heat conductive pase onto the slug, put in place and hand press to make better contact. Glue outside of emitter with epoxy glue. Then just solder wires onto the upper contacs. Is a bit more difficult to normal soldering, needs way more time to heat everything up, heatsink might get hot also, but working. No need to reflow (which is not possible when You use a sink homemade from aluminium)