Surface Mounting an LED?

[Uzzi]

I'd like to mount an emitter directly to the slug of my copper housing. How do you do it? I'm working with XRE's and XPGs.

I was just going to attach them using their 20mm stars using heat sink paste and screws, but thought it would be more thermally efficient to bond them directly to the heat sink slug.

Ive read that emitters can be removed from the star by heating them in a frying pan. I don't want to "fry" them in the process though, so what is the approximate heat to use (IE; Med High, Low?) I don't have an infrared thermometer, so it could be tricky.

 

[BrianMc]

Seems to me some of the flashlight guys do this and Troutie used a heat gun to make a 'stove' for soldering desoldering that had a temperature readout. I think here, but maybe in MTBR. Have a go. If you are not registered at MTBR, I might have a go to find it.

 

[AnAppleSnail]

I'd like to mount an emitter directly to the slug of my copper housing. How do you do it? I'm working with XRE's and XPGs.

I was just going to attach them using their 20mm stars using heat sink paste and screws, but thought it would be more thermally efficient to bond them directly to the heat sink slug.

Ive read that emitters can be removed from the star by heating them in a frying pan. I don't want to "fry" them in the process though, so what is the approximate heat to use (IE; Med High, Low?) I don't have an infrared thermometer, so it could be tricky.

I've seen these options for attaching LEDs to a heatsink, generally from hard/thermally good to easy/thermally bad

Reflow soldering
Thermal-epoxy bare LED to heatsink (Thin layer of epoxy!)
MCBCB with thermal paste or epoxy. Again, thin layers.

Note: With the Crees you can isolate the bare-LED bottom contacts by sanding the ends of each, or lopping the corners off the bare LED - that teensy dot on the solder pads is the wire going through the silicon. I've got pictures of that somewhere.

With reflow soldering, you want to get no hotter than the solder's melting point. I used a brazing torch on a metal plate to heat mine. To avoid damaging the LED, 'preheat' it at about 120F for ten minutes to drive moisture out. To apply the LED, melt solder on the heatsink, place the LED onto the solder (tweezers) and let it cool off. Again, just-melted solder temperatures won't damage it quickly. I've put ice on the heatsink a few inches away to cool it from way-hot to more-LED-friendly temperatures. Most solder melts at temperatures higher than LEDs are designed for, so don't dawdle with it hot.

Thermal epoxy's only finesse is getting a thin layer for best results. Thermal epoxy is much more conductive than air, but rather less so than metal-to-metal contact. I get the surface and the LED back smooth and flat. Flat is important; it gives better contact.

Thermal goo requires some way to press the LED onto the heatsink. Prepare the surfaces the same way as for epoxy. Screws on the LED star, or a reflector/optic/whatever pushing it down.

The thermal benefits of reflow soldering only pay off at the highest power levels, and it helps a little bit with limited heatsinking.

 

[PCC]

I can tell you that those XP-Gs are tiny compared to XR-Es. It's going to be very easy to short both leads out trying to surface mount them to anything other than a star intended to mount them. Sure, you'll lose some efficiency using a star but I think it's a small price to pay compared to shorting out a driver board.

 

[Uzzi]

Thanks for all the advise! It seems that de-soldering, sanding, re-soldering (and then praying) would be nearly impossible on an object the size of a grain of rice, like the Cree XP series. I'll save the surface mounting for a larger LED project. The P7's +/- contacts are isolated from it's core, which would make this task much more practical, I'd think.

 

[AnAppleSnail]

I can tell you that those XP-Gs are tiny compared to XR-Es. It's going to be very easy to short both leads out trying to surface mount them to anything other than a star intended to mount them. Sure, you'll lose some efficiency using a star but I think it's a small price to pay compared to shorting out a driver board.

The Cree XP-G and XR-E chip packages are the same size, about like my pinky fingernail. And you can disable the bottom solder contacts pretty easily.

 

[Illum]

unlike XREs, with the stars XPEs can be pretty much self sufficient as a standalone fixture all the way up to around 175-200ma before additional heatsinking is needed for 24/7 operation. Some people have considered cutting a small T using an exacto knife on a copper clad PCB prior to attempt surface mount soldering. You'll have to keep in mind that solder itself has some thickness to it. Too much solder on one pad and the LED would tip at an angle, too little solder and too long of a heating temperature and the flux [I'm not sure whats the technical term for it] dries out and the solder no longer wants to bond metal. At this point a little dab of new solder does the trick, but for XP series a dab may be too much.

If you can, just buy them on stars

 

[AnAppleSnail]

Some people have considered cutting a small T using an exacto knife on a copper clad PCB prior to attempt surface mount soldering.
If you can, just buy them on stars

This Cree can now be freely soldered to a metal plate without fear of shorting it.

Not my picture but it shows the point.

Evan's thread tells you more, but in short, the bottom pads on Cree LEDs stop being electrical contacts if you sand off the ends of those pads (bottom corner of the LED).

 

[Illum]

well, what you have there is an XR-E LED, you have the option of mounting your wire traces on top. I was referring to LEDs like the rebel or the XP series that do not have solder contacts on top
In those cases its not much the fear of shorting as it is difficult to bias

 

[AnAppleSnail]

well, what you have there is an XR-E LED, you have the option of mounting your wire traces on top. I was referring to LEDs like the rebel or the XP series that do not have solder contacts on top
In those cases its not much the fear of shorting as it is difficult to bias

Ack! I feel dumb. Now that I look at the pictures of it, it does indeed have teensy spots for the wires on the chip. Also, I like the XR-E package better for homebrew stuff

 

[Uzzi]

I'll give the XRE idea a try, for experimentation purposes. I accidentally direct drove mine at 5.5 volts and now the LED has a brown patch on its die. It still lights up, but at half the brightness.

One issue Ive noticed is that my light housing is soldered together in 2 places using lead solder. For future lights, I'll need to use silver solder for all of the joints, for higher temps when surface mounting and better heat transfer. That way I can heat up the whole housing and mount the LED into it without having the housing fall apart.

 

[Pöbel]

well, what you have there is an XR-E LED, you have the option of mounting your wire traces on top. I was referring to LEDs like the rebel or the XP series that do not have solder contacts on top
In those cases its not much the fear of shorting as it is difficult to bias

The XP-G does have solder contacts on top. How did you get the impression that it does not?

 

[Illum]

Ack! I feel dumb. Now that I look at the pictures of it, it does indeed have teensy spots for the wires on the chip. Also, I like the XR-E package better for homebrew stuff

Me too

The XP-G does have solder contacts on top. How did you get the impression that it does not?

I wasn't aware of it until I took macro shots of it this morning
Heres a warm white XPE on star,

Not that it would matter as the top of my solder iron is about the same width as the dome

I am almost positve that Rebels do not have connections on top
ES:

 

[PCC]

I just tried hotwiring those two silver patches on top of an XP-G and I got nothing. I tried the solder points on the star and it lights the LED right up so I know it should work if those patches were connected. I could have sworn that I had read somewhere that those silver patches are not electrically connected and are there for identification purposes.

 

[Pöbel]

They are connected but covered with some kind of clear coat or whatever this is.

You have to scrape it off and then solder the wires to the pads.

I did that many times and it does work.

/edit

sloppy but proof