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How to mount bare emitters?

1304 Views 19 Replies 7 Participants Last post by troutie-mtb, Dec 21, 2008

ocean breathes salty

Discussion Starter · Dec 18, 2008

So I have ordered a bare MCE but don't actually know how to mount it to the pill. What is the most thermally efficient manner? Is it just AA or can you use solder for a better thermal connection? I have browsed around here, CPF and google and came up with NIL. Mostly because i keep getting side tracked in CPF and wander off topic : D

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troutie-mtb

· #2 · Dec 18, 2008

What are you mounting it to . and why did you want bare emitter just curious .

Best is soldered to a star or copper heatsink you also need to allow for the contacts not shorting on the heatsink.

next best would be AA which is how I do it with XRE leds and will be doing with some bare MCE i have ordered also.

gillestugan

· #3 · Dec 18, 2008

I theory it actually a good idea to use bare emitters as every junction (emitter-star-heatsink) gives a loss in thermal conductivity.
But the MC-E is made for surface mount, so it can be tricky to use it without star or some other kind of PCB.

The best is to solder them to the heatsink. This can also be tricky as aluminium is almost impossible to solder with normal solder blends. Copper is good, but spreads the heat so efficiently it is hard to solder with a iron.
You may reflow solder them by using solder paste and a torch or kitchen stove, but it is easy to damage LED if done without proper equipment.

Second thermally best is to use a thin layer of high quality thermal compound, for example Arctic Silver 5. Then fixing it the heatsink with glue on around the edges, or some kind of mechanical fixing. EDIT: I assume Troutie's AA is Arctic Alumina?

Third best, but easiest, is to use thermally conductive glue. Just remember to us as thin layer as possible.

You may bend the connector "legs" slightly upwards and place insulating tape under them to prevent shortening them or use a heatsink with a small rise/knob to place the led on. But it has to be part of the original heatsink, otherwise you could just as well use a star.

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bbgobie

· #4 · Dec 18, 2008

I'm gonna try using solder paste and heating up the heating up the heatsink.

If that doesn't work, I"m just gonna use thermal epoxy.

ocean breathes salty

Discussion Starter · #5 · Dec 18, 2008

troutie-mtb said:
What are you mounting it to . and why did you want bare emitter just curious .

Best is soldered to a star or copper heatsink you also need to allow for the contacts not shorting on the heatsink.

next best would be AA which is how I do it with XRE leds and will be doing with some bare MCE i have ordered also.

http://forums.mtbr.com/showthread.php?t=476243

I just don't have the room in there to fit a star.

ocean breathes salty

Discussion Starter · #6 · Dec 19, 2008

gillestugan said:
The best is to solder them to the heatsink. This can also be tricky as aluminium is almost impossible to solder with normal solder blends. Copper is good, but spreads the heat so efficiently it is hard to solder with a iron.
You may reflow solder them by using solder paste and a torch or kitchen stove, but it is easy to damage LED if done without proper equipment.

I am trying to avoid option 3 as it doesn't sound like a very thermally efficient path, although it may well be what i end up doing.

Looking at Cree's website, They recommend re-flow soldering only and advocate using a template to ensure the solder paste only goes where you want it. Do you think it would be feasible to make a masking tape cutout of the area that i want the star to stick to, cover the whole pill with AA, then remove the taped area so that there is a void for the solder to melt into? It would also electrically insulate the "legs" to protect against shorts.

I was thinking of doing some experimenting with this concept on some scrap AL on the stove top first and if it works OK then progress to actually attempting it with the real emitter.

gillestugan

· #7 · Dec 19, 2008

Oh, I just saw where you are going to put it. Use a star, you just have to make it a little bit smaller to fit inside the pill. Use file to make the diameter 16mm. You may also have to drill the cable cutouts a little deeper.
This is by far the easiest way and probably also gives very good cooling compared to tying to fix it directly to the pill.
The wiring on the individual wired PCB wont be effected. Just make sure the solder pads are not in contact with the pill, shortening some of the dies.

ocean breathes salty

Discussion Starter · #8 · Dec 19, 2008

gillestugan said:
Oh, I just saw where you are going to put it. Use a star, you just have to make it a little bit smaller to fit inside the pill. Use file to make the diameter 16mm. You may also have to drill the cable cutouts a little deeper.
This is by far the easiest way and probably also gives very good cooling compared to tying to fix it directly to the pill.
The wiring on the individual wired PCB wont be effected. Just make sure the solder pads are not in contact with the pill, shortening some of the dies.

I hadn't thought of an individually addressable star, however, the stand off height that the star adds was what had me using the bare emitter, there isn't a whole lot of room in there and the extra mm's the star adds might be critical.

I will have a crack with the bare emitter (have ordered it already) and if i blow it up, i will know i did something wrong

gillestugan

· #9 · Dec 19, 2008

Ok, but prepare for a hard time soldering the aluminium

T
Problem with aluminium is that it oxidise easily and that the oxide is very tough. You have to sand off the oxide before soldering, but then when you heat it up it oxidises even faster. Normal flux cant remove it, but sou may find some special aluminium flux to use.

The stars have a layer of tin(?) on top of the aluminium to make it easy to solder.
I have an extra DX star I can send for free if you need one. Just pm me your address and I'll post.

Regarding the extra mm's: I don't really understand this as the threaded extension of the reflector is very long. (8mm?) The pill would just not screw all the way in, but that is usually not a problem. Sorry for asking, Im just curious.

ocean breathes salty

Discussion Starter · #10 · Dec 20, 2008

gillestugan said:
Regarding the extra mm's: I don't really understand this as the threaded extension of the reflector is very long. (8mm?) The pill would just not screw all the way in, but that is usually not a problem. Sorry for asking, Im just curious.

Ask away, that is what these boards are for!

I don't know that i had a logical reason for it to be honest. I saw the small space and mind straight away went "bare emitter". I figured that torch reflectors with pills are designed for a bare emitter aswell so I was already thinking along that path.

I was also looking at my series wired MCE stars and assumed that the "jumpers" would be compromised if i cut it up (not thinking of the individually addressable stars at the time.)

There was also a thread on CPF about someone who crushed the dome on their MCE by putting to much pressure on it which also made me worry about standoff height. Although the dome will just clear the reflector aperture so it is probably a null issue.

Anyway.. Thanks for the offer of the star but since I have started down this path, I am going to bullishly make it work or blow something up trying. Fun and games await me

gillestugan

· #11 · Dec 20, 2008

The easy trail is usually not the fun one... of course you will be able to mount it properly without a star. Happy soldering!

The flashlight modules are actually designed for stars. You can buy 16mm stars that fit perfectly in the pill. All you need to do is to add some thermal compound between star and pill. Then when you screw it in it will be pressed down and make good contact between pill and star.
Haven't seen any MC-Es on 16mm stars though. Only XR-Es an P4s
The one who crushed the dome probably used a reflector with too small opening, making the reflector rest on the dome.

ocean breathes salty

Discussion Starter · #12 · Dec 20, 2008

gillestugan said:
The easy trail is usually not the fun one... of course you will be able to mount it properly without a star. Happy soldering!

The flashlight modules are actually designed for stars. You can buy 16mm stars that fit perfectly in the pill. All you need to do is to add some thermal compound between star and pill. Then when you screw it in it will be pressed down and make good contact between pill and star.
Haven't seen any MC-Es on 16mm stars though. Only XR-Es an P4s
The one who crushed the dome probably used a reflector with too small opening, making the reflector rest on the dome.

I always wondered what those 16 mm round stars were for! Certainly sounds a lot less ham fisted than what I am about to attempt :lol: Perhaps this will end up merged with the "so angry at DIY" thread.

gmcttr

· #13 · Dec 20, 2008

Aluminum Solder Paste

Scroll down to the aluminum paste. I have never used it, but it shows that specialized solder pastes are available. Good luck.

troutie-mtb

· #14 · Dec 20, 2008

I have been using the arctic alumina method for a while now with starless XRE
and am going to do it with MCE in my next build.

It has got to be as good as the 2 junctions you get with a star if not better.

I have a little press made with a mr 11 quad optic to get the spacing and pressure while the AA sets . the trickiest bit is the soldering which I do before installing in the light.

this will be easier with the MCE if going paralell

Just thought i would throw my penny worth in

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ocean breathes salty

Discussion Starter · #15 · Dec 20, 2008

Thanks Trout,

The ease of AA is mighty appealing and uses equipment I already have at home so I wouldn't have to wait for more parts to arrive after the MCE turns up. Perhaps I will just glue it on and see what results. If the MCE fails, then it might be time to consider specialist soldering techniques. I just wish the MCE would hurry up as this is going to make for a very interesting helmet light.

Thanks to everyone for the advice so far.

ocean breathes salty

Discussion Starter · #16 · Dec 20, 2008

I just played around with a scrap bit of Al, I had to see what i would be dealing with.

First I tried a standard soldering Iron to heat the work piece enough to get the solder to melt directly onto it. I had no hopes of success and wasn't disappointed. The Al was so efficient at heat dispersal that instead of heating up the workpiece, it just cooled the soldering Iron.

Then I tried an old saucepan on a stove top. I sanded the scrap Al immediately prior to putting it in the pan and put a piece of normal soldering wire on to melt. Intriguingly, it acted very similar to mercury as it immediately formed a sphere and would roll about on the surface of the Al which had oxidised almost immediately (as gillestugan said it would). There was no way that an emitter would make any sort of successful mount under those conditions.

This job would certainly need Aluminium soldering paste... so i think AA will do for a first try. Will keep you posted on how it turns out.

bbgobie

· #17 · Dec 20, 2008

Do you have soldering paste?
I figure if you cover the area you want soldered with soldering paste it wont oxidize.
Than you pretty much are doing your own ghetto reflow soldering.
I've read of other ppl having success with this.

TheBigYin

· #18 · Dec 20, 2008

Soldering Paste - 2-5 days, so it's not going to get to you this year though!

texas

· #19 · Dec 20, 2008

series paralell?

I don't know if this will work but it seems like it would.

use one wire with the length of insulation stripped off in the middle for each side of the MCE, then solder one across each side of the leds. This would result in a parrelel conection of the 4 emitters with a set of wires going off the top and another off the bottom. Then cut those 2 wires in the middle resulting in each wire contacting 2 pins. You would then have 2 sets of wires each with 2 die in parallel. Then use AAA to contact the die to your light.

troutie-mtb

· #20 · Dec 21, 2008

Tex :thumbsup:
That is exactly how I am going to do mine only as I plan on using A hipflex I will only want one wire and keep them in paralell and then series the MCEs

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