# Strengths of Different Techniques for Mounting an LED



## Ken_McE (Mar 26, 2011)

I'm aware of three techniques for mounting a high power LED to a heatsink.

1.) using a glue such as Artic Alumina

2.) Using mechanical fasteners (screws, bolts) around the edge and into the radiator

3.) Using a thermal adhesive tape

Anybody know how they compare in performance?


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## hellokitty[hk] (Mar 26, 2011)

...


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## saabluster (Mar 26, 2011)

Ken_McE said:


> I'm aware of three techniques for mounting a high power LED to a heatsink.
> 
> 1.) using a glue such as Artic Alumina
> 
> ...


Well you have left out valuable information. What is your definition of performance? Holding power or thermal conductance? You also state "mounting an LED" when in fact it appears you may mean mounting a star. Thermal tape is dead last in every category. Epoxy is good but can difficult to remove if the need arises and the vast majority of epoxies do not transfer heat as good as the thermal pastes. There can also be issues with CTE mismatches that would cause the star to pop off after a while. The screw and paste method is the best solution both thermally and for reliability.


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## Ken_McE (Mar 26, 2011)

saabluster said:


> What is your definition of performance? Holding power or thermal conductance?



Heat transfer my dear Saabluster, heat transfer.



> You also state "mounting an LED" when in fact it appears you may mean mounting a star.


Yes, a star or square.



> The screw and paste method is the best solution both thermally and for reliability.


Any particular pastes you recommend or disrecommend? How badly do the tapes lag?


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## VegasF6 (Mar 26, 2011)

The data is available from heatsinking paste manufacturers. They are a best case scenario though. A big problem with tape is not only the performance, but also the chance for error. If you have two very smooth surfaces, probably the best performing pastes actually contain diamond. Or synthetic, whatever: http://www.xoxide.com/ic-diamond-7.html 
And of course good old Artic Silver, and god only knows how many others out there. But, if you have a gap to fill, you are probably better off with something like Berquist Gap Pad.
http://www.bergquistcompany.com/thermal_materials/
I bet you can spend a week looking at data there. 

There is one more option, directly soldered would be the best.


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## Mr_Light (Mar 26, 2011)

I have used this Thermal Glue on well over a hundred lights and assorted LED Fixed lighting projects.

http://www.dealextreme.com/p/fujik-silicone-thermal-glue-50ml-grease-like-4579
FUJIK Silicone Thermal Glue (50ml Grease-Like) 
*Price:* *$7.19*


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## purduephotog (Mar 26, 2011)

I always wondered... would pop riveting a star work too?


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## VegasF6 (Mar 26, 2011)

FujiK worked ok for me on stars, but I used it with mixed results on the back of bare leds from SSC and Satistronics. The tricky part was clamping them without crushing the dome and most importantly without lifting one side or the other. How would you refer to the case on those style leds anyhow?

"I always wondered... would pop riveting a star work too? "
It did for me  But I had to buy those little red insulating washers.


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## Ken_McE (Mar 26, 2011)

VegasF6 said:


> There is one more option, directly soldered would be the best.



I do not believe that I could keep the base hot enough to flow the solder without getting the emitter hot enough to damage it.


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## VegasF6 (Mar 27, 2011)

Since you are talking about leds on stars, that pretty much limits you. I have cree xr-e on FR4 that could be soldered directly to copper. I have XM-L from DX on copper star that could be soldered directly. But, of course you can't do it with aluminum boards. I thought we were talking in a more general sense though.

Since you have been around for nearly 8 years, I probably don't need to post links to various you tube videos of people reflow soldering leds with standard hot plates or even kitchen stoves. While not strictly following the profiles published by Cree or other manufacturers, it does work. Does it shorten the life of the led? I don't know. But it certainly doesn't make them into dark emitting diodes instantaneously.
Ok, maybe just one link, hah hah.


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## Mick (Mar 29, 2011)

Another link:
http://www.candlepowerforums.com/vb/showthread.php?299088-Soldering-SST90-to-Copper-Disk-with-Indium


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## MikeAusC (Mar 29, 2011)

The critical fact is that metal to metal thermal conductivity is WAY better than ANY thermal compound. 

If you clamp a star onto a heatsink, you have metal to metal contact, with thermal compound just filling in the airgaps causing by surface irregularities. Even using Lanolin Oil, I had good conductivity if I polished both surfaces !

If you use epoxy and lots of clamping pressure, you may be able to get as much metal to metal contact, but thermal expansion may soon reduce that.

If you use tape . . . . why bother.


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## blasterman (Mar 29, 2011)

Dan's Data did an interesting review of some of these techniques a few years ago and concluded *any* material that had water in it worked better than thermal compounds, including toothpaste. Actually, I believe his results also concluded that a metal to metal interface was improved by pretty much anything water based. It's worth reading.

I don't like to drill and tap so I make my own thermal epoxy out of powdered aluminum and any generic epoxy. When I'm done mixing it the stuff is at least 50% metal by volume, almost black in color, and very hard / brittle when it dries so it won't flex. It easily supports large Bridgelux and 10watt emitters perfectly. I don't waste my money on over the counter thermal grease or epoxies because the metal content is too small.


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## CKOD (Mar 30, 2011)

blasterman said:


> Dan's Data did an interesting review of some of these techniques a few years ago and concluded *any* material that had water in it worked better than thermal compounds, including toothpaste. Actually, I believe his results also concluded that a metal to metal interface was improved by pretty much anything water based. It's worth reading.
> 
> I don't like to drill and tap so I make my own thermal epoxy out of powdered aluminum and any generic epoxy. When I'm done mixing it the stuff is at least 50% metal by volume, almost black in color, and very hard / brittle when it dries so it won't flex. It easily supports large Bridgelux and 10watt emitters perfectly.* I don't waste my money on over the counter thermal grease or epoxies because the metal content is too small.*


 arctic silvers website says 62% to 65% silver by weight for their silver adhesive, so not too bad, or by over the counter do you mean whatever you can find locally, and not have to order online?


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## saabluster (Mar 30, 2011)

blasterman said:


> Dan's Data did an interesting review of some of these techniques a few years ago and concluded *any* material that had water in it worked better than thermal compounds, including toothpaste. Actually, I believe his results also concluded that a metal to metal interface was improved by pretty much anything water based. It's worth reading.
> 
> I don't like to drill and tap so I make my own thermal epoxy out of powdered aluminum and any generic epoxy. When I'm done mixing it the stuff is at least 50% metal by volume, almost black in color, and very hard / brittle when it dries so it won't flex. It easily supports large Bridgelux and 10watt emitters perfectly. I don't waste my money on over the counter thermal grease or epoxies because the metal content is too small.


 Try mixing in a little HBN instead of aluminum if you like making your own brew. 

How about the epoxy that is like soldering? Diemat 3060hk 85% silver by weight and 60W/m°K. I don't know how it would do long term like this but you could use it as a paste and get far better performance(I assume) than any of the normal compounds. I'll have to do a long term test like this to see if it works.


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## MikeAusC (Mar 30, 2011)

blasterman said:


> Dan's Data did an interesting review of some of these techniques a few years ago and concluded *any* material that had water in it worked better than thermal compounds, including toothpaste. Actually, I believe his results also concluded that a metal to metal interface was improved by pretty much anything water based. . . . .




. . . some quotes from Dan ( most commercial compounds scored 0.5°C/W)- 
- "That's right - Toothpaste Superior To Arctic Silver 3!"
- "Vegemite's runny compared with thermal goop, as well; . . . . But the stuff still delivered a solid 0.48°C/W result."
- "To see whether these substances work so well simply because they're wet, I tried some plain tapwater between the heater and the heat sink. And lo, it scored a magnificent 0.41°C/W"
- "It would also appear that loading thermal compound with silver, or exotic oxides, or whatever the heck the thermal goop formulators come up with next, really doesn't seem to help much"


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## evilc66 (Mar 30, 2011)

CKOD said:


> arctic silvers website says 62% to 65% silver by weight for their silver adhesive, so not too bad, or by over the counter do you mean whatever you can find locally, and not have to order online?


Measuring by weight and by volume are two different things. 65% silver by weight is a lot less than 65% silver by volume. If it was by volume, then that paste would cost considerably more. And perform considerably differently.



MikeAusC said:


> . . . some quotes from Dan ( most commercial compounds scored 0.5°C/W)-
> - "That's right - Toothpaste Superior To Arctic Silver 3!"
> - "Vegemite's runny compared with thermal goop, as well; . . . . But the stuff still delivered a solid 0.48°C/W result."
> - "To see whether these substances work so well simply because they're wet, I tried some plain tapwater between the heater and the heat sink. And lo, it scored a magnificent 0.41°C/W"
> - "It would also appear that loading thermal compound with silver, or exotic oxides, or whatever the heck the thermal goop formulators come up with next, really doesn't seem to help much"



While it's true that many common materials may perform better than commercial thermal compounds short term (like that test), they won't keep that performance level long term. Thermal compounds have their solid components suspended in a liquid that either won't evaporate, or evaporate at a low rate. Those that evaporate at a low rate typically contain materials that will actually keep performance constant the longer they are used (up to a point), even though the liquid portion of the compound has dried up.


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## CKOD (Mar 30, 2011)

evilc66 said:


> Measuring by weight and by volume are two different things. 65% silver by weight is a lot less than 65% silver by volume. If it was by volume, then that paste would cost considerably more. And perform considerably differently.


Good point, I missed the by volume/byweight difference. I have a lot of epoxies that I use at work which have quite different ratios for volume or weight.


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## saabluster (Mar 30, 2011)

blasterman said:


> I don't like to drill and tap so I make my own thermal epoxy out of powdered aluminum and any generic epoxy. When I'm done mixing it the stuff is at least 50% metal by volume, almost black in color, and very hard / brittle when it dries so it won't flex.


 
One thing I forgot to touch on is that your home brew is in my opinion a bad idea for anything that you want to last a long time. The hard and brittle aspect is in general a bad thing. You actually want a little bit of flex to allow for differing CTEs. Maybe you already considered this and have perfectly matched CTE in the components you are attaching but I certainly wouldn't go around suggesting others are "wasting" their money by buying off the shelf epoxies that will be far more robust than the home brew version you have there.


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## lightide (Mar 31, 2011)

Glue mounting usually use for some small power led because of heat problem.
Mechnical fastening often use for power led because there is a base under the led. 
Ahhasive tape often use for led strip and some module. It is easy to mount, but keep not long time. 

They are based on where you want to use.


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