Seoul modders-Couple of Questions

[munkybiz_9881]

Just a couple of questions here about modding luxeon stars with seoul emitters

1. The positve polarity of the slug makes me a little nervous. I purchased a few of the bare emitters and didnt know to also buy the copper spacers. I have some .030" aluminum, can this be used instead? I was also thinking that maybe very thin ceramic could also be used, and no conductivity there.....any thoughts?

2. About the copper spacers themselves, what the best way to isolate them?? I've heard fingernail polish, and then the Artic Alumina sandwich method. And them some people don't use them at all, just bed the emitter in AA.

Any thoughts would be helpfull, I probably wouldn't be this cautious, but Im going to mod my gladius first, just want to make sure I get it right the first time cause I use this light everyday.

 

[LukeA]

Aluminum will work just fine as a spacer. You could paint or anodize the aluminum to stop it conducting electricity, or put down a thin layer of AA epoxy on the heatsink, let it cure, and then epoxy everything together. If you can find .030 thick alumina, by all means, use that. I wouldn't recommend putting the emitter in a thick bed of thermal compund or epoxy, because that stuff won't conduct heat like a metal or ceramic solid will.

 

[Lighthouse one]

I had some good results using a very thin mica shim with a circular hole that was smaller than the led base. It is difficult to insulate the led even if you epoxy the parts first and let them dry. It works, but try putting 3 volts to the finished job: Hold a wire on the negative of the Led :than- try putting the positive wire to the ground of the flashlight. No light is what you want.

I've done 8 or more lights, and the results are usually excellent. Stock reflectors work perfect...no other changes needed.

 

[wai king]

I need some help?

I replace an emitter on my Coast flashlight with an Seoul and it shorted out, I didn't know the bottom of the emitter was positive until I've read this thread. Can anyone tell me what is and what's the different between "Artic Alumina" and the themal paste or heatsink compounds. I used heatsink compounds for the modd but I think it was not enough so it shorted out. I did another one with enough of the themal paste and so far it is fine. I hear so many mention about "Artic Alumina" and I don't know what that is. Thanks all.

 

[richdsu]

SSC P4 Emitter bottom slug is electrically conductive.

What I will do === get some Kapton Tape cut it up and stack it up until 0.7mm thick and paste onto the bottom slug. Apply some heat sink compound ( thermal paste ) and put it back on the flashlight body.

Reasons for using Kapton Tape ( as quoted):
1) High dielectric strength
2) 3X heat dissipation factor of normal film
3) Resistance to temperature extremes

http://www2.dupont.com/Kapton/en_US/assets/downloads/pdf/billet-H-78263.pdf
http://www2.dupont.com/Kapton/en_US/uses_apps/insulation/index.html
http://en.wikipedia.org/wiki/Kapton
http://www.fff.saint-gobain.com/Data/Element/Product/product.asp?ele_ch_id=P0000000000000001676

Arctic Alumina Thermal Compound -- look here:

http://www.arcticsilver.com/arctic_alumina.htm

 

[Curious_character]

Ouch - there's some bad advice being given here.

It's vital to keep the LED junction temperature below its maximum limit of 145 degrees C. If you have poor thermal conductivity from the LED to the air, your hand, or some other sink of constant temperature, the LED will overheat and die. There are several thermal resistances in the path from the LED to the ultimate sink. All are in series, and the temperature differentials add.

A convenient and common unit of measurement of thermal resistance is degrees C per watt. If you have, for example, a total thermal resistance from the LED junction to the air of 30 degrees C per watt and the LED is dissipating 3 watts of heat, then its temperature relative to the air will be 30 * 3 = 90 degrees. If the air is 25 C, then, the LED temperature will be 115 degrees C, well within its spec. Note that the internal thermal resistance from the LED junction to its mounting pad is 6.9 degrees C per watt. So you need to keep the pad temperature below 124 C if you're running the LED at 3 watts. Assuming an ambient temperature of 25 C, that means you can tolerate a temperature differential from the pad to the air of 99 C. This in turn means that you can tolerate no more than a total of 33 degrees C per watt thermal resistance from the mounting pad to ambient if you're running the LED at 3 watts. (Strictly speaking, the 3 watts is the heat dissipation, which would require putting a bit more into the LED to account for the light output.)

The thermal resistance of a piece of material like a spacer is directly proportional to the thickness, inversely proportional to the area, and inversely proportional to the thermal conductivity of the material.

So let's look at the thermal resistance of spacers made from several materials that have been mentioned here and a few others. I've calculated the resistance for a spacer thickness of 0.030" and the area of a Seoul P4 LED mounting pad (0.2" diameter). Instead of the resistance in degrees C per watt, the table shows the temperature differential across the spacer for an LED dissipation of 3 watts. This is how much hotter the top of the spacer will be than the bottom with 3 watts of heat passing through it. The resistance in degrees C per watt is 1/3 this value. Remember, you have a total of 99 degrees for the entire path from the LED pad to the outside world. You don't want to use up any more of the allotment in the spacer than you have to.

MATERIAL ---- TEMP RISE FOR 3 WATTS (deg. C)

Copper (pure) -------------- 0.3
Aluminum (pure) ------------ 0.5
Alumina (ceramic) ----------- 6.3
Arctic Silver Epoxy ---------- 5.0
Arctic Silver Compound ----- 12.7
Arctic Alumina Compound --- 28.2
Kapton (HN type) --------- 940

It should be obvious from the list why layered Kapton tape isn't a good choice for a spacer. (And the table value doesn't include interspersed layers of adhesive of unknown thermal conductivity.) Using paint or fingernail polish is a crap shoot -- you'll have no idea of its thickness or thermal conductivity. But remember, the thinner the layer is, the less important its thermal conductivity. If you use copper or aluminum for the spacer itself, you can get by with a thin layer of poorer material on one or both sides. But even 0.001 inch of Kapton tape, with the adhesive removed, will get you a 31 degree rise at 3 watts. That's a full 1/3 of your total thermal budget.

The manufacturer of the Arctic Alumina and Arctic Silver materials gives no thermal conductivity specification for Arctic Alumina epoxy, which is why you don't see it on this list. The lack of any specification is in itself enough to steer me away from it.

I usually use around 0.005" (a wild guess) of Arctic Silver epoxy (about 2.5 degree rise at 3 watts) for electrical insulation, a copper spacer (0.3 degree), and perhaps 0.002" of Arctic Silver compound (about 1 degree) under the LED, for a total of around 4 degrees. This way I can get by with a poorer path from there to the outside than if I had blown more of the thermal budget at the spacer.

Stars have an electrically isolated mounting pad (at least every one I've encountered does), so you only need a very, very thin layer of compound or epoxy on each surface of the spacer, with no worry about electrical insulation.

Of course, you can ignore all this and take the experimental approach. Just try whatever's handy. The Seoul LED will tell you when it gets too hot -- you'll learn to recognize that blue color right away once you've seen it.

c_c

 

[ak645]

Hi Curious_character,

Thanks for the very useful chart.Do you know the thermal resistance of anodizing? As in mounting the emitter on an anodized aluminum heatsink.

Andy

 

[richdsu]

There are 2 objectives needed in SSC P4 Emitter modding :

1) to electrically insulate the emitter slug.
2) to provide good heat sinking.

Can I paste a thin layer of Kapton tape onto the emitter slug, apply heatsink compound and sandwitched a copper disc in between the body.

Any better suggestions and tips would be appreciated.

 

[PhotonFanatic]

Ouch - there's some bad advice being given here.

It's vital to keep the LED junction temperature below its maximum limit of 145 degrees C. If you have poor thermal conductivity from the LED to the air, your hand, or some other sink of constant temperature, the LED will overheat and die. There are several thermal resistances in the path from the LED to the ultimate sink. All are in series, and the temperature differentials add.

A convenient and common unit of measurement of thermal resistance is degrees C per watt. If you have, for example, a total thermal resistance from the LED junction to the air of 30 degrees C per watt and the LED is dissipating 3 watts of heat, then its temperature relative to the air will be 30 * 3 = 90 degrees. If the air is 25 C, then, the LED temperature will be 115 degrees C, well within its spec. Note that the internal thermal resistance from the LED junction to its mounting pad is 6.9 degrees C per watt. So you need to keep the pad temperature below 124 C if you're running the LED at 3 watts. Assuming an ambient temperature of 25 C, that means you can tolerate a temperature differential from the pad to the air of 99 C. This in turn means that you can tolerate no more than a total of 33 degrees C per watt thermal resistance from the mounting pad to ambient if you're running the LED at 3 watts. (Strictly speaking, the 3 watts is the heat dissipation, which would require putting a bit more into the LED to account for the light output.)

The thermal resistance of a piece of material like a spacer is directly proportional to the thickness, inversely proportional to the area, and inversely proportional to the thermal conductivity of the material.

So let's look at the thermal resistance of spacers made from several materials that have been mentioned here and a few others. I've calculated the resistance for a spacer thickness of 0.030" and the area of a Seoul P4 LED mounting pad (0.2" diameter). Instead of the resistance in degrees C per watt, the table shows the temperature differential across the spacer for an LED dissipation of 3 watts. This is how much hotter the top of the spacer will be than the bottom with 3 watts of heat passing through it. The resistance in degrees C per watt is 1/3 this value. Remember, you have a total of 99 degrees for the entire path from the LED pad to the outside world. You don't want to use up any more of the allotment in the spacer than you have to.

MATERIAL ---- TEMP RISE FOR 3 WATTS (deg. C)

Copper (pure) -------------- 0.3
Aluminum (pure) ------------ 0.5
Alumina (ceramic) ----------- 6.3
Arctic Silver Epoxy ---------- 5.0
Arctic Silver Compound ----- 12.7
Arctic Alumina Compound --- 28.2
Kapton (HN type) --------- 940

It should be obvious from the list why layered Kapton tape isn't a good choice for a spacer. (And the table value doesn't include interspersed layers of adhesive of unknown thermal conductivity.) Using paint or fingernail polish is a crap shoot -- you'll have no idea of its thickness or thermal conductivity. But remember, the thinner the layer is, the less important its thermal conductivity. If you use copper or aluminum for the spacer itself, you can get by with a thin layer of poorer material on one or both sides. But even 0.001 inch of Kapton tape, with the adhesive removed, will get you a 31 degree rise at 3 watts. That's a full 1/3 of your total thermal budget.

The manufacturer of the Arctic Alumina and Arctic Silver materials gives no thermal conductivity specification for Arctic Alumina epoxy, which is why you don't see it on this list. The lack of any specification is in itself enough to steer me away from it.

I usually use around 0.005" (a wild guess) of Arctic Silver epoxy (about 2.5 degree rise at 3 watts) for electrical insulation, a copper spacer (0.3 degree), and perhaps 0.002" of Arctic Silver compound (about 1 degree) under the LED, for a total of around 4 degrees. This way I can get by with a poorer path from there to the outside than if I had blown more of the thermal budget at the spacer.

Stars have an electrically isolated mounting pad (at least every one I've encountered does), so you only need a very, very thin layer of compound or epoxy on each surface of the spacer, with no worry about electrical insulation.

Of course, you can ignore all this and take the experimental approach. Just try whatever's handy. The Seoul LED will tell you when it gets too hot -- you'll learn to recognize that blue color right away once you've seen it.

c_c

Curious_character,

Congrats--this is the best single post on thermal management that I've seen on CPF. This should be a sticky in its own right.

As for Arctic Alumina Thermal Adhesive, I'm fairly confident that the thermal resistance would lie somewhere between Artic Silver Epoxy and Arctic Alumina compound, but I will try to see if Arctic Silver can address that question. There are just too many LEDs installed using Arctic Alumina Thermal Adhesive, without problems of overheating, that I feel confident in suggesting that for isolation of the Seoul P4 as well as the Arctic Silver Adhesive.

 

[Ledean]

Thanks Curious_Character for that very useful information.

 

[wai king]

Thanks Richdsu and Curious Character for the info. I really appreciate it. :bow:

 

[Curious_character]

Hi Curious_character,

Thanks for the very useful chart.Do you know the thermal resistance of anodizing? As in mounting the emitter on an anodized aluminum heatsink.

Andy

It would end up being insignificant in this application.

Aluminum forms an extremely thin layer of aluminum oxide (alumina, a ceramic) the instant it's exposed to air, which protects it from further corrosion. That's why aluminum, although chemically very active, doesn't corrode easily. Anodizing is a process that causes the layer of alumina to be thicker than it would naturally be. I don't know what thickness would be typical, but I'm sure it's still extremely thin. Since the table is for 0.030" thickness, the temperature differential due to an anodized surface would be 1/30 its value, or only about 0.2 degree C per 0.001" at 3 watts.

c_c

 

[Curious_character]

There are 2 objectives needed in SSC P4 Emitter modding :

1) to electrically insulate the emitter slug.
2) to provide good heat sinking.

Can I paste a thin layer of Kapton tape onto the emitter slug, apply heatsink compound and sandwitched a copper disc in between the body.

Any better suggestions and tips would be appreciated.

It's up to you. 0.001 inch of Kapton will result in a whopping 31 degree LED temperature rise at three watts. As I mentioned, I use a layer of Arctic silver epoxy. Engr_Paul posted a detailed process some time ago that used this method. I've successfully done over a dozen so far. A while back I experimented with making a donut-shaped pad of Kapton tape, with the hole being filled with thermal compound. It was tricky to apply and use, and I got variable results with regard to thermal conductivity, so I abandoned the approach. If you use the copper spacer, smash it in a vice and sand it to make it very flat before using.

c_c

 

[Gryloc]

Hello all! I found a thread long ago by NYLYTE that addressed this issue. He came up with a very practical technique and I would highly recommend that you try it! Here is the link:

http://www.candlepowerforums.com/vb/showthread.php?t=157064

All it requires is some thermal adhesive/epoxy, some deli containers or lids (made of LDPE plastic), and a little extra curing time. People worry about using two layers of epoxy like in his application, but the way he does it is very good. I have tried it for my quad Seoul mod (all on one aluminum heatsink) and it does great. As long as you spread on the epoxy then press the emitter hard (on the sides of the emitter -not the dome) onto the non-stick plastics, then you should have a nice, thin layer of epoxy. Let it mostly harden (not fully cure), and then apply one more layer of epoxy to the back of the slug and press it into place in the mod.

Curious Character, could you try this method by NYLYTE and test it? I have used it for several Seouls, Luxeon K2s, and Luxeon Rebels with success, but I always wondered how it compares to one dangerous layer of epoxy. Even though you will have a protective layer that protects you from shorting the slug, I hope the layers are spread very thinly. That would be great; thanks! Well, good luck and happy modding!

-Tony

 

[mudman cj]

I have used a similar technique successfully. Just use a fresh razor blade to spread a very thin and uniform layer of the thermally but not electrically conductive epoxy on the emitter and allow it to dry before mounting with a second thin layer. Be aware that Arctic Silver is conductive before it has dried!

Here is a link to an old thread I remembered where some of us were discussing the differences between the Arctic products if anyone is interested.

Oh, and here is a quick range of thicknesses for the three types of Anodizing for reference:
Type I: 0.02 to 0.1 mils (thousandths of an inch)
Type II: up to 1 mil
Type III (HA): 0.5 to 4 mils

This is the source.

The anodizing process creates a lot of porosity, and it is these pores that can be filled with dyes to create different colors of anodizing before they are sealed. Anyway, the porosity further reduces the thermal conductivity of the anodized layer as compared to dense aluminum oxide. I found this source for conductivity of a typical anodized layer and calculated a temperature rise using Fourier's Law of about 4 degrees C for a 1 mil layer or almost 16 degrees C for a layer 4 mils thick. Better surface contact ought to be possible by adding a bit of thermal paste, but it is hard to say if the effect would be noticeable.

 

[Curious_character]

I have used a similar technique successfully. Just use a fresh razor blade to spread a very thin and uniform layer of the thermally but not electrically conductive epoxy on the emitter and allow it to dry before mounting with a second thin layer. Be aware that Arctic Silver is conductive before it has dried!

Very interesting! I always check for continuity with an ohmmeter between the spacer and mounting surface before the epoxy has cured, and get an open circuit indication (unless of course the spacer is contacting the mounting surface). So the portion under the spacer has a resistance of more than 200 ohms, the max for the scale I use. Just how conductive is it, and why should that level of conductivity be a matter of concern?

It's actually pretty tricky to make decently conductive epoxy. The one-part temperature cure stuff we used for hybrid electronic circuits consisted of tiny silver or gold spheres suspended in a viscous liquid. The liquid would shrink as it solidified, pulling the spheres into contact with each other. It wasn't conductive until it cured.

Here is a link to an old thread I remembered where some of us were discussing the differences between the Arctic products if anyone is interested.

Posting #6 is incorrect, and it might be why so few people here at CPF use Arctic Silver in spite of its superior thermal conductivity.

Oh, and here is a quick range of thicknesses for the three types of Anodizing for reference:
Type I: 0.02 to 0.1 mils (thousandths of an inch)
Type II: up to 1 mil
Type III (HA): 0.5 to 4 mils

This is the source.

The anodizing process creates a lot of porosity, and it is these pores that can be filled with dyes to create different colors of anodizing before they are sealed. Anyway, the porosity further reduces the thermal conductivity of the anodized layer as compared to dense aluminum oxide. I found this source for conductivity of a typical anodized layer and calculated a temperature rise using Fourier's Law of about 4 degrees C for a 1 mil layer or almost 16 degrees C for a layer 4 mils thick. Better surface contact ought to be possible by adding a bit of thermal paste, but it is hard to say if the effect would be noticeable.

Thanks very much for that information. I've added it to my collection. I assume that the cross sectional area you used for the calculation was that of a typical LED?

c_c

 

[Curious_character]

. . .I found this source for conductivity of a typical anodized layer and calculated a temperature rise using Fourier's Law of about 4 degrees C for a 1 mil layer or almost 16 degrees C for a layer 4 mils thick. . .

For a 0.2" diameter mounting pad and 3 watts, I get 1.79 degrees C per mil thickness, using the source's average figure of 0.7 W/m/K conductivity. Did I goof up my arithmetic, or is the 4 degrees for some other power or cross sectional area?

c_c

 

[mudman cj]

Hi c_c,

Yeah, 200 Ohms wouldn't be a problem, but I know I remember someone reporting that he fried his emitter by powering it up before the epoxy had dried. I will look for it...

The misunderstanding about Arctic Silver probably comes from the data sheets. They mention that it does have some conductivity at high frequencies, but then of course DC currents are not affected.

I actually used a slightly different size for the pad in my calculation (I can't recall right now and don't have it here). Using a pad of 0.2" I come up with a value of 5.37 degrees C/mil.

 

[Curious_character]

I actually used a slightly different size for the pad in my calculation (I can't recall right now and don't have it here). Using a pad of 0.2" I come up with a value of 5.37 degrees C/mil.

Thanks for taking the time to look at that again. You're right -- the 1.79 degrees I quoted was for one watt, not 3 as I said, and I also get 5.37 C per mil for three watts.

The 1.79 was quickly calculated using the conversion factor I'd worked out for the table, and I was worried that all the values in the table were wrong. Turns out they're ok (for 3 watts) -- I just mistakenly used the one watt factor for the anodized resistance calculation. But this forced me to double check the calculations -- thanks again.

I think I've figured out a decent way to get a reasonable idea of the temperature rise of the LED chip due to the LED mounting, so I can try some different mounting methods. The first results are a bit surprising, but I've got to run some more tests to figure out exactly what's going on.

c_c