# encapsulating the Cree LED dome/lens



## fun4four (Feb 17, 2009)

Hi, Can anyone recommend a optically clear resin/epoxy sealer for my project. 
What I need to do/accomplish is fully submerge the Cree led dome and the little aluminum ring with some kind of resin/epoxy that is half way optically clear as not to loose to much light transmission.
I have tried searching but I couldn't find anything that pertains to submerging the dome completely. Any recommendations would be greatly appreciated....
Bob


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## saabluster (Feb 17, 2009)

fun4four said:


> Hi, Can anyone recommend a optically clear resin/epoxy sealer for my project.
> What I need to do/accomplish is fully submerge the Cree led dome and the little aluminum ring with some kind of resin/epoxy that is half way optically clear as not to loose to much light transmission.
> I have tried searching but I couldn't find anything that pertains to submerging the dome completely. Any recommendations would be greatly appreciated....
> Bob


Why do you want to do it? I have done it and the results were less than spectacular. With a lens anyhow.


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## Morelite (Feb 17, 2009)

Norland Optical adhesives

NOA 61 is the preferred for potting tritium vials and should work the same for you app.


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## WeLight (Feb 17, 2009)

In a word 'DONT' the lens on Cree is a floating lens, if you epoxy it it no longer floats


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## Morelite (Feb 17, 2009)

WeLight said:


> In a word 'DONT' the lens on Cree is a floating lens, if you epoxy it it no longer floats


 
What does that mean? Are you talking about deforming the shape of the dome?


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## likeguymontag (Feb 17, 2009)

Morelite said:


> What does that mean? Are you talking about deforming the shape of the dome?



No, it's solid acrylic. It's attached to the die with a thin layer of gelatinous optical silicone though, and it's fairly easy to knock the dome free if you aren't being careful. At that point, the LED is essentially useless, although it'll still light up.

I believe he means that if you glue something to the dome, you're highly likely to pull the dome free of the die and ruin your LED.


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## saabluster (Feb 18, 2009)

WeLight said:


> In a word 'DONT' the lens on Cree is a floating lens, if you epoxy it it no longer floats


I have absolutely no problems potting the Cree from a mechanical perspective. There is no reason to not pot other than the fact that there can be an associated lumen loss. 






likeguymontag said:


> No, it's solid acrylic. It's attached to the die with a thin layer of gelatinous optical silicone though, and it's fairly easy to knock the dome free if you aren't being careful. At that point, the LED is essentially useless, although it'll still light up.
> 
> I believe he means that if you glue something to the dome, you're highly likely to pull the dome free of the die and ruin your LED.



No it is solid glass. There is no more chance of knocking the dome off than if you were performing any other action with an LED such as changing one out.


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## tebore (Feb 18, 2009)

saabluster said:


> I have absolutely no problems potting the Cree from a mechanical perspective. There is no reason to not pot other than the fact that there can be an associated lumen loss.
> 
> 
> No it is solid glass. There is no more chance of knocking the dome off than if you were performing any other action with an LED such as changing one out.



It's surprising how much misinformation is spread around like it's the truth isn't it? 

Good thing we got guys like you go keep us on the up and up. :twothumbs


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## saabluster (Feb 18, 2009)

tebore said:


> It's surprising how much misinformation is spread around like it's the truth isn't it?
> 
> Good thing we got guys like you go keep us on the up and up. :twothumbs


Thanks. I can't say I haven't done it myself but the shear volume of misinformation is hard to keep up with and correct.


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## SemiMan (Feb 18, 2009)

Well it is actually a bad idea in general to encapsulate the full down for a few reasons:

1) You will change the optical properties considerably.

2) You will change the mechanical properties of the LED. By fixing the dome in space with the encapsulant, you are making the led less able to handle mechanical stress (including that from thermal cycling).

3) Intrusion of solvents, etc. from the encapsulant into the LED.

I would contact Cree directly to see what they have tested and confirmed it works. How long does your product need to work?

Semiman


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## saabluster (Feb 18, 2009)

SemiMan said:


> Well it is actually a bad idea in general to encapsulate the full down for a few reasons:
> 
> 1) You will change the optical properties considerably.


Changing the optical properties is not necessarily a bad thing. In my experiment(with an aspheric) it did not work out and I have a theory as to why but I think it would have been a smashing success with a reflector. Don't know for sure as I have not tested it though.



SemiMan said:


> 2) You will change the mechanical properties of the LED. By fixing the dome in space with the encapsulant, you are making the led less able to handle mechanical stress (including that from thermal cycling).


Have you actually tried it? I have. I ran the encapsulated R2 up to 3.2A. If anything was going to stress it that would. Result? No problems whatsoever. Normally if you knock off even partially the LED dome on the Cree you get what looks like cracks that run through it. This is because air gets in between the tears. If the whole LED is encapsulated the air cannot get in and these tears will *not* result. There certainly is no appreciable increase in stress to the LED die so where is the failure to occur? The only issue I can see is if you did a poor prep job before encapsulation and it did not adhere very well. 



SemiMan said:


> 3) Intrusion of solvents, etc. from the encapsulant into the LED.


What kind of encapsulant would you use? The stuff I use has no such issues. 




SemiMan said:


> I would contact Cree directly to see what they have tested and confirmed it works. How long does your product need to work?
> 
> Semiman


Seriously? You know exactly what they would say. Don't do it! And in all honesty if I was a big company that wanted to use these LEDs I would not do it. But for a personal project I see no problems.


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## WeLight (Feb 18, 2009)

Let me reiterate 'DONT'
As a Cree distributor we sell leds to commercial customers, we have been down the path of encapsulating leds, as I said earlier the lens is a 'FLOATING' lens, it will no longer float if you lock it in place with encapsulant, therefore it can no longer expand and contract with thermal changes, you will inherently alter the fundamental design of the led, we have done it and leds blow up, this is not speculating it is a fact. If you need to encapsulate do so with a small air pocket around the lens, this will work


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## bikerjay (Feb 18, 2009)

I know I saw lens units some where that are made to be encapsulated, they even had a thin chrome colored coating on the back of the tir optic so there was no optical change by encapsulating it. I think it even boasted that it formed a waterproof unit. They were made by L2 or perhaps khatod..... 
I know if this post were a little less vague it would be much more helpful.


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## fun4four (Feb 18, 2009)

Everyone,
Thanks for all of your reply's.
Let me elaborate a little on why I need to do this. This is not for a flashlight application. I would prefer to leave the LED dome clear of anything but I am afraid that the LED and the housing will not be waterproof. I am not using any reflector or optics. I'm just using the LED mounted to it's little pcb board in open air. I am also concerned about popping the LED dome off with a garden hose or pressure washer. As we all know it doesn't take much to pop the dome off. If the led were encapsulated I think the chances of popping the dome off will be almost zero. I have experimented last night with filling the pocket with some resin. It worked extremely well. I lost a little of the blue tint and the led light seems to have a warmer glow to it. It does not appear to have lost any light output but I'm sure it did. I have no way to check that. I don't know what the long term affect are but I will find out. It is waterproof as I had it submerged in a bucket of water for a couple of hours with it being on with no damage.
The reason for the original post was to find the clearest optical transmitted sealer/resin/epoxy available possible.


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## dilbert (Feb 18, 2009)

fun4four said:


> The reason for the original post was to find the clearest optical transmitted sealer/resin/epoxy available possible.


 
I think that would be Norland 61


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## likeguymontag (Feb 18, 2009)

tebore said:


> It's surprising how much misinformation is spread around like it's the truth isn't it?
> 
> Good thing we got guys like you go keep us on the up and up. :twothumbs





saabluster said:


> Thanks. I can't say I haven't done it myself but the shear volume of misinformation is hard to keep up with and correct.



 oops.


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## HarryN (Feb 18, 2009)

Perhaps based on the information from the distributor, it would make sense to consider an alternative LED product. I am reasonably sure that the Lumileds K2 and for sure the rebel are not glass domes.

Ideally, you will want a high refractive index material. Silicones would be better than epoxy (more flexible / more UV resistant) if you can work with it.


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## saabluster (Feb 18, 2009)

WeLight said:


> Let me reiterate 'DONT'
> As a Cree distributor we sell leds to commercial customers, we have been down the path of encapsulating leds, as I said earlier the lens is a 'FLOATING' lens, it will no longer float if you lock it in place with encapsulant, therefore it can no longer expand and contract with thermal changes, you will inherently alter the fundamental design of the led, we have done it and leds blow up, this is not speculating it is a fact. If you need to encapsulate do so with a small air pocket around the lens, this will work


You had very unimaginative engineers working on your project. See the quote below from the genius HarryN. You can't seriously expect us to believe that the LEDs blew up?:shakehead Were you encapsulating with C-4 or something?





HarryN said:


> Silicones would be better than epoxy (more flexible / more UV resistant) if you can work with it.


Congratulations! You got it right. What he needs to look for is a "water clear" silicone encapsulant.


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## HarryN (Feb 18, 2009)

saabluster said:


> See the quote below from the genius HarryN. .



Thanks - but I am not sure that suffering in the past = genius in the present. LOL.

Take care.


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## SemiMan (Feb 21, 2009)

saabluster said:


> Changing the optical properties is not necessarily a bad thing. In my experiment(with an aspheric) it did not work out and I have a theory as to why but I think it would have been a smashing success with a reflector. Don't know for sure as I have not tested it though.
> 
> 
> Have you actually tried it? I have. I ran the encapsulated R2 up to 3.2A. If anything was going to stress it that would. Result? No problems whatsoever. Normally if you knock off even partially the LED dome on the Cree you get what looks like cracks that run through it. This is because air gets in between the tears. If the whole LED is encapsulated the air cannot get in and these tears will *not* result. There certainly is no appreciable increase in stress to the LED die so where is the failure to occur? The only issue I can see is if you did a poor prep job before encapsulation and it did not adhere very well.
> ...



- Speaking of misinformation.....

I have done significant work with the automotive and other vehicle based industries, all of who would love to have an encapsulant over the lens as well as the LED body. Simply running the LED up to 3.2A (and just a few) is not indicative of long term thermal stress. Perhaps if you did that then dropped it into a freezing bath a few times, then perhaps yes..... or maybe a hundred times. As was stated, it absolutely does lower the reliability of the LED, though likely more so for Cree and others who have bond wires versus the TFFC Lumileds products that do not.

The dome does form part of the optical system of the LED. If you use the wrong encapsulant, you can reduce light emission by creating internal reflection between the dome and the encapsulant medium. The next answer will of course be "then get an encapsulant with the same index of refraction as the dome....". That has its own issues.... now the LED/air interface occurs at a different spot and again, depending on the angle of light, you could get internal reflection and lose light.

I would guarentee that Cree has been looking at exactly this issues and quite possibly could offer suggestions on what has been the most effective. They will be researching it as their large customers are interested.

For a one off where life is not critical, I would not worry too much though. Silicon based encapsulants should have the least amount of yellowing and have some give.

Semiman


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## saabluster (Feb 22, 2009)

SemiMan said:


> - Speaking of misinformation.....
> 
> I have done significant work with the automotive and other vehicle based industries, all of who would love to have an encapsulant over the lens as well as the LED body. Simply running the LED up to 3.2A (and just a few) is not indicative of long term thermal stress.


Misinformation? Where? I make no claims as to this solution lasting 50,000 hours. As I said if I was a big company looking to do something like this in a product to be sold to the masses I probably would not risk it.



SemiMan said:


> Perhaps if you did that then dropped it into a freezing bath a few times, then perhaps yes..... or maybe a hundred times. As was stated, it absolutely does lower the reliability of the LED, though likely more so for Cree and others who have bond wires versus the TFFC Lumileds products that do not.


I do not see how encapsulating with silicone will lower the reliability. The LED die and its bond wires are in a very mushy gel. It acts very much like amniotic fluid(please don't get pedantic as concerns this analogy). The silicone will have enough give to allow the normal expansion and contraction of the dome and the ring and will definitely have no effect on the die. The only potential problem I see is keeping the silicone from peeling up and letting air in between the package and silicone. Based on my experience with this stuff you would need a mechanical solution to help seal it.



SemiMan said:


> The dome does form part of the optical system of the LED. If you use the wrong encapsulant, you can reduce light emission by creating internal reflection between the dome and the encapsulant medium. The next answer will of course be "then get an encapsulant with the same index of refraction as the dome....". That has its own issues.... now the LED/air interface occurs at a different spot and again, depending on the angle of light, you could get internal reflection and lose light.


It is not just the wrong encapsulant that can be the problem but the contour that you make with the new encapsulant that can cause more internal reflections. As I have already said I noticed a reduced output from the LED after having encapsulated it. And the next answer for me is not "then get an encapsulant with the same index of refraction as the dome...." it is get rid of the glass dome and encapsulate silicone directly over the existing silicone.



SemiMan said:


> I would guarentee that Cree has been looking at exactly this issues and quite possibly could offer suggestions on what has been the most effective. They will be researching it as their large customers are interested.



I'm sure they could offer suggestions but would they? 
Don't you think that they have looked at running their LEDs at currents higher than the specified maximum of 1A(XR-E)? Yet it is very well documented on this site that these XR-Es can very easily handle more than 1A but if you ask Cree what they recommend what do you think the answer is going to be?



SemiMan said:


> For a one off where life is not critical, I would not worry too much though. Silicon based encapsulants should have the least amount of yellowing and have some give.
> 
> Semiman


 This is exactly my point. People are freaking out about a home encapsulating project. So what if the LED fails in 5000 hours as opposed to 50,000.


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## Gryloc (Feb 24, 2009)

You can always surround the dome of the emitter by a cylindrical piece of plastic, metal, or even paper, and then fill the outside of this cylinder with the epoxy. Then, top the emitter with a flat glass, acrylic, or polycarbonate lens (watch glass, flashlight lens, or cut down acrylic plexi-glass). Finally, you can secure this lens with additional epoxy. This way, you get a permanent water-proof solution, but also a layer of air on the outside of the dome. It may be slightly more time consuming, and a bit more expensive, but it shouldn't be too complicated. However, if you need to have the emitter produce a very wide beam, the cylinder may block a bit of light.

However, I cannot see potting the emitter for a small scale, personal project be much of a problem, unless these have a critical application. If it is for fixed lighting, the emitter may barely ever see power cycling to worry about thermal expansion issues. 



fun4four said:


> I'm just using the LED mounted to it's little pcb board in open air



It sounds like you would have a larger problem with heat buildup causing reduced lifetime before you see a decrease in life from thermal expansion. I am guessing you are referring to metal core PCB (those little metal stars). Do you plan on having any exposed aluminum for heat dissipation, or do you plan on surrounding the emitter entirely by epoxy? You will need some decent heatsinking (an additional aluminum heatsink or block attached to the back of your MCPCB stars) if you plan on sending more than 350mA to the emitter. Without having any metal exposed to open air, you will get heat buildup no matter how much heatsink material you use. If you plan on mounting the emitter on a plain brown PCB board with no additional heatsink on the back (connected with vias), then you will have even more problems with overheating. I suppose that I am not clear on how you plan on heatsinking or powering these emitters.

I hope you find the ideal clear epoxy to work with. Good luck.


Cheers,
-Tony


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## cpf user (Jul 25, 2013)

Old thread, but new info:
www.cree.com/products/pdf/xlamp_chemical_comp.pdf 

Basically, any encapsulant at all, in contact with a silicone dome, alters the chemistry of the dome when light and heat are applied. In worse cases, such as ours, the LED fails. We were having about a 50% failure rate at 5 to 7 days when encasulating 3w Chinese stars (driven at 600ma) with water-clear tin-cured 2-part silicone.


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## SemiMan (Jul 25, 2013)

I would go with a tffc led with no bond wires as that is a major failure point that could be avoided.

Sent from my Nexus 4 using Tapatalk 4 Beta


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## cpf user (Jul 26, 2013)

They look to be too expensive for production. The Chinese 3w stars we use now are $0.60 each in lots of 1000.


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