I am [not really] angry at AA epoxy! Please help...

[Gryloc]

I discovered just how much this stuff sucks after I had an accident with one of my projects! I was cutting some of the heatsink material from my 18 K2 LED headlight cluster and then I later set it aside to finish tomorrow. I was pulling the cord to my soldering iron out from behind my desk when it snagged one of my clusters and caused it to fall face down onto my hard floor from about 2 feet. I was angry about this at first, especially since one of the LEDs were physically damaged, but this only uncovered another huge problem. I hooked up some power to this cluster (the way I usually do), and quickly, a few banks of LEDs turn off. Arghh. I have seen this before and I have already replaced some of the K2s already, so I know this is not anything small.

I hooked up a little battery pack to each LED and found that 3 burned out (the ESD diodes are damaged and ended up causing the whole LED not to light) along with the physically damaged LED. That is 4 more LEDs than what I wanted to replace.

Now why I posted this. I used the Arctic Alumina thermal epoxy to adhere each LED to a copper plate. I had confidence in the epoxy to electrically isolate each LED properly. I trusted it and it let me down. Some of the slugs (actually almost all of them) are electrically attached to the ground (the copper plate). There is little true electrical isolation at all between the slugs and the copper plate. I have been wasting my time and money all along.

I feel very bad about my friend whom I am making this for. This problem is so extensive and will take so much time (and money even) to fix, that I may never get to it by the way I am so busy with class-work.

It is so much work to remove a damaged K2 from the cluster and then prep the surface for the replacement LED. I do not know if I should start from scratch to ensure that each and every one is isolated and have properly working ESD diodes. I could apply a thin layer of nail polish to each LED first, or I could try to find star hex boards to isolate the LEDs. I am afraid to mix methods of truly isolating each LED and depend on the AA epoxy, so do I continue replacing each LED and just epoxy them, or should I re-work everything and try to isolate every LED?

This is so frustrating. If anyone has any ideas or comments, please leave them. I hate venting on the CPF, but this is something pretty major for me as it can cost me too much money. My friend payed for all the parts except for a few replacement parts. I might as well pay him all back and try everything from scratch. Anyone ever get in a strange situation like this? There are few people around here who are making automotive headlights, so it may be difficult to relate. Thanks you all for your help!


-Tony

edit: see my next post...

 

[shiftd]

Re: I am so angry at AA epoxy!

how do you attach the luxeon to the slug? you just apply the AA in between and hope that the AA will isolate the slug with the sink electrically?

And then after you attached them together, did you press the luxeon to the sink so the thermal contact would be maximized?

This is where the problem occurs. By maximizing thermal contact, you also maximizing the electrical contact. AA is electrically non conductive, but it does not make the substance near it becomes non conductive. So by pressing the luxeon against the sink, you are making the slug and the sink touch each other. The AA helps the contact between them stay intact. This is why you cannot expect to have an isolation simply by having AA in the middle of the junction.

What I recommend you is either:
1. Have your sink anodized. The anodizing will help to isolate the sink electrically from the slug.
2. have the luxeon pre-isolated prior to attaching it to the sink. you can do this by applying a thin AA underneath it and press it against a nonstick plastic. After the AA dried down, you should have a thin layer of AA beneath the slug.
After you done with this, you can attach the lux as usual.

Good luck

 

[Handlobraesing]

Re: I am so angry at AA epoxy!

You're blaming user error on arctic alumina. The adhesive itself is non conductive so getting it on electrically active parts won't cause them to short out, but its not designed to be used to isolate the two metal pieces that are being bonded together, because there is always a possibility of metal-to-metal contact, unless you attach it in such way to "float" the LED on heatsink on a pile of epoxy.

 

[chris_m]

Re: I am so angry at AA epoxy!

A poor workman blames his tools.

 

[evan9162]

Re: I am so angry at AA epoxy!

yeah, sounds like user error to me too.

If you expect AA to electrically isolate your LEDs, then you'd better A) have a method that guarantees an insulative layer sets up between the slug and heat sink, or B) manually check each slug with a multimeter before the epoxy sets up.

 

[srvctec]

Re: I am so angry at AA epoxy!

Yup, operator error. I just did my first LED swap for a new SSC P4 and fried it because I did the same thing (didn't realize I needed a thin layer of cured AA down first). I just tried the thin cured layer first and have been happily playing with my newly modded light!

 

[LEDite]

Re: I am so angry at AA epoxy!

Gryloc;

I have been building Cree XRE multiple-LED lights for indoor use for a few months now. I have tried tried copper spreaders and anodized aluminum with a special mixed epoxy for isolation.

Heat management is your main concern.

With 18 K2's, I would expect power dissapation to be very large.

Even lots of copper will have a hard time keeping the temp rise down.

On the loss of isolation, it is probably the roughness of the copper making contact with the bottom of the K2's. It needs to be sanded with 600 grit paper to insure a smooth surface.

Building state-of-the-art lighting systems is a big challenge.

Don't let it discourage you.

Larry Cobb

 

[Illum]

Re: I am so angry at AA epoxy!

make sure you clean both contacts first...its very important to do so no matter how clean you THINK the contacts are..

secondly, make sure your mixing it 1:1...or else a portion of it is rigid and the other quite flexible...I know because my first little syringe of Acrtic Alumina went to experimenting with different ratios

Failure is the root of success

 

[Gryloc]

Re: I am so angry at AA epoxy!

Thanks all. I apologize for how I sounded. I knew I made a mistake, I was just angry at my own workmanship and a whole bunch of other things. It is one of those crappy weekends (you know what I mean), and then this happens to me. I assumed that because I lightly pressed the emitter against the copper plate (even with light enough pressure that I was sure it did not touch), that I would be in the clear.

It is correct that one should not blame his tools. I made a stupid mistake and it is now biting me where it hurts. I know better for my next project to be more careful when I was doing such a critical thing as attaching my prized emitters to the heatsink.

Now I hear about people applying a thin layer of epoxy to the slug then letting it harden first before using epoxy to attach it to your heatsink. This will give you far better isolation. How would you go about doing this? What would qualify as non-stick plastic? Will a small plate of glass or even wax paper work? I bet you have to lightly sand the hardened epoxy next for smoothness.

Does a thin layer of clear paint (like for fingernails) work? I hear about people doing this? Would it be the wisest to buy a bunch of isolated star MCPCB boards? These two solutions have a lot of thermal resistance, though.

Currently, heat is not a huge problem. There are fins and a powerful, temperature-controlled 80mm fan on the back of this copper plate. I wont go into all the details right now. If you are interested in learning more about this, go here:
http://www.candlepowerforums.com/vb/showthread.php?t=117661&page=1&pp=20
Pictures work sometimes. I need to change where the pictures are hosted. It is a really long thread, so you might have to skim through.

Finally, can you guys give me some advice? Do you think I should pull of all of the K2 emitters from both headlight clusters (there are 36 in all) and apply that thin layer of epoxy or paint, then re-epoxy them to the heatsink? This will take so much time that I almost do not have. Should I just replace the damaged emitters and isolate those while leaving the others the way they are? I would be taking the risk of having the same problems down the road if I am not any more careful. Luckily, with the emitters the way they are, they have survived for a while so far.

I do not know. Please leave any ideas and help me if you can. Thanks again and sorry. I will have to give myself more time after something like this happens before I post my frustrations on the CPF.


-Tony

 

[IsaacHayes]

Yeah in series it becomes a problem when they are all on the same heatsink.

And if they are making electrical contact with the heatsink, that means you did a good job and the thermal interface is very thin!! But then again, it's not gonna isolate them.