How do I insulate high power LEDs from my MCPCB?

[michaelpc]

I should even call it a PCB, it's just a bar of aluminum really. I need to put down a dozen or so 3w leds and I'd like to set them on to the bar directly. Is there a non-conductive thermal tape I can use? There is some continuity between one of the pins and the base, so I need to separate it somehow.

 

[itsmoked]

Yes there are many thermally conductive electrically isolated materials. Go to:

http://www.bergquistcompany.com/

They have it all. Not cheap so if you are doing production then you might want to look to other places like Panasonic and 3M.

If you are talking about LEDs mounted on STAR boards then you have something wrong because you can certainly mount all of them onto a conductive heatsink.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[1capybara]

itsmoked is right, ive always seen LEDS mounted directly on the heatink, perhaps you want to look for a different LED than the one you have?

 

[MacGyverS2000]

Quite often the bare LEDs mounted to those star boards require isolation... which is provided by the star board, so you don't need to worry about it come time to mount them. If you're using star boards, you are likely looking at it wrong... if you're not, then Keith's suggestion of Bergquist is a good one.

I have used this one from 3M to good effect in the past:

http://multimedia.3m.com/mws/media/...ctive-adhesive-transfer-tapes-8800-series.pdf

Dan - Owner

http://www.Hi-TecDesigns.com

 

[mcgyvr]

yep.. mount the stars directly to the aluminium with a thin layer of thermal compound between it and the bar just to fill in any small voids..
I've even used 2 part thermal "adhesive" (Arctic Silver) and didn't do anything to mechanically secure them before (no screws,etc..)

http://www.coolerguys.com/840556003...nnelid=FROOG&gclid=CJjiicGwmsoCFQoqHwodJO8Nhg

 

[michaelpc]

Thanks for the tips guys. For clarification, these are the star less but without the star heat sinks as I need to pack them closer together. The perfect solution would be some thermal, elec isolating tape that I can then use a ther a adhesive on top of and between the chip LED and tape. Is that a terrible idea?

 

[mcgyvr]

yes.. sounds like a terrible (HACK) idea..
post the datasheet for the LEDs so we are clear exactly what you have....

Why do you think you need to pack them closer together?

And a "bar" of aluminum is a poor "heatsink".. Surface area is king...
"packing" high power LEDs together on a poor heatsink is just asking for trouble..

 

[michaelpc]

Agreed, it wouldn't be ideal however I'm running them at the lowest possible voltage so they're not getting very warm and if it's relatively successful, I'll go from bar to vented. I'm still just prototyping.


Photo attached (although as mentioned, to get them closer together, I'm using a 6 pin setup sans star heat sinks, so I can wire them in series as close together as possible)

Datasheet:

https://solarbotics.com/product/led-rgb-star43cd/

So the end result would ideally be very tightly packed leds run in series, running at minimum power, on an aluminum heat sink. To complete the test though I need to find something a little thinner than the epoxy I'm using (barely shown on another test in the corner of the picture).

 

[mcgyvr]

why do you need them so close? (I'm gonna guess Star Wars light saber)
And your not running them at the lowest possible voltage.. LEDs brightness is dictated by current through them.
I hope you know that (LEDs) need to be fed a constant current (resistor or other current limiting circuitry) and not from a fixed voltage source.

 

[michaelpc]

Yes, I'm using a voltage booster to get a constant current. But there does seem to be some range available, albiet a bit small, to adjust brightness.

Not a light saber! I've never even seen a Star Wars

 

[zappedagain]

DigiKey has many types thermal gaskets available. They may have an adhesive on one side but that is just to hold the gasket in place until it is securely pressed by the mounting hardware. I wouldn't trust tape to hold components due to the difference in thermal coefficients of expansion of the component material and the heatsink material. They'll break loose after enough thermal cycles.

Z

 

[michaelpc]

Is there perhaps a spray on epoxy type material? Right now I'm using an epoxy that can handle heat for prototyping, but need some kind of fine layer (also thermally conductive) to electrically isolate the beads from the heat sink.

 

[mcgyvr]

How are you attaching wires/traces to the LED leads?
You can get cheap metal clad PCBs made to your specs on ebay,etc.. for a prototype.. sitopway does it

http://stores.ebay.com/sitopway

not sure the intention of your "prototype" is going to be..

as far as thermal/insulating pads,etc.. check out products from bergquist they have the thermally conductive/insulating products you need

http://www.bergquistcompany.com/

(sold at digikey,etc...)

 

[michaelpc]

I'm keeping the LEDs so close together that I'm just soldering the legs together, bypassing trace or wire needs. I've thought about getting some MCPCBs made, but I thought I could get away with a DIY as it's just so basic. The only thing I need to solve is that isolation layer.

I'll look into the thermal pads, thanks for the links.

 

[itsmoked]

Did you read my Bergquist comment? I'm confused because you still seem to be looking for tape.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Did you read my Bergquist comment? I'm confused because you still seem to be looking for tape.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[michaelpc]

itsmoked: Yes, I did read that, thanks much. I'm still confused honestly. Sil-pads all seem to be precut. Gap pads don't seem to fit the bill. Bond-ply would have too much additional adhesive collecting dirt. If you look at the image attached below, you'll see my ideal setup - achieved with a rather thick layer of epoxy and Arctic Silver adhesive between that and the LEDs. The epoxy isn't thermally conductive though and is only serving to help prototype this light bar.

I think the ideal epoxy replacement would be a liquid pour/spray thermally conductive, electrically non-conductive layer, that I fix these LEDs onto.

Thoughts?

 

[michaelpc]

What do you guys think of this?

http://www.amazon.com/MG-Chemicals-...95&sr=1-3&keywords=thermally+conductive+epoxy

 

[itsmoked]

That epoxy may work but you will likely have some rather steep learning curves to deal with. It's not as easy as it seems and it will be messy. As an example those bottles may be settled and if you try to make a small amount they often do not perform like you'd expect. You may end up with something not fully curing.

If you are concerned about a tape that has some surface adhesive still exposed then just cut the shapes you need. If you're just prototyping about, sit down with some scissors and get to work using the Bond-Ply 400 or one of the nice fully characterized 3M tapes MacGyverS2000 linked at you. Cutting little circles out of tape is much much easier than mixing liquids and transferring them here and there in an orderly successful manner with time ticking. Tape is considerably less wasteful ecologically if that matters to you.

If you can't bring yourself to using a scissors use an appropriate washer and and Exacto knife. If you don't want to do that find a piece of pipe with an inside diameter the same as your LEDs and then grind the outside diameter down to make a knife edge of the inside diameter. Using a piece of scrap wood you can punch out dozens of pads just the size you need. Using a pad that works instantly whenever you need them and doesn't take a day to cure, and drip, and run, all over, is way nicer to work with.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[michaelpc]

Thanks for all the thoughts, and the heads-up on the epoxy. The tape does seem a lot easier to deal. I'd still have to run it across the length of the heat sink just to insulate the pins from one another, but it'd be a very narrow strip.

Thanks again! This really clarified the best solution.