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I'm planning on using a thermal adhesive to bond an LED pcb to an aluminum enclosure. The enclosure is meant to serve as a heat sink and wick away any heat generated by the LED board. The aluminum enclosure has a curvature to it, which means that I cannot simply place the PCB flush against its surface. To that end I'm hoping to use a solution that will fill the gap between the two surfaces (at it's maximum the gap is roughly .063").

In the past I've used thermal pastes to help a heat sink wick away heat from an IC. I'd be inclined to use thermal paste, but it never quite hardens and I've found often "drips" away over time. That's generally fine if the chip/board is stationary, but in this case the entire enclosure moves often such that I can already see the thermal paste dripping off the board into some corner of the enclosure. Not to mention that with a .063" gap I can't imagine traditional thermal paste doing a good job here.

Google provided me with some basic info on thermal epoxies, but I've never used them and am wondering if anyone has experience with them. If so any advice/commentary? Are there any solutions that are "resettable" (e.g. can somehow break the bond and redo it)? Beyond epoxies are there solutions commonly used for this kind of stuff?

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    \$\begingroup\$ It looks like you want something more along the lines of thermal pads instead of thermal pastes or adhesives. Pads are not nearly as effective at removing heat, but are pretty much the best you can do when you have a large gap to fill. Very common in consumer computer components like video cards. \$\endgroup\$ – Shamtam Dec 24 '13 at 0:28
  • \$\begingroup\$ Thanks for the comment. Had not heard of thermal pads, but based off the digikey search they look interesting and potentially like the right solution. Do you know anything about the adhesives they typically use? Digikey has a "tacky" or "adhesive" filter. I'm wondering how tacky tacky is and how strong the adhesive options are. Do you have any idea how those bonds compare to an epoxy (not thermal necessarily -- just trying to get a sense of how strong it is)? Thanks again for the tip. \$\endgroup\$ – Doov Dec 24 '13 at 1:05
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    \$\begingroup\$ Judging by what I've come across in computer repair and the terms there... "tacky" is for applications where the pad is solely for thermal connection, where the heatsink will be clamped down on the surface otherwise, either with mounting screws, clips, etc. "Adhesive," on the other hand, would be used where the heatsink is to be attached to the part only with the pad. The distinction sounds similar to the difference between thermal paste and epoxy/adhesive (thermal paste:"tacky"::thermal epoxy:"adhesive") \$\endgroup\$ – Shamtam Dec 24 '13 at 2:32
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    \$\begingroup\$ Additionally, most pads and adhesives I've come across are not permanent, so you can remove the heatsink without damaging it or the component. Much of the time, I reuse thermal pads (for instance, XBox 360) if they stay intact upon removal and reassembly, but it really is best practice to clean the part and the heatsink and apply a new pad if possible. \$\endgroup\$ – Shamtam Dec 24 '13 at 2:34
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    \$\begingroup\$ That sounds like how I would have expected them (and attempted to describe). Note, that both pads will slightly melt (or at least become more malleable) upon a couple of thermal cycles. In this way, an adhesive pad will stick better after the part heats up. \$\endgroup\$ – Shamtam Jan 4 '14 at 5:04
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Almost any adhesive will be suitable. Thermal adhesives are better, but that's missing the point. Some numbers from wikipedia: Air has a thermal conductivity of 0.025 (W.m-1.K-1). Aluminium is about 200, so that's a ratio of 10,000 times. If you can fill the gap with thermal grease (0.9), or a polymer like epoxy (0.3), or even water (0.5), the thermal resistance of the gap will decrease by about 100 times.

3M claims a thermal conductive epoxy with 0.72, but this is not much better than average plastic around 0.3 to 0.5. The important thing is that these are all 100 times more conductive than air, and they fill the gap eliminating the air.

So select an epoxy that meets your mechanical needs, something with the right temperature range and flexibility. You can probably use it somewhat above it's high temperature limit, as you don't need full strength. Prepare the surfaces properly and fill the air gap.

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If you have the time and materials to experiment with . . .

I have had great success using JB Weld metal filled epoxy to fasten ten watt LEDs to flat but not particularly smooth mill bastard hand filed surface heat sinks and driving them at maximum rated current levels. I have some that are five years old still going strong. I firmly clamp them while curing to keep the glue line thin.

I have also successfully used Elmer's polyurethane glue on flat but not particularly smooth mill bastard hand filed surfaces for a breakable bond as well, but the gap you have probably makes this a non-starter.

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