cooling - Experience with thermal glues/adhesives?

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?

Answer

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.