It’s always good to look for improvements, but a proper thermal paste joint is pretty good. With a 20mm MCPCB, a thermal paste thickness of 0.075mm, and thermal paste conductivity of 5 W/m*K, the thermal resistance of the thermal paste joint is 0.05 K/W. This could certainly be improved upon, but it is already small compared to other thermal resistances in the path such as the LED junction to solder point resistance (1-3 K/W).
I used Coollaboratory Ultra in my last PC build (on a nickel plated CPU cooler) with very good results.
Unfortunately, gallium absolutely cannot be used on aluminium , and apparently it’s not wise to use it on copper either (although the reaction isn’t anywhere near as dramatic as it with aluminium). So that pretty much rules it out for flashlight builds…
Only possible downside for flashlight use is that it’s likely electrically conductive, but that’s not a big deal if you’re careful.
(I once used ‘Permatex copper anti-seize’ in a pinch on a laptop cpu cooler, and it hasn’t exploded in the 2 years since)
You can use tin-zinc, tin-zinc-gallium or tin-indium solder to just solder copper to aluminium without any major hassle (outside of obtaining or mixing the solder itself and the aluminium-special flux, though you can get by without one). The latter two compositions even have lower melting point than the common tin-lead solder. And the tin-zinc-gallim should bond easily to aluminium surface.
If you’re talking about the charts in the previous post then where do you see 160W figure? I don’t think modern CPU’s simply can take that much power for any prolonged period of time.
And shouldn’t the surface area of their hood be bigger than a 20mm LED MCPCB?
Well, right now I have some special aluminium soldering flux in the mail; check out the thread Aluminum soldering fluxes: The Way It's Meant to be Soldered :D for related info. I'm yet to decide what kind of solder to use among Sn63/Pb37 and Sn99'3/Cu0'7; I'm somehow leaning towards the lead free alloy, as it is said to work better on aluminium, though I don't know why.