Monochrome 2: custom fanless 7.5L Strix Halo system (WIP)

Currently I have to wait, the case is ready, the custom cooling solution, including custom bent heat pipes is as well. It could take a number of more months though until I can finally get my hands on that Framework board. I read rumours about Strix Halo supply being scarce so maybe it will take longer than "Q3" that is officially the shipping window. We will see. Once I have the board I can see if my contact plate is actually working out or if I have to last minute modify it and then I can finally attempt assembling the heatsink and heat pipes with proper thermal compound.

Will the thermal concept work out? Only one way to find out. Theoretically, the heatsink should be capable of dissipating 140W when I manage to get it up to 60°C, which of course necessitates that the APU is running very hot. I would hope it will work also under load without too much thermal throttling but that is what I shall find out.

To illustrate it better that is the actual cooling system without the side cover:

And some more pictures with the side cover assembled, the PSU in place but obviously without the board:

Wow, this is truly a work of art. Can you show us some of the design and manufacturing steps? I'd be particularly interested in how you determined the size of the heat sink and how it was manufactured.

Thanks!

Well, it is a bit early for that I guess. I don't even know if it will work out yet.

What I do know is how my fanless Mini PC is working like: That system has a 28W APU (7640U). With two flat heat pipes and those heat pipes have not too efficient contact with the heat sink via 1 mm thermal pads. That is good enough. There is thermothrottling during full single core load and its hitting thermal limit during lasting full multicore load. Interestingly with full iGPU load during games temperatures stay pretty decent at less than 70°C (even though RAM is hitting thermal limit). From that system I also know that the heatsink can manage to reach 55-60°C or so (just a bit too hot to touch for longer than a few seconds).

The Mini-PC heat sink is actually a LED heatsink (PADLED-13080) and has a technical data sheet: https://wakefieldthermal.com/content/data_sheets/PADLED_WT_v2.pdf

At a deltaT to the room temperature of app. 30°C, the data sheet would specify a power dissipation of 33W. Close enough I think to what I see in reality. The data sheet is listing a thermal resistance of 1.5 °C/W. But based on the actual numbers it would look more like 1.0 °C/W. Anyhow, it is a starting point.

I found extruded heatsinks with technical data sheets and took the profile of one as inspiration for my heat sink. The commercial one would have a width of 200 mm and be 5 mm less deep with a thermal resistance of 0.3 °C/W. My heat sink is on one side 250 mm long and on the other 70 mm long. That should estimated get me somewhere around 120-140 with 30°C dT. If I can keep thermal losses low enough from the die to the heatsink it should theoretically work out.

What should work in my favour is that my heat pipes are distributing the heat along the height of the heat sink and there also 3 dedicated heat pipes to the front where there is more heatsink surface. What works against me is that some heat pipes are rather long at 250 mm. However there is additionally also a pretty large direct aluminum link to the heat sink which should also transfer some of the heat.

That's my reasoning. How well I estimated it all i will only see once I have it completed and I can test it.

Manufacturing

Ok, I possibly did not go the most effective route and most definitely not a cheap one. But I have to say I did enjoy to design CNC parts and see them turning out just as I wanted them. I was already aware that my design with slanted fairly narrow fins (for CNC) would probably need more advanced tools. I was ordering it form JLCCNC and they used wire cutting to cut the heat sink fins and then milled the rest. That came at a rather steep price admittedly. But at least I got app. 5 kg heat sink in return for it.

The heat pipes are another story. Those I bent myself and every individual heat pipe is bent differently. This was a real pain.
In the end I needed 3-4 bending shapes per tube to get a somewhat decent curve, while veeeery slowly bending it.

Those 3d printed shapes were screwed into the base and I had a metal tube as lever.

A first test assembly:

I replaced some of those pipes in the end. Lessons learned: Go for heat pipes with mesh if you want nice curves. Sintered pipes do still work out but grooved pipes are an absolute nightmare. Those groves stabilise the pipe and make it very challening to bend without knicking them, at least at minimal radii.

Thanks.

I have to make a small correction. The commercial reference heat sink with 0.3 K/W thermal resistance was 250 mm wide, not 200 mm. So my 250+70=320mm width would have roughly 0.234 K/W if I assume that it all simply adds up. So, to cool 140W the dT of the heatsink to room temperature would have to be 33 K. In other words, at 25°C room temperature, a heat sink with 58°C.

It is a bit on the tight side but I the reference was under the assumption of black anodized surface and vertical orientation but no heat pipes and with 5 mm less fin depth.

The real question mark is how high the temperature loss will be at the interface of die to copper spreader, from there to the heat pipe, in the heat pipe and then from the heat pipe to the heat sink.