Chip reversal, heatsink on the bottom

Hi there,

Another idea to make the Radxa boards more useful in products is to do with heatsinking. If the RK chips were on the bottom of the board, then it would be possible to affix a heatsink to the board quite easily.

Being able to attach SBCs to the chassis of products (by having the heatsink on the bottom of the board) is a great advantage to cooling.

Just my 2 cents worth on this topic !

thanks for making great products

There is always a debate about the CPU/heatsink at the bottom of on top. From our point of view, 10W is the max power the CPU at the bottom. Think about this, why wouldn’t Apple do that with such a simple mode?

Easy: since they would get sued by customers in the US for burning their skin since unlike a SBC a laptop is supposed to be operated on top of your lap :slight_smile:


Considering Apple has released products such as the 2019 MacBook Air, that didn’t have a heatpipe from the cpu heatsink to a finstack (thermal throttling complaints and cooked to death CPUs resulted), and that macbook that couldn’t be opened beyond 90deg, if that, without breaking an internal cable, and such, I wouldn’t exactly look to them for good engineering design.

They would release the product. State that burned users weren’t using the product properly, and weren’t thinking differently enough during use, and the Apple/Mac community would defend their chosen lifestyle brand. Hell they would probably look towards the burns as a badge of honor.

Also, regardless of the size, you need a thick plate to spread the heat in a wide surface. That’s not compatible with laptop characteristics nowadays where they sacrifice ergonomy to thinness as if thinner was always better.

I even have a simpler view in that heat rises and a bottom mounted cpu is heating a load of components on the top side that likely don’t need to be.
The hottest part of a bottom mount cpu is board side and your fan and heatsink is on the cooler side and that just doesn’t make much sense to me, but hey.

“heat rises” is a common misconception comming from the fact that hot hair is lighter than cold air and rises. But heat doesn’t rise, fall or whatever, it radiates and spreads in any direction. You’ll even note that in a number of places (large halls etc) heating is provided by heat plates on on the ceiling below which you really feel the heat. In a board where everything is solid, heat spreads in every direction, there are no moving parts which will change position once hot (unless the board melts of course :-)).

One difficulty with a CPU at the top is to cool it inside an enclosure where there is no air flow. However for developers it’s more convenient this way, when the board lays on the table, for sure. When at the bottom, it will need a thick plate that increases the total height (and that’s a problem for a number of deployments, though here it would allow to swap M2 and the plate and would probably not change anything), plus it doesn’t allow us to replace it with whatever cheap tiny heat sink, the board-large plate is really needed when the board is on the table.

I agree with @hipboi that for a bare SBC with no enclosure and just a plate as large as the board, it’s difficult to go higher than 10W with the CPU at the bottom. On the other hand, once in an enclosure it becomes the opposite, spreading 15-20W into an aluminum enclosure is easier when the CPU is touching the case than when there are small holes and a fan inside.

FriendlyElec managed to find a good compromise with the latest R4S/R5S, with the CPU at the top and an extruded aluminum case with a large part directly touching the SoC. However that means you must use exclusively that case (which has no hole nor room for a console port).

There’s no single solution that satisfies everyone in the end.

What surrounds everything that heat is radiated in?

The board, then the enclosure, then everything that’s around it (i.e. table at the bottom, air at the top). Or maybe I don’t understand your question ?

It doesn’t matter that much. thx for the replies.