Large Heat Sink on Banana Pi M3

So while I was playing with the idea of running BOINC - on my otherwise mostly idle - octa-core Banana Pi M3, I remembered that the previously added tiny heat sink will probably not do much good. The CPU is still easily heating to 65-67 degrees Celsius even with a CPU clock rate set to 1GHz and under moderate load.

Small heat sink installed on my Banana Pi M3

Sizing

I started looking around in my "random parts box" and found a roughly 5*5 cm aluminium heat sink, with a slight plateau on the bottom, in the centre line where something used to be attached. Unfortunately, there isn't any mounting opportunity on the board and everything is tightly packed. The CPU and the RAM are the same height, but some parts directly next to them are slightly higher and this puts a limit on how large the heat sink can be. I measured and a roughly 3.5*2.5 cm heat sink should give enough clearance from nearby parts. I had to cut my heat sink into pieces, but it worked.

Heat sink cut into 3 parts. Bottom one is scrap, left and right are the right size.

Mounting

I cleaned and polished the cut edges a bit and made sure it fits on the board. I put a grain of thermal paste on both the CPU and RAM and aligned the part carefully. It fits just right and the paste also gives a bit of mechanical stability, although not much. It is not visible on the pictures, but to hold the heat sink in place I have used 2 pea-sized balls of glue on the side of the heat sink close to the GPIO pins. Ideally I would have used a glue gun, but since I do not have one, I used the 2 component Pattex glue paste.

As you can see below, the heat sink has an odd-shape with one "arm" extending over the eMMC storage (to the right), whereas contact is only made on the left side with the CPU and RAM. I realize it is an oddly shaped heat sink, but the shape allowed me to use a surface area as large as possible.

Large heat sink mounted on the Banana Pi M3.

Large heat sink mounted on the Banana Pi M3. Side view.

 It is an odd shape, yes, but it is the added thermal mass and the surface area that matters.

Result

• I have been running BOINC on 6 cores at 100% load, using a clock frequency of 1GHz for about 45 minutes and the temperature seems to have maxed out at 62-63 degrees Celsius. Ambient temperature of 27 degrees Celsius.
• The heat sink seems to sit tightly, despite the agreeably dodgy mounting. I wish I could have added some screws, but there simply isn't enough space.
• The possible drawback is that since the heat sink covers both the RAM and CPU, it is quite likely that it is in fact heating the RAM. I am not sure if this is the case - or to what degree as the RAM was usually hot as well, just checking by touching - nevertheless, I'll keep an eye on this.

I will do a bunch of testing to see what temperatures I get at different CPU speeds and number of cores used. Stay tuned.