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  • Author Author: shabaz
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Raspberry Pi 3 Cooling / Heat Sink Ideas

shabaz

Note: This is part 1 of a 2-part post. For part 2, click here: Raspberry Pi 3 Dynamic Current Consumption, Power and Temperature Tests

To see an implementation using a heat pipe, see the Outdoor Pi 3 Image Recognition Security Camera project (aka HAL-CAM 9001)

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Inspired by Christopher's   Raspberry Pi Operating Temperature report (with thermal photos from Gareth Halfacree) regarding the heat dissipated by the Broadcom chip on the Raspberry Pi 3, and the discussion with we started searching around for ways to keep the Pi cool.

Ceramic heat sinks have an innovative structure (vias or micro-pores) which allow a heightened thermal conductivity compared to traditional aluminium heat sinks. They also can have the advantage that they won't affect radio frequency (RF) communications as much, when positioned close to the wireless antenna that is present on the Pi 3.

Furthermore, ceramic heat sinks are not electrically conductive and therefore there is no risk of accidentally shorting something on the Pi.

Back-of-the-envelope calculations (we don't have all parameters since we don't have a copy of the Broadcom datasheet to examine the device operating conditions in detail, nor a copy of the schematic to examine if we can measure device power consumption isolated from the remainder circuitry) suggests that a heat sink with thermal resistance of around 10 degrees C/W might be effective to keep the Broadcom chip's internal temperature below 120 degrees C when the ambient temperature is below around 40 degrees C. These are guesstimates until practical measurements have been taken.

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Armed with this information, I searched for a suitable sized heat sink and I found a cheap aluminium one. However and suggested that there may be issues with the heatsink getting in the way of connected HAT boards on top of the Pi, and Brian suggested examining ceramic heatsinks.

In summary, I think these parts may be suitable although measurements still need to be done:

22x22x2.5mm MPC222225T22x22x2.5mm MPC222225T

15x15x2.5mm MPC151525T15x15x2.5mm MPC151525T

(Optional) 20x20x10mm 5V DC fan MC33873(Optional) 20x20x10mm 5V DC fan MC33873

(EDIT: after some discussions below, it looks like this Sunon 'DR-MagLev' design fanSunon 'DR-MagLev' design fan is a far better choice, it is 25x25x10mm, quieter, higher throughput, and more efficient with overall lower power consumption. There is also a 25x25mm finger guard25x25mm finger guard).

 

The Broadcom chip is about 14x14mm in size, and ceramic heat sinks do exist in approximately that size. However it is possible to attach a larger heat sink if desired.

The photo below shows the parts that were examined. All of these ceramic heat sinks come with adhesive tape on the underside; the protective tape is removed and the heat sink will stick to the top of the integrated circuit.

The photo at the top of the blog post shows the largest 22x22mm ceramic heatsink.

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The photo below shows a heat sink closer to the size of the Broadcom chip, 15x15mm. It also shows a 10x10mm heat sink on top of the USB hub/Ethernet controller chip, however this is really not needed. It doesn't get very hot according to the thermal photos in the previous blog post.

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The memory chip on the underside gets hot too. If desired, the 15x15mm heat sink could  be attached there too. The 22x22mm one is too large for that location due to nearby components (the memory chip has a lower height than the Broadcom chip on the top side).

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So, I plan to attach a 22x22mm part to the Broadcom chip on the top of the board, and possibly a 15x15mm heat sink to the memory chip on the underside. I don't think a fan will be needed unless a very small enclosure with no natural ventilation was used, or if the Pi was in a very warm environment.

In that case, a fan may be an option. I tried the MC33873 fanMC33873 fan and it generates a usable level of air that can be felt from a distance of 10cm or more. However it does generate a small amount of noise too (possibly inaudible if the Pi will be behind other items such as a TV, but I think there is still a risk it could be audible. The voltage could be reduced from 5V to lessen the noise). It could be mounted on the heat sink as shown here although I think a small gap would be good to allow forced air to hit the entire top face of the heat sink. (The red dot on the fan was placed by me so that I could see it spinning). The fan could be secured with epoxy adhesive. The overall height is less than the height of the USB connectors on the Pi 3. The problem with this is that a HAT board cannot be plugged on top if there is a fan in the way.

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A very nice solution would have been to put the fan on the side (to the left side of the photo above) so that the fan could blast air across the entire top face, and the underside of the board. However the display connector (the long white thing on the edge of the board in the photo above) is in the way and would block the flow of air which is extremely unfortunate. The Pi 3 wasn't designed with air flow in mind : ( However it might be possible with some 3D-printed duct design to achieve something that could work.

 

To summarize, some heat sinking ideas have been suggested however it is for further examination to see how well they perform. It will be good to see what solutions people come up with over time.

  • in reply to shabaz

    Just thinking, the other problem with a large copper or aluminium heat sink is that it will impact wireless performance, because the wireless antenna is just about 2-3cm away depending on the size of it.

    The fancy materials like ceramic/diamond won't have that issue.

    In fact, it would be very cool to reuse the heat sink as part of the antenna too, they really should consider that for Pi 4 (or any of the other SBCs).

    (e.g. antenna printed or glued onto the ceramic substrate, or embedded in it).

  • in reply to shabaz

    shabaz wrote:

    Still, for initial measurements, I might drill a small hole in the top of the heat sink and put that thermistor inside anyway. It is tiny, smaller than a match head. Don't want to mess up and snap the ceramic though..!

    I found drilling the ceramic very easy by just twisting the drill bit with my fingers, being so full of micro holes it makes it quite soft.

  • in reply to shabaz

    Yes I just looked at the raw data for test 1 No cover , No fan, No heatsink  and compared with test 5 No cover,No fan With heatsink and the time taken to reach temperatures shows quite significant effect the heatsink has 

     

    T1  reached 60'C at 30 seconds and 80'C at 267 seconds

    T5  reached 60'C at 58 seconds and 80'C at 493 seconds

  • in reply to e14 Contributor

    Hi Manuel,

     

    I found this data, which shows that for near-like-for-like (i.e. near-similar size and shape), the ceramic one has a substantial win over the copper heat sink. The ceramic one in the table below is 0.5mm thicker unfortunately so the comparison isn't truly like-for-like, but close for practical purposes.

    The ceramic one out-performs the others because although the thermal conductivity is lower, it has a 30% higher surface area due to the micro-pores.

    image

     

    However, if size is not important, then a heat sink with fins could be used. Here there is a comparison with an aluminium one that is 1.7 times larger than the ceramic one

    (8.6mm high versus 5mm high) for near-similar performance (0.7 degree difference, so within experimental error perhaps). So, a  copper heat sink could perform

    better than a ceramic one, but would be larger.

     

    image

    But, it is possible to go beyond a copper heat sink too, e.g. copper foam apparently exists too, or as has found, diamond has an extremely high thermal conductivity compared to even copper.

    And identified that it is possible to obtain small, flat heat pipes at low cost, which could allow pushing the heat sinking out beyond the case.

     

    Regarding availability of  off-the-shelf 14mm copper heat sinks, they may be harder to find perhaps due to cost (aluminium ones are cheaper) so maybe relegated to desktop PC use where the size/weight/performance mix might make the copper heat sink more popular despite the increased cost. Also even though the thermal conductivity is high, usually it can be a copper alloy (to make it easier to manufacture/machine) and also perhaps plated to reduce oxidisation, so it probably cannot be assumed that the material of the heatsink has the same thermal conductivity as pure copper.

  • in reply to cstanton

    Christopher Stanton wrote:

     

    It also makes me consider that using a different thermally conductive grease/adhesive would help pull the heat away better.

    A good point, but if using a none adhesive thermal compound this would require a bracket or clip of some sort to hold the heatsink in place as is common in PC motherboard designs.