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Thermal Dynamic of foam?

colporteur

When did foam become a good thermal conductor?

 

I studied electronics when Marconi was developing wireless technology. Well maybe not that far back. My career started on the cusp of the transition from glass transistors to semiconductor transistors. The one course in technical school that I had to pass and stood in the way of me graduating from college was Thermal Dynamics, the design consideration for heat transfer in the electronic components. The instructor was an Engineer and the Dean. I found him a poor instructor and struggled in the course. Our relationship deteriorated the more I struggled. I did mange to graduate. The Dean signed off on my thesis paper to graduate.

 

I recall it was important to span the air gap between a component and the heat sink. Sizing the heat sink was important but that air gap left unaddressed impacted the design. Liquid thermal compound we called paste was the go to solution. It filled the air gap to increased the thermal conductivity between the components and heat sink. About ten years ago I came across a thermal tape to replace the thermal paste. It was cleaner. Now I discover a foam?

 

image

 

I purchased a knock-off Pi4B metal case that doubles as a heat sink. The air gap between the Pi components and the metal case was close to a 16 of an inch. Wow! Included in the kit was foam pads. More pads (red circle) than required to cover the components that needed a heat sink. I was sceptical it would worked. Using the stress test posted here https://core-electronics.com.au/tutorials/stress-testing-your-raspberry-pi.html I discovered the metal case warms up but Hey it works.

 

My question to those individuals that never heard of a glass transistor, meaning your learning is newer than mine, on a scale of best to worse thermal conductivity solutions what is used today for bridging the thermal gap?

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  • 0

    Thermally insulating foam insulates by trapping air and preventing it from circulating because stagnant air is a good thermal insulator. The plastic in the foam actually conducts heat much better than air. In a situation where the gap is a couple of mm, the air would be stagnant and there would be no convection, so inserting insulating foam actually improves thermal transfer across the gap. Thermal pads and pastes are often not great thermal conductors, but they are way better than stagnant air. Materials designed to be thermal conductors range from low density foam to higher density materials, to higher conductivity materials all the way through metals to pure diamond. You need to examine the datasheets to discover exactly how well they do in various situations.

    Some materials like pyrolytic graphite sheets can have much better thermal conductivity than copper. I expect we will see graphene materials when they become economic.

  • 0 in reply to dougw

    Awesome commentary DW.

     

    The CPU stress test results in throttling without a heat sink. Fitting the Pi with the full metal jacket case and the foam kept the CPU temperature well below throttling temperature. My test using the foam supplied in the kit demonstrated it works.

     

    I am surprised at your suggestion of the poor performance for paste. I know there are good and bad paste. I recall purchasing CPU's and the vendor marketing different flavours of paste with an ever increasing cost. I have a small jar of what I believed was the good stuff. It doesn't dry out even after continuous use. The downside it doesn't come off if you wipe any excess on your clothes:(

  • 0 in reply to colporteur

    During my education, I learned that paste is only supposed to fill holes. No more paste should be applied than that.

  • 0 in reply to Jan Cumps

    I recall a caution to use the compound sparingly. The coating of paste was suppose to be just sufficient to fill the air space between two surfaces. A quantity of paste that resulted in extra squirting out the sides when pressure was applied to secure a component to a heat sink was frowned upon. The small jar I have, has been in my possession for close to 25years. I leaving it as part of my estate for my children's inheritance:)

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  • 0 in reply to Jan Cumps

    I recall a caution to use the compound sparingly. The coating of paste was suppose to be just sufficient to fill the air space between two surfaces. A quantity of paste that resulted in extra squirting out the sides when pressure was applied to secure a component to a heat sink was frowned upon. The small jar I have, has been in my possession for close to 25years. I leaving it as part of my estate for my children's inheritance:)

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