7 Replies Latest reply on Jun 21, 2020 6:13 PM by colporteur

    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?

       

       

      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?

        • Re: Thermal Dynamic of foam?
          fmilburn

          I have worked extensively with heat transfer but not in the context of electronics.  Curious, I looked at what Newark offers.  They list 224 different products that can be sorted by thermal conductivity.  https://www.newark.com/c/cooling-thermal-management/thermal-interface-materials/thermally-conductive-materials

           

          Thermally Conductive Materials

          Looks like there is a lot of choice :-)

          2 of 2 people found this helpful
          • Re: Thermal Dynamic of foam?
            shabaz

            Hi Sean,

             

            I've seen 'SIL-pads', those are available in different thicknesses (up to about 4mm) and are squishy but spread out, so are more like very soft rubber in feel, and don't feel like they have an air foam. (EDIT - got the name wrong.. GAP PAD is a brand name for the thicker more squishy ones. Other manufacturers have different names for similar stuff).

            Those are the more orthodox way of handling varying air gaps, but I guess foam may work to an extent.

            My Pi 4 is in a large plastic case and has no heatsink (I don't do much processing with it), and the case gets warm, through conduction through the PCB and so on. I've not got around to finding a heatsink for it yet, and due to the particular enclosure I'm using I might need a fan eventually : (

            However for now (with the light processing) it's been running non-stop for weeks as-is, so I'm only going to attach the fan as a last resort.

            2 of 2 people found this helpful
            • Re: Thermal Dynamic of foam?
              dougw

              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.

              3 of 3 people found this helpful
                • Re: Thermal Dynamic of foam?
                  colporteur

                  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:(

                    • Re: Thermal Dynamic of foam?
                      Jan Cumps

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

                      1 of 1 people found this helpful
                        • Re: Thermal Dynamic of foam?
                          colporteur

                          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:)