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I'm working on a circuit which will rest on top of the lithium polymer cell that powers it. A large portion of this circuit would be better off with minimal temperature drift so I'm planning to place a thin silicone sheet between the two to solve this.

Of course air would be a better insulator (I think so at least) but the height requirements are quite stringent and this would be difficult to achieve with good mechanical stability.

I am quite confused as I see silicone used for both thermal insulation as well as conduction in different applications.

For instance, I have seen them used to connect heat sinks to ICs (see 3M thermally conductive interface pads for example.) I have also seen silicone used for insulating applications, and a quick google for "is silicone a thermal insulator" returns multiple pages stating "silicone has a low thermal conductivity."

What am I not understanding here? Is a silicone sheet a good idea to keep my circuit insulated from the battery temperature to some extent or would it achieve exactly the opposite of that? If so, what is a cheap, thin and widely available material that could achieve this function?

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  • \$\begingroup\$ "Silicone" could mean a lot of different things. I think you need to be more specific. \$\endgroup\$
    – Lundin
    Oct 28, 2022 at 12:27
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    \$\begingroup\$ A silicone (or similar material) can be both an electrical insulator and a thermal conductor. For an example, look at some of the products from Aptek (not an endorsement). \$\endgroup\$
    – SteveSh
    Oct 28, 2022 at 12:29
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    \$\begingroup\$ Thermal pads aren’t just made of silicone - this is just a binder for a ceramic powder or whatever is used to improve the thermal conductivity. Same with thermal paste. \$\endgroup\$
    – Kartman
    Oct 28, 2022 at 12:33
  • \$\begingroup\$ Same with thermal heatsink paste and conductive epoxy - very specific contaminants are introduced (to increase some property) while still retaining original function. \$\endgroup\$
    – rdtsc
    Oct 28, 2022 at 12:36
  • \$\begingroup\$ Sometimes if you really need it to be electrically insulating but thermally insulating such as to electrically isolate a heatsink, you use a thin material that is electrically insulating, even if it's also thermally insulating because you're making it really thin. \$\endgroup\$
    – DKNguyen
    Oct 28, 2022 at 15:23

2 Answers 2

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CES Selector Version 4.7.0 lists three unfilled silicone elastomers with a thermal conductivity of range 0.38 W / m K to 2.55 W / m K. Anything below 1 W / m K is a pretty poor thermal insulator conductor.

The thermal conductivity versus cost per volume is for all the materials in the database is shown below:

enter image description here

If you zoom in on the lower left hand corner, it's mostly polymer foams (light green):

enter image description here

In general, thin things don't make great thermal insulators due to their low thermal resistance (https://en.wikipedia.org/wiki/Thermal_resistance). You either need something thick, something with air gaps, or both.

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  • \$\begingroup\$ Thank you for the detailed answer, however I'm not sure I follow, you say that "anything below 1 W / m K is a pretty poor thermal insulator" but the materials you list are on the lower end of the chart well below that value. Did I misunderstand your answer or did you make a mistake in that sentence ? \$\endgroup\$
    – Ryan
    Oct 28, 2022 at 19:21
  • \$\begingroup\$ @Ryan sorry about that. I meant anything below 1 W / m K is a pretty poor thermal conductor. Sometimes I type faster than I think :) \$\endgroup\$
    – C. Dunn
    Nov 1, 2022 at 14:39
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If the thermal insulation requires either a layer of gas or vacuum, then a solid or liquid layer would need to be order(s) of magnitude thicker than the gas/vacuum layer to have just as low thermal conductivity.

Any polymer you put in a thin layer between the battery and the PCB will only improve thermal conductivity from battery to PCB. An uncontrolled layer of air - with occasional contact areas - will work way better than a layer of polymer that is guaranteed to be in intimate contact with both.

Your best bet for a mechanically stable insulator is a grid-shaped spacer. The area covered by the grid material must be 3+ orders of magnitude smaller than the area covered by the gas pockets between the grid lines. Then it'll act as-if the spacer wasn't there.

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