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I would like fo know how do thermal pads work, more specifically why does their thickness matter so much even more so than their actual conductivity numbers sometimes and how do I choose the proper thickness, or rather how do manufacturers choose it for the various products/applications.

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    \$\begingroup\$ Thermal pad is a vague term. Do you mean a sheet of material that goes between the part and a heatsink (or copper on a PWB used as a heatsink)? \$\endgroup\$
    – Mattman944
    Commented Oct 11, 2020 at 22:43
  • \$\begingroup\$ Both that and thermal pads for example on top of PCB modules, VRAMs, VRMs etc, but ultimately thermal pads that dissipatiate heat. \$\endgroup\$
    – appwizcpl
    Commented Oct 11, 2020 at 22:49
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    \$\begingroup\$ Your comment made the question more vague, not less. Do you know how to calculate the temperature differentials for the conductive path? This is fairly straightforward. The convection from heatsink is more complicated. \$\endgroup\$
    – Mattman944
    Commented Oct 11, 2020 at 23:14

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Thermal pads work basically on the basis of the fact that they're more conductive then the material they replace (air).

They don't work too well because they generally need be soft enough to conform to the surface they're trying to mate, which places limits on their composition.

Basically, you want the thinnest thermal pad you can manage. The obvious end-point for this is thermal paste (where the thermal paste can squeeze out allowing the thinnest bond-line possible).

Generally, you design for the thinnest gap possible. You only go thicker due to mechanical constraints.

Thermal pads are basically a compromise that you only take due to mechanical constraints. They're generally never better then a mechanically clamped interface with thermal paste. A thermal pad provides assembly convenience (and substantial gap filling in some cases), but poorer performance.

more specifically why does their thickness matter so much even more so than their actual conductivity numbers

This is pretty apparent if you consider it a bit. If you reduce the thickness of a thermal pad by 1/2, you've effectively cut it's thermal impedance by half, which would require double the thermal conductivity to equal. It's much easier to thin the thermal pad then to improve the thermal conductivity beyond material limits.

Both that and thermal pads for example on top of PCB modules, VRAMs, VRMs etc, but ultimately thermal pads that dissipatiate heat.

In any application I've seen, thermal pads do not dissipate heat. They move heat from a device to a dissipative radiator.

Making a heatsink out of thermal pad material would be silly, because they're really quite crappy in terms of actual thermal conductivity.

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  • \$\begingroup\$ You say they move heat from a device to a radiator, but what if there is no radiator. E.g. thermal pad on a NVMe drive? \$\endgroup\$
    – appwizcpl
    Commented Oct 12, 2020 at 6:54
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    \$\begingroup\$ It's not going to do anything substantial? Fortunately, most NVMe drives have heat spreaders (which are basically really crappy heatsinks), rather then just thermal pads. \$\endgroup\$ Commented Oct 15, 2020 at 23:51
  • \$\begingroup\$ So a thermal pad can act as a really bad passive heatsink? If that is the case what thickness is better, if we are comparing 0.5mm, 1mm, 1.5mm and all the up to 3mm? \$\endgroup\$
    – appwizcpl
    Commented Oct 19, 2020 at 7:06
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    \$\begingroup\$ No, my point was that the things on NVMe drives aren't just thermal pads, there's copper embedded in a lot of them. And the thickness of a material used as a heatsink has no effect on it's utility as a heatsink, it's entirely a function of it's surface area. If you're trying to use a thermal pad as a heatsink, well, 1: why, and 2: a thicker thermal pad would mean slightly more surface area, but the poor thermal conductivity of the pad material would lead to diminishing returns probably fairly quickly. \$\endgroup\$ Commented Oct 19, 2020 at 9:42
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    \$\begingroup\$ Basically, anything that's exposed (generally to the air, though radiative thermal transfer is also a thing) and not at ambient temperature is a heatsink, at least to some extent. It's not like you only get thermal relief from the thing you label "this is the heatsink". You have to think in the context of thermal energy transfer in a entire system. \$\endgroup\$ Commented Oct 19, 2020 at 9:45

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