Thermal Conductance 101 with Dr. Fylladitakis

Simplified, thermal conductance is the ability of a material or arrangement to conduct heat. Thermal conductance is inversely related to the absolute thermal resistance, meaning that a lower absolute thermal resistance will improve thermal conductance. In our particular case study, that arrangement is the CPU/Cooler setup. When studying arrangements, the absolute thermal resistance of the entire arrangement is the sum of the thermal resistance that each individual part has. Anything that lowers the absolute thermal resistance of the CPU/Cooler arrangement will result to better thermal conductance, i.e. lower operating temperatures. Vice versa, if the absolute thermal resistance of the arrangement increases, the thermal conductance will be lowered.

The figure above displays a simplified CPU/Cooler arrangement. At the bottom layer we have our CPU die(s), the middle layer is the CPU’s copper lid and, finally, the top layer is the CPU cooler. With such a setup, we have three individual thermal resistances: R1) the CPU’s die (heat conduction from the CPU die to the CPU lid), R2) the CPU’s lid (heat conduction from the CPU’s lid to the cooler), and R3) the cooler’s absolute thermal resistance. These three resistances sum up to make the total absolute thermal resistance of the CPU/Cooler arrangement. Simplified and assuming one-dimensional conduction, each absolute thermal resistance depends on three things: a) the length of the material in parallel to the heat flow (i.e. its thickness), b) the thermal conductivity of the material, and c) the cross-sectional contact area.

The cooler’s absolute thermal resistance obviously depends on the cooler itself (size, materials, design, air flow, etc.). However, no matter how good a cooler is, the absolute thermal resistance of the entire arrangement can still be poor if any other thermal resistance is too high. For example, it is known amongst enthusiasts that some of Intel’s previous processor generations had poor thermal performance that was unjustified given their very low power requirements. That was because the CPU’s lid was making poor contact with the CPU’s die(s), greatly decreasing the thermal flow between the die and the lid. Hardcore enthusiasts were “delidding” their processors in order to fix this issue.

Introduction AMD Threadripper Processors & Cooler Compatibility
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  • Chaotic42 - Thursday, March 15, 2018 - link

    Out of curiosity I ran AIDA64's CPU/FPU/Cache stress tests for 30 minutes on my 1950X with a Corsair H115i and an ambient temperature of 21C and my CPU temperature pegged out at 58C the entire time. Reply
  • Phylyp - Wednesday, March 21, 2018 - link

    Minor quibble about the presentation (not content, top-notch article there!). In the bar charts for idle & load temperatures, the subtitle reads "Ambient at ~21°C" but the tilde almost looks like a negative sign at that size of font, which threw me for a minute. May I suggest that in future it be replaced by "approx."? Reply
  • vpz - Thursday, March 22, 2018 - link

    Thermal results do represent the delta ?
    What was the ambient temperature ?
    Reply

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