The steady state method is commonly adopted to measure thermal resistance and conductivity of thermal interface materials or TIMs. A heat flow through the TIM is generated and the temperature gradient across the TIM is measured, and the thermal resistance or conductivity can be calculated through the procedure defined by ASTM standard ASTM D5470.
Testing setup and principle
The above figure schematically shows the setup and measuring principle for thermal resistance. NTC: Negative Temperature Coefficient Thermistors; DUT: Device under test, Rth: thermal resistance. Effective thermal resistance for a TIM is the sum of bulk resistance and interfacial resistance.
BLT: board line thickness, or TIM thickness for simplicity. A: area, lama is thermal conductivity. Effective resistance can be measured by ASTM D5470 or similar ways. Plot effective thermal resistance Vs TIM thickness, TIM bulk thermal conductivity and interface resistance can be extracted from slop and y-axis intercept as shown in the following picture.
Measurement error
The major errors of the measurement come from the measurement errors of heat flow and temperature gradience. The relative error of a highly conductive material to be tested is high because of the small temperature gradient along the sample. And it is high as well for materials with very low thermal conductivity due to less heat flow.
The concept can be illustrated by the above figure. Huge error can be expected if sample’s thermal resistance does not match with reference body (RB, typically copper and aluminum). Another way is to reduce the area ratio of sample and RB in order to improve measurement accuracy.
As an examples, the following figure shows the shift of measurement error Vs sample thermal resistance.
The above content obtained from the reference (DOI: 10.1109/EuroSimE.2014.6813766). For more detailed information, please refer to the original publication.