Thermal Conductivity Testing of Thermal Interface Materials (TIM)

Characterization of the thermal performance of thermal interface materials (TIM) which are utilized in electronics packaging can be crucial to preserving reliable performance, particularly with the quickly growing heat dissipation needs of every new generation of electronic components.

Light- or Laser-Flash Methods (ASTM E1461) can be relied on to perform routine measurements on TIM’s quicker, without the impact of contact resistance. A large scope of materials are available, usually to decrease the thermal resistance of the joint between a heat-generating component and its heat sink. These materials include tapes, filled elastomer pads, greases, and phase-change materials.

Established Technique for TIM-Testing: ASTM D5470

The absolute steady-state technology with a guarded-hot-plate based on ASTM C177 or ISO 8302 is a frequently employed method for thermal conductivity testing. Using this technique, a sample is heated by a heat-source with a known steady-state power input and the temperature drop which results from it across a given length is measured using thermocouples.

The main challenge of this technology is to minimize parasitic heat losses to an extremely low level and overcome the effects of contact resistance between sample and bars. In the standard ASTM D5470, the steady-state technique from ASTM C177 and ISO8302 is adjusted for thermal interface materials.

TIM-Tester Principle.

In order to establish the heat flux, and to extrapolate the temperature to the surface of the meter bar in contact with the specimen, this design calculates the temperatures in the meter bars. The heat flux in the experiment is established from a known conductivity of the meter bar material – copper alloy 110, and the slope of this temperature gradient.

The surface temperature is gathered from the extrapolation of the plot of temperature vs. location to the zero position, or surface, of the meter bar.

Light- or Laser-Flash Methods (ASTM E1461) can be relied on to perform routine measurements on thermal interface materials faster. The flash diffusivity technique (LFA) is frequently used to calculate the thermal diffusivity and conductivity of a wide array of materials.

This transient technique entails heating a surface of a sample rapidly, with a single pulse from a flash lamp or laser and observing the arrival of the resulting temperature disturbance as a function of time on the opposite surface, typically by using an IR detector.

LFA-Principle

Calculation of thermal conductivity from LFA.

It can overcome two of the major disadvantages of the steady-state technique. LFA can help to:

1. Remove the influence of contact resistance between the metal bars and the TIM from the measurement results
2. Decrease the measurement time – an LFA shot only requires seconds

Measurements on a thermal grease and a TIM-Tape from 3M (8800) at different thicknesses are utilized for the calculation of the thermal conductivity from the slope of the thermal resistance plotted over thickness. Extremely similar results in slope (the true thermal conductivity) can be observed in the results. An impact on contact resistance (Y-axis-offset) can be observed in the ASTM D5470 measurements.

As long as the results measured by ASTM C5470 are calculated with the knowledge about contact resistance, which can only be achieved with a series of measurements with a variation of the thickness, the thermal conductivities gathered using both techniques are in good agreement.

LFA vs. ASTM D5470 on 3M 8800 Tape.

LFA vs. ASTM D5470 on thermal grease.

This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.

For more information on this source, please visit NETZSCH-Gerätebau GmbH.