Numerous factors impact thermal performance. For best performance of thermal interface materials conductivity, wetting, and reliability should be high, whereas resistance, impedance, cost, weight, and bond line thickness should all be low. Reliability and other application concerns are important, but thermal properties themselves are what cool overheated electronics. Three key terms describe TIM thermal properties: conductivity is one key variable, a material property of independent of thickness or area, and resistance and impedance are two variables related to the inverse of conductivity.

Conductivity

To compare thermal performance with a value not specific to a certain thickness or area, use conductivity in W/m-K. Bulk conductivity is a property of the material that describes the relative heat-conducting ability of a material at a particular temperature differential:

Bulk conductivity ignores potential interface resistance. "Apparent thermal conductivity" does include the effect of interface resistance:

Interface resistance is highly mitigated by materials excellent at wetting interface surfaces. For this reason, bulk conductivity can generally compare effective grease, phase change, or PCMA materials. For non-wetting TIMs, however, apparent thermal conductivity takes into account the average interface resistance due to air voids. It is therefore important to know which "conductivity" is being referenced. With laser flash diffusivity testing, for instance, 'conductivity' refers to bulk conductivity.

Efforts to maximize apparent conductivity arose from problems with thermal tapes and pads. These materials risk creating air voids along the surfaces, and such air voids dramatically block heat transfer. Phase changing polymers and greases were thereby developed to wet the surfaces, but each has costly inconveniences (curing and containment respectively). Liquid metal alloys have historically suffered from reliability problems, but can provide even higher conductivity with easier wetting and containment processes.

In order to determine how much heat the material will transfer in an application given a certain heat differential (Q/T), multiply application surface area and divide thickness from bulk conductivity. Interface materials with a liquid operating phase (grease, phase change polymers, PCMA) may also be affected by temperature-dependent thickness change.

Resistance

The term resistance can invite confusion due to multiple definitions. One definition deserves mention but is not commonly used in regards to thermal interface material: measured in K/W,
R = dT/Q (temperature differential divided by watts). That resistance only applies to a specific thickness and area of material. The more prominent definition for resistance is measured
in K-m2/W:

The above is also called "area-normalized thermal resistance" of the TIM material, which means that the resistance value is not affected by the surface area under consideration. Some in the industry add area-normalized resistances of the interface junctions, which increase with presence of air voids:

These last two elaborations of area-normalized resistance have been adopted under the term "impedance". Perhaps this was to avoid confusion with resistance specific to an area (as with R= dT/Q). Whether dependent on a particular area or not, resistance is specific to a thickness. In other words, resistance would double if the thickness doubles. Area-normalized resistance stays the same value at any surface area. Conductivity, in contrast, remains the same value regardless of the measured substance's thickness.

Impedance

Impedance, as mentioned above, is a term commonly used to refer to area-normalized thermal resistance of TIMs. For non-wetting TIMs, it should be clarified if an impedance value includes interface (contact) impedance or not. The ASTM D5470 standard testing method gives an impedance value with interface effects included. Measurement of impedance is established at the bond line thickness for the target application. Two formulas (units K-m2/W) include:

While impedance may be a clearer term to use than area-normalized resistance, "impedance" itself may be confused with a historical definition. Impedance has previously referred to time-dependent thermal resistance measured in K/W. That definition is not in use here, and all references to thermal impedance by Enerdyne can be alternatively defined as area-normalized thermal resistance.

Related Terms

Bond line thickness (BLT) - average thickness of the thermal interface material.
Wetting - effectiveness of a material at liquid phase to coat a surface, dispelling air voids.
Voids - gaps of air between the thermal interface material and a surface; voids have extremely low conductivity, almost halting heat transfer across voids.