Measurement of the ability of a substance to conduct heat, Measured in W/mK.
A material property, meaning that it does not change when the dimensions of the material change, as long as it is made up uniformly.
For example: the thermal conductivity of a cm3 of gold is exactly the same as the thermal conductivity of a 100m3 of gold.
Generally obtained in the industry using one of two tests, either the D-5470 test, or the E-1461 standard ASTM tests.
The D-5470 test measures the thermal impedance in Ccm2/W (Celsius, centimeters squared per Watt) of the sample and gets a value of the thermal conductivity through the relation:
Thermal Conductivity = Thermal Diffusivity * Specific Heat Capacity * Density
This is the opposite of thermal conductivity. It is a measurement of the ability of a material to oppose the flow of heat, so from a PCB point of view, we want this value to be low. The lower the thermal impedance, the quicker heat flows through the PCB and to the heat sink where it is dissipated.
Its value is dependent on the thermal conductivity of the material and its thickness; in other words, this is not a material property, but is an object property, as changing the thickness of the material will change this value. Saying that, changing the area of the material will not change this value (as long as the thickness stays constant).
For example: the thermal impedance of a sheet of laminate is the same as the thermal impedance of a cut piece of the laminate, say a cm2 of it. Whereas the thermal impedance of a sheet of gold of 1mm thickness is different to the thermal impedance of a sheet of 2mm thickness.
This is generally obtained using the D-5470 test mentioned above and relates to the thermal conductivity via the relation:
Thermal Impedance = Thickness / Thermal Conductivity
Thermal resistance (measured in Kelvin / Watt) is basically the same as the thermal impedance. The difference is that it takes into account the area of the sample as well as the thickness and conductivity.
Changing either the thickness or the area of the material will change the associated value of the thermal resistance.
Thermal Resistance = Thickness / (Thermal Conductivity * Sample Area)
(excerpted from Chapter 2 of "Thermal Management: A Fabricator's Perspective." A link to download your own free copy is directly below.)