What is Thermal Conductivity Testing?
Thermal conductivity is a fundamental material property associated with the ease with which heat energy is conducted through a specific material. Materials that conduct heat readily with a small imposed temperature gradient have a higher thermal conductivity than materials that are more resistant (more insulating) to the flow of heat. Accurate knowledge of a material's thermal conductivity is essential for predicting heat transfer by conduction.
Thermal conductivity is a pure material property independent of conduction-area or the material thickness. ASTM D5470 standardizes a method two-sided, one-dimensional heat flow method for thermal conductivity measurement so that the resulting data will reflect only the material properties without regard to the specific test equipment utilized.
ASTM D5470 deals with the thermal conductivity in "thin" materials that are often described as "thermal interface materials". "Thin" materials are roughly those less than 1-2 cm thick. This method defines thermal conductivity as the ratio of heat flux to the associated thermal gradient under one-dimensional heat conduction conditions. This measurement can be envisioned as thermal conduction between two parallel, isothermal surfaces of area A at temperatures TH and TC separated by a layer of the material-under-test having a thickness X with a steady state power of Q. Thermal conductivity, k, is thus defined as
k = Q * X / ((TH - TC) * A)
The most popular units for thermal conductivity are Watt/meter Kelvin (W/mK). Thermal resistivity is simply defined as the reciprocal of thermal conductivity. An alternate expression for k can be made by using thermal resistance, R, of the material sample between the parallel hot and cold surfaces, defined:
R =(TH - TC) / Q
Combining these yields a simpler form of the definition of thermal conductivity:
k = X / (R * A)
Frequently, thermal interface materials are supplied in sheets of nominal thickness where the thermal conductivity specifications for the material provided in the form of RA rather than k. For these situations,
RA = X / k
Thermal conductivity is a material property that is independent of any particular application since material properties are ideally "application-independent". When a particular material is implemented as a thermal interface in an electronic packaging application, the impact of the material on the overall thermal performance of the assembled package depends on many engineering details, only one of which is interface material conductivity. Issues such as geometry of the design, surface contact resistances, and the thermal resistance of other elements in the assembly are critical when evaluating the overall thermal performance of any particular electronics packaging application. For this purpose, Analysis Tech Semiconductor Thermal Testing can determine the overall thermal resistance of a packaged semiconductor device as a function of type of thermal interface material utilized.