This standard measures the steady state thermal impedance of electrical insulating materials used to enhance heat transfer in electrical and electronic applications. This standard is especially useful for measuring thermal transmission properties of specimens that are either too thin or have insufficient mechanical stability to allow placement of temperature sensors in the specimen as in Test Method E1225.

This standard imposes an idealized heat flow pattern and specifies an average specimen test temperature. The thermal impedances thus measured cannot be directly applied to most practical applications where these required uniform, parallel heat conduction conditions do not exist.

This standard is useful for measuring the thermal impedance of the following material types.

Type I-Viscous liquids that exhibit unlimited deformation when a stress is applied. These include liquid compounds such as greases, pastes, and phase change materials. These materials exhibit no evidence of elastic behavior or the tendency to return to initial shape after deflection stresses are removed.

Type II-Viscoelastic solids where stresses of deformation are ultimately balanced by internal material stresses thus limiting further deformation. Examples include gels, soft, and hard rubbers. These materials exhibit linear elastic properties with significant deflection relative to material thickness.

Type III-Elastic solids which exhibit negligible deflection. Examples include ceramics, metals, and some types of plastics.

The apparent thermal conductivity of a specimen can be calculated from the measured thermal impedance and measured specimen thickness if the interfacial thermal resistance is insignificantly small (nominally less than 1 %) compared to the thermal resistance of the specimen.

The apparent thermal conductivity of a sample material can be accurately determined by excluding the interfacial thermal resistance. This is accomplished by measuring the thermal impedance of different thicknesses of the material under test and plotting thermal impedance versus thickness. The inverse of the slope of the resulting straight line is the apparent thermal conductivity. The intercept at zero thickness is the sum of the contact resistances at the two surfaces.

The contact resistance can be reduced by applying thermal grease or oil to the test surfaces of rigid test specimens (Type III).