The time rate of heat flow through a unit area producing a unit temperature difference across a unit thickness.
Thermal conductivity is an inherent or absolute property of the material.
The opposition to the flow of heat through a unit area of material across an undefined thickness.
Thermal resistance varies with thickness.
A property of a particular assembly measured by the ratio of the temperature difference between two surfaces to the steady state heat flow through them.
Factors affecting thermal impedance include:
Area: Increasing the area of thermal contact decreases thermal impedance.
Thickness: Increasing the insulator thickness increases thermal impedance.
Pressure: Increasing mounting pressure under ideal conditions decreases thermal impedance.
Time: Thermal impedance decreases over time.
Thermal impedance is affected by the method of temperature measurement.
Test Methods – ASTM D5470
Thermal Impedance per Bergquist TO-220 Thermal Performance (25 degrees C. Colde Plate Testing