Thermal resistance is a measurement of a material’s or a component’s resistance to heat flow. It is the reciprocal of thermal conductance, which is the ability to conduct heat. Thermal resistance is used in PCB circuit design to measure a package’s heat dissipation and avoid overheating.
There are two values related to thermal resistance that refer to a material’s ability to conduct heat versus a component’s ability to conduct heat: specific thermal resistance and absolute thermal resistance.
Specific thermal resistance (Rλ) is a material constant, measured in K∙m/W or ft2∙h∙oF/Btu, that is useful for comparing materials. It is the reciprocal of the material’s thermal conductivity, and is the absolute thermal resistance per unit area. An R-value is the specific thermal resistance, given in British units of degree Fahrenheit square-foot hour per British thermal unit (ft2∙h∙oF/Btu). For example, a thermal resistance given as R-1 means 1 ft2∙h∙oF/Btu. When using the SI units of kelvin meters per watt K∙m/W, this value is denoted as the RSI-value.
Absolute thermal resistance (Rθ), measured in kelvins per watt (K/W) or degrees Celsius per watt (oC/W), is a property of a determined quantity of a material. Once a material and size have been chosen, absolute thermal resistance is the measure of that component’s ability to resist heat flow. Absolute thermal resistance is an extensive property, meaning it depends on the amount of the material.
Thermal resistance is an important value in PCB circuit design to prevent overheating. IC manufacturers specify a device’s junction-to-ambient thermal resistance (θJA), which is a measurement of absolute thermal resistance and is usually given in oC/W. Other values that are sometimes used are junction-to-case thermal resistance (θJC) and case-to-ambient thermal resistance (θCA). See App Note 3500: Monitor Heat Dissipation In Electronic Systems by Measuring Active Component Die Temperature for how to measure θJA, and Tutorial 4083: IC Package Thermal Resistance Characteristics for more on θJA, θJC, and θCA.