Metal Core Resource Center

Thermal Resistance


Thermal Resistance (Rth) is defined as the difference in temperature between two closed isothermal surfaces divided by the total heat flow between them.

Heat Transfer Basics for LED Applications

In a DC electrical circuit, Ohm’s law describes the relations between the voltages and the currents. It states that a voltage difference over a resistor causes an electrical current, which is proportional to the voltage difference: ?V = I * R. In steady state heat transfer, a temperature difference causes a heat flow which is proportional to the temperature difference as is seen in equations (1, 2). Both equations can be written in the form ?T = q * Rth, with Rth the thermal resistance (also commonly noted as R when there is no chance for misreading it as an electrical resistance). This is analogous to Ohm’s law. In both the electrical and the thermal case we observe that a driving force exists (either voltage difference or temperature difference), which causes a flow (of current, or of heat) over a resistor. The thermal resistance per unit area is equal to the ratio between thickness (t) and thermal conductivity (k) and is often used to allow for a direct comparison of the heat transfer performance of commercially available TIMs.

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thermal-resistance


Total Thermal Resistance (Rth total) is the sum of components and its thermal resistance value.


total-thermal-resistance


Thermal Impedance

Nomenclature: the confusing situation regarding 'thermal impedance'

‘Electrical impedance’ is historically reserved to describe time-dependent electrical resistance. In the limit of steady state, thermal impedance equals thermal resistance;  therefore, units should be the same. Hence, Thermal impedance , as used by U.S. vendors, violates the electro-thermal analogy, because:

  • Unit does not correspond (K/W vs. m2K/W)
  • Definition does not correspond (time-dependent vs. steady state)

Why is this a problem?
time-dependent (dynamic) test methods will be increasingly used, one output of which is the ‘correct’ thermal impedance.

Use thermal resistance per unit area, or unit Rth.


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