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