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Printed Circuit Boards with Aluminum Core or Carrier - LeitOn-Alu

Heat Discharge Calcuation

The heat path can be classified into three parts:

  1. Heat transfer from semi-conductor material to packaging
  2. Heat transport from casing to the heat sink
  3. Heat discharge from heat sink to the environment

Developers have little influence on the first part of the heat path (producer´s declaration), however, adequate measure can positively influence perceding parts of the heat path.

1. In analogy to electricity, an Ohm´s law can be defined in thermodynamics:

Whereas:

equals the heat quantity per unit of time.

2. Usually data sheets of the various materials will include the thermal conductivity:

The larger k, the better the thermal conductivity.

3. This specification allows calculating the thermal resistance at a given material thickness and surface of the thermal conduction:

An example with common values shall exemplify this::

We will use a specific resistor of k=0,38, a 100mm² surface and a foil thickness of 50µm. The foil thickness is equivalent to the insolation thickness. Resistance than equals 1.32?. Applying formula (1) results in a temperature difference of 13,2°C at 10W heat flux.

Continuing with formula (2) and (3) enable you to recognize that the specific thermal conductivity is independent of thickness and surface. In contrast the thermal resistance value realistically reflects the area of assembly.

4. Thermal resistance calculation always assumes homogenous materials:

The situation is different at the interface of two massive bodies:

At a standardized surface of e.g. 100mm² the values are specified in K/W. With a RTÜ =3,7 at 10W heat flux the temperature difference therefore equals 37°C.

According to the manufactur´s information the specific thermal conductivity for FR4 laminates and prepreg is between 0,26 and 0,38. At W/mK = 0,38 and adoption of a prepreg with 0,1mm (100µ) thickness for pressing a one layer circuitboard results to:

It must be kept in mind that the quantity or total thickness must be considered next to the thermal conductivity as multilayer circuits often make use of 2 prepregs.