Thermal Model

Many considerations go into the construction of an accurate thermal model. The physical properties of each different materiel in the path of heat flow and the characteristics of the interfaces between each of these materials must be identified and calibrated. For example, some of the physical properties that might be modeled for the various materials in the path of heat flow are:

· Thermal conductivity - The time rate of steady heat flow/unit area through unit thickness of a homogeneous material in a direction perpendicular to the surface induced by a unit temperature difference.

· Thermal diffusivity - A heat transport property term obtained in the solution of the basic heat transfer equation and given by the thermal conductivity divided by the product of the density and heat capacity (thermal mass).

· Thermal effusivity - A heat transport property term given by the square root of the product of thermal conductivity and thermal mass.

· Specific heat capacity - The amount of heat per unit mass required to raise the temperature by one degree Celsius.

Each interface between a device’s junction and the outside world makes or breaks the thermal path between the two, enabling or preventing the desired heat transfer. This includes the interfaces between different materials within the path of heat flow (e.g. silicon-to-mounting pad, mounting pad-to-substrate, substrate-to-case, case-to-heat sink). Each of these interfaces introduces a component of the overall thermal impedance for the circuit.

Fortunately there is computer software available to assist in the thermal modeling of electronic products. Problems can be simulated geometrically using 2D and 3D techniques. Certain applications can accurately represent skewed elements. Most make use of a finite element analysis (FEA) method to convert geometry into an accurate resistor/capacitor representation which is then solved using a finite difference method. Typical thermal modeling tools that use FEA are NASTRAN, Ansys and TAS.


		Thermal Modeling Software