Thermal Resistance is a property of a printed circuit board that specifies its resistance to heat dispersion. A low thermal resistance in a PCB makes the dispersion of heat easier. This is basically the inverse of thermal conductivity. The thermal resistance of a PCB can be calculated by evaluating all the layers of the board and the heat parameters of the material.

To find the total thermal resistance for your board, you must include all layers of the board and the associated heat parameters for the type of material through which heat will flow.

[Formula]

R_\theta = absolute thermal resistance (K/W) across the thickness of the sample

\Delta x = thickness (m) of the sample (measured on a path parallel to the heat flow)

k = thermal conductivity (W/(K·m)) of the sample

A = cross-sectional area (m2) perpendicular to the path of heat flow

In addition to the thermal resistance of the board. The Thermal resistance of Vias must also be calculated. This usually depends on the copper trance, the laminate and the substrate and their respective thermal resistivities.

How can you reduce the Thermal Resistance of a PCB?

• Thermal resistance can be reduced by increasing the thickness of the copper traces. 
• Another method to decrease the thermal resistance is to place copper pads below the hot components. The high thermal conductivity of copper provides a low resistance path for the dispersion of heat. These pads can be connected to an internal ground plane through vias, which have good thermal conductivity and thus help move heat away from the component.
• Using Heat Sinks can help lower the thermal resistivity of the board.