To understand the heat transfer properties
of thermal interface materials (TIMs) we need to understand the meaning of
thermal conduction, convection and resistance.
The following is a sneak peak from Clemens Lasance's Thermal Interface
Material Basics for Electronic Engineers. Clemens recently presented this
information in relation to the role of the PCB in Thermal LED Applications at
the MCPCB Design and Fabrication webinar.

Conduction

The notion of thermal conduction is not very old. Biot (1804) and Fourier (1822) were the first to quantitatively study the heat flow through a piece of solid material. Fourier observed that the heat flow q was proportional to the temperature difference ΔT over the test piece, proportional to the cross sectional area A of the bar, and inversely proportional to the length or thickness ℓ., known as Fourier’s law:

The proportionality constant k is called
the thermal conductivity in W/mK. It is a material property and a measure for
the ability of a material to conduct heat. The range for engineering materials
is from air (0.03W/mK), via plastics (0.2 W/mK), glass (1 W/mK), aluminum PCB
board (200 W/mK) to copper board (400 W/mK). Typical TIM values cover the range
0.4-4 W/mK.

Convection

The heat generated in an electronic device
is usually transported by conduction to a heat sink or an area where the heat
is transferred to a fluid which is called convection. The fluid can be a gas
such as air, or a ‘real’ fluid such as water. As a result, the convection heat
is proportional to the area A and the temperature difference between the wall
and the main stream flow:

This equation is commonly known as “

Resistance

The last term to discuss shortly is the
thermal resistance. 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: ΔV = 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 ΔT = 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.