Principles of Thermal Management

Why do we need Thermal Management?

As power density and complexity increases in electronic devices the amount of heat also increases, effective thermal management is required to increase product performance and longevity. If heat is not managed in an appropriate manner then this can lead to reduced life-times, product failures and safety issues.

What is Thermal Management?

All electrical energy supplied to electronic devices is dissipated as heat. Thermal management is the controlled dissipation of this heat. This is achieved  by providing an effective thermal path from the junction to the ambient environment using all modes of heat transfer:

  • Conduction
  • Convection
  • Radiation

Heat Transfer Definitions:

Use the toggles below to find out more about the principles of heat transfer.

Thermal Management No TIM
Thermal Management with TIM

Thermal Conductivity (k) – is an intrinsic property of a material’s ability to conduct heat.

This is independent of material size, shape or orientation. Heat transfer across materials of high thermal conductivity occurs at a faster rate than across materials of low thermal conductivity. The thermal conductivity of a material is heavily dependent on the conditions under which it is used:

First, we define heat conduction, ‘’H’’: H=\frac{\Delta Q}{\Delta t} = k A\frac{\Delta T}{x} where \frac{\Delta Q}{\Delta t} is the rate of heat flow, ‘’k’’ is the thermal conductivity, ‘’A’’ is the total cross sectional area of conducting surface, Δ’’T’’ is temperature difference, and ‘’x’’ is the thickness of conducting surface separating the two temperatures. Dimension of thermal conductivity = M1L1T−3K−1

Rearranging the equation gives thermal conductivity:

k=\frac{\Delta Q}{\Delta t} \frac{1}{A} \frac{x}{\Delta T} (Note: \Delta T/x is the temperature gradient)

I.E. It is defined as the quantity of heat, Δ’’Q’’, transmitted during time Δ’’t’’ through a thickness ‘’x’’, in a direction normal to a surface of area ‘’A’’, per unit area of A, due to a temperature difference Δ’’T’’, under steady state conditions and when the heat transfer is dependent only on the temperature gradient.

Alternatively, it can be thought of as a [[flux]] of heat (energy per unit area per unit time) divided by a temperature gradient (temperature difference per unit length): k=\frac{\Delta Q}{A \Delta t}\frac{x}{\Delta T}.

Thermal resistance is a measure of the ability of the material to conduct heat only after heat has entered the material across a specific thickness.

From Fournier’s law for heat conduction, the following equation can be derived, and is valid as long as all of the parameters (X, A, and K) are constant throughout the sample.

Where:

  • Rθ is the thermal resistance (across the length of the material) (k/W)
  • x is the length of the material (measured on a path parallel to the heat flow) (m)
  • k is the thermal conductivity of the material ( W/(k·M) )
  • A is the total cross sectional area of the material (measured perpendicular to the heat flow) (m2)

This is usually quoted as the thermal resistance from the unction to case of the semiconductor device. The units are °C/W.

For example, a heatsink rated at 10 K/W will get 10K hotter than the surrounding air when it dissipates 1 Watt of heat. Thus, a heatsink with a low K/W value is more efficient than a heatsink with a high K/W value.

The thermal impedance (ø) of a material is defined as the sum of its thermal resistance and any contact resistance between it and the contacting surfaces. As defined by the following equation:

(ø) = R material + R contact

Thermal Management No TIM
Thermal Management with TIM

Optimising Heat Transfer:

Any engineering surface is rough on a microscopic scale level, due to the presence of infinitesimal asperities. When two such rough surfaces come into contact, the actual contact occurs only at a few discrete spots, usually at the high points of the two surfaces Heat flowing from one body into the other is constricted to flow through the actual contact spots, because the thermal conductivity of the solid contact spots is much higher than that of the surrounding gap which is filled with air in most engineering applications. Thermal interface materials (TIMs) are often inserted between the surfaces of a contact pair to reduce the thermal contact resistance and improve thermal conductivity. Click the button below to read more on Thermal Interface Materials.