Heat dissipation in electronic systems

The evolution of electronics is leading to ever greater integration. Whereas in 1958 there was just one transistor per circuit, there are now at least half a million per integrated circuit silicon wafer (chip). While the technology used to make transistors for chips has evolved to reduce power consumption per transistor, the power dissipated per unit area of chip has increased. The very rapid increase in the number of transistors has not been offset by the reduction in thermal dissipation. In reality, the thermal problems encountered in equipment are not related to power but to temperature, which is a direct function of power density. In some computers, the power dissipated per unit area of chip is of the order of 500 kW/m ² , i.e. quite comparable to the flux densities encountered at the nose of a space shuttle as it re-enters the atmosphere.

In the life of any piece of equipment, failures have two main causes:

• those due to its design and/or component parts;

• those due to the environment in which it is placed.

Climatic constraints result mainly from the effects of :

• temperature ;

• the action of humidity: it promotes corrosion, leads to changes in insulation resistance, and affects thermal exchanges in terms of thermal conductivity, which varies with water content;

• at atmospheric pressure: involved in ventilation to evacuate dissipated power and in air renewal;

• solar radiation, which can cause significant additional heating.

Other climatic conditions may also come into play, such as rain, wind, snow, freezing rain, dew, fog, etc., depending on the location and period of use.

All electronic components are temperature-sensitive: they perform poorly outside certain temperature limits, and can be destroyed if the temperature is well outside this operating range. The operating ranges are specified by the manufacturers and are commonly as follows:

• industry: 0 to 70 ˚C ;

• civil: - 20 to + 85 ˚C ;

• military: -55 to 125 ˚C.

The maximum guaranteed operating temperature is always specified by the manufacturer. The influence of temperature can be seen in :

• electrical performance: temperature can be a limiting value beyond which operation is no longer guaranteed, and parameter drifts can lead to a reduction in performance, which can be more...