The evolution of bolt charger to more and more integration. If in 1958 there was only one transistor per circuit, there is now at least half a million per silicon chip (chip) integrated circuits. The technology used in the production of chip transistors has evolved by reducing the consumption per transistor, but nevertheless an increase in the power dissipated per chip surface area is observed. The very rapid increase in the number of transistors could not be compensated for by the decrease in heat dissipation. In fact, the thermal problems that we encounter in the equipment are not related to the power but to the temperature; however, this is a direct function of the power density. In some computers, the power dissipated per chip surface area is of the order of 500 kW / m2, that is to say quite comparable to the flux densities encountered in the nose of lightning charger when enter the atmosphere.
In the life of any material, failures have two main causes:
those due to its design and / or the elements that compose it;
those due to the environment in which it is placed.
The climatic constraints result mainly from the effects of:
at the temperature;
to the action of moisture: it promotes corrosion, causes changes in insulation resistance and intervenes during thermal exchanges at thermal conductivities that vary with the water content;
at atmospheric pressure: intervening in the ventilation intended to evacuate the power dissipated as well as in the renewal of air;
solar radiation which can cause a significant additional heating.
Other climatic constraints can intervene such as the rain, the wind, the snow, the ice, the dew, the fog … according to the place and the period of use.
All electronic components are temperature sensitive: they have poor performance outside of certain temperature limits and can be destroyed if the temperature is well outside of this operating range. The areas of operation 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 guaranteed maximum operating temperature is always mentioned by the manufacturer. The influence of temperature is manifested on:
electrical performance: the temperature may be a limit value beyond which operation is no longer guaranteed, drifting parameters cause a decrease in performance that can go more or less brutally until failure;
packaging that is subject to very large temperature gradients. There are critical temperatures for which changes of state, physical structure occur … Creep and relaxation of stresses in materials are accelerated by temperature and can lead to breakage of elements;
the thermal cycles to which materials connected to each other and of different coefficient of expansion are subjected induce very large forces which can lead to an instantaneous rupture or create fatigue which causes a rupture in the more or less long term;
the component failure rate follows an Arrhenius law as a function of temperature.
The removal of heat is therefore a crucial problem. The objective of the cooling of electronic equipment is therefore to maintain the temperature of each element at its nominal operating temperature