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http://www.cea.fr/gb/institutions/Clefs44/an-clefs44/clefs4485a.html— THE RATIONAL USE OF ENERGY ————————————————————————————
The problem of electronic cooling,
and solutions
Microprocessor with improved performance,
covered by its system of heat evacuation.
Evergreen Technologies
Heat control is a key point in the design of electronic equipment, as the quality and reliability of equipment components are highly reliant upon the temperature. Miniaturization only serves to increase the sensitivity of heat dissipation. This is why the CEA, in coordination with the Grenoble polytechnic institute, is studying solutions to different problems posed by the rational use of energy in this field. Just limiting the temperature of an electronic chip below a critical level reduces heat movement in the semiconductor network. The component's connector temperature, which represents the average nominal heat level of the chip, is around 125° C for silicon. Homogeneity of temperature in the volume of the component also limits thermo-mechanical stress.
The architecture of a conventional component (figure) shows the heat path and the way that calories are dissipated from the base unit. This unit is the chip made of semi-conductor material and housing the electrical function toward the card or toward the printed circuit via the substrate. The substrate, made up of numerous levels of different materials, assures the mechanical hold, or the electrical insulation between the chip and the casing and the transmission of electrical signals outward. The chip/substrate combination, constituting the casing, is generally coated with a protective resin. In power electronics (see Toward low-tension, energy saving and low cost power electronics), there is no card, and the casing is threaded together with other elements.
An increasingly tricky problem
Electronic applications are everywhere and have seized the public's interest. In all fields – military, space, industrial, household – an increase in response speed is called for, as are size reduction, more complex operating methods, and greater reliability. Moore's Law, that predicts that semi-conductor performance will double every eighteen months, continues to hold out since the start of the 1970s for all components – microprocessors, memories, logic circuits, power components.
The problem of heat evacuation is present today at the very level of the component, because of the strong increase in flow density, owing to miniaturization and the increase in operating frequencies. Heat flux densities of 50 W/cm2 are the rule for the new generations of microprocessors. As for electronic power converters used for electric traction on rails and future hybrid vehicles, their volume is impressively reduced (by several grades). The IGBT (Insulated Gate Bipolar Transistor), with a surface area of around cm2, transfers high voltage and current, works at high frequencies and with flow densities up to 400 W/cm2. Laser diodes dissipate 500 W/cm2 and more.
New technologies to be implemented
There is thus a real need for innovation in mounting, connecting and cooling of components. In recent years, studies on micro-exchangers in copper integrated in the substrate of the power components, and functioning in forced convection with a cooling fluid in a laminar flow regimen or in diphase form (liquid and vapor phases) have been carried out. The studies showed the usefulness of such methods of cooling when the densities of evacuated flows reach 400 W/cm2 with water. However, these systems have several flaws, in particular the lack of electric insulation when water is used, and the overall thermo-mechanical ageing when an insulating ceramic is inserted between the chip and the exchanger.
Infra-red photograph showing the network
of the microchannel, the two collectors and
the two fluid power supply holes of
a silicon-etched micro-exchanger.
CEA
Another solution is to design micro-coolers in the silicon, using deep etching techniques (rectangular and hexagonal channels with internal hydraulic diameter of around 250 mm) and automatic soldering of silicon wafers, techniques with which the CEA's electronics and information technology laboratory (Leti) are highly familiar. In this type of structure, which serves both as support and casing, the cooling is carried out in forced laminar convection, with a single-phase fluid. It is pertinent for applications in which the component dissipates power greater than 100 W. The electrical insulation is facilitated through the insertion of a thin layer of silicon oxide, which reduces to two the number of interfaces between the neighboring expansibility materials (silicon and silicon oxide). Its small volume also allows for a more compact and lighter micro-cooler. The flow densities measured range from 200 to 400 W/cm2 (depending upon the shape of the channel) with a variation of 40°C between the extraction fluid and the channel wall. Coolers with hexagonal channels are less efficient but more flexible and less expensive.
For lower-power applications, i.e. less than one hundred watts, it would not be appropriate to implement a structure in silicon with powerful heat evacuation capacity, but rather to put into place a structure with strong diffusion capabilities. The concept of a passive, silicon with integrated heat pipe type exchanger, functioning in dual phase, and with a flow density potential of 100 W/cm2, is very promising. Numerous research laboratories are now investigating this, among them the CEA laboratories.
Alain Bricard
Research Group on Heat Exchangers
(GRETh)
Technological Research Division
CEA/Grenoble
and Christian Schaeffer
Grenoble electrotechnical laboratory
ENSIEG (Grenoble national college of
electrical engineers)
Grenoble Polytechnic Institute
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