Finishing with conduction

We thus saw the factors that come into play in thermal conduction, translated thanks to the concept of thermal resistance which is the only parameter that makes possible the expression of the performance of a cooling system. As a summary, we can make a brief recap of these resistances :

• R : pure conduction, related to the thickness of materials
• R_c : contact, related to the state of surfaces in contact
• R_s : spreading, related to the difference of the surfaces areas between the devices in contact

In the final analysis, what it will be necessary to keep in mind for the application of interest, i.e. cooling a processor, includes only the case of a plate in contact on one side with a cpu whose surface area is smaller and has given 'state' ( the pioneering days of lapped cpus seems finished ), just as the surface of the plate has a controllable but imperfect state.
We can thus write the relation which will give the temperature of the cpu :

Here T_x represents the temperature of the upper face of the plate ( generally taken equal to the average temperature ), but what is the meaning of this x ? Eh well quite simply that this temperature is, a priori, unknown ! The plate only constitutes a part of the cooling system : the base plate. The determination of this temperature requires knowing what occurs above the plate, this is the subject of the following article : convection ...