gradient in the pipe forcing the vapour to flow along a middle part 4 of the pipe towards the condenser 3 the vapour is adiabatically conveyed whereby the variations in pressure and temperature are small in the condenser the vapour condenses back into liquid conveying the evaporation heat to the heat sinks of the condenser the working fluid is returned in liquid form to the evaporator along a porous prime coat 5 forming the external thread of the heat pipe by capillary force the basic function of a heat pipe is known in the prior art and will therefore not be explained here in greater detail alternatively micro heat pipes can preferably be used in the cooling apparatus of the invention in which case the capillary force is caused by the geometry of the pipe the basic function of a micro heat pipe is also known in the art and will not be described here in greater detail fig2 shows an embodiment of the cooling apparatus based on working fluid in phase transition of the invention in accordance with the invention the cooling apparatus is formed from a matrix of elongated cooling elements i e heat pipes the circuit board level is marked with the letter a in this example the heat pipes are two dimensional some.