olycu!'bc,note
4 General conclusions and recommendations
As microelectronics become faster and smaller, the power density increases and the generated heat becomes a problem. In addition, the area available for heat removal decreases. As a result, thermal management becomes increasingly critica} to the
electrollies industry. Power densities ranging from 80-200 W/cm2 are expected for future consumer applications. The maximum temperature should be maintained below 120
oe.
Two new cooling techniques have been investigated, namely single phase forced convection through microchannels and the pulsating heat pipe, which is a two-phase cooling method. The goal ofthis research is todetermine whether these two techniques can play a role in cooling of future microelectronics.
Microchannels have been etched in silicon. Water and air were employed as cooling fluids. With water as cooling fluid 105 Watts could be dissipated on a square centimetre while the temperature remained well below the critical value. The thermal resistance varies with the flow. At a flow rate of 0.075 1/min and a pressure drop of 0.1 bar the measured thermal resistance equals 0.32 KIW. At a flow rate of 1 1/min and a pressure drop of 2.6 bar the thermal resistance equals 0.05 KIW.
Water cooling of the microchannel structure is also simulated with Flotherm. The temperature distribution calculated with Flotherm showed a maximum temperature of 57
oe
at a flow rate of 0.1 1/min, which is much lower than the maximal allowabletemperature. This makes microchannel cooling with water as cooling fluid an attractive possibility for cooling of microelectronics.
Air has also been used as cooling fluid since the combination of water and electronics can cause hazardous situations. With air as cooling fluid 13.5 W was dissipated on a square centimetre. The measured thermal resistance varies from 5.4 KIW at a flow rate of 9 1/min (pressure drop
=
0.3 bar) to 1.8 KIW at a flow rate of 33 1/min (pressure drop is 1.2 bar). The thermal resistance in case of air cooling is thus much higher. The main reason is the relatively high temperature rise of the air along the channel due to the smaller cp and density.The second technique that is investigated is the pulsating heat pipe. Experiments have been conducted on a pulsating heat pipe (PHP) structure made in aluminium. The main advantage of a PHP is the fact that no extemal pump is required. The fluid motion is caused by thermally driven pressure differences within the system.
It is seen that the oscillations of the fluid in horizontal orientation do not contribute to the heat transfer.
However when placed vertical the thermal resistance drops when pulsating fluid motion commences. The thermal resistance is 40 % lower when compared to the thermal
resistance of an unfilled heat pipe. The lowest measured thermal resistance is 2.4 KIW. In this case the heat souree temperature is 85
oe
at a power input of 28 W on a surface of 5 cm2. The power density occurring in microelectronics is in the region of 80-200 W/cm2. These power densities were not achievable while maintaining the maximum temperature below 120oe.
It can fmally be concluded that forced convection through microchannels can play a role in cooling of microelectronics. Water is recommended as cooling fluid since the cooling performance with water is superior to the cooling performance with air. The
measurements and calculations indicate that heat fluxes of 100 W/cm2 can be obtained while the maximum temperature is not even close to the critica! temperature. This is not the case for pulsating heat pipes.
It is therefore recommended to focus further research on microchannels cooling with water as cooling fluid. More information on design constraints, such as maximal allowable flow rate and pressure drop, should be gained. Further research can then be conducted to optimise the microchannel structure. Height and width of the channels can be optimised to reduce convective and conductive resistance. Another interesting possibility is non-rectangular channels.
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