Advanced Thermal Management Techniques for High Power Electronic Devices

The introduction of more densely populated electronic processing systems utilising high power processing chips  with heat fluxes up to 250Wcm^-2 has caused the limits of conventional forced air convection cooling techniques to move towards their thermal limits and a requirement for advanced thermal management systems to dissipate in excess of 20kW per server cabinet in 3G and 4G systems has emerged.

In collaboration with Industry (Thermacore Europe) a project is being carried out to develop passive cooling systems using two-phase heat pipe technology to transport heat to a remote location where it can be dissipated more easily using liquid cooling or enhanced forced air convection techniques (fig. 1).

Fig. 1 - Forced air convection therma bus

Minichannel Liquid Cold Plates

 An on-chip minichannel cold plate prototype (fig. 2) manufactured from 1.5mm thick, 42mm wide Aluminium extrusion with 35 x Ø750µm circular microchannels running through its cross-section doubles the heat transfer surface area available for forced liquid convection when compared to the surface area of the flat plate.  Laminar flow investigations of water through the channels found that the temperature of a simulated chip could be maintained below the specified maximum temperature of 70°C whilst the chip power was able to be increased from 120W to 320W.

 Fig 2 - An on-chip minichannel cold plate prototype

In heat pipes, the transport of heat between the evaporator and condenser sections is limited by opposing pressured of the vapour and the returning condensed liquid.  Loop thermosyphons overcome this limitation by having separate liquid and vapour lines to form a loop.  Flow of vapour and  liquid causes a circulation around the loop allowing the fluids to to circulate in the same direction, enhancing the performance of the device.  The industrial 'Therma BusTM' project uses the LTS to transport heat from the processing components to the rack level cooling system.  Figure 3 shows two LTS's transferring heat from two processing chips to a liquid cold plate at the top of the board.  The largest thermal resistances in this system are across the thermal interface between the chip and the LTS evaporator and from the LTS condenser to the Liquid Cold Plate.

Loop Thermosyphon (LTS)

Fig. 3 - PCB with 2 x LTS and Liquid Cold Plate

Combined Liquid Loop Thermosyphon

By combining a minichannel cold plate with an LTS to form a vapour to liquid heat exchanger, a more compact system is produced with a lower thermal resistance due to the elimination of the thermal interface Layer (fig. 4). 

Fig. 4 - Minichannel cooled LTS 

For more information please contact Dr Jon Lee or Prof David Reay.