Thermal Performance and Key Challenges for Future Cpu Cooling Technologies

Over the past few years, thermal design for cooling microprocessors has become increasingly challenging mainly because of an increase in both average power density and local power density, commonly referred to as “hot spots”. The current air cooling technologies present diminishing returns, thus it is strategically important for the microelectronics industry to establish the research and development focus for future non air-cooling technologies. This paper presents the thermal performance capability for enabling and package based cooling technologies using a range of “reasonable” boundary conditions. In the enabling area a few key main building blocks are considered: air cooling, high conductivity materials, liquid cooling (single and two-phase), thermoelectric modules integrated with heat pipes/vapor chambers, refrigeration based devices and the thermal interface materials performance. For package based technologies we present only the microchannel building block (cold plate in contact with the back-side of the die). It will be shown that as the hot spot density factor increases, package based cooling technologies should be considered for more significant cooling improvements. In addition to thermal performance, a summary of the key technical challenges are presented in the paper. NOMENCLATURE A Area (m) COP coefficient of thermal performance CFD Computational Fluid Dynamics cp specific heat (J/kg-K) DFjc Density factor from junction to case (1/cm) Dh hydrodynamic diameter of the channel (m) harea equivalent heat transfer coefficient of the fins based on heat sink base area (W/ m°C) HX remote heat exchanger h average heat transfer coefficient (W/ m°C) k thermal conductivity (W/m-K) m& mass flow rate (kg/s) h D Nu Nusselt number for the rectangular channel subject to constant heat flux boundary condition Q power (W) Rjc Normalized Thermal Impedance from junction to case (oC-cm/W) T temperature (°C) Ta Ambient temperature local to the heat sink (oC) Tc Package case temperature (oC) TDP Thermal Design Power (W) TIM Thermal Interface Material impedance (°C cm /W) Tj Junction or die temperature (oC) Tj, max Junction temperature at the hottest point of the die (oC) Ts Heat sink temperature (oC) Tx Temperature at any location on the thermal solution (oC) ZT Thermoelectric Figure of Merit