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Experimental results of the thermal and hydraulic performances of silicon-based, low aspect ratio micropin-fin cold plates under cross flow conditions are reported. The pins were both circular and square in shape with dimensions (diameter for circular and sides for square) ranging from 50μm to 150μm. The test chip contained 20 integral 75×75μm temperature sensors which were used to determine the thermal resistance (KW−1) of the cold plates. The experiments were conducted using water, over a Reynolds number (Re) ranging from 40 to 1000. The data show that the average Nusselt number (Nu) based on the fin diameter varies as Re0.84 for Re<100 and as Re0.73 for Re>100, where Re is the Reynolds number based on maximum velocity and the fin diameter. Analysis of the Fanning friction factor (f) data shows that f varies as Re−1.35 for Re<100 and as Re−0.1 for Re>100.

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.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.

Experimental results of the thermal and hydraulic performances of silicon-based, low aspect ratio micro-pin-fin cold plates under cross flow conditions are reported. The pins were both circular and square in shape with dimensions (diameter for circular and sides for square) ranging from 50 to 150 μm. The test chip contained 20 integral 75×75 μm temperature sensors which were used to determine the thermal resistance (K W-1) of the cold plates. The experiments were conducted using water, over a Reynolds number (Re) ranging from 40 to 1000. The data show that the average Nusselt number (Nu) based on the fin diameter varies as Re0.84 for Re &lt; 100 and as Re0.73 for Re &gt; 100, where Re is the Reynolds number based on maximum velocity and the fin diameter. Analysis of the Fanning friction factor (f) data shows that f varies as Re-1.35 for Re &lt; 100 and as Re-0.1 for Re &gt; 100.

This paper reports the test results of vapor chambers using copper post heaters and silicon die heaters. Experiments were conducted to understand the effects of nonuniform heating conditions (hot spots) on the evaporative thermal performance of vapor chambers. In contrast to the copper post heater, which provides ideal heating, a silicon chip package was developed to replicate more realistic heat source boundary conditions of microprocessors. The vapor chambers were tested for hot spot heat fluxes as high as 746 W/cm2. The experimental results show that evaporator thermal resistance is not sensitive to nonuniform heat conditions, i.e., it is the same as in the uniform heating case. In addition, a model was developed to predict the effective thickness of a sintered-wick layer saturated with water at the evaporator. The model assumes that the pore sizes in the sintered particle wick layer are distributed nonuniformly. With an increase of heat flux, liquid in the larger size pores are dried out first, followed by drying of smaller size pores. Statistical analysis of the pore size distribution is used to calculate the fraction of the pores that remain saturated with liquid at a given heat flux condition. The model successfully predicts the experimental results of evaporative thermal resistance of vapor chambers for both uniform and nonuniform heat fluxes.

Microchannel heat exchangers using two-phase convective boiling is one of the most promising future technologies for the cooling of microprocessors. Heat generation on microprocessors is highly non-uniform due to the presence of multiple time-varying localized hotspots. Previous literature has primarily been focused on the performance of microchannels under uniform heating conditions. In this paper we report the performance of microchannel heat exchanger under both uniform and hotspot (non-uniform) heating conditions. We performed these experiments using a novel test setup where the test chip has multiple temperature sensors, one heater to provide uniform heating and a hotspot heater of size 400 μm × 400 μm. We report some of the preliminary results on the thermal performance of the heat exchanger. Results show that fluctuation in the wall temperature is different under hotspot heating conditions as compared to the uniform heating conditions.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.

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