Created: 10/24/95 Updated: 8/4/98

Passive Cooling Techniques for Thermal
Management of High Power Electronics

Project Number : C96-16

Point of Contact:

Dr. Yogi Joshi
CALCE EPRC
email: joshi@calce.umd.edu
Phone: (301) 405-5528
Fax: (301) 314-9269


Objectives Background Work Accomplished


Objectives

Examine passive cooling approaches for the removal of moderately high surface heat fluxes (upto 50W/in2) in electronic packages.


Background

Heat removal from electronic components has become a problem of significant interest due to the continuing reduction in feature size and increase in functional performance. A number of direct liquid immersion cooling techniques have recently been examined for removal of very high heat fluxes. For example, heat flux handling capabilities on the order of 200 W/cm2 have been demonstrated with micro-channel heat sinks under flow boiling conditions. These high heat flux removal immersion cooling methods require an intimate contact between the coolant liquid and electronic components. Also, the use of forced convection, employed in many cases, increases design complexity and costs. These constraints are often unacceptable during product design and a strong need exists for the development of passive cooling techniques for removal of moderately high heat fluxes ( up to 50 W/in2).

Recent advances in material technology have resulted in several new materials with extremely high thermal conductivity. Fibers with unidirectional thermal conductivity of almost 1100W/m-K have been developed and used to produce composite materials in sheet form as potential substrate materials for electronic thermal management.In parallel to these advances in high thermal conductivity materials, efforts have been made to adapt several high performance thermal technologies to electronics cooling. Passive technologies offering promise include:

  1. thermosyphon loops
  2. dielectric liquid filled bags
  3. miniature heat pipes
  4. high performance heat sinks.


Work Accomplished

The project began by analyzing the capabilities of the various passive cooling techniques available. Based on these studies, techniques suitable for handling heat fluxes of 50 W/in2 at the package level were identified. Various combinations of advanced materials and improved cooling technology were considered. These included: (i) high thermal conductivity substrates and heat sinks, (ii) heat pipes and fluorinert filled bags and (iii) two phase thermosyphons. The use of thermosyphons was identified as an appropriate cooling option. During the second part of the project, an experimental facility was designed and constructed to assess the thermal performance of the thermosyphon. Thermofoil heaters were employed with the capabilities of producing heat fluxes in the desired range. The module thermal performance, in a passive environment, was measured for various heat fluxes.


LITERATURE REVIEW

The techniques for passive cooling of high power electronic components and systems using thermosyphons and heat pipes were studied, with special emphasis on the enhancement in heat transfer obtained. A comparison of single-phase thermosyphons and two-phase thermosyphons (liquid-vapour) showed that there is a significant enhancement in heat transfer in the latter case. The latent heat transfer in the two-phase systems allows the same amount of heat transfer at a much lower temperature difference (between the evaporator and the condenser). However, due to the high vapour pressures and volumes, the two-phase thermosyphons are slightly more complex.

A variety of working fluids, for the thermosyphon, have been investigated in the past (e.g. water, water-methanol mixture and fluorocarbon liquids). These fluids can operate in either single phase or two-phase mode. A new working fluid which shows promise is an aqueous emulsion of a phase change material (PCM). PCMs transform from solid to liquid and absorb the latent heat in the process. PCMs have been studied for heat storage systems in the past. All the studies in literature have looked at forced convection cooling, though.

DESIGN OF EXPERIMENTAL SETUP FOR THERMOSYPHON COOLING WITH PCM EMULSION

An approximate sizing of the experimental setup was performed using a resistance network. Two fin configurations were considered for the condenser, one with horizontal radial fins and the other with vertical longitudinal fins

The thermosyphon working fluids included air, de-ionized water, n-triacontane-water emulsion and FC-72 (fluorinert)

The objective of dissipating 50W/in2 in a passive environment was achieved with the vertical finned setup and deionized water as the working fluid. The PCM emulsion did not show any enhancement in the thermal performance compared to water for the single chamber construction. FC-72 resulted in higher heater temperatures for the same power levels compared to water. Thus it is not a good candidate for single-phase operation. In the two-phase mode for FC-72, while considerable thermal enhancement was observed compared to single phase operation, vapor containment was a concern. With the horizontal finned setup, the maximum heat dissipation achieved was about 30W/in2, with water as the working fluid. The vertical finned setup was found to be better for passive cooling compared to one with horizontal fins.