Micro and Miniature Heat Pipes

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Cotter ^[1] was the first to propose the micro heat pipe concept for cooling of electronic devices. The micro heat pipe was defined as a heat pipe in which the mean curvature of the liquid vapor interface is comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel. Typically, micro heat pipes (Fig. 1) have convex but cusped cross sections ^[2] (for example, a polygon), with hydraulic diameter in range of 10 to 500 μm ^[3]. Miniature heat pipe is defined as a heat pipe with a hydraulic diameter in the range of 0.5 to 5 mm ^[4].

References to micro and miniature heat pipes in open literature are sometimes not properly differentiated as defined above ^[5][6]. For example, miniature heat pipes with micro grooves are sometimes improperly referred to as micro heat pipes. It is also worth noting the other structural differences between micro and miniature heat pipes in addition to the size ranges noted above. A true micro heat pipe, for both metallic and silicon applications, has a single non-circular channel for both liquid and vapor phases where the liquid return from condenser to evaporator by capillary force occurs in the sharp corner of the channel. These types of micro heat pipes are generally placed in an array of parallel micro heat pipes on the substrate surface in order to increase the overall heat transfer area. Miniature heat pipes can be designed based on micro axially grooved structure (1D capillary structure), meshes or cross grooves (2D capillary structure).

Attempts have been made to etch micro heat pipes directly into the silicon and use them as thermal spreaders ^[7][8][9](Peterson and Ma, 1999; Le Berre et al., 2003; Launay et al., 2004a). The performance characteristics of the micro heat pipes are different from those conventional heat pipes ^[10][9].

In order for the micro heat pipe to find more commercial applications in microelectric cooling, it must compete with other cooling methods, such as forced convection, impingement and two phase direct cooling in areas such as manufacturing cost and reliability. Because of the extremely small dimensions of micro heat pipes, difficulties in manufacturing and the subsequent degassing and charging are expected. Furthermore, the “true” micro heat pipe built directly into the silicon must compete with the miniature heat pipe attached on the chip surface in this area, which is much less costly.

Miniature heat pipes with micro axial grooves can be successfully used in electronic components cooling systems. Fabrication and experimental data on the performance characteristics of the flat water-copper heat pipe with external dimensions 2x7x120 mm have been reported by Hopkins et al. ^[4] with radial heat fluxes of 90 W/cm² and 150 W/cm² for horizontal and vertical applications, respectively.

References

1. ↑ Cotter, T. P., 1984, "Principles and Prospects for Micro Heat Pipes," Proceedings of 5th International Heat Pipe Conference, Tsukuba, Japan, 328-335.
2. ↑ Peterson, G. P., 1992, "Overview of Micro Heat Pipe Research and Development," Applied Mechanics Reviews, 45(5), 175-189. http://dx.doi.org/10.1115/1.3119755
3. ↑ Cao, Y., and Faghri, A., 1994a, "Micro/Miniature Heat Pipes and Operating Limitations," Journal of Enhanced Heat Transfer, 1(3), 265-274.
4. ↑ ^{4.0 4.1} Hopkins, R., Faghri, A., and Khrustalev, D., 1999, "Flat Miniature Heat Pipes with Micro Capillary Grooves," Journal of Heat Transfer, 121(1), 102-109. http://dx.doi.org/10.1115/1.2825922
5. ↑ Faghri, A., 2012, "Review and Advances in Heat Pipe Science and Technology," Journal of Heat Transfer, 134(12), 123001. http://dx.doi.org/10.1115/1.4007407
6. ↑ Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C.
7. ↑ Peterson, G. P., and Ma, H. B., 1999, "Temperature Response of Heat Transport in a Micro Heat Pipe," Journal of Heat Transfer, 121(2), 438-445. http://dx.doi.org/10.1115/1.2825997
8. ↑ Le Berre, M., Launay, S., Sartre, V., and Lallemand, M., 2003, "Fabrication and Experimental Investigation of Silicon Micro Heat Pipes for Cooling Electronics," Journal of Micromechanics and Microengineering, 13(3), 436-441. http://dx.doi.org/10.1088/0960-1317/13/3/313
9. ↑ ^{9.0 9.1} Launay, S., Sartre, V., and Lallemand, M., 2004a, "Experimental Study on Silicon Micro-Heat Pipe Arrays," Applied Thermal Engineering, 24(2-3), 233-243. http://dx.doi.org/10.1016/j.applthermaleng.2003.08.003
10. ↑ Khrustalev, D., and Faghri, A., 1994, "Thermal Analysis of a Micro Heat Pipe," Journal of Heat Transfer, 116(1), 189-198. http://dx.doi.org/10.1115/1.2910855