Compositions – Vaporization – or expansion – refrigeration or heat or energy...
Reexamination Certificate
1995-01-30
2001-02-27
Einsmann, Margaret (Department: 1751)
Compositions
Vaporization, or expansion, refrigeration or heat or energy...
C252S073000, C252S077000
Reexamination Certificate
active
06193905
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid coolant for immersion cooling, and an electronic device using the coolant.
2. Description of the Related Art
When a heat generator is cooled by a direct immersion in a coolant, relationships between the difference AT of the temperatures of the heat generator and the coolant (superheat; ° C.) and the heat flux (W/cm
2
) removed from a unit area of the heat generator by the coolant generally exist as shown in
FIGS. 1A and 1B
. As the temperature of the heat generator is elevated, the heat flux is increased in accordance with the AT, but the coolant tends to bump and the heat flux is not increased when the phase shifts from the natural convection to the nucleated boiling phase, and then the nucleated boiling phase continues for a while and is shifted to a film boiling phase. The heat flux reaches the maximum when the nucleated boiling phase is being shifted to the film boiling phase.
It is known to use a coolant having a low boiling point, mixed with another coolant having a higher boiling point, for cooling by a direct immersion of a semiconductor element in the coolant.
The inventors investigated and developed methods of cooling a semiconductor element by a direct immersion in a liquid coolant, and these methods are disclosed or published in (1) “Evaporation Cooling Module for Semiconductor” (U.S. Pat. No. 4,704,658), (2) “Cooling Computers by Direct Immersing LSIs in Liquid”, Nikkei Electronics No. 425, Jul. 13, 1987 p 167-176, (3) “Overheat Phenomena in Boiling Cooling”, 1982 Autumn 43rd Applied Physics Society Conference Proceedings, Sep. 28-30, p 569, 29-F-3, (4) “Liquid Cooling Type Electronic Device” (Japanese Unexamined Patent Publication No. 59-125643), (5) “Immersion Cooling for High-Density Packaging” IEEE TRANSACTION ON COMPONENTS, HYBRIDS, AND MANUFACTURE TECHNOLOGY, vol. CHMT-12, No. 4, Dec. 1987, p 643-646, (6) “STUDIES ON IMMERSION COOLING FOR HIGH DENSITY PACKAGING” ISHM '87 Proceedings p 175-180, (7) “Cooling Technique for Semiconductor Element” Semiconductor Integrated Circuit Techniques 24th Symposium Conference Papers, Jun. 2-3, p 30-35, etc.
U.S. Pat. No. 4,704,658 describes fleons C
2
Cl
3
F
3
(b.p. 49° C.), C
5 F
12
(b.p. 30° C.), C
6
F
14
(b.p. 56° C.), etc. as the coolant, and discloses cooling modules corresponding to
FIGS. 10
to
18
attached to this specification.
Nikkei Electronics No. 425 states that fluorocarbons having a molecular weight of several hundreds, and chemically stable as a liquid coolant for a direct immersion cooling of LSIs, are a colorless transparent liquid and have a boiling point of 30-150° C. (an example is a fluorocarbon having a boiling point of 56° C.; p-fluorohexane) and discloses that a coolant mixture does not have a specific boiling point and does have a temperature range of boiling, and that overshoot can be reduced by combining a plurality of coolants; the minimum overshoot is obtained by mixing two fluorocarbons having boiling points of 56° C. (p-fluorohexane) and 101° C. (p-fluoro-2-octanone) in a ratio of 20:80.
1982 Autumn 43rd Applied Physics Society Conference Proceedings states that a mixture of two coolants having boiling points of 50° C. and 100° C. provides substantially no overheating, i.e., a deviation from the ideal starting point of the nucleated boiling.
Japanese Unexamined Patent Publication No. 59-125643 describes a coolant comprising two fluorocarbons having boiling points which are at least 10° C. different from each other, in respective amounts of at least 10% by weight; specifically, fluorocarbons having boiling points of 50° C. and 102° C.
IEEE TRANSACTION ON COMPONENTS, HYBRIDS, AND MANUFACTURE TECHNOLOGY, vol. CHMT-12 describes a mixture of fluorocarbons having boiling points of 56° C. and 102° C., for minimizing overshoot.
ISHM '87 Proceedings states that perfluoro-carbon C
6
F
14
(b.p. 56° C.) is suitable as a coolant at room temperature, and that the heat flux of C
6
F
14
(b.p. 56° C.) is 10 W/cm
2
when the film boiling occurs.
Semiconductor Integrated Circuit Techniques 24th Symposium Conference Papers disclose that a feon has a cooling capability of 20 W/cm by boiling cooling, the maximum heat flux relates to the gasification heat of a liquid coolant, and the size of bubbles due to boiling of a coolant relates to a surface tension of the coolant, for example, 0.5 mm for C
6
F
14
(b.p. 560° C.), and is small, i.e., 0.05 mm, when the coolant is liquid helium having a small surface tension of 0.12 dyne/cm, whereby a three-dimensional high density packaging is possible.
The present invention is based on the results of the above investigation, and the object of the present invention is to improve the maximum heat flux of a coolant, and a cooling capability at a unit area of a semiconductor element by a coolant, while the temperature of film boiling is maintained as low as a temperature range allowable for a semiconductor element.
SUMMARY OF THE INVENTION
The present invention provides a coolant for cooling a semiconductor element by direct immersion, comprising a low boiling point fluorocarbon having a boiling point of 30° C. to 100° C. and a high boiling point fluorocarbon having a boiling point higher than that of the low boiling point fluorocarbon by at least 100° C., an amount of the high boiling point fluorocarbon being less than 20% by volume based on the volume of the low boiling point fluorocarbon.
The present invention is based on the finding that, by adding a high boiling point fluorocarbon having a boiling point of 215° C. or 253° C. to a low boiling point fluorocarbon having a boiling point of 30° C. to 100° C., the surface tension of the coolant for boiling cooling is surprisingly lowered. As the surface tension of the coolant is lowered, the size of bubbles formed by boiling is reduced and the cooling capability of the coolant is increased at a temperature where the phase is shifted to the film boiling. Since the size of bubbles formed by boiling is smaller, the space between semiconductor elements mounted on a substrate can be narrowed without lowering the maximum heat flux of the coolant, thereby allowing a high density packaging of semiconductor elements.
Thus, in accordance with the present invention, there is also provided a boiling cooling-type electronic device comprising an electronic element having a heat generating portion immersed in a liquid coolant, the liquid coolant comprising a low boiling point fluorocarbon having a boiling point of 30° C. to 100° C. and a high boiling point fluorocarbon having a boiling point higher than that of the low boiling point fluorocarbon by at least 100° C.; an amount of the high boiling point fluorocarbon being less than 20% by volume based on the volume of the low boiling point fluorocarbon.
Typically, the structure of the device can be expressed as a module for evaporation cooling of a plurality of semiconductor chips mounted on a common, generally planar surface of a circuit board and immersed in a liquid coolant contained within the module, said liquid coolant including, during cooling, a first gas of bubbles of evaporated coolant and a second gas such as air, comprising:
(a) a case forming a chamber and having at least one sidewall with an opening formed therein in communication with said chamber, said sidewall being adapted for receiving a planar circuit board thereon in hermetically sealed relationship for closing said opening and with the common, generally planar surface thereof disposed inwardly with respect to said chamber for positioning the plurality of semiconductor chips mounted thereon within said chamber, said case, with said opening enclosed by the circuit board, defining a sealed interior cooling chamber for receiving sufficient liquid coolant therewithin to immerse the plurality of semiconductor chips within the liquid coolant;
(b) at least one heat exchanger within the liquid coolant and mounted at a predetermined position within said cooling chamber, adjacent corresponding, immersed semiconductor chips
Kamehara Nobuo
Niwa Koichi
Yamada Mitsutaka
Yokouchi Kishio
Armstrong, Westerman Hattori, McLeland & Naughton
Einsmann Margaret
Fujitsu Limited
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