Note: Descriptions are shown in the official language in which they were submitted.
FJ-8473
2 ~J~
-- 1 --
IMMERSION COOLING COOLANT AND ELECTRONIC
DEVICE USING THIS COOLANT
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 ~T of the temperatures of the heat generator
and the coolant (superheat; C) and the heat flux
(W/cm2) removed from a unit area of the heat generator
by the coolant generally exist as shown in Figs. lA and
lB. As the temperature of the heat generator is
elevated, the heat flux is increased in accordance with
the ~T, 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.P. 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
- 2 -
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.P. 4,704,658 describes fleons C2C13F3
), 5F12 (b-p- 30 C), C6F14 (b.p. 56C), 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
- 150C (an example is a fluorocarbon having a boiling
point of 56C; 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 56C
(p-fluorohexane) and 101C (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 S0C and 100C
provides substantially no overheating, i.e., a deviation
from the ideal starting point of the nucleated boiling.
Japanese Unexamined Patent Publication No.
- 3 - ~ s:~
59-125643 describes a coolant comprising two
fluorocarbons having boiling points which are at least
10C different from each other, in respective amounts of
at least 10% by weight; specifically, fluorocarbons
having boiling points of 50C and 102C.
IEEE TRANSACTION ON COMPONENTS, HY~RIDS, AND
MANUFACTURE TECHNOLOGY, vol. CHMT-12 describes a mixture
of fluorocarbons having boiling points of 56C and
102C, for minimizing overshoot.
ISHM '87 Proceedings states that perfluoro-
carbon C6F14 (b.p. 56C) is suitable as a coolant at
room temperature, and that the heat flux of C6F14 (b.p.
56C) is 10 W/cm when the film boiling occurs.
Semiconductor Integrated Circuit Techniques
24th Symposium Conference Papers disclose that a fleon
has a cooling capability of 20 W/cm2 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 C6F14 (b.p. 56 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 30C to 100C and a high boiling point
fluorocarbon having a boiling point higher than that of
- 4 -
the low boiling point fluorocarbon by at least 100C, 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 215C or 253C to a low boiling point
fluorocarbon having a boiling point of 30C to 100C,
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 30C to 100C and
a high boiling point fluorocarbon having a boiling point
higher than that of the low boiling point fluorocarbon
by at least 100C; 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
C~,f~.,
-- 5 --
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 of
a plurality thereof mounted on the circuit
board enclosing the sidewall openings of the
case, for cooling the liquid coolant and
reliquifying the first gas; and
(c) porous metal means associated with each said
heat exchanger and immersed within the
coolant, for trapping the first gas, allowing
the second gas to pass therethrough, and
maintaining contact of the first gas with said
associated heat exchanger, and optionally
(d) means associated with each said bubble
trapping means, immersed within the coolant
and mounted within said cooling chamber, for
guiding said fist gas toward said associated
trapping means.
The liquid coolant components or low and high
~ 6 ~ J ;J
boiling point components are selected from fluoro-
carbons, because they are chemically stable. The term
"fluorocarbons" in this context means a compound
composed of carbon and fluorine atoms, optionally with
oxygen, nitrogen, sulfur or other atoms, or a
hydrocarbon having a skeleton mainly composed of carbon
atoms in which all or most hydrogen atoms are
substituted by fluorine atoms. Such a compound is
chemically stable because the carbon-fluorine bond is
very strong and stable.
The low boiling point fluorocarbon is selected from
fluorocarbons having a boiling point of 30C to 100C.
The semiconductor element should be cooled to a
temperature not higher than about 80C, preferably not
higher than 70C, because the maximum junction
temperature is about 85C, with a cooling margin of
15C. To obtain such a cooling by room temperature
cooling, the low boiling point fluorocarbon should have
a boiling point in the above temperature range. A
preferable range of the boiling point is 30 to 100C,
with a more preferable range being 30 to 60C. If the
boiling point of the l-ow boiling point fluorocarbon is
lower than 30C or room temperature, particularly lower
than the ice point, the coolant would be cooled to lower
than the room temperature or the ice point, and thus the
cooling efficiency undesirably lowered.
Examples of the low boiling point fluorocarbon
include CF3(CF2)4CF3 (FC-86, b-p- 56 C);
CIF2 C 1 2
C ~ 2 (FC-78, b.p. 84C);
(C5FloO)
~ C4F9 (FC-77, b.p, 97C); etc,
o
(C8Fl6o)
The high boiling point fluorocarbon is selected
from fluorocarbons having a boiling point higher than
that of the low boiling point fluorocarbon by at least
100C. It was found that the maximum heat flux of the
S coolant can be remarkably increased by adding a small,
amount of a fluorocarbon having such a boiling point
difference. The maximum heat flux of a coolant is
obtained at a point whereat the nucleated boiling is
shifted to the film boiling. By adding such a high
boiling point fluorocarbon in a small amount, the
cooling capability of the coolant is unexpectedly
increased. If the difference of the boiling points of
fluorocarbons is not higher than 100C, a desired effect
of increasing the cooling capability cannot be obtained.
A preferred difference of the boiling points of
fluorocar~ons is at least 100C, more preferably 150C
or more.
Examples of the high boiling point fluorocarbon
include C13F26 (FC-40, b-p- 155 C);
FC-43 tb.p. 174C); FC-71 (b.p. 253C);
(FC-5311, b.p. 215C)
(C14F24 )
C F
/ 5 11
C5 ll N\ (FC-70, b.p. 215C); etc.
C5F1 1
(C15F33N)
Each of the low and high boiling point fluoro-
carbons may be a single fluorocarbon or a mixture
thereof.
The amount of the high boiling point fluorocarbon
is up to 20% by volume based on the volume of the low
boiling point fluorocarbon, preferably less than 9~ by
volume, more preferably 1.5 to 6% by volume. If the
amount of the high boiling point fluorocarbon is too
high, the film boiling temperature is undesirably
elevated and the reliability of a semiconductor element
is lost.
As described before with reference to Fig. l, when
a heat generator is cooled by a direct immersion in a
coolant, with an increase of the temperature of the heat
generator, the natural convection is shifted to the
nucleated boiling, and further, to the film boiling.
g ~ fi 1~
The heat flux is remarkably higher in the nucleated
boiling phase than in the natural convection phase, and
is lowered after the nucleated boiling is shifted to the
film boiling. Therefore, preferably the cooling is
conducted in a temperature range of from the natural
convection to the nucleated boiling, particularly in a
temperature range of the nucleated boiling, and it is
preferable to use a coolant having a maximum or critical
heat flux of a coolant, since as large as possible a
heat flux at the nucleated boiling is shifted to the
film boiling. The coolant of the present invention is
used for cooling a semiconductor element in a
temperature range of from the natural convection to the
nucleated boiling, mainly in a temperature range of the
nucleated boiling, and the heat flux thereof at a unit
area of a body to be cooled is remarkably improved. It
is considered that the heat flux is increased by
lowering the surface tension when a small amount of a
high boiling point fluorocarbon is added.
Thus, the coolant of fluorocarbon mixture of the
present invention is suitable as a coolant used at a
near room temperature,-because of a high stability and a
high heat transfer capability thereof. Further, the
space between LSIs mounted on a substrate depends on the
nature of the coolant, more specifically, the size of
bubbles formed by boiling, and the space between LSIs or
semiconductor elements can be reduced to 2 to 0.8 mm,
without causing film boiling, by using a fluorocarbon
mixture coolant of the present invention. As a result,
a dense packaging while maintaining a high cooling
capability is possible, and a compaction of a liquid
cooling device as a whole is possible.
The above is an outline of the development of the
present invention, and the effects of the present
invention, and known liquid coolants found by the
inventors are briefly described below.
Toshiba discloses a liquid coolant suitable for
:
~, .
-- 10 --
cooling transistors and thyristors, etc. (Japanese
Examined Patent Publication No. 51-20743), a liquid
coolant suitable for cooling oil-immersed transforms,
etc. (Japanese Examined Patent Publication No.
5 53-20319), and a liquid coolant with a controlled
boiling point for transistors, thyristors, LSI's,
transformers, etc. (Japanese Unexamined Patent
Publication No. 59-58081). These all comprise a liquid
coolant composed of a fXeon (R113, R112) mixed with a
silicone oil, and the cooling mechanism used is a gas
condensation liquefying. The silicone oil is added to
suppress bubbling, to thus prevent a lowering of a
breakdown voltage caused by bubbles, or to prevent a
chemical action of the fleon, or to control the boiling
point, etc. These methods are different from that of
the present invention, in that a silicone oil is added
to the coolant.
Hitachi discloses semiconductor devices in which
two coolants having different boiling points are
enclosed (Japanese ~nexamined Patent Publication Nos.
49-68674, 52-38662 and 61-104696). Particularly,
Japanese Unexamined Pa~ent Publication No. 61-104696
describes a non-azeotropic mixed coolant comprising two
or more components having a difference of the boiling
point of at least 70C. The use of the non-azeotropic
mixed coolant is intended to improve the latent
evaporation heat which determines the cooling capability
of the coolant. Therefore, these coolants are not to be
used for cooling by the nucleated boiling, as is the
coolant of the present invention. Also, the coolants of
JPP 61-104696 comprise a low boiling point coolant
having a boiling point lower than the ice point, which
is excluded from the present invention because it is not
suitable for cooling a semiconductor element which
generates a large amount of heat.
The coolant of the present invention is different
from the above coolants in that the mixed coolant
components are composed of fluorocarbons, the coolant is
used at room temperature, mainly by the nucleated
boiling, the low boiling point component has a boiling
point of 30C to 100C, and the high boiling point
component has a boiling point higher than that of the
low boiling point component by at least 100C.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures lA and lB show relationships between the
temperature difference of the coolant and heat generator
and the heat flux removed from the heat generator;
Fig. 2 is a sectional view of a device for
measuring the cooling capability of a boiling cooling
coolant;
Figs. 3A and 3B are sectional views of cooling
devices in which two sides or one side of a semi-
conductor element are cooled by a coolant;
Figs. 4 and 5 show relationships between the heat
flux of a coolant and the temperature of the coolant in
the double-sided cooling;
Fig. 6 shows relationships between the overheating
of a semiconductor element and the heat flux of a
coolant in the double-sided cooling;
Fig. 7 shows relationships between the heat flux of
a coolant and the temperature of the coolant in the
double-sided cooling;
Fig. 8 shows relationships between the added amount
of high boiling point coolant and the maximum heat flux
of the coolant and relationships between the added
amount of high boiling point coolant and the temperature
of chip;
Fig. 9 shows relationships between the added amount
of high boiling point coolant and the maximum heat flux
of the coolant;
Fig. 10 is a perspective view of a liquid cooling
module;
Fig. 11 is a sectional view of a liquid cooling
module;
f ~
~ 12 -
Fig. 12 is an enlarged sectional view of a liquid
cooling module;
Fig. 13 is a sectional view of a first liquid
cooling structure;
Fig. 14 is a sectional view of a second liquid
cooling structure;
Fig. 15 is a sectional view of a third liquid
cooling structure;
Fig. 16 is a sectional view of a fourth liquid
cooling structure;
Fig. 17 is a sectional view of a fifth liquid
cooling structure; and
Fig. 18 is a perspective view of a system involving
a plurality of liquid cooling modules.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 2 illustrates a section of a device for
measuring the cooling capability of a boiling cooling
coolant. The device comprises a cooling case 1, a
circulating cooler 2, and pipes 3 and 4 for supplying
and discharging water as a coolant. The cooling case 1
has a double wall structure in which a fluorocarbon
coolant to be measured-is contained in an inside
case 11, water cooled to a pre~etermined temperature by
the circulating cooler 2 is circulated in an outside
case 12 through the pipes 3 and 4, and a fin 5 is
provided between the inside and outside cases 11 and 12
for improving the heat transfer efficiency.
A total amount of 400 ml of a fluorocarbon having a
boiling point of 56C mixed with a certain amount of a
high boiling point fluorocarbon having a boiling point
of 215C or 253C was charged as a coolant in the inside
case 11 of the cooling case 1, and an Si chip, 13 mm
square, as a model of an LSI, was immersed in the
coolant. The Si chip was immersed in the coolant in
such a manner that both sides of the chip were boiling
cooled by supporting the chip 21 at an end thereof
(Fig. 3A), and such that a single side of the chip 21
- 13 _ ~ ~S'~ 3
was boiling cooled by mounting the chip 21 on a ceramic
substrate 23 (Fig. 3B). In the double-sided cooling,
the heat flux was almost double that of the single-sided
cooling. The heat generator was made by forming a NiCr
film 24 on both or one surface of the 13 mm square Si
chip 21 and was immersed in the coolant in the inside
case 11 while the Si chip was supported by a support 22,
so that the chip 21 was horizontally arranged. The heat
flux of the heat generator was varied and the
temperature of the Si chip 21 was measured. The
measurement of the temperature of the surface of the
heat generator was carried out by a copper-constantan
thermocouple 25.
Figure 4 shows relationships, in the double-sided
cooling, between the heat flux and the temperature of a
mixed fluorocarbon coolant in which a low boiling point
component was FX3250 (C6F14; sold by 3M) having a
boiling point of 56C, with which a high boiling point
component FC-70 (3M) having a boiling point of 215C was
mixed in an amount of 0, 3, 6 or 9% by volume. The
temperature of the coolant was measured by a
copper-constantan ther-mocouple with a diameter of
0.1 mm. The temperature of thé cooling water at the
inlet of the outside case 12 of the cooling case 1
supplied from the circulating cooler 2 was 5 to 18C,
the temperature of the cooling water discharged at the
outlet of the outside case 12 was 8 to 22C, and the
feed of the cooling water was 250 ml/min. ~he Si chip
was cooled from both sides in this case.
As seen from Fig. 4, the heat fluxes of the mixed
coolants comprising FX3250 (b.p. 56C) with 3 to 9 vol%
FC-70 (b.p. 215C) were about 1.5 to 1.8 times that of
FX3250 (b.p. 56C) alone, at the same coolant
temperature, specifically 1.47 to 1.56 times at 10C and
1.70 to 1.80 times at 20C, and thus the cooling
capability of the fluorocarbon coolant is remarkably
improved by the mixing of the coolants.
- 14 ~ f~ ~f,
Figure 5 shows relationships, in the double-sided
cooling, between the heat f lux and the temperature of a
mixed f luorocarbon coolant in which a low boiling point
component was FX3250 (3M) having a boiling point of
56C, with which a high boiling point component FC-71
(3M) having a boiling point of 253C was mixed in an
amount of 0, 1 . 5, 3, 4.5, 6 or 9% by volume. As seen in
Fig. 6, the heat f luxes of the mixed coolants comprising
FX3250 (b.p. 56C) with 1.5 to 9 vol% FC-71 (b.p. 253C)
were remarkably improved to 1.43 to 1.85 times that of
FX3250 (b.p. 56C) at 10C and 1.60 to 2.33 times at
20C, and thus the cooling capability of the
f luorocarbon coolant was remarkably improved by the
mixing of the coolants.
Figure 6 shows an overheating of a heat
generator(LIST chip) when the natural convection is
shifted to the nucleated boiling. The heat generator
having a NiCr film on the surface thereof, as described
above, was immersed in a coolant in the same manner as
shown in Fig. 4A (double-sided cooling), the heat flux
of the heat generator was varied, and the temperature of
the surface of the Si -chip heat generator and the
saturation temperature of the coolant were measured, to
determine the amount of overheating of the heat
generator just before the nucleated boiling.
The coolant used was a mixed fluorocarbon
comprising FX3250 (3M) having a boiling point of 56C
and 1.5 vol~ FC-71 (3M) having a boiling point of 253C,
and the comparative coolant was FX3250 (3M) alone. In
Fig. 6, the ordinate shows the heat f lux and the
abscissa shows the overheating of the chip.
As seen in Fig. 6, when the mixed coolant
containing 1.5 vol% FC-71 was controlled to a
temperature of 10C, the heat flux was 50 W/cm2 and the
overheating of the chip was 54C. When the low boiling
point FX3250 alone was used and controlled to 10C, the
heat flux was 27 W/cm and the overheating of the chip
was 42C. The temperature of the chip was therefore
64C in the mixed coolant case and 52C in the FX3250
alone case.
Thus, the heat flux was increased by about 1.85
times, i.e., the cooling capability is improved,
although the degree of overheating of the Si chip heat
generator was slightly increased.
Figure 7 shows relationships between the heat flux
and the temperature of a mixed fluorocarbon coolant in
which a low boiling point component was C5F12 (3M or
Daikin) having a boiling point of 30C, with which a
high boiling point component FC-71 (3M) having a boiling
point of 253C was mixed in an amount of 0, 3, or 6% by
volume. As seen in Fig. 7, the heat fluxes of the mixed
coolants comprising C5F12 (b.p. 30C) with 3 to 6 vol%
FC-71 (b.p. 253C) were remarkably improved to 1.71 to
1.87 times that of C5F12 (b.p. 30C) at 10C and 1. 53
to 1.65 times at 20C, and thus the cooling capability
of the fluorocarbon coolant was remarkably improved by
the mixing of the coolants.
Figure 8 shows relationships between the maximum
heat flux and the amou-nt of high boiling point
fluorocarbon, and relationship-between the temperature
of the chip and the amount of the added high boiling
point fluorocarbon, when using a mixed fluorocarbon
coolant in which a low boiling point component was C5F12
(Daikin) having a boiling point of 30C, to which a high
boiling point component FC-71 (3M) having a boiling
point of 253C was mixed in an amount of 0, l.S, 3, 4.5,
6 or 9% by volume. The temperature of the coolant was
controlled to 15C.
As seen in Fig. 8, as the added amount of the high
b.p. component FC-71 was increased, the maximum heat
flux was rapidly increased from 35 W/cm2 to 65 W/cm2, in
a range up to 3 vol% FC-71, but the increase of the heat
flux was gradual in a range of 3 to 9 vol%; the
temperature of the chip was increased almost in relation
.
.
- 16 - ~ c-
to the added amount of FC-71, from 47C at 0 vol% FC-71
to 84C at 6 vol% FC-71.
A semiconductor element mounted on a substrate was
directly immersed in a mixed coolant comprising a low
b.p. coolant mixed with varied amounts of a high b.p.
coolant, and the mixed coolant was controlled to a
temperature of 20C. The heat generated by the
semiconductor element at a temperature at which the
nucleated boiling was shifted to the film boiling was
measured, to obtain the maximum heat flux for a unit
area, and then the relationships of the maximum heat
flux with the added amount of the high b.p. coolant,
this amount being based on the volume of the low b.p.
coolant, were obtained. In every case, the low b.p.
coolant used was FX-3250 having a b.p. of 56C. The
high b.p. coolant was FC-71 tb.p. 253C) in case A,
FC-70 (b.p. 215C) in case B and FC-5311 (b.p. 165C) in
case C, and as a comparison, FC-104 (b.p. 101C), which
is the same as the high b.p. coolant used in Japanese
Unexamined Patent Publication No. 59-125643, was used.
The results are shown in Fig. 9. In cases A and B,
due to the lowering of-the surface tension, the size of
bubbles leaving the semiconductor element became smaller
and the maximum heat flux at a temperature of a shift to
the film boiling was increased to about 30 W/cm2. In
case C, the maximum heat flux was increased to about
24 W/cm2. In contrast, in case D, where the high b.p.
coolant was the same as used in JPP ~643, the maximum
heat flux was about 20 W/cm , which is little increased
from 18 W/cm2 where no high b.p. coolant was added.
When the added amount of high b.p. coolant was
about 3 to 5% by volume, the maximum heat flux became
almost constant, and when the added amount of high b.p.
coolant was over 20% by volume, the film boiling
temperature became too high, and the reliability of a
semiconductor element was undesirably lowered.
Figures 10 to 12 illustrate a liquid cooling module
- 17 - h t i j~ "
as an example of the present invention; wherein Fig. 10
is a perspective view, Fig. 11 is a sectional view, and
Fig. 12 is an enlarged sectional view.
The liquid cooling module 31 comprises a metal
case 32, having openings on both sides, in which ceramic
substrates 34 with a plurality of semiconductor chips 33
such as LSIs mounted thereon are sealed from the inside
of the case 32.
Pipes 35 for supplying cooling water are provided
at an upper portion of the module 31. In the module ~1,
a plurality of heat exchange pipes 36 in which the
cooling water is circulated are disposed, and bubble
traps 38 made of a porous metal are arranged above the
pipes 36 in the coolant 37. Bubble guides are arranged
between the chips 33 mounted on the ceramic
substrates 34, and extend laterally inwardly from the
ceramic substrate, to thereby guide a coolant gas formed
on the chips 33 to the bubble traps 38.
The ceramic substrates 34 may be multi-layer and
have a plurality of lead pinis 40 buried on the back side
for connection with connectors. The plurality of
semiconductor chips 33 are bonded onto wiring patterns
on the ceramic substrate 34 by a flip chip bonding, and
the wiring patterns are connected to the lead pins 40
through via holes.
The bubbles traps 38 are provided for an effective
reliquefying of the gas gasified by the chips 33. The
- bubble traps 38 have pores at a pitch of 20/cm3, a
porosity of 20 pores per cm3, corresponding to a pore
size of approximately 0.5 to 0.6 mm in diameter. In
general, the bubble trap material should have a porosity
within a range from about 15 to 50 pores per cm3, to
achieve the intended beneficial effect, and the cooling
water flows in the heat exchange pipes 36 at a flow rate
of 1 Q/min.
By providing bubble traps, the efficiency of the
liquid cooling module becomes superior to that of liquid
~ r~
cooling module without bubble traps, and the
reliquefaction of the bubbles is accelerated so that an
elevation of the temperatuxe of the chips is suppressed.
Preferably the heat exchange pipes 36 are arranged
in the coolant, not in the open space above the coolant,
as if they are arranged in the open space, the
efficiency of the heat exchange is lowered by a
deposition onto the pipes of spray formed by breaking
bubbles, etc. Therefore, the traps are preferably
arranged in the coolant whereat the collected bubbles
are liquefied, to obtain a maximum efficiency.
By using porous metal traps, although the bubbles
are trapped, the coolant can easily pass through the
traps, and therefore, the circulation of the coolant is
accelerated.
The bubble guides 39 are provided for an effective
guiding of bubbles, generated on the surface of the
chips 33, to the traps 38. The bubble guides 39, which
are made of a material not soluble in the coolant, are
arranged just above and adjacent to the chips 33,
whereby almost all bubbles 38 can be guided to the
bubble traps 38. By p~oviding bubble guides, the
efficiency of the liquid cooling module becomes superior
to that of liquid cooling modules without the bubble
guides, and an elevation of the temperature of the chips
is suppressed.
Figure 13 illustrates another liquid cooling
structure of the present invention, in which the
semiconductor elements (LSIs) 63 are mounted on a
ceramic substrate 64 by flip chip bonding, etc. and the
substrate 64 is connected to an outer circuit through a
connector (not shown) on the back of the substrate 64.
The cooling case 65 has a double wall structure in
which the coolant 66 is charged inside the double walled
case, and a heat exchanger 67 is provided to divide the
coolant into up and down sections. The space formed by
the double wall structure is constituted such that a
~ ~ ~ 2 ~J '~
flow of cooling water passing through the heat
exchanger 67 is circulated.
The heat exchanger 67 has a structure by which the
bubbles generated by boiling are trapped, but the
coolant 66 can be easily pass through. An example is a
structure of a plurality of horizontally corrugated
copper pipes bonded with a copper foam, in which about
15 to 50 pores are provided per 1 cm3.
In such a construction, since all bubbles formed by
the LSIs 63 are trapped and liquefied, the cooling
capability is improved.
Figure 14 illustrates a third liquid cooling
structure of the present invention. In a closed
case 75, a circuit board 74 is inserted and fixed to a
connector 78, and a plurality of LSIs 73 are mounted on
the circuit board 74 by flip chip bonding.
A heat exchange pipe 79 constitutes the heat
exchanger and has a structure in which the pipe is
folded and extends into spaces between and above the
LSIs. In this structure, bubbles generated by the LSIs
are in contact with a plurality of portions of the heat
exchange pipe 79, whil-e floating, and are thereby
cooled, and thus the cooling capability is improved and
the cooling is uniform.
Figure 15 illustrates a fourth liquid cooling
structure of the present invention, which is an
improvement of the third structure of Fig. 14. In this
structure, the circuit board 74 is inserted and fixed to
a side wall of the cooling case 75, and LSIs 73 are
mounted on the circuit board 74.
Bubble guides 80 with bubble traps 8l are arranged
between and above the vertically arranged LSIs 73, and a
plurality of heat exchange pipes 79 are disposed below
the bubble guides 80 and the bubble traps 81. The
bubble traps 81 are provided with pores so that the
coolant 76 can easily pass through the bubble traps 81
but the bubbles are trapped by the bubble traps 8l. In
-..-. . ' ~ -
--
- 20 -
one example, the bubble traps 81 are formed from a
stainless steel sheet, are 0.1 mm thick, and have 0.5 to
2 mm pores.
In this structure, bubbles formed by the LSIs float
and are turned by the bubble guides 80 and guided to the
bubble traps 81 where they are cooled to be liquefied by
the heat exchange pipes 79, whereby the cooling
capability is improved.
Figure 16 illustrates a fourth liquid cooling
structure of the present invention. The cooling case 95
has a double structure in which a circuit board 94 with
a plurality of LSIs 93 mounted thereon are immersed in a
coolant 96 and heat pipes 92 are horizontally arranged
between the LSIs 93. An end of the heat pipes 92
protrudes from an inner wall of the case 95 toward a
space 97 between the double walls, where the heat pipes
92 are contact with heat radiator 94 cooled by cooling
water. A preferable cooling media in the heat pipes 92
is methanol.
Fig. 17 is a schematic, elevational view, partially
in cross-section, of a module 100 in accordance with a
fifth liquid cooling s~ructure, suitable for
incorporation in a system 130. The module 100
comprises a main cooling chamber 120 containing a liquid
coolant 105 therewithin, the latter being circulated by
a pump 18 to pass from the chamber 100 through outlet
122b, as shown by the arrow therein, into an external
heat exchanger 121, and thereafter through a return
conduit 131 to a gas extractor 124 in which any released
gas present in the coolant 105 is extracted. Pump 118
pressurizes the liquid coolant from the gas
extractor 125 and returns the liquid coolant through
inlet 122a to the main cooling chamber 120. The heat
exchangers 106 with the module 100 supplement the
primary reliquification function performed by the
external heat exchanger 121 and significantly, correct
for a deficiency which otherwise would exist if only the
- 21 - 2 ~ - f ~
external heat exchanger 121 were employed, as occurs in
prior art systems. Particularly, the system 130
provides forced circulation of the liquid coolant 105 by
virtue of utilizing the pump 118; thus, within the
module 100, the required circulation of the coolant 105
over the IC chips lQ3 mounted on the circuit boards 104
does not depend on convection currents. However, if
sufficient heat dissipation occurs with resultant,
relatively high levels of evaporation of the
coolant 105, a thermal difference may develop between
the upstream and downstream regions of the circuit
boards 104 and the correspondingly positioned IC
chips 103 mounted thereon, i.e., regions with reference
to the flow of the coolant 105 from the inlet 122 and
through the internal chamber of the module 100 and
across the IC chips 103 to the outlet 122b.
Figure 18 illustrates a system in which a plurality
of liquid cooling modules are installed. Each cooling
module 35 is similar to that shown in Fig. 16 and is
covered with a box 32 of aluminum or other various
materials. The modules 31 are arranged three
dimensionally.
