U.S. patent application number 11/573002 was filed with the patent office on 2009-01-08 for cooling system for electronic substrates.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS, N.V.. Invention is credited to Jean-Christophe Baret, Michel Marcel Jose Decre, Clemens J.M. Lasance, Celine Nicole, Menno Willem Jose Prins.
Application Number | 20090008064 11/573002 |
Document ID | / |
Family ID | 35457651 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090008064 |
Kind Code |
A1 |
Nicole; Celine ; et
al. |
January 8, 2009 |
Cooling System for Electronic Substrates
Abstract
The present invention relates to a cooling system for an
electronic substrate comprising a heat transfer fluid (4, 10). The
heat transfer fluid (4, 10) is arranged to flow along a path (5,
11, 12) by capillary force.
Inventors: |
Nicole; Celine; (Eindhoven,
NL) ; Lasance; Clemens J.M.; (Eindhoven, NL) ;
Prins; Menno Willem Jose; (Eindhoven, NL) ; Baret;
Jean-Christophe; (Eindhoven, NL) ; Decre; Michel
Marcel Jose; (Eindhoven, NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS,
N.V.
EINDHOVEN
NL
|
Family ID: |
35457651 |
Appl. No.: |
11/573002 |
Filed: |
July 21, 2005 |
PCT Filed: |
July 21, 2005 |
PCT NO: |
PCT/IB05/52461 |
371 Date: |
January 31, 2007 |
Current U.S.
Class: |
165/104.28 |
Current CPC
Class: |
F28D 15/04 20130101;
F28D 15/06 20130101; F28F 13/16 20130101 |
Class at
Publication: |
165/104.28 |
International
Class: |
F28D 15/00 20060101
F28D015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 2004 |
EP |
04103775.5 |
Claims
1. A cooling system for an electronic substrate comprising a heat
transfer fluid (4, 10), characterized in that the heat transfer
fluid (4, 10) is arranged to flow along a path (5, 11, 12) by
capillary force.
2. A cooling system according to claim 1, wherein the system
further comprises an electrode (6) arranged for applying a voltage
to the heat transfer fluid (4, 10) for changing the surface tension
of the heat transfer fluid (4, 10).
3. A cooling system according to claim 1, wherein at least one
micro channel (5, 11, 12) is connected to a heat transfer fluid
reservoir (3).
4. A cooling system according to claim 2, wherein the electrode (6)
is situated outside the heated region (7).
5. A cooling system according to claim 1, wherein the fluid system
is a closed system.
6. A cooling system according to claim 1, wherein the cooling
system comprises two immiscible fluids (9, 10).
7. A cooling system according to claim 1, wherein the system is
arranged such that the fluid (4, 10) is actuated in a
bi-directional manner.
8. A cooling system according to claim 1, wherein the system
comprises two sets of micro channels (11) arranged in a counter
flow relationship.
9. A method for cooling an electronic substrate using a system
according to claim 1, by applying a voltage to micro channels (5,
11, 12) comprising a heat transfer fluid (4, 10) such that the
surface tension of the heat transfer fluid (4, 10) is changed.
10. A method according to claim 9, wherein a pulsating voltage is
applied.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a cooling system for an
electronic substrate comprising a heat transfer fluid.
BACKGROUND OF THE INVENTION
[0002] Control of component temperature and temperature gradient is
essential for successful operation and reliability of electronic
products, such as electronic circuits. As consumer products become
smaller with a continuing need for higher heat removal, thermal
management will play a crucial role. Thus, novel cooling methods
are required to face the demands of the market. The trend towards
smaller components imposes an increase in power density, requiring
more sophisticated cooling methods than pure radiation or
convective cooling with, for instance, fans. Important aspects as
noise and reliability limit the use of fan cooling. Therefore,
there is a need for advanced cooling methods.
[0003] Liquid cooling is a method already used in portable
computers, i.e. laptops. A particular application that requires
cooling is solid-state lighting. White light or colour controlled
solid-state lighting requires the use of a multi-chip module
wherein several LEDs are placed very close to each other in order
to define an optical point source. This design causes high power
densities in the silicon submount, in the order 100 W/cm.sup.2.
Liquids are significantly better heat transfer media than air,
because their thermal conductivity and thermal capacity are higher
(10 to 1000 times better). Forced convection micro channels liquid
cooling has proven to be highly efficient in the industrial world
(metallic micro channel structures) and in industrial research
(silicon micro channel device). The main inconvenience of this
technique is that liquid is pumped through the channels by a pump,
which makes it less suited for miniaturized and integrated consumer
products and electronics.
[0004] O'Connor et al disclose an example of a cooling system with
a pump in US 2002/0039280 A1. The invention by O'Connor et al.
concerns a micro fluidic heat exchange system for cooling an
electronic component internal to a device, such as a computer. The
heat exchange device is substantially in interfacial contact with a
heat-generating electronic component and supplies an internal
operating fluid to a heat exchange zone. Operating fluids flows
into the heat exchange zone at a first fluid temperature that is
lower than the component temperature, and then exits the zone at a
second fluid temperature higher than the first fluid
temperature.
SUMMARY OF THE INVENTION
[0005] It is an object of the present invention to provide a
cooling system for electronic components that do not utilize a
pump. This is achieved with a cooling system according to claim 1.
Preferred embodiments are indicated in the dependent claims.
[0006] According to the present invention, a cooling system for an
electronic substrate comprises a heat transfer fluid, wherein the
heat transfer fluid is arranged to flow along a path by capillary
force. Thus, an advantage is that this cooling system does not have
moving parts, apart from the heat transfer fluid, and the power
consumption is relatively low. This increases the reliability,
flexibility and makes it a robust construction compared to systems
that require a mechanical pump or piezo-actuated pumps.
[0007] The essential feature of the present invention is the use of
a technique to move fluids at a micro scale in order to achieve
integrated and compact cooling of an electronic component, for
instance a chip submount.
[0008] To facilitate understanding of the technique an example is
used where it is desired to dissipate 100 W/cm.sup.2 with a
temperature drop of 90.degree. C. An estimation, as to what the
velocity of water should be through a pair of parallel plates, will
follow. The heat flux of the system is given by
Q = k D .DELTA. T N u ##EQU00001##
where k is the thermal conductivity of water (0.628
Wm.sup.-1K.sup.-1), D is the hydraulic diameter (10.sup.-3 m) and
Nu is the Nusselt number, which is given by
Nu=CRe.sup.mPr.sup.nK
where Re is the Reynolds number and Pr is the Prandtl number given
by
Re = .rho. v av D .mu. ##EQU00002## and ##EQU00002.2## Pr = C p
.mu. k . ##EQU00002.3##
respectively. C=1.85, m=1/3, n=1/3, K=0.368, Cp=4178
Jkg.sup.-1K.sup.-1, .rho.=995 kg/m.sup.3, and .mu.=65110.sup.-6
kgm.sup.-1 s.sup.-1. v.sub.av is estimated to be in the order of 1
m/s, which gives, depending on the cross-sectional area, a
volumetric flow in the order of 10 to 100 .mu.l/s.
[0009] In a preferred embodiment of the present invention the
system further comprises an electrode arranged for applying a
voltage to the heat transfer fluid for changing the surface tension
of the heat transfer fluid. The electrowetting principle allows
moving several hundreds of .mu.l/s in a sequence of droplets.
Stated in a slightly different way, the energy transport rate P
(J/s) is given by
P = .DELTA. V .DELTA. t .rho. C p .DELTA. T ##EQU00003##
where .DELTA.V/.DELTA.t is the volumetric flow rate of fluid
passing the heat source. For P=30 W and .DELTA.T=50.degree. C. the
flow rate will be 140 .mu.l/s. Electrowetting involves a change of
surface tension by electrostatic charges, resulting in a movement
of a fluid/fluid meniscus. The movement can be provided in at least
two different ways, namely (i) by actuating a fluid/fluid meniscus
in one or several channels or slits or (ii) by transporting
droplets over a surface.
[0010] The maximum meniscus speed demonstrated by electrowetting is
0.1 m/s or a little higher. The maximum pressure modulation that
can be generated by electrowetting is given by 2.DELTA..gamma./R,
where .DELTA..gamma. is the change of surface tension and R the
curvature of the meniscus. .DELTA..gamma. can be of the order of
0.1 N/m. For a curvature of 100 .mu.m, the maximum pressure is
about 2000 Pa.
[0011] To ensure orientational freedom of an electro-wetting
device, the gravitational pressure drop in the system has to be
lower than the maximum modulation of electrowetting pressure. The
gravitational pressure drop equals .DELTA..rho.gL, with L the
projected length. Maximum orientational freedom can be ensured by
using fluids with similar mass density, by minimizing the column
heights of one of the fluids, and by using balanced geometries.
[0012] It is known that flow velocities of 0.1 n/s can be reached
in channels with a length of 2 cm and a diameter of 300 .mu.m. This
gives a volumetric flow rate of 7 .mu.l/s per micro channel. In
other words, a volumetric flow rate of 140 .mu.l/s can be achieved
in an actuated-actuated system with about 20 micro channels.
[0013] Preferably, at least one micro channel is connected to a
heat transfer fluid reservoir. With fluid reservoirs disposed
around the heat source and proper heat sinking of these reservoirs,
heat is efficiently removed.
[0014] In an embodiment of the present invention the electrode is
situated outside the heated region. The actuated-actuated flow
generates a transport of energy in the device, from a concentrated
heating region to a larger cooling region. Preferably the actuated
electrodes are situated outside the heated region, because this
will improve the lifetime of the device. Further, the fluid system
is preferably a closed system. This will decrease the risk for
evaporation and leakage of fluids.
[0015] In a further embodiment the cooling system comprises two
immiscible fluids with different electrical conductivity, for
instance, air/water, water/oil, etc. Actuated actuation requires
that an electrode is present in the vicinity of the fluid/fluid
meniscus. The electrode generally consists of a material with
metallic conductance, coated with an insulating layer. The
insulating coating can for example be 1 .mu.m-10 .mu.m of parylene,
or 10 nm-1 .mu.m of a fluoropolymer layer, or a combination of such
layers.
[0016] The different micro channels can be hydrostatically
separated from each other or they can join in certain junctions or
channels (e.g. common channels or reservoirs). Care should be taken
to ensure integrity of the menisci in the micro channels, e.g. to
avoid fluid of one type to enter a reservoir for fluid of the
second type.
[0017] In yet another embodiment the system is arranged such that
the fluid is actuated in a bi-directional manner. The fluid flow
can be uni-directional or bi-directional. Preferably, the fluid is
actuated in a bi-directional manner, so that fluid contact to the
heated region can be limited to only one type of fluid. This will
improve the lifetime of the device. A bi-directional flow is
achieved by applying a pulsating voltage that will result in a
reciprocating flow of heat transfer fluid.
[0018] In order to reduce the temperature gradient across the
device to be cooled the system preferably comprises two sets of
micro channels arranged in a counter flow relationship.
SHORT DESCRIPTION OF ACCOMPANYING FIGURES
[0019] The present invention will now be further described with
reference to the figures showing different embodiments of the
invention.
[0020] FIG. 1 shows an example of a multi chip module for a solid
state lighting application.
[0021] FIG. 2 shows an example of droplet flow.
[0022] FIG. 3 shows one micro channel for fluid transport between a
hot and a cold region.
[0023] FIGS. 4a and 4b show a cooling unit with several micro
channels connected to a reservoir.
[0024] FIG. 5a shows a system according to the invention with ring
geometry.
[0025] FIG. 5b shows an enlarged view of a portion of the system in
FIG. 5a.
[0026] FIG. 6a shows a system according to the invention with a
counterflow arrangement.
[0027] FIG. 6b shows an enlarged view of a portion of the system in
FIG. 6a.
[0028] FIG. 7a shows a radial system according to the
invention.
[0029] FIG. 7b shows an enlarged view of a portion of the system in
FIG. 7a.
[0030] FIG. 8 shows a radial system with channels having
non-continuous width.
DESCRIPTION OF A PREFERRED EMBODIMENT
[0031] FIG. 1 shows a general view of a multi chip module with 9
LEDs 1 (light emitting diodes). White light or colour controlled
solid state lighting requires the need of a multi chip module where
several LEDs 1 are placed very close to each other in order to
define an optical point source. This design causes high power
densities on the silicon submount 2. By integrating an active
liquid cooling droplet based actuated pump in the silicon submount
2, the required cooling can be achieved. In the example shown in
FIG. 1 the size of the silicon submount is 5 mm.times.6 mm and the
submount 2 is arranged adjacent to reservoirs/collectors 3. The
reservoirs/collectors 3 comprises a heat transfer fluid for removal
of the heat energy produced by the LEDs 1.
[0032] In FIG. 2 the principle of the droplet transport is shown.
Heat transfer fluid droplets 4 are flowing in a channel 5 from a
reservoir/collector 3. The droplets are made to move by a voltage
applied on the fluid via electrodes 6. In this way heat will now be
transferred from the silicon submount 2 to the droplets 4. The
droplets 4 will subsequently be cooled in a reservoir/collector 3.
The energy absorbed by the reservoirs/collectors 3 will
subsequently be transferred from the reservoirs/collectors 3 with
the help of a separate cooling system (not shown). The heated chip
is part of a larger device consisting for example of printed
circuit board material, a moulded-interconnect-device (MID), a
glass, a metal device, etc. Each of these materials can contain
electrodes and channel structures. A hole can be provided so that
the silicon chip can be exposed to the heat transfer fluid as well
as being electrically interconnected.
[0033] In one embodiment of the present invention the heat transfer
fluid channel 5 is filled with two fluids. FIG. 3 is a diagrammatic
drawing of one micro channel 5 for fluid transport between a hot
region 7 and a cold region 8. The electrodes are not drawn. A plug
9 of one of the fluids is used to "push" the other fluid 10 that
acts primarily as the heat transfer fluid. The electrodes
preferably applies pulsating voltage to the plug 9 such that the
plug 9, and consequently the heat transfer fluid 10, is actuated in
a bi-directional manner, i.e. a reciprocating flow of the heat
transfer fluid, in order to avoid the plug entering the hot region
7. This will improve the lifetime of the device. For this to
function, a requirement is that the two fluids are immiscible
fluids, for instance a plug of oil in a surplus of water.
[0034] FIGS. 4a and 4b illustrates a multi channel system
comprising a heat transfer fluid reservoir 3. In FIG. 4a no voltage
is applied and all heat transfer fluid remains in the reservoir 3.
In FIG. 4b a voltage has been applied and the heat transfer fluid
has started to flow in the micro channels 11. When the applied
voltage is turned off, the heat transfer fluid returns to the
reservoir 3.
[0035] An example of channels 11 made in a "loop" shape can be seen
in FIGS. 5a and 5b, the latter being an enlarged view of a portion
of the former. The embodiment comprises two reservoirs 3 as heat
sinks for the heat transfer fluid.
[0036] FIG. 6a shows a cooling system according to the invention
comprising two reservoirs 3 of heat transfer fluid arranged with
separate sets of channels 11 arranged in a counter current
relationship. This arrangement helps in reducing the temperature
gradient across the silicon chip and consequently the lifetime of
the silicon chip is increased due to a more even heat load. FIG. 6b
is an enlarged view of a portion of the embodiment shown in FIG.
6a.
[0037] In FIGS. 7a and 7b, 7b being an enlarged view of a portion
of the embodiment in FIG. 7a, is shown an embodiment according to
the present invention with radial cooling and the heat source in
the centre. Thus, the heat transfer fluid travels from the
reservoir 3 in the micro channels 11 towards the centre. Outside
the reservoir a heat sink is connected (not shown).
[0038] FIG. 8 shows yet another embodiment of a system according to
the present invention. The system comprises two reservoirs 3
interconnected with channels 12. The channel width varies between
the two reservoirs 3 for optimising the capillary flow of the heat
transfer fluid.
[0039] In order to take advantage of the invention the filling of
the liquid should be done at the latest stage by a hole/channel in
the device. Preferably all micro channels are filled
simultaneously, e.g. via filling channels that run perpendicular to
the micro channels. Also, the whole liquid device should be
entirely sealed after filling. A pressure damper could be included
to avoid pressure built up in the set-up. Further, a flexible
reservoir can be included (e.g. with a membrane, or a pocket
containing an air bubble) to allow expansion and contraction of
fluids.
[0040] The person skilled in the art realizes that the present
invention by no means is limited to the embodiments described
above. On the contrary, many modifications and variations are
possible within the scope of the appended claims. For example, the
shape of the channel systems is not limited to the embodiments in
the appended figures.
* * * * *