U.S. patent application number 13/049707 was filed with the patent office on 2012-09-20 for low-profile heat sink with fine-structure patterned fins for increased heat transfer.
Invention is credited to Ho-Shang Lee.
Application Number | 20120234519 13/049707 |
Document ID | / |
Family ID | 46827532 |
Filed Date | 2012-09-20 |
United States Patent
Application |
20120234519 |
Kind Code |
A1 |
Lee; Ho-Shang |
September 20, 2012 |
Low-Profile Heat Sink with Fine-Structure Patterned Fins for
Increased Heat Transfer
Abstract
In one embodiment, a device for transferring heat comprises a
base member and a first array of pin fins supported by the base
member, the pin fins having an aspect ratio of not less than about
10, and the pin fins being not more than about 0.3 mm in equivalent
diameter and not more than about 3 mm in length, either one or both
of the base member and pin fins comprising a metallic or
semiconductor material. To form this device, a substrate is
provided. A pattern is formed on the substrate, the pattern having
holes therein or in the form of dots with cross-sectional
dimensions of not more than about 0.3 mm. Pin fins supported by the
substrate are formed, where the pin fins have an aspect ratio of
not less than about 10, and not more than about 0.3 mm in
equivalent diameter and not more than about 3 mm in length. Either
one or both of the base member and pin fins comprise a metallic or
semiconductor material. The pattern is then removed.
Inventors: |
Lee; Ho-Shang; (El Sobrante,
CA) |
Family ID: |
46827532 |
Appl. No.: |
13/049707 |
Filed: |
March 16, 2011 |
Current U.S.
Class: |
165/121 ;
165/185; 29/890.03 |
Current CPC
Class: |
B23P 15/26 20130101;
Y10T 29/4935 20150115; H01L 2924/0002 20130101; H01L 2924/0002
20130101; H01L 23/3677 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
165/121 ;
165/185; 29/890.03 |
International
Class: |
F28F 13/00 20060101
F28F013/00; B21D 53/02 20060101 B21D053/02; F28F 7/00 20060101
F28F007/00 |
Claims
1. A device for transferring heat comprising: a base member; and a
first array of pin fins supported by said base member, said pin
fins having an aspect ratio of not less than about 10, and said pin
fins being not more than about 0.3 mm in equivalent diameter and
not more than about 3 mm in length, either one or both of said base
member and pin fins comprising a metallic or semiconductor
material.
2. The device of claim 1, said pin fins having an aspect ratio of
not less than about 20.
3. The device of claim 1, said pin fins having a perimeter P,
wherein said equivalent diameter of the pin fins is P/.pi..
4. The device of claim 1, said pin fins having a length or lengths
less than about 1 mm.
5. The device of claim 1, said pin fins having an equivalent
diameter less than about 0.1 mm.
6. The device of claim 1, said pin fins created by a Deep Reactive
Ion Etching process.
7. The device of claim 1, further comprising a second array of pin
fins supported by the base member.
8. The device of claim 1, the base member comprising a layer of
silicon material, said device further comprising a second member
bonded to the base member for conducting heat between the pin fins
and the second member through the base member.
9. The device of claim 1, wherein said base member defines one or
more holes therein for enhancing air flow turbulence and heat
convection to transfer heat.
10. The device of claim 1, wherein said first array of pin fins
arranged in sub-arrays with gutters between the sub-arrays for
enhancing heat convection to transfer heat.
11. The device of claim 1, further comprising a fan for generating
air flow in spacings between the pin fins.
12. A device for transferring heat comprising: a heat spreader; and
a plurality of heat transfer elements supported by and thermally
connected to said on said heat spreader, each element comprising a
base member, and an array of pin fins supported by said base
member, said pin fins having an aspect ratio of not less than about
10 and said pin fins being not more than about 0.3 mm in equivalent
diameter and not more than about 3 mm in length, said base members
and pin fins comprising a metallic or semiconductor material.
13. The device of claim 12, said base members comprising plates
with edges, said plurality of heat transfer elements supported by
and thermally connected to said on said heat spreader through the
edges of said base members of the elements.
14. A device for transferring heat comprising: a set of fins
arranged in a radial pattern; and a plurality of heat transfer
elements supported on said fins, each element comprising a base
member, and an array of pin fins supported by said base member,
said pin fins having an aspect ratio of not less than about 10, and
said pin fins being not more than about 0.3 mm in equivalent
diameter and not more than about 3 mm in length, said base members
and pin fins comprising a metallic or semiconductor material.
15. A method for making a device that dissipates heat in air,
comprising: providing a substrate; forming a pattern on the
substrate, said pattern having holes therein or in the form of dots
with cross-sectional dimensions of not more than about 0.3 mm; and
causing pin fins supported by the substrate to be formed, said pin
fins having an aspect ratio of not less than about 10, and said pin
fins being not more than about 0.3 mm in equivalent diameter and
not more than about 3 mm in length, said base member and pin fins
comprising a metallic or semiconductor material; and removing said
pattern.
16. The method of claim 15, wherein said pattern is in the form of
dots and formed by means of a photolithographic process, and said
pin fins are formed by means of an etching process.
17. The method of claim 15, wherein said pin fins are formed by
means of a Deep Reactive Ion etching process.
18. The method of claim 15, wherein said pattern have holes therein
and formed by means of an UV photolithographic or nano-imprinting
process, said causing including depositing growing seeds in said
holes, and growing nanotubes or nanowires on top of the seeds.
19. The method of claim 18, wherein said nanotubes or nanowires
have diameters not more than 1 micron.
20. A method for transferring heat, comprising: providing a device
for transferring heat which comprises: a base member; and a first
array of pin fins supported by said base member, said pin fins
having an aspect ratio of not less than about 10, and said pin fins
being not more than about 0.3 mm in equivalent diameter and not
more than about 3 mm in length, said base member and pin fins
comprising a metallic or semiconductor material; locating said base
member relative to an object to transfer heat between the pin fins
and the object and so that said pin fins are in contact with a
gaseous environment to enable heat transfer between the pin fins
and the gaseous environment.
21. The method of claim 20, wherein heat is transferred from the
object to the pin fins, and said pin fins are in contact with
air.
22. The method of claim 20, wherein heat is transferred from the
pin fins to the object, and said pin fins are in contact with
air.
23. The method of claim 20, wherein heat is by means of conduction
or radiation or both.
Description
BACKGROUND
[0001] Electronic components or devices generate heat locally. It
is desirable to extract or remove this heat, to bring down the
temperature of the components or devices, in order to increase
their performance and enhance component reliability. The heat
generated by electronic components can be transported to other
places or locations through the use of thermally conductive
materials or devices such as heat pipes. However, eventually the
heat has to be dumped to the surrounding fluid medium, via some
form of heat sink. The efficiency of the heat transfer to the
surrounding medium (for simplicity, air is used as the example in
the description of the present patent application; however, other
fluid media such as water or other liquids and gases are also
applicable) by the heat sink depends on the geometry of the heat
sink, the contact surface with the air (or other fluid medium), the
flow field around the heat sink, and the material properties of
air. The transfer of heat from the heat sink to the air is usually
one of the major thermal resistances of the full thermal
system.
[0002] The heat transfer between a heat sink and air is governed
by:
Q=h.sub.t A(T.sub.HS-T.infin.) Equation I
where Q is the amount of heat transferred to the air, h.sub.t is
the average heat transfer coefficient, A is the contact surface of
the heat sink with the medium, T.sub.HS is the average heat sink
temperature, and T.infin. is the air temperature in free
stream.
[0003] Many types of geometries for heat sinks have been introduced
for forced-air and natural convection systems, respectively.
However, no matter how the geometry is arranged, the contact
surface with the air medium is either limited, or it hinders the
flow field such .sub.that the heat transfer coefficient is herein
reduced. The length of the fins of a typical heat sink for
electronic components is typically on the order of ten times (or
more) of the fin diameter. Since the fins are typically made using
an extrusion or forging process, their diameters cannot be too
small, since very thin fins may break in the process. Hence the fin
length is typically on the order of tens of millimeters, resulting
in bulky heat sinks. The present invention introduces
fine-structure designs and their manufacturing methods to heat
sinks, to tremendously increase the contact surface with air, to
increase heat flux density across the heat sink, to reduce the drag
force on the flow of .sub.the fluid medium around the heat sink,
and at the same time to keep the heat sink compact, with a low
profile. As a result of their low profile, multiple fine-structure
patterned (herein FSP) heat sinks of the present invention can be
stacked up for greater overall heat transfer, while retaining
compact dimensions. The present invention creates fine-structure
patterned fins that protrude from the base surfaces of a heat
sink.
[0004] Research has also been performed on the use of pin fin
geometry in heat sinks immersed in liquids such as water. Such heat
sinks, however, have small aspect ratios since water cools down the
pin fins rapidly. For example, see J. J. Wei, "Effects of Fin
Geometry on Boiling Heat Transfer from Silicon Chips with
Micro-Pin-Fins Immersed in FC-72," International Journal of Heat
and Mass Transfer, 46 (2003) 4059-4070. Such heat sinks are not
suitable for use for cooling in air which is a poor heat
conductor.
SUMMARY
[0005] One embodiment of the invention is directed to a device for
transferring heat comprises a base member and a first array of pin
fins supported by the base member, the pin fins having an aspect
ratio of not less than about 10, and the pin fins being not more
than about 0.3 mm in equivalent diameter and not more than about 3
mm in length, either one or both of the base member and pin fins
comprising a metallic or semiconductor material.
[0006] Another embodiment of the invention is directed to a device
for transferring heat comprises a heat spreader; and a plurality of
heat transfer elements supported by and thermally connected to the
heat spreader, each element comprising a base member, and an array
of pin fins supported by the base member, the pin fins having an
aspect ratio of not less than about 10 and the pin fins being not
more than about 0.3 mm in equivalent diameter and not more than
about 3 mm in length, the base members and/or pin fins comprising a
metallic or semiconductor material.
[0007] Yet another embodiment of the invention is directed to a
device for transferring heat comprises a set of fins arranged in a
radial pattern; and a plurality of heat transfer elements supported
on the fins, each element comprising a base member, and an array of
pin fins supported by the base member, the pin fins having an
aspect ratio of not less than about 10, and the pin fins being not
more than about 0.3 mm in equivalent diameter and not more than
about 3 mm in length, the base members and/or pin fins comprising a
metallic or semiconductor material.
[0008] According to yet another embodiment of the invention, a
device that dissipates heat in air is made as follows. A substrate
is provided. A pattern is formed on the substrate, the pattern
having holes therein or in the form of dots with cross-sectional
dimensions of not more than about 0.3 mm. Pin fins supported by the
substrate are formed, where the pin fins have an aspect ratio of
not less than about 1.0, and not more than about 0.3 mm in
equivalent diameter and not more than about 3 mm in length. Either
one or both of the base member and pin fins comprise a metallic or
semiconductor material. The pattern is then removed.
[0009] One more embodiment of the invention is directed to a method
for transferring heat. The method comprises providing a device for
transferring heat which comprises a base member and a first array
of pin fins supported by the base member, the pin fins having an
aspect ratio of not less than about 10, and the pin fins being not
more than about 0.3 mm in equivalent diameter and not more than
about 3 mm in length, the base member and/or pin fins comprising a
metallic or semiconductor material. The method includes locating
the base member relative to an object to transfer heat between the
pin fins and the object and so that the pin fins are in contact
with a gaseous environment to enable heat transfer between the pin
fins and the gaseous environment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1A exemplifies a conventional pin-fin heat sink of the
prior art, made of a thermally-conductive metal such as Aluminum
Alloy 6063.
[0011] FIG. 1B is a rear view of the conventional pin-fin heat sink
of FIG. 1A.
[0012] FIG. 1C illustrates a conventional radial-type prior art
heat sink.
[0013] FIG. 2 illustrates another prior art heat transfer mechanism
that transports heat via a heat pipe to the finned heat sinks,
where the heat is dissipated to air.
[0014] FIGS. 3A, 3B, and 3C illustrate a representative geometrical
model of a heat sink of one embodiment of the present invention,
whose base surface is patterned with fine-scale pin fins.
[0015] FIGS. 3D, 3E, and 3F illustrate three kinds of pin fin
cross-sections that are intended to increase the contact surface
with air in embodiments of the present invention.
[0016] FIG. 3G illustrates one kind of pin fin cross-section to
introduce the definition of equivalent diameter.
[0017] FIG. 3H shows a perspective view of an FSP heat sink of an
embodiment of the present invention.
[0018] FIG. 3I presents a single pin fin from an FSP heat sink of
an embodiment of the present invention, for the purpose of
depicting its thermal conduction properties.
[0019] FIG. 4 illustrates a semiconductor fabrication process flow
for making a micro-structure pin fin heat sink of an embodiment of
the present invention.
[0020] FIG. 5 exemplifies a process for fabricating nano-scale
carbon tubes as pin fins on a thermally conductive substrate.
[0021] FIG. 6 shows an FSP heat sink of an embodiment of the
present invention, mounted to a heat spreader for transferring heat
to air.
[0022] FIG. 7A shows multiple FSP heat sinks inserted or attached
to a U-shaped heat spreader to multiply the heat dissipation
capacity to air.
[0023] FIG. 7B shows composite metal layers deposited on the back
side of an FSP heat sink for purposes of metal bonding.
[0024] FIG. 7C shows a highly thermally conductive material that is
metallically bonded to an FSP heat sink.
[0025] FIG. 7D illustrates another embodiment of the present
invention, in which an assembly of multiple FSP heat sinks is used
to multiply the heat dissipation capacity of the system to air.
[0026] FIG. 8 shows a fan that has been added to the heat sink bank
of FIG. 7A to further increase heat convection.
[0027] FIG. 9A illustrates the momentum and thermal boundary
layers, respectively, of a natural convection flow along a smooth
vertical wall.
[0028] FIG. 9 B shows the natural convection flow between two FSP
heat sinks.
[0029] FIG. 10 shows the forced convection flow between two FSP
heat sinks.
[0030] FIGS. 11A and 11B show an embodiment of the present
invention in which the base of the heat sink has perforated holes
for increasing flow velocity perpendicular to the base, in order to
enhance the heat transfer.
[0031] FIGS. 12A and 12B show an embodiment of the present
invention in which heat gutters are created to facilitate carrying
heat away from the pin fins.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] At least some of the embodiments herein include a compact,
low-profile heat sink with fine-structure patterned pin fins for
increased heat transfer. These embodiments introduce fine-structure
designs and their manufacturing methods to the field of heat sinks,
to tremendously increase the contact surface with air, to increase
heat flux density across the heat sink, to reduce the drag force on
the flow of the fluid medium around the heat sink, and at the same
time to keep the heat sink compact, with a low profile. The
embodiments of the present invention have applications in the
removal of heat from electronic components and devices, as well as
any application where efficient heat transfer is desired. The
fine-structure patterned (FSP) pin fins retain the typical aspect
ratio of conventional pin fin heat sinks of the prior art, and also
have a similar relationship between pin fin diameter and pin fin
spacing. However, with much smaller pin fin diameter, the FSP pin
fin heat sink of the present invention has many more pin fins, when
compared to a conventional heat sink of similar base area, and the
FSP pin fins are of greatly reduced height. This results in the FSP
pin fin heat sink achieving heat transfer that is comparable to
that of a conventional heat sink of similar base area, but with the
FSP pin fins having a greatly reduced height dimension.
Alternatively, an FSP pin fin heat sink can achieve much greater
heat transfer, when compared to a conventional heat sink of similar
volume. Multiple FSP pin fin heat sinks can be affixed to a common
heat spreader, and it is also possible to fabricate FSP pin fin
heat sinks with pin fins on both sides of a common base plate.
[0033] In some embodiments, a method is described for using
semiconductor processing techniques to fabricate micro-structure
PSP fin pin heat sinks, from a variety of thermally conductive
substrate materials, including metals and semiconductor material.
An alternative method is described for fabricating nano-scale
carbon nano-tubes, or metallic nano-wires as pin fins on a
thermally conductive substrate.
[0034] FIG. 1A shows a perspective view of a conventional prior art
heat sink that has pin fins protruding out of a metal base. FIG. 1B
is a rear view of the heat sink of FIG. 1A. The diameter or
thickness of the pin fins is in the range of a few millimeters. The
aspect ratio S of a pin is defined as the length L of a pin divided
by its characteristic diameter D.sub.e. The aspect ratio for the
pin fins of a typical prior art heat sink with large diameters (of
the order of a millimeter or more) obtained by means of forging or
extrusion is on the order of ten. Therefore the typical length of
the pin fins is in the range of tens of millimeters, resulting in
conventional heat sinks being bulky. Especially for natural
convection systems that need larger contact surface with air, heat
sinks typically dominate the overall volume of a power generating
electronic device. For example, about 75% of the electrical power
consumed by Light Emitting Diode (LED) chips becomes a localized
thermal heat source. It is critical for LEDs to reduce temperature,
in order to ensure their performance and reliability. Heat sinks
are usually used to dissipate heat from the heat source to air.
[0035] FIG. 1C shows another conventional prior art heat sink,
using a radial arrangement of fins. The width and length of the
pins are on the order of tens of millimeters.
[0036] FIG. 2 illustrates another prior art heat transfer mechanism
that transports heat via one or more heat pipes to the finned heat
sinks where the heat is dissipated to air.
[0037] FIGS. 3A (front view) and 3B (side view) show an embodiment
of the present invention, with a large number of pin fins (301)
protruding out of a thermally conductive plate or base (303).
Either one or both of the pin fins (301) plate (or base) (303) may
comprise a metallic or semiconductor material. The length of each
pin fin is L. FIG. 3C shows another embodiment of the present
invention in which pin fins protrude from both the top and bottom
surfaces of the thermally conductive base plate.
[0038] FIGS. 3D to 3F illustrate multiple embodiments of the pin
fins of the present invention, with a variety of cross-sections.
Non-circular cross-sections are implemented to increase the area of
the contact surface with air. The pitch between pin fins is shown
as dimension b. P denotes the length or dimension of the perimeter
of a cross-section of the pin fin. For non-circular pin fin, the
equivalent diameter D.sub.e is defined as P divided by .pi.. Thus,
pin fins with differing cross-section shapes, but with the same
perimeter dimension P, will have the same equivalent diameter
D.sub.e, and will also have the same equivalent diameter D.sub.e as
a pin fin with a circular cross section that has a circumference
dimension equal to P. Note that for non-circular cross-section
shapes, the effective diameter D.sub.e will be larger than that of
a minimal-size circle that just barely encloses the cross-section
shape, as depicted in FIG. 3G.
[0039] The lattice configurations shown in FIGS. 3D to 3F are
square for illustrative purpose. Other lattice configurations such
as triangles and rectangles or other polygons are within the scope
of the present invention.
[0040] Suppose the base plate (303) is a square with dimension W
for each side.
Define M = total surface area contacting air surface area of base
plate = ( W b ) 2 .pi. D e L + W 2 W 2 = 1 + .pi. S ( b D e ) 2
where S = aspect ratio = L D e .apprxeq. .pi. S ( b D e ) 2 for
fins of large S , Equation ( 2 ) ##EQU00001##
As an example, if
b D e = 3 2 ##EQU00002##
and S=15, then M=22.
[0041] Equation (2) shows that for two heat sink designs that are
required to have the same air contact surface per unit base area,
the required length of the pin fin (L) is inversely proportional to
the equivalent diameter of the pin fin (D.sub.e), if the lattice
ratio b/D.sub.e is fixed. For example, if the same contact surface
with air is desired for both a conventional prior-art heat sink and
the fine-structure patterned heat sink of the present invention,
both of them having the same base size, the same pin fin aspect
ratio of 15, and the same lattice ratio, then the length of the
conventional pin fin of diameter 2 mm is 30 mm, but that of a pin
fin having a diameter of 50 microns from a fine-structure pattern
(FPS) heat sink is only 0.75 mm. Therefore the height of an FPS
heat sink in the embodiment of FIGS. 3A-3H of the present invention
is tremendously reduced without sacrificing the amount of contact
surface with the surrounding air.
[0042] The fine-structure pin fins of the present invention can be
created by either etching or chemical growth processes. Here two
methods are introduced as examples, to illustrate processes for
making fine-structure pin fins from a piece of thermally-conductive
plate as a base.
Semiconductor Processes for Making Fine-Structure Patterned Pin
Fins:
[0043] As illustrated in FIG. 4, a highly thermally-conductive
material (401) such as silicon, having a thermal conductivity of
about 149 w/m.degree. C., is selected as a substrate, as indicated
in Step 1. The thickness of silicon or poly-silicon is of the order
of a few millimeters. On top of the substrate is formed a silicon
dioxide layer (408) of a thickness of a few micrometers grown by
thermal oxidation or chemical vapor deposition. Photoresist (403)
is spin-coated onto one surface of the silicon wafer as indicated
in Step 2. Then photolithography is used to expose a fine-structure
pattern into the photoresist, as indicated in Step 3. The optically
exposed area of the photoresist (if positive photoresist is used)
is washed out during the development. It is fairly easy to create
photoresist dots (indicated by 405) with sizes ranging from a few
microns to a few tens of microns, by using modern lithography
technology. In Step 4, hydroflouride acid is used to chemically
etch the silicon dioxide layer (408) that is not covered by the
photoresist (405), which is then removed afterwards. Then a dry
etching method, such as Deep Reactive Ion Etching (DRIE) is applied
to dig deep recesses (as indicated by 406 in Step 5) in the area
not covered by silicon dioxide (408). Current high-rate DRIE that
directionally etches away silicon out of substrate 401 to create
deep and steep holes, walls and trenches is able to etch pin fins
having aspect ratios of 10 to 30. Bosch process that alternates
repeatedly between isotropic ion etching and side-wall passivation
is one of the most recognized DRIE technologies. Chemical wet
etching is a cheaper alternative process that can also be
implemented for aspect ratios less than 10. Finally, the remaining
silicon dioxide is removed and a fine-structure patterned plate
with an array of pin fins is then created, as shown in the
perspective view of FIG. 3H. The above procedures can be applied
again to the other side of the substrate (401) to create FSP pin
fins on both sides of the base plate or substrate, as shown in FIG.
3C. The resulting double-sided FSP heat sink is even more compact
and more heat efficient.
Carbon Nano-Tubes Grown on a Thermally Conductive Plate:
[0044] A thermally conductive plate or wafer (501), either made of
metals such as copper (thermal conductivity K=398 w/m k) or
aluminum 1100 (K=220 w/398/mk), or semiconductor materials such as
silicon and SiC, is first selected, as indicated by Step 1 in FIG.
5. In Step 2 a thin polymer film (503) is coated onto the plate
surface. In Step 3 a nano-scale pattern is introduced onto the
polymer film (503) by either nano-scaled methods such as
nano-imprinting from a template with a nano-scale pattern, or via
UV photolithography. The patterned polymer film is shown as item
505 with voids (506) in the film. Then growing seeds (507) such as
Fe.sub.2O.sub.3 for later growing of nano-tubes are deposited into
the voids (506), as shown in Step 4. The unwanted polymer residual
is removed in Step 5, leaving the growing seeds (507) exposed on
the surface of the substrate (501). Then single-wall or multi-wall
carbon nano-tubes are grown on the top of the growing seeds (507)
by either chemical vapor deposition or wet chemical growth as shown
in Step 6. It is well-known that carbon nano-tubes have thermal
conductivity on the order of a few times to tens of times that of
copper. The diameter of a carbon nano-tube ranges from a few
nanometers to tens of nanometers. The above growing procedure can
also be applied to the other side of the substrate (501). Also
represented in FIG. 3C is a heat sink with nano-tubes grown on both
sides, that is expected to provide super-efficient heat
transfer.
[0045] Processes similar to the ones described above for growing
carbon nano-tubes can also be applied to growing nano-wires,
composed of metals such as transition metals, copper (Cu), silver
(Ag, and gold (Au), as well as semiconductors such as silicon (Si),
germanium (Ge), and indium arsenide (InAs), all having good thermal
conductivity. Methods capable of growing the pin fins (301) in FIG.
3B, with diameters ranging from a few nanometers up to the
sub-millimeter range, with a pin fin aspect ratio greater than
about three or about ten, are all within the scope of the present
invention. Preferably the nanotubes and nanowires have diameters of
not more than 1 micron.
[0046] Molding methods may also be used for making pin fins having
aspect ratio less than 3, but have a challenge for larger aspect
ratio.
[0047] Laser ablation that sends high power laser pulses to ablate
material over a surface line by line can also be a feasible method
to create high aspect fine-structure fin pins out of a thermal
conductive planar material though the production speed may be only
moderate.
[0048] Other methods that are able to create fine-structure fins
are also within the scope of the present inventions.
[0049] FIG. 3I shows a single pin fin taken from the heat sink in
FIG. 3H. The temperature of the heat sink base (306) is T.sub.b,
whereas the temperature of the air or fluid around the heat sink is
T.sub..infin.. The temperature distribution along the length of the
pin fin, the heat flux from the heat sink base to the pin fin, and
the pin fin heat transfer efficiency, respectively, are defined as
follows (See Gregory Nellis & Sanford Klein, Heat Transfer,
Cambridge University Press, 2009.):
T - T .infin. T b - T .infin. = cosh ( m ( L - x ) ) + h _ mk sinh
( m ( L - x ) ) cosh ( mL ) + h _ mk sinh ( mL ) Equation ( 3 ) q .
fin = ( T b - T .infin. h _ per k A c sinh ( mL ) + h _ mk cosh (
mL ) cosh ( mL ) + h _ mk sinh ( mL ) Equation ( 4 ) .eta. fin = [
tanh ( mL ) + mL AR tip ] mL [ 1 + mL AR tip tanh ( mL ) ] ( 1 + AR
tip ) Equation ( 5 ) ##EQU00003##
where: [0050] T.sub.b=base temperature [0051] T.sub..infin.=fluid
(air) temperature [0052] per=perimeter of the pin fin [0053]
L=length of the pin fin [0054] T=temperature
[0054] mL = per h _ k A c L = pin fin constant ##EQU00004## [0055]
h=heat transfer coefficient [0056] A.sub.c=cross-sectional area of
the pin fin [0057] k=thermal conductivity [0058] {dot over
(q)}.sub.fin=pin fin heat transfer rate [0059] x=position (relative
to base of pin fin)
[0059] AR tip = A c per L = tip area ratio ##EQU00005##
[0060] Generally the aspect ratio of a pin fin is large. The heat
convection from the end tip of a pin fin is much smaller than that
along the pin. Therefore, equations (3)-(5) can be simplified
to:
T - T .infin. T b - T .infin. = cosh ( m ( L - x ) ) cosh ( mL )
Equation ( 6 ) q . fin = ( T b - T .infin. ) h _ per k A c tanh (
mL ) Equation ( 7 ) .eta. fin = tanh ( mL ) ( mL ) Equation ( 8 )
##EQU00006##
[0061] For free convection air flow whose heat transfer coefficient
is less than 2 w/m.sup.2k and forced air flow whose heat transfer
coefficient is generally less than 200 w/m.sup.2k, mL in equation
(7) is much less than 1. Equation (7) and (8) are therefore
approximated to
{dot over (q)}.sub.fin=.pi.(T.sub.b-T.sub..infin.)S h D.sub.e.sup.2
Equation (9)
.eta..sub.fin.fwdarw.1 Equation (10)
[0062] Multiplying b.sup.-2 to Equation (9), the heat flux across
the heat sink's unit base area to the pin fins is
.pi.(T.sub.b-T.sub..infin.)S h(D.sub.e/b).sup.2, which is dependent
on the lattice ratio b/D.sub.e. This concludes that the heat flux
across the heat sink's unit base area for a FSP heat sink is same
as that for a conventional bulk heat sink as long as the lattice
ratio is kept same. Furthermore Equation 9 illustrates the heat
dissipation to air per pin fin is proportional to aspect ratio S
for free convection and forced air flows. Therefore it is desirable
in the present invention to make aspect ratio as large as possible
by methods such as modern DRIE, or by using nanotubes and nanowires
as described above. In one embodiment, the aspect ratio S of the
pin fins is not less than about 10. In another embodiment, the
aspect ratio S of the pin fins is not less than about 20. The pin
fins preferably have equivalent diameter of not more than about 0.3
mm, and length of not more than about 3 mm. In one embodiment, the
length of the pin fins are less than about 1 mm. In still another
embodiment, the pin fins preferably have equivalent diameter of not
more than about 0.1 mm. The heat transfer coefficient of a liquid
flow is generally one or two orders higher in magnitude than that
of an air flow. Thus the temperature quickly drops along the pin
fin so that a high aspect ratio (>5) has little additional
benefit for heat dissipation to surrounding liquid.
[0063] Air blowing across the pin fins either by forced flow or by
natural convection has a pressure drop due to the drag force
induced by the fins. It is desirable that the pressure drop be as
small as possible for a heat transfer device. The total drag force
F.sub.D induced by the pin fins is
F D = C D A P 1 2 .rho. U .infin. 2 ##EQU00007##
where C.sub.D=drag coefficient depending on geometry of pin fins,
Reynold's number, as well as other factors [0064] A.sub.P=total
projected area of the pin fins, facing the flow [0065] .rho.=air
density [0066] U.sub..infin.=air free stream velocity
[0067] The projected area of the pin fins per unit length of heat
sink base is
.about. ( D b ) L ##EQU00008##
Therefore, the drag force per unit length of heat sink base is
proportional to L, assuming that the pin fin shapes are the same,
and that the lattice ratio
b D ##EQU00009##
is held constant for both bulk and FSP heat sinks, which is both
achievable and practical. That reveals that the FSP heat sink has
much less drag force than that of the conventional prior art heat
sink.
[0068] In summary, based on the above illustrations and equations,
the FSP heat sink of the present invention is not only one to two
orders of magnitude smaller in pin fin length when compared to
conventional prior art heat sinks, but is also superior in drag
force reduction. An FSP heat sink as shown in FIG. 3H with pin fin
length less than 1.0 mm should perform in heat transfer as good as
a conventional prior art heat sink with pin fin length on the order
of tens of millimeters, as shown in FIG. 1A.
[0069] FIG. 6 shows an FSP heat sink (601) mounted directly onto
the back side of a heat spreader (603) that has a heat source (605)
on its front side. As compared to FIG. 1A, the FSP heat sink (601)
is low-profiled and has a much smaller thickness. Because of their
low profile, multiple FSP heat sinks (703), as shown in FIG. 7A,
can be stacked up by tightly affixing them, or metallically bonding
them to a highly thermally conductive metal U-shaped heat spreader
(701), as long as the space between the FSP heat sinks is large
enough to prevent degradation of their individual heat transfer
coefficients.
[0070] The densely populated pin fins of an FSP heat sink are
capable of dissipating a large amount of heat to air. In some
situations the heat is transferred to the pin fins via the edges of
the heat sink base such as is illustrated by item 705 in FIG. 7A
and item 710 in FIGS. 7B and 7C. In some cases, as illustrated in
FIGS. 7B and 7C, the base (720) of the FSP heat sink may be very
thin, especially if a semiconductor wafer is being used as the base
material, so that heat conduction is limited to the edge areas of
the base (720). In this case, where such contact arrangement of the
base (720) becomes the bottle neck for heat conduction, a highly
thermally conductive material (715) such as copper can be
metallically bonded (712) to the base (720) to increase the area
for heat conduction to the pin fins (713). Thin composite metallic
layers (719) such as Ti/Au/Sn may be deposited by either
electrochemical plating or e-beam evaporation to the back side of
the FSP heat sink as shown in FIG. 7B. Then the FSP heat sink
assemblies of FIG. 7C can be used to replace the FSP heat sinks
(703) in FIG. 7A.
[0071] The FSP heat sink bank in FIG. 7A is able to draw a lot of
heat from the heat source (706). A fan (801) in FIG. 8 can be
mounted on one side of the heat sink bank in the embodiment of FIG.
7A as well as other embodiments herein to increase the flow of air
past the heat sink bank, thereby providing further improvement in
heat transfer.
[0072] The flow field and the material properties of the
surrounding fluid, as well as other minor parameters, determine the
heat transfer coefficient h.sub.t in equation (1), and its
corresponding dimensionless Nusselt number Nu. FIG. 9A pictorially
represents the flow field and temperature profile of a laminar
natural convection air flow along a heated vertical plate (901)
with a temperature T.sub.w. .delta..sub.m (903) and .delta..sub.t
(905) indicate the momentum and thermal boundary layers,
respectively. The velocity profile and the thermal distribution are
indicated by items 907 and 909, respectively. As the surface of
plate is densely populated with fine-structure patterned fins, the
momentum and thermal boundary layer are expected to grow thicker,
more quickly than that of a smooth plate, due to more vigorous
transport vertical to the plates. FIG. 9B sketches the flow field
profiles. FIG. 10 pictorially illustrates how forced convection
flow fields are developed between two FSP plates. By scaling laws
and empirical data, the heat transfer coefficient of an FSP plate
is expected to be of the same order of magnitude as that of smooth
plate. However, the FSP plate has a contact surface area with air
that is at least two orders of magnitude greater than that of a
smooth plate, leading to a proportional increase in overall heat
transfer.
[0073] Perforated holes, as indicated by item 1101 in FIG. 11A, may
be drilled through the base plate (1105) of the FSP heat sink
(1102), to generate a flow perpendicular to the base plate's
surface, thereby causing thermal boundary layer bursting or
circulation, leading to increased flow turbulence for enhanced heat
convection.
[0074] In addition to the perforated holes (1101) in FIG. 11A, the
heat gutters (1201) shown in FIGS. 12A and 12B are created between
sub-arrays or groupings of densely populated pin fins (1202) to
facilitate carrying heat away from pins. Resulting from patterning
during the fabrication process, heat gutter shape and geometry can
be arranged arbitrarily among the sub-arrays or groupings of
densely populated pin fins (1202) over the heat sink's base plate
(1204), for optimal heat transfer.
[0075] The above FSP heat sink can be used conversely to suck heat
from the surrounding media to provide heat to an object. Due to
large contact surface provided by FSP fins, the transient time for
the heated body to reach thermal equilibrium is reduced. For
example, FSP heat sink in the present invention can be attached to
a biological culture tube or a chemical beaker that endo-thermal
reaction is taking place to timely keep the testing sample in
constant temperature by quickly absorbing heat from the surrounding
heat reservoir.
[0076] Resulting from that fact of that the surface area of the FSP
heat sink is one to two orders larger in magnitude larger than that
of a plane surface. As FSP heat sink is attached to an elevated hot
body, a FSP heat sink can radiate a significant amount of heat by
thermal radiation as it is attached to an elevated hot body.
Conversely, the FSP can be used to absorb radiation energy from the
environment to heat up a cooler body. Thus, the heat transfer can
take place by radiation, as in black body radiation, as well as by
conduction and/or convection. All in all the FSP heat sink is
benefited by its low profile.
[0077] Pin fins have been used in the present invention to
illustrate the advantages of the fine-structure patterned heat
sink. Other fin shapes such as straight plate fins and curved plate
fins with either or both of their width and thickness less than one
millimeter and high aspect ratio (defined by height divided by
either of width and thickness, whichever is the smaller) and are
directed built by patterning from a thermal conductive substrate
are also within the scope of the present invention.
[0078] While the invention has been described above by reference to
various embodiments, it will be understood that changes and
modifications may be made without departing from the scope of the
invention, which is to be defined only by the appended claims and
their equivalents.
* * * * *