Low-Profile Heat Sink with Fine-Structure Patterned Fins for Increased Heat Transfer
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.
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.
The heat transfer between a heat sink and air is governed by:
Q=ht A(THS−T∞) Equation I
where Q is the amount of heat transferred to the air, ht is the average heat transfer coefficient, A is the contact surface of the heat sink with the medium, THS is the average heat sink temperature, and T∞ is the air temperature in free stream.
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 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 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.
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.
SUMMARYOne 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.
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.
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.
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.
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.
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.
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.
The lattice configurations shown in
Suppose the base plate (303) is a square with dimension W for each side.
As an example, if
and S=15, then M=22.
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 (De), if the lattice ratio b/De 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
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:As illustrated in
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
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
Molding methods may also be used for making pin fins having aspect ratio less than 3, but have a challenge for larger aspect ratio.
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.
Other methods that are able to create fine-structure fins are also within the scope of the present inventions.
where:
- Tb=base temperature
- T∞=fluid (air) temperature
- per=perimeter of the pin fin
- L=length of the pin fin
- T=temperature
-
h =heat transfer coefficient - Ac=cross-sectional area of the pin fin
- k=thermal conductivity
- {dot over (q)}fin=pin fin heat transfer rate
- x=position (relative to base of pin fin)
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:
For free convection air flow whose heat transfer coefficient is less than 2 w/m2k and forced air flow whose heat transfer coefficient is generally less than 200 w/m2k, mL in equation (7) is much less than 1. Equation (7) and (8) are therefore approximated to
{dot over (q)}fin=π(Tb−T∞)S
ηfin→1 Equation (10)
Multiplying b−2 to Equation (9), the heat flux across the heat sink's unit base area to the pin fins is π(Tb−T∞)S
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 FD induced by the pin fins is
where CD=drag coefficient depending on geometry of pin fins, Reynold's number, as well as other factors
-
- AP=total projected area of the pin fins, facing the flow
- ρ=air density
- U∞=air free stream velocity
The projected area of the pin fins per unit length of heat sink base is
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
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.
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
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
The FSP heat sink bank in
The flow field and the material properties of the surrounding fluid, as well as other minor parameters, determine the heat transfer coefficient ht in equation (1), and its corresponding dimensionless Nusselt number Nu.
Perforated holes, as indicated by item 1101 in
In addition to the perforated holes (1101) in
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.
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.
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.
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.
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/π.
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.
Type: Application
Filed: Mar 16, 2011
Publication Date: Sep 20, 2012
Inventor: Ho-Shang Lee (El Sobrante, CA)
Application Number: 13/049,707
International Classification: F28F 13/00 (20060101); B21D 53/02 (20060101); F28F 7/00 (20060101);