Cooling System for Electronic Substrates
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.
Latest KONINKLIJKE PHILIPS ELECTRONICS, N.V. Patents:
- METHOD AND ADJUSTMENT SYSTEM FOR ADJUSTING SUPPLY POWERS FOR SOURCES OF ARTIFICIAL LIGHT
- BODY ILLUMINATION SYSTEM USING BLUE LIGHT
- System and method for extracting physiological information from remotely detected electromagnetic radiation
- Device, system and method for verifying the authenticity integrity and/or physical condition of an item
- Barcode scanning device for determining a physiological quantity of a patient
The present invention relates to a cooling system for an electronic substrate comprising a heat transfer fluid.
BACKGROUND OF THE INVENTIONControl 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.
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/cm2. 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.
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 INVENTIONIt 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.
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.
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.
To facilitate understanding of the technique an example is used where it is desired to dissipate 100 W/cm2 with a temperature drop of 90° 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
where k is the thermal conductivity of water (0.628 Wm−1K−1), D is the hydraulic diameter (10−3 m) and Nu is the Nusselt number, which is given by
Nu=C·Rem·Prn·K
where Re is the Reynolds number and Pr is the Prandtl number given by
respectively. C=1.85, m=⅓, n=⅓, K=0.368, Cp=4178 Jkg−1K−1, ρ=995 kg/m3, and μ=651·10−6 kgm−1 s−1.
vav 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 μl/s.
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 μl/s in a sequence of droplets. Stated in a slightly different way, the energy transport rate P (J/s) is given by
where ΔV/Δt is the volumetric flow rate of fluid passing the heat source. For P=30 W and ΔT=50° C. the flow rate will be 140 μl/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.
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 2Δγ/R, where Δγ is the change of surface tension and R the curvature of the meniscus. Δγ can be of the order of 0.1 N/m. For a curvature of 100 μm, the maximum pressure is about 2000 Pa.
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 ΔρgL, 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.
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 μm. This gives a volumetric flow rate of 7 μl/s per micro channel. In other words, a volumetric flow rate of 140 μl/s can be achieved in an actuated-actuated system with about 20 micro channels.
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.
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.
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 μm-10 μm of parylene, or 10 nm-1 μm of a fluoropolymer layer, or a combination of such layers.
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.
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.
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.
The present invention will now be further described with reference to the figures showing different embodiments of the invention.
In
In one embodiment of the present invention the heat transfer fluid channel 5 is filled with two fluids.
An example of channels 11 made in a “loop” shape can be seen in
In
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.
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.
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.
Type: Application
Filed: Jul 21, 2005
Publication Date: Jan 8, 2009
Applicant: KONINKLIJKE PHILIPS ELECTRONICS, N.V. (EINDHOVEN)
Inventors: Celine Nicole (Eindhoven), Clemens J.M. Lasance (Eindhoven), Menno Willem Jose Prins (Eindhoven), Jean-Christophe Baret (Eindhoven), Michel Marcel Jose Decre (Eindhoven)
Application Number: 11/573,002
International Classification: F28D 15/00 (20060101);