KR20070040835A - A cooling system for electronic substrates - Google Patents

Abstract

The present invention relates to a cooling system for an electronic substrate comprising a heat transfer fluid (4, 10). The heat transfer fluids 4, 10 are configured to flow along the paths 5, 11, 12 by capillary forces.

Heat transfer fluid, cooling system, electronic circuit

Description

Cooling system for electronic board {A COOLING SYSTEM FOR ELECTRONIC SUBSTRATES}

The present invention relates to a cooling system for an electronic substrate comprising a heat transfer fluid.

Component temperature and temperature rate control are essential for the reliable and successful operation of electronic products such as electronic circuits. As consumer products continue to require high heat removal, thermal management will play an important role. Therefore, a new cooling method is needed to cope with the market demand. The tendency of component miniaturization leads to an increase in power density, requiring a more sophisticated cooling method than for example pure radiation or convective cooling by a fan. Important aspects such as noise and reliability limit the use of fan cooling. Thus, there is a need for an improved cooling method.

Liquid cooling is a method already used in portable computers, ie laptops. A particular application requiring cooling is solid state lighting. White light or color controlled solid state lighting requires the use of multichip modules, where several LEDs are placed very close to each other to define the light point source. This design increases the power density of the silicon submount to as high as 100 W / cm 2. Liquids are a much better heat transfer medium than air because their thermal conductivity and heat capacity are higher (by 10 to 1000 times) than air. Forced circulation microchannel liquid cooling has proven to be very efficient in this industry (metal microchannel structure) and industrial research (silicon microchannel devices). The main inconvenience with this technique is that it pumps liquid by pumps throughout the channel, making it less suitable for miniaturized and integrated consumer and consumer electronics.

O'Connor et al. Disclose an example of a cooling system using a pump in US 2002/0039280 A1. The invention by O'Connor et al. Relates to a compact fluid heat exchange system for cooling electronic components inside devices such as computers. Heat exchange devices are generally in interfacial contact with heat generating electronic components and provide an internal operating fluid to the heat exchange zone. The working fluid flows into the heat exchange zone at a first fluid temperature lower than the part temperature and then exits this zone at a second fluid temperature higher than the first fluid temperature.

It is an object of the present invention to provide a cooling system for electronic components that does not use a pump. The object of the invention is achieved by a cooling system according to claim 1. In the dependent claims, preferred embodiments are shown.

According to the present invention, a cooling system for an electronic substrate comprises a heat transfer fluid, said heat transfer fluid being configured to flow along the path by capillary forces. Therefore, this cooling system does not have a moving part separated from the heat transfer fluid, and has the advantage that the power consumption is relatively low. This enhances reliability and flexibility, and makes the construction more robust than systems requiring mechanical pumps or piezoelectric drive pumps.

An essential feature of the present invention is the use of microscale fluid transfer technology to achieve integrated and compact cooling of electronic components such as chip submounts.

으로 구하는데 여기에서 k는 물의 열 전도율(0.628 Wm^-1K^-1)이고, D는 수력 직경(10^-3m)이고,

로 구해지는 Nu는 너셀수(Nusselt number)인데, 여기에서 Re는

로 구해지는 레이놀드수(Reynolds number)이며 Pr은

로 구해지는 프란틀수(Prandtl number)이다.

이고,

이다.In order to facilitate understanding of this technique, an example in which it is desirable to dissipate 100 W / cm 2 having a temperature reduction of 90 ° C. has been used. The estimate of the speed of water through a pair of parallel plates is: The heat flux of the system

Where k is the thermal conductivity of water (0.628 Wm ^-1 K ^-1 ), D is the hydraulic diameter (10 ^-3 m),

Where Nu is the Nusselt number, where Re is

Reynolds number obtained by Pr is

The Prandtl number obtained by.

ego,

to be.

는 단면적에 따라서, 10 내지 100 ㎕/s 정도의 부피 흐름을 제공하는 1m/s정도로 추정된다.

Is estimated to be on the order of 1 m / s, providing a volume flow on the order of 10 to 100 μl / s, depending on the cross-sectional area.

로 구하여지는데, 여기서

는 열원을 지나는 유체의 부피 흐름 속도이다.

이고

라면 흐름 속도는

가 될 것이다. 일렉트로웨팅은 정전하에 의한 표면 장력의 변경을 수반하므로, 유체/유체 메니스커스(meniscus)를 이동시킨다. 이 이동은 적어도 2개의 서로 다른 방식, 즉 (i) 하나 또는 몇몇의 채널 또는 슬릿에서 유체/유체 메니스커스를 구동함으로써, 또는 (ii) 표면 상에 작은 방울을 수송함으로써 제공될 수 있다.In a preferred embodiment of the invention the system of the invention further comprises an electrode configured to apply a voltage to the heat transfer fluid to change the surface tension of the heat transfer fluid. The principle of electroweting makes it possible to move hundreds of μl / s into a series of droplets. In a slightly different way, the energy transport rate P (J / s)

Is obtained from where

Is the volumetric flow rate of the fluid through the heat source.

ego

Ramen flow rate is

Will be. Electrowetting involves the change of surface tension due to electrostatic charge, thus moving the fluid / fluid meniscus. This movement can be provided by at least two different ways, i.

로 구하는데, 여기서

는 메니스커스의 곡률 R과 표면 장력의 변경치이다.

은 0.1 N/m 정도일 수 있다. 100㎛ 곡률에 대하여, 최대 압력은 약 2000 Pa이다.The maximum meniscus speed described by the electrowetting is 0.1 m / s or slightly higher. The maximum pressure modulation that can be produced by electrowetting is

Where you get

Is the change in the curvature R of the meniscus and the surface tension.

May be about 0.1 N / m. For a 100 μm curvature, the maximum pressure is about 2000 Pa.

과 같으며 여기서 L은 투사된 길이이다. 유사한 매스 밀도를 가지는 유체를 이용함으로써, 유체들 중 하나의 열(column) 높이를 최소화함으로써, 균형잡힌 기하학적 도형을 이용함으로써 최대 배향 자유도를 보장할 수 있다.In order to ensure the orientation freedom of the electrowetting device, the gravity pressure drop in the system must be lower than the maximum modulation of the electrowetting pressure. Gravity pressure drop

Where L is the projected length. By using a fluid having a similar mass density, minimizing the column height of one of the fluids, it is possible to ensure maximum orientation freedom by using a balanced geometry.

It is known that a flow rate of 0.1 m / s can reach channels of 2 cm in length and 300 μm in diameter. This gives a flow rate of 7 μl / s volume per microchannel. In other words, a flow rate of 140 μl / s can be achieved in an actuated-actuated system with about 20 microchannels.

Preferably at least one microchannel is connected to the heat transfer fluid reservoir. With the use of fluid reservoirs disposed around heat sources and proper heat sinking of these reservoirs, heat is effectively removed.

In one embodiment of the invention the electrode is located outside the heating zone. Drive-driven flow generates energy transfer from the concentrated heating zones in the device to the extended cooling zones. The driven electrode is preferably located outside the heating zone, since it will increase the life of the device. In addition, the fluid system is preferably a circulating system. This will reduce the risk for fluid evaporation and leakage.

In other embodiments, the cooling system includes two immiscible fluids having different electrical conductivity, such as, for example, air / water, water / oil, and the like. The drive-drive type requires that the electrode be near the fluid / fluid meniscus. Electrodes coated with an insulating layer generally consist of a material having a metal conductance. The insulating coating can be, for example, parylene of 1 μm-10 μm, or a fluoropolymer layer of 10 nm-1 μm, or a combination of these layers.

Different microchannels may be hydrostatically separated from each other, or may be coupled to a specific junction or channel (eg, a regular channel or reservoir). Note that the integrity of the meniscus in the microchannel is prevented to prevent any type of fluid from entering the reservoir for the second type of fluid.

In another embodiment, the system of the present invention is configured such that the fluid is driven in both directions. Fluid flow may be unidirectional or bidirectional. Preferably, the fluid is preferably driven in both directions so that fluid contact to the heating zone can be limited to only one type of fluid. This will extend the life of the device. Bi-directional flow is achieved by applying a pulsating voltage to send and receive the flow of heat transfer fluid.

In order to reduce the rate of temperature change on the device to be cooled, the system of the present invention preferably includes two sets of microchannels arranged in counter flow relationship.

1 illustrates an example of a multi chip module for a solid state lighting application.

2 shows an example of a droplet flow.

3 illustrates a given microchannel for fluid transfer between on and cold zones.

4a and 4b show a cooling unit with several microchannels connected to the reservoir.

5a shows a system according to the invention with a ring geometry.

5B is an enlarged view of a portion of the system of FIG. 5A.

6a shows a system according to the invention with a countercurrent arrangement.

6B is an enlarged view of a portion of the system of FIG. 6A.

7a shows a radial system according to the invention.

7B is an enlarged view of a portion of the system of FIG. 7A.

8 illustrates a radial system having channels that are not continuous in width.

The invention will be further described below with reference to the drawings showing different embodiments of the invention.

1 is a general view of a multi-chip module having nine LEDs (light emitting diodes) 1. White light or color controlled solid state lighting requires a multi-chip module in which several LEDs 1 are placed in close proximity to one another to define the light point source. This design results in a higher power density on the silicon submount 2. By collecting the activated liquid cooling droplets using a pump driven in the silicon submount 2, the required cooling can be performed. In the example shown in FIG. 1 the size of the silicon submount is 5 mm x 6 mm and this submount 2 is arranged adjacent to the reservoir / collector 3. The reservoir / collector 3 comprises a heat transfer fluid for the removal of the thermal energy produced by the LED 1.

In Fig. 2 the principle of droplet delivery is shown. The heat transfer fluid droplet 4 is flowing in the channel 5 from the reservoir / collector 3. The droplets are moved by the voltage applied to the fluid through the electrode 6. In this way heat will now be transferred from the silicon submount 2 to the droplet 4. The droplets 4 will then cool in the reservoir / collector 3. The energy absorbed by the reservoir / collector 3 will then be delivered from the reservoir / collector 3 with the aid of a separate cooling system (not shown). The heated chip is part of a larger device made of, for example, a printed circuit board material, a molded-interconnect device (MID), a glass, a metal device, or the like. Each of these materials may comprise an electrode and channel structure. Holes may be provided so that the silicon chips may be exposed to the heat transfer fluid and electrically interconnected.

In one embodiment of the invention the heat transfer fluid channel 5 is filled with two rapeseeds. FIG. 3 is a schematic view of a predetermined microchannel 5 for fluid transfer between the on region 7 and the cold region 8. The electrode is not shown. A plug 9 of one of the fluids is used to "push out" another fluid 10 which mainly acts as a heat transfer fluid. The electrode 9 allows the plug 9 and, consequently, the heat transfer fluid 10 to be driven in both directions, ie with a reciprocating flow of the heat transfer fluid, in order to prevent the plug 9 from entering the on-region 7. It is preferable to apply the pulsation voltage to This will extend the life of the device. In order to function like this, the two fluids need to be plugged with a fluid which cannot be mixed, for example oil in excess water.

4a and 4b show 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 was applied and heat transfer fluid began to flow in the microchannel 11. When the applied voltage is turned off, the heat transfer fluid returns to the reservoir 3.

An example of a channel 11 in a "loop" shape can be seen in Figures 5a and 5b, where 5b is an enlarged view of a portion of 5a. This embodiment comprises two reservoirs 3 which are heat sinks for heat transfer fluid.

6a shows a cooling system according to the invention comprising two reservoirs 3 of heat transfer fluid arranged in a set of separate channels 11 arranged in a countercurrent relationship. This arrangement helps to reduce the rate of temperature change on the silicon chip, resulting in longer life of the silicon chip due to more uniform thermal loads. FIG. 6B is an enlarged view of a portion of the embodiment shown in FIG. 6A.

In FIG. 7B, which enlarges a part of the embodiment of FIGS. 7A and 7A, an embodiment according to the invention is shown with radial cooling with a heat source in the center. Therefore, the heat transfer fluid moves from the reservoir 3 in the microchannel 11 towards the center. A heat sink is connected outside the reservoir (not shown).

8 shows another embodiment of a system according to the invention. The system includes two reservoirs 3 interconnected with channels 12. The channel width is varied between the two reservoirs 3 to optimize the capillary flow of the heat transfer fluid.

In order to use the present invention, liquid filling must be accomplished by the holes / channels of the device of the present invention in the last step. All microchannels are filled at the same time, for example, via filling channels extending perpendicular to the microchannel. In addition, the entire liquid device must be completely sealed after filling. A pressure damper may be included to remove the pressure created during setup. In addition, a fluid reservoir (eg, having a membrane or a bag containing air bubbles) may be included that allows for expansion and contraction of the fluid.

Those skilled in the art clearly understand that the present invention is by no means limited to the above described embodiments. On the contrary, various modifications and variations are possible within the scope of the appended claims. For example, the form of the channel system is not limited to the embodiment of the accompanying drawings.

Claims (10)

In a cooling system for an electronic substrate, A heat transfer fluid (4, 10), said heat transfer fluid (4, 10) being configured to flow along the path (5, 11, 12) by capillary forces. The method of claim 1, The system further comprises an electrode (6) configured to apply a voltage to the heat transfer fluid (4, 10) to change the surface tension of the heat transfer fluid (4, 10). The method according to claim 1 or 2, Cooling system in which at least one microchannel (5, 11, 12) is connected to a heat transfer fluid reservoir (3). The method of claim 2, The electrode (6) is located outside the heating zone (7). The method according to any one of claims 1 to 4, The fluid system is a circulatory system. The method according to any one of claims 1 to 5, The cooling system comprises two immiscible fluids (9, 10). The method according to any one of claims 1 to 6, The system is configured such that the fluid (4, 10) is driven in both directions. The method according to any one of claims 1 to 7, The system comprises two sets of microchannels (11) arranged in a counter flow relationship. An electronic substrate cooling method using the system according to any one of claims 1 to 8, Applying a voltage to a microchannel (5, 11, 12) comprising a heat transfer fluid (4, 10) such that the surface tension of the heat transfer fluid (4, 10) is changed. The method of claim 9, How pulsating voltage is applied.

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