Apparatus and Method for Cooling ICs Using Nano-Rod Based Chip-Level Heat Sinks

BACKGROUND OF THE INVENTION

DESCRIPTION OF THE PRIOR ART

Electronic components are known to dissipate heat during their active and standby operation. The heat generated may cause the ambient temperature around such components to increase to levels that deteriorate the performance of the components, at best, or permanently damage the component. Various cooling solutions including heat sinks, fans, and more exotic approaches, such as liquid or gas cooling, are known used for the purpose of keeping the temperature of the components at an ambient temperature level, which is close to optimal for the components' operation.

Nonetheless, the current art for cooling electronic assemblies at the chip, chip set, board, and rack levels, suffers from a number of limitations that are the result of: a) nano-scale heat transfer bottlenecks at transistor level; b) lack of sufficient surface area required by a heat exchanger to dissipate the large amounts of heat generated by dense packing of chips, chip sets, and/or stacked boards; c) the small form-factor of electronic boxes, which limits the volume required for a cooling solution; d) established limits on noise levels generated by a fan, pump, or other devices required by the heat exchanger; and, e) the amount of power available for cooling the assembly.

Together, the above limitations create severe limitations in providing cooling solutions having lower complexity, lower cost of extremely dense, hot microelectronic assemblies, such as boards, racks, etc., which must fit in small boxes and enclosures. In the current art, the cost and the complexity of cooling grows very fast with ever increasing density of micro-electronic assemblies, e.g. chip, chip set, board and board assemblies. Aside from cost considerations, the reliability of the micro-electronic assemblies suffers from

high, local temperatures which translate into lower mean-time-between failure (MBTF) performance.

SUMMARY OF THE INVENTION

A method and apparatus for overcoming the problems of rapidly increasing complexity and cost and degrading reliability measures in connection with the cooling of a multi-chip mounted on an electronic printed circuit board. Accordingly, there are combined a) nano-structures materials for micro or nano-scale heat transfer from a substrate; b) small dimension heat sinks or heat spreaders matched to the mico-scale heat transfer to control the spread resistance; c) nano-scale cooling channel surfaces or micro-channel heat exchangers to improve heat transfer coefficients of the hot components to the cooling agent, air or liquid; and d) sharing of the active device such as a fan, pump, compressor, etc., that are responsible for moving the cooling agent in an active cooling embodiment. By providing appropriate passage for the cooling agent an effective and efficient cooling of the hot surfaces is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

TECHNICAL FIELD

The invention relates to integrate circuits, more particularly, the invention relates to an apparatus and method for cooling ICs using nano-rod based CNIP-level heat sinks.

Figure 1 shows a layout of an electronic circuit having a cooling system in accordance with the disclosed invention;

Figure 2 shows a cross section of a chip level heat sink (CLHS) in accordance with the disclosed invention;

Figure 3 shows an upper level view of the CLHS;

Figure 4 shows a CLHS structure further comprising an expansion chamber; and

Figure 5 shows a cross section of a portion of an electronic circuit that is cooled in accordance with the disclosed invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the disclosed invention, physical dimensions of the active component responsible for movement of a cooling fluid, e.g. gas or liquid is vastly reduced by deploying a set of components that also combined to improve the thermal performance, and reduce costs, of the overall cooling apparatus. This is accomplished by combining: a) a vastly improved thermal interface resistance; that is made possible by new nano-materials which have excellent thermal properties, e.g. nano rods, nano-wires, carbon nanotubes, carbon nanofibers, etc.; b) a small foot-print, micro-channel heat evaporator or heat exchanger; and c) a small dimension fan(s), pump(s) such as those known-in-the-art as MEMs or conventional micro-pumps, micro-fans, micro- compressors, etc. The advantages of the disclosed invention are achieved by matching the dimensions of chip micro-electronics, e.g. nano-scale dimension transistor heat source, micron sized hot spots, millimeter chip sizes, centimeter sized card or printed circuit boards, etc., to the smallest dimension cooling components that are still compatible to micro-electronic cards. Furthermore, the reliability measure is improved by minimizing local, high- temperature spots. This becomes possible when dimensions of relatively the same scale are applied to each and every element of the cooling hierarchy.

The following components comprise an exemplary embodiment of the novel heat removing system: a) a compressed air tank and/or an array of micro-

valves and/or a micro-compressor pump and/or a liquid micro-pump; b) a chip-level heat sink or heat spreader (CLHS); c) access to a cold source, e.g. cold gas or liquid; and d) a system-level heat sink (SLHS), also known as a heat exchanger. Fig. 1 is an exemplary and non-limiting top view of an electronic circuit 100 cooled in accordance with the disclosed invention. On top of a printed circuit board (PCB) 140 there are mounted a plurality of integrated circuits (ICs) (not shown). Each IC is coupled to a CLHS 120. The CHLS 120 is described in more detail below. Each CHLS 120 is equipped with an inlet and an outlet that allows the flow of a cold source, e.g. a gas or a liquid, to flow through the CHLS 120 and thereby remove heat that is transferred from the respective IC that is being cooled. CHLS 120, for example CHLS 120-1 through 120-6 is connected inlet to outlet, in series by means of conduits 130, in series to allow the flow of the cold source through a number of components. The total number of components to be connected in series in this way is only limited by the amount of heat that is necessary to be removed from the components. A control unit 110 is comprised of a compressed air tank and/or an array of micro-valves and/or a micro- compressor pump and/or a liquid micro-pump, and an SLHS. The control unit 110 provides the cold source under ample pressure to flow through the CHLS 120 and remove the heat therefrom and then, using the SLHS, get rid of the heat from the cold source, such that the card source may be used to reflow through the system, or otherwise remove the heat to a distance from the electronic circuit being cooled.

An exemplary and non-limiting CLHS 200, having small dimensions for use with ICs, is shown in Figs. 2 and 3. One advantage of the CLHS is the amount of surface area provided for heat transfer which is drastically increased through the use of a plurality nano-scale rods 220 that are spaced appropriately to allow for the flow of the cold source. Spacing between the nano-rods is, for example, 400 nanometers in each direction thus allowing for a flow of the cold source. Nano-rods are grown on a substrate 210, at a distance from each other to allow for the flow of a compressed cold source, and thereby form a channel in between the nano-rods, the nano-scale

channel having very small hydraulic diameters. To those skilled in the art, it is readily apparent that a higher heat transfer coefficient is achieved with smaller hydraulic diameters. In addition to increased heat transfer coefficients, a vast increase of surface area is achieved by growing nano- rods 220 on substrate the 210 that have a high aspect ratio. The presently preferred aspect ration of a nano-rod 220 is measured as the ratio between its diameter 'd' and length T. In an exemplary and non-limiting embodiment, where d=100 nano-meters and l=50 micro-meters, the nano-rod aspect ratio is 500, and the increase of surface area achieved thereby is approximately 90:1.

The current literature with regard to creating functionalized carbon nanotubes teaches various methods of creating conformal coating of nanotubes with various materials, e.g., metals, conductive polymers, etc.. Usually, the carbon nanotubes need a pre-treatment, e.g. high temperature annealing, to remove amorphous carbon found on the nanotubes, nanowires, nanofibers, or nanotowers. Furthermore, the nano-rods 220 may be coated by a coating 225 for the purpose of better heat transfer between the nano-rod 220 and the cold source. The conformal coating 225 of the nano-rods 220 may be achieved using highly thermally conductive materials, e.g. metals such as Pd, Au, Ag, Cu, and the like. For optimal functionality of the CLHS it is enclosed from all sides and further equipped with an inlet and an outlet, such that the cold source can flow in a known direction from the inlet to the outlet.

Fig. 4 shows an exemplary and non-limiting CLHS structure 400 further comprising an expansion chamber 420. The expansion chamber 420 is placed immediately after the inlet 410 thus allowing rapid expansion of the cold source that, as is well-known in the art, causes a decrease of temperature of the cold source by several degrees, further contributing to the cooling process. The design and proportions of the expansion chamber 420 may vary to suit the specific characteristics of the CLHS 400 and the cold source used, without departure from the spirit of the disclosed invention.

Fig. 5 shows a cross section of a PCB 520 on top of which an IC 510 is mounted. On top of the IC 510 there is mounted a CLHS, for example CHLS 400, that is used for the purpose of removing heat from the hot surface of the IC 510.

Returning to Fig. 1 , the discussion continues with respect to the control unit 110. One example of a suitably designed micro-pump for the liquid coolant that may be found is a 3.5 cm x 4.5 cm x 3.0 cm micro-pump with pressure drops in the range of 6 kPascal and 400 ml/ min flow rates. Similarly, an example of a suitably designed air micro-compressor of similar dimensions is readily found in the current art. Furthermore, the current art of MEMs (micro electronic mechanical systems) provides for a number of examples of micro- vales to control fluid, i.e. micro-fluid applications. Therefore, constructing the control unit 110 from such components is readily within the capabilities of those skilled-in-the-art.

As noted above, in the small dimensions of micro-electronic enclosure, e.g. electronic boxes, heat density is high. It is therefore important to have control of fluid coolant temperature at all points of the thermal path between the heat source, for example 120-2 and the heat exchanger in the control unit 110. The invention disclosed herein provides advantages over the prior art in various aspects. Cost of electronic cooling is minimized by sharing the cold source compressor, for example an air compressor, storage tank, and micro- liquid pumps required to cool the IC set at the board level. The compressor, the tank, and liquid pumps provide the control needed to achieve the cost/performance required at the heat dissipation level by the electronic box. The air compressor and storage tank can be, in some embodiments, replaced by one or more fans that blows air to the heat exchanger's heat sink when a shared micro-pump is used to pump the liquid cooling in serial mode thru an individual hot chip's heat sink or heat spreader. The embodiments disclosed herein provide for area minimization by sharing coolant moving devices, e.g. fan, pump, or blower, to move small volumes of the cold source used in the electronic enclosure..

Further advantages of the invention include, for example, the minimization of noise generated by the coolant-moving-devices due to the sharing of such devices for a plurality of ICs. As heat density generated by each chip increases, the cooling capacity is increased by increasing the coolant, e.g. air or liquid, flow and further by increasing the pressure required to move such coolant across the thermal path, as shown with respect to Fig. 1. The cooling loop may be a closed loop when a liquid or target gas are used, or an open loop when air is used. Pressure is controlled, for example, by the air storage tank, by micro-valves, or by a liquid pump in the cooling circuit. The path seen by air flow is controlled via pipes and conduits leading to appropriate heat sink fins/pins depending upon the air temperature required at these heat sink locations. The temperature of coolant fluid is under control of the control unit 110. The cooling components 120 are deployed in a manner depicted in Fig. 1 according to physical layout available from constraints in the design of PCBs where the micro-electronic chips are laid out. The physical location of the cold air inlet to the pump into the storage tank is chosen in a way to access the coldest possible source of ambient temperature air. Similarly, the hot air from heat exchanger in the control unit 110 is directed to regions of the PCB layout where no heating damage is inflicted to other micro-electronic chips. The storage tank is deployed in such a manner to provide an air flow at the appropriate orientation, as may be required by fins or pin direction of heat sinks, at a quantity and pressure required, and as allowed by physical constraints or spacing available for cooling components. This similarly applies to the pumps and the associated chip-level heat sinks.

Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the Claims included below.