Patent Application
20060131003
Embodiments of the present invention are generally directed to cooling systems and, more particularly, to an apparatus and associated methods for cooling a microelectronic device.
Microelectronic devices generate heat as a result of the electrical activity of the internal circuitry. In order to minimize the damaging effects of this heat, thermal management systems have been developed to remove the heat. Such thermal management systems have included heat sinks, heat spreaders, and fans, and various combinations that are adapted to thermally couple with the microelectronic device. With the development of faster, more powerful, and more densely packed microelectronic devices such as processors, improved cooling technology is needed to remove the generated heat to prevent overheating.
Embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements and in which:
Embodiments of a microelectronic cooling apparatus and corresponding methods are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.
Microchannel heat exchangers and associated techniques are emerging as an improved thermal solution for high-power, densely populated microelectronic devices such as processors and other integrated circuit (IC) dies. One such technique employs a microchannel heat exchanger in a single-phase liquid cooling system. The single-phase system uses a coolant, or cooling solution, in the liquid phase for heat transfer. Another technique employs a microchannel heat exchanger in a dual-phase, liquid-vapor cooling system, wherein the coolant undergoes partial vaporization during the heat transfer process.
Two challenges impede widespread adoption of microchannel cooling systems that rely on a liquid coolant in microelectronic devices. First, the coolant may freeze (e.g., during shipment of the device), damaging the cooling system and/or the microelectronic device. Second, traditional antifreeze solutions that are added to coolants to mitigate the freezing problem have relatively poor thermal and pump working fluid characteristics that render the coolant less effective and may be very corrosive to the elements of the microelectronic cooling system. Therefore, a coolant that simultaneously provides effective antifreeze characteristics and improved thermal and pump working fluid characteristics without corroding the elements of the cooling system is needed.
In one embodiment, the pump 114 moves a coolant through the microchannel heat exchanger 112 and the heat removal system 118. The pump may cycle the coolant through the elements of the cooling system 100 continuously, periodically, or intermittently. Pumps that are generally used in accordance with embodiments described herein may comprise any one or more of an electromechanical (i.e.—microelectromechanical system or MEMS-based) or electro-osmotic pumps (i.e.—electric kinetic or “E-K” pumps). However, the pump is not limited to these types and may comprise gear, piston, peristaltic, sliding vane, and centrifugal technologies among others without deviating from the scope and spirit of the present invention.
According to one embodiment, the coolant that moves between the heat removal system 118 and the microchannel heat exchanger 112 is designated ‘cold’ coolant 120, whereas the coolant that moves between the microchannel heat exchanger and the heat removal system is designated ‘hot’ coolant 116. ‘Cold’ is not intended to imply that the coolant is actually cold, although it may be. Rather, use of the term is relative to the ‘hot’ coolant, in that ‘cold’ coolant refers to the coolant before it has absorbed heat 104 generated by the IC die 102 within the microchannel heat exchanger 112. Conversely, ‘hot’ is not intended to necessarily imply that the coolant is actually hot, although it may be. Rather, ‘hot’ coolant refers to the coolant after it has absorbed heat 104 generated by the IC die 102 through the microchannel heat exchanger 112 and before passing through the heat removal system 118. The words ‘hot’ and ‘cold’ when used in reference to the coolant generally describe the relative heat of the coolant as it is processed through the cooling system 100.
Those skilled in the relevant art will appreciate that a variety of factors may affect a coolant's heat transfer ability and working fluid efficiency. In one embodiment, typical characteristics that affect a coolant's heat transfer and working fluid effectiveness include specific heat, conductivity, and viscosity among others. Higher specific heat, higher conductivity, and lower viscosity improve a coolant's ability to transfer heat and efficiently operate as a pump working fluid. These characteristics allow for lower pressure drop and flow rate requirements from a pump for a given thermal resistance. Improved heat transfer and pump efficiency save energy and effectively reduce the size and cost of the pump needed to provide sufficient heat removal. This benefit is particularly relevant in a technology where the size and cost of a pump are important considerations for cost-effective implementation of cooling systems for microelectronic devices.
In one example embodiment, the coolant 116 and 120, which may be the same coolant carrying different levels of relative heat, comprises a solution including Potassium Formate. The solution may include water or any another suitable medium or solvent or combination of suitable solvents. A Potassium Formate solution, designed as disclosed herein, prevents freezing and has very good thermal and pump working fluid characteristics.
In its native state, Potassium Formate can be extremely corrosive. According to one embodiment, the Potassium Formate solution disclosed herein may include corrosion inhibitors, which effectively prevent damage to the microchannel heat exchanger 112 and other elements of the cooling system 100. According to one embodiment, biocide may also be added to the Potassium Formate solution to inhibit bio-growth in the solution and the elements of the cooling system 100. Inhibitors may comprise one or more of corrosion inhibitor(s) and/or biocide(s).
The Potassium Formate solution may be selectively processed through a microchannel heat exchanger 112 and other elements of a cooling system 100. The Potassium Formate solution is particularly well-suited for use in microchannel geometries where flow stability is dependent on thermal and pump working fluid characteristics. Those skilled in the art will appreciate that the coolant flow rate may be varied to change the heat transfer rate and uniformity within the microchannel environment and within the cooling system 100.
According to one embodiment, the pump 114, when actuated, moves cold coolant 120 through the microchannel heat exchanger 112, which transfers heat 104 generated by the IC die 102 to the cold coolant 120. The cold coolant 120 becomes hot coolant 116 after it absorbs the generated heat 104. The pump 114 moves the hot coolant 116 out of the microchannel heat exchanger 112 into a heat removal system 118.
The heat removal system 118 may remove heat from the hot coolant 116 in a variety of ways. In one embodiment, the heat removal system 118 may include fins 126 that increase surface area contact with the incoming hot coolant 116 to enhance heat transfer 122 away from the hot coolant 116. Fins 126 are intended to represent any feature that increases the surface area contact with the coolant to enhance heat transfer. In addition, a fan 124 or similar device may increase the flow of air or another fluid across the surface area 126 of the heat removal system 118 to further enhance heat removal 122.
The hot coolant 116 becomes cold coolant 120 after the heat has been removed from the hot coolant 116 in the heat removal system 118. The pump 114 may circulate the cold coolant 120 leaving the heat removal system 118 back through the microchannel heat exchanger 112 to absorb more heat 104 generated by the IC die 102. This process may be continuous.
The coolant may pass through a plurality of microchannels 214 of width, W, and length, M. Microchannels 214 may be micromachined heat sinks separated by microchannel walls 216 of thickness, t, and length, N. The microchannels may be arranged in an array 2221 . . . n of a repeating structure comprising a microchannel 214 adjacent to a microchannel wall 216 such that n represents a variable number of repeating structures. As the coolant passes through the microchannel array 2221 . . . n, heat transfer may occur between the microchannel walls 216 and the coolant. Heat transfer may also occur between the base of the microchannel trough and the coolant. The coolant may pass through an outlet reservoir 218 with width, R, and may exit through an outlet 220 with diameter, d, and distance, H, from the wall 206 exterior.
The length X 224 and width Y 226 of the microchannel heat exchanger 200 may be designed to accommodate different sizes. During design, Length X 224 and width Y 226 may be adjusted by varying the dimensions of the heat exchanger's 200 subcomponents in each direction. Those skilled in the art will appreciate that dimensions s, Ts, d, H, R, W, t, N may be designed to improve heat transfer in accordance with coolant properties. Though the microchannel heat exchanger 200 depicted in this embodiment is rectangular, the shape will generally correspond to the die to which the heat exchanger is thermally coupled and is adaptable. Likewise the microchannels 214 may vary in shape such that the channels may be rounded as opposed to rectangular or may embody other geometries to accomodate heat transfer characteristics such as temperature and pressure drop profiles.
Turning now to
When coolant enters inlet 210, it passes into an inlet reservoir 212, which may operate as a manifold for the coolant before it flows through the microchannels 214. The coolant flows through one or more microchannels 214 with height, p, absorbing heat from the microchannel walls and floor. The coolant passes from the microchannels 214 into an outlet reservoir 218 and exits through outlet 220.
Those skilled in the art will appreciate that the thickness, width, length, height, radius, and other geometries of the microchannel heat exchanger's features may be designed to improve heat transfer rates and uniformity. One embodiment may be designed to include microchannels of varying width, length, and/or height in relationship with each other. In general, a configuration will be a function of the heat transfer characteristics of the coolant, the heat transfer characteristics of the material used for the heat exchanger, the pumping characteristics of the coolant, and the desired heat exchanger area.
The elements of the microchannel heat exchanger, including the microchannel walls 216, bottom 302, and walls 202, 204, 206, and 208 may be made of any suitable material including materials or combinations of materials from the following group: silicon, copper, diamond, and metals. Cover plate 304 may also be made of any suitable material including materials or combinations of materials from the following group: silicon, copper, glass, diamond, plastic, and metals.
As introduced above, according to one embodiment, a microelectronic cooling system may use a cooling solution containing Potassium Formate. The Potassium Formate coolant referenced herein is capable of withstanding temperatures down to at least −45° C. for at least 24 hours without freezing, which is a typical storage and shipping requirement for microelectronic devices. Inhibitors may be added to prevent corrosion and bio-growth in the cooling system environment. In one embodiment, corrosion inhibitors and biocide are added to the Potassium Formate coolant to prevent corrosion and bio-growth within the cooling environment. Those skilled in the art will appreciate that the composition design of an effective antifreeze will depend on several factors including freeze temperature, thermal properties, volume expansion, corrosiveness, and bio-growth limit. Moreover, the Potassium Formate coolant has improved thermal and pump working fluid properties that generally make it more suitable than traditional coolants to operate in a microelectronic cooling system and specifically in a microchannel environment.
Graph 400 depicts water 406 as a baseline coolant for comparison because of its superior pump requirement properties. One coolant comprising 50% Propylene Glycol 410 exhibits a comparatively high pressure drop when compared to water 406. Another coolant comprising liquid metal 412 demands a very high flow rate in comparison to water 406. The coolant comprising 40% Potassium Formate 408, however, is significantly more closely-matched to the pressure drop 402 and flow rate 404 of water 406 than the 50% Propylene Glycol 410 and liquid metal 412 making it a better coolant based on pump requirements. The 40% Potassium Formate 408 may be a better-suited coolant than water 406 in microelectronic devices, however, because it can withstand much colder temperatures without freezing. Research indicates that 30-50% Potassium Formate may be a suitable range for antifreeze and coolant applications in microelectronic devices, however, the composition is not limited to this range.
Though the flowchart 500 depicts that the processes are attached with unidirectional arrows, this is solely for ease of illustration purposes and does not necessarily limit or imply the ordering of the processes. For example, the process or processes may be continuous. The method may be part of a closed-loop or open-loop process. These steps may occur out of sequence or not at all. Heat transfer may continue to occur after the elements of the process, such as the microelectronic device or the pump, have been turned off and so forth.
The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
