Abstract:
A circuit board cooler employs a closed fluid delivery system to transfer heat from electronic components to a cooling fluid. One or more channels embedded within the circuit board carry cooling fluid to the components where the fluid absorbs heat from the component, then away from the component to deposit the excess thermal energy in a thermal sink, such as a heat exchanger. The cooling system may employ conductive thermal transfer elements, such as “thermal vias”, to enhance heat transfer from electronic components to the cooling fluid.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the cooling of electronics, and more particularly to high capacity cooling systems for electronics located on circuit boards. 
     BACKGROUND OF THE INVENTION 
     As electronics systems continue to decrease in size and increase in performance (with a concomitant increase in power consumption), power density becomes a major design issue. That is, electronic circuits operate efficiently and effectively only over a prescribed temperature range. Operating outside this range can degrade and even destroy the circuit. Although there are many performance and price motivations for packing as much circuitry into as small a volume as possible, packing too much circuitry into a given volume creates a power density that could destroy the electronic circuitry. Many electronic circuit cooling systems, both active and passive have been employed over the years to varying degrees of effectiveness. U.S. Pat. Nos. 5,719,444; 6,313,992 B1; 5,880,931; 5,701,751; 4,392,153; 4,573,067; 5,239,200; 5,345,107; 5,049,973; 5,373,417; 6,141,214; 6,105,661; 6,190,941; 6,101,094; and 3,746,942 disclose various cooling systems and methods for electronics systems and are all hereby incorporated by reference. 
     Notwithstanding the performance afforded by conventional circuit board cooling systems, a circuit board cooling system that provides high capacity cooling at a relatively low cost, and which occupies very little space would be highly desirable. 
     SUMMARY OF THE INVENTION 
     A circuit board cooler employs a closed fluid delivery system to transfer heat from electronic components to a cooling fluid. A fluid cooling system in accordance with the principles of the present invention includes one or more channels embedded within a circuit board that supports electronic components to be cooled. The electronic components may be discrete components, such as capacitors, resistors, or inductors, or integrated circuits, for example. Additionally, the components may be surface-mounted or through-hole mounted components. Each channel carries cooling fluid to and from at least one component. The cooling fluid absorbs excess thermal energy from the component and disposes of the excess heat using any of a variety of methods including the use of active or passive devices, such as heat exchangers or heatsinks, for example. After cooling, the fluid may be re-circulated for further cooling of the component of interest. Each circuit board layer may include a substrate having first and second surfaces and composed of a dielectric material, for example, with copper deposited on one or both of the first and second surfaces (also referred to herein as “top” and “bottom” surfaces). The deposited copper may also be referred to herein as a layer, although it may be patterned to form, not a sold layer of copper as for a power or ground plane, but, traces for the inter-connection of electronic devices. 
     Each cooling channel may be formed as a void in an inner layer of a multi-layer circuit board, with inlet and outlet ports formed in one or more adjoining layers for the introduction and evacuation, respectively, of the cooling fluid. A plurality of cooling channels may be formed in an individual circuit board. Circuit boards having a plurality of channels may have one or more channels formed in each of a plurality of layers. One or more thermal conduction enhancements, such as “thermal vias”, for example, may be employed to improve the heat transfer efficiency between the electronic component being cooled and the cooling fluid within a channel. 
     A circuit board cooler in accordance with the principles of the present invention may be particularly effective at cooling electronic components supported on both sides of a circuit board. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings as described below. 
     FIG. 1 is an exploded view of a multi-layer circuit board with an embedded cooling channel in accordance with the principles of the present invention; 
     FIG. 2 is an exploded view of a multi-layer circuit board with a deep embedded cooling channel in accordance with the principles of the present invention; 
     FIG. 3 is an exploded view of a multi-layer circuit board with an embedded cooling channel in accordance with the principles of the present invention; 
     FIG. 4 is an exploded view of a multi-layer circuit board with a plurality of embedded cooling channels in accordance with the principles of the present invention; and 
     FIG. 5 is a top plan view of a circuit board layer having voids formed therein such as may be employed to form one or more embedded cooling channels within a multi-layer circuit board. 
    
    
     DETAILED DESCRIPTION 
     A circuit board cooler employs a closed fluid delivery system to transfer heat from electronic components to a cooling fluid. A fluid cooling system in accordance with the principles of the present invention includes one or more channels embedded within a circuit board that supports electronic components to be cooled. The electronic components may be discrete components, such as capacitors, resistors, or inductors, or integrated circuits, for example. The exploded side view of FIG. 1 illustrates a segment of a multi-layer circuit board  100  in accordance with the principles of the present invention. In this illustrative embodiment the circuit board  100  includes three layers  102 ,  104 , and  106 . Each layer includes a substrate  108  that supports “top” and “bottom” copper layers,  110  and  112 , respectively. A substrate  108  may be composed of a dielectric material and may be included within a multi-layer circuit board with no copper layers, with a copper layer on only one surface, or with a copper layer on both major surfaces. Since each copper layer may be patterned to form a sold layer of copper, as for a power or ground plane, or traces for the inter-connection of electronic devices, which may be surface-mounted or through-hole mounted components, for example. The conductive layers supported by a substrate need not be composed of copper, but copper, or a copper alloy, is typically employed for various reasons, including a relatively high conductivity-to-cost ratio. 
     An epoxy resin  114  may be used to join a plurality of layers to form a multi-layer circuit board. Such a circuit board formation may be accomplished using any of a number of processes known in the art, one of which is discussed in U.S. Pat. No. 5,478,972 issued to Mizutani et al., which is hereby incorporated by reference. Electronic components, such as components  116  and  118 , may be placed on the top  120  and/or bottom  122  surfaces of the circuit board  100 . The electronic components may be discrete components, such as capacitors, resistors, or inductors, or integrated circuits, and they may be surface-mounted or through-hole mounted components, for example. 
     Although the dielectric, copper, and resin layers may be of any thickness, they are typically on the order of 490, 1.4, and 3 mils thick, respectively. A void  124  is formed in the middle layer  104  of the multi-layer circuit board before the layers are joined to form a multi-layer circuit board. The void is such that it forms a channel between the two outer layers  102  and  106  when the multi-layer board is formed. Channels may be formed by creating voids that do not extend to a layer&#39;s edge, thereby leaving intact portions of the layer (that is, dielectric substrate and copper cladding, if cladding is used), to form channel stops. A void may be formed by cutting a portion out of the layer  104 , for example. All channels/voids illustrated in FIGS. 1 through 4 are shown from an “end on” view. The width, W, of a channel may vary according to the mechanical, electrical and thermal properties of a specific board layout and the height, H, will typically be that of a layer&#39;s thickness, approximately 500 mil, for the example of layer  104 . 
     One or more inlets  140  and outlets  141  (as shown in FIG. 5, and not shown in FIGS. 1-4) may be formed through one or more of the layers adjoining the void to thereby provide fluid inlet and outlet ports to the cooling channel. Inlet and outlet ports may be formed through the same adjoining layer or through opposite adjoining layers. Each channel carries cooling fluid to and from at least one component. The cooling fluid absorbs excess thermal energy from the component and disposes of the excess heat using any of a variety of methods including the use of active or passive devices, such as heat exchangers or heatsinks, for example. After cooling, the fluid may be re-circulated for further cooling of the component of interest. A pump may be used to impel cooling fluid from the one or more inlet ports, through the channel, then out the one or more outlet ports. The cooling fluid may be a nonflammable, nonconductive fluid, such as Flourinert™, available from 3M Company, St. Paul Minn. 
     A plurality of cooling channels may be formed in an individual circuit board. Circuit boards having a plurality of channels may have one or more channels formed in each of a plurality of layers. A single channel may be formed by connecting one or more voids, such as void  124 , within a layer with one or more voids in one or more other layers to form a multi-layer channel. A channel may be formed in one or both exterior layers  102 ,  106  by creating a void within respective interior copper  112  and  110  and dielectric  108  sub-layers, while leaving respective exterior  110  and  112  copper layers intact. The respective exterior  110  and  112  intact copper layers would then form a “cap” for the channel. Additionally, the close proximity of cooling fluid to the copper cap and the high thermal conductivity of the copper cap would increase the thermal transfer efficiency from the one or more components being cooled to the cooling fluid contained within the channel. By “intact copper layer” we mean that at least enough copper to seal the channel remains after the void is formed. Additionally, the channel “cap” may be formed of any material, but, the use of copper would be particularly convenient, especially in an application where the copper also forms a circuit element, such as a ground or power plane. One or more thermal conduction enhancements, such as “thermal vias”  126 ,  128  in FIGS. 1,  2 , and  4 , and  126 ,  128 ,  130 , and  132  in FIG. 3, for example, may be employed to improve the heat transfer efficiency between the electronic component being cooled and the cooling fluid within a channel. Various types of thermal vias are known. Some are described, for example, in U.S. Pat. No. 6,190,941 issued Feb. 20, 2001 to Heinz, et al., which is hereby incorporated by reference in its entirety. 
     The exploded sectional view of FIG. 2 illustrates the basic elements of a multi-layer circuit board with embedded cooling channel in accordance with the principles of the present invention. In this illustrative embodiment, a plurality of interior layers include voids that, when all the layers are combined into a single printed circuit board, form a deeper channel than would be possible using a single layer of the same thickness. That is, a layer  105  includes a void  125  that coincides with the void  124 . The voids  124  and  125  may have substantially the same outline and substantially overlap to form a continuous, deep, channel, for example. Additionally, whether the voids have substantially the same overall outline or not, each void may be broken up into smaller voids by “inclusions” of the layer within which the void is formed. That is, rather than cutting the same pattern out of all layers, one or more portions of the pattern may be cut from one or more layers to channel segments in one or more layers. The channel segments may be formed in proximity to the location of electronic components that have a critical cooling requirement, thereby permitting the cooling fluid to absorb heat from locations of critical need, without absorbing heat from components whose thermal dissipation may be handled by other means. 
     The exploded sectional view of FIG. 3 illustrates a multi-layer circuit board having a plurality of cooling channels within the same circuit-board layer in accordance with the principles of the present invention. The channel-forming voids  124 , 127  have thermal vias  126 , 128  and  130 ,  132  respectively associated with them. As with other embodiments, each channel may have more than one thermal vias and the thermal vias may be formed in either of the adjoining layers  102  or  106 . Additionally, the channels formed by the voids may be linked to form a single channel, in which case, a single inlet port and a single outlet port may be employed to supply cooling fluid throughout the entire, combined, channel. 
     The exploded sectional view of FIG. 4 illustrates a multi-layer circuit board having non-overlapping channels formed in a plurality of inner layers. Voids  129  and  131  are respectively formed in layers  104  and  111 , and the board includes a layer  109  that intervenes between layers  104  and  111 . If a void (not show) within the intervening layer  109  connects the voids  129  and  131 , a single multi-layer channel may be formed and an inlet port on one exterior surface may be used to supply fluid coolant through the channel to an outlet port on the other exterior surface. Either port,  140  or  141 , may act as an inlet or an outlet port. 
     The top plan view of FIG. 5 provides a view of a circuit board having a variety of buried cooling channels in accordance with the principles of the present invention. As is meant to be demonstrated by this illustrative view, embedded cooling channels may take on an infinite variety of shapes and sizes. For example channel  502  is a straight channel, channel  504  is a curved channel, and channel  506  is a double-curved channel. 
     Although various exemplary embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. It will be apparent to those reasonably skilled in the art that other components performing the same functions may be suitably substituted. The foregoing description of specific embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described to best explain the principles of the invention and its practical application, and to thereby enable others skilled in the art to best utilize the invention. It is intended that the scope of the invention be limited only by the claims appended hereto.