Abstract:
Cold plates, permanently plumbed into card cage, cool circuit boards that are equipped with personality plates to improve thermal transfer. The personality plates have a contoured surface that is the complement of the circuit board&#39;s landscape. The personality plate made of heat transmissive material therefore contacts each heat generating component, regardless of its height. The opposite surface of the personality plate is flat, and, when installed with the circuit board in the card cage, lies parallel to the surfaces of the cold plate. A card ejector applies force on the entire board bringing the flat surfaces into intimate contact for excellent heat transfer. The assembly of circuit board and personality plates is augmented by interposing thermal interface material at each contact surface and fastening this assembly into a unit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to provisional patent application serial No. 60/305,480 filed Jul. 13, 2001, the disclosure of which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to cooling of circuit boards and, more specifically, to an apparatus and methods used for liquid-cooling circuit boards. 
     Electronic components mounted on circuit boards generate heat that must be dissipated to assure proper functioning of the components. Air is typically used to cool the circuit board when the total power dissipated is low or when the power density is low. The use of fans, ducting and/or heatsinks to improve air cooling is well understood and widely used in industry. 
     Air provides insufficient cooling when the total power used is high and in high density power applications because of air&#39;s relatively low thermal capacity. In high power applications, liquid can be used to provide significantly improved cooling, but at an added complexity level, since provision must be made to contain the liquid so it does not contact the components directly. 
     A way to contain cooling liquid is to use a liquid-cooled cold plate. A liquid-cooled cold plate is typically made of copper, aluminum or their alloys, although other materials can be used. It has channels within it that distribute the cooling liquid and has inlets and outlets that enable the liquid to enter and exit the cold plate. The cold plate is then mated to the electronic circuit board that requires cooling. Electrical components on the circuit board that touch the cold plate are cooled. They become cool because of their close proximity to the cold plate that transfers the heat to the cooling liquid, but at no time do they actually touch the cooling liquid directly. 
     The fact that cold plates are co-mounted with the circuit boards implies that these cold plates must be removable when the circuit board is removed. Removing the cold plate implies severing the liquid cooling connections. When leaks occur in cold plates, they do not occur within the cold plate but rather occur where the plumbing connections are made to the plate. The plumbing connection is especially challenging when there is a need for the connection to be temporary. 
     While liquids cool much more effectively than air, systems using cooling liquids are subject to leaking. If a liquid leak occurs in the proximity of an electronic circuit board, it can create a serious problem. This problem can be mitigated by choice of liquid. Water has an excellent heat transfer function, but a water leak can short out electrical components and cause permanent damage. High dielectric fluids, such as fluorocarbons, have been used in liquid cooling to limit the damage caused by a leak. The disadvantages to this approach are that fluorocarbons have a less advantageous heat transfer function than water and fluorocarbons are expensive. In addition, fluorocarbons easily leak through conventional pump seals increasing the number of leaks. In addition, fluorocarbons can still cause damage to electronic systems. 
     There is a wide variety of cold plate technologies currently in use. Lower performance cold plates commonly use internal metal tubes to distribute the liquid. Higher performance cold plates commonly use vacuum brazed construction. Vacuum brazing allows the use of high performance fins placed within the liquid channel at locations where better heat transfer is required with the cold plate surface. 
     A further difficulty with cold plate cooling is that the components on a circuit board are seldom at the same height. If the cold plate is substantially planar, only the tallest components will touch the cold plate and be cooled. For small variations in height (˜0.01″), compliant thermal interface material can be applied to lower components allowing the components to touch the cold plate through the interface. However, when the variations are larger, the compliant thermal interface material is not sufficient to bridge the gap. 
     A cold plate having a “personality profile” impressed on its side to mate with the component board is one solution to the larger gap issue. This personality profiled cold plate is thicker in places where short components are on the circuit board and is thinner where the taller components are located. Such personalized cold plates must be custom made and can only be used with one type of board. They are difficult to manufacture because the internal liquid path must be adapted to the personality contours of the cold plate. Typically these plates must be inserted and extracted with the circuit board, requiring the use of temporary connections to the cooling liquid source. 
     BRIEF SUMMARY OF THE INVENTION 
     A populated printed circuit board cooling system comprises a cold plate and a set of personality plates that assure good contact between the printed circuit board and the cold plate. The cold plate is formed of a heat transmissive material that has internal liquid circulating paths. The cold plate must have an inlet and an outlet port for the circulating liquids. These ports are incorporated in at least one header portion of the cold plate for permanent connection to a liquid circulating source and return. If only one header is used, it is formed as a split header separating the supply and return. The ports may also be split between two headers. The cold plate has a header portion and a flat portion having two flat planar surfaces. The flat planar surfaces may be the opposite sides of a rectangular block or may be joined to form a wedge. The header portion is connected to the top and/or bottom of the flat portion. The remaining sides are sealed to provide a closed environment for the circulating liquid. Each personality plate is formed of a heat transmissive material and has a flat side and a contoured side. The contoured side conforms to the contour of one side of the populated printed circuit board. When the flat surfaces of the cold plate and a personality plate are butted against each other and the side of the printed circuit board is cradled in the contoured side of its personality plate, heat is transferred from components on the printed circuit board through the personality plate to the cold plate to be removed by a circulating liquid. 
     The heat transmissive materials available include copper, aluminum and their alloys. One or two headers can be used, and each header can have baffles and gates to direct the liquid in a particular circuit through the cold plate. Thermal interface material may be used within the system wherever heat transference is desired. The thermal interface material improves the thermal contact between the heat sources and the cooling mechanisms. 
     An apparatus for cooling the components on printed circuit boards is made up of a card cage incorporating cold plates and circuit board assemblies that adapt the printed circuit boards to the cold plates and card cage. The plurality of cold plates are each connected to a mechanism that is circulating cooling liquid through a plurality of headers mounted to the top and/or bottom of the cold plates. Each cold plate is formed as a wedge shape with the flat sides oriented towards the sides of the card cage, a narrow end oriented toward the front of the card cage and a wider end oriented toward the rear of the card cage. The cold plates are made of a heat transmissive material. 
     Each circuit board assembly is custom formed based on its printed circuit board. A pair of personality plates are tailored each to a respective side of the printed circuit board. One side of each personality plate is flat; the other is contoured. The first plate contoured surface is complementary to the contours of a first side of its printed circuit board and the second plate contoured surface is complementary to the contours of the second side of its printed circuit board. The personality plate flat surfaces are angled such that each completed circuit board assembly with personality plates is wedge shaped with a wedge angle complementary to the cold plates&#39; angle. A fastening mechanism holds the personality plates and printed circuit board tightly pressed together forming the circuit board assembly. The circuit board assemblies are mounted in the card cage with the narrow end facing the rear of the card cage, so that the flat surfaces of the personality plates and their matching cold plate are parallel when installed. Interface material may cover the plate flat surfaces to improve thermal transfer. A plurality of clamping mechanisms, each structured to fully seat a circuit board assembly between a pair of cold plates, press the flat surfaces of the cold plates and the personality plates together forming a thermally conductive path from the components, through the personality plates to the cold plates. 
     A method of cooling populated printed circuit boards comprises fabricating a card cage incorporating wedge-shaped cold plates that are connected, permanently or otherwise, to a liquid circulating mechanism. The cold plates are arranged to allow board mounting between the cold plates. Printed circuit board assemblies are formed by sandwiching each populated printed circuit board between personality plates. One side of each personality plate conforms to the contours of one side of a particular populated printed circuit board and the other side has a flat surface. The personality plates are made of a heat transmissive material. When the printed circuit board assemblies are inserted into the card cage with the flat sides of the personality plates parallel to the flat sides of the cold plates heat transfer is enabled. By applying force to press the printed circuit board assemblies and cold plates together, the heat transfer is improved and circulating liquid removes the heat from the cold plates. Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be understood from the following detailed description in conjunction with the drawings, of which: 
     FIG. 1 is a perspective drawing of an embodiment of a cold plate according to the invention; 
     FIG. 2 is a perspective drawing of a card cage adapted for the invention; 
     FIGS. 3A,  3 B, and  3 C are a side view of a portion of a circuit board assembly according to the invention; 
     FIGS. 3D and 3E are a top view of circuit board assemblies and cold plates intermixed according to the invention; 
     FIG. 4A is a section view of the card cage of FIG. 2; 
     FIG. 4B is a detail view of an interface between a cold plate and a circuit board assembly according to the invention; 
     FIG. 5 is a second section view of the card cage of FIG. 2; 
     FIG. 6A is a perspective drawing of a card cage using both liquid cooling and air cooling according to the invention; 
     FIG. 6B is a top view of a circuit board assembly for used in the card cage of FIG. 6A; and 
     FIG. 7 is a side view of an embodiment of and assembly to provide contact between cold plates and circuit board assemblies according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of a cold plate according to the invention is shown in FIG.  1 . The cold-plate  8  is a heat-transmissive structure, cooled by a circulating liquid, adapted to be placed near a heat source to cool the heat source. The heat-transmissive material may be copper, aluminum, and alloys based on copper or aluminum. The cold plate  8  is composed of at least two main parts, the flat portion  10  and the header portion  20 . The flat portion is commonly wedge shaped as shown in FIG. 1, but can also be box shaped. Flat side  12  of the flat portion  10  is most usefully rectangular shaped, although other outlines can be accommodated. Wedge shaped flat portion  10  has two plate-like flat sides that are joined at the edges  18  to form a wedge where front face  14  is narrower than rear face  16 . 
     FIG. 1 illustrates a single header  20  permanently attached to the top  19  of the flat portion  10  typically by a brazing, welding, soldering or similar process. The single header  20  is split to perform a dual function, bringing cooling liquid to the interior of the flat portion  10  and removing heated cooling liquid from the flat portion  10 . Internal to the header  20  are channels and baffles that direct the liquid in a circuit from the source, past expected hot spots of the flat portion  10 , and back to a return. In FIG. 1, the front end  22  of header  20  is closed off and the rear end  24  is available to connect to a liquid circulating system (not shown). The single split header  20  can be connected to either the top or bottom of the flat portion  10 . The bottom of the flat portion is sealed when one header  20  is used at the top. While the cold plate of FIG. 1 is shown with one header  20 , other embodiments with a top header and a bottom header bracketing the flat portion  10  allow for alternate circulation patterns. When two headers are used at the top and bottom, they may each be split headers, allowing a complex circulation pattern, or one may be dedicated to the source connection and the other dedicated to the return connection. 
     An item to be cooled could be placed on the flat portion  10  of cold plate  8 . If the item to be cooled is not flat enough to allow good heat transfer, a heat transmissive interface material may be interposed between the item and the flat portion  10 . The industry makes interface material that exhibits varying thermal transmissivity characteristics combined with other characteristics such as compressibility and electrical insulation. A designer charged with cooling the item specifies the characteristics of the interface material to be used. It is preferable to provide compression to assure a good contact between the item to be cooled and the cold plate. Such compression can be provided by weight or pressure on the item and interface material pressing the to the cold plate. 
     An adaptive personality plate for the item could be made to improve the interface between the cold plate and the item. The adaptive personality plate conforms to the item shape on one side and presents a flat surface on the other. Such an adaptive personality plate may be made of aluminum, copper, alloys of copper or aluminum, metal filled composite or other moldable material with high thermal transmissivity. Interface material may be interposed between the item and the personality plate and/or between the flat surface of the personality plate and the flat portion  10 . The personality plate may in addition be fastened to the item providing compression to assure a good thermal contact. 
     When numbers of circuit boards are to be cooled by cold plates, card cages are typically designed to hold the boards and the cold plates in close contact. The design of an appropriate card cage requires consideration of such factors as height of components to be used on the circuit boards, spacing between the cold plates, amount of heat to be rejected, location of hot spots on the circuit boards and cost. 
     An embodiment of a card cage designed to use cold plates to cool particular circuit boards fitted with personality plates is shown in FIG.  2 . In FIG. 2, a card cage  30  is shown having an approximately solid top  30 , bottom  44  and sides  32 . Circuit board assemblies  36  are inserted in the front  42  and electrical connections are made through the back  40  or front  42  as appropriate. Cold plates  34  and their connections to a liquid cooling system (not shown) are built into the card cage  30 . The liquid-cooled cold plate  34  can be permanently installed in the card cage  30  allowing permanent plumbing connections. The permanent plumbing connections reduce the risk of leaks allowing the use of water as the cooling medium, greatly improving the thermal performance and providing a much lower system cost than when using fluorocarbons. The cold plates  34  are arranged so that they are interleaved with the circuit board assemblies  36 . 
     FIGS. 3A-3C illustrate the make-up of the circuit board assemblies  36 . FIG. 3A shows a side view of the circuit board  51  comprising the board  50  with components  52  mounted on it. The components  52  mounted on this circuit board are of various heights but none is higher than component  52   a.  The majority of the heat being generated by the circuit board is generated by the components. In FIG. 3B, interface material  56  has been placed on the tops of all heat generating components  52 . The interface material  56  will assure good thermal contact between the board  50 , components  52  and an adjacent structure. Other implementations may use a thin interface material that covers the entire board including the components, but contact between the board itself and the interface material is usually not required. 
     In FIG. 3C, a completed circuit board assembly  36  is shown. Adaptive personality plates  60 ,  62  have been fit to the opposite sides of the circuit board. Each adaptive personality plate  60 ,  62  is made of a thermally transmissive material and has a contoured side  66  and a flat side  68 . The heat sources  52  contact the contoured side  66  and the flat side  68  is on the outside of the circuit board assembly  36 . The adaptive personality plates  60 ,  62  have an approximate tapered wedge shape at an angle a to dotted line parallel to the board  50 . The contoured surface  66  has a nominal surface  70  parallel to the plane of the board  50 . Sunk into the nominal surface  70  are cavities  72  that correspond to the components  52 , each cavity being sized for its component&#39;s shape and height. A minimum thickness  74  of personality plate is maintained, even over the tallest component  52   a  to assure adequate structural integrity and thermal transmissivity. 
     A fastening device  58 , such as a recessed screw or bolt, holds the circuit board assembly  36  together. A layer of interface material (not shown) may be attached to the flat surfaces  68  of the circuit board assembly  36  before it is inserted in the card cage  30 . FIGS. 3D and 3E are a top view of interleaved cold plates  140  and extremely tapered circuit board assemblies  150  to illustrate the operation of the tapered shapes in assembling the system. The actual taper is usually very slight 3-5°, but has been exaggerated in the figures. The cold plates  140  are plumbed to the cooling liquid source  152  and fixed in position with the narrow edge  154  facing the front of the card cage. Circuit board assemblies  150  are composed of a circuit board  142  and a set of personality plates  144 ,  146  clamped together with thermal interface material placed as needed. In FIG. 3D, the circuit board assemblies  150  are partially placed in the card cage. The flat sides  158  of the cold plates and the flat sides  156  of the personality plates  144 ,  146  are parallel and spaced apart allowing insertion of the circuit board assembly  150 . In FIG. 3E, the circuit board assemblies  150  are seated in position. The respective flat sides  158 ,  156  are parallel and butted against each other forming a good thermal transmission path. 
     FIG. 4A shows a view along section A—A of card cage  30 . In this view, the front  80  of the card cage  36  is at the left and the back  82  of the card cage  30  is at the right. Sides  83  of the card cage  30  are to the top and bottom respectively. The cold plates  34  are shown with the tapered portion  96  having a brazed connection  84  into a cold plate manifold  94  for distributing the liquid. In the tapered portion  96 , the narrower end  95  is positioned toward the front  80  and the wider end  97  is positioned toward the rear  82 . FIG. 4A shows the fins  99 , for rejecting heat into the liquid, internal to the tapered portion  96 . Circuit assembly  36  is mounted between two cold plates  96 ,  100 . The circuit assembly  36  is tapered with the narrow end  102  toward the rear  82  of the card cage  30  and the wider end  104  toward the front  80 . Electrical connection  92  to the circuit assemblies  36  is at the rear  82  of the card cage  30  in this embodiment. 
     When the circuit assemblies  36  are inserted in the card cage  30 , they are first inserted into the slots between the cold plates  34  by hand. The tapers of the tapered portion  96  and the personality plates  86  are such that the flat surfaces of the cold plates  34  and the circuit assemblies  36  are parallel during this insertion, but the clearance is sufficient that the placement of the circuit assemblies  36  is allowed. The ejection handles  90 , incorporating a soft element such as a spring, provide sufficient force to seat the circuit assemblies  36  into position compressing the cold plates and circuit board assemblies together improving the thermal contact. Such ejection handles  90  provide a pressure that is distributed along the entire edge of the circuit board assembly and engages the thermal plates and any electrical connection in a backplane when the handles are closed. FIG. 4B illustrates the contact where the personality plate  86  is intimately contacting the tapered portion  96  through an interface material  88  between them. 
     FIG. 5 is a view along section B—B of the card cage  30 . The tapered portions  96  of the cold plates  10  are a uniform thickness in this direction. The area for a liquid, supplied through manifolds  110 , is shown internal to the tapered portion  96 . Circuit board assemblies  36  are interleaved between the taper portions  96  with personality plates  86  on each side of the printed circuit board. 
     The cold plate  34 , through which liquid is circulating, then cools off the personality plate  86 , which in turn, then cools off the components  52  of the electronic circuit board  51 . Interface material improves the thermal connection between the cold plate and the personality plate but may be omitted. Because of the narrow angle of the taper, it is possible to achieve a relatively large clamping force between the personality plate and the cold plate. The large clamping force further improves the thermal connection between the circuit board and he personality plate. 
     The card cage  30  and circuit board assembly  36  embodiment above are illustrative of possible configurations. Alternate configuration could utilize printed circuit boards with components having a greater height necessitating a wider spacing between cold plates  8 . For configurations utilizing single sided boards, only one personality plate would be needed, but the taper for that personality plate would have to equal the taper of the tapered portion of the cold plate contacting the board assembly. Interface material could be applied to the non-component side of the circuit assembly to improve heat transfer. Cold plates can be made having a flat area with an area of up to 24″×20″ but there is significant challenge to producing sides with sufficient flatness. There are several methods to fabricate cold plates with each having different size and cost limitations. The more economical cold plates can be formed by an extrusion process. 
     FIG. 6A illustrates a configuration that might be created for a set of circuit boards that cluster high power/high heat components in one area of the board. Here, a card cage  120  that supports both liquid and air-cooling is created. Cold plates and liquid manifolds (not shown) are installed in rearward section  126  of the card cage  120 . Provisions for air cooling, such as a fan  130  or other air cooling mechanism, and perforations in the top  127  of the card cage  120  are installed in the air-cooled portion  128  of the card cage  120 . The complexity of this hybrid cooling is justified by the high cost of liquid cooling and the reduced cost when a smaller area must be cooled this way. A circuit board assembly  124  for this system is illustrated in FIG.  6 B. The portion of the board  124  requiring liquid cooling is fit with personality plates  122  as described above while the portion to be air-cooled  123  is not. 
     When the flat portion is formed into a box shape, the insertion process for the card cage previously described is not operational. Instead, an embodiment like the one illustrated in FIG. 7 is used to compress the circuit board assemblies and cold plates together. Dual liquid manifolds  172  are illustrated. One cold plate  161  is held fixedly between the manifolds  172  and has a permanent plumbed connection  164  to the manifolds  172 . The remaining cold plates  163  are spaced apart and held between the manifolds  172  by a flexible connection  166 . Except for the plumbing connection, cold plates  161  and  163  are identical having dual headers  162  and flat portions  160  of the same size and thermal capacity. Circuit board assemblies  169 , composed of a printed circuit board  168  clamped between box-like personality plates  170 ,  171 , are inserted between the flat portions  160  of the cold plates. Once all the circuit board assemblies  169  are inserted, a compression mechanism, such as the illustrated press, exerts force on the stacked cold plates  161 ,  163  and circuit board assemblies  169  to bring them into tight thermal contact. As a result of the pressure, cold plates  163  will be displaced toward the fixed cold plate  161  and the flexible connections  166  to the manifolds  172  will flex. 
     The separation of the prior art&#39;s personalized cold plate into a standard cold plate and personality plates for each circuit board is useful because the standard cold plates can be continuously connected to a liquid source removing the prime potential for leaks. With the possibility of leaks reduced, water may be used as the cooling liquid replacing the much more expensive high dielectric liquids. Personality plates need to be thermally conductive but do not need to have passageways for liquid. It is much less expensive to create solid personality plates that to create a cold plate with a personality face. 
     In one embodiment, two phase flow of the liquid is used. In this case, a refrigerant is used as the liquid and evaporates in the cold plate as heat is absorbed. The resultant gas is reliquefied in an external system. This embodiment provides an even greater degree of cooling than can be effected with just circulation of a cooling liquid. 
     Having described preferred embodiments of the invention it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Accordingly, it is submitted that the invention should not be limited by the described embodiments but rather should only be limited by the spirit and scope of the appended claims.