Abstract:
Circuit board assemblies and methods that employ integrated heatspreaders to cool the assemblies and serve as electrical ground planes for the assemblies. Such a circuit board assembly includes a substrate having at least one circuit device on at least a first surface thereof and an electrical ground plane. The circuit device has a first set of solder connections electrically connected to the electrical ground plane and a second set of solder connections electrically connected to power and signal traces on the first surface of the substrate. The assembly further includes a heatspreader embedded in the substrate and defining an electrical element of the electrical ground plane as a result of being electrically connected to the first set of solder connections. The heatspreader is configured as a plate-mesh-plate laminate that defines a cavity containing a fluid for transferring heat from the circuit device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/829,325, filed Oct. 13, 2006, the contents of which are incorporated herein by reference. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to circuit board assemblies. More particularly, this invention relates to circuit board assemblies with enhanced thermal management capabilities. 
         [0003]    With the evolution of electronic devices, integrated circuits (ICs) have become increasingly condensed with respect to overall power density. Contributing factors are the migration to smaller design processes that shrink the physical dimensions of devices, including transistors and capacitors, as well as metal layer interconnects. In addition, the power consumption linearly follows the number of switching events, which, in turn, is a direct function of the operating frequency. The result is the ubiquity of ICs that feature transistor counts at orders of magnitude higher than their predecessors, with operating frequencies at only a fraction of the footprint of their predecessors. Though a portion of the increased power demand can be offset by lower operating voltages, from the above it is evident that lower voltages can only be a partial remedy for increasing power density. 
         [0004]    Thermal management of ICs has evolved greatly over the past few years. Whereas a simple metal plate integrated into an IC package previously sufficed, current ICs are finding more and more sophisticated methods to offload heat. Historically, ICs were packaged with the active silicon of the chip facing down and cooling applied primarily to the backside of the chip substrate. This approach incurs the thermal resistance of the substrate, resulting in reduced heat dissipation since the substrate behaves as a heat barrier to at least some degree. An improved solution was the development of flip-chip designs in which the active die faces up and can be in direct contact with a heatspreader. 
         [0005]    Further improvements in cooling techniques have been achieved with factory-preinstalled heat slugs over the die. This step solves two problems, namely, it eliminates the risk of accidental damage to the surface of the chip during mounting and, more importantly, by using a low-temperature solder to attach the heat slug, a highly efficient heat transfer path with increased surface area can be established. The heat slug can then be interfaced with relative ease to any secondary cooling device using standard thermal interface materials. 
         [0006]    Heatspreaders that contain a cooling fluid have also been proposed, as taught in commonly-assigned U.S. Pat. No. 7,219,715 to Popovich and commonly-assigned U.S. patent application Ser. No. 11/861,810 to Schuette, the contents of which are incorporated herein by reference. The cooling fluids of Popovich and Schuette flow through microchannels formed by interstices of a woven metal screen or mesh sandwiched between two foils or plates. Popovich discloses an open fluid cooling system in which the cooling fluid is in direct contact with an integrated circuit device, whereas Schuette discloses a fully-sealed, self-contained fluid cooling system in which thermal energy is initially absorbed by the foil nearest a heat source, propagated through the mesh into a cooling fluid within the microchannels, and then removed by displacement of the fluid. At a distance from the heat source, the thermal transfer process is reversed, namely, the heat absorbed by the fluid is transferred to the mesh and finally to the second foil for dissipation into the environment. 
         [0007]    Other types of microchannels for coolant fluids have also been known for some time, as evidenced by U.S. Pat. No. 4,450,472 to Tuckerman et al. The preferred embodiment featured in this patent integrated microchannels into the die of the microchip to be cooled and coolant chambers. U.S. Pat. No. 5,801,442 also describes a similar approach. Still other approaches have focused on the combined use of coolant phase change (condensation) and microchannels, an example of which is U.S. Pat. No. 6,812,563. U.S. Pat. No. 6,934,154 describes a similar two-phase approach including an enhanced interface between an IC die and a heatspreader based on a flip-chip design and the use of a thermal interface material. U.S. Pat. Nos. 6,991,024, 6,942,018, and 6,785,134 describe electroosmotic pump mechanisms and vertical channels for increased heat transfer efficiencies. Variations of microchannel designs include vertical stacking of different orientational channel blocks as described in U.S. Pat. No. 6,675,875, flexible microchannel designs using patterned polyimide sheets as described in U.S. Pat. No. 6,904,966, and integrated heating/cooling pads for thermal regulation as described in U.S. Pat. No. 6,692,700. 
         [0008]    Additional efforts have been directed to the manufacturing of microchannels. U.S. Pat. Nos. 7,000,684, 6,793,831, 6,672,502, and 6,989,134 are representative examples, and disclose forming microchannels by sawing, stamping, crosscutting, laser drilling, soft lithography, injection molding, electrodeposition, microetching, photoablation chemical micromachining, electrochemical micromachining, through-mask electrochemical micromachining, plasma etching, water jet, abrasive water jet, electrodischarge machining (EDM), pressing, folding, twisting, stretching, shrinking, deforming, and combinations thereof. All of these methods, however, share the drawback of requiring a more or less elaborate and expensive manufacturing process. 
         [0009]    A parallel development has occurred in the electrical interfacing of ICs with the substrates to which they are mounted. Most older ICs used edge pins to receive power as well as for communicating with the electrical system on a substrate, such as a printed circuit board (PCB). Exemplary designs were PDIP, QFP, SOP, and TSOP, among others, wherein the die is interfaced through bond wires to a lead frame, with the latter extending to form lateral feet that are soldered to a circuit board. Advantages of this design include the relative ease of mounting as well as the facilitation of potential manual reworks. Recently, the trend has moved to a more sophisticated interfacing scheme known as a ball grid array (BGA), in which IC chips are housed in a package with contacts distributed on one of its surfaces for use as interconnects to a conductor pattern on a substrate. An important factor to consider in this context is the fact that in almost every case, a large number of contacts is dedicated to providing distributed power and ground to the IC. In particular, power and ground buses of BGAs are typically relatively solid structures as opposed to the much finer signal traces. As a result, the ground plane of a circuit board is capable of absorbing heat from its ICs through the solder ball connections of the ICs. Some circuit board designs, especially in the field of lower power devices such as memory modules, specifically take advantage of augmented copper ground planes to transfer heat from ICs to blank areas of the circuit board. In this case, the ground plane is typically located within an inner layer of the circuit board to avoid interference with signal routing through the circuit board. Inherently, this has the disadvantage of encapsulating the heatspreader and, as a result, a connection must be provided to a terminal heatspreader external to the circuit board, typically through the use of vias. Furthermore, heat conductance is often limited by the very small cross-sectional area typical of ground planes. Consequently, a ground plane used as an internal heatspreader within a circuit board is rather limited in its ability to dissipate heat generated by ICs on the circuit board. While thermal conduction through a ground plane used as an internal heatspreader can be enhanced by increasing the thickness of the ground plane, doing so comes at a severe cost of material and weight disadvantage, since a highly electrical-conductive metal such as copper or silver must normally be used as the material for ground planes. In view of these limitations, there is a continuing need for circuit board assemblies with enhanced thermal management capabilities. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention provides circuit board assemblies and methods that employ integrated heatspreaders to cool the assemblies and serve as electrical ground planes for the assemblies. 
         [0011]    According to a first aspect of the invention, a circuit board assembly includes a circuit board substrate having at least one circuit device on at least a first surface thereof and an electrical ground plane. The circuit device has a first set of solder connections electrically connected to the electrical ground plane and a second set of solder connections electrically connected to power and signal traces on the first surface of the substrate. The assembly further includes a heatspreader embedded in the substrate and defining an electrical element of the electrical ground plane as a result of being electrically connected to the first set of solder connections. The heatspreader is configured as a plate-mesh-plate laminate that defines a cavity containing a fluid for transferring heat from the circuit device. 
         [0012]    According to a second aspect of the invention, a method is provided for combining an electrical ground plane of a circuit board substrate with heat dissipation from a circuit device on a first surface of the substrate. The method entails fabricating the substrate to have an embedded heatspreader comprising a plate-mesh-plate laminate filled with coolant fluid. 
         [0013]    In view of the above, heatspreaders employed by this invention are sealed, fluid-filled laminates integrated into a circuit board assembly to concurrently act as an electrical ground plane and a thermal management device, by which the fluid within the heatspreader transfers heat away from a heat source on the circuit board substrate. The heat source may be an IC chip or package mounted to the circuit board substrate, and the heat path from the heat source to the heatspreader may include solder connections of an IC package or IC die that are part of the ground bus of the circuit board. As such, the heatspreader also serves as the electrical ground plane of the circuit board assembly. 
         [0014]    The fluid within the heatspreader is preferably contained in microchannels defined by a screen or mesh within the cavity, which is preferably defined between two foils or plates. The fluid may flow through the microchannels by natural convection or forced convention, the latter of which includes forced flow with a pump. Because the heatspreader carries current as a result of being part of the ground plane of the circuit board, the current can be used to move an ionically-charged fluid through the microchannels by electroosmotic flow. 
         [0015]    The heatspreader can be located at or beneath a surface of a circuit board substrate and locally restricted to exclude power and signaling traces. Alternatively, the heatspreader can be located in a layer different from those containing signals and power traces, in which case the heatspreader is preferably situated within an internal layer of the substrate. If located within an internal layer (i.e., beneath the surface) of the substrate, a circuit device can be thermally connected to the heatspreader through elongated solder bumps, for example, longer solder bumps of a staggered solder bump array. The heatspreader can be thermally connected to a heat exchanger to dissipate the heat into the environment. Functional connectivity in this case is meant to specify thermal conductivity, which, in the simplest case, may be through vias or folded edge extensions. 
         [0016]    In view of the above, notable advantages of the invention include heat absorption from a circuit device through its electrical ground connections, rapid heat removal from the circuit device and the surrounding vicinity with a fluid, enhanced heat transfer as a result of the fluid being contained and flowing within microchannels, and a light-weight design with high rigidity. In addition, because the heatspreader is part of the ground plane of the circuit board, current in the heatspreader can be used to drive electroosmotic flow of the coolant through the microchannels. 
         [0017]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  schematically shows a ball grid array (BGA) package on a circuit board substrate and electrically connected with ground connections to a sealed fluid-filled heatspreader, which is located at a surface of the substrate and forms part of the ground plane of the substrate in accordance with a first embodiment of the invention. 
           [0019]      FIG. 2A  schematically shows a BGA package on a circuit board substrate and a sealed fluid-filled heatspreader located at a surface of the substrate and forming part of the ground plane of the substrate, wherein ground connections of the package are electrically connected to an extension of the heatspreader that is separated from surface areas of the substrate containing power and signal traces in accordance with a second embodiment of the invention. 
           [0020]      FIG. 2B  is a fragmentary top view of  FIG. 2A , with the package represented in phantom to show the region containing the ground, power, and signal connections between the package, the heatspreader, and power/signal traces on the surface of the substrate. 
           [0021]      FIG. 3  schematically shows a BGA package on a circuit board substrate and a sealed fluid-filled heatspreader located beneath a surface of the substrate and forming part of the ground plane of the substrate, wherein the package has a staggered array of solder bumps and the package are electrically connected to the heatspreader through longer solder bumps that extend through an outer layer of the substrate in accordance with a third embodiment of the invention. 
           [0022]      FIG. 4  is similar to  FIG. 3 , but further includes a second BGA package on an opposite surface of the circuit board substrate and electrically connected to the heatspreader through longer solder bumps that extend through an outer layer of the substrate in accordance with a fourth embodiment of the invention. 
           [0023]      FIG. 5  is similar to  FIG. 3 , but further shows the heatspreader as having an extension that protrudes from the circuit board substrate, and fins on the extension to promote heat transfer from the package to the environment in accordance with a fifth embodiment of the invention. 
           [0024]      FIG. 6  is similar to  FIG. 3 , but further shows the heatspreader as having two extensions that protrude from and wrap around an edge of the circuit board substrate, and fins on one of the extensions to promote heat transfer from the package to the environment in accordance with a sixth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIGS. 1 through 6  depict multiple configurations of heatspreaders in accordance with various embodiments of this invention. For convenience, consistent reference numbers are used to identify functionally similar structures throughout these Figures. 
         [0026]    The present invention is represented in  FIGS. 1 through 6  as a heatspreader  20  that also serves as part of the ground plane of a circuit board assembly  10 . As described in more detail below, the heatspreader  20  is adapted for dissipating heat from electronic components mounted to the circuit board substrate  12 , which may be a printed circuit board (PCB) or another suitable substrate. The electronic components may include various devices, the example shown in the Figures being a BGA package  14  carrying an IC die  16  and attached to the substrate  12  with solder connections  18  (only one row of which is visible in  FIG. 1 ). The heatspreader  20  is particularly beneficial if the package  14  has a high power density. The heatspreader  20  is integrated into the circuit board substrate  12  to enable the heatspreader  20  to simultaneously function as a ground plane in the power and ground buses. Heat exchange between the package  14  and heatspreader  20  is through ground connections of the package  14 , which in  FIG. 1  are the visible solder connections  18  contacting the upper/exposed surface  22  of the heatspreader  20 . As such, additional electrical connections (e.g.,  18 B in  FIG. 2 ) are necessary to electrically connect the package  14  to power and signal traces (e.g.,  50  in  FIG. 2 ) on the substrate  12 . For other types of electronic components, additional packaging may be omitted and solder bumps on an IC die may be directly bonded to the heatspreader  20 . 
         [0027]    As shown in  FIG. 1 , the heatspreader  20  is a self-contained, closed-loop, fluid-cooling device having a composite laminate construction, in which a relatively pliant screen or mesh  26  is sandwiched between two foils or plates  28  and  30  that are substantially parallel to each other. The mesh  26  is represented as being composed of individual strands  32  that are woven together, generally transverse to each other and conventionally referred to as warp and weft strands  32 . The mesh  26  and plates  28  and  30  are preferably formed of materials having physically and chemically compatible properties, including materials having the same composition, though various material combinations are possible. For example, individual strands  32  of the mesh  26  can be formed by an individual wire, braided wires, bundled wires, etc., of copper, silver, aluminum, carbon, or alloys thereof, and the plates  28  and  30  can be formed of the same or similar materials. As discussed below, heat transfer occurs by conduction through the plates  28  and  30  and mesh  26 , such that preferred materials for these components are thermally conductive, though the use of other materials including polymeric and nonmetallic materials is also foreseeable. Suitable thicknesses for the plates  28  and  30  and mesh  26 , suitable cross-sectional shapes and dimensions for the mesh strands  32 , and suitable weaves (including strands per inch) for the mesh  26  may depend on the particular application and the materials from which these components are formed. 
         [0028]    As evident from  FIG. 1 , peripheral edge portions  34  of both plates  28  and  30  are preferably raised relative to the remainder of the plates  28  and  30 , such as by embossing, to form a relief in each plate  28  and  30  that promotes their rigidity and further defines a continuous peripheral surface at which the plates  28  and  30  can be bonded to each other, such as with a solder alloy, braze alloy, adhesive, etc. With the plates  28  and  30  laminated together, the reliefs define a cavity  36  between the plates  28  and  30  that contains the cooling fluid of the heatspreader  20 . Additional embossing can be performed on one or both plates  28  and  30  to define within the cavity  36  a channel system (not shown) between the plates  28  and  30 , by which particular flow routes can be established within the heatspreader  20 . Three-dimensional structures formed by such additional embossing have the further advantage of increasing the mechanical stability of the heatspreader  20 . 
         [0029]    As evident from  FIG. 1 , the mesh  26  within the cavity  36  may have approximately the same thickness as the height of the cavity  36  (as measured in the direction normal to the surface  22  of the plate  28 ). The peaks  38  projecting from both sides of the mesh  26  are preferably bonded, such as by soldering or brazing, to the plates  28  and  30  to establish a highly-conductive thermal contact between the mesh  26  and both plates  28  and  30 . Bonding also serves to cross-link the plates  28  and  30 , which resists any shearing forces to which the plates  28  and  30  are subjected and contributes additional mechanical stability and rigidity to the heatspreader  20 . The warp and weft strands  32  of the mesh  26  form interstices that are more or less freely penetrable by any fluid, yet define tortuous paths that avoid laminar flow conditions within the cavity  36  that would reduce the heat transfer rate between the cooling fluid, the plates  28  and  30 , and the mesh  26 . 
         [0030]    As generally known in the art, suitable coolant fluids include liquids such as water, mineral spirits/oils, alcohols, and fluorocarbonate derivatives, though various other fluids could also be used, including air, vapor, etc., depending on the required temperature range of operation. For example, in extremely cold environments, a fluid with lower viscosity is a better choice than in extremely hot environments. Various other parameters for choosing a cooling fluid exist and are well known, and therefore will not be discussed in any further detail here. 
         [0031]    As evident from  FIG. 1 , the heatspreader  20  is self-contained with the cooling fluid being hermetically sealed within the cavity  36 , such that cooling of the package  14  is achieved by providing a thermal conductive path between the package  14  with one of the plates  28 / 30  (plate  28  in the embodiment of  FIG. 1 ). With the plate  28  in thermal contact with the package  14  as shown in  FIG. 1 , heat transfer from the package  14  is through the ground connections  18  and into the plate  28 , the cavity  36  containing the mesh  26  and fluid, and the plate  28 , which together cooperate to conduct heat away from the package  14 , for example, to an edge (not shown) of the circuit board substrate  12 . More particularly, heat transfer through the heatspreader  20  is by thermal conduction through the plate  28 , mesh  26 , and plate  30 , and by convention between the plate  28  and the cooling fluid and between the cooling fluid and the plate  30 , as well as convection through the cooling fluid from the plate  28  to the mesh  26  and convection through the cooling fluid from the mesh  26  to the plate  30 . Accordingly, heat transfer is generally in a direction parallel to the plane of the heatspreader  20 , and the fluid acts as a secondary heat absorbent and a thermal transport media capable of transporting thermal energy to the mesh  26  at a distance from the plate  28  nearest the heat source (the BGA package  14 ). 
         [0032]    The cooling fluid may be recirculated through the cavity  36  with a pump (not shown) mounted on the substrate  12  or external to the circuit board assembly  10 . A wide variety of pumps are possible and suitable for use in the heatspreader  20 , and the choice of which will be primarily dependent on the specific application since pressure and noise requirements need to be taken into consideration. Notable but nonlimiting examples of suitable pump types include centrifugal, positive displacement, rotary, and osmotic pumps that are commercially available and have been used in prior cooling systems for electronic components. 
         [0033]    Because the cooling fluid assists the plates  28  and  30  in conducting heat from the package  14 , the coefficient of thermal conductance of the material(s) used to form the plates  28  and  30  is less important than in structures that rely on passive heat transfer. As such, a wider variety of materials could be used to form the heatspreader  20  and its individual components. Moreover, because the heatspreader  20  is hollow, the total amount of material used is substantially lower than in a comparable solid structure, resulting in reduced material costs for manufacturing the heatspreader  20 . A related issue is the mechanical stability of the heatspreader  20 . Hollow structures generally exhibit only a minor reduction in rigidity as compared to a solid body of the same dimensions. The rigidity of the heatspreader  20  is promoted as a result of the peripheral edge portions  34  of the plates  28  and  30  being bonded together, as well as bonding of the mesh  26  to both plates  28  and  30 . Consequently, the heatspreader  20  can be much lighter but yet nearly as strong and rigid as a solid heatspreader of comparable size. 
         [0034]    In the embodiment of  FIG. 1 , the heatspreader  20  is shown embedded in a surface layer  40  and an immediately adjacent subsurface layer  42  of the circuit board substrate  12 , such that the surface  22  of the heatspreader  20  is generally flush with the substrate surface  44  at which the package  14  is mounted. The opposite surface  24  of the heatspreader  20  is buried within the substrate  12 , and not exposed at the surface  46  of the substrate  12  opposite the surface  44 . 
         [0035]    In  FIG. 2A , the solder connections  18  of the BGA package  14  are shown as being arranged as a group of ground solder connections  18 A and signal and power solder connections  18 B. The ground solder connections  18 A directly contact an extension  48  of the heatspreader  20  that is formed by the plate  28  and contiguous with the surface  22  of the heatspreader  20 . The extension  48  extends beneath a limited portion of the package  14  corresponding to the ground solder connections  18 A, but not beneath portions of the package  14  where the signal/power solder connections  18 B and their traces  50  are located. As in  FIG. 1 , the surface  22  of the heatspreader  20  is approximately flush with the substrate surface  44 , such that the ground and power/signal solder connections  18 A and  18 B have approximately equal heights (lengths perpendicular to the plane of the substrate  12 ). The heatspreader  20  is further shown as including fins  52  that project above the surface  44  of the substrate  12  and promote convective and radiative heat transfer to the surrounding environment. 
         [0036]    As represented in  FIG. 2B , the extension  48  of the heatspreader  20  is one of multiple finger-like extensions  48  that collect heat from the ground solder connections  18 A. The extensions  48  are preferably interdigitated with the signal/power solder connections  18 B and their traces  50  (not shown) to avoid electrical shorting between the ground plane, power, and signal lines of the substrate  12 . The embodiment of  FIG. 1  will also typically require physical separation between the heatspreader  20  and the signal/power solder connections  18 B and their traces  50 , depending on the manner in which the package  14  is electrically connected to its power and signal traces on the substrate  12 . The embodiments of  FIGS. 3 through 6  are capable of avoiding this limitation. 
         [0037]      FIG. 3  shows the integration of the heatspreader  20  into subsurface (internal) layers of the circuit board substrate  12 , such that both surfaces  22  and  24  of the heatspreader  20  are embedded in the substrate  12  and the surface  22  nearest the heat source (package  14 ) is beneath the outer surface  44  of the substrate  12 . The solder connections  18  of the BGA package  14  are shown as being vertically staggered, with the ground solder connections  18 A being slightly elongated compared to the signal/power solder connections  18 B in order to penetrate the surface layer  40  of the circuit board substrate  12 . An advantage of this configuration is that the shape of the heatspreader  20  in the plane of the substrate  12  can be relatively simple, since there is no need to meander around the signal and power traces  50  to avoid electrical shorting with the ground plane. As such, the signal and power traces  50  can be located on the outer surface  44  of the substrate  12  between the package  14  and heatspreader  20 , and the package  14  can lie entirely above the heatspreader  20 . Furthermore, the ground solder connections  18 A are situated directly beneath the IC die  16  for optimal heat transfer to the heatspreader  20 . 
         [0038]    The embodiment of  FIG. 4  is similar to that of  FIG. 3 , but further includes a second BGA package  15  on the lower surface  46  of the circuit board substrate  12 . As in  FIG. 3 , the package  15  is electrically connected to the heatspreader  20  through longer ground solder bumps  19 A that extend through an outer layer  54  of the substrate  12 , while shorter signal/power solder bumps  19 B contact signal and power traces  56  located on the substrate&#39;s lower surface  46 . The packages  14  and  15  can mounted to the substrate  12  in a clamshell configuration. 
         [0039]    In the embodiment of  FIG. 5 , a pair of the peripheral edge portions  34  of the plates  28  and  30  are shown elongated and protruding beyond an edge  56  of the substrate  12 , providing a location for two oppositely-disposed sets of fins  52  that promote convection heat transfer to the environment. 
         [0040]    Finally,  FIG. 6  shows a configuration in which a pair of the peripheral edge portions  34  of the plates  28  and  30  are elongated and wrapped around an edge  56  of the circuit board substrate  12 , with fins  52  provided on one of the edge portions  34 . With this configuration, the heatspreader  20  and its fins  52  do not significantly increase the length of the circuit board assembly  10  beyond that of the substrate  12 . 
         [0041]    While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, additional embodiments could be constructed that differ in appearance and construction from the embodiments shown in the Figures, and appropriate materials could be substituted for those noted. Therefore, the scope of the invention is to be limited only by the following claims.