Abstract:
A circuit module is provided that includes a system for reducing thermal variation and cooling the circuit module. The module includes a thermally-conductive rigid substrate having first and second lateral sides and an edge. Flex circuitry populated with a plurality of ICs and exhibiting a connective facility that comprises plural contacts for use with an edge connector is wrapped about the edge of the thermally-conductive substrate. Heat from the plurality of ICs is thermally-conducted by the thermally-conductive substrate. The module also includes one or more heat pipes. Each heat pipe is sealed water-tight and includes a wick and a vaporizable fluid.

Description:
FIELD 
       [0001]    The present invention relates to high density circuit modules, particularly reducing thermal variation and cooling circuit modules. 
       BACKGROUND 
       [0002]    The well-known DIMM (Dual In-line Memory Module) board has been used for years, in various forms, to provide memory expansion. A typical DIMM includes a conventional PCB (printed circuit board) with memory devices and supporting digital logic devices mounted on both sides. The DIMM is typically mounted in the host computer system by inserting a contact-bearing edge of the DIMM into a card edge connector. Systems that employ DIMMs provide, however, very limited profile space for such devices and conventional DIMM-based solutions have typically provided only a moderate amount of memory expansion. 
         [0003]    As bus speeds have increased, fewer devices per channel can be reliably addressed with a DIMM-based solution. For example,  288  ICs or devices per channel may be addressed using the SDRAM-100 bus protocol with an unbuffered DIMM. Using the DDR-200 bus protocol, approximately 144 devices may be address per channel. With the DDR2-400 bus protocol, only 72 devices per channel may be addressed. This constraint has led to the development of the fully-buffered DIMM (FB-DIMM) with buffered C/A and data in which 288 devices per channel may be addressed. With the FB-DIMM, not only has capacity increased, pin count has declined to approximately 69 signal pins from the approximately 240 pins previously required. 
         [0004]    The FB-DIMM circuit solution is expected to offer practical motherboard memory capacities of up to about 192 gigabytes with six channels and eight DIMMs per channel and two ranks per DIMM using one gigabyte DRAMs. This solution should also be adaptable to next generation technologies and should exhibit significant downward compatibility. 
         [0005]    In a traditional DIMM typology, two circuit board surfaces are available for placement of memory devices. Consequently, the capacity of a traditional DIMMs is area-limited. There are several known methods to improve the limited capacity of a DIMM or other circuit board. In one strategy, for example, small circuit boards (daughter cards) are connected to the DIMM to provide extra mounting space. The additional connection may cause, however, flawed signal integrity for the data signals passing from the DIMM to the daughter card and the additional thickness of the daughter card(s) increases the profile of the DIMM. 
         [0006]    Multiple die packages (MDP) are also used to increase DIMM capacity while preserving profile conformity. This scheme increases the capacity of the memory devices on the DIMM by including multiple semiconductor die in a single device package. The additional heat generated by the multiple die typically requires, however, additional cooling capabilities to operate at maximum operating speed. Further, the MDP scheme may exhibit increased costs because of increased yield loss from packaging together multiple die that are not fully pre-tested. 
         [0007]    Stacked packages are yet another strategy used to increase circuit board capacity. This scheme increases capacity by stacking packaged integrated circuits to create a high-density circuit module for mounting on the circuit board. In some techniques, flexible conductors are used to selectively interconnect packaged integrated circuits. Staktek Group L.P. has developed numerous systems for aggregating CSP (chipscale packaged) devices in space saving topologies. The increased component height of some stacking techniques may alter, however, system requirements such as, for example, required cooling airflow or the minimum spacing around a circuit board on its host system. 
         [0008]    As DIMM capacities and memory densities increase, however, thermal issues become more important in DIMM design and applications. Because of the directional air flow from a system fan, the heat generated in a typical DIMM is not evenly distributed. Consequently, different parts of the DIMM exhibit different temperatures during typical operations. As is well known, circuit performance and timing can be affected by temperature. Consequently, some circuitry on-board the DIMM will have different timing characteristics than other circuitry located closer to or further from the cooling air flow. In short, there will be a thermally-induced timing skew between constituent devices. This may not affect performance at slower speeds where timing windows are larger but as bus and RAM speeds increase, the thermally-induced skew between devices on a DIMM becomes more significant reducing the timing window or eye. 
         [0009]    Thermal energy management in modules is an issue of increasing importance. What is needed, therefore, are systems and methods that provide enhanced module cooling and minimization of thermally-induced skew amongst module devices. 
       SUMMARY  
       [0010]    A circuit module is provided that includes a system for reducing thermal variation and cooling the circuit module. The module includes a thermally-conductive rigid substrate having first and second lateral sides and an edge. Flex circuitry populated with a plurality of ICs and exhibiting a connective facility that comprises plural contacts for use with an edge connector is wrapped about the edge of the thermally-conductive substrate. Heat from the plurality of ICs is thermally-conducted by the thermally-conductive substrate. The module also includes one or more heat pipes. Each heat pipe is sealed water-tight and includes a wick and a vaporizable fluid. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1A and 1B  are cross-sectional and side-view depictions of an embodiment of a system for reducing thermal variation and cooling high density circuit modules. 
           [0012]      FIG. 2  is a cross-sectional depiction of an embodiment of a system for reducing thermal variation and cooling high density circuit modules. 
           [0013]      FIGS. 3A and 3B  are cross-sectional and perspective depictions of an embodiment of a system for reducing thermal variation and cooling high density circuit modules. 
           [0014]      FIG. 4  is a cross-sectional perspective depiction of an embodiment of a system for reducing thermal variation and cooling high density circuit modules. 
           [0015]      FIG. 5  is a cross-sectional depiction of an embodiment of a system for reducing thermal variation and cooling high density circuit modules. 
           [0016]      FIG. 6  is a cross-sectional depiction of an embodiment of a system for reducing thermal variation and cooling high density circuit modules. 
           [0017]      FIGS. 7A ,  7 B and  7 C are cross-sectional and side-view depictions of an embodiment of a system for reducing thermal variation and cooling high density circuit modules. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Embodiments of a system for reducing thermal variation and cooling high-density circuit modules may take advantage of flex-based circuit technology. The embodiments described herein may be incorporated in flex-based circuit modules, such as flex-based circuit modules, described in U.S. patent application Ser. No. 11/007,551, filed Dec. 8, 2004 and U.S. patent application Ser. No. 11/193,954, filed Jul. 24, 2005, both of which are owned by assignee Staktek Group LP and hereby incorporated by reference. Embodiments may also utilize non-flex-based circuit modules. 
         [0019]    With reference now to  FIGS. 1A and 1B , shown are diagrams illustrating an embodiment of a system for reducing thermal variation and cooling circuit modules.  FIG. 1A  illustrates a cross-section of a circuit module  10 . Circuit module  10  may be a flex-based module devised to supplant traditional DIMMs. Circuit module  10  includes integrated circuits (ICs)  18  disposed along each of two sides of flex circuitry  12  that is wrapped about substrate  14 . ICs  18  may be, for example, memory CSPs. Flex circuitry  12  exhibits a connective facility that comprises plural contacts  21  for use with an edge connector (see discussion of edge  24  below). Substrate  14  may also be referred to as the core of the module  10 . Substrate  14  may be comprised of thermally-conductive material and may be, for example, comprised of a metallic material or thermally-conductive plastic or carbon material. In the embodiment shown, substrate  14  exhibits enclosed fins  16 . Enclosing fins  16  creates sealed chamber  20  which may be devised from multiple components or extruded, for example. The plural edges  16 E of fins  16  may be faced with plates to create sealed chamber  20  within which may be confined or, preferably, circulated cooling fluid  17 . Fins  16  and sealed chamber  20  are positioned on an axis of substrate  14 , with fins  16  extending perpendicularly from the orientation of the axis. The system for reducing thermal variation and cooling high-density circuit modules utilizes the additional surface area provided by fins  16  and optionally circulating cooling fluid  17  (e.g., water or other fluid) in sealed chamber  20 , to increase the removal of heat from circuit module  10 . Those of skill will, however, recognize that while not limited to a perpendicular arrangement for fins  16 , most applications impose limitations on module profiles that will typically cause perpendicular arrangements to be preferred. 
         [0020]    Edge or end  24  of substrate  14  is shaped to function as a male side edge of an edge card connector. Edge  24  may take on other shapes devised to mate with various connectors or sockets. Flex circuitry  12  is preferably wrapped around edge  24  of substrate  14  and may be laminated or adhesively connected to substrate  14 . In other embodiments multiple flex circuits may be employed. 
         [0021]      FIG. 1B  illustrates a side view of circuit module  10  with a partial cross-sectional view of sealed chamber  20  and fins  16 . As shown, either after enclosing fins  16  or prior, substrate  14  may be machined so that at each end of sealed chamber  20  the spaces between fins  16  are open to each other. Fittings  22  may be then inserted into each end of sealed chamber  20  so that a fluid may be pumped into sealed chamber  20  and the seal maintained. The open space at one end of sealed chamber  20  is shown in the partial cross-sectional view. Also shown are plurality of ICs  18  disposed along one side of flex circuitry  12  on one side of substrate  14 . 
         [0022]    Fittings  22  may be used to couple circuit module  10  to a recirculating system. For example, if circuit module  10  were a DIMM, edge  24  may be inserted into a socket on a mother board, connecting the DIMM with the mother board, and a recirculating system coupled to fittings  22  to re-circulate fluid in sealed chamber  20 , reducing thermal variation while cooling the DIMM. Conversely, the DIMM may be disconnected from the mother board and removed from the recirculating system. The embodiment shown in  FIGS. 1A-1B  provides active cooling for circuit module  10 , integrated cooling inside substrate (core)  14  (e.g., instead of a heat exchanger applied to the outside), and, by maintaining a thin profile of circuit module, enables circuit module  10  to get standard air cooling as well as the circulated cooling. 
         [0023]    With reference now to  FIG. 2 , shown is another embodiment of the system for reducing thermal variation and cooling circuit modules.  FIG. 2  illustrates a cross-sectional view of circuit module  10 . As above, circuit module  10  may be a flex-based DIMM. Circuit module  10  includes ICs  18  disposed along each of two sides of flex circuitry  12  that is wrapped about substrate (or core)  14 . Depicted module  10  further includes IC  19  which is depicted as an advanced memory buffer (AMB). Substrate  14  may be comprised of thermally-conductive material and may be, for example, comprised of a metallic material or thermally-conductive plastic or carbon material. The size of circuit module  10  may be modified as per end-use requirements. 
         [0024]    In the embodiment shown, substrate  14  includes hollow cavity  26  that may be extruded from substrate  14 . Alternatively, substrate  14  may be comprised of multiple pieces of, e.g., aluminum that when assembled create hollow cavity  26 . Hollow cavity  26  may be sealed at the ends of circuit module  10  or left open to allow air flow through cavity  26 . Substrate may include a cap  28  (at the top of cavity  26 ) and fittings  30 , similar to fittings  22  above, so that a fluid may be circulated through cavity  26  to remove heat if the ends of cavity  26  are also sealed. Cavity  26  may be coupled to a recirculating system through fittings  30 , as described above. 
         [0025]    The positioning of cavity  26  in the center of circuit module  10 , near the two folded sides of flex circuitry  12  and ICs  18  and IC  19  disposed on flex circuitry  12 , enables fluid to be circulated very close to the heat sources (i.e., ICs). Those of skill will recognize from the depiction of  FIG. 2  that module  10  include ICs of a variety of functions including but not limited to memory, such as ICs  18  and AMB  19 . Consequently, at the expense of minimal added thickness, the embodiment shown in  FIG. 2  provides enhanced cooling and heat distribution. The embodiment shown in  FIG. 2  provides passive and/or active cooling and integrated cooling inside substrate (core)  14  (e.g., instead of a heat exchanger applied to the outside). 
         [0026]    With reference now to  FIGS. 3A-3B , another embodiment of the system for reducing thermal variation and cooling circuit modules is depicted.  FIG. 3A  illustrates a cross-sectional view of circuit module  10 . Circuit module  10  may supplant a traditional DIMM. Circuit module  10  includes ICs  18  disposed along each of two sides of flex circuitry  12  that is wrapped about substrate (or core)  14  and may include other ICs such as an AMB or logic, for example. Substrate  14  may be comprised of thermally-conductive material and may be, for example, comprised of a metallic material or thermally-conductive plastic or carbon material. Substrate  14  includes hollow cavity  26 , which may be formed as discussed above. 
         [0027]    In the depicted embodiment, cavity  26  houses heat exchanger  32 . Heat exchanger  32  includes a series of parallel pipes. Other types of heat exchangers  32  may be used, such, for example, as a coiled continuous pipe. As shown in the perspective, cross-sectional view in  FIG. 3B , heat exchanger  32  may be coupled to a recirculating system with short tubes  34  with fittings  36  extending away from a side or sides of circuit module  10  (only one short tube  34  with fitting  36  is shown in  FIG. 3B ). Alternatively, tubes  34  may extend from the top of circuit module  10 . Other couplings to a recirculating system, such as fittings held in brackets on circuit module  10  may be used. Substrate  14  may also include a cap  28  that covers the top of cavity  26 . 
         [0028]    Fluid may be circulated through heat exchanger  32  (e.g., by a recirculating system) to cool circuit module  10  and minimize thermal variation. By utilizing heat exchanger  32  positioned within cavity  26 , the necessity of sealing cavity  26  is avoided. This simplifies the manufacturing and assembly process of circuit module  10 . As with the embodiment shown in  FIG. 2 , by positioning heat exchanger  32  in cavity  26  near flex circuitry  12  and the ICs, the embodiment in  FIGS. 3A and 3B  provides enhanced cooling and heat distribution at the expense of minimal added thickness. As above, the embodiment shown in  FIGS. 3A and 3B  provides active cooling and integrated cooling inside substrate (core)  14  (e.g., instead of a heat exchanger applied to the outside). 
         [0029]    With reference now to  FIG. 4 , another embodiment of the system for reducing thermal variation and cooling circuit modules is shown.  FIG. 4  illustrates a perspective, cross-sectional view of an exemplar circuit module  10 . In the depicted preferred embodiment, circuit module  10  is devised as a flex-based replacement for a traditional DIMM. Circuit module  10  includes ICs such as ICs  18  disposed along each of two sides of flex circuitry  12  that is wrapped about substrate (or core)  14 . Substrate  14  may be comprised of thermally-conductive material and may be, for example, comprised of a metallic material or thermally-conductive plastic or carbon material. Substrate  14  includes hollow cavity  26 , which may be formed as discussed above or by bonding plates together, for example, so that a channel routed in the substrate becomes a sealed chamber. Substrate  14  includes a cap  28  that covers the top of hollow cavity  26 . 
         [0030]    In the embodiment shown here, circuit module  10  includes fittings  38  on top of cap  28  (only one fitting  38  is shown in  FIG. 4 ). Cavity  26  houses routed channel  40  that is routed from one fitting  38  to the other. If substrate  14  is formed from separate pieces, the pieces may be laminated together to form a watertight path. Routed channel  40  may be a series of parallel paths or a single channel meandering from one side of circuit module  10  to another. Fittings  38  may couple routed channel  40  to a recirculating system as described above. As above, positioning routed channel  40  in cavity  26  near flex circuitry  12  and the ICs which are the heat sources, provides enhanced cooling and heat distribution at the expense of minimal added thickness. The embodiment shown in  FIG. 4  provides active cooling and integrated cooling inside substrate (core)  14  (e.g., instead of a heat exchanger applied to the outside). 
         [0031]    With reference now to  FIG. 5 , shown is another embodiment of the system for reducing thermal variation and cooling circuit modules.  FIG. 5  illustrates a perspective, cross-sectional view of circuit module  10 . Circuit module  10  includes ICs  18  disposed along each of two sides of flex circuitry  12  that is wrapped about substrate (or core)  14 . Substrate  14  includes hollow cavity  26 , which may be formed as discussed above. 
         [0032]    In the embodiment shown in  FIG. 5 , the ends of hollow cavity  26  are left open and air circulator  42  is installed in hollow cavity  26 . Air circulator  42  is installed to increase the air flow and the velocity of the air in the cavity  26  to improve cooling. Air circulator  42  provides greater air circulation than embodiments that simply have hollow cavity  26  with open ends. Air circulator  42  may be, e.g., a small fan, a piezo-electric crystal, or a turbulence generator. Other air circulators  42  known to those of skill in the art may be used. Multiple air circulators  42  may be installed in hollow cavity  26 . Although, circuit module  10  may include a cap (not shown), leaving hollow cavity  26  open on top may increase passive and active cooling effects (e.g., from circuit board fan). The embodiment shown in  FIG. 5  provides active cooling and integrated cooling inside substrate (core)  14  (e.g., instead of a heat exchanger applied to the outside). Because the cavity  26  is positioned near the heat sources, the embodiment provides effective cooling. 
         [0033]    Yet another embodiment of the system for reducing thermal variation and cooling circuit modules includes a semiconductor heat pump installed between substrate  14  and ICs  18  disposed on the side of flex circuit  12  facing substrate  14 . A semiconductor heat pump may be installed on a circuit module with a simple substrate around which the flex circuit wraps or in combination with any of substrate  10  in the embodiments shown and described herein (e.g., see  FIGS. 1-5 ). For example, with reference now to  FIG. 6 , shown is an embodiment of the system for reducing thermal variation and cooling high-density circuit modules that includes a semiconductor heat pump  44 .  FIG. 6  illustrates a perspective, cross-sectional view of circuit module  10 . Circuit module  10  includes hollow cavity  26 . Semiconductor heat pump  44  is installed between substrate  14  and IC(s)  18  disposed on one side of flex circuit  12  facing substrate  14 . Additional semiconductor heat pumps  44  may be installed between substrate  14  and ICs  18  disposed on sides of flex circuit  12  facing substrate  14 . Hollow cavity  26  may be configured, e.g., as described above, with fluid, couplings to a recirculating system, internal heat exchanger, routed channels, or air circulator. 
         [0034]    Substrate  14  and, if present, hollow cavity  26  act as a heat sink for semiconductor heat pump  44 . Semiconductor heat pump  44  reduces the temperature of adjacent ICs  18  and circuit module  10 . The embodiment shown provides active cooling and integrated cooling inside substrate (core)  14  and inside circuit module  10  (e.g., instead of a heat exchanger applied to the outside). 
         [0035]    Preferred embodiments of the system for reducing thermal variation and cooling circuit modules use a fluid to transfer heat from circuit modules  10  (e.g., DIMMs) to a remote component (e.g., recirculating system) that removes the heat so that a cool fluid may be re-circulated to circuit modules  10 . A fluid, due to its increased mass over air, provides an efficient medium to take heat away from circuit module  10 . 
         [0036]    With reference now to  FIGS. 7A-7C , shown is another embodiment of the system for reducing thermal variation and cooling circuit modules that includes heat pipe  46 . A basic heat pipe is a closed container including a capillary wick structure and a small amount of vaporizable fluid. A heat pipe acts like a high conductance thermal conductor, employing an evaporating-condensing cycle which accepts heat from an external source, uses this heat to evaporate the liquid, and then releases latent heat by condensation (reverse transformation) at a heat sink region. This process is repeated continuously by a feed mechanism (e.g., capillary wick structure) of the condensed fluid back to the heat zone. 
         [0037]      FIG. 7A  illustrates a cross-sectional view of a heat pipe  46 . Heat pipe  46  is a sealed tube manufactured into the structure of circuit module  10 . For example, heat pipe  46  may be manufactured into substrate  14  of circuit module  10 . Heat pipe  46  includes porous tubular wick  48  and fluid  50 . Wick  48  is of an appropriate diameter to remain in contact with the inner walls of heat pipe  46 . Wick  48  may comprise porous structures made of materials such as, for example, steel, aluminum, nickel, copper, metal foams, felts, fibrous materials such as ceramics, carbon fibers, etc. Pipe  46  is partially filled with fluid  50  whose boiling point at the pressure within heat pipe  46  is the temperature at or above which cooling is required. Heat pipe  46  is sealed to retain fluid  50  and maintain the pressure selected at manufacture of heat pipe  46 . 
         [0038]      FIGS. 7B-7C  show heat pipe  46  incorporated into substrate  14  of circuit module  10 . As shown, heat pipe  46  preferably spans the length of circuit module  10 . Circuit module  10  may be a flex-based circuit module, as above. When circuit module  10  is fully assembled, portions of flex circuit  12  and ICs  18  along flex circuit  12  will be adjacent to heat pipe  46 . In operation, heat applied along the surface of heat pipe  46  by conduction from flex circuit  12  and ICs  18  causes vaporization of fluid  50  in an adjacent region of heat pipe  46 . The vapor moves by its own pressure to cooler portions of heat pipe  46 , where it condenses on the cooler surfaces of cooler portions of heat pipe  46 . The condensate fluid  50  is absorbed by wick  48  and transported by capillary action back to the heated adjacent region. 
         [0039]    The net effect of the above-described operation of heat pipe  46  is that if any portion of heat pipe  46  (as well as the flex circuit  12  and the ICs in thermal contact with heat pipe  46 ) is warmer than any other area, the heat from the warmer region is absorbed as heat of vaporization by fluid  50  and transported by wicking action to the cooler portions of heat pipe  46 . This mechanism effectively distributes heat across circuit module  10 , thereby encouraging maintenance of a uniform temperature across the span of heat pipe  46  and the adjacent span of circuit module  10 . By distributing heat and removing heat from heat sources, heat pipe  46  also has a cooling effect on circuit module  10 . 
         [0040]    With continued reference to  FIGS. 7A-7C , the embodiment shown effectively reduces thermal variation in circuit module  10 . Heat pipe  46  can move larger amounts of heat with lower temperature differential than conduction through substrate  14  alone would normally allow. Heat pipe  46  may also be incorporated with the other embodiments shown and described herein (e.g., see  FIGS. 1-6 ). Heat pipe  46  may also be placed at various positions and orientations in circuit module  10 , including elsewhere on substrate  14 , on flex circuit  12 , on ICs  18 , in hollow cavity  26 , etc. Multiple heat pipes  46  may also be used. The number of heat pipes  46  and positioning of heat pipes  46  may be chosen in order to obtain optimal results based on component placement and cooling flow. Placement of heat pipe  46  close to heat sources (e.g., ICs) and across major cooling surface (e.g., substrate  14  surrounding hollow cavity  26 ) tends to optimize heat pipe  46  performance. Heat pipe  46  may be used with circuit module  10  or other circuit cards made of any material, as long as there is reasonable heat conduction through the material into the heat-spreading pipe. Placement of heat pipe  46  close to the source of the heat and across the major cooling surface tends to optimize its performance. 
         [0041]    Although the present invention has been described in detail, it will be apparent to those skilled in the art that many embodiments taking a variety of specific forms and reflecting changes, substitutions and alterations can be made without departing from the spirit and scope of the invention. Therefore, the described embodiments illustrate but do not restrict the scope of the claims.