Patent Publication Number: US-2002008963-A1

Title: Inter-circuit encapsulated packaging

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims benefit of the following U.S. Provisional Patent Applications, each of which are incorporated by reference herein:  
     [0002] Application Serial No. 60/196,059, entitled “EMI FRAME WITH POWER FEED-THROUGHS AND THERMAL INTERFACE MATERIAL IN AN AGGREGATE DIAMOND MIXTURE,” by Joseph T. DiBene II and David H. Hartke, filed Apr. 10, 2000;  
     [0003] Application Serial No. 60/219,813, entitled “HIGH CURRENT MICROPROCESSOR POWER DELIVERY SYSTEMS,” by Joseph T. DiBene II, filed Jul. 21, 2000;  
     [0004] Application Serial No. 60/232,971, entitled “INTEGRATED POWER DISTRIBUTION AND SEMICONDUCTOR PACKAGE,” by Joseph T. DiBene II and James J. Hjerpe, filed Sep. 14, 2000;  
     [0005] Application Serial No. 60/251,222, entitled “INTEGRATED POWER DELIVERY WITH FLEX CIRCUIT INTERCONNECTION FOR HIGH DENSITY POWER CIRCUITS FOR INTEGRATED CIRCUITS AND SYSTEMS,” by Joseph T. DiBene II and David H. Hartke, filed Dec. 4, 2000;  
     [0006] Application Serial No. 60/251,223, entitled “MICRO-I-PAK FOR POWER DELIVERY TO MICROELECTRONICS,” by Joseph T. DiBene II and Carl E. Hoge, filed Dec. 4, 2000; and  
     [0007] Application Serial No. 60/251,184, entitled “MICROPROCESSOR INTEGRATED PACKAGING,” by Joseph T. DiBene II, filed Dec. 4, 2000.  
     [0008] This patent application is also continuation-in-part of the following co-pending and commonly assigned patent applications, each of which applications are hereby incorporated by reference herein:  
     [0009] application Ser. No. 09/353,428, entitled “INTER-CIRCUIT ENCAPSULATED PACKAGING,” by Joseph T. DiBene II and David H. Hartke, filed Jul. 15, 1999;  
     [0010] application Ser. No. 09/432,878, entitled “INTER-CIRCUIT ENCAPSULATED PACKAGING FOR POWER DELIVERY,” by Joseph T. DiBene II and David H. Hartke, filed Nov. 2, 1999;  
     [0011] application Ser. No. 09/727,016, entitled “EMI CONTAINMENT USING INTER-CIRCUIT ENCAPSULATED PACKAGING TECHNOLOGY” by Joseph T. DiBene II and David Hartke, filed Nov. 28, 2000;  
     [0012] application Ser. No. 09/785,892, entitled “METHOD AND APPARATUS FOR PROVIDING POWER TO A MICROPROCESSOR WITH INTEGRATED THERMAL AND EMI MANAGEMENT,” by Joseph T. DiBene II, David H. Hartke, James J. Hjerpe Kaskade, and Carl E. Hoge, filed Feb. 16, 2001; and  
     [0013] application Ser. No. 09/798,541, entitled “THERMAL/MECHANICAL SPRINGBEAM MECHANISM FOR HEAT TRANSFER FROM HEAT SOURCE TO HEAT DISSIPATING DEVICE,” by Joseph T. DiBene II, David H. Hartke, Wendell C. Johnson, and Edward J. Derian, filed Mar. 2, 2001;  
     [0014] application Ser. No. 09/801,437, entitled “METHOD AND APPARATUS FOR DELIVERING POWER TO HIGH PERFORMANCE ELECTRONIC ASSEMBLIES” by Joseph T. DiBene II, David H. Hartke, Carl E. Hoge, James M. Broder, Edward J. Derian, Joseph S. Riel, and Jose B. San Andres, filed Mar. 8, 2001; and  
     [0015] application Ser. No. 09/802,329, entitled “METHOD AND APPARATUS FOR THERMAL AND MECHANICAL MANAGEMENT OF A POWER REGULATOR MODULE AND MICROPROCESSOR IN CONTACT WITH A THERMALLY CONDUCTING PLATE” by Joseph T. DiBene II and David H. Hartke, filed Mar. 8, 2001.  
    
    
     
       BACKGROUND OF THE INVENTION  
       [0016] 1. Field of the Invention  
       [0017] This invention relates in general to a methodology to improve thermal and mechanical issues created by increased interconnect density, increased power levels by electronic circuits and increased levels of integrated electronic packaging. The present invention addresses these issues by encapsulating the circuitry within a circuit board structure which improves thermal, mechanical and integrated circuit device management over existing technologies known in the art today.  
       [0018] 2. Description of Related Art  
       [0019] As circuitry in electronics becomes more and more complex, packaging of the circuitry has become more difficult. The common method for packaging integrated circuits and other electronic components is to mount them on Printed Circuit Boards (PCBs).  
       [0020] Recently, the application of new organic laminates in the construction of Multi-Chip-Modules (MCMs) has brought about significant improvements in the packaging cost and density of electronic circuits. Throughout this patent reference will be made to PCBs which shall be meant to include technologies associated with MCMs as well.  
       [0021] Computer chip clocking speeds have also increased. This increase in speed has made it difficult to couple chips together in such a way that the chip speeds are completely useable. Further, heat generated by integrated circuits has increased because of the increased number of signals travelling through the integrated circuits. In addition, as die size increases interconnect delays on the die are beginning to limit the circuit speeds within the die. Typically, the limitations of a system are contributed to, in part, by the packaging of the system itself. These effects are forcing greater attention to methods of efficiently coupling high-speed circuits.  
       [0022] Packaging the integrated circuits onto PCBs has become increasingly more difficult because of the signal density within integrated circuits and the requirements of heat dissipation. Typical interconnections on a PCB are made using traces that are etched or pattern plated onto a layer of the PCB. To create shorter interconnections, Surface Mount Technology (SMT) chips, Very Large Scale Integration (VLSI) circuits, flip chip bonding, Application Specific Integrated Circuits (ASICs), Ball Grid Arrays (BGAs), and the like, have been used to shorten the transit time and interconnection lengths between chips on a PCB. However, this technology has also not completely overcome the needs for higher signal speeds both intra-PCB and inter-PCB, because of thermal considerations, EMI concerns, and other packaging problems.  
       [0023] In any given system, PCB area (also known as PCB “real estate”) is at a premium. With smaller packaging envelopes becoming the norm in electronics, e.g., laptop computers, spacecraft, cellular telephones, etc., large PCBs are not available for use to mount SMT chips, BGAs, flip chips or other devices. Newer methods are emerging to decrease the size of PCBs such as Build-Up-Multilayer technology, improved organic laminate materials with reduced thicknesses and dielectric constants and laser beam photo imaging. These technologies produce greater pressure to maintain the functionality of the PCB assembly in thermal, EMI and power application to the semiconductor devices. It can be seen, then, that there is a need in the art for a method for decreasing the size of PCBs while maintaining the functionality of PCBs. Further, there is a need for reducing the size of PCBs while using present-day manufacturing techniques to maintain low cost packaging. There is further a need to provide for a compact package of one or more PCBs that provides for integrated thermal and EMI management, while providing high-current/low-voltage power signals to chips mounted on the PCBs.  
       [0024] Designers have attempted to address such needs with designs such as that which is illustrated in U.S. Pat. No. 5,734,555, issued to McMahon. This design uses a collocated second circuit board that may include voltage regulation or power conversion capability. For cooling purposes, both the first PCB (to which the IC is mounted) and the second PCB include an aperture. A heat plug is inserted through the apertures to make thermal connectivity with the component and to provide a path for heat to dissipate from the component away from the package. Unfortunately, this package has several disadvantages and only partially addresses the problem of integrated EMI, thermal, and power management. First, the package requires an aperture to be located in both the first PCB and the second PCB. This reduces the real estate in the second circuit board available for signal routing and increases fabrication costs. Second, the package does not allow the entire surface of the component to be thermally coupled to the heat plug (since the component is larger than the aperture in the first circuit board). Third, the package routes power from the motherboard, through pins and traces in the first circuit board to the second circuit board for power conditioning, then back to the first circuit board and to the component. This circuitous route induces substantial impedance and can also contribute to EMI generation. Finally the McMahon reference discloses a package that uses pins which must be soldered or otherwise permanently connected to the holes in the circuit boards. Hence, the assembly is non-modular, and cannot be easily disassembled.  
       SUMMARY OF THE INVENTION  
       [0025] To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a modular circuit board assembly having a substrate, a circuit board, and a heat dissipating component that is disposed between the circuit board and the substrate and is physically and electrically coupled to the substrate. In one embodiment, the modular circuit board assembly includes a heat sink or other heat dissipation device having a mesa extending through an aperture in a VRM circuit board disposed between the heat sink and the component.  
       [0026] An object of the present invention is to provide more efficient usage of printed circuit board real estate. Another object of the present invention is to increase the density of electronics on printed circuit boards. Another object of the present invention is to provide heat transfer from devices on printed circuit boards.  
       [0027] These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying detailed description, in which there is illustrated and described specific examples of a method, apparatus, and article of manufacture in accordance with the invention.  
       [0028] The foregoing design has particular advantages over prior art designs. For example, by placing the component on the same side of the substrate as the heat dissipation device, the substrate itself does not require an aperture and a heat slug to efficiently transfer thermal energy away from the component. This simplifies the design of the conductive paths in the substrate layers, and if desired, permits the substrate to include a greater number of circuit paths. It also reduces substrate fabrication costs. Further, this design provides a greater physical and thermal contact area between the heat dissipation device and the component, reducing the thermal impedance of the energy path from the component to the heat dissipation device. This design also permits the use of heat sinks with mesas to further reduce thermal impedance as well as the use of special location and/or retention features to assure structural integrity and ease of assembly. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0029] Referring now to the drawings in which like reference numbers represent corresponding parts throughout:  
     [0030] FIGS.  1 A- 1 D illustrate the construction of a printed circuit board assembly using the present invention;  
     [0031] FIGS.  2 A- 2 C illustrates the construction of a printed circuit board assembly using the present invention for multiple heat generating integrated circuit devices;  
     [0032]FIG. 3 illustrates a spacer which is used in conjunction with the present invention;  
     [0033] FIGS.  4 A- 4 C illustrate the construction of a printed circuit board using the present invention wherein the thermal heat sink is located outboard the active circuit area;  
     [0034]FIGS. 5A and 5B illustrate the thermal considerations of a printed circuit board embodying the present invention;  
     [0035]FIG. 6 illustrates a flow chart describing the steps used in practicing the present invention;  
     [0036]FIG. 7 is a diagram illustrating an embodiment of the present invention wherein the second PCB includes an aperture;  
     [0037]FIGS. 8A and 8B are diagrams illustrating an embodiment of the present invention wherein the second PCB includes an aperture and the heat sink includes a mesa;  
     [0038] FIGS.  8 C- 8 F are diagrams illustrating an embodiment of the present invention wherein the heat sink or the component include surface features for location and/or retention;  
     [0039]FIG. 9 is a diagram illustrating an embodiment of the present invention wherein the second PCB is disposed adjacent the component; and  
     [0040]FIG. 10 is a diagram illustrating exemplary method steps used to practice one embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0041] In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.  
     [0042] Overview  
     [0043] The present invention discloses an encapsulated circuit assembly and a method for making such an assembly. The assembly comprises a first printed circuit board, a second printed circuit board, and heat transfer devices. The second printed circuit board comprises a heatsink or secondary heat transfer mechanism such as heat pipes and heat transfer devices imbedded within the second printed circuit board which thermally couples devices mounted on the first printed circuit board and the thermal heat sink of the second printed circuit board.  
     [0044] The present invention provides a method and apparatus for mounting integrated circuit devices onto PCBs that removes the heat from those devices that generate large amounts of heat. The present invention allows for air cooling, heat pipe cooling, or other methods of cooling devices, as well as a compact packaging design to allow for heat generating devices to be packaged into small volumes. Furthermore, the present invention can be expanded to provide beneficial aspects to the art of power distribution, containment of electromagnetic interference and electronic signal interconnect.  
     [0045] Encapsulated Circuit Assembly  
     [0046] FIGS.  1 A- 1 D illustrate the construction of an encapsulated circuit assembly using the present invention. FIG. 1A illustrates an exploded view of assembly  100 . Assembly  100  comprises first printed circuit board (PCB)  102 , second PCB  104 , and heat transfer device  106 . First PCB  102  can be a single layer PCB or multi-layer PCB, where the multi-layer PCB is comprised of alternating layers of conducting and non-conducting materials to allow electrical signals to be routed from device to device on the first PCB  102 . Devices  108 - 116  are shown mounted on first PCB  102 . Devices  114  and  116  are shown as being mounted on the opposite side of first PCB  102  as devices  108 - 112 . This illustrates that first PCB  102  can have devices  108 - 116  mounted on both sides.  
     [0047] Device  108  is coupled to first PCB  102  via a Ball Grid Array (BGA)  118 . BGA  118  provides electrical contacts between device  108  and first PCB  102 . Other methods of electrical coupling between device  108  and first PCB  102  are possible, e.g., wire bonding, solder connections, etc. Further, there can also be thermal coupling between device  108  and PCB  102  if desired.  
     [0048] Heat transfer device  106  couples device  108  to second PCB  104 . Heat transfer device  106  is typically a thermally conductive material, e.g., thermal grease, thermal epoxy, or a commercially produced material such as THERMA-GAP™. Heat transfer device  106  provides a thermal interface between device  108  and the second PCB  104 . Heat transfer device  106  is typically a mechanically compliant material to allow for minimal applied pressure to the device  108  such that device  108  is not subjected to additional stress through use of heat transfer device  108 .  
     [0049] Spacers  141  and fasteners  142  provide for a precision alignment between boards  102  and  104  and the device  108  such that a controlled gap exists in which heat transfer device  106  can properly be accommodated without deleterious air gaps nor excessive pressure applied to device  108 . Additionally, the location of the spacers  141  adjacent to the device  108  reduce variations in spacing caused by bow and warpage of board  102  and, to some extent, board  104 .  
     [0050] Devices  110 - 116  that are thermally active but do not require heat transfer device  106  to cool the devices  110 - 116  and are cooled by conduction through first PCB  102 , or through convection should air flow be available across first PCB  102 . Otherwise, additional devices  110 - 116  can be coupled to second PCB  104  through additional heat transfer devices  106 . The present invention is not limited to a single device  108  that is cooled through the use of heat transfer device  106 . Any number of devices  108 - 116  can be cooled through the use of single or multiple heat transfer devices  106 .  
     [0051] Second PCB  104  is mechanically coupled to first PCB  102  through the use of fasteners  120  and standoffs  122 . Fasteners  120  are typically screws, but can be other types of fasteners such as rivets, hollow feedthroughs, connectors, or other fasteners. Standoffs  122  are typically unthreaded inserts with a height equal to the height of spacer  141 . The fasteners  120  and standoffs  122  are located at mechanically and/or electrically desirable locations on first PCB  102 . These locations are typically at the periphery of first PCB  102 , but can be anywhere on first PCB  102 .  
     [0052] Second PCB  104  has areas  124  that are designed to facilitate the transfer of heat from device  108 , through heat transfer device  106 , to a heat sink. Areas  124  comprise plated through holes (PTHs)  126 , consisting of holes in board  104  with interior walls of plated copper or other high thermally conductive material. In addition, the region within the hole may be filled with metal, liquid filled areas, or other thermal transfer devices or mechanisms to enhance thermal conduction between the material  106  and the heatsink  130 . Areas  124  can be designed to be the same size, a larger size, or a smaller size than the device  108 , depending on the heat dissipation requirements for device  108  and the size of second PCB  104 . An additional benefit of PTHs  126  is to provide a means of reducing air pockets in material  106  and to provide a volume where excesses of material  106  may flow in the case of a reduced gap between device  108  and board  104 . Still another benefit of PTHs  126  can be to adjust the thermal conductivity of the paths of multiple devices  108  on a single first PCB  102  to the common “isothermal” heatsink  130  such that if the two devices  108  have differing heat flow then the conductivity in each thermal path can be adjusted such that the junction temperature of each device  108  will be the same. This can be beneficial in improving timing margins of digital devices.  
     [0053] A thermal interface such as a plate  128  is coupled to second PCB  104  to equalize and transfer heat from device  108 , through heat transfer device  106  and second PCB  104  area  124  to heat sink  130 . Although shown as a finned heat sink, heatsink  130  can be any device, e.g., a heat pipe, or a layer on second PCB  104  that acts as an isothermal conduction layer to properly remove the heat generated by device  108 . Thermal interface  128  can be electrically conductive, or non-electrically conductive, depending on the design for second PCB  104 . For example, if devices  302 - 308  need to be mounted on second PCB  104 , thermal interface  128  should be electrically non-conductive so as not to interfere with signals travelling between devices  108 - 116  that are mounted on second PCB  104 . Thermal interface  128  can be thermal epoxy or any other material which thermally and mechanically bonds second PCB  104  to heatsink  130 .  
     [0054]FIG. 1B illustrates the assembly  100  as a completed assembly. The thermal coupling of device  108 , heat transfer device  106 , second PCB  104  in conjunction with PTHs  126 , thermal interface  128 , and heatsink  130  provide a thermal path for heat generated by device  108  to be dissipated by heatsink  130 . Further, airflow can be provided to further cool device  108  and devices  110 - 116 . Although shown as covering the entire area of second PCB  104 , heatsink  130  can be larger or smaller than the area of second PCB  104 . Heatsink  130  also acts as a mechanical stabilizer for assembly  100 , to provide additional mechanical stability for assemblies  100  that will experience more severe mechanical environments, e.g., vibration.  
     [0055]FIG. 1C illustrates assembly  100  in an isometric view. Heatsink  130  is shown as smaller than second PCB  104  and thermal interface  128  to illustrate the flexibility of the design of the present invention. Airflow can again be provided to increase the heat dissipation capabilities of assembly  100 .  
     [0056]FIG. 1D illustrates an embodiment of the assembly  100  comprising a heat pipe  160 .  
     [0057] Multiple Device Encapsulated Circuit Assembly  
     [0058] FIGS.  2 A- 2 B illustrate the construction of an encapsulated circuit assembly using the present invention for multiple heat generating integrated circuit devices. FIG. 2A illustrates an exploded view of assembly  100 . Assembly  100  comprises first printed circuit board (PCB)  102 , second PCB  104 , and heat transfer device  106 . First PCB  102  can be a single layer PCB or multi-layer PCB, where the multi-layer PCB is comprised of alternating layers of conducting and non-conducting materials to allow electrical signals to be routed from device to device on the first PCB  102 . Devices  108 ,  114 - 116 , and  132  are shown mounted on first PCB  102 . Devices  114  and  116  are shown as being mounted on the opposite side of first PCB  102  as devices  108  and  132 . This illustrates that first PCB  102  can have devices  108 ,  114 - 116 , and  132  mounted on both sides.  
     [0059] Devices  108  and  132  are coupled to first PCB  102  via a Ball Grid Array (BGA)  118 . BGA  118  provides electrical contacts between devices  108  and  132  and first PCB  102 . Other methods of electrical coupling between devices  108  and  132  and first PCB  102  are possible, e.g., Tape Automated Bonding (TAB), SMT, flip chip, etc. Further, there can also be thermal coupling between devices  108  and  132  and PCB  102  if desired.  
     [0060] Heat transfer device  106  couples device  108  to second PCB  104 . Heat transfer device  106  is typically a thermally conductive material, e.g., thermal grease, thermal epoxy, or a commercially produced material such as THERMA-GAP™. Heat transfer device  106  provides a thermal interface between device  108  and the second PCB  104 . Heat transfer device  106  is typically a mechanically compliant material to allow for minimal applied pressure to the device  108  such that device  108  is not subjected to additional stress through use of heat transfer device  108 .  
     [0061] Spacers  141  and fasteners  142  provide for a precision alignment between boards  102  and  104  and the device  108  such that a controlled gap exists in which heat transfer device  106  can properly be accommodated without deleterious air gaps nor excessive pressure applied to device  108 . Additionally, the location of the spacers  141  adjacent to the device  108  reduce variations in spacing caused by bow and warpage of board  102  and, to some extent, board  104 .  
     [0062] Devices  114 - 116  that are thermally active but do not require heat transfer device  106  to cool the devices  114 - 116  are cooled by conduction through first PCB  102 , or through convection should air flow be available across first PCB  102 .  
     [0063] Device  132  is another heat generating device similar to device  108 . However, all devices  108  and  132  that will require additional cooling through heat transfer device  106 , second PCB  104 , and heatsink  130  are not the same size and/or height. Therefore, each device  108  and  132  must be treated individually using the present invention to best provide heat dissipation for each device  108  and  132 . In FIG. 2A, device  132  is shown as having a height  134  smaller than height  136  of device  108 . There can be many devices  108  and  132  of varying heights mounted on first PCB  102 , all of which can be cooled by the assembly  100  of the present invention, through use of an additional thermal interface  138  and a thermally conductive spacer  140 .  
     [0064] Thermal interface  138  provides a thermal path for device  132  that will allow heat generated by device  132  to be dissipated by heatsink  130 . Thermal interface  138  can be similar to heat transfer device  106 , but can also be a different thermal transfer material to provide a proper thermal dissipative path. As an example thermal interface  138  need not be mechanically compliant so long as thermal interface  106  above it is. Thus, the use of a hardening thermal epoxy may be useful to hold spacer  140  in place during assembly.  
     [0065] Spacer  140  is provided to increase height  134  to approximate height  136 . This allows device  108  and device  132  to contact heat transfer device  106 , which in turn contacts second PCB  104  and heatsink  130  to transfer heat from devices  108  and  132  to heatsink  130 . Spacer  140  is shown as larger in size than device  132 , which can provide for heat spreading of the heat generated by device  132  to heatsink  130 . Spacer  140  can be of any size relative to device  132 . Further, there can be spacers  140  on more than one device  108  and  132 .  
     [0066] Where height differences between devices are relatively small and power levels modest these height differences may beneficially be accommodated by selecting varying thicknesses of heat transfer device  106  rather than utilizing thermal interface  138  and spacer  140 .  
     [0067] Second PCB  104  is coupled mechanically to first PCB  102  through the use of fasteners  120  and standoffs  122 . Fasteners  120  are typically screws, but can be other types of fasteners such as rivets, feedthroughs that are hollow, connectors, or other fasteners. Standoffs  122  are typically unthreaded inserts with a height equal to the height of spacer  141 . The fasteners  120  and standoffs  122  are located at mechanically and/or electrically desirable locations on first PCB  102 . These locations are typically at the periphery of first PCB  102 , but can be anywhere on first PCB  102 . Second PCB  104  has areas  124  that are designed to facilitate the transfer of heat from devices  108  and  132 , through heat transfer device  106 , to a heat sink. Areas  124  comprise plated through holes (PTHs)  126 , consisting of holes in board  104  with interior walls of plated copper or other high thermally conductive material. In addition, the region within the hole may be filled with metal, liquid filled areas, or other thermal transfer devices or mechanisms to enhance thermal conduction between material  106  and heatsink  130 . Areas  124  can be designed to be the same size, a larger size, or a smaller size than the device  108 , depending on the heat dissipation requirements for device  108  and the size of second PCB  104 . An additional benefit of PTHs  126  is to provide a means of reducing air pockets in material  106  and to provide a volume where excesses of material  106  may flow in the case of a reduced gap between device  108  and  104 .  
     [0068] Thermal interface  128  is coupled to second PCB  104  to equalize and transfer heat from device  108 , through heat transfer device  106  and second PCB  104  area  124  to heat sink  130 . Although shown as a finned heat sink, heatsink  130  can be any device, e.g., a heat pipe, or a layer on second PCB  104  that acts as an isothermal conduction layer to properly remove the heat generated by device  108 . Thermal interface  128  can be electrically conductive, or non-electrically conductive, depending on the design for second PCB  104 . For example, if devices  108 - 116  need to be mounted on second PCB  104 , thermal interface  128  can be electrically non-conductive so as not to interfere with signals travelling between devices  108 - 116  that are mounted on second PCB  104 . Thermal interface  128  can be thermal epoxy or any other material which thermally and mechanically bonds board  104  to heatsink  130 .  
     [0069]FIG. 2B illustrates the assembly  100  of FIG. 2A as a completed assembly. The thermal coupling of devices  108  and  132 , heat transfer device  106 , thermal interface  138 , spacer  140 , second PCB  104  in conjunction with PTHs  126 , thermal interface  128 , and heatsink  130  provide thermal paths for heat generated by devices  108  and  132  to be dissipated by heatsink  130 . Further, airflow can be provided to further cool devices  108  and  132 , as well as devices  110 - 116 . Although shown as covering the entire area of second PCB  104 , heatsink  130  can be larger or smaller than the area of second PCB  104 . Heatsink  130  also acts as a mechanical stabilizer for assembly  100 , to provide additional mechanical stability for assemblies  100  that will experience more severe mechanical environments, e.g., vibration.  
     [0070]FIG. 3A illustrates in plan and section views a molded plastic spacer  143  that may be used in place of spacers  141  around a device that must be thermally coupled to board  104 . This spacer has clearance holes  145  for fasteners  142 . Although spacer  143  is shown with four clearance holes  145 , spacer  143  can have any number of clearance holes  145  without departing from the scope of the present invention. Imbedded metal spacers may be molded into holes  145  where it may be desirous to provide electrical contact between board  102  and board  104 . Spacer  143  substantially surrounds device  108 , but can take any shape desired. A feature of the spacer is pins  144  that engage in mating holes of board  102  and act to hold spacer  143  in place until final assembly of assembly  100 . An additional benefit of spacer  143  is that it provides complete enclosure of device  108  to prevent accidental damage. Furthermore, spacer  143  may be used to provide thermal isolation between device  108  and the remainder of the board assembly  100 .  
     [0071]FIG. 3B illustrates a molded plastic spacer  147  that may be used in place of spacers  141  which have been previously described as used to couple second PCB  104  to first PCB  102 . This spacer  147  is shown as having ten clearance holes  150  for fasteners  120 , however a larger or smaller number of fasteners may be used as the need and size of the PCBs  102  and  104  require. Imbedded metal spacers may be molded into holes  150  where it may be desirous to provide electrical contact between board  102  and board  104 . Furthermore, the entire molded assembly may be formed as a cast metal structure or other metallic form which may be useful in the containment of electromagnetic radiation. A feature of the spacer  147  is pins  149  that engage in mating holes of board  102  and act to hold in place spacer  147  until final assembly of  100 . An additional benefit of spacer  147  is that it provides complete enclosure of device  108  to prevent accidental damage. Furthermore, spacer  147  may be used to provide environmental isolation to the internal components of assembly  100 .  
     [0072] Embodiments of the Present Invention  
     [0073] FIGS.  4 A- 4 C illustrate the construction of a printed circuit board using the present invention. FIG. 4A illustrates an exploded view of assembly  100 . Assembly  100  comprises first printed circuit board (PCB)  102 , second PCB  104 , and heat transfer device  106 . First PCB  102  can be a single layer PCB or multi-layer PCB, where the multi-layer PCB is comprised of alternating layers of conducting and non-conducting materials to allow electrical signals to be routed from device to device on the first PCB  102 . Devices  108 ,  114 , and  116  are shown mounted on first PCB  102 . Devices  114  and  116  are shown as being mounted on the opposite side of first PCB  102  as device  108 . This illustrates that first PCB  102  can have devices  108 ,  114 , and  116  mounted on both sides.  
     [0074] Device  108  is coupled to first PCB  102  via a Ball Grid Array (BGA)  118 . BGA  118  provides electrical contacts between device  108  and first PCB  102 . Other methods of electrical coupling between device  108  and first PCB  102  are possible, e.g., wire bonding, solder connections, etc. Further, there can also be thermal coupling between device  108  and PCB  102  if desired.  
     [0075] Heat transfer device  106  couples device  108  to second PCB  104 . Heat transfer device  106  is typically a thermally conductive material, e.g., thermal grease, thermal epoxy, or a commercially produced material such as THERMA-GAP™. Heat transfer device  106  provides a thermal interface between device  108  and the second PCB  104 . Heat transfer device  106  is typically a mechanically compliant material to allow for minimal applied pressure to the device  108  such that device  108  is not subjected to additional stress through use of heat transfer device  108 .  
     [0076] Spacers  141  and fasteners  142  provide for a precision alignment between boards  102  and  104  and the device  108  such that a controlled gap exists in which heat transfer device  106  can properly be accommodated without deleterious air gaps not excessive pressure applied to device  108 . Additionally, the location of the spacers  141  adjacent to the device  108  reduce variations in spacing caused by bow and warpage of board  102  and, to some extent, board  104 .  
     [0077] Devices  114 - 116  that are thermally active but do not require heat transfer device  106  to cool the devices  114 - 116  are cooled by conduction through first PCB  102 , or through convection should air flow be available across first PCB  102 . Otherwise, additional devices  114 - 116  can be coupled to second PCB  104  through additional heat transfer devices  106 . The present invention is not limited to a single device  108  that is cooled through the use of heat transfer device  106 . Any number of devices  108  can be cooled through the use of single or multiple heat transfer devices  106 .  
     [0078] Second PCB  104  is coupled mechanically to first PCB  102  through the use of fasteners  120  and standoffs  122 . Fasteners  120  are typically screws, but can be other types of fasteners such as rivets, hollow feedthroughs, connectors, or other fasteners. Standoffs  122  are typically unthreaded inserts with a height equal to the height of spacer  141 . The fasteners  120  and standoffs  122  are located at mechanically and/or electrically desirable locations on first PCB  102 . These locations are typically at the periphery of first PCB  102 , but can be anywhere on first PCB  102 .  
     [0079] Second PCB  104  has areas  124  that are designed to facilitate the transfer of heat from device  108 , through heat transfer device  106 , to a heat sink. Areas  124  comprise plated though holes (PTHs)  126 , consisting of holes in board  104  with interior walls of plated copper or other high thermally conductive material. In addition, the region within the hole may be filled with metal or other thermal transfer devices or mechanisms to enhance thermal conduction between the material  106  and the heatsink  130 . Areas  124  can be designed to be the same size, a larger size, or a smaller size than the device  108 , depending on the heat dissipation requirements for device  108  and the size of second PCB  104 . An additional benefit of PTHs  126  is to provide a means of reducing air pockets in material  106  and to provide a volume where excesses of material  106  may flow in the case of a reduced gap between device  108  and board  104 . Still another benefit of PTHs  126  can be to adjust the thermal conductivity of the paths of devices  108  and  132  to the common “isothermal” lateral heat spreader block  146  such that if the two devices have differing heat flow then the conductivity in each path can be adjusted such that the junction temperature of each device will be the same. This can be beneficial in improving timing margins of digital devices.  
     [0080] Thermal interface  128  is coupled to second PCB  104  to equalize and transfer heat from device  108 , through heat transfer device  106  and second PCB  104  area  124  to lateral heat spreader block  146 . Heat spreader block  146  is desirably of a thermally high conductivity material such as aluminum which allows the heat emanating from devices  108  and  132  to flow to heat sink  130  which is located outside of the volume used by boards  102  and  104 . Additionally, heat spreader block  146  may incorporate imbedded heat pipes to enhance lateral thermal conduction and/or reduce height. Although shown as a finned heat sink, heatsink  130  can be any device, e.g., a heat pipe, that can conduct heat out of the heat spreader block  146 . Thermal interface  128  can be electrically conductive, or non-electrically conductive, depending on the design for second PCB  104 . For example, if devices  108 - 116  need to be mounted on second PCB  104 , thermal interface  128  should be electrically non-conductive so as not to interfere with signals travelling between devices  108 - 116  that are mounted on second PCB  104 . Thermal interface  128  can be thermal epoxy or any other material which thermally and mechanically bonds board  104  to heatsink  130  and between heatsink  130  and heat spreader block  146 .  
     [0081] As opposed to FIG. 1A, heatsink  130  is now shown as being mounted outboard the volume occupied by PCB  102  and second PCB  104 . This flexibility of the present invention to mount the heatsink  130  at multiple locations provides additional design capabilities, i.e., the height of assembly  100  is now independent of the height of heatsink  130 . Thus, heat dissipative capability is provided without additional volume requirements for assembly  100  other than the height of heat spreader block  146 .  
     [0082]FIG. 4B illustrates the assembly  100  as a completed assembly. The thermal coupling of device  108 , heat transfer device  106 , second PCB  104 , thermal interface  128 , heat spreader block  146  and heatsink  130  provide a thermal path for heat generated by device  108  to be dissipated by heatsink  130 . Further, airflow can be provided to further cool device  108  and devices  114 - 116 . Heatsink  130  can be larger or smaller than the height of PCB  102 , PCB  104  and heat spreader block  146 . Heat spreader block  146  also acts as a mechanical stabilizer for assembly  100 , to provide additional mechanical stability for assemblies  100  that will experience more severe mechanical environments, e.g., vibration.  
     [0083]FIG. 4C illustrates assembly  100  in an isometric view. Heatsink  130  is shown as residing outboard of first PCB  102  and second PCB  104 . Thermal interface  128  is shown on the opposite side of second PCB  104 , and is shown as smaller than second PCB  104  to illustrate the flexibility of the design of the present invention. Airflow can again be provided to increase the heat dissipation capabilities of assembly  100 .  
     [0084] The design of FIGS.  4 A- 4 C can be used where assembly  100  height is at a premium, or, where the heatsink  130  would be more efficient located outboard first PCB  102  and second PCB  104  than it would be if heatsink  130  sat atop second PCB  104 . This might occur when it is desirous to locate assembly  100  adjacent to similar assemblies  100  as close as practical to minimize electrical interconnect lengths, where airflow over the top of second PCB  104  is less than airflow outboard of assembly  100 . Further, the placement of heatsink  130  outboard first PCB  102  and second PCB  104  allows heatsink  130  to be electrically grounded, or placed at a desired potential, using both first PCB  102  and second PCB  104 .  
     [0085] Thermal Considerations  
     [0086]FIGS. 5A and 5B illustrate the thermal considerations of a printed circuit board embodying the present invention.  
     [0087]FIG. 5A illustrates assembly  100  with the various thermal interfaces described for the present invention. The silicon die is represented as die  148 . Thermal Interface  1  (TI 1 )  172  is the thermal interface internal to the device  108  between device heatspreader  178  and silicon die  148 . Heatspreader  178  may not always be present in which case thermal interface  172  would be used to represent the thermal resistance of the outside package surface to the silicon die  148 , e.g. molding compound. Thermal Interface  2  (TI 2 )  174  is the interface between second PCB  104  and device  108 . Thermal Interface  3  (TI 3 )  176  is the interface between second PCB  104  and heatsink  130 .  
     [0088] Plated through holes (PTH)  180  is the area  124  of PCB  104  that allows thermal conduction through the board  104 . Heatsink (HSK)  130  is the device that couples the heat flow to the air or in some cases to thermal pipes to remote radiators. FIG. 5B illustrates the thermal schematic for the assembly  100  shown in FIG. 5A. Starting from die  148 , TI 1   172  receives a thermal resistance value, theta TI 1  (θ TI1 )  186 , HS 1   178  receives a thermal resistance value theta HS 1  (θ HS1 )  188 , TI 2   174  receives a thermal resistance value, theta TI 2  (θ TI2 )  190 , HV  180  receives a thermal resistance value, theta HV (θ HV )  192 , TI 3   176  receives a thermal resistance value, theta TI 3  (θ TI3 )  194 , and HSK  130  receives a thermal resistance value, theta HSK (θ HSK )  202 . The thermal resistances of the assembly  100  are determined in terms of degrees centigrade per watt (° C./W). To determine the total temperature rise across the interface from silicon die  148  to ambient air, the total power of the device is multiplied by the total thermal resistance:  
         Δ                 T     =       ∑     i   =   1     n          θ                 i   *   W                     
 
     [0089] For example, a 1 ° C./W total thermal resistance for a 50-Watt device would yield a total temperature change of 50° C.  
     [0090]FIG. 6 illustrates a flow chart describing the steps used in practicing the present invention.  
     [0091] Block  204  represents the step of mounting a heat generating device on a first printed circuit board.  
     [0092] Block  206  represents the step of thermally coupling the heat generating device to a heatsink coupled to a second printed circuit board, wherein a thermal path passes through the second printed circuit board.  
     [0093] Further Heat Sink and PCB Embodiments  
     [0094]FIG. 7 is a diagram illustrating an embodiment of the present invention. In this embodiment, the modular circuit board assembly  700  comprises a substrate  702 , a circuit board  704 , and a component  706  such as an integrated circuit or other heat-dissipating component, disposed between the circuit board  704  and the substrate  702 . The component  706  is physically and electrically coupled to the substrate  702 . The substrate  702  may be physically and electrically coupled to a socket  708 , thereby providing a path for signals between one or more of the layers  710  of a motherboard  712  and the component  706 .  
     [0095] In one embodiment, an aperture  714  is disposed at least partially through the circuit board  704 . At least a portion of the component  706  extends to within the aperture  714  and thermally communicates with a heat dissipation device such as a heat sink. In one embodiment, a thermal interface material  718  such as a thermal grease, is interposed between the top surface of the component  706  and the bottom surface of the heat sink. Standoffs  720  are disposed between the motherboard  712  and the circuit board  704 . In one embodiment, the circuit board  704  includes a one or more passive and/or active components assembled together to form a power conditioning or voltage regulation module (VRM). Power can be supplied to one or more of conductive surfaces in the layers  722  of the circuit board  704  from the motherboard  712  using the coaxial power standoffs described in the related applications referenced in the beginning of this disclosure. In one embodiment, the circuit board and the substrates of the modular assembly  700  are impermanently coupled together. That is, the modular assembly  700  can be assembled without permanent press-fit or solder connections, and can be therefore disassembled if desired.  
     [0096]FIG. 8A is a diagram illustrating another embodiment of the present invention. In this embodiment, the heat dissipating device or heat sink  716  or the modular circuit board assembly  800  includes a mesa  802 . The mesa  802  extends to within the aperture  714 , where it provides thermal connectivity with the component  706 . As with the embodiment illustrated in FIG. 7, a thermal interface material  718  can be disposed between the mesa  802  and the component  706 .  
     [0097]FIG. 8B is a diagram illustrating another embodiment of the present invention. In this embodiment, the mesa  802  extends all the way through the aperture  714  to the side of the circuit board  704  opposing the heat sink  716 .  
     [0098] In addition to the mesa  802  disclosed above, the heat sink  716  may also comprise a depressed portion, sized and shaped to accept the component  706  or a member thermally attached to the component  706 . The depressed portion can include the location and/or retention features discussed below.  
     [0099]FIGS. 8C and 8D are diagrams depicting further embodiments of the present invention. In these embodiments, heat sink  716  includes features  804  and  806  that can be used as location and/or retention features. As shown in FIG. 8C, first feature  804  includes an elevated portion that is shaped and sized so as to accept the periphery of the component  706  therebetween, thus providing location and/or retention for the component  706  and/or related devices relative to the heat sink  716  and the components affixed thereto. As shown in FIG. 8D, a second feature  806  can be used such that the surfaces of the second features  806  contact one or more outer surfaces of the component.  
     [0100]FIGS. 8E and 8F are diagrams depicting another embodiment of the present invention in which the features  808 ,  810  interface with matching features  812 ,  814  disposed on an external surface of the component  706  or a member physically or thermally coupled to the component  706 . While the illustrations presented in FIGS. 8E and 8F show the heat sink  716  with male features  808 ,  810  and the component  706  with female features  812 ,  814 , this need not be the case . . . male features may instead be disposed on the component  706 . Further, the scope of the applicants&#39; invention includes other location and/or retention features that may be utilized.  
     [0101]FIG. 9 is a diagram illustrating another embodiment of the present invention. In this embodiment, a modular circuit board assembly  900  includes a component  906  die mounted on and in electrical communication with a substrate  914 . The substrate  914  is mounted on an interposer circuit board  904 , which makes electrical contact with a motherboard (not shown), thus providing an electrical path for communication between the motherboard and the die. A thermal interface material  908  may be placed on an upper surface of the component  906  die to provide for improved thermal communication between the component  906  die and the heat sink mesa  802 . In one embodiment, an external surface of the heat sink mesa  802  includes location and/or retention features, as described above. The heat sink  716  is mounted to a frame  902 , which supports the structure of the modular circuit board assembly  900 . A second circuit board  912  (such as a voltage regulation module, or VRM) adjacent the component  906  die is communicatively coupled to the interposer circuit board  904 . In one embodiment, this is accomplished by the use of coaxial conductors  910  described fully in the cross-referenced patent applications.  
     [0102] The second circuit board  912  can be thermally coupled to the heat sink  716  by direct content, or contact thorough a thermal interface material. The heat sink  716  may also comprise a second mesa, for making thermal contact with the second circuit board  912 . If desired, elements on the second circuit board  912  and/or the second mesa external surface can include location and/or retention features.  
     [0103] In one embodiment, the second circuit board  912  is disposed adjacent to the component  906  die, thus minimizing size and conserving space in the z (vertical) axis. If desired, the top surface of the second circuit board  912  can be disposed substantially co-planar with that of the top surface of the component  906  die, or thermal transfer element thermally coupled to the die. In another embodiment, the “height” of the mesa  802  is selected to account for any differences in the height of the component  906  die and related assemblies, and the second circuit board  912 .  
     [0104]FIG. 10 is a flow chart illustrating exemplary method steps used to practice one embodiment of the present invention. A first surface of a component  706  is mounted on a substrate  702 , as shown in block  1002 . A second surface of the component  706  which opposes the first surface of the component  706  is then thermally coupled to a heat sink  716  via an aperture  714  in a second circuit board  704  disposed between the heat sink  716  and the component  706 , as shown in block  1004 .  
     [0105] Conclusion  
     [0106] This concludes the description of the preferred embodiment of the invention. The following describes some alternative embodiments for accomplishing the present invention. Assembly  100  can have both rigid and flexible layers to accommodate the needs of PCB designers without departing from the scope of the present invention. Further, the thicknesses of assembly  100  can be modified to accommodate components as needed.  
     [0107] Although described with respect to thermal considerations, the present invention can also be used to shield device  108  from outside radiative effects, e.g., radiation, electromagnetic interference, etc. Further, device  108  can be shielded from emitting radiation and/or electromagnetic signals to the outside world through the use of the present invention. The present invention can also be used to provide power to devices through the second PCB  104  by contacting the device  108  through spacers  124  or standoffs  122 .  
     [0108] In summary, the present invention discloses an encapuslated circuit assembly and a method for making such an assembly. The assembly comprises a first printed circuit board, a second printed circuit board, and a heat transfer device. The second printed circuit board comprises a heatsink, and the heat transfer device couples between a device mounted on the first printed circuit board and the second printed circuit board for transferring heat from the device to the heatsink of the second printed circuit board.  
     [0109] The foregoing description of the preferred embodiment 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 form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.