Patent Abstract:
A Micro Blade is described for implementing an electronic assembly having a thin profile; it is a miniaturized stand-alone unit that is mechanically and thermally rugged, and connects to external components using a cable. The electronic assembly is preferably fabricated on a copper foil substrate including an interconnection circuit, a special assembly layer, and directly attached components. The components are preferably in bare die form, and are preferably attached using plated copper spring elements inserted into wells filled with solder. The copper foil substrate may be folded to form a compact system in package (SIP) inside of the Micro Blade. A jacket comprised of thermally conductive members is formed around the electronic assembly using hermetic seams. The Micro Blade is preferably cooled by immersion in water contained in a tank; the water is cooled and circulated using an external pumping system.

Full Description:
RELATED APPLICATIONS 
   This application claims priority to Provisional Application Ser. No. 60/570,199 filed May 11, 2004. 

   THE INVENTION 
   This invention relates to cooling of electronic components and systems, and more particularly to a thermal architecture for liquid cooling of electronic sub-assemblies connected to a motherboard or backplane. 
   BACKGROUND OF THE INVENTION 
   Blade components have a characteristic form factor; they have a thickness dimension substantially smaller that their height and width. This provides a convenient packaging arrangement for a large system, wherein multiple blades can be inserted in slots in a chassis, each blade providing a standalone capability, and each blade replaceable if problems develop. 
   Advanced packaging technology may be applied to reduce the dimensions of such a blade assembly. However, dissipating the heat generated in such a miniaturized version is difficult. It is helpful to make the blade as thin as possible. This provides short paths for the heat to escape from the integrated circuit chips inside to an external heat sink. In this manner, and employing advanced cooling methods, a Micro Blade assembly can effectively handle the approximately 500 watts of power associated with a high-end 4-way server. The Micro Blade package typically occupies less than 1% of the volume of a conventional blade device that is fabricated using packaged devices mounted on printed circuit boards. This type of result provides motivation for the Micro Blade form factor. 
   An example blade server is the HS40 manufactured by International Business Machines; it is 2.3 inches thick, 9.7 inches high, and 17.6 inches wide. The HS40 can be inserted into a chassis having 16 slots for similarly shaped blade components. The components may have different functions such as processor modules, or switch modules for high data rate communications to or from other components. 
   The HS40 includes up to 4 Xeon processors. Because these processors each generate up to 85 watts of heat during operation, they are provided with large and bulky finned aluminum heat sinks; and forced air passes over the fins to cool the processors. These heat sinks occupy approximately 40% of the HS40 blade volume. The total blade electronics requires 480 watts of cooling when four 3 GHz processors are installed. The HS40 also includes a power supply board, a controller board, and memory provided within serial in-line packages; it weighs 15.4 pounds and occupies 393 cubic inches. 
   SUMMARY OF THE INVENTION 
   The current invention implements the functions of a board or a blade in a micro-sized version. This micro-sized version preferably has a small thickness dimension and is referred to herein as a Micro Blade. The Micro Blade concept can be applied to a broad class of electronic components, including printed circuit boards, subsystems, system-in-package (SIP), and complete systems. It can be of any size. However, for the purpose of illustration, a particular size is described; this particular size resulting from shrinking the example HS40 device using advanced packaging techniques. 
   The Micro Blade version implements the same functions as the HS40 except for some minor differences in power distribution. The size of the preferred embodiment is 45×45×3.2 mm, with a corresponding weight of approximately 0.1 pounds. The corresponding volume is 0.4 cubic inches. Its thinness contributes to effective cooling. 
   The Micro Blade embodiment described herein is designed to use the same integrated circuit (IC) chips as the system it replaces. Consequently, it must dissipate the same amount of power, 480 watts in the case of the HS40 server example. In the preferred embodiment, this is achieved using a folded system-in-package (SIP) built on a copper foil substrate; copper comprises approximately 78% of the SIP volume, providing good heat transfer characteristics for cooling. By wrapping the SIP in a hermetic copper jacket it becomes a Micro Blade; a miniaturized stand-alone unit that can be connected to outside elements using a cable. It is mechanically rugged and also thermally rugged; “thermally rugged” means that it can effectively distribute localized hot spots and can also withstand brief surges in overall heat generation. It is preferably cooled in water or other liquid coolant, although air cooling is also a viable option by adding finned elements and forced air, as is known in the art. Yet another option is to provide heat pipes that carry heat produced in the copper jackets to remote radiators where it is dissipated using large area surfaces. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of the invention will be more clearly understood from the accompanying drawings and description of the invention: 
       FIG. 1  is a cross-sectional view of a Micro Blade of the current invention at approximately 4× scale; 
       FIG. 2  shows a side view of the Micro Blade of  FIG. 1  at approximately 2× scale; 
       FIG. 3A  shows a fragmentary cross-section of a circuit assembly fabricated on a copper substrate, that can be folded to form a system-in-package inside a Micro Blade; 
       FIG. 3B  is an expanded cross-sectional view of region B of  FIG. 3A , including details of the flip chip connectors; 
       FIG. 4  is a top view of a system-in-package fabricated on a copper wafer, prior to folding; 
       FIG. 5  shows a fragmentary cross-section of a cable aligned with a circuit assembly, just prior to physical attachment; 
       FIG. 6A  is a schematic top view of a hermetic cable of the current invention; 
       FIG. 6B  is a cross-section shown as BB in  FIG. 6A ; 
       FIG. 6C  is a cross-section shown as CC in  FIG. 6A ; and 
       FIG. 7  is a cross-sectional view of a water-cooled system of the current invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The cross-section of  FIG. 1  illustrates Micro Blade  10  of the current invention, at a scale of approximately 4×. Micro Blade  10  can be of any size, but preferably has a thin form factor for ease of cooling the active circuits inside. Copper jacket  11  is soldered, welded, or brazed at crimped edges  12  to form a hermetic package. Inside the copper jacket is an electronic assembly that is preferably a folded system in package (SIP)  13  including IC chips  14 . At the top of Micro Blade  10  is a semi-hermetic seal  15  that also provides a strain relief for cable  16 . Semi-hermetic seal  15  may be formed using an epoxy adhesive or a potting compound, as is known in the art. Cable  16  connects to electronic assembly  13  using a direct attachment (like flip chip), as will be further described. Cable  16  also includes a section that is hermetic, as will be further described. 
     FIG. 2  is a side view of Micro Blade  10 , showing cable  16 . Cable  16  typically carries power and high speed signals, as will be further described. Although any width and height are covered under the current invention, W  21  and H  22  are both 45 mm in the preferred embodiment. Pressure is applied to the copper sheets of hermetic jacket  11  to assure intimate contact with the circuit assembly inside, while the edges  23  are crimped together. Mating surfaces at the crimped edges are coated with solder paste, or a dry film of solder alloy is laminated between them. Heat and pressure are applied to melt the solder and make a watertight seal. 
     FIG. 3A  is a cross-sectional view of a fragment of a circuit assembly  13 A. After fabrication, testing, and any necessary rework, assembly  13 A will be folded to make circuit assembly  13  of  FIG. 1 , as will be further described. Assembly  13 A includes foldable circuit board  31  formed on a copper foil substrate  32 . Interconnection circuit  33  is preferably fabricated using build up dielectric and conducting layers, as is known in the art. In the preferred embodiment it includes dual damascene copper and dielectric structures that implement transmission lines having a characteristic impedance. The dual damascene structures may be fabricated using a combination of imprinting, electroplating, and chemical mechanical polishing (CMP). Further details of these processes are described in co-pending application Ser. No. 10/783,921 which is incorporated herein by reference. 
     FIG. 3B  is an expanded view of region B of  FIG. 3A . It shows a fragment of IC chip  34  having copper spring bumps  35  attached at input/output pads  36 . The copper spring bumps  35  are inserted into wells  37  containing solder paste that has been heated to form melted solder  38  in the wells. The solder in each well captures the end of a copper spring bump as shown, providing a strong mechanical connection and a low-resistance electrical connection. This flip chip connector is labeled  39  in  FIG. 3B , and is a good stress reliever in all three dimensions. Such mechanical compliance is desirable for relaxing shear stresses caused by un-matched thermal expansion of IC chips versus circuit boards during temperature excursions. Vertical compliance of the spring elements is provided by a bend in spring element  35  as shown; this is additionally useful for accommodating imperfections in the components and in the assembly process. Co-pending application Ser. No. 11/015,213 describes the flip chip connectors in more detail, including methods for manufacture and assembly, and is incorporated herein by reference. Interconnection circuit  33  is shown built up on copper substrate  32 , and includes a special assembly layer  40  in which the wells  37  are formed in a dielectric material  41 . The walls  42  of the wells are coated with titanium/copper to provide a solder-wetting surface having good adhesion to dielectric material  41 . Dielectric material  41  is preferably benzo-cyclo-butene (BCB). Flip chip connector  39  is shown connecting between an input/output pad  36  on IC chip  34  and a copper trace  43  in interconnection circuit  33 . The minimum pitch  44  of flip chip connectors  39  is preferably around 80 μm. 
     FIG. 4  shows a complete layout for electronic assembly  13 A of  FIG. 3A . Assembly  13 A is built on copper foil substrate  32  of  FIG. 3A . For manufacturing convenience, substrate  32  may have the same shape and thickness as a silicon wafer. In this case the preferred wafer diameter is 150 mm; however, the wafer can be of any size. Interconnection circuit  33  of  FIG. 3  has been fabricated on copper substrate  32 , with clear areas surrounding alignment targets  50 . The set of chips in assembly  13 A is a chipset that implements a 4-way server in this example, including 4 processor chips  51 , arrays of memory chips  52 , a test chip  53 , integrated passives  54 , and power distribution devices  55 . An area  56  is shown for attaching cable  16  of  FIG. 1 . Assembly  13 A is tested and any defective chips are replaced. Test chip  53  and cable  16  are used during testing, employing methods described in co-pending application Ser. No. 10/448,611 incorporated herein by reference. A back grinding and lapping procedure is preferably employed to reduce the thickness of all of the chips to approximately 100 microns. Copper substrate  32  may also be thinned using a grinding/lapping procedure, and the wings  57  of assembly  13 A are folded at fold lines  58  to form electronic assembly  13  of  FIG. 1 . More details about the folding and the associated system-in-package are described in co-pending application Ser. No. 10/783,163, incorporated herein by reference. 
     FIG. 5  illustrates in cross-section the situation just prior to bonding cable  16  of  FIG. 1  to foldable circuit board  31  of  FIG. 3A  using flip chip connectors  39  described in reference to  FIG. 3B . The attachment procedure is similar to that used for attaching an IC chip like  34  of  FIG. 3A . A flip chip bonding machine employing split beam optics is used, having an alignment accuracy of approximately ±1 micron. Copper spring bumps  35  are shown aligned to wells  37  containing solder paste  60  that has been deposited in the wells using a squeegee. Copper substrate  32  of foldable circuit board  31  and copper substrate  61  of cable  16  are preferably connected to ground (GND). For high-speed signals, offset coplanar striplines are implemented in interconnection circuit  62  of cable  16 , providing a preferred characteristic impedance of 50 ohms. The preferred pitch between flip chip connectors is around 80 microns, as discussed in reference to  FIG. 3B . Having a preferred height of approximately 100 μm, the flip chip connectors have an inductance of approximately 0.1 nH, supporting signaling at multi-gigahertz rates between the Micro Blade and external devices. 
     FIG. 6A  shows hermetic cable  65  including cable  16  of  FIG. 1 . Cable  16  has arrays  66   a  and  66   b  of copper spring elements as described in  FIG. 3B . Array  66   a  connects to the electronic assembly or SIP inside Micro Blade  10  of  FIG. 1 , and array  66   b  typically connects to a back plane, to be further described. A copper sheath  67  encloses a center portion of cable  16  as shown, and is crimped at the edges  68 .  FIG. 6B  illustrates section BB of  FIG. 6A . Copper spring elements  35  of array  66   a  are shown. Copper sheath  67  is preferably fabricated from sheets of copper foil approximately 600 microns thick.  FIG. 6C  illustrates section CC of  FIG. 6A , and shows that the seam in copper sheath  67  is hermetically sealed, preferably using solder  69 . The center hermetic portion of hermetic cable  65  can be used to traverse a damp or steam-laden path of cable  16 , as will be further described; each end will preferably be dry, where the flip chip connections are made. 
     FIG. 7  shows a water-cooled electronic system  80  of the current invention in cross-section. System  80  includes a tank  81  having a lid  82 . Tank  81  is filled with water  83  to a controlled level  84 . An array of Micro Blade elements  10  is inserted in tank  80  as shown. Although Micro Blade elements have a preferred form factor, any hermetic sub-assembly can be similarly immersed for cooling. Partial immersion is preferred as shown, wherein a hermetic jacket protects on all sides against water intrusion. A water seal  85  is provided for tank  81  as shown, preferably consisting of potting material. Back plane printed circuit board  86  is shown in a dry environment; this board may be a conventional laminate board constructed from glass fibers and epoxy, or it may be constructed on a copper substrate as shown in  FIG. 3A ; in either case it preferably includes wells filled with solder for accepting copper spring elements  66   b  of  FIG. 6A  at the end of cable  16 . Circuit board  86  may also be a motherboard for integrating electronic activity among all of the Micro Blades or sub-assemblies. Circuit board  86  preferably has slots  87  through which the Micro Blade cables pass. Section  88  of cable  16  of  FIG. 1  passes through the potting material of water seal  85 . Such potting materials are not totally impervious to moisture, and this is why the seal is only “semi-hermetic”. Residual moisture will likely cause a reliability problem with copper conductors on a cable passing through such a potting material; eventually the metallic conductors will corrode. This is the motivation for creating a cable  16  having a center section that is fully hermetic, as described in reference to  FIG. 6A . The center hermetic section of the cable protects the cable conductors from residual moisture, either in potting material  15  of  FIG. 1  or material  85  of  FIG. 7 . Connections  89  between Micro Blade cables  16  of  FIG. 1  and back plane  86  are preferably constructed as flip chip connections, as described in reference to  FIG. 5 . Other types of flip chip connectors may also be used. For example, solder bumps may replace copper spring elements at the ends of cable  16 ; they may connect to wells filled with solder, or to corresponding lands on circuit board  86 . A further alternative is to provide pin-and-socket connectors at this end of the cable attachment. 
   For corrosion protection, it may be desirable to plate the outer surfaces of the hermetic jackets  11  of  FIG. 1  with a thin layer of nickel followed by a thin layer of gold, as is known in the art. 
   The water  83  in tank  80  is preferably circulated through a cooling system (not shown), as is known in the art. Water entry ports  90  and exit ports  91  are shown. Since water has a specific heat of 4.186 Joules per gram per degree Centigrade, a flow rate of 20 liters per minute will provide over 62 kilowatts of cooling if the water temperature rises by 45° C. The Micro Blades are thermally well coupled to the coolant, since the water is circulating in contact with the jacket surfaces. A typical desired maximum junction temperature for the electronic circuits contained inside of a Micro Blade is 85° C. and a typical temperature for the chilled water is 15° C. A thermal resistance θ JC  from junction to case of less than 0.05° C./W is achievable for a Micro Blade of the current invention, as well as heat dissipation exceeding 5 watts per square millimeter of the Micro Blade jacket. 
   Maintenance of Micro Blades  10  in water-cooled system  80  is difficult; the Micro Blades are semi-permanently attached using potting material  85 . Accordingly, a preferred maintenance philosophy includes monitoring the health of the Micro Blades and adding isolation circuits to each; defective ones are switched out of operation without adversely affecting the remaining good units. In a data center for example, stacks of water-cooled systems  80  may be provided. Their total compute and switching power will depend on the total number of Micro Blades in service. This is preferably managed by adding or subtracting systems  80  to meet the peak demand over the long term.

Technology Classification (CPC): 7