Abstract:
A multi-chip module (MCM) structure comprises more than one semiconductor chip lying in a horizontal plane, the MCM having individual chip contact patches on the chips and a flexible heat sink having lateral compliance and extending in a plane in the MCM and secured in a heat exchange relation to the chips through the contact patches. The MCM has a mismatch between the coefficient of thermal expansion of the heat sink and the MCM and also has chip tilt and chip height mismatches. The flexible heat sink with lateral compliance minimizes or eliminates shear stress and shear strain developed in the horizontal direction at the interface between the heat sink and the chip contact patches by allowing for horizontal expansion and contraction of the heat sink relative to the MCM without moving the individual chip contact patches in a horizontal direction.

Description:
FIELD OF THE INVENTION 
       [0001]    The filed of the invention in one aspect comprises a flexible heat sink with lateral compliance 
       BACKGROUND OF THE INVENTION 
       [0002]    The so-called “silicon revolution” brought about the development of faster and larger computers beginning in the early 1960&#39;s with predictions of rapid growth because of the increasing numbers of transistors packed into integrated circuits with estimates they would double every two years. Since 1975, however, they doubled about every 18 months. 
         [0003]    An active period of innovation in the 1970&#39;s followed in the areas of circuit design, chip architecture, design aids, processes, tools, testing, manufacturing architecture, and manufacturing discipline. The combination of these disciplines brought about the VLSI era and the ability to mass-produce chips with 100,000 transistors per chip at the end of the 1980&#39;s, succeeding the large scale Integration (“LSI”) era of the 1970&#39;s with only 1,000 transistors per chip. (Carre, H. et al. “Semiconductor Manufacturing Technology at IBM”, IBM J. RES. DEVELOP., VOL. 26, no. 5, September 1982). Mescia et al. also describe the industrial scale manufacture of these VLSI devices. (Mescia, N.C. et al. “Plant Automation in a Structured Distributed System Environment,” IBM J. RES. DEVELOP., VOL. 26, no. 4, July 1982). 
         [0004]    The release of IBM&#39;s Power6™ chip in 2007, noted “miniaturization has allowed chipmakers to make chips faster by cramming more transistors on a single slice of silicon, to the point where high-end processors have hundreds of millions of transistors. But the process also tends to make chips run hotter, and engineers have been trying to figure out how to keep shrinking chips down while avoiding them frying their own circuitry.” (http://www.nytimes.com/reuters/technology/tech-ibm-power.html?pagewanted=print (Jul. 7, 2006)) 
         [0005]    Technology scaling of semiconductor devices to 90 nm and below has provided many benefits in the field of microelectronics, but has introduced new considerations as well. While smaller chip geometries result in higher levels of on-chip integration and performance, higher current and power densities, increased leakage currents, and low-k dielectrics with poorer heat conductivity occur that present new challenges to package and heat dissipation designs. 
         [0006]    Thus CMOS power density is increasing. Recently the industry has seen it rise from100 W/sq cm to 200 W/sq cm, beyond that of bipolar technology in the early 1990&#39;s. This increase in power density also increases the operating temperature of the device. 
         [0007]    Compliant heat sinks which cover more than one semiconductor chip on a multi-chip module (MCM) have often been proposed as a cooling solution for such modules. As there is generally a mismatch between the coefficient of thermal expansion (CTE) of the heat sink and the MCM, shear stress and/or strain can develop in the interface between the heat sink and the chips, as the “compliant” heat sinks generally are only significantly compliant in the vertical (perpendicular to the chip faces) direction to accommodate chip tilt and/or height mismatch. This stress and/or strain is typically absorbed by a low-modulus thermal interface material (TIM) used to connect the heat sink to the chips on the MCM. 
         [0008]    These “compliant” heat sinks generally are not significantly compliant in the horizontal (parallel to the chip faces) direction and as a result do not adequately address shear stress and/or strain developed horizontally in the interface between the heat sink and the chips. 
         [0009]    When a rigid or semi-rigid TIM such as a stiff gel, Indium, or solder is used as a heat sink, it can fail under thermal cycling due to this stress/strain. Thermal greases employed to connect chip components to conventional heat sinks heat sinks do not provide a complete solution to the problem since their heat conductivity is at best about 4.5 W/m K. These greases function by loading thermally conducting particles such as copper, silver, carbon nanotubes or other materials into the grease to lower its inherent thermal resistance, however, a common problem of this approach lies in effecting thermal conduction between the particles via proximity of the particles to one another. The particles are neither chemically nor metallurgically bonded to one another therefore high thermal resistance is invariably present in these interface materials. The TIM material typically would sit underneath “contact patches,” in the device being cooled, helping to thermally bridge the gap between the device and the “contact patches” Movement of the two surfaces interfacing with the grease therefore can “pump” the grease out of the interface, resulting in failure of the interface. 
       RELATED ART 
       [0010]    Shirikami et al., U.S. Pat. No. 7,663,883, show a heat transfer mechanism for dissipating heat from a heat generating body to a heat dissipating part, but do not show elements for relieving lateral stress between separate heat removal areas. 
         [0011]    Thompson et al., U.S. Pat. No. 7,646,608, illustrate an apparatus for coupling a heat-generating device to a heat-removing device, but does not show elements for relieving lateral stress between separate heat removal areas. 
         [0012]    Han et al., U.S. Pat. No. 7,443,678, show a circuit board with heat dissipation ability, but do not show elements for relieving lateral stress between separate heat removal areas. 
         [0013]    Farrow et al., U.S. Pat. No. 7,200,006, illustrate a metallic sheet to bridge a gap between a top surface of a processor package and a bottom surface of a heat sink. The sheet is positioned between the processor package and heat sink before securing the heat sink to the processor package, but do not show elements for relieving lateral stress between separate heat removal areas. 
         [0014]    Gelormeet al., U.S. Pat. No. 7,063,127, show thermal interface enclosure apparatus adapted to be placed between an IC chip and a heat sink to enhance heat transfer from the chip to the heat sink, but do not show areas or elements for relieving lateral stress between separate heat removal areas. 
         [0015]    Sen et al., U.S. Pat. No. 7,007,741, illustrate a heat spreader apparatus for cooling an electronic component but do not show elements for relieving lateral stress between separate heat removal areas. 
         [0016]    Herbert, U.S. Pat. No. 6,549,411, shows a flexible heat sink and/or object from which heat is to be removed that may have significant flexibility, but does not show elements for relieving lateral stress between separate heat removal areas. 
         [0017]    McCullough, U.S. Pat. No. 6,367,541, illustrates a heat sink assembly for removing heat from heat generating components, but does elements for relieving lateral stress between separate heat removal areas. 
         [0018]    Kelley, U.S. Pat. No. 6,131,646, shows a heat conductive interface, a heat generating device and a support platform, but does not illustrate elements for relieving lateral stress between separate heat removal areas. 
         [0019]    Harris et al., U.S. Pat. No. 5,794,454, illustrate a mechanism for cooling high-powered electronic circuit chip modules which cannot otherwise be cooled by conventional means because of their non-coplanarity and their inaccessibility, but do not elements for relieving lateral stress between separate heat removal areas. 
         [0020]    Chu et al., U.S. Pat. No. 5,168,348, show a heat sink arranged to extract heat from an array of computer chips in an electric module, but do not illustrate elements for relieving lateral stress between separate heat removal areas. 
         [0021]    Flint et al., U.S. Pat. No. 4,964,458, show a heat exchanger for cooling an array of electric circuit chips disposed on a common substrate do not, but do not address or a thermal interface material with limited ability to absorb shear forces. 
       SUMMARY OF THE INVENTION 
       [0022]    The present invention provides structures, articles of manufacture and processes that address these needs to not only provide advantages over the related art, but also to substantially obviate one or more of the foregoing and other limitations and disadvantages of the related art by providing a flexible heat sink with lateral compliance. Incorporating lateral compliance into a vertically flexible/compliant heat sink allows CTE driven strain to be absorbed by the heat sink rather than the TIM for stiff TIM materials. 
         [0023]    Not only do the written description, claims, and abstract of the disclosure set forth various features, objectives, and advantages of the invention and how they may be realized and obtained, but these features, objectives, and advantages will also become apparent by practicing the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The accompanying drawings are not necessarily drawn to scale but nonetheless set out the invention, and are included to illustrate various embodiments of the invention, and together with this specification also serve to explain the principles of the invention. These drawings comprise various Figures that illustrate, inter alia, flexible heat sink structures having lateral compliance 
           [0025]      FIG. 1  is a side elevation in cross section illustrating a prior art structure for cooling an MCM and consists of a thin bottom metal sheet, a thin top metal sheet and heat transfer elements (HTM) separating them. The metal sheets in this structure do not have lateral compliance features. 
           [0026]      FIG. 2  Illustrates the present invention comprising a side elevation in cross-section of a structure for cooling an MCM and comprises a thin bottom thermally conductive sheet, a thin top thermally conductive sheet and heat transfer elements (HTM) separating them. The thermally conductive sheets in this structure include lateral compliance features comprising bends in the thermally conductive sheets. 
           [0027]      FIG. 3  Illustrates the present invention comprising a side elevation in cross-section of a structure for cooling an MCM and comprises separate thin thermally conductive sheets at the bottom of the structure, and separate thin thermally conductive sheets at the top of the structure and heat transfer elements (HTM) separating the thin thermally conductive sheets. The thermally conductive sheets in this structure include lateral compliance features comprising an elastomer that joins them along their horizontal edges. 
           [0028]      FIG. 4  Comprises a plan view of a cooling structure with the top cover removed illustrating the present invention comprising a structure for cooling an MCM where the structure comprises lateral compliance features between MCM components. 
           [0029]      FIG. 5  is a side elevation in cross section of a structure for cooling an MCM consisting of a prior art rigid cold plate assembly associated with a cooling/chip structure as illustrated in  FIG. 1 . 
           [0030]      FIG. 6  Illustrates the present invention comprising a side elevation in cross-section of a structure for cooling an MCM and comprises a flexible, direct attach cold plate assembly associated with a cooling/chip structure as illustrated in  FIG. 2 . 
           [0031]      FIG. 7  Illustrates the present invention comprising an isometric projection of a structure for cooling an MCM with the top cover removed and comprises a bottom flexible plate with lateral compliance features and a number of heat transfer elements associated with the chips on the MCM. 
           [0032]      FIG. 8  Illustrates the present invention and comprises a plan view of a cooling apparatus with the top cover removed where the apparatus comprises a flexible cold plate design for cooling an MCM and incorporates parallel coolant feeds to a flexible cold plate structure where the structure includes lateral compliance features between MCM components. 
           [0033]      FIG. 9  Illustrates the present invention and comprises a plan view of a cooling apparatus with the top cover removed where the apparatus comprises a flexible cold plate design for cooling an MCM and incorporates seria/parallel coolant feeds to obtain higher velocity coolant flow to a flexible cold plate structure where the structure includes lateral compliance features between MCM components. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    To achieve the foregoing and other advantages, and in accordance with the purpose of this invention as embodied and broadly described herein, the following detailed description comprises disclosed examples of the invention that can be embodied in various forms. 
         [0035]    The specific processes, compounds, compositions, and structural details set out herein not only comprise a basis for the claims and a basis for teaching one skilled in the art to employ the present invention in any novel and useful way, but also provide a description of how to make and use this invention. The written description, claims, abstract of the disclosure, and the drawings that follow set forth various features, objectives, and advantages of the invention and how they may be realized and obtained. These features, objectives, and advantages will also become apparent by practicing the invention. 
         [0036]    The invention comprises a flexible heat sink extending in a plane in an MCM where the heat sink comprises a thermally conductive sheet having lateral compliance. 
         [0037]    The heat sink is made from a thin thermally conductive sheet, e.g., a metal sheet from about 100 micrometers to about 350 micrometers thick. Flexible sheets thinner or thicker than this are possible, but this would be a typical range. Although we describe the sheet as being made from a metal such as Cu, Al, Au, Ag, combinations thereof and alloys thereof, and the like, for the purpose of this invention, “thermally conductive” is also intended to include graphene or carbon nanotubes, both of which are commercially available in sheet form. Other materials for MCM cooling in this regard comprise any other thermally conductive mechanically flexible material. 
         [0038]    A structure provided according to the invention comprises a heat sink having a thin top sheet which may or may not be thermally conductive and a thin bottom thermally conductive sheet with heat transfer elements separating them to form a gap between the two. Cooling fluids, such as gases or liquids known in the art can be circulated or passed through this gap. Thin top and bottom sheets provide flexibility to accommodate tilted chips and level differences between chips. Adding lateral compliance features to at least the bottom (chipside) sheet allows for lateral expansion and contraction of the heat sink relative to the substrate without substantially moving or substantially stressing the individual chip contact patches. In one instance the lateral compliance feature comprise a bend in the thermally conductive sheet; in another it comprises an elastomer joining the horizontal edges of two or more of these sheets. 
         [0039]    Flexible cold plates with lateral compliance features can absorb CTE -driven strain and stress in the cold plate rather than in the TIM. 
         [0040]    Lateral compliance features can also improve the flexibility of the flexible cold plate by disconnecting the linked parallel plate structure present in “standard” flexible cold plate designs. With linked parallel plate structures as shown in  FIG. 1 , neighboring heat transfer elements  60  can be constrained to be essentially parallel to each other even if the elements end up at different vertical locations. Lateral compliance features in both top and bottom sheets remove this constraint. 
         [0041]    The invention will reduce the cost of an MCM by eliminating the second heat spreader seen in some prior art cooling solutions such as cold plate  590  in  FIG. 5 , and expand material selections for MCM cooling. 
         [0042]    By adding lateral compliance features to the flexible heat sink we allow movement of the heat sink surface attached or semi-attached to one chip site relative to the surface attached to another chip site. This allows “direct attach” using a TIM such as indium without requiring the indium to absorb chip-to-chip driven strains and stresses. A TIM material would sit between the chip contact patch of the heat sink and the heat sink itself as in  FIG. 6 . TIMs in addition to indium can be used such as silicone gels, thermal pastes, other solders (e.g. Pb/Sn), thermal epoxies, thermal pads, graphite sheets or pads. “Semi-attached” as distinguished from “direct attach.” refers to softer TIM&#39;s like silicone gels which allow for some movement of the interface without necessarily degrading the interface 
         [0043]    Referring to the drawings,  FIG. 1  is an illustration of a prior art heat sink without lateral compliance consisting of a substrate  10  of a MCM device with semiconductor chips  20  terminating in chip contact patches  30 . A thin metal heat sink sheet  40  extends in a plane over the chips  20  and is secured to chips  20  through chip contact patches  30 . typically utilizing a thermal interface material (TIM, [not shown) Heat transfer elements (HTE)  60  such as fins, pins or other heat transfer structures are mounted on heat sink sheet  40  and support a thin metal heat sink sheet  50 . Heat sink material  40 , HTE  60 , and heat sink material  50  define channels  80  through which a cooling fluid or heat exchange fluid flows to carry away any heat developed in the MCM. Typical fluids would include water and air. 
         [0044]      FIG. 2  comprises an illustration of a heat sink device of the invention with lateral compliance, and comprises a substrate  210  of a MCM device with semiconductor chips  220  terminating in chip contact patches  230 . A thin thermally conductive heat sink sheet  240  extends in a plane over the chips  220  and is secured to chips  220  through chip contact patches  230  utilizing a TIM (not shown). Heat transfer elements (HTE)  260  such as fins, pins or other heat transfer structures are mounted on heat sink sheet  240  and support a thin thermally conductive heat sink sheet  250  that also extends in a plane above the MOM device. Heat sink sheet  240 , HTE  260 , and heat sink sheet  250  define channels  280  through which a cooling fluid or heat exchange fluid flows to carry away any heat developed in the MCM. 
         [0045]    As noted before, thin top and bottom heat sink sheets  250  and  240  provide flexibility to accommodate tilted chips and level differences between chips. Adding lateral compliance features  270 , i.e., bends to at least the bottom (chipside) sheet  240 , and optionally sheet  250 , allows for lateral expansion and contraction of the heat sink relative to the substrate without moving or significantly stressing the individual chip contact patches  230 . 
         [0046]      FIG. 3  comprises an illustration of a heat sink device of the invention with lateral compliance, and comprises a substrate  310  of a MCM device with semiconductor chips  320  terminating in chip contact patches  330 . A thin thermally conductive heat sink sheet  340  extends in a plane over the chips  320  and is secured to chips  320  through chip contact patches  330  utilizing a TIM (not shown) Heat transfer elements (HTE)  360  such as fins, pins or other heat transfer structures are mounted on heat sink sheet  340  and support a thin thermally conductive sheet heat sink sheet  350  that also extends in a plane over the MCM device. Heat sink sheet  340 , HTE  360 , and heat sink sheet  350  define channels  380  through which a cooling fluid or heat exchange fluid flows to carry away any heat developed in the MCM. 
         [0047]    In one embodiment, the chips of the MCM structure, and the heat sink sheets extend in planes substantially parallel to one another. 
         [0048]    Thin top and bottom heat sink sheets  350  and  340  provide flexibility to accommodate tilted chips and level differences between chips. Adding lateral compliance features  370 , i.e., an elastomer to secure the facing, substantially parallel edges of adjoining separate heat sink sheets allows for lateral expansion and contraction of the heat sink relative to the substrate without moving or significantly stressing the individual chip contact patches. 
         [0049]    Hawlev&#39;s Condensed Chemical Dictionary, 12 th  Edition 1993, Hackh&#39;s Chemical Dictionary, Third Edition 1953, and Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, define “elastomers” as we use the term in this specification. 
         [0050]      FIG. 4  comprises an illustration of a heat sink device of the invention with lateral compliance, and comprises a thin thermally conductive sheet heat sink sheet  440  that extends in a plane over chips in the device. Thin heat sink sheet  440  provides flexibility to accommodate tilted chips and level differences between chips. Lateral compliance features  470 , i.e., bends in sheet  440  or an elastomer to secure the parallel edges of adjoining separate heat sink sheets as we describe in  FIGS. 2 and 3 , allows for lateral expansion and contraction of the heat sink relative to the substrate without substantially moving or substantially stressing the individual chip contact patches. These lateral compliance features will also serve to reduce the horizontal stiffness of the flexible heat sink. 
         [0051]      FIG. 5  is an illustration of a prior art rigid cold plate assembly without lateral compliance consisting of a substrate  510  of a MCM device with semiconductor chips  520 . A cold plate  590  connects to chip  520  in a heat exchange relation by means of heat exchange pistons  560  and TIM  570  and TIM  530 . 
         [0052]    By contrast, the present invention provides a flexible cold plate directly attached to a chip assembly as illustrated in  FIG. 6  which illustrates a substrate  610  for a MCM assembly comprising chips  620  mounted on substrate  610  and secured by means of TIM  630  to a flexible heat sink  690  with lateral compliance which we also describe as a flexible cold plate. Heat sink  690  is substantially the same as the flexible heat sink we illustrate in  FIG. 2  or  FIG. 3  of this invention. 
         [0053]      FIG. 7  illustrates another aspect of the invention and comprises rigid cold plate base  710 , coolant carrying tube  720 , coolant Inlet  730 , and Coolant outlet  740 . 
         [0054]      FIG. 8  illustrates a baffle arrangement to provide parallel flow of coolant through a flexible cold plate of the invention and comprises a cold plate rim structure  810 , top and bottom sheets  820 , lateral compliance features  870  as illustrated in  FIGS. 2  and/or  3 , flow blockers  880  such as a plurality of fins substantially normal to the surface of at least one of the sheets  820 , an inlet port  885 , heat transfer elements (fins)  890 , and outlet port  895 . 
         [0055]      FIG. 9  illustrates a flexible cold plate design of the invention baffled to provide serial/parallel flow of a coolant through the flexible heat sink and comprises, a cold plate rim structure  910 , top and bottom sheets  920 , lateral compliance features  970  as illustrated in  FIGS. 2  and/or  3 , flow blockers  980  such as a plurality of fins substantially normal to at least one of the surface of sheets  920 , inlet port  985 , heat transfer elements (fins)  990 , and outlet port  995 . 
         [0056]    Throughout this specification, and abstract of the disclosure, the inventors have set out equivalents, of various materials as well as combinations of elements, materials, compounds, compositions, conditions, processes, structures and the like, and even though set out individually, also include combinations of these equivalents such as the two component, three component, or four component combinations, or more as well as combinations of such equivalent elements, materials, compositions conditions, processes, structures and the like in any ratios or in any manner. 
         [0057]    Additionally, the various numerical ranges describing the invention as set forth throughout the specification also includes any combination of the lower ends of the ranges with the higher ends of the ranges, and any single numerical value, or any single numerical value that will reduce the scope of the lower limits of the range or the scope of the higher limits of the range, and also includes ranges falling within any of these ranges. 
         [0058]    The terms “about,” “substantial,” or “substantially” as applied to any claim or any parameters herein, such as a numerical value, including values used to describe numerical ranges, means slight variations in the parameter or the meaning ordinarily ascribed to these terms by a person with ordinary skill in the art. In another embodiment, the terms “about,” “substantial,” or “substantially,” when employed to define numerical parameter include, e.g., a variation up to five per-cent, ten per-cent, or 15 percent, or somewhat higher. 
         [0059]    All scientific journal articles and other articles, including internet sites, as well as issued and pending patents that this written description or applicants&#39; Invention Disclosure Statements mention, including the references cited in such scientific journal articles and other articles, including internet sites, and such patents, are incorporated herein by reference in their entirety and for the purpose cited in this written description and for all other disclosures contained in such scientific journal articles and other articles, including internet sites as well as patents and the references cited therein, as all or any one may bear on or apply in whole or in part, not only to the foregoing written description, but also the following claims, and abstract of the disclosure. 
         [0060]    Although we describe the invention by reference to some embodiments, other embodiments defined by the doctrine of equivalents are intended to be included as falling within the broad scope and spirit of the foregoing written description, and the following claims, abstract of the disclosure, and drawings.