Abstract:
Embodiments of the invention comprise a homogeneous heat spreading cap element in chip packages to facilitate better heat spreading and dissipation. The heat spreading cap comprises a single high-K graphite layer supported by a copper frame for increased stability and reduced thermal warpage during handling and operation while minimizing thermal penalty by reducing the amount of material having a relatively low heat conductivity that is needed in conventional heat spreading caps.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of controlling thermal conduction in computer chip packaging, and more specifically to a heat spreading cap structure having high thermal conductivity. 
       BACKGROUND 
       [0002]    Innovations in semiconductor fabrication and packaging technologies have enabled development of high performance, densely integrated semiconductor chip modules. The downscaling of chip geometries and the increase in operating speeds lead to increased power densities, resulting in more heat generation per unit area. The increased power density poses practical limitations to the level of integration density and performance that may be achieved. The ability to implement chip modules with higher densities and higher performance is limited primarily by the ability to effectively cool the chip modules during normal operation. For instance, as heat is generated by integrated circuit (“IC”) chips during normal operation, cooling structures must be employed to provide sufficiently low thermal resistance paths between the chips and ambient air or a circulating liquid coolant to adequately remove heat and maintain the operating temperature of the chips low enough to assure continued reliable operation. 
         [0003]    In conventional packaging technologies, chip level packages can be constructed with one or more chips mounted on a thin flexible first level package substrate, such as an organic laminate build up package substrate, using micro solder bump connections, referred to as C4&#39;s (controlled collapse chip connection). The packages are often bonded to a cap, which provides structural stability to the package by reducing warpage, and by spreading heat along a plane, thereby improving the package thermal performance. The cap, in turn, is attached to a secondary cooling structure, such as a heat sink or a liquid cooling system. 
         [0004]    Referring now to  FIG. 1 , a flip chip ball grid array (BGA) package  100 , according to the prior art, comprises a chip die  112  having an operational surface mounted onto a substrate  106  via a series of C4&#39;s  108  encased in an underfill layer  110 . These components are operatively connected to a circuit board  102  via a separate series of solder balls  104 . The die  112  is said to be flipped because its operational connections face down towards the substrate  106  and the circuit board  102 , and its other side is connected to a heat sink  140  via a protective heat spreading cap  114 . Several layers of the package  100  are connected using layers of material, including, for example, the thermal interface material (TIM1)  132  connecting the die  112  to the cap  114 , the adhesive layer  130  connecting the cap  114  to the substrate  106 , and the thermal interface material (TIM2)  134  connecting the cap  114  the heat sink  140 . 
         [0005]    In conventional implementations, the heat spreading cap  114  may be made from copper, having a thermal conductivity of 400 W/m-° C., at 300° K. Although copper has relatively high structural stability and extends that stability to the package  100 , it is not the most thermally conductive material available. Some forms of graphite, for example, have much higher thermal conductivity across at least one spatial plane. Other forms of graphite exhibit thermal conductivity above 1200 W/m-° C. These materials generally are referred to as high-k materials, or high-k graphite. 
         [0006]    According to an aspect of the prior art (for example, as disclosed by U.S. Pat. No. 6,758,263 B2 entitled “HEAT DISSIPATING COMPONENT USING HIGH CONDUCTIVE INSERTS”), a heterogeneous planar graphite element includes a high-conductivity graphite layer having a cavity for housing an insert. The graphite layer exhibits high thermal conductivity in the x and y planes, but low thermal conductivity in the z plane. The insert layer has relatively higher thermal conductivity across the z plane, but not across the x or y planes. 
         [0007]    Traditional designs and methods in the prior art face significant challenges, particularly because known structures using high-k material exhibit weak structural stability, and experience warpage under normal operating temperatures. This warpage leads to device defects and even to device failure. In fact, the relative instability of these structures also makes more prone to damage during manufacturing processes. Furthermore, prior art solutions using graphite employ one or more heterogeneous layers, adding to device complexity but taking advantage of graphite&#39;s thermal conductivity in a limited way. 
         [0008]    Therefore, it is desirable to manage heat spreading and dissipation in semiconductor packaging technology by taking advantage of the relatively high thermal conductivity of available material, such as graphite, while at the same time maintaining structural integrity of the package and its constituent components. 
       SUMMARY 
       [0009]    Embodiments of the invention include a cooling system for a semiconductor package comprising a heat spreading layer partially encased in a supporting frame along an outer perimeter, the supporting frame encasing the perimeter and an adjacent portion of the heat spreading layer to define centrally exposed top and bottom portions of the heat spreading layer; and a heat generating element thermally connected to the centrally exposed bottom portion of the heat spreading layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Elements of the figures are not necessarily to scale and are not intended to portray specific parameters of the invention. For clarity and ease of illustration, scale of elements may be exaggerated. The detailed description should be consulted for accurate dimensions. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements. 
           [0011]      FIG. 1  is a cross sectional front elevational view of a flip chip ball grid array package assembly, according to the prior art; 
           [0012]      FIG. 2  is a partial cross sectional front elevational view of a flip chip package assembly, having a heat spreading layer encased in a supporting frame, wherein the supporting frame extends over a portion of the heat spreading layer, according to an embodiment of the disclosed invention; 
           [0013]      FIG. 3  is a cross sectional front elevational view of the flip chip package assembly depicted in  FIG. 2 , and further depicts a heat sink, according to an embodiment of the disclosed invention; 
           [0014]      FIG. 4  is a cross sectional front elevational view of the flip chip package assembly depicted in  FIG. 2 , wherein the supporting frame comprises a plurality of interconnected pieces, according to an embodiment of the disclosed invention; 
           [0015]      FIG. 5  is a cross sectional front elevational view of the flip chip package assembly depicted in  FIG. 2 , wherein the supporting frame comprises a plurality of interconnected pieces, according to an embodiment of the disclosed invention; 
           [0016]      FIG. 6  is a partial cross sectional front elevational view of a flip chip package assembly, having a heat spreading layer encased in a supporting frame, wherein the supporting frame extends over a portion of the heat spreading layer, according to an embodiment of the disclosed invention; 
           [0017]      FIG. 7  is a partial cross sectional front elevational view of a flip chip package assembly, having a heat spreading layer encased in a supporting frame, wherein the edges of a top surface of the heat spreading layer are chamfered, and a top surface of the supporting frame is flush with a top surface of the heat spreading layer, according to an embodiment of the disclosed invention; 
           [0018]      FIG. 8  is a partial cross sectional front elevational view of a flip chip package assembly, having a heat spreading layer encased in a supporting frame, wherein the edges of a top and a bottom surface of the heat spreading layer are chamfered, and a top surface of the supporting frame is flush with a top surface of the heat spreading layer, according to an embodiment of the disclosed invention; 
           [0019]      FIG. 9  is a partial cross sectional front elevational view of a flip chip package assembly, having a heat spreading layer encased in a supporting frame, wherein the edges of a top and bottom surface of the heat spreading layer are chamfered, and a top surface of the supporting frame is flush with a top surface of the heat spreading layer, according to an embodiment of the disclosed invention; 
           [0020]      FIG. 10  is a partial cross sectional front elevational view of a flip chip package assembly, having a heat spreading layer encased in a supporting frame, wherein the edges of a top surface of the heat spreading layer are modified to form a rabbet, and a central portion of a top surface of the supporting frame is flush with the top surface of the heat spreading layer, according to an embodiment of the disclosed invention; 
           [0021]      FIG. 11  is a partial cross sectional front elevational view of a wire bonding chip assembly package, having a heat spreading layer encased in a supporting frame, wherein the supporting frame extends over a portion of the heat spreading layer, and central portion of a bottom surface of the heat spreading layer is connected to a central portion of a chip, according to an embodiment of the disclosed invention; 
           [0022]      FIG. 12  is an aerial view of a multi-core processor having a series of heat spreading layers encased in a supporting frame, according to an embodiment of the disclosed invention; and 
           [0023]      FIG. 13  is a partial cross sectional front elevational view of the multi-core processor depicted in  FIG. 12 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Referring now to  FIG. 2 , a subset  200  of a flip chip package, according to an embodiment of the disclosed invention, comprises a chip die  112  mounted onto a substrate  106  via a series of C4&#39;s  108  encased in an underfill layer  110 . A top surface of the chip die  112  is connected to a lid comprising a homogeneous, thermally conductive heat spreading layer  114   a  housed within a supporting frame  114   b.  A bottom surface of the heat spreading layer  114   a  is connected to the top surface of the chip die  112  via a thermal interface material (“TIM1”)  132 . A bottom surface of the supporting frame  114   b  is connected to the top surface of the substrate  106  via an adhesive layer  130 . The heat spreading layer  114   a  and the interior cavity of the supporting frame  114   b  are also connected via an adhesive layer  136 . The purpose the adhesive layer  136  is, in part, to form a strong interaction between the heat spreading layer  114   a  and the supporting frame  114   b,  and to maintain the integrity of these two components, particularly in light of structural stresses, such as heat fluctuations, exerted upon the chip package during manufacturing and operation. One example of the material that can be used to form the layer  136  is epoxy based adhesive. 
         [0025]    Further referring to  FIG. 2 , the supporting frame  114   b  is formed as a single piece, as is the heat spreading layer  114   a.  It is not necessary to bond these two components via the adhesive layer  136 , particularly if the supporting frame  114   b  is molded onto and around the heat spreading layer  114   a  during the fabrication process. According to an aspect of the disclosed invention, the heat spreading layer  114   a  is made from a material having relatively high thermal conductivity, such as graphite, some forms of which exhibit conductivity above 1200 W/m-° C. 
         [0026]    The graphite heat spreading layer  114   a  is effective in spreading heat generated by the chip die  112  and its surrounding components across the z plane, and at least one of the x or y planes. By using a high-k material such as graphite, the heat spreading layer  114   a  provides greater heat spreading and dissipation compared to the prior art where a customary choice of chip cap/lid material is copper. In fact, the graphite heat spreading layer  114   a  has double the thermal conductivity of copper given an equal shape and thickness. Consequently, it is a suitable choice for forming the heat spreading layer  114   a.    
         [0027]    According to an aspect of the disclosed invention, the graphite heat spreading layer  114   a  may be formed using two smaller graphite pieces joined at two adjacent edges by an adhesive (not shown) to form a single planar graphite layer. Each pyrolytic graphite piece may be grown to a thickness of about 25 mm, where its high thermal conductivity is in the horizontal plane and its low thermal conductivity is in the vertical, or growth, direction. Thereafter, each piece is orientated such that the low thermal conductivity of the graphite is in the x direction. A 50×50 mm graphite heat spreader may be constructed by cutting out two pieces that are 25 mm tall, 4 mm thick, and 50 mm long where the thermal conductivity is high in the 4 mm and 50 mm directions. These pieces are glued together side-by-side (i.e. by joining their respective 4×50 mm faces) to produce the 50×50×4 mm single, planar layer heat spreader. Other embodiments of the invention may have additional pieces in the single planar layer and oriented and joined in a manner different from the described configuration, without departing from the scope or the spirit of the invention. 
         [0028]    Although graphite has desirable high thermal conductivity, it is generally susceptible to thermal warpage and cracking or breaking during device operation. Failure of the heat spreading layer  114   a  due to warpage in typical operational conditions may be virtually eliminated by encasing it in the supporting frame  114   b.  According to an embodiment of the invention, the supporting frame  114   b  is made from copper, and encompasses the side, and parts of the top and bottom surfaces of the layer  114   a,  leaving a central portion of both the top and bottom surfaces of the graphite exposed. By retaining an exposed portion of graphite, the heat spreading layer  114   a  remains twice as effective as conventional copper caps in thermal conductivity at the exposed area. The supporting frame  114   b  gives the heat spreading layer  114   a  sufficient rigidity at its outer surfaces and edges without creating a thermal penalty at its exposed regions. 
         [0029]    Other embodiments may employ other metals, such as stainless steel, or polymers that are sufficiently strong to provide structural stability to the highly conductive layer  114   a.    
         [0030]    Referring now to  FIG. 3 , a subset  300  of a flip chip package is similar to the elements recited above in connection with  FIG. 2 , wherein like elements have been similarly marked. Additionally, the subset  300  comprises a heat sink  140  having a bottom surface contoured to fit into the opening of the exposed area of the top surface of the heat spreading layer  114   a  and the top surface of the frame  114   b,  via a TIM2 layer  134 . Contouring the bottom surface of the heat sink  140  to the top surface of the lid is beneficial because it fills a gap that would otherwise form between the heat sink  140  and the top surface of the heat spreading layer  114   a  due to the asymmetrical shapes of the two surfaces. Without contouring the bottom surface of the heat sink  140 , the gap may be filled with a TIM2 layer, a solution that likely would create an increased thermal penalty. 
         [0031]    Referring now to  FIG. 4 , a further embodiment of the disclosed invention comprises a subset  400  of a flip chip package, as described and depicted in  FIG. 2 . Additionally, rather than using a single-piece supporting frame (the supporting frame  114   b  as shown in  FIG. 2 ), the supporting frame of the subset  400  comprises a top frame  114   c  and a foot frame  114   d,  joined at a junction point  114   e.  The two layers may be joined using an adhesive layer (not shown), or mechanically. Embodiments of the disclosed invention may comprise top frames  114   c  and foot frames  114   d  made from, for example, copper or stainless steel. 
         [0032]    Further referring to  FIG. 4 , the design of the two-piece supporting frame ( 114   c  and  114   d ) may be used where manufacturing considerations make it less desirable to form a single piece frame. For example, it may not be feasible to implement a molding mechanism that would form a single-piece frame around the layer  114   a  due to equipment cost, or the unavailability of a polymer ideal for manufacturing a molded frame. 
         [0033]    Referring now to  FIG. 5 , a subset  500  of a flip chip package, according to an embodiment of the disclosed invention, comprises the elements recited and described with respect to  FIG. 4 , except that the junction point  114   e  between the top frame  114   c  and the foot frame  114   d  is vertically aligned with the outer edges of the heat spreading layer  114   a,  and the foot frame  114   d  forms an “L” shape in a cross sectional view. 
         [0034]    Further referring to  FIGS. 4 and 5 , the position of the junction point  114   e  may be changed according to cost, ease of design, and other manufacturing considerations, without departing from the spirit and scope of the disclosed invention. 
         [0035]    Referring now to  FIG. 6 , a further embodiment of the disclosed invention comprises a subset  600  of a flip chip package, as described in connection with  FIG. 2 . In the disclosed embodiment, the supporting frame  114   b  is a single piece having a top portion that is substantially thinner than the side or foot portions of the structure. Moreover, the top portion of the supporting frame  114   b  extends onto the top surface of the heat spreading layer  114   a  at a fraction of the distance it covers on the bottom surface of the heat spreading layer  114   a.  Consequently, the disclosed embodiment allows for greater spreading and dissipation of heat generated by the chip die  112  by allowing a larger surface area of the heat spreading layer  114   a  to connect to a heat sink. 
         [0036]    Referring now to  FIG. 7 , a further embodiment of the disclosed invention comprises a subset  700  of a flip chip package, as recited and described with respect to  FIG. 2 . In the depicted embodiment, the top surface of the heat spreading layer  114   a  is chamfered such that the frame  114   b  extends over a portion of the layer  114   a.  The top surface of the supporting frame  114   b  is flush with a central portion of the top surface of the heat spreading layer  114   a,  and extends over its chamfered edges. The bottom surface of the heat spreading layer  114   a  is substantially flat. Chamfering the heat spreading layer  114   a  increases the surface area at which the supporting frame  114   b  contacts the heat spreading layer  114   a,  improving the supporting frame&#39;s  114   b  grip, without necessarily reducing the surface area of the heat layer  114   a  that remains exposed. Since the exposed portion of the heat spreading layer  114   a  contacts the heat sink  140 , it is desirable to maintain as much contact area as possible. Moreover, according to the disclosed embodiment, the heat sink  140  has a flat bottom surface, since the layer  114   a  is flush with the supporting frame  114   b.    
         [0037]    Referring now to  FIG. 8 , a further embodiment of the disclosed invention comprises a subset  800  of a flip chip package, as recited and described with respect to  FIG. 2 . In the depicted embodiment, the heat spreading layer  114   a  is chamfered along its top surface, and housed within the supporting frame  114   b,  as depicted in  FIG. 7 , and described in connection therewith. Additionally, the bottom surface of the heat spreading layer  114   a  is also chamfered along its outer edges, with the supporting frame  114   b  extending over the chamfered edges. In the depicted embodiment, the chamfered surface on the top surface of the heat spreading layer  114   a  is larger than the chamfered surface of the bottom surface. 
         [0038]    Referring now to  FIG. 9 , a further embodiment of the disclosed invention comprises a subset  900  of a flip chip package, as recited and described with respect to  FIG. 2 . Furthermore, in the depicted embodiment, the top and bottom surfaces of the heat spreading layer  114   a  are symmetrically chamfered and encased by the supporting frame  114   b.  The top surface of the supporting frame  114   b  is flush with a central portion of the top surface of the heat spreading layer  114   a.    
         [0039]    Referring now to  FIG. 10 , a further embodiment of the disclosed invention comprises a subset  1000  of a flip chip package, as recited and described with respect to  FIG. 2 . Furthermore, in the depicted embodiment, the top surface of the heat spreading layer  114   a  is processed to form rabbet edges, and encased in the supporting frame  114   b,  such that the supporting frame  114   b  extends over the rabbet edges. Moreover, the top surface of the supporting frame  114   b  is flush with a central portion of the top surface of the heat spreading layer  114   a.    
         [0040]    Referring now to  FIG. 11 , a further embodiment of the disclosed invention comprises a subset  1100  of a wire bonding chip assembly package similar to the flip chip assembly package depicted in  FIG. 6 , wherein like elements are similarly referenced. However, according to the depicted embodiment, the heat spreading layer  114   a  further comprises a protruding central portion on its lower surface, whereby the heat spreading layer  114   a  connects to a central portion of a top surface of the chip  112 . The chip  112  is operatively connected to the substrate layer  106  via a plurality of wire connections  142 , with the active side of the chip  112  facing the heat spreading layer  114   a.  According to the depicted embodiment, the heat spreading layer  114   a  is in contact with a central region of the top surface of the chip  112  so as to provide space for the wire connections  142  in a typical wire bonding package. The wire connections  142  may be encapsulated in a molding compound. 
         [0041]    Although the embodiments depicted in  FIGS. 2-11  reference an adhesive layer  136  connecting the heat spreading layer  114   a  to the supporting frame  114   b  (or to the top frame  114   c  and the foot frame  114   d  in embodiments having a two-piece frame), forming the adhesive layer  136  is not necessary to practice the invention. Whether the adhesive layer  136  is used and at what thickness may be varied without departing from the spirit or the scope of the disclosed invention. 
         [0042]    Furthermore, although the embodiments of the disclosed invention depicted in  FIGS. 2-10  are based primarily on a flip chip package, it will be understood by a person of ordinary skill in the art, in light of the present disclosure, that the features of the depicted embodiments (including, without limitation, the shape of the heat spreading layer  114   a  or the supporting frame  114   b ) can be incorporated into corresponding embodiments using a wire bonding assembly package, as described with respect to  FIG. 11 , without departing from the spirit or scope of the disclosed invention. 
         [0043]    In addition, although the embodiments of the disclosed invention depicted in  FIGS. 2-11  are discussed in reference to the package  100  using BGA interconnections between the substrate  106  and the circuit board  102 , as depicted in  FIG. 1 , these embodiments can be incorporated into a land grid array (LGA) package or a pin grid array (PGA) package. In contrast to a BGA package, wherein the substrate  106  is electrically connected to the circuit board  102  via a grid of solder ball connections  104 , LGA packages use surface contact areas on the underside of the substrate  106  to connect to a corresponding grid of contacts on the circuit board  102 . The connection may be implemented using a variety of methods such as LGA sockets, or solder paste. The substrate  106  surface contacts in LGA packages are relatively flat compared to the solder ball connections in BGA packages. In PGA packages, the surface contacts on the underside of the substrate  106  are not flat, but instead are pin shaped protrusions arranged in an array, and may connect to a socket mounted on the circuit board  102 . 
         [0044]    Furthermore, although most of the disclosed embodiments depict a single-piece frame, it will be understood by a person of ordinary skill in the art that the multi-pieced frames, such as those depicted in  FIGS. 4 and 5 , may be employed in the other depicted embodiments, and any other embodiments as claimed, without departing from the spirit or scope of the disclosed invention. 
         [0045]    Referring now to  FIG. 12 , a multichip module (“MCM”)  1200  comprises four chips  112  (not shown) mounted onto a substrate layer  1200  (via, for example, a flip chip package) and thermally connected to respective heat spreading layers  114   a,  wherein the heat spreading layers  114   a  are encased in a supporting frame  114   b.  As described with respect to other embodiments of the invention (and  FIGS. 4 and 5  in particular), above, the supporting frame may be formed as a single piece through, for example, a molding manufacturing process using a polymer; or it may be formed as multiple pieces using additional manufacturing methods, such as a high pressure metal press. According to one embodiment, the supporting frame  114   b  may comprise a top frame and a foot frame (not shown), similar to similarly referenced components depicted in  FIGS. 4 and 5 . 
         [0046]    Referring now to  FIG. 13 , the multichip module (“MCM”)  1200  comprises additional sets of components beneath top surfaces of the supporting frame  114   b  and heat spreading layers  114   a  depicted in  FIG. 12 . According to the depicted embodiment, each set of components in the MCM  1200  is similar to the embodiment shown in  FIG. 6 , wherein like elements have been similarly referenced. According to the embodiment depicted in  FIG. 13 , each shared portion of the supporting frame  114   b  situated between each pair of chips  112  (making a total of four shared portions) is a continuous portion of the entire supporting frame  114   b  structure. According to related embodiments, each heat spreading layer  114   a  may be encased by a corresponding supporting frame  114   b,  wherein the supporting frames  114   b  are thermally or mechanically joined to one another to form the MCM  1200 . 
         [0047]    The disclosed invention, including the depicted embodiments, employ a single high-k layer supported by a frame having a stronger structural stability. By using a single graphite layer, for example, the disclosed invention allows for high heat dissipation across at least two planes, including the z plane, and at least the x or y planes. Consequently, the disclosed invention allows for effective thermal conduction overall across the entirety of the heat spreading layer, without sacrificing structural integrity or incurring a thermal penalty by covering the high-k material with another material having relatively low thermal conductivity. Embodiments of the disclosed invention increase the thermal conductivity of semiconductor packages in which they are employed, and allow for reliable operation in hygrothermal operating conditions, which include temperatures between −40 ° C. and 125 ° C., and as much as 85% relative humidity.