Abstract:
A semiconductor package including a conical or pyramidal vapor chamber body coupled to a package bottom to enclose a vapor chamber within which are disposed a semiconductor die and working fluid. A matching conical or pyramidal heatsink is coupled to the vapor chamber body. The conical or pyramidal shape allows a tight fit and good thermal performance, without undue force being applied to the package bottom, and further allows a variety of heatsinks to be used with a standardized shape vapor chamber body.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention relates generally to thermal solutions for semiconductor packaging, and more specifically to thermal solutions using a vapor chamber such as in heatpipes. 
     2. Background Art 
     Semiconductor packages presently rely upon a primarily conductive model for transmitting heat from the surface of the semiconductor die to the exterior of the package. This model includes a number of interfaces, each of which drives an increase in the thermal resistance of the package. 
     FIG. 1 illustrates a generalized semiconductor packaging system  10  such as is known in the prior art. The system includes a package having a package bottom  12  and a package top  14  or integrated heat spreader which encompass a cavity  16  within which a semiconductor die  18  is contained. The semiconductor die is electrically connected through the package bottom to a number of pins or contacts  22  on the exterior of the packaging system. The semiconductor die is thermally interfaced to the package top with a quantity of a first thermal interface material  24  which improves thermal transfer from the die to the interior surface of the package top. The exterior surface of the package top is thermally coupled to a heatsink  26  with a quantity of a second thermal interface material (TIM)  28 . The reader will appreciate that this is a generalized and simplified view of the packaging system, for purposes of discussion. 
     Table 1 lists exemplary typical ratios of the various components of the aggregate thermal resistance of the system. Thermal resistance is measured in degrees Celsius per watt from the junction of the semiconductor device to the ambient air, in the case of an air-cooled system. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Thermal Component 
                 % of Resistance 
               
               
                   
                   
               
             
             
               
                   
                 Package 
                 49% 
               
               
                   
                 TIM 2 
                  7% 
               
               
                   
                 Heatsink 
                 42% 
               
               
                   
                 Heatsink Variation 
                  2% 
               
               
                   
                 TOTAL 
                 100%  
               
               
                   
                   
               
             
          
         
       
     
     In order to achieve good thermal contact across the second thermal interface layer  28 , the heatsink is mechanically fastened to the package or to the computer chassis or motherboard (not shown), with strong springs or with bolts. This creates a large force (shown as Fn in FIG. 1) which is normal to the mating surfaces of the heatsink and the package top. This force is necessary to ensure a thin layer of TIM2 material with optimal thermal characteristics. However, this force can cause electrical and/or mechanical breakdown of the die, the package, connection to the package contacts, and so forth. 
     FIG. 2 illustrates a generalized, simplified heatpipe system  30  as is known in the prior art. The system includes a heatpipe  32  coupled to conduct heat from a hot device  34  to a cooling device  36 . The body of the heatpipe encloses a vapor chamber  38  which contains a quantity of a working fluid  40  which does not completely fill the vapor chamber. The vapor chamber may also include a wicking material  42 , which improves performance in configurations or orientations in which the working fluid is not held by gravity against the hot device end of the vapor chamber. As the hot device boils the working fluid, the working fluid vaporizes. As the vapor condenses elsewhere in the heatpipe nearer the cooling device, the significant amount of heat captured by the phase change is released, effecting an efficient thermal transfer from the hot end of the heatpipe to the cooler end. Any suitable cooling fluid can be used, such as water, alcohol, flourinert, or the like. Any suitable active or passive cooling device can be used, such as a heatsink, heat spreader, peltier device, refrigerator coil, or the like. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only. 
     FIG. 1 shows an exemplary semiconductor packaging and cooling system according to the 
     FIG. 2 shows an exemplary heatpipe system according to the prior art. 
     FIG. 3 shows one embodiment of a semiconductor packaging and cooling system according to this invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 shows, in cross-section, one exemplary embodiment of a system  50  utilizing the teachings of this invention. The system includes a semiconductor die  18  coupled to a package bottom which has electrical contacts  22 . 
     A vapor chamber body  52  is sealed to the package to enclose a vapor chamber  54 . The vapor chamber body includes a sidewall member  53  whose exterior surface has a substantially conical or pyramidal shape, which is wider at the open end (shown as being attached to the package body) than at the closed end (shown as the top end where the heatsink is attached). In the case of a pyramidal shape, the shape may include any suitable number of sides, each having any suitably flat or curved surface. In some embodiments, the overall shape is a truncated cone or pyramid, including a top member  55 . In some embodiments, the top member and the sidewall member are one monolithic structure. In some embodiments, the sidewall member may have a lateral extension or lip  56  extending around parts or all of its perimeter at the open end, to provide increased sealing surface area for mating to the package. 
     The vapor chamber contains a quantity of suitable working fluid  40  and, optionally, a suitable wicking material  42 . The semiconductor die may, in some cases, need to be suitably encapsulated so it does not come into e.g. direct electrical or chemical contact with the working fluid, while remaining in significantly direct thermal contact with the working fluid by being immersed therein (in configurations in which gravity holds the working fluid in the package end of the vapor chamber). The skilled reader will appreciate that the wicking material is to be appropriately placed within the chamber, such as by locating it along the inner surface of the chamber and extending onto or near to the semiconductor die. It is for ease of illustration only that the wick material is shown as residing in the central area of the chamber. 
     A heatsink  58  or other suitable cooling device is coupled to the exterior surface of the vapor chamber body by any suitable means, such as by a fastener  60  (such as a bolt, screw, rivet, or other means) which engages a threaded recess  62  in the upper portion of the vapor chamber body but does not compromise the seal of the vapor chamber. The heatsink may advantageously include a number of fins  64  or other structures for increasing its surface area. In one embodiment, these fins may extend laterally; in other embodiments, they may be differently oriented. In some embodiments, the fins may extend only partway along the heatsink, leaving a portion  66  without fins. This portion may be at the bottom of the heatsink as shown, or at any other suitable location, and its location may in part be determined by required keep-out zones in the system, such as if the package has one or more other devices coupled to it outside the vapor chamber body, for example an on-package voltage regulator  68  which may have its own heatsink  70 . In some embodiments, these voids or finless portions may not be symmetrical around the vapor chamber body. In some embodiments, fins of differing length can be used to accommodate keep-out zones. 
     The portion of the heatsink that is not the fins may be termed the body of the heatsink. In one embodiment, the heatsink may be a monolithic structure, such as one formed by machining a block of metal. In other embodiments, the fins may be separate structures suitably affixed to the body of the heatsink, such as by brazing or by friction fit. In some such embodiments, the fins may be a set of plates having different diameter holes, such that they slip down a conical exterior surface of the heatsink to different positions, where they can be affixed. 
     The convex exterior surface of the vapor chamber body and the concave interior surface of the heatsink have substantially similar shapes and dimensions, such that they make good thermal contact with each other. The vapor chamber body may be considered a male structure and the heatsink may be considered a female structure; as shown in FIG. 3, the exterior surface of the vapor chamber body mates with the interior surface of the heatsink. This shape is generally conical or pyramidal, such that an upper portion of the vapor chamber body is narrower than a lower portion. This helps ensure a very close tolerance fit between the mating sides of the vapor chamber body and heatsink, thereby reducing the thermal resistance of a thermal interface material (not shown) between the heatsink and the vapor chamber body. The heatsink will simply slip down onto the vapor chamber body until the sides come into direct mechanical contact, as long as the top surface of the vapor chamber body does not contact the under side of the heatsink first. Additionally, the conical or pyramidal shape provides for a high normal force Fn between the mating surfaces, without creating a high force on the package, die, etc. This high normal force helps reduce the thermal resistance of the thermal interface material between the mating surfaces. Furthermore, because the die is not in a “mechanical stack” with the package top as in the prior art, mismatched thermal coefficients of expansion of the die and of the package top or integrated heat spreader will not cause mechanical stress on the die; the die is free to expand in the vapor chamber, and the heatsink is free to expand on top of the vapor chamber body, without physically impinging on the die. Moreover, since there is no direct contact between the vapor chamber and the die, the die may be thinned to bring the working fluid in closer proximity to the actual heat-generating transistors, which are typically located on the side of the die facing the package bottom. The die may also be machined to increase its effective surface area, to enhance the efficiency of the vapor generation (boiling) process. The vapor chamber heat spreader can, in some embodiments, have a reduced weight as compared to a more traditional solid copper base plate heatsink. 
     The generally conical shape of the chamber vessel facilitates easy removal of the heatsink. This allows customized heatsinks with a standard vapor chamber implementation. 
     Typically, the vapor chamber body may include a fill tube or other such mechanism through which the working fluid is injected. The size and placement of this mechanism, as well has how the chamber is evacuated and the fill mechanism sealed, are well within the ordinary skill of those in this field, and thus have been omitted from the drawings. 
     Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the invention. The various appearances “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. 
     If the specification states a component, feature, structure, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
     Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.