Abstract:
Disclosed herein is a module cooling system, comprising, a module in operable communication with a circuit board, a stiffener abutting the circuit board, a heatsink abutting the module, a first biasing member biasing the heatsink towards the module, a plurality of non-influencing fasteners positionally fixing the heatsink, and a second biasing member biasing the circuit board and module towards the heatsink. Further disclosed herein is a method of mounting a module cooling system, comprising, connecting electrically a module to a circuit board, abutting a stiffener to the circuit board, abutting a heatsink to the module, biasing with a biasing member the heatsink in a direction towards the module, fixing the heatsink with non-influencing fasteners, and biasing with a second biasing member the circuit board and module towards the heatsink.

Description:
TRADEMARKS 
   IBM® is a registered trademark of International Business Machines Corporation, Armonk, N.Y., U.S.A. Other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation or other companies. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to efficiently cooling electronic circuits, and particularly to cooling circuits through the use of heatsinks. 
   2. Description of Background 
   Electronic components, such as microprocessors and integrated circuits, must operate within certain specified temperature ranges to perform efficiently. Excessive heat degrades electronic component performance, reliability, life expectancy, and can even cause failure. Heatsinks are widely used for controlling excessive heat. Typically, heatsinks are formed with fins, pins or other similar structures to increase the surface area of the heatsink and thereby enhance heat dissipation as air passes over the heatsink. In addition, it is not uncommon for heatsinks to contain high performance structures, such as vapor chambers and/or heat pipes, to further enhance heat transfer. Heatsinks are typically formed of metals, such as copper or aluminum. More recently, graphite-based materials have been used for heatsinks because such materials offer several advantages, such as improved thermal conductivity and reduced weight. 
   Electronic components are generally packaged using electronic packages (i.e., modules) that include a module substrate to which the electronic component is electronically connected. In some cases, the module includes a cap (i.e., a capped module), which seals the electronic component within the module. In other cases, the module does not include a cap (i.e., a bare die module). 
   Bare die modules are generally preferred over capped modules from a thermal performance perspective. In the case of a capped module, a heatsink is typically attached with a thermal interface between a bottom surface of the heatsink and a top surface of the cap, and another thermal interface between a bottom surface of the cap and a top surface of the electronic component. In the case of a bare die module, a heatsink is typically attached with a thermal interface between a bottom surface of the heatsink and a top surface of the electronic component. Bare die modules typically exhibit better thermal performance than capped modules because bare die modules eliminate two sources of thermal resistance present in capped modules, i.e., the thermal resistance of the cap and the thermal resistance of the thermal interface between the cap and the electronic component. Accordingly, bare die modules are typically used to package electronic components that require high total power dissipation. 
   Heatsinks are attached to modules using a variety of attachment mechanisms, such as clamps, screws, and other hardware. The attachment mechanism typically applies a force that maintains a thermal interface gap, i.e., the thickness of the thermal interface extending between the heatsink and the module. In the case of a capped module, the cap protects the electronic component from physical damage from the applied force. In the case of a bare die module, however, the applied force is transferred directly through the electronic component itself. Consequently, when bare die modules are used, the attachment mechanism typically applies a compliant force to decrease stresses on the electronic component. 
   Typical methods and designs used to control the thermal interface gap, while not putting excessive mechanical loads onto the module, include many components and are thus complex, expensive and take up valuable real-estate that could be put to better use by packaging more circuit components. Accordingly, there is a need in the art for a smaller, less complex and less expensive module-to-heatsink mounting arrangement. 
   SUMMARY OF THE INVENTION 
   The shortcomings of the prior art are overcome and additional advantages are provided through the provision of a module cooling system, comprising, a module with a center in operable communication with a circuit board, a stiffener abutting the circuit board on a side of the circuit board opposite a side at which the module is disposed, a heatsink abutting the module on a side of the module opposite a side at which the circuit board is disposed, a first biasing member biasing the heatsink relative to the stiffener towards the center of the module, a plurality of non-influencing fasteners positionally fixing the heatsink relative to the stiffener, and a second biasing member biasing the circuit board and module towards the heatsink relative to the stiffener. 
   Further disclosed herein is a method of mounting a module cooling system, comprising, connecting electrically a module with a center to a circuit board, abutting a stiffener to the circuit board on a side of the circuit board opposite a side at which the module is disposed, abutting a heatsink to the module on a side of the module opposite a side at which the circuit board is connected, biasing with a first biasing member the heatsink in a direction towards the center of the module relative to the stiffener, fixing the heatsink relative to the stiffener with non-influencing fasteners, and biasing with a second biasing member the circuit board and module towards the heatsink relative to the stiffener. 
   Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
   TECHNICAL EFFECTS 
   As a result of the summarized invention, technically we have achieved a solution, which efficiently couples a heatsink to a circuit for dissipation of heat therefrom. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  illustrates one example of a plan view of a module cooling system disclosed herein; 
       FIG. 2  illustrates one example of a cross sectional front view of the module cooling system of  FIG. 1  taken at arrows  2 - 2 ; 
       FIG. 3  illustrates one example of a plan view of a biasing spring disclosed herein; and 
       FIG. 4  illustrates one example of a cross sectional view of a non-influencing fastener positionally fixing a heatsink to a stiffener disclosed herein. 
   

   The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1 and 2 , a module cooling system according to an embodiment of the invention is shown generally at  10 . A bare die module  12  comprising an electronic component such as a semiconductor chip  14 , a module substrate  18 , and an electronic connector  22  generates heat that requires dissipation. The bare die module shown in  FIG. 1  is a single-chip module (SCM); however, those skilled in the art will recognize that the spirit and scope of the present invention is not limited to SCMs. For example, those skilled in the art will recognize that the present invention may be practiced using a multi-chip module (MCM) or other electronic components/heat sources. The semiconductor chip  14  is electrically connected to the module substrate  18  in a center  16  of the module  12 . Electronic connector  22 , which electrically connects printed circuit board  24  to module substrate  18 , may be a pin grid array (PGA), a ceramic column grid array (CCGA), a land grid array (LGA), or the like. 
   Referring to  FIGS. 1 and 2 , a module cooling system according to an embodiment of the invention is shown generally at  10 . A bare die module  12  comprising an electronic component such as a semiconductor chip  14 , a module substrate  18 , and an electronic connector  22  generates heat that requires dissipation. The bare die module shown in  FIG. 1  is a single-chip module (SCM); however, those skilled in the art will recognize that the spirit and scope of the present invention is not limited to SCMs. For example, those skilled in the art will recognize that the present invention may be practiced using a multi-chip module (MCM) or other electronic components/heat sources. The semiconductor chip  14  is electrically connected to the module substrate  18  in a center  16  of the module  12 . Electronic connector  22 , which electrically connects printed circuit board  24  to module substrate  18 , may be a pin grid array (PGA), a ceramic column grid array (CCGA), a land grid array (LGA), or the like. 
   In order to dissipate the heat generated in the module  12  a heatsink  28  is pressed against the module  12  with a thermally conductive material therebetween forming a thermal interface  36 . The thermal interface  36  is made of a thermally conductive material such as thermal gel, grease, paste, oil, gas, solid or other material with a high thermal conductivity. Typically, the thermal interface  36  is relatively thin so that it may easily transfer heat away from semiconductor chip  14  and toward the heatsink&#39;s base plate  40 . The thickness of thermal interface  36  extending between the bottom of the heatsink&#39;s base plate  40  and the top surface of semiconductor chip  14  is referred to as the thermal interface gap  44 . In one embodiment, the thermal interface gap  44  is about 1.2 mil. 
   In addition to providing uniform heat dissipation for the module  12  the thermal interface gap  44  provides a damping effect. This damping effect reduces the vibration, and other mechanical transient loads, that impacts the heatsink  28  before it reaches the module  12 . The mounting and loading of the heatsink  28  relative to the module  12  is therefore very important and is described in the following embodiments in detail. 
   A stiffener  50 , made of a strong material such as stamped metal, is abutted to the circuit board  24  on the side opposite of the module  12 . Threaded non-influencing fastener (NIF) standoffs  54  screw into the stiffener  50  through holes  56  in the board  24  thereby fixing the board  24  to the stiffener  50 . Four holes  58  in the comers of the heatsink  28  slidably engage the NIF standoffs  54  and positionally center the heatsink  28  above the module  12 . A spring  62 , as shown in  FIG. 3 , slidably engages center cooling fins  66  of the heatsink  28  and is thereby centered relative to the heatsink  28 . The spring  62  includes a threaded hole  70  at its center for receiving a screw  74 . The centering of the spring  62  relative to the heatsink  28  and the centering of the heatsink  28  relative to the module  12  assures that the screw  74  and its receiving threaded hole  70  are centered above the module  12 . 
   Ends of the spring  62 , as best seen in  FIG. 3 , receive heads  78  on heatsink load posts  82 . A first end  86  of the spring  62  has a keyhole  90 , while a second end  94  has a slot  98 . Since the round portion  102  of the keyhole  90  is larger than the head  78  of the lead posts  82  the spring  62  can be placed over the heads  78  of two load posts  82  and then moved relative to the lead posts  82  thereby locking the ends  86  and  94  under the heads  78 . A pair of heatsink load posts  82  threadably engaged with holes  106  in the stiffener  50 , through holes  56  in the board  24 , are positioned apart by the same distance as the ends  86  and  94  of the spring  62 , and are positioned such that the spring  62  fits between the center fins  66  of the heatsink  28 . 
   The aforementioned structure allows the heatsink  28  to be centrally spring loaded over the module  12  by tightening a screw  74  into the center hole  70  of the spring  62 . The further the screw  74  is screwed into the spring  62 , the more the center of the spring  62  deflects, and the higher the force applied to the heatsink  28 , and, correspondingly, the higher the force between the heatsink  28  and the module  12 . The central location of the force application assures uniformity of pressure of the thermally conductive material and the corresponding uniformity of the thermal interface gap  44 . Several methods may be employed to create and control the force, such as turning the screw  74  a predefined number of rotations, or turning the screw  74  until its head bottoms out against the spring  62 , for example. 
   The heatsink  28  may be massive enough to require additional structural attachment, to for example the stiffener  50 , than is provided by the spring  62  alone. Therefore, embodiments may lock the heatsink  28  to the stiffener  50  through the use of non-influencing fasteners (NIF)  120 . Referring to  FIG. 4  the NIF  120  includes a NIF screw  122 , a resilient member  124  and, optionally, a washer  128 . The NIF screw  122  threads into a threaded hole  132  in the NIF standoffs  54  which are slidably engaged with holes  58  in the base plate  40  of the heatsink  28 . Tightening of the NIF screw  122  causes the resilient member  124  to compress and expand radially outward until it frictionally engages with the hole  58  thereby positionally locking the heatsink  28  to the stiffener  50 . Structurally supporting the heatsink  28  through the NIFs  120  to the stiffener  50  may remove the potentially damaging dynamic loads that the heatsink  28  could place upon the module  12 . 
   Referring again to  FIG. 2 , a biasing load is applied between the stiffener  50  and the circuit board  24  through the raised surface  140  in the stiffener  50 . The flexing of the stiffener  28 , the board  24  or both, generates this biasing load. By locating the raised surface  140  central relative to the module  12 , the biasing load will act to maintain a uniform thermal interface gap  44 . The amount of force created by the raised surface  140  can be accurately set by the design of the raised surface  140  relative to points where the NIF standoffs  54  attach to the stiffener  50 . Additional ribbing of the stiffener  50  or the addition of a separate pad of alternate material (not shown) could also be incorporated to create specific characteristics of the biasing load. 
   Embodiments of the invention may have some of the following advantages: a first biasing force applied to a heatsink centrally relative to a module, accurate control of the first biasing force, removal of the first biasing force upon completion of the circuit board assembly, structural support of the heatsink to a stiffener in multiple locations, and a continuously applied centrally loaded biasing force after completion of the assembly. 
   While the embodiments of the disclosed system and method have been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the embodiments of the disclosed system and method. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiments of the disclosed system and method without departing from the essential scope thereof. Therefore, it is intended that the embodiments of the disclosed system and method not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the embodiments of the disclosed system and method, but that the embodiments of the disclosed system and method will include all embodiments falling within the scope of the appended claims. 
   While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.