Patent Publication Number: US-2005133907-A1

Title: Mechanism for maintaining consistent thermal interface layer in an integrated circuit assembly

Description:
BACKGROUND  
      1. Field of the Present Invention  
      The present invention is in the field of integrated circuits and, more particularly to integrated circuits that use heat dissipation hardware.  
      2. History of Related Art  
      Mobile processors including notebook and desktop computers have historically used a thermal interface pad to participate in the heat transfer process. Thermal interface pads are characterized by a one-sided adhesive, which makes it relatively easy to apply and to remove the thermal solution. A drawback is that the thermal interface pad has poor thermal conductivity. As the maximum designed thermal power of CPUs has increased, this pad has proven to be insufficient for most applications.  
      The inadequacy of thermal interface pads as a heat transfer mechanism forced mobile processing device manufacturers to employ thermal greases or “phase change” materials that had been in use on desktops machines and server-class computers for some time. Standard phase change materials are typically a polymer/carrier filled with a thermally conductive filler, which changes from a solid to a high-viscosity liquid (or semi-solid) state at a certain transition temperature typically in the range of 50 to 70° C. These materials conform well to irregular surfaces and have wetting properties similar to thermal greases, which significantly reduces the contact resistance at the different interfaces. Due to this composite structure, phase change materials are able to withstand mechanical forces during shock and vibration, protecting the die or component from mechanical damage. When heated to the transition temperature, the material significantly softens to a near liquid-like physical state in which the thermally conductive material slightly expands in volume. This volumetric expansion forces the more thermally conductive material to flow into and replace the microscopic air gaps present in between the heat sink and electronic component. With the air gaps filled between the thermal surfaces, a high degree of wetting of the two surfaces minimizes the contact resistance.  
      Unfortunately, phase change materials and thermal greases are difficult to use in a manufacturing environment. Specifically, the quasi-liquid characteristics of phase change materials and thermal greases are sensitive to any gradient in the pressure applied to the film. Typically, the phase change material is situated between a thermal interface such as the bottom of a heat sink, a fan sink assembly, a vapor chamber, or a heat spreader (with or without heat pipes) surface and an upper surface of the device itself. In either case, maintaining a uniformly thick film is important and difficult. It is important to ensure the best heat transfer characteristics possible and thereby improve device performance, reliability, and lifetime. It would be desirable, therefore, to implement an integrated circuit assembly that permitted the use of these advanced heat transfer materials and addressed the difficulty of maintaining a uniform thickness inherent with these materials.  
     SUMMARY OF THE INVENTION  
      The objective identified above is achieved according to the present invention by an integrated circuit assembly that includes an integrated circuit overlying a printed circuit board and a thermal solution interface such as a fan sink or a heat pipe overlying the integrated circuit. The lower surface of the thermal solution interface has a plurality of spacer structures to enforce a uniform displacement between the lower surface and an underlying surface contacted by the spacers. A heat transfer material, such as a thermal phase change material or a thermal grease, is positioned between the thermal solution interface and the underlying surface contacted by the spacers. The assembly may include a socket connected to the printed circuit board into which the integrated circuit is inserted. The integrated circuit may include a thermally conductive heat spreader attached to its upper surface such that the spacers contact an upper surface of the heat spreader. In other embodiments, the heat transfer material directly contacts an upper surface of the integrated circuit die. The spacers likely enforce a uniform displacement in the range of approximately 0.001 to 0.005 (dependent on ideal properties of the interface material) inches. The spacers may be configured as a set of substantially hemispherical protrusions or a set of substantially parallel elongated ridge protrusions.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:  
       FIG. 1  is a diagram of an integrated circuit assembly according to one embodiment of the invention emphasizing a thermal interface having a set of spacer structures;  
       FIG. 2  is a bottom view of one embodiment of the thermal interface of  FIG. 1 ;  
       FIG. 3  is a bottom view of an alternative embodiment of the thermal interface of  FIG. 1 ;  
       FIG. 4  is a diagram of an alternative implementation of the integrated circuit assembly of  FIG. 1 ;  
       FIG. 5  is a diagram of an alternative implementation of the integrated circuit assembly of  FIG. 1 ; and  
       FIG. 6  is a diagram of an alternative embodiment of the integrated circuit assembly of  FIG. 1 . 
    
    
      While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description presented herein are not intended to limit the invention to the particular embodiment disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.  
     DETAILED DESCRIPTION OF THE INVENTION  
      Generally speaking, the present invention is concerned with an integrated circuit assembly that employs semi-liquid heat transfer materials such as thermal grease and/or phase change materials. The semi-liquid material is formed between a pair of surfaces of the assembly. At least one of the surfaces includes spacer structures formed thereon. The spacer structures are preferably of a uniform dimension and, in one embodiment, are hemispherical. The presence of the spacer structures positioned across the face of the surface enforces a uniform separation between the two plates between which the phase change material is present. Any gradient in pressure across the face of the surface will not result in a heat transfer material thickness gradient or non-uniformity. By maintaining a uniformly thick phase change material across a heat transfer interface of an integrated circuit, the invention beneficially improves the heat transfer properties of the integrated circuit by eliminating localized “hot spots” that may occur when portions of the heat transfer material are thinner than others.  
      Turning now to the drawings,  FIG. 1  is a plan view of an integrated circuit assembly  100  according to one embodiment of the invention for use with a data processing system. In the depicted embodiment, which is likely implemented in conjunction with a desktop or server-class data processing system, assembly  100  includes a printed a socket  104  overlying a printed circuit board  102 . Printed circuit board  102 , which is connected to a chassis  101  of the system, is exemplified by the system&#39;s motherboard in a desktop PC implementation or a processor planer in a server class machine. An integrated circuit  106  is positioned with the socket  104 . Integrated circuit  106  may be any integrated circuit that employs active or passive heat transfer hardware. In a likely embodiment, integrated circuit  106  is a general purpose microprocessor or central processing unit (CPU) such as PowerPC® family of processors from IBM Corporation or an x86 family processor. CPU&#39;s represent the integrated circuits most likely to employ heat transfer hardware because of the amount of heat such devices generate. Increasingly, however, other integrated circuits including graphics controllers, chipsets, memory devices, and other integrated circuits are employing heat transfer mechanisms to combat the ever increasing performance demands. Integrated circuit  106  may be a packaged device in which the integrated circuit die is located within a plastic or ceramic package. Alternatively, integrated circuit  106  may comprise an un-packaged die inserted into socket  104 .  
      A heat spreader  108  is shown as attached to an upper surface of integrated circuit  106 . Heat spreader  108  (also referred to as an integrated circuit cap) is preferably a thermally conductive material that facilitates the transfer of heat from integrated circuit  106 . A preferred implementation of heat spreader  108  is made of copper. A thermal paste or grease  111  is likely placed between heat spreader  108  and an upper surface of integrated circuit  106  to further enhance the efficiency of heat transfer.  
      A thermal solution interface  114  is shown as resting on an upper surface of heat spreader  108 . Interface  114  may comprise the lower portion of a heat sink or fan sink assembly that is attached to printed circuit board  102 . In the preferred implementation, thermal interface  114  is secured from above using a spring force (not shown) or other securing mechanism (such as a set of screws), to maintain the thermal interface in proximity to the underlying integrated circuit  106 .  
      In the depicted embodiment, thermal solution interface includes a plurality of spacers  112  formed on the surface of interface  114 . As depicted in the bottom view of  FIG. 2 , spacers  112  are uniformly dimensioned, hemispherical structures that protrude from the surface of thermal solution interface  114 . The depicted implementation of spacers  112  includes a spacer  112  positioned to contact heat spreader  108  at each corner of the die and a fifth spacer centered among the other four, but this pattern is implementation specific and other patterns of spacers  112  are within the scope of the invention. In applications requiring a larger surface area or higher pressure, as examples, the number of spacers may be increased. In the embodiment depicted in  FIG. 3 , spacers  112  are implemented as a set of three evenly spaced, elongated ridged protrusions. In either embodiment, the uniform dimension of the spacers enforces a uniform separation between the surface of interface  114  and the upper surface of heat spreader  108 . In one embodiment, spacers  112  are formed from (i.e., are integral with) thermal solution interface  114  so that spacers  112  have the same composition as interface  114 . Spacers  112  may be formed by any of a variety of methods including milling, stamping, and chemical etching. The critical dimension of spacers  112  is the amount of displacement that spacers  112  enforce between thermal interface  114  and the surface that spacers  112  contact. In a likely integrated circuit application, this dimension is in the range of 1 to 5 mils (thousandths of an inch).  
      Returning to  FIG. 1 , a heat transfer material  110  is applied between thermal solution interface  114  and heat spreader  108 . The heat transfer material  110  is preferably a thermal phase change material or a thermal grease. Thermal phase change materials are exemplified by the Hi-Flow® family of phase change materials from the Bergquist Company. Thermal greases suitable for use in assembly  100  include the Cooler Master thermal compound from Shin Etsu Chemical Company. In any of these embodiments, heat transfer material  110  may exhibit liquid or quasi-liquid properties at certain temperatures. Specifically, the heat transfer material may be unable to withstand a pressure gradient without conforming or yielding to the pressure-applying surface. When this is the case, maintaining a uniformly thick heat transfer material is difficult in the absence of spacers  112 .  
      Referring to  FIG. 6 , an alternative embodiment of system  100  extends the spacer concept by incorporating spacers  109  affixed to a lower surface of heat spreader  108  and introducing a heat transfer material  107  between heat spreader  108  and CPU die  106 . Like heat transfer material  110 , heat transfer material  107  may include a thermal phase change material or a thermal grease.  
      The embodiment of assembly  100  depicted in  FIG. 1 , is characteristic of a desktop or server application in which a heat spreader or thermal cap  108  covers the CPU die  106 . Referring now to  FIG. 4  and  FIG. 5 , alternative embodiments of integrated circuit assemblies according to the present invention are depicted to emphasize applications more suitable for mobile computing applications that require lower profile assemblies. In  FIG. 4 , an integrated circuit assembly  400  includes a socket  404  overlying a printed circuit board  402 . An integrated circuit die  406  is positioned within socket  404 . Heat transfer material  410 , which is analogous to heat transfer material  110  of  FIG. 1 , is located between a thermal solution interface  414  and an upper surface of integrated circuit die  406 . Thermal solution interface  414  includes spacers  412  that are equivalent to the spacers  112  of  FIG. 1 . In this application, spacers  412  and heat transfer material  410  are in direct contact with the upper surface of integrated circuit die  406 . This embodiment achieves a slightly reduced profile while still employing a socket  404  that enables customers to replace the socketed device (integrated circuit die  406 ).  
      An even lower profile is achieved with the integrated circuit assembly  500  depicted in  FIG. 5 . In this implementation, the integrated circuit die  506  is connected (soldered) directly to the underlying printed circuit board  502 . The thermal solution interface  514  includes spacers  512 , equivalent to the spacers  112  of  FIG. 1  and  412  of  FIG. 4 , that contact an upper surface of integrated circuit die  506 . The heat transfer material  510 , analogous to materials  110  and  410 , is located between thermal solution interface  514  and integrated circuit die  506 .  
      It will be apparent to those skilled in the art having the benefit of this disclosure that the present invention contemplates a mechanism for maintaining a uniform dimension of a thermal interface material in an integrated circuit assembly. It is understood that the forms of the invention shown and described in the detailed description and the drawings are to be taken merely as presently preferred examples. It is intended that the following claims be interpreted broadly to embrace all the variations of the preferred embodiments disclosed.