Patent Publication Number: US-2004042178-A1

Title: Heat spreader with surface cavity

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates generally to the field of electronic component heat dissipation. More particularly, this invention relates to an improved heat spreader for use with heat-producing electronic components.  
       [0003] 2. Description of the Relevant Art  
       [0004] Thermal elements such as heat sinks are commonly attached to electronic components to facilitate heat dissipation from the components. A heat sink is typically composed of a thermally conductive material, such as aluminum, with a plurality of fins or pins on its exposed side. Heat is dissipated from the fins or pins to the surrounding air principally by thermal convection. Heat sinks may be coupled to a component by a variety of means known to those skilled in the art. For example, the heat sink may be coupled to the component by an adhesive or by various types of retainers such as clamps, brackets, or screws.  
       [0005] Some heat sinks are mounted to, consist of, or incorporate a thermally conductive element that facilitates distribution of heat from the surface of the component to the heat sink. The intermediary thermally conductive element may be referred to as a “heat spreader.” In some cases, the thermally conductive element may also function as a lid to physically protect the electronic component.  
       [0006] When a thermal element is mounted in direct contact with an electronic component, air gaps exist at the interface because of roughness and nonplanarity of the mating surfaces. These air gaps reduce the effectiveness of the thermal element because air has a relatively low thermal conductivity. For this reason, a thermal interface material, such as a thermal grease or paste, an elastomeric thermal pad, or a phase change material, is often placed between the thermal element and the electronic component to improve the performance of the thermal element.  
       [0007] A problem that arises from the use of thermal grease or similar materials is a phenomenon known as “pump-out.” “Pump-out” occurs when an electronic component is subjected to cyclic load conditions. Under cyclic loads, thermo-mechanical stresses between the electronic component and a heat sink or heat spreader may cause a loss of grease material from the interface. The loss of grease material may result in an increase in thermal resistance at the interface.  
       [0008] Elastomeric thermal pads may be used as an alternative to thermal grease. Elastomeric pads are not susceptible to pump-out. However, such pads may be difficult to position and maintain in place. Improper placement or slippage may result in increased thermal resistance at the interface. In some applications, special frames or similar components are used to facilitate placement and retention of the thermal interface material. However, such components add cost and complexity to the system in which they are used.  
       [0009] Regardless of the cause, increased thermal resistance at the interface between an electronic component and a thermal element results in higher operating temperatures of the component. Higher operating temperatures are associated with decreased reliability of electronic components. Accordingly, there is a need for an apparatus to improve thermal performance between an electronic component and a thermal element.  
       SUMMARY OF THE INVENTION  
       [0010] In an embodiment, the body of a heat spreader includes a cavity that has an opening on the surface of the body. A thermal interface material, such as a resilient thermal pad, a thermal grease, or phase change material, may be disposed in the cavity to facilitate heat transfer from the component to the body of the heat spreader. The size and location of the opening of the cavity may be such that the cavity is at least partially enclosed by a surface of the component when the heat spreader is coupled to the component.  
       [0011] In an embodiment, the thermal interface material may be a resilient material, such as a thermally conductive elastomeric pad. The thickness of the thermal interface material may be chosen so that the thermal interface material is compressed between enclosing surface and the inner surfaces of the cavity when the heat spreader is coupled with the component.  
       [0012] In another embodiment, the thermal interface material may be a thermal grease. The thermal grease may substantially fill the cavity. The thermal grease may be at least partially contained by the inner surfaces of the cavity and the enclosing surface of the component during operation of the component. Containment of the thermal interface material within cavity may inhibit pump-out of the thermal interface material.  
       [0013] In another embodiment, the body of a heat spreader may include a cavity to accommodate thermal interface material between the heat spreader and a heat sink. In another embodiment, the thermal interface material can be disposed in a cavity formed in a heat sink instead of, or in addition to, a cavity in the body of the heat spreader. In another embodiment, a heat spreader may be incorporated into a computer system to facilitate heat transfer from one or more electronic components of the system.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014] 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:  
     [0015]FIG. 1 shows an exploded perspective view of a heat sink, a heat spreader and an electronic component.  
     [0016]FIG. 2 shows a cross-sectional view of a heat spreader having two cavities positioned on a component.  
     [0017]FIG. 3 shows a cross-sectional view of a resilient thermal interface material disposed in a heat spreader body before it is coupled with a heat sink.  
     [0018]FIG. 4 shows a cross-sectional view of a heat transfer device, wherein the enclosing surfaces of a heat sink include side surfaces that face sidewalls of a heat spreader cavity.  
     [0019]FIG. 5 shows a cross-sectional view of a thermal interface material disposed in a cavity in a heat sink.  
     [0020]FIG. 6 shows a cross-sectional view of a thermal interface material disposed in matching cavities in a heat spreader and a heat sink.  
     [0021]FIG. 7 shows a block diagram of a computer system that incorporates one or more heat spreaders. 
    
    
     [0022] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawing and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form 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  
     [0023] Referring now to FIG. 1, a heat spreader  20  may be mounted to a component  22  to facilitate cooling of the component. Component  22  may be any electronic component that produces heat during use, including, but not limited to, a surface-mounted integrated circuit, dual in-line memory module, or a single transistor housed in a can. Component  22  may be mounted to substrate  25 , which may in turn be mounted to a printed circuit board (not shown). The package design of component  22  may be of a lidded or lidless type. Electrical connection of component  22  to substrate  25  may be by any means, including, but not limited to, ball grid array or pin grid array. Heat spreader  20  may be mounted either directly to component  22  or to another element, such as substrate  25  or a printed circuit board to which component  22  is also mounted.  
     [0024] Heat spreader  20  may include a body  24 . Body  24  may be of a thermally conductive material. Examples of such materials include, but are not limited to, aluminum, copper, or brass. A cavity  26  may be disposed within body  24  having an opening on an outer surface  28  of body  24 . Cavity  26  may have one or more inner surfaces  30 , including sidewalls  32 . Extended members such as fins or pins (not shown) for increasing the exposed surface area of the heat spreader  20  may be integral features of body  24 . Alternatively, extended members may be part of a separate heat sink  34  that is coupled to a heat spreader  20  by screws, rivets, an adhesive, or other means known to those skilled in the art, to form a heat transfer device.  
     [0025] A thermal interface material  36  may be disposed in cavity  26  to facilitate heat transfer from component  22  to body  24  of heat spreader  20 . Thermal interface material  36  may be any of a variety of thermal interface materials known to those skilled in the art, including, but not limited to, a resilient thermal pad, a thermal grease, a thermal paste, or a phase change film. Thermal interface material  36  may substantially fill cavity  26 . Sidewalls  32  may inhibit movement of thermal interface material  36  during installation of heat spreader  20  or use of the electronic component, which may obviate the need for separate components to position and retain thermal interface material  36  in place on component  22 .  
     [0026] The dimensions of the opening of cavity  26  may be smaller than dimensions of an enclosing surface  37  of component  22  (e.g., the upper surface of the component, not visible in FIG. 1) so that cavity  26  is at least partially enclosed by enclosing surface  37  when heat spreader  20  is coupled to the component. In some embodiments, thermal interface material  36  may be sealed in cavity  26  by installation of heat spreader  20  on component  22 . In other embodiments, thermal interface material  36  may not be sealed in cavity  26 , but merely inhibited from movement by inner surfaces  30  and enclosing surface  37 .  
     [0027] Heat spreader  20  may be coupled to component  22  by a variety of means. Such means include, but are not limited to, an adhesive, a retainer, or both. Adhesive  40  may be any of various thermally conductive epoxies or other bonding materials known to those skilled in the art. Referring to FIG. 2, adhesive  40  may be used to attach heat spreader  20  to substrate  25 . A retainer may be any of various retaining elements known to those skilled in the art, including, but is not limited to, a clamp, a clip, a bracket, or one or more screws. FIG. 2 shows a heat transfer device  42  mounted to a printed circuit board  50  by a plurality of screws  52 . Heat transfer device  42  includes heat spreader  20  and a heat sink  34 . Screws  52  may extend through a matching pattern of holes  54  in printed circuit board  50 . Screws  52  may be affixed to a bolster plate  56 , and the parts maintained in relative position to each other by nuts  58 .  
     [0028] In an embodiment, thermal interface material  36  may be a resilient material. For example, thermal interface material  36  may be a thermally conductive elastomeric pad. As shown in FIG. 3, the free thickness T of thermal interface material  36  may be chosen so that thermal interface material  36  is compressed between enclosing surface  37  and inner surfaces  30  when heat spreader  20  is coupled with component  22 . For example, the depth D of cavity  26  may be 25% less than free thickness T of thermal interface material  36 . Compression of the resilient material in cavity  26  may reduce thermal contact resistance between thermal interface material  36  and enclosing surface  37  and between thermal interface material  36  and inner surfaces  30 , which may improve the thermal performance of heat spreader  20 .  
     [0029] In another embodiment, thermal interface material may be a thermal grease. As shown in FIG. 2, the thermal grease may substantially fill cavity  26 . The thermal grease may be at least partially contained by inner surfaces  30  and enclosing surface  37  during operation of component  22 . Containment of the thermal grease within cavity  26  may inhibit pump-out of the thermal grease.  
     [0030] In an embodiment, a heat spreader may have a cavity for accommodating thermal interface material on a surface that faces a heat sink. FIG. 2 shows a heat spreader  20  having a cavity  126  in which thermal interface material  136  is disposed. When heat sink  34  is coupled to heat spreader  20 , thermal interface material  136  may be partially enclosed by enclosing surface  44  of heat sink  34 .  
     [0031] In an embodiment, a cavity may be at least partially enclosed by a plurality of surfaces that face the sidewalls of the cavity. FIG. 4 shows a heat sink  34  having enclosing surfaces  145  that include side surfaces  146  and front surface  147 . Side surfaces  146  face a portion of sidewalls  132 . Stop  148  of heat sink  34  may control spacing between front surface  147  and bottom surface  149  of cavity  126  so that the desired level of compression of thermal interface material  136  may be achieved.  
     [0032] In some embodiments, the thermal interface material can be disposed in a cavity formed in a heat sink instead of, or in addition to, a cavity in the body of a heat spreader. FIG. 5 shows thermal interface material  136  enclosed in a cavity  226  of a heat sink  34  by enclosing surface  237  of heat spreader  20 . FIG. 6 shows thermal interface material  136  enclosed in a cavity  126  of heat spreader  20  and a cavity  226  of heat sink  34 .  
     [0033] In an embodiment, heat spreader  20  may be incorporated into a computer system to facilitate heat transfer from one or more electronic components of the system. FIG. 7 is a high-level component diagram of such a computer system. Computer system  70  may include a central processing unit  72  and other electronic components  74 . Heat transfer devices  42  may be coupled to central processing unit  72  and to electronic components  74 . Heat transfer devices  42  include a heat spreader  20 , as described herein, and a heat sink  34 .  
     [0034] Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.