Abstract:
A heat sink comprises a side including a structural member defining a distance between a heat generating structure and the second side of the heat sink.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/474,791, filed Dec. 29, 1999 now U.S. Pat. No. 6,570,764, by Intel Corporation, entitled LOW THERMAL RESISTANCE INTERFACE FOR ATTACHMENT OF THERMAL MATERIALS TO A PROCESSOR DIE. 
    
    
     FIELD 
     The embodiments disclosed herein relate to electronic devices and more particularly to the dissipation of heat generated by microprocessors. 
     BACKGROUND 
     In operation, microprocessors and other electronic devices generate heat. Excess heat can damage the device if it is not dissipated. Therefore, generally microprocessors and other heat-generating electronic devices utilize heat dissipating structures or heat sinks as a conductor to dissipate excess heat. A conventional configuration for dissipating heat from a microprocessor is to mount a heat sink of a metal material (such as aluminum or copper) over the microprocessor. Mounting a metal heat sink directly over the microprocessor is not a favored practice, because of the poor conductivity achieved by the union of the metal heat sink and the microprocessor. In addition, the surface of the heat sink material is generally comprised of micro-pores or surface roughness and the surface of the microprocessor has a crown shape. Accordingly, the union of a heat sink and the microprocessor is not uniform leading to the presence of air pockets and poor thermal conductivity. Therefore, a thermal interface material, such as thermal grease, a thermal elastomer, or a phase-change material is interposed between the microprocessor and the heat sink. The thermal interface material provides improved thermal conductivity between the processor and the heat sink. The thermal interface material tends to fill the micro-pores and therefore makes the transition between the microprocessor and the heat sink more uniform. 
     A microprocessor or other heat-generating electronic device generally is affixed to a printed circuit board (PCB). In the case of a microprocessor, a heat sink is usually affixed to the PCB through bolts or screws with an established gap or bond line thickness between the heat sink and the microprocessor. In portable computer applications, for example, the bond line thickness associated with conventional microprocessor packaging is approximately 5 mils±2 mils, the difference generally attributable to differences in microprocessor heights. It is desirable, in one sense, to establish a consistent bond line thickness. One way this is established is by securing the heat sink to the PCB under pressure. The amount of pressure that may be applied to heat sink affixation is limited, however, to about 20 to 100 pounds per square inch to avoid damage to the microprocessor. The amount of compression that a thermal interface material can withstand is also limited. Thermal interface material under compression tends to flow out of the gap between the heat sink and the microprocessor under compression and additionally tends to dry out with power cycling. The compressive limitation of the thermal interface material reduces the reliability of the thermal interface material. 
     Despite its limitations, it is desirable to use thermal interface material between a heat sink and a microprocessor or other heat-generating electronic device. What is needed is a configuration whereby thermal interface material may be utilized and the reliability issues present in prior art configurations can be avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of a packaged microprocessor according to an embodiment of the invention. 
     FIG. 2 is a planar bottom view of the structure of FIG.  1 . 
     FIG. 3 is a schematic side view of a second embodiment of a heat sink according to the invention. 
     FIG. 4 is a planar bottom view of the structure of FIG.  3 . 
     FIG. 5 is a schematic side view of a portion of a heat sink over a microprocessor according to a third embodiment of the invention. 
     FIG. 6 is a planar bottom view of the structure of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     The embodiments disclosed herein relate to a heat sink comprising a protuberance having a thickness defining a distance between a heat generating structure and a heat sink. In this manner, utilizing the heat sink as a heat dissipating element in conjunction with a microprocessor affixed to a printed circuit board, the protuberance defines a volume for a thermal interface material between the heat sink and a heat generating electronic device such as the microprocessor. A desired bond line thickness may be established between a heat sink and a heat generating electronic device to improve the thermal resistance of the thermal interface material and the heat sink and provide consistency of thermal resistance between packages. An apparatus incorporating such a heat sink is also described. 
     FIG. 1 shows a schematic side view of a packaged electronic device such as a microprocessor. In FIG. 1, microprocessor  10  (or other heat generating device) includes socket  12  that is mounted on printed circuit board  14  utilizing, for example, ball grid array  16 . Heat sink  18  is attached to printed circuit board  14  through supports  27 . Heat sink  18  is, for example, a block or plate of a metal such as aluminum or copper. Heat sink  18  is attached to printed circuit board  14  in a position that defines bond line thickness or gap  20  between bottom surface  22  of heat sink  18  and top surface  24  of microprocessor  10 . Thermal interface material  26 , such as a thermal grease, elastomer, or phase-change material, or other thermally conductive material  26  spans gap  20  and defines heat conducting path  28  from microprocessor  10  to heat sink  18 . A quantity of thermal interface material such as a thermal grease, elastomer, phase-change material or other material sufficient to fill thermal gap  20  and provide an adequate thermal path for heat generated by microprocessor  10  is shown. 
     In the embodiment shown in FIG. 1, heat sink  18  includes protuberances  28  defining gap  20  between heat sink  18  and microprocessor  10 . Protuberances  28  establish gap  20  at a desired fixed height. In this manner, thermal interface material  26  may be positioned between heat sink  18  and microprocessor  10  without being subject to compression that can cause squeezing out and drying of the material. Protuberances  28  also establish a consistent bond line thickness or gap  20  between different units, so that the same thermal gap is consistently established to consequently establish a consistent adequate thermal path among packaged microprocessors. 
     In the embodiment shown in FIG. 1, supports  27  such as pins or bolts are securely attached to heat sink  18  at the upper end and pass through four corresponding holes in printed circuit board  14  to affix heat sink  18  to printed circuit board  14 . In one embodiment, supports  27  utilize locking clips and coil spacer springs surrounding the supports to provide a consistent tension between heat sink  18  and printed circuit board  14 . This tension is not reflected against thermal interface material  26  as protuberances  28  shield thermal interface material  26  from any pressure applied by supports  27 . 
     FIG. 2 shows a planar bottom side view of heat sink  18  having protuberances  28 . In one embodiment, heat sink  18  is a metal such as aluminum or copper formed by a die-casting method. Protuberances  28  may also be formed according to die-casting techniques known in the art. Protuberances are formed to a height or thickness, in one embodiment, of approximately 5 mils for use with modern microprocessors and a desirable bond line thickness as known in the art. In this manner, protuberances  28  may be considered dimples in a surface of heat sink  18 . Cooling mechanism  25  such as a chain transfer mechanism as known in the art may be incorporated in heat sink  18  to dissipate heat from heat sink  18  to a fan or the environment as known in the art. 
     FIG. 3 shows a second embodiment of a heat sink according to the invention. In this embodiment, a surface of heat sink  180  includes protuberance  280  that is a frame having four sides extending from a surface of heat sink  180 . FIG. 4 shows a bottom planar view of the second embodiment of the invention. As shown in FIG. 4, protuberance  280  consists of a frame having four sides defining opening  285  for thermal interface material. Similar to the embodiment shown in FIGS. 1 and 2, protuberance  280  allows thermal interface material to reside in opening  285  between a microprocessor and the bottom surface of heat sink  180  without being subject to compression. Protuberance  280  of a frame, in one embodiment, is established at a bond line thickness of approximately 5 mils. In one embodiment, protuberance  280  is formed utilizing die-casting techniques along with at least the bottom surface of heat sink  180 . 
     It is to be appreciated that the embodiment illustrated in the figures represent, in particular, two configurations of a heat sink having a suitable protuberance or protuberances to establish a bond line thickness and allow thermal interface material to be placed between the heat sink and the microprocessor without compression. Many other configurations of protuberances, including protuberances that are not die-cast in the heat sink but are separate components may be utilized. 
     FIG. 5 shows still another embodiment wherein a recess is formed in the heat sink to provide a die-referenced bond line thickness between the heat sink and a microprocessor. FIG. 5 shows heat sink  380  having recess  375  over a portion of microprocessor  310 . Between heat sink  380  and microprocessor  310  in recess  375  is thermal interface material  320  such as a thermal grease. The recess defines a volume and the walls of the recess trap the thermal interface material over microprocessor  310 , inhibiting grease migration during power or temperature cycles. Heat sink  380  contacts microprocessor  310  at contact points  370 . FIG. 6 shows a planar bottom view of heat sink  380 . 
     In the preceding detailed description, the invention is described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.