Patent Publication Number: US-6707673-B2

Title: Heat sink for chip stacking applications

Description:
This application is a divisional application of application Ser. No. 09/741,820, filed Dec. 22, 2000, now U.S. Pat. No. 6,449,161, which is a continuation application of application Ser. No. 09/178,480, filed on Oct. 26, 1998, now U.S. Pat. No. 6,201,695, both of which are hereby incorporated by reference. This application is a also related to application Ser. No. 09/741,822, filed Dec. 22, 2000, now U.S. Pat. No. 6,319,756, which is a continuation application of application Ser. No. 09/178,480 U.S. Pat. No. 6,201,695, filed on Oct. 26, 1998. All of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of integrated circuits. More particularly, the invention provides a heat sink for use with stacks of integrated circuits. 
     2. Description of the Related Art 
     As computer manufacturers have attempted to build more powerful machines, the use of chip stacks in modern computing applications has become increasingly desirable. The term ‘chips’ used with the present invention is intended to include any packaged integrated circuit device including processing devices e.g. microprocessors etc., memory devices e.g. DRAMS, SRAMS, etc., and the like. In essence, a chip stack comprises multiple integrated circuit packages which are stacked together (back-to-front or back-to-back). The chip stacks may be oriented either in face up position or in a side-to-side orientation with chip edges down. 
     There are a number of advantages to the chip stack configuration over conventional single chip mounting arrangements. In particular, the chip stacks provide a more compact circuit arrangement for computers and other high speed electronic systems. 
     In addition, chip stacks particularly allow for more efficient use of space on circuit boards. The stack takes advantage of relatively less valuable space above the circuit board, while at the same time leaving a small footprint on a circuit board or card, thereby increasing the space available for other components or chip stacks. 
     While there are numerous advantages to a stacked chip configuration, there are also associated problems. Specifically, larger and larger chip stacks create unique cooling problems. Because the chip stacks contain multiple chips, they generate more heat per unit volume, requiring greater heat dissipation, while at the same time providing significantly smaller surface areas which may be used as a heat sink. In view of this problem, the general response in the industry to the need for cooling chip-stacks has been to immerse the entire chip-stack in liquid or to operate at greatly reduced power levels. This is often an unwelcome solution because of technical concerns and also because of customer and user preferences. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed at providing a relatively low cost heat sink for dissipating heat generated within chip stacks (sometimes referred to as ‘chip cubes’, although a cubic structure is not necessary). The invention provides a heat absorbing surface between at least a first and second chip within a chip stack which is connected to a heat dissipating surface located outside the stack. According to a preferred embodiment, the heat sink includes one or more heat absorbing sections for respective insertion between chips within one or more chip stacks; a heat transfer section for transferring heat away from the absorbing sections; and a heat dissipating section for commonly dissipating heat transferred from the heat absorbing sections. 
     These and other features and advantages of the invention will become more readily apparent from the following detailed description of preferred embodiments of the invention which are provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a heat sink of a first embodiment of the invention. 
     FIG. 2 is a side view of the heat sink shown in FIG.  1 . 
     FIG. 3 is a perspective view of the heat sink of FIG. 1 secured to chips on a chip mounting surface. 
     FIG. 4 is a side view of the heat sink configuration shown in FIG.  3 . 
     FIG. 5 is a side view of a first alternative heat sink configuration of the invention. 
     FIG. 6 is a side view of a second alternative heat sink configuration of the invention. 
     FIG. 7 is a side view of a third alternative heat sink configuration of the invention. 
     FIG. 8 is a perspective view of a preferred embodiment of the invention. 
     FIG. 9 is a perspective view of a fourth alternative embodiment of the invention. 
     FIG. 10 is a perspective view of a fifth alternative embodiment of the invention. 
     FIG. 11 is a perspective view of a sixth alternative embodiment of the invention. 
     FIG. 12 is a perspective view of a second preferred embodiment of the invention. 
     FIG. 13 is a side view of the second preferred embodiment shown in FIG.  12 . 
     FIG. 14 is a side view of an alternative embodiment of the second preferred embodiment shown in FIGS. 12 and 13. 
     FIG. 15 is a block diagram of a processor system in which the invention may be utilized. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a planar heat sink  20  in accordance with a preferred embodiment of the invention will now be described. Heat sink  20  is shown including three interconnected co-planar sections: a heat absorbing section  22 , a heat transfer section  24 , and a heat dissipating section  26 . FIG. 2 is a side view of heat sink  20  showing the relative lateral dimensions of the heat absorbing section  22 , heat transfer section  24 , and heat dissipating section  26 . 
     Heat absorbing section  22  includes one or more fingers  22   a - 22   h,  each of which is configured to be insertable between chips of a chip stack. Heat transfer section  24  includes heat transfer elements  24   a - 24   h.  The fingers  22   a - 22   h  remove heat from the chip stack, which flows through respective heat transfer sections  24   a - 24   h  to dissipation section  26 . Preferably, fingers  22   a - 22   h  are of a generally rectangular shape and sized to maximize heat absorption from a target chip&#39;s surface. As shown in FIG. 1, the width of the fingers  22   a - 22   h  is larger than the width of the heat transfer elements  24   a - 24   h.  Alternatively, the width of the fingers  22   a - 22   h  may be the same, narrower or wider than the width of the heat transfer elements  24   a - 24   h.    
     Preferably, heat sink  20  is comprised of a thermally conductive material having a thermal rate of expansion approximately equal to the thermal expansion rate of the stacked chips. In accordance with a preferred embodiment, heat sink  20  is comprised of a metal such as aluminum or copper and may be easily stamped out of plate metal. In alternative embodiments, it is possible for each of the three sections of heat sink  20  to be formed of different materials in accordance with each section&#39;s functional constraints. 
     With reference to FIGS. 3,  4 , and  8 , heat sink  20  is utilized by placing each heat absorbing finger  22   a   14   22   h  over a first layer chip  30  secured on a mounting surface  28  such as a plug-in board having edge connectors  27 . A second layer of chips is then secured over each heat absorbing finger  22   a - 22   h . Each finger  22   a - 22   h  may be affixed to the first and second layer chips  30 ,  32  using thermally conductivity enhancing mediums such as a thermal paste or epoxy. With reference to FIGS. 3 and 4, heat sink  20  is shown with each finger  22   a - 22   h  placed over a respective first layer chip  30  with the heat transfer elements  24   a - 24   h  and heat dissipating section  26  extending away from the location of the chips  30 ,  32 . As shown in FIG. 8, a second layer of chips  32  is provided over the first layer of chips  30  with each respective finger  22   a - 22   h  positioned between each pair of first and second layer chips  30 ,  32 . As shown, the heat transfer elements  24   a - 24   h  and heat dissipating section  26  are provided outside the chip stack  33  created by the first and second layers of chips  30 ,  32 . In an alternative embodiment, the heat transfer elements  24   a - 24   h  may be provide so that at least a portion of the heat transfer elements  24   a - 24   h  lie within the chip stack  33 . 
     With reference to FIGS. 3,  4 , and  8 , the heat transfer and heat dissipating sections  24 ,  26  are shown provided coplanar with heat absorbing section  22 . As shown in FIGS. 5-7, the heat transfer and heat dissipating sections  24 ,  26  may extend from the heat absorbing section  22  at any angle necessary to take advantage of unused space above and below the chip stack. With reference to FIG. 5, an alternative embodiment is shown in which heat dissipating section  26  is at approximately a 45 degree angle to the heat absorbing section  22 . With reference to FIG. 6, an alternative embodiment is shown in which the heat dissipating section  26  is orthogonal to the heat absorbing section  22 . With reference to FIG. 7, an alternative embodiment is shown in which the heat dissipating section  26  is initially orthogonal to the heat absorbing section  22  and then is bent again be in parallel with the heat absorbing section  22  at a point above the heat absorbing section. 
     With reference to FIG. 9, an alternative embodiment is shown in which the heat dissipating section  26  is comprised of heat dissipating fins  34  in order to further enhance heat dissipation by enlarging the surface area of section  26 . With reference to FIG. 10, an additional alternative embodiment is shown in which the heat dissipating section  26  is formed as corrugation waves  36  in order to increase surface area and heat dissipation. 
     With reference to FIG. 11, an alternative embodiment is shown in which heat sink  27  includes a heat dissipating section  26  in thermal contact with a pair of heat transfer sections  24 ,  25  and a pair of heat absorbing sections  22 ,  23 , which extend along both sides of heat dissipating section  26 . As shown in FIG. 11, heat transfer sections  24  and  25  respectively contain heat transfer elements  24   a - 24   h  and  25   a - 25   h,  and heat absorbing sections  22  and  23  respectively contain heat absorbing elements  22   a - 22   h  and  23   a - 23   h.    
     With reference to FIGS. 12 and 13, a second preferred embodiment is shown in which a pair of planar heat sinks  37 ,  39  are used together to dissipate heat from chip stacks  33 ,  35  positioned on each side of mounting surface  28 . Alternatively, as shown in FIG. 14, a single continuous heat sink  41  may be used to dissipate heat from chip stacks  33 ,  35  positioned on each side of mounting surface  28 . 
     One particular environment in which the heat sink of the invention may be used is within a memory module for a processor-based system. In this case, the integrated circuits  30 ,  32  may be integrated circuit memory devices such as DRAMS, SRAMS, EEPROM, etc. and the mounting surface  28  may be constructed as a plug-in board such as a SIMM (Single In-Line Memory Module), DIMM (Dual In-Line Memory Module), SO-SIMM (Small Outline-Single In-Line Memory Module), SO-DIMM (Small Outline-Dual In-Line Memory Module), RIMM (Rambus In-Line Memory Module) or other plug-in module, for receipt in a system memory socket. 
     A typical processor-based system, which includes the present invention formed as a memory module, is illustrated generally at  640  in FIG. 15. A processor-based system typically includes a processor, which connects through a bus structure with memory modules, which contain data and instructions. The data in the memory modules is accessed during operation of the processor. This type of processor-based system is used in general purpose computer systems and in other types of dedicated processing systems, e.g. radio systems, television systems, GPS receiver systems, telephones and telephone systems to name a few. 
     Referring to FIG. 15, such a processor-based system generally comprises a central processing unit (CPU)  644 , e.g. microprocessor, that communicates to at least one input/output (I/O) device  642  over a bus  652 . A second (I/O) device  646  is illustrated, but may not be necessary depending upon the system requirements. The processor-based system  640  also may include a static or dynamic random access memory (SRAM, DRAM)  648  in the form of memory modules of the kind described and illustrated above, a read only memory (ROM)  650  which may also be formed in the form of memory modules of the kind described above. The processor-based system may also include peripheral devices such as a floppy disk drive  654  and a compact disk (CD) ROM drive  656 , which also communicate with CPU  644  over the bus  652 . It must be noted that the exact architecture of the processor-based system  600  is not important and that any combination of processor compatible devices may be incorporated into the system. Each of the memories  648  and  650  may be constructed as plug-in modules employing a heat sink constructed in accordance with the teachings of the invention. 
     The above description and accompanying drawings are only illustrative of preferred embodiments, which can achieve and provide the features and advantages of the present invention. It is not intended that the invention be limited to the embodiments shown and described in detail herein. For instance, the present invention is described only with respect to stack of two chips stacked vertically. Alternatively, the present invention may be used with any number of stacked chips, which may be stacked in a vertical, horizontal, or in side-by-side fashion. Accordingly, the invention is not limited by the foregoing description but is limited only by the spirit and scope of the appended claims.