Abstract:
A low profile heat removal system suitable for removing excess heat generated by an integrated circuit operating in a compact computing environment is disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation of U.S. application Ser. No. 13/223,224 filed Aug. 31, 2011 entitled CONSOLIDATED THERMAL MODULE, and claims priority to and the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/503,512, filed Jun. 30, 2011, entitled CONSOLIDATED THERMAL MODULE which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present invention relates generally to the cooling of computer components. More particularly an apparatus is described for reducing the size, weight, footprint, and cost of a thermal module. 
       BACKGROUND 
       [0003]    Computer cooling keeps components within safe operating limits by removing waste heat. In some cases the Central Processing Unit (CPU) alone needs over 100 W of power, which must then be dissipated. Most computers remove the waste heat by using at least one of the following thermal modules: heat sinks, fans, water cooling, heat pipes, or phase change cooling. Conventional desktop computer designs have a relatively enough space for a large heat sink, and fan for regulating the operating temperature of an Integrated Circuit (IC). These conventional designs also include an independent loading mechanism (ILM), which when fastened secures the IC into an IC socket. Unfortunately, the ILM only comes into contact with the IC at 2 discrete points, resulting in uneven loading on the IC. The combination of both the thermal module and the ILM also requires multiple attachment positions on the printed circuit board (PCB) it is attached to. The attachment positions for the ILM fall outside of the footprint of the IC as they typically screw into a steel backer plate located below the PCB. The attachment positions for the thermal module fall even farther from the IC since they must fall outside of the footprint of the ILM. Unfortunately, because the screw attachments are located significantly outside the footprint of the IC they put a significant amount of torque on the PCB. Unopposed torque on the PCB below the IC could result in bending or crowning of the PCB, and could also prevent IC pins from seating properly. This means it is crucial for the backer plate to be strong enough to oppose the torque created at the attachment points. In addition to being rather tall, this attachment configuration also takes up a lot of board space on the PCB. 
         [0004]    Small form factor computers typically use the same processors as their larger desktop counterparts. Unfortunately, as discussed above, all the components that are required to cool a desktop class CPU take up a significant amount of room. Space or volume is at a premium in small form factor computer environments and it is essential that any heat removal system must be able to maximize heat transfer while minimizing the space occupied Therefore a way to reduce the space taken up by the CPU cooling components in a small form factor computer is desired. 
       SUMMARY 
       [0005]    This paper describes many embodiments that relate to a method and apparatus for the manufacture and implementation of a consolidated thermal module. 
         [0006]    A low Z profile consolidated thermal module (CTM) is disclosed. The CTM is designed to both secure and cool an integrated circuit (IC) mounted to a printed circuit board (PCB). The CTM includes a number of components including: a heat removal assembly having a reduced footprint, a retaining mechanism, a backer plate and at least one fastener. The heat removal assembly is disposed on a first surface of the PCB, and in thermal contact with the integrated circuit. The retaining mechanism is disposed on a second surface of the PCB. The backer plate is disposed between the retaining mechanism and the PCB. At least one fastener is used to secure the heat removal assembly to the retaining mechanism, where the retaining mechanism causes a substantially uniform retaining force to be applied across the backer plate thereby minimizing an amount of torque applied to the IC. 
         [0007]    In another embodiment a method for installing a consolidated thermal module (CTM) in a small form factor computer is described. The method can be carried out by performing at least the following operations: receiving a number of CTM components including at least a backer plate, a retaining mechanism, and a heat removal assembly; receiving a printed circuit board (PCB) for a small form factor computer; placing the backer plate on a first side of the PCB; placing the retaining mechanism beneath the backer plate; placing the heat removal assembly on a second side of the PCB, in thermal contact with the IC; and securing the retaining mechanism to the heat removal assembly. 
         [0008]    In yet another embodiment, an apparatus for installing a consolidated thermal module in a computing device during an assembly operation is disclosed. The apparatus includes at least means for receiving a plurality of CTM components including at least a backer plate, a retaining mechanism, and a heat removal assembly, means for receiving a printed circuit board (PCB) for a small form factor computer, means for placing the backer plate on a first side of the PCB, means for placing the retaining mechanism beneath the backer plate, means for placing the heat removal assembly on a second side of the PCB, in thermal contact with the IC, and means for securing the retaining mechanism to the heat removal assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention and the advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings. 
           [0010]      FIG. 1A  shows a cross-section of the conventional design. 
           [0011]      FIG. 1B  shows a perspective view of the forces exerted by the conventional design. 
           [0012]      FIG. 2A  shows a cross-section of the consolidated thermal module m accordance with the described embodiments. 
           [0013]      FIG. 2B  shows a perspective view of the forces exerted by the consolidated thermal module design, in accordance with the described embodiments. 
           [0014]      FIGS. 3A and 3B  show perspective views of both the top and bottom of the consolidated thermal module. 
           [0015]      FIGS. 4A and 4B  show two examples of design limitations for small form factor computer enclosures. 
           [0016]      FIG. 5  shows a flowchart detailing a manufacturing process  500  for installing a consolidated thermal module into a small form factor computer. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The present invention relates generally to the cooling of computer components. More particularly an apparatus is described for reducing the size, weight, footprint and cost of a thermal module. 
         [0018]    In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the present invention. 
         [0019]    Computer cooling is used to keep components within safe operating limits by removing waste heat. In some cases a Central Processing Unit (CPU) alone needs over 100 W of power, which must then be dissipated. Cooling methods generally include at least one of a number of available thermal modules including: heat sinks; fans; water cooling; heat pipes; or phase change cooling. Conventional desktop computer designs have plenty of space for a large heat sink, and fan for regulating the operating temperature of an Integrated Circuit (IC). These conventional designs also include an independent loading mechanism (ILM), which when fastened secures the IC into an IC socket with about 100 pounds of force. The ILM is useful in a desktop computer as it allows the IC to be relatively easily removed by an end user. Unfortunately, in order to leave room for contact between the discrete thermal module and the IC, the ILM concentrates about 100 pounds of force on the IC in 2 small contact positions, resulting in uneven loading on the IC. The combination of both the thermal module and the ILM also requires multiple attachment positions on the printed circuit board (PCB). The attachment positions for the ILM fall outside of the footprint of the IC as they simply screw into a backer plate located below the PCB. The attachment positions for the thermal module fall even farther from the IC since it has to fit around the ILM. Unfortunately, because the fasteners are located significantly outside the footprint of the IC they put a significant amount of torque on the PCB. This means that even a steel backer plate must be rather thick, since it must be strong enough to oppose the large torque moment created by the force applied at the attachment points. This attachment configuration also takes up a lot of space on the PCB, and prevents supporting components from being place close to the CPU. 
         [0020]    Small form factor computers use desktop class ICs in enclosures much smaller than the desktop cases they were designed for. In many designs the enclosure is not only smaller but is also integrated into a display unit. The reduction in enclosure size, and proximity to the heat producing display unit, make thermal management much more challenging than in conventional desktop computers. Not only is there more heat in a more limited space but desktop class ICs must typically be maintained at significantly lower temperatures than their mobile counterparts. Because, thermal management is so challenging in the design of small form factor computers, heat pipes are used to help efficiently remove waste heat from ICs. A heat pipe is a heat transfer mechanism that can transport large quantities of heat with a very small difference in temperature between the hotter and colder interfaces and is therefore well suited for compact computing environments. A typical heat pipe consists of a sealed pipe or tube made of a material with high thermal conductivity such as copper or aluminum. The heat pipe includes a working fluid, (or coolant), chosen to match the operating temperature of the compact computing device. Some example fluids are water, ethanol, acetone, sodium, or mercury. (Clearly, due to the benign nature and excellent thermal characteristics, water is used as the working fluid in consumer products such as laptop computers.) Inside the heat pipe&#39;s walls, an optional wick structure exerts a capillary pressure on the liquid phase of the working fluid. The wick structure is typically a sintered metal powder or a series of grooves parallel to the heat pipe axis, but it may be any material capable of exerting capillary pressure on the condensed liquid to wick it back to the heated end. It should be noted, however, that the heat pipe may not need a wick structure if gravity or some other source of acceleration is sufficient to overcome surface tension and cause the condensed liquid to flow back to the heated end. 
         [0021]    Space or volume is at a premium in compact computer environments and it is essential that any heat removal system must be able to maximize heat transfer while minimizing the space occupied. One way to further maximize space inside the small form factor computer would be to reduce the size of the cooling unit, while maintaining the amount of heat removed. Unfortunately, there has not been much effort made towards reducing the size of the cooling unit, since in most desktop applications the current overall size is not a problem. A design which consolidates the ILM and thermal module of the conventional unit solves the following problems: (1) it can allow an overall size and weight reduction of up to 50%; (2) it can significantly lower the overall production costs; and (3) it can increase the reliability of the system by placing more uniform loads on the IC and PCB. 
         [0022]      FIG. 1A  show a cross-section of the conventional thermal cooling system that includes at least ILM  102 . ILM  102  is attached to PCB  104  by a number of fasteners such as screws which are anchored through PCB  104  and backer plate  106 . As can be seen, backer plate  106  has a substantially greater footprint than does IC  108 . It should be further noted that the overall footprint of ILM  102  and backer plate  106  takes up substantial amounts of PCB real estate and reduces the flexibility of circuit traces formed on the PCB. Note how force  110  imparted by ILM  102  is imparted in the middle of integrated circuit (IC)  108  at about midpoint of IC  108 , and at ILM attachment positions  112  that is outside the footprint of IC  108 ; these forces can result in a significant amount of torque on PCB  104 . Discrete thermal module  114  is also connected to PCB  104  and backer plate  106 . Discrete thermal module  114  typically includes springs inside of the spring receiver channels  116  used to impart a precise amount of force at thermal module attachment points  118 . The attachment of discrete thermal module  114  puts additional torque on PCB  104  at thermal module attachment points  118  because the position of thermal module attachment points  118  at distance d 2  from the center of IC  108  creates a large moment arm that is only opposed directly over IC  108  by distribution of force  120 . Distances d 1  and d 2  clearly show the length of the moment arms created by the force at ILM positions  112 , and thermal module attachment points  118 . All this torque on PCB  104  is especially problematic as IC  108  is electrically attached to socket  122  with a number of signal pins  124 . In many cases IC  108  can have over a thousand signal pins  124 . If PCB  104  is bent or deformed by the applied moments the deformation can cause undesirable forces to be exerted on signal pins  124  which can dislodge or interrupt the signal they carry. In another more extreme case PCB  104  itself could crack from the applied moments. Thus the selection of a thick backer plate  106  is clearly critical for the conventional configuration. 
         [0023]      FIG. 1B  shows a perspective view of the forces being exerted by the conventional design. In  FIG. 1B  a top layer represents the top side of IC  108  and a bottom layer represents the bottom side of backer plate  106 . In  FIG. 1B  the large discrete loading of the ILM can be seen in conjunction with distribution of force  120  imparted by the discrete thermal module. In the bottom layer of  FIG. 1B  note particularly how much bigger the area of the PCB being acted upon is than the area occupied by CPU  108  itself. Also note that the PCB is being acted on at 7 discrete points; 3 from the ILM and 4 more from the discrete thermal module. 
         [0024]      FIG. 2A  shows a cross-section of the consolidated thermal module (CTM) in accordance with the described embodiments. Thermally conductive casting  202  of the CTM can be made from many possible materials such as cast aluminum. Thermally conductive slug  204  and heat pipe  206  are both attached to thermally conductive casting  202 . Thermally conductive slug  204  can be made of thermally conductive material along the lines of copper and can be in direct contact with the top side of IC  208 . Slug  204  conducts heat from IC  208  to heat pipe  206 . Heat pipe  206  can be arranged to overlay the entire area of thermally conductive slug  204  that is in contact with IC  208 . This allows for an efficient conduction of heat from IC  208  to heat pipe  206 . Since a retaining mechanism, embodied here by leaf spring  210 , provides the requisite tension there is no need for springs like those used to set tension at the conventional design&#39;s thermal module attachment points  118 . In this way, fastener attachment points  212 , which connect leaf spring  210  and thermally conductive casting  202  through PCB  214 , can have a much lower Z profile than the spring receiver channels  116  required in the conventional design. As shown more clearly below in  FIG. 3B , leaf spring  210  can be designed to exert an evenly distributed, predetermined leaf spring force  216  across backer plate  218 . In response to leaf spring  210  exerting leaf spring forces  216 , thermally conductive slug  204  can respond by exerting distributed CTM force  220  to IC  208 . Since components of leaf spring force  216  are exerted directly onto IC  208 , the distances d 1  and d 2  from the conventional design are significantly reduced to a single, much shorter distance d CTM . The application of leaf spring force  216  at distance d CTM  advantageously positions leaf spring force  216  directly beneath a number of signal pins  222 . Because leaf spring force  216  is exerted directly below signal pins  222 , the likelihood of dislodging or disrupting the connections is much reduced. 
         [0025]      FIG. 2B  more clearly shows force distribution  216  across the bottom of backer plate  218  from a perspective view. In  FIG. 2B  a top layer represents the top side of IC  208  and a bottom layer represents the bottom side of backer plate  218 . The bottom layer of  FIG. 2B  shows how much the introduction of the leaf spring helps to even out the load across the bottom of the backer plate. Instead of having point forces like the conventional design, leaf spring  210  (not shown) spreads the force out over two lines of force. The two rows of exerted leaf spring force  216  are within the confines of IC  208  (not shown). Therefore, the use of leaf spring  210  (not shown) enhances the flexibility in determining the force pattern, and although the described embodiment shows two lines of force under the chip, many other force configurations might be desirable when applied to different computing environments and should be considered a part of this disclosure. Another advantage of having two solid lines of force is that the torque is limited to single torque axis  224 . This could allow further refinement and size reduction of backer plate  218  since backer plate  218  can be specifically designed to counter torque in that specific axis. 
         [0026]      FIGS. 3A and 3B  show perspective views of both the top and bottom of the CTM. In  FIG. 3A  heat pipe  206  can be seen running along a channel in thermally conductive casting  202 . In one embodiment the other end of heat pipe  206  could run over to a heat exchanger. Also visible in  FIG. 3A  are fastener attachment points  212 . Fastener attachment points  212  each pass through a hole in PCB  214 . It should be noted that fastener attachment points  212  only attach thermally conductive casting  202  and leaf spring  210 ; they do not exert any force on PCB  214 . Socket  302  for IC  208  is visible beneath thermally conductive casting  202 . Socket  302  has channels for securing signal pins  222  of IC  208 . In  FIG. 3B  the relative size and thickness of leaf spring  210  and backer plate  218  can be seen. Notice that in this embodiment backer plate  218  is positioned primarily underneath socket  302 . Fastener attachment points  212  are also visible, although in this diagram they are shown in an unfastened position. This helps to demonstrate that leaf spring  210  must bend to attach to fastener attachment points  212 . The bending of the leaf spring at each of the four fastener attachment points  212  is what actually creates leaf spring force  216 . 
         [0027]      FIGS. 4A and 4B  show two examples of design limitations for small form factor computer enclosures such as the iMac® manufactured by Apple Inc. of Cupertino, Calif. Layouts for components inside of two different small form factor computer enclosures are shown. In each enclosure there is a critical stack height that cannot be exceeded. A critical stack might include components such as integrated circuits, memory modules, and heat exchangers. In  FIG. 4A  exceeding the critical X stack height  402  would result in having to extend the length of PCB  404 , to conform closely to the outline of component  406 . As can be seen in  FIG. 4A  the size of CTM  408  directly affects the stack height. In addition to having to pay for a larger PCB  404 , the bottom portion of the board would have to be shaved off to get it to fit inside the enclosure, thus creating an extra manufacturing step and wasting materials. In  FIG. 4B , critical Y stack height  410  must be achieved in order to properly position component  412  in the enclosure. In both examples by reducing the size of CTM  408 , the critical X or critical Y stack heights can be achieved. 
         [0028]    In summary, a design which consolidates the ILM and thermal module of the conventional unit solves many problems. As shown in  FIGS. 4A and 4B , reducing the size of the CTM achieves important objectives. Specifically the significantly reduced footprint taken up on the board (compare  FIGS. 1A and 2A ) allows for the critical stack heights to be achieved. A smaller footprint also allows more flexibility in placing the CTM on the PCB since it becomes easier to fit the CTM between other important components. The shortened distance between the edge of the IC and the end of the CTM makes it possible to position IC power supplies closer to the IC. The reduction in the number of attachment screws creates fewer holes in the PCB, allowing for more flexibility in routing electrical traces. Additionally, the reduced height of the CTM itself allows a thinner enclosure size in the Z direction. Finally, production costs are reduced for the following reasons: the backer plate can require less material; the cost of producing the ILM is recovered; the attachment process involves fewer fasteners; and the aluminum casting can be smaller. 
         [0029]      FIG. 5  shows a flowchart detailing a manufacturing process  500  for installing a CTM. Process  500  begins at step  502  by receiving a CTM. In step  504  a PCB for a small form factor computer is received. In step  506  the backer plate is placed on the back side of the PCB. In step  508  the leaf spring is placed on the backer plate. In step  510  the heat removal assembly is placed on the front side of the PCB. In step  512  the leaf spring is secured to the heat removal assembly. 
         [0030]    The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. 
         [0031]    The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.