Abstract:
This application relates to a low profile, small footprint cooling stack that does not extend substantially beyond a footprint of an integrated circuit to which it is affixed. The cooling stack utilizes a number of beam springs that supply a seating force to the integrated circuit by way of a metal slug. In some embodiments, a bottom surface of the metal slug can be contoured in accordance with a top surface of the integrated circuit and/or socket. In other embodiments a gap between peripheral portion of a bottom surface of the metal slug and an associated printed circuit board can be filled by a layer of foam to reduce auditory signals generated by the integrated circuit.

Description:
FIELD 
     The described embodiments relate generally to methods for removing heat from integrated circuits within compact computing device housings. More particularly, the present embodiments relate to low profile heat removal device. 
     BACKGROUND 
     Integration of graphics processing units and central processing units into a single integrated circuit package has caused substantial increases in die sizes. In most computing devices, integrated circuits require some kind of cooling component to dissipate heat from the integrated circuits. Components for attaching cooling component to the integrated circuit can cause substantial increases in a stack height above the integrated circuit. While this may not be problematic in more traditional tower or desktop computing applications, impact upon a slim form factor of portable computing devices can be quite problematic. While some solutions have been utilized that reduce an overall stack height above the integrated circuit, these solutions tend to require additional board area. Unfortunately, when the printed circuit board to which the integrated circuit is mounted has a high packing density, board space for such a mounting component may not be available. 
     Therefore, what is desired is a low profile, small footprint cooling stack. 
     SUMMARY 
     This paper describes various embodiments that relate to a low profile, small footprint cooling stack. 
     A cooling stack for removing heat from an integrated circuit mounted on a printed circuit board (PCB) is disclosed. The cooling stack includes at least the following: a metal slug including a top surface and a channel arranged along the top surface; a heat pipe disposed within and coupled to the channel of the metal slug; a beam spring exerting a force directly to the top surface of the metal slug; and a number of fasteners, each of the fasteners configured to fasten an end of the four point beam spring to a fastening feature disposed next to a periphery of the integrated circuit. 
     A heat removal system configured to transfer heat generated by an operating component to the external environment is disclosed. The heat removal system including at least the following: a slug having a bottom surface in contact with a top surface of the operating component, the slug including a channel disposed along a top surface of the slug and extending from a first side of the slug to a second side of the slug, the second side opposite the first side; a heat pipe disposed within the channel, the heat pipe comprising lateral surfaces coupled to sidewalls defining the channel; and a number of beam springs configured to exert a force on the operating component by way of the slug, each of the beam springs having a first end and a second end, the first end coupled to a securing feature disposed proximate the first side of the slug and the second end coupled to a securing feature disposed proximate the second side of the slug. 
     A portable computing device is disclosed. The portable computing device includes at least the following: a printed circuit board (PCB); a plurality of fastening features coupled to a top surface of the PCB; an integrated circuit electrically coupled to the top surface of the PCB by a socket; a metal slug in direct contact with a top surface of the integrated circuit; a beam spring coupled to the top surface of the PCB by a number of fasteners that engage corresponding ones of the fastening features and exerting a seating force on the integrated circuit by way of the metal slug; and a heat pipe disposed within a channel arranged along a top surface of the metal slug, the heat pipe including lateral surfaces soldered to sidewalls defining the channel. 
     Other aspects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the described embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG. 1  shows a perspective view of an exemplary cooling stack suitable for use with a portable computing device; 
         FIG. 2  shows a cross-sectional view of the cooling stack of  FIG. 1 ; 
         FIG. 3  shows another cross-sectional view of the cooling stack of  FIG. 1 ; 
         FIG. 4  shows a perspective view of another cantilevered spring prior to compressing the spring against the cooling stack; 
         FIG. 5  shows a top view of a heat pipe configured to draw heat away from a number of heat emitting components; 
         FIGS. 6A-6C  show cross-sectional side views of various cooling stack embodiments; 
         FIGS. 7A-7B  show perspective views of a bottom surface of the slug depicted in  FIGS. 6B and 6C ; 
         FIGS. 8A-8E  show cross-sectional side views of a fixture configured to concurrently install two beam springs against a slug component; and 
         FIG. 9  shows a block diagram representing a method for assembling a cooling stack. 
     
    
    
     DETAILED DESCRIPTION 
     Representative applications of methods and apparatus according to the present application are described in this section. These examples are being provided solely to add context and aid in the understanding of the described embodiments. It will thus be apparent to one skilled in the art that the described embodiments 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 described embodiments. Other applications are possible, such that the following examples should not be taken as limiting. 
     In the following detailed description, references are made to the accompanying drawings, which form a part of the description and in which are shown, by way of illustration, specific embodiments in accordance with the described embodiments. Although these embodiments are described in sufficient detail to enable one skilled in the art to practice the described embodiments, it is understood that these examples are not limiting; such that other embodiments may be used, and changes may be made without departing from the spirit and scope of the described embodiments. 
     A cooling mechanism is used to keep electrical components within safe operating limits by removing waste heat. In some cases a Central Processing Unit (CPU) alone needs a substantial amount of power that must be dissipated to keep the CPU operating within normal operating parameters. The cooling mechanism generally includes at least one of a number of heat removal components including: heat sinks; fans; water cooling; heat pipes; or phase change cooling. While some computing device designs have sufficient space for a large heat removal component or components for heat removal, smaller devices may not have room for relatively large heat rejection components. In addition to size taken up by the cooling components themselves, mounting components can also occupy substantial room. Some integrated circuits require a substantial amount of force to properly seat pins of the integrated circuit within a socket. A cantilevered beam spring can be configured to couple heat removal components to an integrated circuit without adding substantially to a height of the integrated circuit. Unfortunately, a force provided by a cantilevered beam spring is directly proportional to a length of the cantilevered beam spring. For this reason, while the cantilevered beam spring can provide a low profile structure for applying force to the integrated circuit the mounting structure can extend well outside of a footprint of the integrated circuit to provide a requisite amount of force for seating the integrated circuit. 
     In one embodiment, a cantilevered beam spring can extend across the integrated circuit itself. In this way, the length of the cantilevered beam spring can be substantially contained within a footprint of the integrated circuit, thereby increasing an amount of force that can be provided by a cantilevered beam spring disposed within a constrained footprint. This configuration provides a number of advantages over a more traditional arrangement. First, an overall footprint is substantially reduced when compared with a configuration in which the cantilevered beam springs are outboard of the integrated circuit. Second, by locating mounting points closer to the integrated circuit a moment exerted upon the PCB is substantially less, thereby reducing an amount of strain experienced by the PCB. Third, when a four-point spring is utilized, a force exerted upon the integrated circuit by the cantilevered beam spring is self-leveling, substantially preventing irregular force distribution upon the integrated circuit. Fourth, in some embodiments, the cantilevered beam spring can be configured so that it does not extend above other cooling components preventing the cantilevered beam spring from adding height to a cooling stack (sometimes referred to as a heat removal system) associated with the integrated circuit. Finally, since an amount of force applied by the beam spring varies substantially linearly with an amount of bending of the beam spring, tuning the amount of force applied to the die can be accomplished by for example, increasing a height of the standoffs or changing a geometry or shape of the spring. For example, by increasing a curvature of the beam spring an amount of force exerted by the spring when flattened generally increases. 
     These and other embodiments are discussed below with reference to  FIGS. 1-9 ; however, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. 
       FIG. 1  shows a perspective view of a cooling stack in accordance with the described embodiments. The cooling stack is mounted upon printed circuit board (PCB)  100 . Standoffs  102  are mounted directly to PCB  100 . Standoffs  102  can be coupled to PCB  100  in any number of ways including by soldering, or even screwing standoffs  102  to PCB  100 . Standoffs  102  are configured to receive fasteners  104 . Fasteners  104  are configured to engage standoffs  102  to secure cantilevered beam springs  106  a fixed distance above PCB  100 , the fixed distance defined by standoffs  102 . In some embodiments, cantilevered beam springs  106  can be formed of precipitation hardened steel. Formation of beam springs  106  from precipitation hardened steel can increase an amount of pressure provided by beam springs  106  for a given lever arm length. Each of beam springs  106  can include a number of stress concentration features  108  configured to contact slug  110  and fastens  104  at predetermined positions. The depicted beam springs  106  are four point beam springs. Two of the points facilitate even engagement of a head portion of fasteners  104  with outer stress concentrators  108  of beam spring  106 . The other two points of contact are between a top surface of slug  110  and centrally disposed beam concentrators  108 . In this way a force distribution across slug  110  can be consistently applied to an integrated circuit disposed below slug  110 . 
     Slug  110  can be formed from a highly conductive material such as copper. In some embodiments, a copper alloy made from about 98% copper can be utilized, imparting a high thermal conductivity to slug  110 . Slug  110  can be further configured to conduct thermal energy from the integrated circuit to a heat distribution member. In one embodiment, as depicted, the heat distribution member is embodied as heat pipe  112 . Heat pipe  112  in turn is configured to conduct heat away from the integrated circuit. In some embodiments, each end of heat pipe  112  can be in thermally conductive contact with an array of cooling fins across which the transported heat is dissipated. Heat pipe  112  can be coupled with slug  110  by soldering heat pipe  112  to slug  110  at solder joint  114 . In some embodiments, solder joint  114  can include solder solids to enhance a robustness of solder joint  114 . This can be especially helpful to protect a thinned portion of slug  110  from experiencing a bending force that would cause the thinned portion to break of deform. Solder joints  114  essentially joins lateral edges of the heat pipe and curved sidewalls of the channel formed in the slug to reinforce the thinned portions of slug  110  under loading conditions. 
       FIG. 2  shows a cross-sectional side view of the cooling stack depicted in  FIG. 1  in accordance with section line A-A.  FIG. 2  depicts how slug  110  interacts with integrated circuit  202 . Beam springs  106  apply a seating force to die  202  through or by way of slug  110 . In this way, connectors disposed along a bottom surface of die  202  can be electrically and mechanically coupled with PCB  100  by attachment layer  204 . Attachment layer  204  can be any suitable attachment mechanism. For example, in some embodiments attachment layer  204  can be a socket with electrical contacts or in other embodiments die  202  can be SMT&#39;d directly to PCB  100 . In one particular embodiment, attachment layer  204  can be a ball grid array coupling connector pins of die  202  to electrical traces disposed upon PCB  100 . Slug  110  In some embodiments, slug  110  can have a channel, which allows heat pipe  112  to be in closer proximity to die  202 . The channel is also operable to prevent heat pipe  112  from adding to an overall height of the cooling stack. In one particular embodiment, an overall height b of slug  110  can be about 0.8 mm while a height a of a portion of the slug disposed between heat pipe  112  and die  202  can be about 0.25 mm. Solder joint  114  is also visible in this depiction. By including the aforementioned solder solids in the formation of solder joint  114 , adhesion between heat pipe  112  and slug  110  can be improved. Solder joint  114  also allows heat pipe  112  to reinforce a structural integrity of slug  110 . 
       FIG. 3  shows a cross-sectional side view of the cooling stack depicted in  FIG. 1  in accordance with section line B-B. In this view, a force distribution diagram is depicted showing forces F S  applied by beam spring  106  through stress concentrators  108  and forces F B  applied through stand offs  102 . Because the effective lever arm of beam spring  106  extends over die  202  and attachment layer  204 , the standoffs  102 , which apply force F B  to PCB  100 , can be in close proximity to a peripheral edge of attachment layer  204 . In this way, a moment applied to PCB  100  can be minimized, thereby reducing an amount of flex placed upon PCB  100 . A reduction in stress to PCB  100  can prevent early failure of PCB  100  or in some cases allow a thinner or less expensive PCB design to be used, thereby saving vertical height and/or money on fabrication of PCB  100 . It should also be noted that a curvature of stress concentrators  108  associated with fasteners  104  remove any influences on moments that could be due to a head portion of one of fasteners  104  engaging one side of beam spring  106  before another side. The curvature of stress concentrators  108  associated with fasteners  108  helps to allow fasteners  104  to concurrently engage both sides of the head portion during an attachment operation. This configuration also keeps an effective beam length or lever arm associated with beam spring  106  substantially constant while fasteners  104  engages standoffs  102 . Furthermore, in some embodiments, this configuration allows an engagement height of fasteners  104  to beam spring  106  to be adjusted without making design changes to beam springs  106 . Because the lever arms remain substantially constant some reductions and increases in engagement height can provide changes to an amount of force applied to die  202  without making substantial design changes to the cooling stack components. For example, this can be particularly advantageous in rework situations where in one case where it is subsequently determined increased force would allow increased thermal dissipation, or in another case where it is subsequently determined that the PCB is overstressed by the cooling stack. 
       FIG. 4  shows a perspective view of a partially assembled cooling stack. To provide a comparison between pre-bent and bent states of beam springs  106 , one of beam springs  106  is shown attached to standoffs  102 , while the other beam spring  106  is shown in a pre-bent state before being attached to corresponding standoffs  102 . Generally both beam springs  106  are concurrently coupled with slug  110  to prevent an application of asymmetric force to die  202  (not shown) by way of slug  110 . One end of pre-bent beam spring  106  can be positioned a distance c above a top surface of corresponding standoffs  102 . In one specific embodiment, distance c can be about three millimeters. By deforming each end of beam spring  106  a distance c and securing the ends to corresponding standoffs  102 , beam spring  106  can apply a predetermined amount of force to slug  110 . 
       FIG. 5  shows a top view of one particular configuration of a cooling stack in contact with a heat pipe  112 . Opposing ends of heat pipe  112  are in contact with fin stack  502  and fin stack  504 . In some embodiments, cooling fans can be configured to force air across fin stacks  502  and  504 .  FIG. 5  also depicts how beam spring  106 - 1  has very little clearance from protrusion  506  of heat sink  508 . In this way, a reduced size foot print of the depicted cooling stack provided by this particular configuration allows an integrated circuit associated with the cooling stack to be in close proximity to another integrated circuit associated with heat sink  508 . 
       FIGS. 6A-6C  show cross-sectional side views of a number of alternative configurations of a cooling stack.  FIG. 6A  shows a configuration in which the cooling stack includes insulation to reduce an auditory output of die  202 . This auditory output reduction can be particularly helpful when die  202  includes high frequency switching components, such as a voltage regulator with a high frequency switching component. In some embodiments, the insulation can be formed from conductive foam  602  and positioned between a bottom peripheral surface of slug  110  and a top surface of PCB  100 . Conductive foam  602  can cooperate with slug  110  to surround die  202 , thereby preventing or at least subduing an amount of sound emitted from the cooling stack. The conductive foam  602  can also be configured to create a faraday cage around die  202  when it is grounded to a top surface of PCB  100 . In this way, in addition to reducing a volume of sound output from the cooling stack, die  202  can be electrically isolated from other electrical components proximate the cooling stack. 
       FIG. 6B  shows how slug  110  can be shaped to conform with a top surface of die  202 . One set of features that can be added to slug  110  during a forming operation are cavities  604  that are complementary to protrusions of die  202 . When cavities  604  conform to a top shape of die  202 , thermal conduction of heat from die  202  to slug  110  can be increased since a larger amount of surface area of slug  119  contacts die  202 . In addition to increasing heat transfer, the conformal features of slug  119  can also be operable to align slug  119  to die  202 . In this way misalignment of slug  119  with respect to die  202  can be avoided. Because die  202  is located in a predetermined position by virtue of a fixed position of attachment layer  204 , the alignment of slug  110  with die  202  allows beam springs  106  to contact slug  110  at known positions. Another advantage of the embodiment displayed in  FIG. 6B  is that a volume of slug  119  can be increased allowing slug  119  to distribute received heat across a larger volume. In this way, slug  119  can be configured to store a relatively larger amount of heat at a given overall temperature. In this way, a temperature differential between die  202  and slug  119  can be maintained for a longer period of time, allowing efficient transfer of heat from die  202  to slug  110  for a longer period of time. Finally  FIG. 6C  shows yet another alternative embodiment in which slug  119  is shaped to conform with die  202  and includes a conductive foam  202  that provides both auditory and electrical isolation of die  202  from a surrounding system. In this way, the advantages of  FIGS. 6A and 6B  can be included in a single embodiment. 
       FIG. 7A  shows a bottom perspective view of slug  110  in accordance with the embodiment depicted in  FIG. 6C . A bottom surface of slug  110  defines cavities  604  that can have a geometry complementary to a top surface of die  202  (not shown). Slug  110  can be formed in a number of ways. In one embodiment slug  110  can be formed from a single metal block. Cavities  604  can be machined from a bottom surface of slug  110  and channel  702  (associated with heat pipe  112 ) can be machined from a top surface of slug  110 .  FIG. 7B  shows an embodiment in accordance with the embodiment depicted in  FIG. 6B . Here a ridge  704  is left along a periphery of slug  110 . In some embodiments, ridge  704  can be positioned outside of a footprint of a socket to which the die is electrically coupled. 
       FIGS. 8A-8E  show cross-sectional side views of a cooling stack during an assembly operation in which a fixturing device positions, bends and secures beam springs against a slug portion of the cooling stack. Fixturing device  800  includes body portion  802 . Body portion  802  has curved portion  804  disposed along a bottom portion of body portion  802 . In some embodiments, as depicted, curved portion  804  can have a curvature in accordance with an unbent geometry of beam spring  106 . Curved portion  804  includes magnetic elements  806  which can be configured to retain beam spring  106  against an outside surface of curved portion  804 . In some embodiments magnetic elements  806  can be electro magnets so that fixturing device  800  can pick up and put down beam springs  106  when desired. In other embodiments magnetic elements  806  can be permanent magnets along the lines of neodymium magnets. Curved portion  804  also includes a number of plungers  808  disposed within a channel extending into body portion  802 . It should be noted that while not depicted fixturing device  800  can be configured to install two or more beam springs  106  concurrently. 
     In  FIG. 8A  fixturing device  800  is depicted positioning a beam spring  106  above slug  110  so that openings in beam spring  106  line up with threaded openings in standoffs  102 . In  FIG. 8B  fixturing device  806  places a bottom surface of beam spring  106  in contact with or at least in close proximity to a top surface of slug  110 . In  FIG. 8C  plungers  808  are depicted as having moved from the retracted position shown in  FIGS. 8A-8B  to an extended position. By extending plungers  808  and drawing body portion  802  away from slug  110 , plungers  808  can begin bending beam spring  106  into position against slug  110  and magnetic elements  806  and beam spring  106  can be decoupled. It should be noted that ends of beam spring  106  can retain a slight curvature at this point in an assembly process. In  FIG. 8D  fasteners  104  are depicted being inserted through openings in beam spring  106 . The openings in beam spring  106  can be longer than a diameter of fasteners  104  so that fasteners  104  can have a wider tolerance for insertion within the openings. Furthermore, since four point beam spring  104  is self-leveling, its use precludes force distribution irregularities when beam spring  106  is slightly misaligned. Finally in  FIG. 8E  fasteners  104  are shown full engaged with standoffs  102 . In some embodiments fixturing device  800  can include drivers for engaging fasteners  104  with standoffs  102 , while in other embodiments a separate driver can be applied to engage fasteners  104  with standoffs  102 . In this way, beam spring  106  can be fully engaged with slug  110  and substantially parallel with a top surface of slug  110 . In some embodiments, an amount of force exerted by fasteners  104  can be reduced by backing fasteners slightly away from a top surface of beam spring  106 , thereby beam spring  106  to retain a slight bend. In other embodiments, a height of standoffs  102  can be adjusted to change an amount of force applied when fasteners  104  are fully engaged against standoffs  102 . Plungers  808  are shown partially retracted as body portion  802  extends farther above the assembled cooling stack. 
       FIG. 9  shows a block diagram representing a number of steps in a method for assembling a cooling stack. In a first step  902 , a heat pipe is soldered to a channel disposed along a top surface of a slug. A joint between the slug and the heat pipe can be reinforced by adding solder solids to the joint during the soldering operation. In step  904  the slug is placed in direct contact with a top surface of an integrated circuit. In some embodiments, a layer of thermal grease can be disposed between the integrated circuit and the heat pipe to facilitate the conduction of heat between the integrated circuit and the heat pipe. It should be noted that when the heat pipe is said to be in direct contact with the slug, an intervening layer of thermal grease can be present between the integrated circuit and the heat pipe. In step  906  a number of beam springs are pressed against a top surface of the slug. Distal ends of the beam springs can be fastened to a top surface of a printed circuit board (PCB) to which the integrated circuit is attached. Fastening the ends of the beam springs to the PCB causes the beam springs to deform changing from a bent state to a substantially flat state. While fixed to the PCB the beam springs exert a force to the top surface of the slug that causes the slug to be compressed against the integrated circuit, thereby minimizing a thickness of a layer of thermal grease disposed between the integrated circuit. The force compressing the integrated circuit and the die helps to increase an efficiency of heat transfer between the two components. In this way the assembled cooling stack can both remove heat from the integrated circuit by way of the slug and heat pipe, and provide the force to improve thermal efficiency of the cooling stack. In certain embodiments, the force can also assist in electrically coupling the integrated circuit when the integrated circuit is coupled to the PCB by a socket. 
     The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. The described embodiments can also be embodied as computer readable code on a computer readable medium for controlling manufacturing operations or as computer readable code on a computer readable medium for controlling a manufacturing line. The computer readable medium is any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, HDDs, DVDs, magnetic tape, and optical data storage devices. The computer readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of specific embodiments are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the described embodiments 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.