Abstract:
A single-piece, high performance, inexpensively fabricated heat sink and electromagnetic interference (EMI) shield makes thermal contact with a heat generating device. Capillary forces exerted on the heat sink by cooling flowed solder draw the heat sink toward the PCB. The heat sink attaches directly to a printed circuit board (PCB), thus not stressing ball grid array (BGA) solder joints between the heat generating device and the PCB in applications with BGAs. The heat sink does not require any special tools for installation or removal nor any additional PCB space. The heat sink is designed to allow automated surface mounting techniques, such as pick and place. The single-piece construction eliminates the need for a separate clip, thereby increasing heat transfer area.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    As integrated circuit technology has improved, substantially greater functionality has been incorporated into devices. Along with this expanded functionality, the size of devices has diminished resulting in higher clocking frequencies and increased power consumption. As a consequence, the integrated circuit devices of today generate more heat while possessing smaller surface areas to dissipate the heat. Therefore, it is important to have a high rate of heat transfer from the integrated circuit package to maintain the temperature of the integrated circuit within safe operating limits. Excessive heat may adversely affect the performance of the circuit, cause permanent degradation of its components and increase failure rates. 
         [0002]    A heat sink may be used for transferring heat away from a heat source, such as an electronic component or printed circuit board (PCB), to maintain the component within an optimum or safe operating temperature range, so that the component can operate continuously within safe thermal operating limits. 
         [0003]    Conventional heat sinks typically contain a plurality of fins or rods that extend from a base that contacts the heat generating integrated circuit. Some heat sinks employ securing mechanisms that enlarge or exceed the heat sink envelope. This increases the footprint of the electronic component/heat sink assembly, thereby reducing the surface area of the PCB available for other circuit elements and potentially imposing a limit on the height of nearby elements on the PCB. 
         [0004]    In addition to producing heat, integrated circuits radiate radio frequency (RF) emissions which may cause electromagnetic interference (EMI). EMI can cause other devices to malfunction. Typically, an EMI shield is used to reduce EMI. EMI shields typically take a form of a chassis element which extends around the entire electronics compartment of the computer or other device producing RF emissions. EMI shields limit electromagnetic radiation from entering or exiting sections of the PCB containing electrical components. 
       SUMMARY OF THE INVENTION 
       [0005]    One example embodiment of a heat sink, and corresponding method for securing the heat sink to a component, includes a contact structure with a conduction side and a convection side. The conduction side is configured to be in thermal communication with a heat generating device. Extending outward from the convection side of the contact structure are a plurality of thermally conductive elements. An attachment structure is configured to be coupled to a support structure, by less than or equal to a thickness of the attachment structure, to which the heat generating device is coupled. The attachment structure extends outward from the conduction side of the contract structure. The attachment structure defines a cavity having a length and a width defined by the length and the width of the attachment structure and the conduction side of the contact structure. The cavity also has a depth defined by the height of the heat generating device when coupled to the support structure. The conduction side of the contact structure contacts the heat generating device in a configuration in which both the attachment structure and the heat generating device are coupled to the support structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating example embodiments of the present invention. 
           [0007]      FIG. 1A  is an isometric diagram of a heat sink. 
           [0008]      FIG. 1B  is an isometric diagram of the heat sink of  FIG. 1A  in application with a heat generating device configured to be secured via a ball grid array (BGA) to a printed circuit board (PCB). 
           [0009]      FIG. 1C  is an isometric diagram of the heat sink, heat generating device and PCB of  FIG. 1B  illustrating additional features of the heat sink. 
           [0010]      FIG. 2  is a cross sectional diagram of the heat sink and the heat generating device illustrating solder wicking features of an attachment structure to draw flowed solder by capillary, or other, forces. 
           [0011]      FIG. 3  is a force diagram of a combination of the attachment structure and the contact structure in an example embodiment of the present invention annotated with forces exerted by the attachment structure on the PCB and forces exerted by the contact structure on the heat generating device. 
           [0012]      FIG. 4  is a diagram of the heat sink further including a gap pad material to account for any excess height of the cavity and ensure thermal communication between the heat generating device and the heat sink. 
           [0013]      FIG. 5  is a top view diagram of a heat sink with an attachment structure having faces that extend from a contact structure at an angle. 
           [0014]      FIG. 6  is a flow diagram illustrating a method by which a heat sink may be secured. 
           [0015]      FIG. 7  is a flow diagram illustrating a method by which heat may be dissipated. 
           [0016]      FIG. 8  is a flow diagram illustrating a method of manufacturing a heat sink. 
           [0017]      FIG. 9  is a flow diagram illustrating a method for preparing a circuit board for receiving an electronic component. 
           [0018]      FIG. 10  is a diagram of an example mask, according to an example embodiment of the present invention, with a solder paste pattern corresponding to a pattern of structural components on an attachment structure of a heat sink. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    A description of example embodiments of the invention follows. 
         [0020]    Conventional heat sinks and clips for their attachment are inefficient, expensive to produce and bulky. Conventional heat sinks typically include both a body and a fastener. The fastener may be a curved piece that applies pressure on an electronic component to which the heat sink is applied when installed. However, such fasteners reduce the available convection surface area of heat sinks, thereby reducing their efficiency. Other conventional heat sinks employ spring action for securing the body to the electronic component, but require special modification of the printed circuit board (PCB). Further, conventional heat sinks require screws, push pins, clips, anchors or other forms of mechanical attachment that are additional parts beyond the body, itself, which takes up valuable surface area on the PCB, and block air flow to the convection surfaces of the heat sink. 
         [0021]    Further, conventional methods of securing a heat sink to an integrated circuit or other heat generating device can weaken ball grid array (BGA) solder connections between the integrated circuit and the PCB through application of stresses. In a BGA, balls of solder may be adhered to the bottom of the package, e.g., the integrated circuit package, which is placed on a PCB that carries copper pads in a pattern corresponding to the solder ball pattern. The assembly is then heated, either in a reflow oven or by an infrared heater, causing the solder balls to melt. Surface tension causes the molten solder to hold the package in alignment with the circuit board, at the correct separation distance, while the solder cools and solidifies. Conventional clip-on heat sinks apply a bending moment to the heat generating device, which stresses the solder balls-to-PCB connections and decreases component reliability or useful life. Without a heat sink, the outer balls generally fail over time due to heating and inherent stresses. By adding more stress through an application of a heat sink, failures occur even quicker and more often. 
         [0022]    Moreover, some heat sinks take an additional role of shielding electrical components from electromagnetic interference (EMI). These heat sinks may rely on many different plates, with springs and screws securing them to a PCB. However, such heat sinks require many small, custom parts with a certain amount of tooling or machining, thereby making them expensive to produce. Other heat sinks provide EMI shielding by being soldered directly to the PCB. However, such heat sinks can take up significant PCB surface and also require certain amounts of tooling or machining. 
         [0023]    A heat sink according to an example embodiment of the present invention includes a contact structure with a conduction side and a convection side. The conduction side is configured to be in thermal communication with a heat generating device, and the convection side is configured to be in thermal communication with the heat generating device via the conduction side. The heat sink includes a plurality of thermally conductive elements extending outward from the convection side of the contact structure and an attachment structure extending from the conduction side of the contact structure defining a cavity. The attachment structure is configured to be connected, by less than or equal to a thickness of the attachment structure, to a surface of a member to which the heat generating device is connected. The cavity has a volume defined by the length and width of the contact structure and at least the height of the heat generating device as coupled to the member. 
         [0024]    The attachment structure may also define a solder wicking feature configured to wick solder in a state of flow and draw the attachment structure toward the member during a transition of the solder from the state of flow to a state of being a solid. The wicking feature may be within a thickness of the attachment structure. The attachment structure may also define a flange extending substantially parallel to the surface of the member. 
         [0025]    The attachment structure may extend substantially perpendicularly from the conduction side to the member, or may extend non-perpendicularly at an angle from the conduction side to the member. 
         [0026]    The thermally conductive elements may provide more convection surface area than plate elements with flat surfaces. The thermally conductive elements may be configured to increase airflow across them. The thermally conductive elements may define a pick and place feature. A pick and place feature may be included in place of at least one thermally conductive element. 
         [0027]    The contact structure, in combination with the attachment structure, may substantially encompass the heat generating device on five sides to contain electromagnetic interference (EMI) radiation generated by the heat generating device and provide EMI immunity for the heat generating device. 
         [0028]    The attachment structure may include structural components selected from a group consisting of pegs, slots, ridge or solder wicking. 
         [0029]    As deployed on a circuit board member, the attachment structure may have a thickness sufficiently thin to be mechanically between the heat generating device and AC filter capacitors coupled to the heat generating device. The cavity may also be configured to encompass other devices in addition to the heat generating device, such as the AC filter capacitors. 
         [0030]    In another example embodiment, a heat sink includes a contact structure with a conduction side and a convection side. The conduction side is configured to be in thermal communication with a heat generating device, and the convection side is configured to be in thermal communication with the heat generating device via the conduction side. The heat sink includes a plurality of thermally conductive elements extending outward from the convection side of the contact structure and an attachment structure extending from the conduction side of the contact structure defining a cavity. The attachment structure is configured to be coupled to a surface of a member to which the heat generating device is coupled. The cavity has a volume defined by the length and width of the contact structure and at least the height of the heat generating device as coupled to the member. The attachment structure also defines a solder wicking feature configured to wick solder in a state of flow and draw the attachment structure toward the member during a transition of the solder from the state of flow to a state of being a solid. 
         [0031]    In some example embodiments, the heat sink may be secured by positioning the heat sink relative to a heat generating device in a configuration in which the heat sink contacts a solder paste pattern deposited on a surface of a member to which the heat generating device is coupled. The solder is then caused to enter a state of flow. The solder draws the heat sink toward the member during a transition of the solder from the state of flow to a state of being a solid, wherein simultaneous contact between the heat sink with the heat generating device and the surface of the member is maintained. The heat sink may be positioned via a pick and place feature. 
         [0032]    According to an example embodiment of the present invention, a heat sink may be manufactured by forming a contact structure with a conduction side and a convection side, forming a plurality of thermally conductive elements extending outward from the convection side of the contact structure, and forming an attachment structure extending from the conduction side of the contact structure for a distance defining a cavity. The attachment structure is configured to be connected, by less than or equal to a thickness of the attachment structure, to a surface of a member to which the heat generating device is connected. The cavity has a volume defined by the length and width of the contact structure and at least the height of the heat generating device as coupled to the member. 
         [0033]    According to another example embodiment of the present invention, a circuit board may be prepared for receiving an electronic component by applying a first surface mount solder paste pattern to a surface of the circuit board to receive an electronic component, and applying a second surface mount solder paste pattern to the surface of the circuit board, in proximity to the first surface mount solder paste pattern, to receive a heat sink configured to be surface mounted in an arrangement to draw heat from the electronic component. The second surface mount solder paste pattern has a thickness corresponding to the thickness of a solder wicking feature of the heat sink configured to wick solder in a state of flow and draw the heat sink toward the circuit board during a transition of the solder from the state of flow to a state of being a solid. 
         [0034]    In one example embodiment, a mask may define a solder paste pattern corresponding to a pattern of structural components on an attachment structure of a heat sink. The pattern may have a thickness substantially corresponding to a thickness of a solder wicking feature of the heat sink. The solder paste pattern may further correspond to a pattern of structural components on a heat generating device. 
         [0035]      FIG. 1A  is an isometric diagram of a heat sink. The heat sink  100  includes a contact structure  105 , having a conduction side  107  and a convection side  108 . The conduction side  107  is configured to be in thermal communication via direct or indirect contact with a heat generating device (not shown), such as an integrated circuit or a variety of other optical, electrical, or mechanical components. The heat sink  100  further includes a plurality of thermally conductive elements  110  extending outward from the convection side  108  of the contact structure  105 . In this example embodiment, the thermally conductive elements  110  are flat surfaces  112 ; however, they may be wavy fins, having an undulating curvature across the convection side  108  or across their distance extending outward from the convection side, to provide more convection surface area (not shown) as understood in the art. Further, the thermally conductive elements  110  may be configured to increase airflow across them by defining narrowing airflow paths in a direction of airflow across them or being airfoil shaped, thereby increasing the efficiency of the heat sink  100 . Here, the example thermally conductive elements  110  are aligned in a parallel manner such that air flows freely through the channels created by opposing thermally conductive elements  110 . 
         [0036]    The heat sink  100  further includes an attachment structure  115  that extends substantially perpendicularly from the conduction side  107  of the contact structure  105 . The attachment structure  115  further has a plurality of solder wicking features  120 , in this example embodiment along a thickness of the attachment structure  115 . The solder wicking features  120  may help secure the attachment structure  115  to a PCB and are described in greater detail with reference to  FIG. 2 . The attachment structure  115 , in an additional example embodiment, may also extend non-perpendicularly at an angle from conduction side  107 . In both of these example embodiments, the attachment structure  115  may further include a flange extending at an angle from the attachment structure  115  with the flange having wicking features  120  on a surface substantially parallel to the surface to which the attachment structure  115  will be attached (not shown). 
         [0037]      FIG. 1B  is an isometric diagram of the heat sink  100  of  FIG. 1A  illustrated in application with a heat generating device  150  secured via a BGA  155  to a PCB  145 , the attachment structure  115  extends downward for a length defining a cavity  117 . The cavity  117  has a length and width defined by the length and width of the attachment structure  115  and the conduction side  107  of the contact structure  105  and has a depth defined by the height of the heat generating device  150  when coupled to the PCB  145  via the BGA  155 . The solder wicking features  120  may help secure the attachment structure  115  to the PCB  145  and are described in greater detail with reference to  FIG. 2 . 
         [0038]      FIG. 1C  is an isometric diagram of the heat sink  100 , heat generating device  150 , and PCB  145  of  FIG. 1B  illustrating additional features of the heat sink  100 . The heat sink  100  is designed to allow automated surface mounting techniques In this example embodiment, a pick and place feature  160  provides a surface for a suction-based pick and place machine to appropriately position the heat sink  100  for soldering to the PCB  145 . 
         [0039]    During preparation of the PCB  145  for receiving the heat generating device  150 , a first surface mount solder paste pattern  157  is applied to the PCB  145  for soldering the heat generating device  150  to the PCB  145 . A second surface mount solder paste pattern  140  may be applied to the PCB  145  in proximity to the first surface mount solder paste pattern  157  for soldering the heat sink  100  to the PCB  145 , typically in electrical connection with a ground plane that spans beneath the heat generating device  150  to provide EMI shielding below the device  150 . The second solder paste pattern  140  corresponds to the locations of the solder wicking features ( 120  of  FIGS. 1A and 1B ) of the attachment structure  115 . The heat sink  100  may be placed over solder paste pattern  140  by the pick and place machine. The solder paste patterns  140 ,  157  may be heated through a reflow process, thereby allowing the solder of the solder paste patterns  140 ,  157  to flow so that the heat generating device  150  and the heat sink  100  may be soldered to the PCB  145 . 
         [0040]    Because the heat sink  100  is substantially connected to the PCB  145 , the heat sink  100  provides EMI shielding around the heat generating device  150 . The heat sink  100  may also provide heat transfer and EMI shielding to other devices  170  within the cavity  117 . Moreover, the heat attachment structure  115  may have a thickness ( 116  of  FIG. 1A ) sufficiently thin to be mechanically between the heat generating device  150  and AC filter capacitors  165  coupled to the heat generating device  150 . 
         [0041]    The heat sink  100  may be secured to and removed from the PCB  145  without use of special tools other than a heating mechanism and grip mechanism and uses substantially no additional PCB surface area beyond the heat-generating device  150 . The heat sink  100  may be black anodized so that its surface is non-electrically conductive and may be fabricated through an extrusion process, machining process, or die casting process, for example, and is inexpensive to fabricate, in part, because it may be a single piece. The heat sink  100  may be tin plated to facilitate soldering the heat sink  100  to the PCB  145 . Moreover, use of the heat sink  100  does not require special modification to the PCB  145  or any other member to which the heat generating device  150  is secured. Because the heat sink  100  is attached to the PCB  145 , the heat sink  100  is able to draw additional heat from the PCB  145  directly, thereby providing increased cooling over traditional heat sinks. 
         [0042]      FIG. 2  is a cross sectional diagram of the heat sink  200  and heat generating device  250  that illustrates a combination of solder  242  and the wicking features  220  of the attachment structure  215 . When heated, the solder  242  flows and is caused to be drawn into the solder wicking features  220  by capillary forces or other forces, such as forced vacuum forces (active vacuum device not shown). As understood in the art, when solder cools, it shrinks. When the solder  240  cools, the shrinking that occurs causes the heat sink  200  to be pulled toward the PCB  245  and held tightly against the heat generating device  250 . The conduction side  207  of the contact structure  205  may then be in direct (or indirect) thermal communication with the heat generating device  250  to provide improved heat transfer characteristics. 
         [0043]      FIG. 3  is a force diagram of a combination of an attachment structure  315  and a contact structure  305  in an example embodiment of the present invention annotated with forces exerted by the attachment structure  315  on the heat generating device  350 . In this example embodiment, the heat generating device  350  is an integrated circuit secured to a PCB  345  via a BGA  355 . In some example embodiments of the present invention, the forces may be exerted in a manner that does not affect a mechanical arrangement between the heat generating device  350  and a member  345  to which the heat generating device  350  is coupled. In the example embodiment of  FIG. 3 , the combination of the attachment structure  315  and the contact structure  305  exerts forces substantially uniformly across all solder joints  357  of the BGA  355 . Each arrow indicating a force is only intended to indicate the direction component of the force vector and not the magnitude relative to the other arrows. 
         [0044]    The heat sink  300  increases reliability of the heat generating device  350  by not exerting stresses non-uniformly upward, or downward, or a combination thereof, on the solder joints of the BGA  355 . By attaching the heat sink  300  to the PCB  345 , the forces are supported by the PCB  345 . This makes the heat sink  300  suitable for high vibration and shock environments. 
         [0045]      FIG. 4  is a cross sectional diagram of a heat sink  400 , PCB  445 , heat generating device  450 , and gap pad material  418 . The gap pad material  418  may be used in some applications to account for any excess height of the cavity  417  above the heat generating device  450  to ensure thermal communication between the heat generating device  450  and the heat sink  400 . The gap pad material  418  may be of a springing or flexible nature with good thermal conduction properties such that it creates a positive mechanical force toward a contact plate  405  of the heat sink  400  and the heat generating device  450  and be a suitable thermal interface between the heat generating device  450  and the heat sink  400 . 
         [0046]      FIG. 5  is a top view diagram of a heat sink  500  with an attachment structure  515  having faces that extend from a contact structure  105  at an angle. The heat sink  500  may also have thermally conductive elements  510  extending from the contact structure  505 . In this arrangement, the heat sink  500  may provide heat transfer and EMI shielding to other drives with the cavity  517  while reducing the volume of the cavity  517  as well as the amount of material used to form the heat sink  500 . The heat sink  500  may be secured, as discussed above with reference to  FIG. 2 , by solder wicking features  520  which are not plainly visible in this top view but are located on the attachment structure  515  where it comes in contact with the member, such as a PCB, to which the heat generating device is coupled. 
         [0047]    It should be understood that the solder wicking features may be any one of or a combination of pegs, slots, ridges, long and narrow grooves, or any other type of structural feature that allows for the securing of a heat sink to a PCB while maintaining thermal communication with a heat generating device by forces exerted on the heat sink by flowed and cooled solder. 
         [0048]      FIG. 6  is a flow diagram  600  illustrating a method by which a heat sink may be secured. First, the heat sink must be positioned  605  relative to a heat generating device in a configuration in which the heat sink contacts a solder paste pattern deposited on a surface of a member to which the heat generating device is coupled. Next, the solder must be caused to flow  610 , such as by heating. The solder is then caused to draw the heat sink toward the member  615  during a transition of the solder from the state of flow to a state of being a solid. Simultaneous contact between the heat sink with the heat generating device and the surface of the member is maintained. The heat sink may be positioned via a pick and place feature. 
         [0049]      FIG. 7  is a flow diagram  700  illustrating a method by which heat may be dissipated. First, simultaneous contact must be maintained with a heat generating device and a surface of a member to which the heat generating device is coupled  705 . Heat may then be drawn from the heat generating device  710 . Next, the heat drawn from the heat generating device may be dissipated via convection away from a plurality of thermally conductive elements. 
         [0050]      FIG. 8  is a flow diagram  800  illustrating a method of manufacturing a heat sink. First, a contact structure is formed  805  have a conduction side and a convection side. Next, a plurality of thermally conductive elements are formed  810  extending outward from the convection side of the contact structure. Finally, an attachment structure is formed  815  extending from the conduction side of the contact structure for a distance defining a cavity, the attachment structure configured to be coupled, by less than or equal to a thickness of the attachment structure, to a surface of a member to which a heat generating device is coupled, the cavity having a volume defined by the length and the width of the contact structure and at least the height of the heat generating device as coupled to the member. 
         [0051]      FIG. 9  is a flow diagram  900  illustrating a method for preparing a circuit board for receiving an electronic component. A first surface mount solder paste pattern is applied  905  to a surface of the circuit board to receive an electronic component. Next, a second surface mount solder paste pattern is applied  910  to the surface of the circuit board, in proximity to the first surface mount solder paste pattern, to receive a heat sink configured to be surface mounted in an arrangement to draw heat from the electronic component, wherein the second surface mount solder paste pattern has a thickness corresponding to the thickness of a solder wicking feature of the heat sink configured to wick solder in a state of flow and draw the heat sink toward the circuit board during a transition of the solder from the state of flow to a state of being a solid. 
         [0052]      FIG. 10  is a diagram of an example mask  1000 , according to an example embodiment of the present invention, with a solder paste pattern  1020  corresponding to a pattern of structural components on an attachment structure of a heat sink. The pattern  1020  has a thickness  1022  substantially corresponding to a thickness of a solder wicking feature of the heat sink. The thickness  1022  may be less than the thickness of the pattern of structural components on the attachment structure of the heat sink with the mask  1000  covering the portions actually contacting a circuit board. The mask  1000  may also have a solder paste pattern  1055  corresponding to a pattern of structural components on a heat generating device. 
         [0053]      FIGS. 6-9  are flow diagrams illustrating methods according to example embodiments of the present invention. The techniques illustrated in these figures may be performed sequentially, in parallel or in an order other than that which is described. It should be appreciated that not all of the techniques described are required to be performed, that additional techniques may be added, and that some of the illustrated techniques may be substituted with other techniques. 
         [0054]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.