Abstract:
A thermal connector configured to be placed within a recess of a heat sink between the heat sink and a heat generating component and transfer heat from the component to the heat sink, including a heat spreader configured to fit within the recess of the heat sink, a spring configured to sit between the heat spreader and with the heat sink and bias the heat spreader towards and away from the heat sink, a flexible membrane attached to the heat sink and the heat spreader and seal off the recess, and a phase change material that fills the recess, wherein the flexible membrane contains the phase change material and allows it to contract or expand in response to the movement of the heat spreader towards or away from the heat sink.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. Patent Application Ser. No. 13/750,078 filed on Jan. 25, 2013, which claims the benefit of U.S. Provisional Patent Application No. 61/559,191, filed on Feb. 15, 2012. The entire contents of each of these applications is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates generally to a system and method for heat dissipation. More particularly, the subject matter disclosed herein relates to thermal connectors between heat generating devices and heat sinks. 
       BACKGROUND 
       [0003]    Electronic devices and other devices often produce heat during operation that needs to be dissipated away from the device. Heat sinks are often used for this purpose; a heat sink is a passive component that cools a device by dissipating heat into the surrounding environment. In order for the heat sink to operate properly, the heat from the device must be transferred to the heat sink over a thermal connection. While the term heat sink is used herein it should be understood that the term refers to all types of heat dissipating devices, including heat pipe modules and thermal ground planes. 
         [0004]    A common arrangement for electronic devices is a plurality of electronic components attached to a printed circuit board (PCB). Heat from these multiple components is transferred to one or more heat sinks using thermal connections. Each component on the PCB is a particular distance from the heat sink (tolerance) and the heat must be effectively transferred across the tolerance from the component to the heat sink. Accordingly, the tolerance is often filled with a thermal connector, such as a heat spreader and/or thermal interface material. 
         [0005]    When a single heat sink serves multiple components, the thermal connectors often must accommodate several different tolerances. Some of the proposed solutions for this issue include the use of thermal pastes, thermal greases, and thermally conductive pads that are compressible and expandable. These thermal connectors typically have fairly low thermal conductivities, in the range of 3 watts per meter kelvin (W/mK). Some thermal pads have conductivity as high as 17 W/mK but they can only be compressed to 10%-20% between the heat sink and the component or the component will be damaged. This limits the size of the starting gap between the component and the heat sink and makes it more difficult to assemble the device. 
         [0006]    Accordingly, there is a need for better thermal connectors to transfer heat between heat generating components and heat sinks. More particularly, there is a need for thermal connectors that accommodate a variety of tolerances between multiple components of a heat generating device and a heat sink. 
       SUMMARY 
       [0007]    In at least one aspect, the present disclosure provides a thermal connector configured to be placed within a recess of a heat sink between the heat sink and a heat generating component and transfer heat from the component to the heat sink. The thermal connector includes a heat spreader configured to fit within the recess of the heat sink, a spring configured to sit between the heat spreader and with the heat sink and bias the heat spreader towards and away from the heat sink, a flexible membrane attached to the heat sink and the heat spreader and seal off the recess, and a phase change material that fills the recess, wherein the flexible membrane contains the phase change material and allows it to contract or expand in response to the movement of the heat spreader towards or away from the heat sink. 
         [0008]    In at least another aspect, the present disclosure provides a cartridge for placing between a heat generating component and a heat sink for facilitating transfer of heat from the component to the heat sink. The cartridge includes a heat spreader and a spring attached to the heat spreader. The heat spreader and spring are circumferentially enclosed by a frame and a flexible membrane is attached to the frame and the heat spreader to define an open topped void filled by a phase change material. The spring, flexible membrane, and phase change material expand or contract to accommodate the tolerance between the heat sink and component. 
         [0009]    In yet another aspect, the present disclosure provides a method for transferring heat from a heat generating component to a heat sink. The method includes the steps of providing the thermal connector or cartridge as described above and positioning the thermal connector or cartridge between a heat sink and a heat generating component. The thermal connector or cartridge can accommodate several different components. 
         [0010]    Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates a cross-sectional schematic view of a thermal connector in accordance with at least one embodiment of the present disclosure. 
           [0012]      FIG. 2  illustrates a top plan view of the spring in accordance with at least one embodiment of the present disclosure. 
           [0013]      FIG. 3  illustrates a cross-sectional schematic view of a thermal connector cartridge in accordance with at least one embodiment of the present disclosure in exploded view with a heat sink and heat generating component. 
           [0014]      FIG. 4  is a perspective view of the thermal connector cartridge of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The following detailed description is merely exemplary in nature and is not intended to limit the applications and uses disclosed herein. Further, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description. While embodiments of the present technology are described herein primarily in connection with dissipation of heat from an electrical circuit board to a heat sink, the concepts are also applicable to other types of systems where it is desirable to transfer thermal energy from a heat generating component to a heat dissipation device. 
         [0016]    In at least one aspect, the present disclosure provides a device and method for the transfer of thermal energy from components on an electrical circuit board to a heat sink. 
         [0017]    Thermal conductor  10  as applied in an electrical device is shown in  FIG. 1 . A portion of an electrical device includes a printed circuit board (PCB)  12  having an electrical device component  14  attached thereto via a ball grid array (BGA)  16 . Silicon die  15  is a part of component  14  and a thermal interface material (TIM)  24  is connected to silicon die  15  to provide for thermal conductivity between silicon die  15  and the thermal conductor  10 . 
         [0018]    Heat sink  18  is displaced away from the PCB  12  and component  14  and there is a tolerance or gap (not numbered) there between that is filled here with the thermal conductor  10 . 
         [0019]    The thermal conductor  10  includes heat spreader  22  and spring  26 . Desirably spring  26  includes body portion  27  and legs  28  extending away from the body portion  27  (shown more clearly in  FIG. 2 ). Desirably, spring  26  is attached to heat spreader  22  at its body portion  27 , leaving leg portions  28  free (as further illustrated by  FIG. 2  and described below). 
         [0020]    Heat sink  18  has a recess (not numbered) into which the assembly of the spring  26  and heat spreader  22  fits. A flexible membrane  32  is fixed to the edges of heat spreader  22  and heat sink  18  and seals off the void  29  between the heat sink  18  and heat spreader  22 . The flexible membrane  32  can extend across the heat spreader  22  or simply to the edges thereof in order to adequately retain the phase change material. 
         [0021]    Void  29  is filled by phase change material  30 . Phase change material  30  is a material, most preferably a low melting point alloy, which melts at a particular temperature. 
         [0022]    To assemble the thermal connector, the heat spreader  22  and spring  26  assembly is placed in the heat sink recess. The spring  26  is compressed to its smallest height while the phase change material  30  is in a melted state, and then the phase change material  30  is hardened by lowering the temperature. 
         [0023]    The thermal connector is deployed by raising the temperature to the melting point of the phase change material  30 . The melting of the phase change material  30  allows the spring  26  to expand and the spring  26  will push heat spreader  22  into thermal contact with component  14  (via TIM  24 ). 
         [0024]    The tolerance between a heat sink and heat generating component in an electrical device ranges from about 0.1 mm to about 3 mm, more specifically about 0.3 to 1 mm, and is typically about 0.8 mm. Accordingly, the thermal conductor should be able to expand to fit this range. 
         [0025]      FIG. 2  illustrates one embodiment of the spring  34 , having a main body  36  and legs  38 . Here, spring  34  is shown having three legs  38  but it could have more or less legs. Spring main body  36  is fixed to a heat spreader  39  such as by spot welding or soldering. The spring  34  is preferably made out of an alloy such as beryllium copper (BeCu). It can be from about 0.10 mm to 0.40 mm in thickness, or more preferably from about 0.20 mm to 0.30 mm in thickness. The spring thickness and dimensions are chosen to achieve the correct loading for the device when compressed. In other words, and referring to  FIG. 1 , the spring  26  should exert sufficient pressure on the heat spreader  22  to hold it against TIM  24  and achieve good thermal contact. However, spring  26  should not cause heat spreader  22  to exert an amount of pressure that damages TIM  24  or the component  14 . In one embodiment, the spring  34  is nickel plated, to increase its wettability with the phase change material, as described below. 
         [0026]    Phase change material  30  is desirably a material that is solid at near room temperature and melts at a temperature to deploy the spring. As one example, for many electronic devices, a phase change material having a melting point between about 40° C. to 250° C. is appropriate, more preferably from about 60° C. to 160° C. One preferred metal alloy is 52 In 48 Sn which has a melting point of 118° C. and a thermal conductivity of 35 W/mK. This alloy is available from Indium Corporation under the trademark INDALLOY® 1E. Eutectic alloys are preferred but are not required. Mixtures or pastes could also be used. 
         [0027]    Other metals and metal alloys that might be useful for certain applications include In, InBi, variations of InSn, BiSn, PbSn, SnAg, InPbAg, InAg, InSnBi, InGa, SnBiZn, SnInAg, SnAgCu, SnAgBi, and InPb. 
         [0028]    In general, phase change materials having a thermal conductivity between about 20 W/mK and 400 W/mK are preferred, most desirably about 30 W/mK to 100 W/mK. 
         [0029]    The flexible membrane  32  functions to retain the phase change material  30  within the void defined by the heat spreader  22  and the heat sink  18 . Flexible membrane  32  is preferably a plastic film that can withstand the highest temperature reached by the operating device. For many electronic devices, a flexible membrane stable up to at least between about 150 to 200° C. is desirable, preferably up to at least 160° C. Options for the flexible membrane include polymers, silicon, urethane, rubbers, and metal foil. One specific example is DUREFLEX® U073 125 μm which is a polyether-based thermoplastic polyurethane film. Flexible membrane  32  can be attached to heat sink  18  and heat spreader  22  with an appropriate adhesive. 
         [0030]    Heat sink  18  can be a typical heat sink as used in the art, such as an aluminum alloy plate. As discussed above, other heat dissipating devices such as heat pipe modules and thermal ground planes can be used with the thermal conductors as described herein. As an example, the recess in the heat sink  18  can be about 2.25 mm. 
         [0031]    Heat spreader  22  can be a typical heat spreader as used in the art, such as a copper plate. Other materials can be used as well, such as aluminum nitride (AlN) plates. Copper offers a higher thermal conductivity but aluminum nitride offers electrical isolation of the heat generating component from the heat sink. The heat spreader can be of a variety of sizes, such as those presently used in the art. 
         [0032]    Thermal interface material  24  can also be a material typically used in the art, such as a paste or thermal grease. 
         [0033]    The metal parts that are in contact with the phase change material (heat sink, heat spreader, spring) may be treated to increase their wettability by the phase change material  30 . One treatment is a nickel plating with gold flash which increases the wettability of the parts with the metal alloy 52 In 48 Sn. This treatment is known in the art. 
         [0034]      FIG. 3  illustrates another embodiment of a thermal conductor in accordance with the disclosure. In this embodiment, the thermal conductor is assembled as a cartridge  40  including the heat spreader  44  and spring  46 . As in the prior embodiment, spring  46  is desirably connected to heat spreader  44  at its body portion  48 , leaving legs  50  free. 
         [0035]    A frame  42  circumferentially surrounds the heat spreader  44 /spring  46  assembly and a flexible membrane  52  extends from the frame to the heat spreader. Phase change material  56  fills the void created by frame  42 , flexible membrane  52 , and heat spreader  44 . The cartridge  40  is open on the top, so that the phase change material  56  is exposed. 
         [0036]    In one embodiment of assembling the heat connector cartridge  40 , the spring  46  is attached to heat spreader  44  and flexible membrane  52  is attached to the frame  42  and heat spreader  44 . The phase change material  56  is melted and placed in the void created by frame  42 , flexible membrane  52 , and heat spreader  44 . The spring  46  is flattened to its lowest height and the temperature lowered to harden the phase change material  56 . The cartridge  40  can then be attached to the heat sink  58  using a sealant or adhesive. 
         [0037]    As shown in exploded form in  FIG. 3 , the cartridge  40  is installed between a heat sink  58  and electrical device. The electrical device includes PCB  60  having an electrical device component  62  attached thereto via a ball grid array (BGA)  64 . Silicon die  66  is a part of component  62  and a thermal interface material (TIM)  68  is connected to silicon die  66  to provide for thermal conductivity between silicon die  66  and the thermal conductor  40 . 
         [0038]    In use, the thermal connector cartridge  40  is placed between the heat sink  58  and the device component  62  (or multiple components). Desirably the cartridge is attached to the heat sink  58  such as by adhesive or other mechanical means such as fasteners. The assembly is deployed by heating to the melting point of the phase change material  56 , which allows the spring  46  to expand or contract and engage the heat spreader  44  with the TIM  68  or component  62  on the other side. The flexible membrane  52  will expand or contract as needed to accommodate this expansion or contraction of the phase change material  56 . 
         [0039]    As discussed above, the tolerance between a heat sink and heat generating component in an electrical device ranges from about 0.1 mm to about 3 mm and is typically 0.8 mm. Accordingly, the cartridge should be an appropriate thickness to fit within the gap and the spring should be able to expand to fill the gap. 
         [0040]    The elements in this embodiment can have essentially the same properties as in the embodiment discussed above. Frame  42  can be made out of a number of materials. One option is aluminum and another option is a high melting point plastic. The frame can be a variety of sizes and is at least partially dependent on the size of the heat spreader. For an example, a frame that is about 35 mm square and having 3 mm thick walls works well with a heat spreader that is 20 mm square. 
         [0041]      FIG. 4  further illustrates the cartridge  40  of the embodiment shown in  FIG. 3 . Frame  42  circumferentially surrounds the heat spreader  44  which has spring  46  attached thereto. Flexible membrane  52  encloses the void between the heat spreader  44  and frame  42  on one side thereof. The cartridge  40  as shown here has not yet been filed with phase change material. 
         [0042]    Alternative embodiments, examples, and modifications which would still be encompassed by the disclosure may be made by those skilled in the art, particularly in light of the foregoing teachings. Further, it should be understood that the terminology used to describe the disclosure is intended to be in the nature of words of description rather than of limitation. 
         [0043]    Those skilled in the art will also appreciate that various adaptations and modifications of the preferred and alternative embodiments described above can be configured without departing from the scope and spirit of the disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the disclosure may be practiced other than as specifically described herein.