Abstract:
A thermal interposer for use in providing a mating interface between a heat sink and an electronic module includes an elongated body portion having two opposing surfaces. On one surface, a plurality of press-fit pegs are defined that extend upwardly and outwardly away from the interposer body portion. On the other, opposing surface, a plurality of contact arms are defined that extend, in cantilevered fashion, downwardly and away from the interposer body portion. The press-fit pins are configured to enter grooves formed in a heat sink in a manner to form intimate, metal to metal, contact while the contact arms are configured to contact the top surface of an electronic module with reliable normal force.

Description:
CROSS REFERENCE To RELATED APPLICATION 
       [0001]    The Present Disclosure claims priority to U.S. Provisional Patent Application No. 61/839,414, entitled “Thermal Interposer Suitable For Electronic Modules,” and filed 26 Jun. 2013 with the United States Patent And Trademark Office. The content of this Application is incorporated in its entirety herein. 
         [0002]    The Present Disclosure claims the benefit of U.S. Provisional Patent Application No. 61/839,412 (Molex Docket No. B2-022 US PRO), entitled “Ganged Shielding Cage With Thermal Passages,” filed on the same day as the priority claim listed above, the content of which is hereby incorporated herein. 
         [0003]    Finally, the Present Disclosure also claims the benefit of Co-Pending U.S. Patent Cooperation Treaty Patent Application No. ______ (Molex Docket No. B2-022 WO), entitled “Ganged Shielding Cage With Thermal Passages,” filed on the same day as the Present Disclosure, the content of which is hereby incorporated herein. 
     
    
     BACKGROUND OF THE PRESENT DISCLOSURE 
       [0004]    The Present Disclosure relates, generally, to thermal solutions to heat transfer of electronic modules, and, more particularly, to a thermal interposer having improved contact characteristics to improve heat transfer between an electronic module and a heat sink. 
         [0005]    There are many different styles of heat sinks used in the field of electronics. In many electronic devices, such as routers, servers and the like, different sets of circuits need to connect to associated circuits of other electronic devices. This is commonly accomplished by way of cable assemblies that typically include an electronic module terminated to each end of the cable. The modules serve to connect the cable to a corresponding connector on a circuit board within the device and with respect to routers and servers, these cable assemblies operate at high data transfer speeds, the operation of which generates heat. 
         [0006]    Heat sinks are utilized to transfer heat generated by the module to the exterior of shielding cage in which the module is inserted. Most of these heat sinks are applied directly to the surface of the module and thus require particular configuration for each module. Others rely upon a thermal interface material, referred to as a “TIM,” and these TIM materials increase the thermal resistance of the overall assembly, as well as the cost. Other heat sinks are rigidly attached by adherent materials, such as solder, which may add to the overall thermal resistance and also may affect the structural and mechanical operation of the attachment. Solder also creates problems with certain materials used in heat sinks, such as aluminum as oxide barrier may form on the aluminum during soldering. Still further, due to the dissimilarity of solder with aluminum, galvanic corrosion may occur in the finished heat sink. 
         [0007]    Some have developed a thermal interposer that utilizes a plurality of cantilevered contact arms arranged in a pattern on the interposer. The interposer is rigidly attached to the heat sink by soldering, which inhibits the contact arms from operating in an elastic manner. This rigid attachment results in a permanent set across the face of the interposer and induces plastic strain in the contact arms. This plastic strain does not promote good Hertzian contact, and diminishes the elasticity of the contact arms. When this occurs, the normal force between the contact arms and the opposing surface of the module is reduced. 
         [0008]    The Present Disclosure is therefore directed to an improved thermal interposer that does not require a continuous rigid attachment and which is particularly suitable for use with electronic modules, the interposer having an attachment structure that retains a reliable normal force and good Hertzian contact between the interposer and the electronic module. 
       SUMMARY OF THE PRESENT DISCLOSURE 
       [0009]    Accordingly, there is provided a thermal interposer suitable for electronic module applications, providing a reduced cost structure for attachment to a heat sink and further providing reliable, beneficial contact between the interposer and the electronic module. In accordance with an embodiment of the following Present Disclosure, a thermal interposer is provided for positioning between a heat sink and an electronic module and the interposer is provided with a structure that permits good, reliable contact with both the heat sink and the module, without the need to use any thermal interface material. 
         [0010]    The interposers of the Present Disclosure are formed from a flat plate-like member that has a width matching or exceeding to some extent, the width of the electronic module. On one surface of the module, preselected, discrete portions of the plate-like member are bent upwardly. These bent portions define a series of pegs or the like that are configured to fit within selected grooves, or channels, that are formed in the bottom surface of a heat sink member. Such a fit is a press fit attachment accomplished with high mechanical pressure, creating in effect, a solid joint between the interposer pegs and the heat sink. Such a joint has low thermal resistance, much lower, and typically minimal, at best, than that obtained using a thermal interface material. The press fit application also serves to remove oxides from the aluminum surfaces of the heat sink grooves which would otherwise increase the thermal resistance and thereby improves heat transfer between the heat sink and the module, by way of the interposer. 
         [0011]    The press-fit pegs eliminate the need for a continuous rigid manner of attachment of the interposer to the heat sink. This is important because the interposer has a series of cantilevered contact arms stamped, or otherwise, formed therein and these contact arms have their free ends bent downwardly toward an opposing surface of the electronic module. These arms are intended to be elastic and they remain so due to the press-fit attachment. If the interposer were to be rigidly attached to the heat sink, such as by way of solder, welding or the like, the solder would form an attachment to the contact arms, especially near the radius around which the contact rams flex. The presence of the continuous rigid attachment would cause the contact arms to become plastic, rather than elastic, and this condition would inhibit the application of reliable normal forces by the contact arms onto the module surface. 
         [0012]    The press-fit pegs are arranged in a pattern that separates them into two distinct groups. A first group of such pegs are arranged around a portion of the perimeter of the interposer body portion and in one embodiment described herein, along two opposing, longitudinal edges of the interposer. The second group of press-fit pegs are disposed interior of the perimeter and are arranged between adjacent rows of contact arms. The base portion of the press-fit pegs are arrange longitudinally as are the contact arms but the press-fit pegs have their base portion oriented perpendicular to the based portions of the contact arms. In this manner, as described in one embodiment of the Present Disclosure, a series of L-shaped heat transfer paths are defined between pairs of associated press-fit pegs and contact arms. 
         [0013]    These and other objects, features and advantages of the Present Disclosure will be clearly understood through a consideration of the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0014]    The organization and manner of the structure and operation of the Present Disclosure, together with further objects and advantages thereof, may best be understood by reference to the following Detailed Description, taken in connection with the accompanying Figures, wherein like reference numerals identify like elements, and in which: 
           [0015]      FIG. 1  is a perspective view of a single electronic module, in place within one bay of a ganged shielding cage, utilizing an interposer constructed in accordance the Present Disclosure; 
           [0016]      FIG. 2A  is a perspective view, taken from the bottom of the shielding cage-module assembly of  FIG. 1 , with the bottom and some of the side walls of the shielding cage removed for clarity, along with the bottom half of the electronic module; 
           [0017]      FIG. 2B  is the same view as  FIG. 2A , but with the electronic module removed for clarity and illustrating the thermal interposer in place on the bottom surface of the heat sink; 
           [0018]      FIG. 3A  is the same view as  FIG. 2B , but with the cage side wall removed and enlarged to illustrate the array of contact arms formed on the thermal interposer; 
           [0019]      FIG. 3B  is a front elevational view of the thermal interposer in place upon the heat sink and illustrating the manner of connection therebetween; 
           [0020]      FIG. 4A  is a perspective view of the thermal interposer taken from the bottom surface thereof; 
           [0021]      FIG. 4B  is a perspective view of the thermal interposer of  FIG. 4A , but in an inverted fashion, illustrating the top surface thereof with the heat sink engaging pegs; 
           [0022]      FIG. 4C  is a top plan view of the thermal interposer of  FIG. 4A ; 
           [0023]      FIG. 4D  is a side elevational view of the thermal interposer of  FIG. 4A , along Line D-D; 
           [0024]      FIG. 4E  is an end elevational view of the thermal interposer of  FIG. 4A , along Line E-E; 
           [0025]      FIG. 4F  is an enlarged perspective view of a single contact arm used in a known interposer attached to the heat sink rigidly by way of solder in the shaded area; and 
           [0026]      FIG. 4G  is an enlarged view of a contact arm used in the interposers of the Present Disclosure attached to the heat sink by way of the attachment members. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    While the Present Disclosure may be susceptible to embodiment in different forms, there is shown in the Figures, and will be described herein in detail, specific embodiments, with the understanding that the Present Disclosure is to be considered an exemplification of the principles of the Present Disclosure, and is not intended to limit the Present Disclosure to that as illustrated. 
         [0028]    As such, references to a feature or aspect are intended to describe a feature or aspect of an example of the Present Disclosure, not to imply that every embodiment thereof must have the described feature or aspect. Furthermore, it should be noted that the description illustrates a number of features. While certain features have been combined together to illustrate potential system designs, those features may also be used in other combinations not expressly disclosed. Thus, the depicted combinations are not intended to be limiting, unless otherwise noted. 
         [0029]    In the embodiments illustrated in the Figures, representations of directions such as up, down, left, right, front and rear, used for explaining the structure and movement of the various elements of the Present Disclosure, are not absolute, but relative. These representations are appropriate when the elements are in the position shown in the Figures. If the description of the position of the elements changes, however, these representations are to be changed accordingly. 
         [0030]      FIG. 1  illustrates a partial shielding cage  10  typically mounted to a circuit board  11 . The shielding cage  10  is of the ganged type, meaning it has a plurality of bays  12  defined therein between side walls  14  of the cage. Each bay  12  is configured to receive an electronic module  15  therein that provides a connection between a cable containing a plurality of wires (not shown) and a connector mounted to the circuit board  11  and disposed within the bay  12  of the cage  10 . The electronic module  15  may be designed for high speed data transmission and as such, generates heat during its operation. This heat must be dissipated and therefore a heat sink  16  is provided that either lies on top of the cage  10 , or forms a top wall, or ceiling  17 , thereof. 
         [0031]    As illustrated in  FIG. 29 , an interposer, or leadframe,  20  is provided for the bay  12  in which the module  15  resides. The module  15  is shown removed in  FIG. 2B , as is the cage bottom, for clarity. The interposer  20  can be seen to have an elongated body portion  21 , illustrated as a rectangle in the Figures. The interposer has a plurality of side edges  22   a - d  that cooperatively define the body portion  21 . The interposer further has two opposing surfaces, shown as top and bottom surfaces  23 ,  24 , respectively and these surfaces make contact with the heat sink  16  and the electronic module  15  as explained in further detail below. 
         [0032]    In order for the interposer  20  to function as a thermal interposer—that is, one that transfers heat from the module  15  to the heat sink  16  the interposer  20  is firstly provided with a plurality of contact members, illustrated as cantilevered contact arms  25  that may be stamped and formed in the interposer body portion. These contact arms  25  are defined by U-shaped openings  26  formed in the interposer body portion; three parts of the openings  26  provide the cantilevered configuration to the contact arms  25 . The contact arms  25  have elongated base portion  27  aligned lengthwise within the interposer body portion  21 , and which terminate in free ends  28 , which may be coined, or otherwise treated, to form. contact surfaces  29  at the free ends. In use, these contact surfaces  29  make contact with the top surface  15   c  of the electronic module  15 . 
         [0033]    A plurality of attachment members  30  are disposed on the other (top) surface of the interposer. These attachment members  30  are illustrated as press-fit pegs  32  having base portions  33  where they are bent up from the interposer body portion. These base portions  33  terminate in pointed ends  34  having a generally triangular configuration, although other configurations may be suitable. The interior attachment members  30  have U-shaped openings that define their shape and permit them to be bent out of the plane of the interposer body portion into the desired upright shape. These attachment members  30  are configured to be received within grooves  40  formed in the bottom surface  16   b  of the heat sink  16 . The pointed ends  34  of the attachment members  30  permits the attachment members to be reliably inserted into the heat sink grooves  40  in such a manner that good and intimate metal-to-metal contact is made, with good heat transfer capabilities and low thermal resistance properties, about equal to that Obtained from a solid attachment. Thus, it is preferred that the attachment members  30  are slightly thicker than the width of the heat sink grooves  40 . 
         [0034]    As shown in the Figures, the grooves  40  run lengthwise within the heat sink  16  and the spacing between the grooves  40  defines an intended spacing between the attachment members  30 . It can be seen that the contact arms are arranged on the interposer body portion in a manner that defines a plurality of rows, running both lengthwise and crosswise (transversely) within the perimeter of the interposer  20 . The attachment members  30  are arranged in what may be considered as two distinct groups of attachment members  30 . The first group of attachment members  30  are those that are disposed substantially around the perimeter of the interposer, shown as positioned on side edges  22   a,    22   b,    22   c  in  FIG. 4C  and will be referred to herein as an “exterior” group of attachment members  30 . 
         [0035]    The second group of attachment members  30  are those remaining members disposed inwardly from the side edges of the interposer and will be referred to herein as an “interior” group of attachment members  30 . The interior attachment members  30  are disposed in rows that are positioned between rows of contact arms in  FIG. 4C . As such, the interior attachment members serve to divide the contact arms  25  into groups. In  FIG. 4C , two imaginary lines LA and CA are drawn in respective longitudinal and crosswise directions, interconnecting interior attachment members for the CA line and both interior and exterior attachment members for the LA line. The CA lines define crosswise rows of contact arms, while the LA lines define lengthwise rows of contact arms  25 . Cooperatively, the lines define imaginary boxes CAB that surround groups of contact arms  25 . These groups can either be arranged in the lengthwise or crosswise direction. Likewise, the imaginary lines separate adjacent contact arms  25  from each other. Still further it is preferred that the interior attachment members  30  are disposed close to where the contact arm body portions meet the interposer body portion. As illustrated, the location of the attachment members  30  with respect to the contact arms defines a series of individual thermal transfer paths “TP” between associated pairs of contact arms and attachment members  30 . As shown, the thermal transfer paths are L-shaped. 
         [0036]    The structure of this interposer and the grooves of the heat sink provide for a semi-rigid attachment of the interposer that differs from other rigid attachment structures, such as solders. With interposers  20  of the Present Disclosure, heat generated within the module  15  is transferred to the interposer  20  by way of conduction between the contact arms  25  and the interposer body portion  21 . The heat then travels from the interposer body portion  21  to the attachment members via the thermal transfer paths TP, and into the body of the heat sink by way of contact with the walls of the heat sink grooves  40 . Most heat sinks  16  are made out of aluminum, which is prone to oxidation, and the use of dissimilar metals promotes galvanic corrosion. The oxidation that occurs on aluminum surfaces makes soldering difficult and moreover, increases the thermal resistance of the overall structure, as does any thermal interface material such as adhesive, tape, gap filling pads, etc. Still further, as shown in  FIG. 4F , a solder attachment method creates problems with the contact arms of such an interposer in that the body portion of the interposer and part of the base of the contact arm are attached to the heat sink as shown in the shaded area of  FIG. 4F . Because of this area of attachment, plastic strains will occur along the entire width of the contact arm where it is joined to the body of the interposer, at arrow Z. Deflection of the interposer contact arms in this structure when the module is inserted into the shielding cage bay will cause plastic strain and the contact arm no longer becomes entirely elastic. This will detrimentally affect the normal forces required between the contact arms and the electronic module top surfaces. 
         [0037]    Utilizing interposers of the Present Disclosure eliminates these problems. The plastic strains which occur in the interposer contact arms occur in the body portion  21  of the interposer  20  as shown by Arrow Z in  FIG. 4G , thereby reducing, if not altogether eliminating, permanent set in the contact arms. This will maintain the normal force applied by the contact arms in the range desired by the designer to achieve good Hertzian contact. The attachment between the interposer and the heat sink is metal-to-metal and thus there is an overall reduced thermal resistance. 
         [0038]    While a preferred embodiment of the Present Disclosure is shown and described, it is envisioned that those skilled in the art may devise various modifications without departing from the spirit and scope of the foregoing Description and the appended Claims.