Abstract:
An exemplary bridging inter-connector establishes electrical connections between conductors on a PCB and aligned conductors on a first board mounted to the PCB. A flexible non-conductive sheet covers at least a portion of these conductors. Separated conductive strips on the sheet that are dimensioned to align with and engage at least a portion of both the aligned conductors. A thin film of a bonding agent is disposed on the separated conductive strips and located to engage at least a portion of both aligned conductors to form a conductive connection.

Description:
BACKGROUND 
       [0001]    This invention relates to the inter-connection of two physically separated electronic circuits and more specifically relates an interposer that connects conductive traces/conductors on two physically different boards with each other. 
         [0002]    Various types of inter-connections have been utilized to connect electronic circuits on different physical media. For example, wires and cables have been commonly used to connect one circuit/conductive path with another circuit/conductive path. These techniques are commonly used to connect one circuit on a printed circuit board (PCB) with another circuit on a separate PCB. For smaller scales, e.g. integrated circuits (IC), other techniques such as wire bonding have been utilized. For example, an integrated circuit component/die may have a plurality of electronic interconnections made by utilizing a wire bond between a conductive path on the component and a conductive runner/lead associated with the packaged IC. Although wire bonds have been successfully utilized, wire bonding typically requires costly equipment to make the wire bonds. Additional challenges are present where inter-connections are required to couple circuits carrying very high frequency signals which may be adversely impacted if excessive changes in impedance, resistivity or inductive/capacitance effects are encountered due to the inter-connections. There exists a need for a less expensive inter-connector than wire bonding that can accommodate the demands of high-frequency circuits. 
       SUMMARY 
       [0003]    It is an object of the present invention to satisfy this need. 
         [0004]    An exemplary bridging inter-connector establishes electrical connections between conductors on a PCB and aligned conductors on a first board mounted to the PCB. A flexible non-conductive sheet covers at least a portion of these conductors. Separated conductive strips on the sheet that are dimensioned to align with and engage at least a portion of both the aligned conductors. A thin film of a bonding agent is disposed on the separated conductive strips and located to engage at least a portion of both aligned conductors to form a conductive connection. 
         [0005]    In another embodiment, an electronic assembly includes a PCB, an electronic module on a first board is mounted with the PCB, and a bridging inter-connector as described above establishes connections between conductors on the PCB and first board. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which: 
           [0007]      FIG. 1  is an exploded view showing an embodiment of an interposer in accordance with the present invention utilized for an exemplary application. 
           [0008]      FIG. 2  shows the interposer of  FIG. 1  assembled with the exemplary application. 
           [0009]      FIG. 3  is a partial cross-sectional view showing a conductive strip on the interposer engaging and forming a conductive connection between respective conductive strips on two separated boards. 
           [0010]      FIG. 4  shows an enlarged view of an exemplary conductive element of the interposer suited to effectively carry a high frequency signal. 
           [0011]      FIG. 5  is a partial view of an interposer in accordance with the present invention illustrating a connection between vertically offset surfaces with conductors to be interconnected. 
           [0012]      FIG. 6  is a partial view of an interposer showing an exemplary conductive element for which perfect alignment with the conductors to be interconnected may not be achieved. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    One aspect of the present invention resides in the recognition of the difficulties and expense associated with using wire bonds to connect an electronic/IC module with conductors on a PCB, especially where high frequency and/or high power RF signals must be transported over the connection. In accordance with the recognition of this problem, a completely different approach to establishing such inter-connections was conceived and is illustrated in accordance with the embodiments of the invention as described below. 
         [0014]      FIG. 1  shows an exploded view of an embodiment of an interposer  105  in accordance with the present invention as utilized for an exemplary inter-connection application. Interposer  105  is preferably constructed from a thin, low loss, resilient material such as a polyimide film, e.g. DuPont copper-clad laminated composite LF8510R. The interposer  105  contains an interior cut out or opening  110  dimensioned to accept the passage of any parts extending from the board or substrate to be connected to a PCB. Two inwardly extending and opposing projections  112  of the interposer  105  each contain, in the exemplary embodiment, two sets of four conductive strips  115  and  120  on the bottom surface of the interposer  105 . In the exemplary embodiment, each of the conductive strips  115  and  120  are formed of copper that is clad to the polyimide film. Similarly, two opposing conductive strips  125  and  130  are also formed on the other sides of interposer  105 . As will be explained in more detail below, each of the conductive strips preferably has disposed thereon a thin conductive bonding agent for establishing a conductive attachment between conductive surfaces on the board/substrate and corresponding conductive surfaces on the PCB. 
         [0015]    In the exemplary application, the interposer  105  is utilized to establish electrical connections between circuitry on an exemplary electronic module  145 , e.g. monolithic microwave integrated circuit (MMIC), and conductors on a PCB  170 . Although referenced in the description as being an MMIC, the electronic module can contain any type of electronic circuitry or electronic elements, e.g. one or more antenna elements. The MMIC  145  includes a rigid planar board  150  to which a variety of active and passive components forming various RF circuitry are supported under the housing and shield  155 . Capacitors  157  are mounted to board  150  external of the housing as shown. Corresponding to and in alignment to engage with conductive strips  115  and  120  (when assembled), board  150  contains two sets of four conductors  160  which may be utilized to provide input and/or output DC or low frequency AC signals. Similarly, two opposing conductors  165  on board  150  may couple a high frequency RF input signal to and a high frequency and/or high power RF output signal from the MMIC  145 . Conductors  125  and  130  of interposer  105  correspond to and are in alignment to engage with the RF input and output conductors, respectively, on board  150  when assembled. 
         [0016]    A PCB  170  includes a opening or recess  175  dimensioned to receive the peripheral edges of board  150  in it. A heat sink  180  engages the bottom surface of PCB  170  and preferably engages the bottom surface of board  150  when the latter is disposed within the opening  175 . Two sets of four opposing conductive traces  185  are contained on PCB  170  and are dimensioned to align with and engage the conductive strips  115  and  120  on interposer  105  and are also aligned with the respective conductive strips  160  on board  150  of the MIMIC  145 . Two opposing microstrip transmission lines  192  and  194  are also contained on the PCB  170  and are disposed to be in alignment with and to be engaged by the conductive strips  125  and  130  of interposer  105  and are in alignment with the respective input and output conductive strips  165  of board  150  when assembled. The PCB  170  and heatsink  180  as shown represents only a portion of a larger PCB and heatsink assembly that will contain a plurality of other active and passive components that will interface and connect with the MIMIC  145  via connections on the PCB  170 . 
         [0017]      FIG. 2  shows the interposer  105  of  FIG. 1  assembled in the exemplary application in which board  150  is seated within the opening  175  of PCB  170 . Preferably the depth of the opening  175  in PCB  170  is substantially equal to the thickness of the board  150  so that the top surface of the board  150  and the top surface of PCB  170  are level. Alternatively, filler materials could be used to adjust the height of the surfaces so that the top surfaces of the boards are substantially level. The interposer  105  is dimensioned so that each of the conductive strips  115 ,  120 ,  125  and  130  engage both the corresponding conductive strips  160 ,  165  on board  150  and the corresponding conductive strips  185 ,  190 ,  192  and  194  on the PCB  170 . Thus, the conductive strips on interposer  105  form a conductive bridge between the respective conductive strips on board  105  and on the PCB  170 . In order for board  150  to be easily seated within the opening  175  of the PCB  170 , the external dimensions of the board  150  are preferably a little less than the dimensions of the internal edges of the PCB defining the opening  175 . This, of course, means that there will exist a small air gap between the edges of the board  150  and the respective edges of the PCB  170  that define the opening  175 . The conductive strips of the interposer  105  serves to provide continuity between the respectively aligned conductive strips on the board  150  and the conductive strips on the PCB  170 . 
         [0018]      FIG. 3  is a partial cross-sectional view showing an exemplary elongated conductive strip  305  such as made of copper on the interposer  105  engaging and forming a conductive connection between conductive strip  310  on PCB  170  and conductive strip  315  on board  150 . The external distal end of conductive strip  305  contains a segment  320  of a bonding agent that extends from the distal end of the conductive strip  305  but stops short of the air gap  330 . Similarly, the internal distal end of conductive strip  305  contains a segment  325  of a bonding agent that extends from the internal distal end of the conductive strip  305  but again stops short of the air gap  330 . Leaving an intermediate portion of the conductive strip  305  opposite and adjacent the air gap  330  of the bonding agent maintains increased flexibility of the intermediate portion of the interposer  105  and prevents undesired bridges or filling-in of the air gap  330 . The length of this gap in bonding agent should be approximately three times the thickness of the conductive strip  305 . This may be preferred in order to accommodate a potential vertical misalignment between the top surface of the PCB  170  and the top surface of board  150 . Alternatively, the bonding agent may be distributed over the entire length of the conductive strip  305  of interposer  105  if maximum flexibility is not a concern. The bonding agent may consist of solder, a conductive epoxy, or a suitable conductive adhesive. The bonding agent may be predisposed on the external surface of the conductive strip  305  prior to assembly or, alternatively, could be placed on the external surfaces of conductive strips  310  and  315  in an area to be engaged by the conductive strip  305  when assembled. In order to minimize adverse impacts that might be caused by such connections where high-frequency and/or high power RF signals traverse the connections, including consideration of impedance variations and high power handling capability, a preferred thickness of bonding agent is 0.5 mil to 2.0 mils. However, if such considerations are not relevant to a particular circuit module to be attached to the PCB, other thicknesses of bonding agents could be utilized. If solder is used as the bonding agent, it may be preferable to electroplate the solder on the conductors  305  of the interposer  105  in order to maintain better control of the thickness of the applied solder. As will be apparent, if solder is utilized as the bonding agent, heat will need to be applied to at least the regions containing the solder following the assembly of the MIMIC  140  into the recess in the PCB  170 , and following the placing of the interposer  105  in its final assembled position to melt and adhere the solder to both surfaces. 
         [0019]      FIG. 4  shows an enlarged view of an exemplary conductor  405  of the interposer  105  especially suited for a carrying a high power and/or high frequency RF signal. In the illustrative application, conductor  405  preferably represents each of conductors  125  and  130  as best seen in  FIG. 1 . The conductor  405  extends perpendicularly inward from the edge of interposer  105  towards the board  150 . The conductor  405  includes a section  410  extending from the external distal end to reference line  415  forming an elongated rectangle having a width parallel to the edge of interposer  105  slightly smaller than or equal to the width of the conductor of microstrip  192 ,  194  disposed on PCB  170 . The conductor  405  includes two symmetrical tapering sides  420  extending from reference line  415  to the internal end  425  with the taper having an exemplary angle  430  of 78° between reference line  415  and perpendicular reference line  435 . In an exemplary embodiment, the length of section  410  from the reference line  415  to the distal end at the edge of interposer  105  is 0.013 inches and the length of the end  425  parallel to reference line  415  and centered about the longitudinal axis is 0.006 inches. The width of the exemplary corresponding conductor  115  engaged by conductor  405  is 19 mils. Width of section  410  is 16 mils. All dimensions are preferably determined based on an electromagnetic simulation of the structure dependent on the user&#39;s particular choices of substrate materials. Simulations of high frequency signals, e.g. 20 GHz and above, have shown that the conductor  405  with the tapered portion provides improved performance in terms of minimizing adverse impedance changes as compared with providing only a conductor having a rectangular shape. In order to maintain increased flexibility to accommodate vertical surface mismatches, it is preferred that the intermediate section of conductor  405  between reference lines  415  and  440  not contain the bonding agent. 
         [0020]      FIG. 5  is a partial view of an exemplary interposer assembly in accordance with the present invention illustrating an RF connection made by conductor  405  between conductors  194  of PCB  170  and conductor  165  on board  150  where the external surfaces of PCB  170  and board  150  are vertically offset to each other. In the illustrated embodiment, the portion of conductor  405  with tapered sides  420  is bonded to the conductor  165  of board  150  and the rectangular portion of conductor  405  to the right of reference line  440  ( FIG. 4 ) is bonded to conductor  194  of PCB  170 . In this example, the surface of board  150  is 2 mils higher than the surface of PCB  170 . As seen, section  505  of interposer  105  is sufficiently resilient to flex over this offset so as to permit the bonded end portions of the conductor  405  to make a flat/parallel contact with the respective conductors on board  150  and PCB  170 . This flexibility also allows for an amount of subsequent physical movement of the boards relative to each other without causing a break or fracture of the connection. The exemplary conductor  405  maintains good RF transfer characteristics even with such an offset. For example, for an RF signal centered at approximately 28 GHz, the conductor  405  even with a two mil vertical offset between boards, has been simulated to provide a return loss exceeding −30 dB over the range from 25 GHz to 32 GHz. Over this same frequency range, the conductor  405  was simulated to provide an insertion loss of less than −0.257 dB. 
         [0021]      FIG. 6  is a partial view of an interposer  105  useful in considering lateral misalignment of an exemplary conductive element  405  with the conductors on the PCB  170  and board  150 . A lateral direction is indicated by the reference arrows  605 . That is, this represents the potential direction for a lateral misalignment of conductor  405  on either side of conductors  194  and  165 , which for this example, are assumed to be in alignment. A simulation of RF performance for the exemplary conductor  405  indicates that for a lateral misalignment of the conductor  405  of two mils resulted in a return loss of better than 29 dB across a frequency band of 25 GHz to 32 GHz. A similar simulation showed a return loss of less than 20 dB across the same frequency band with five mils of misalignment. For a lateral misalignment of five mils or less, an insertion loss for the connection made by conductor  405  was simulated to be better than −0.26 dB across this frequency band and had less than 0.05 dB of insertion loss variation. Thus, the conductor  405  as used to bridge connections between the MMIC board and PCB provides an effective connection mechanism with good RF performance even in view of potential misalignment during assembly. 
         [0022]    The exemplary interposer provides improvements and advantages for supporting high-frequency and high power RF connections as compared with conventional wire bonds. A simulation shows that a double wire bond is required to substantially equal the performance of the above-described RF connection provided by the interposer in accordance with the present invention. A double wire bond consists of two parallel wire bonds where each provides the same connection between two points. Typically, because of higher inductance provided by a wire bond as contrasted to the RF conductor of the present invention, more complex circuitry is required in a high-frequency RF environment to compensate for the inductance inherently present with the wire bond. Also, a typical wire bond has less RF power and/or DC current carrying capability as contrasted with the RF conductor of the present invention. In addition, the connections provided in accordance with the interposer of the present invention do not require complex and expensive equipment which would be required to install wire bonds. 
         [0023]    Making an exemplary interposer in accordance with the present invention may begin by utilizing a flexible film material as suggested above that is copper-clad on one surface. Except for the copper strips to be utilized to provide electrical connections, the remainder of the copper-clad material may be removed such as by etching. If a high-volume is required for a particular interposer, a customized punch may be utilized to make the interior opening in the interposer as required to fit the outline of the circuitry/board to be installed on a PCB. A bonding agent may be disposed on each copper strip, preferably as described above leaving an intermediate section of the copper strip without any bonding agent in order to enhance resiliency in the intermediate section. For example, solder may be electroplated on to the desired areas of each copper strip. As explained above, the geometric configuration of copper strips that will be transporting high frequency RF signals may be advantageously shaped to minimize adverse characteristics associated with the interconnection. After the module to be connected to the PCB is installed on the PCB, the interposer is placed around the module so that the respective conductive strips on the interposer align with and engage the respective conductive paths on the module and PCB. If the bonding agent utilized is solder, an appropriate amount of heat is applied to at least the conductive strip areas of the interposer in order to establish soldered connections. 
         [0024]    Although exemplary implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention. For example, the interposer may have an external geometric shape that is other than rectangular and may be utilized to conform to the general shape of the module to be connected to the PCB. As will be apparent, the interposer need not have a closed geometric shape if utilized to connect a module that has connections to be made to the PCB only in a limited area or on a specific side. That is, the interposer could be formed as a single elongated strip with no internal opening where the module to be connected to the PCB has only connections to be made in a region that can be encompassed by the single elongated strip. Also the interposer could be used to couple conductors on adjacent edges of two boards where no portion of either board overlaps or lies within the other board. Alternatively, an interposer could have a plurality of separated interior openings for providing a plurality of interconnections with multiple electronic elements, e.g. making connections with multiple elements of a high frequency RF antenna array where corresponding openings in the interposer accommodate individual elements of the antenna and/or mechanical structures. 
         [0025]    The scope of the invention is defined in the following claims.