Abstract:
The invention described herein provides a core modem assembly for a communication circuit that is well suited for mobile communication devices such as cellular phones, personal digital assistants, laptop computers, and hand-held computers. The core modem assembly comprises a baseband module, an RF module, and a socket connector that provides electrical isolation between the baseband module and RF module. The core modem assembly achieves a very small form factor and provides shielding between the baseband module and RF module. Further, the core modem assembly uses a “plug and play” design allowing late customization and interchangeability of the modules.

Description:
BACKGROUND 
     The present invention relates generally to the mounting of circuit components to a circuit board in mobile communication devices or other electronic devices and, more particularly, to a method and apparatus for mounting circuit components to provide electrical isolation between a radio frequency module and baseband module in a mobile communication device. 
     Electromagnetic interference is a common and well known problem associated with mobile communication devices such as cellular phones, personal digital assistants, laptop computers, and hand-held computers. In normal operation, the RF (radio frequency) module in a mobile communication device creates electromagnetic waves which induce undesirable signals that interfere with the normal operation of baseband circuits and other circuit elements in the mobile communication device. These undesirable signals, termed electromagnetic or RF interference, can be minimized by shielding the RF module within a conductive shield at a low or ground potential. The grounded electromagnetic shield dissipates electrostatic build up and absorbs the electromagnetic field thereby isolating the RF module from other circuit components. 
     One of the challenges facing mobile phone manufacturers is providing adequate shielding between the RF module and baseband module of the mobile phone while maintaining a small form factor. 
     SUMMARY 
     The present invention relates to a communication circuit with a core modem assembly that is well suited for mobile communication devices such as cellular phones, personal digital assistants, laptop computers, and hand-held computers. The core modem assembly comprises a baseband module, an RF module, and a socket connector that provides electrical isolation between the baseband module and RF module. The core modem assembly achieves a very small form factor and provides shielding between the baseband module and RF module. Further, the core modem assembly uses a “plug and play” design allowing late customization and interchangeability of the modules. 
     Exemplary embodiments of the invention comprise a core modem assembly for a mobile communication device. One exemplary core modem assembly comprises a radio frequency module comprising a first substrate and one or more radio frequency components mounted to a bottom surface of the first substrate; a baseband module comprising a second substrate and one or more baseband components mounted to a top surface of the second substrate; and a socket connector for mounting the radio frequency module and the baseband module to a main circuit. The socket connector may comprise a frame including a first mounting surface for mounting the radio frequency module and a second mounting surface higher than the first mounting surface for mounting the baseband module above the radio frequency module to provide a predetermined vertical separation between the radio frequency module and the baseband module, a first set of signal contacts disposed on the first mounting surface for making electrical connections with the radio frequency module, and a second set of contacts on the second mounting surface for making electrical connections with the baseband module. 
     Other exemplary embodiments of the invention comprise a communication circuit for a mobile communication device. One exemplary communication circuit comprises a main circuit; a radio frequency module comprising a first substrate and one or more radio frequency components mounted to a bottom surface of the first substrate; a baseband module comprising a second substrate and one or more baseband components mounted to a top surface of the second substrate; and a socket connector for mounting the radio frequency module and the baseband module to the main circuit. The socket connector may comprise a frame including a first mounting surface disposed adjacent an interior opening of the socket connector for mounting the radio frequency module and a second mounting surface disposed adjacent the outer perimeter of the socket connector and higher than the first mounting surface for mounting the baseband module above the radio frequency module to provide a predetermined vertical separation between the radio frequency module and the baseband module, a first set of signal contacts disposed on the first mounting surface for making electrical connections with the radio frequency module, and a second set of contacts on the second mounting surface for making electrical connections with the baseband module; and a set of ground contacts on the second mounting surface to provide ground connections for the baseband module. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a communication circuit including a core modem assembly according to one embodiment of the present invention. 
         FIG. 2  is an exploded section view of the core modem assembly according to a first embodiment. 
         FIG. 3  is a section view of the core modem assembly according to the first embodiment with the components assembled together. 
         FIG. 4  is an exploded section view of the core modem assembly according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings,  FIGS. 1-3  illustrate a communication circuit and core modem assembly  10  suitable for a mobile communication device, e.g., a cellular phone, personal digital assistant, laptop computer, hand-held computer, etc., according to one exemplary embodiment of the present invention. The communication circuit comprises a main circuit in a printed circuit board or other substrate (e.g., flex circuit, LTCC, etc.)  12  and a core modem assembly  10  mounted to the printed circuit board  12 . Core modem assembly  10  generally comprises an RF module  20 , a baseband module  40 , and a socket connector  60  for mounting the RF module  20  and baseband module  40  to the printed circuit board  12 . As described in greater detail below, the core modem assembly  10  provides electrical isolation between the baseband module  40  and RF module  20  while maintaining a small form factor. 
     RF module  20  contains the RF components  24  of a radio frequency transceiver. The RF module  20  comprises a substrate  22  having a bottom surface and a top surface. Substrate  22  may comprise, for example, a printed circuit board, flex circuit, or LTCC. RF components  24  mount to the bottom surface of the substrate  22 . In some embodiments of the invention, a copper foil layer on the top surface of the substrate  22  forms a ground plane  26 , which helps to electrically isolate the RF components after the core modem assembly is assembled. A ball grid array (BGA)  28  is formed on the bottom surface of the substrate  22  along the perimeter thereof. The ball grid array  28  provides electrical connection with the socket connector  60  as will be hereinafter described. In the exemplary embodiment shown, ball grid array  28  includes a single row of solder balls disposed around the perimeter of the substrate  22 . 
     Baseband module  40  contains the baseband components (analog and digital)  44  for processing signals transmitted and received by the mobile communication device. The baseband module  40  may also include memory and other circuit components. The baseband module  40  comprises a substrate (e.g., printed circuit board, flex circuit, LTCC, etc.)  42  having a bottom surface and a top surface. Baseband components  44  mount to the top surface of the substrate  22 . Memory components and other circuit components may also mount to the top surface of the substrate  42 . In some embodiments, a layer of copper foil may be applied to the bottom surface of the substrate  42  to provide a ground plane for electrically isolating the baseband components  44  from the RF module  20 . A ball grid array  48  is disposed on the bottom surface of the substrate  42  adjacent the perimeter thereof to provide means for making electrical connections between the baseband module  40  and the socket connector  60 . The ball grid array  48  includes two rows of solder balls. The outer row of the ball grid array  48  is used to connect to signal lines in the socket connector  60  as hereinafter described. The inner row of the ball grid array  38  is used to connect to ground lines in the socket connector  60 . 
     Socket connector  60  comprises a generally square or rectangular frame  62  having an interior opening  64  to receive the RF module  20 . The socket connector  60  includes two mounting surfaces  66 ,  68  for mounting the RF module  20  and baseband module  40  respectively. The first mounting surface  66  comprises a recessed mounting surface disposed adjacent the interior opening  64  of the socket connector  60 . The top surface of the frame  62  adjacent the outer perimeter of the socket connector  60  forms the second mounting surface  68 . The mounting surfaces  66  and  68  are vertically offset to provide a predetermined vertical separation between the RF module  20  and baseband module  40 . The vertical separation helps provide isolation between the RF module  20  and baseband module  40 . 
     The socket connector  60 , in addition to providing support for the RF module  20  and baseband module  40 , also electrically connects the RF module  20  and baseband module  40  to the main circuit on the printed circuit board  12 . A first set of signal contacts or pads  70  are disposed on the first mounting surface  66 . The first set of contacts  70  form an electrical connection with the ball grid array  28  on the RF module  20  when the RF module is inserted into the socket connector  60 . Similarly, the second mounting surface  68  includes a row of second signal contacts  72  and a row of ground contacts  74  that form an electrical connection with the ball grid array  48  on the baseband module  40  when the baseband module  40  is inserted into the socket connector  60 . 
     A ball grid array  76  is disposed on a bottom surface of the socket connector  60  for forming electrical connections with the printed circuit board  12 . The ball grid array  76  comprises three rows of solder balls  78 ,  80 , and  82 . A first set of signal lines  84  for the RF module (hereinafter the RF signal lines  84 ) connect the innermost row  78  of solder balls in the ball grid array  76  to the contact pads  70  on the first mounting surface  66 . A second set of signal lines  86  for the baseband module  20  (hereinafter the baseband signal lines  86 ) connect the outermost row  80  of solder balls in the ball grid array  76  to the second set of contacts  72  on the second mounting surface  68 . A set of ground lines  88  electrically connect the middle row  82  of solder balls to the ground contacts  74  on the second mounting surface  68  to ground the baseband module  20 . Ground lines  88  form a ground wall between the first and second sets of signal lines  84 ,  86  to prevent unwanted coupling between the RF signal lines  84  and the baseband signal lines  86 . 
     In use, socket connector  60  surface mounts to the printed circuit board  12  so that the solder balls in the ball grid array  76  make electrical contact with pads on the printed circuit board  12 . Heat may be applied to reflow the solder balls in the ball grid array  76 . Alternatively, clips or other retaining devices may be used to retain the socket connector  60  on the printed circuit board  12 . The printed circuit board  12  may include a ground plane  14  that is disposed below the RF module  20 . In some exemplary embodiments, ground plane  14  may be recessed from the top surface of the printed circuit board  12 . 
     After the socket connector  60  is mounted, the RF module  20  and baseband module  40  are mounted to the socket connector  60 . RF module  20  inserts into the interior opening  64  of the socket connector  60  and is sized to rest on the recessed mounting surface  66 . The ball grid array  28  on the RF module  20  makes electrical contact with the signal contacts  70  on the first mounting surface  66 . Heat may be applied to reflow the solder balls in the ball grid array  28 . Alternatively, clips or other retaining devices may be used to retain the RF module  20  in the socket connector  60 . 
     The final step in the assembly of the core modem assembly  10  is to mount the baseband module  40 . After the RF module  20  is securely in place, the baseband module  40  is mounted to the second mounting surface  68 . The outer row of solder balls in the ball grid array  48  on the baseband module  20  make electrical contact with the signal contacts  72  on the second mounting surface  68 . The inner row of solder balls make electrical contact with the ground contacts  74 . Heat may be applied to reflow the solder balls in the ball grid array  48 . Alternatively, clips or other retaining devices may be used to retain the baseband module  40  in the socket connector  60 . Another alternative is to provide a lid or cover (not shown) to retain the baseband module  40  in the socket connector  60 . 
     The core modem assembly  10  according to the present invention provides isolation between the RF module  20  and the baseband module  40  when the core modem assembly  10  is assembled on the printed circuit board  12  as shown in  FIG. 3 . More particularly, the ground plane  14 , ground plane  26 , and the ground wall  88  of the socket connector  60  form a shielded enclosure around the RF module  20  that dissipates the undesired electromagnetic signals emanating from the RF module  20 . The ground wall  88  also provides electrical isolation between the signaling paths for the RF module  20  and baseband module  40 . 
       FIG. 4  illustrates an alternate embodiment of the present invention. In this embodiment, the socket connector  60  is integrally formed with the printed circuit board  12 . The RF module  20  and baseband module  40  mount directly to the printed circuit board  12 . More particularly, the printed circuit board  12  includes a first recessed mounting surface  66  to support the RF module  20  and a second mounting surface  68  (e.g., top surface) to support the baseband module  40 . The RF module  20  and baseband module  40  connect to the integrally formed socket connector  60  as described above. 
     The present invention may, of course, of course, be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.