Patent Publication Number: US-6903934-B2

Title: Circuit board construction for use in small form factor fiber optic communication system transponders

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the use of optoelectronic components in fiber optic digital communication networks and, more particularly, to transponder assemblies and the electrical and circuit components within them. 
   2. Discussion of the Background 
   The majority of computer and communication networks in use today rely on copper wiring to transmit data between network nodes. However, electrical signals carried by copper wiring have a limited capacity for carrying digital data. 
   Many computer and communication networks, including large parts of the Internet, are now being built using fiber optic cabling that can be used to transmit much greater amounts of data. With fiber optic elements data is transmitted using optical signals (sometimes referred to as photonic signals) having greater data carrying capacity. 
   However, since computers use electrical signals as opposed to optical signals, the light signals used to transmit data over fiber optic links must be translated to electrical signals and vice-versa during the optical communication process. Building such fiber optic networks therefore requires optoelectronic modules which electrically and optically interface optical transmission mediums such as fiber optic cables with electronic computing and communications devices. Further, in order to provide the required bandwidth for very high-speed communications, fiber optic ribbon cables having multiple fiber optic elements and so-called “parallel optics” modules adapted for transmitting or receiving multiple signals over such may also be used. 
   The optoelectronic modules associated with fiber optic networks should therefore be adapted for accommodating fiber optic ribbon cables having multiple fibers. Further, it is desirable for separate parallel optic transmitter and receiver modules to be incorporated in transponder assemblies which can separately transmit and receive optical data over separate cables using the transmitter and receiver modules. However, for most applications these transponder assemblies must be compact and should utilize only the smallest possible footprint on the circuit boards within the electronic computing or communications devices with which the fiber optic network is interfacing. Accordingly, the components and circuitry within the transponder assembly must be designed to have the smallest possible dimensions. 
   SUMMARY OF THE INVENTION 
   The transponder assembly of the present invention comprises a small form factor system for interfacing between computer and communication systems and fiber optic cables having multiple fiber elements while providing concurrent electrical-to-optical and optical-to-electrical conversion functionality. The transponder assembly features a pair of optical communication ports one of which functions as a transmitter port and the other of which functions as a receiver port for interconnecting with parallel optics fiber optic ribbon cables. The transponder assembly also features a circuit board having a pin-array electrical connector and a semiconductor chip adapted for signal processing such as a Serializer/Deserializer (SerDes) chip mounted on different surfaces of the board. 
   The transponder assembly includes a parallel optics transmitter mddule and a parallel optics receiver module having pluggable edge connectors. A flexible printed circuit or Flex circuit is used to flexibly connect the circuit board to a connector board mounting a pair of connection jacks which interconnect with the edge connectors of the parallel optics modules. The semiconductor chip and pin-array connector are mounted directly across from one another on opposite surfaces of the circuit board using ball grid arrays having areas in which their attachment structures overlap. The vias associated with the two ball grid array attachments are positioned in between one another in the overlapping areas. The solder pads associated with the vias having the same connection functionality are interconnected by surface traces and redundant vias are eliminated in order to allow adequate pathways for circuit traces to within the circuit board structure to extend through the overlapping areas of the ball grid array attachments. Placement of the pin-array connector and semiconductor chip across from one another on the circuit board allows for an assembly having minimum lateral dimensions. 
   It is an object of the present invention to provide a transponder assembly adapted for use with fiber optic ribbon connectors having multiple fiber optic elements. 
   It is another object of the present invention to provide a transponder assembly which utilizes separate parallel optic transmitter and receiver modules in an assembly having a compact size and minimum footprint. 
   It is a further object of the present invention to provide for the internal components of a parallel optics transponder assembly to be flexibly interconnected whereby a compact size can be achieved. 
   It is yet another object of the present invention to provide for the a semiconductor chip and an electrical connector to be positioned directly across from one another on the same circuit board using ball grid array attachments on opposite surfaces of the same circuit board to electrically interconnect the semiconductor chip, electrical connector and circuit board. 
   It is a yet further object to provide an efficient and effective parallel optics transponder assembly featuring a compact size and small footprint. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and its advantages may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is an overhead, perspective view of a transponder assembly constructed in accordance with the principles of the present invention primarily showing the communication ports by which the assembly can be optically connected to a pair of fiber optic ribbon cables for separately transmitting and receiving digital data over separate cables. 
       FIG. 2  is a perspective view of the underside of the transponder assembly shown in  FIG. 1  primarily showing the pin-array connector for electrically the assembly to a circuit board or the like of a computer or communications system. 
       FIG. 3  is an overhead, perspective view of the transponder assembly of the present invention with the top section of the assembly housing removed showing the internal components within the transponder assembly and showing, among other things, the parallel optic transmitter and receiver modules within the assembly. 
       FIG. 4  is a side view of the internal components of the transponder assembly of the present invention showing, among other things, the parallel optic modules, a connector board with connection jacks, a Flex circuit and a rigid circuit board on which a SerDes chip and pin-array connector are mounted on opposing surfaces of the board. 
       FIG. 5  is a close-up side view along lines  5 — 5  in  FIG. 4  of the connector board, Flex circuit and rigid circuit board components primarily illustrating how these components are flexibly interconnected and aligned. 
       FIG. 6  is a close-up cross-sectional view of a typical section of the rigid circuit board on which the SerDes chip and pin-array connector are mounted showing an area in the vicinity of one of the ball grid arrays associated with either the chip or the connector. 
       FIG. 7  is an overhead, plan view of the circuit board on which the SerDes chip and pin array connector are mounted showing the ball grid array attachment structures associated with the SerDes chip and the pin array connector. 
       FIG. 8  is a close-up cross-sectional view of a typical section of the rigid circuit board on which the SerDes chip and pin-array connector are mounted showing an area of overlap between both of the ball grid arrays associated with the SerDes chip and pin-array connector. 
       FIG. 9  is a close-up overhead, plan view of the circuit board on which the SerDes chip and pin array connector are mounted exclusively showing ball grid array attachment structures associated with both the SerDes chip and pin-array connector in one of the overlapping areas between the ball grid arrays and their attachment structures. 
       FIG. 10  is an exploded, overhead perspective view of a parallel optics module which comprises one of the primary components of the transponder assembly of the present invention showing, among other things, how the receptacle, lens and alignment frame, carrier frame section, circuit board, edge connector and the other components and subcomponents of the module relate to one another. 
       FIG. 11  is an enlarged exploded, overhead perspective view of the forward portion of the module shown in  FIG. 10  primarily showing, how the receptacle, lens and alignment frame, carrier frame section and their subcomponents relate to one another. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will now be described in detail with reference to preferred embodiments as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it should be apparent to one skilled in the art that the present invention may be practiced without some or all of these specific details. In other instances it should be appreciated that well-known process steps have not been described in detail in order to not obscure the present invention. 
   Referring now to  FIGS. 1 and 2 , a transponder assembly  10  is shown as including a two-piece housing  12 , two communication ports  14  and  16 , a pin-array type electrical connector  18  and a heat sink  20 . The pin-array connector  18  is preferably a 300-pin MEG-Array™ electrical connector (which may be purchased from FCI USA, AREVA Group, having US offices at 825 Old Trail Road in Etters, Pa., 17319). The two-piece housing  12  includes a top section  22  and a bottom section  24  which are fitted together to form a small form factor case enclosing the operational modules of the transponder assembly  10 . The communication ports  14  and  16  reside on its front side of the assembly  10  and include parallel optical link receptacles  26  and  28  which are part of parallel optics modules  30  and  32  (not otherwise shown in  FIGS. 1 and 2 ) deployed within the transponder  10 . The receptacles  26  and  28  are designed to mate with standard MTP™ (MPO) connectors (Optical Internetworking Forum, OIF-VSR4-03.0) of fiber optic ribbon cables containing 12 parallel fibers and presenting 12 fiber optic terminations for optical interconnection. The pin-array connector  18  resides on the underside of the transponder assembly  10  and includes a 300-pin plug housing  25  for connecting to a printed circuit board or other electrical assembly having a matching Pin-array receptacle housing. The port  14  is associated with the parallel transmitter module  30  which is adapted for converting electrical signals into optical signals and transmitting these signals into a fiber optic ribbon cable connector. The port  16  is associated with the parallel optics receiver module  32  which is adapted for receiving optical signals from a fiber optic ribbon cable connector and converting the same into electrical signals. The ports  14  and  16  thereby provide transmitter and receiver links so that the transponder  10  can both receive and transmit data over separate 12 fiber element parallel optic ribbon cables. The heat sink  20  includes a set of ribs  35  to allow for improved heat dissipation through the surface of the housing  12  from the active elements deployed inside the assembly  10 . 
   The transponder  10  is designed to be compliant with the OIF (Optical Internetworking Forum)—VSR4-01.0 implementation standard for optically transmitting OC-192 data and providing a parallel optics based VSR (Very Short Reach) OC-192/STM-64 interface. The transponder assembly  10  provides a bi-directional interface for multiplexing a sixteen-bit wide bus of 622 Mb/s electrical LVDS (Low-Voltage Differential Signal) signal data supplied from the pin-array connector  18  into 12 channels of optical signal data at 1.24 Gb/s for transmission as photonic signals over a twelve element optical ribbon cable and also for receiving 12 channels of optical signal data at 1.24 Gb/s and demultiplexing this data into a sixteen-bit wide bus of 622 Mb/s electrical LVDS signal data for supply to the pin-array connecter  18 . The transponder assembly  10  also includes an error detection channel and a protection channel among the 12 optical channels. The pin-array connector  18  is intended to interface to an OC-192 framer on the electrical assembly to which the transponder assembly  10  is connected. 
   Referring now to  FIG. 3 , the transponder assembly  10  includes a parallel optics transmitter module  30 , a parallel optics receiver module  32 , a Flexible printed circuit or Flex circuit  34 , a rigid circuit board  36  and connector mounting board  38  all of which are mounted onto the bottom section  24  of the housing  12 . The parallel optics transmitter module  30  provides an optical interface to a twelve fiber optic ribbon cable and includes a set of twelve VCSELs (Vertical Cavity Surface Emitting Lasers) for emitting optical signals in response to electrical signals which are directed into the twelve fibers of the ribbon cable and includes electrical circuitry for processing electrical signals received from a pluggable transmitter edge connector  40  and converting these signals into forms suitable for driving the VCSELs. The parallel optics receiver module  32  provides an optical interface to a twelve fiber optic ribbon cable and includes a set of twelve PIN diodes for responding to optical signals from the twelve fibers of the ribbon cable which are directed onto the diodes and converting these signals into electrical signals and includes electrical circuitry for processing the electrical signals into suitable forms for general use and supplying them to a pluggable receiver edge connector  42 . The transmitter module  30  includes a frame  44  on which the receptacle  26 , heat sink  46  and a small circuit board  48  are mounted. The circuit board  48  extends along the bottom of the module  30  and carries the edge connector  40  at its far end opposite the receptacle  26 . The receiver module  32  is similarly constructed and includes a frame  45  on which the receptacle  28 , heat sink  50  and a small circuit board  52  are mounted. The circuit board  52  extends along the bottom of the module  32  and carries the edge connector  42  at its far end opposite the receptacle  28 . The Flex circuit  34  extends from the circuit board  36  to the connector board  38  and includes a large number of signal lines which interconnect the circuit board  36  to the connector board  38 . The Flex circuit  34  is comprised of a flexible material such as a Polyimide material supporting thin metal traces as signal lines that can be curled into arcuate shapes while still preserving the integrity of its signal lines. The rigid circuit board  36  provides electrical circuitry for use in signal processing and includes a SerDes (Serializer/Deserializer) semiconductor chip  54  as well as other circuitry. The connector mounting board  38  includes a pair of electrical connection jacks  60  and  62  mounted on its underside for interconnecting with the edge connectors  40  and  42  of the parallel optic modules  30  and  32 . 
   Referring now to  FIGS. 4 and 5 , the parallel optic modules  30  and  32  are connected to the connector board  38  by the edge connectors  40  and  42  carried on their circuit boards  48  and  52  which are plugged into the connection jacks  60  and  62  mounted on the connector board  38 . It should be noted that the module  32 , edge connector  42 , circuit board  52  and connection jack  62  are not shown in  FIGS. 4 and 5  but are positioned directly behind the module  30 , edge connector  40 , circuit board  48  and connection jack  60 . However, discussion of the invention with reference to the module  30 , edge connector  40 , circuit board  48  and connection jack  60  as illustrated in the drawings should serve to fully explain the invention. The connector board  38  is in turn connected to the circuit board  36  by the Flex circuit  34 . However, the Flex circuit  34  is curled into a multi-curved shape between the connector board  38  and circuit board  36  in order to allow the circuit board  36  to be deployed at a different level from the connector board  38 . The plane defined by the position of circuit board  36  is intermediate between the planes defined by the positions of the connector board  38  and the circuit boards  48  and  52 . Thus, the planes defined by the circuit board  36 , the connector board  38 , and the circuit boards  48 , and  52 , are not co-planar. This is of advantage since it allows the top of the circuit board  38  to define the top of the internal assembly within the housing  12  which otherwise would be defined at a higher level by the circuit board  36  thereby adding to the overall height of the module. Use of the Flex circuit  34  in interconnecting the modules  30  and  32  to the circuit board allows for a more compact assembly to be achieved which in turn provides a smaller form factor to the transponder assembly  10 . 
   Referring now again to  FIG. 4 , the circuit board  36  includes the SerDes semiconductor chip  54  mounted on its top surface  64  and the pin-array connector  18  mounted on its bottom surface  66 . The SerDes chip  54  and pin-array connector  18  are mounted directly across from one another on opposing surfaces of the circuit board  36 . Both the SerDes chip  54  and the pin-array connector  18  are connected to the circuit board  36  by ball grid array attachments  70  and  72  on the opposite surfaces  64  and  66  of the circuit board  36 . 
   Referring now to  FIG. 6 , a small section  75  of the circuit board  36  is shown in the vicinity of one of the ball grid array attachments. The circuit board has multiple layers  68  through which circuit traces may extend at multiple levels. This section  75  of the circuit board  36  includes two spaced apart attachment pads  74  on which solder may be deployed on the surface of the board  36  which are associated with the ball grid array. The pads  74  are connected by short circuit traces  76  to a metal plated vias  80  which extend entirely through the board  36  from surface to opposite surface. This construction is characteristic of ball grid arrays and allows the pads  74  to be selectively connected to circuit traces at any of the layers  68  of the board  36  by way of the traces  76  and metal plated vias  80 . 
   Referring now to  FIG. 7 , the attachment pads  74 , connection traces  76  and vias  80  associated with each of the ball grids array attachments are shown. It should be borne in mind that the circuit board  36  is not shown in this view and that the vias  80  extend through the circuit board  36  while the attachment pads  74  and traces  76  for each of the attachments  70  and  72  are resident on opposite surfaces of circuit board  36 . The ball grid array attachment  70  for the SerDes chip is shown as having two-hundred-fifty-six pads in a square pattern. The ball grid array attachment  72  for the pin-array connector  18  is shown as having three-hundred pads in a rectangular pattern. The patterns formed by the attachments  70  and  72  define two overlapping areas  84  and  86  characterized by high densities of pads and associated traces and vias. Surface traces such as traces  76  may be used to connect to discrete surface mounted components. 
   Referring now to  FIG. 8 , a small section  85  of the circuit board  36  is shown in the overlapping area  84 . This section  85  of the circuit board  36  includes two spaced apart pads  74   a  on the surface  64  of the board  36  associated with the ball grid array attachment  70 . The pads  74   a  are connected by short circuit traces  76   a  to a metal plated vias  80   a  which extend entirely through the board  36  from one surface to its opposite surface. This section  85  of the circuit board  36  also includes two spaced apart pads  74   b  (in phantom) on the surface  66  of the board  36  associated with the ball grid array attachment  72 . The pads  74   b  are connected by short circuit traces  76   b  (in phantom) to a metal plated vias  80   b  which extend entirely through the board  36  from one surface to its opposite surface. The vias  80   a  and  80   b  are deployed in an alternating pattern so that the vias of one array attachment are positioned in between the vias of the other array attachment. In this manner all of the vias  80  for ball grid arrays attachments  70  and  72  can be accommodated in the available space. 
   Referring now to  FIG. 9 , the overlapping area  84  is shown as including vias  80  associated with each of the ball grid array attachments  70  and  72  but only pads  74   a  and traces  76   a  associated with the attachment  70 . The vias  80   c  are connected to mutiple pads  74   a  in cases where the circuit functionality allows for such interconnections. Certain vias may thereby be eliminated as redundant which allows for simpler construction. More importantly it allows circuit traces at multiple levels in the circuit board  36  to extend through the via-free pathways, such as space  86 , thereby created for establishing connections within the board  36 . 
   Referring now to  FIGS. 10 and 11 , a typical parallel optic (transmitter) module  30  is shown in greater detail as including the receptacle  26 , metal support frame  44 , lens and alignment frame  92 , carrier assembly  94 , and heat sink  46 . It should be noted that the receiver module  32  would be similar in design and construction to the transmitter module  30  except that it would contain photoactive elements operative for receiving signals such as PIN diodes and associated circuitry as opposed to transmitter elements such as VCSELs. The receptacle  26  is mounted in the recess  98  in the support frame  44  so that it abuts the back wall  102  of the frame  44 . The carrier assembly  94  includes the printed circuit board  48 , the module Flex circuit  106  and the carrier frame section  108 . The lens and alignment frame  92  is mounted in between the frame section  108  of the carrier assembly  94  and the back wall  102  of the support frame  44 so that it is immediately adjacent to the fiber ends on the proximal end of the connector ferrule for the fiber optic ribbon cable when the ferrule is latched into the module  30 . The Flex circuit  106  connects the frame section  108  to the circuit board  48  serving as a medium for providing a large number of connection lines between components on the carrier frame section  108  and the circuit board  48  including the microcontroller chip  110  and the edge connector  40 . The circuit board  48  fits along the back shelf  112  of the support frame  44  underneath the heat sink  46 . The front end  114  of the heat sink  46  abuts the backside of the carrier frame section  108  for dissipating heat generated during operation by the electrical components mounted onto the frame section. The bolts  116  help retain the heat sink  46  and circuit board  48  in position within the frame  44 . The support frame  44  includes a window  118  in its back wall  102 . The lens and alignment frame  92  includes a mostly planar base  120  and a rectangular tower structure  122  projecting forward of the base  120  on which guide pins  124  and a lens array  126  are mounted. The tower structure  122  of the lens and alignment frame  92  fits through the window  118  of the support frame  44  in the assembled device. The lens and alignment frame  92  is a one-piece precision plastic injection-molded part including the tower structure  122 , guide pins  124  and lens array  126 . The carrier frame section  108  of the carrier assembly  94  preferably includes one or more layers of printed circuit board material including a layer of Flex circuit material which is an extended part of the Flex circuit  106 . An optoelectronic device  130  containing photoactive semiconductor components is precisely mounted on the frame section  108 . The device  130  comprises an integrated circuit chip which contains twelve VCSELs which are deployed on and as part of the chip. The photoactive components are disposed in a linear array at regular intervals corresponding to the lens array  126  and the array of fibers in the fiber optic ribbon cable connector. When the lens and alignment frame  92  is mounted on the frame section  108  the optoelectronic device  130  and its photoactive components (VCSELs) are precisely aligned with the lens array  126  and the guide pins  124 . One or more signal processing chips  132  may be mounted on the carrier frame section  108  for communicating with the optoelectronic device  130  and more particularly providing drive signals to transmitter elements (or providing signal amplification and conditioning in the case of receiver elements). 
   Although only a few embodiments of the present inventions have been described in detail, it should be understood that the present invention may be embodied in other forms without departing from the overall spirit or scope of the present invention.