Abstract:
The present invention is to prevent a snapping of wiring patterns formed on a flexible printed circuit (FPC) board by bending the FPC board and to make the length of the pattern short to suppress the degradation of high frequency signals transmitted on the wiring pattern. The FPC board provides a via hole in the land region formed on a primary surface of the FPC board and to be attached to the host board. The wiring pattern, which is formed on a secondary surface opposite to the primary surface and is made of copper foil, is drawn from the via hole at the secondary surface. The wiring pattern is covered by a cover layer. Bending the FPC board such that the primary surface is outside, the FPC board is bent at a boundary of the land region at the primary surface, while; the FPC board provides the cover layer on a region of the secondary surface corresponding to the land region, which prevents the wiring pattern on the second region from snapping.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical module with a flexible printed circuit board, in particular, the invention relates to an optical module with a specific connecting structure with a host board. 
   2. Related Prior Art 
   An optical module generally includes optical subassemblies (OSA) installing a laser diode (LD) or a photodiode (PD) and a circuit board installing electronic circuits to control the LD or the PD. The module is required with a small sized appearance and with a high transmission speed. The OSA provides a plurality of lead pins to guide electrical signals from/to the electronic circuits. In order to maintain the signal quality from/to the OSA, it is necessary to satisfy an impedance matching condition around the lead pins, namely, the lead pin itself and the wirings connected to the lead pins. 
     FIG. 8  shows a conventional wiring configuration between the circuit board  200  and the OSA  10  with lead pins  11 . Because the lead pins  11  inevitably accompany with parasitic inductance, which causes an impedance mismatching condition, it is necessary to compensate this impedance mismatching with additional circuit components. When the transmission speed of the optical module enters Giga Hertz band and reaches 10 GHz, the impedance mismatching of the wiring pattern may be fatal to transmit a signal with good quality. One solution for such subject is to shorten the length of the lead pin and to use a flexible printed circuit (FPC) board between the lead pin  11  and the circuit board  200  on which impedance matched wiring patterns are implemented. 
   However, when the FPC board  100  connects the OSA  10  with the circuit board  200  and a portion of the FPC board  100  bent as the cupper foil of the wiring pattern is exposed, the copper foil is occasionally snapped.  FIG. 8  shows an example to prevent the copper foil of the FPC board  100  from being snapped, where an additional adhesive  30  covers and supports the copper foil so as to bend the FPC board  100  at a portion R not covered by the adhesive  30 . 
   A Japanese Patent Application published as JP-2005-050971A has disclosed a FPC board with additional cover film on a region between a bonding pad and an original cover film such that, when the FPC board is inserted with the lead pin of the electronic component into the bonding pad and bent at the region aside the bonding pad, the copper foil electrically connected to the bonding pad is not snapped. Another Japanese Patent Application published as JP-H05-061063A has disclosed a configuration to reinforce a portion where the FPC board is connected to the liquid-crystal panel by covering the contact portion with a resin curable with ultraviolet rays. Still another Japanese Patent Application published as JP-2004-193466A has disclosed a configuration of the FPC board in which, by covering a portion of the wiring pattern on the FPC board with silicone resin, stress likely to concentrate on the bent portion at an edge of the cover film may be relaxed. 
   In a conventional method where an additional film covers a boundary portion between the pad and the original cover film securing the pad, it is restricted in one direction for the FPC board to be bent, that is, the FPC board must be bent to a direction where the cover film comes inside. Moreover, the bending diameter of the FPC board must be large, which prolongs the wiring pattern on the FPC board and restricts the miniaturization of the optical module. 
   The method that uses the resin curable with the ultraviolet rays requires additional processes for irradiating the ultraviolet rays. Moreover, it is hard to bend a portion where the additional resin is cured, which makes a gap between the OSA and the substrate wide and prevent the miniaturization of the optical module. 
   The present invention is to provide a new arrangement of the FPC board connecting the OSA to the circuit board, which is configured to prevent the snapping of the wiring patterns formed on the FPC board by bending and to shorten the patterns to suppress the degradation of the high frequency signals transmitted on the wiring pattern. The invention also provides an optical module installing such new arrangement of the FPC board. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention relates to a configuration of a flexible printed circuit (FPC) board. The FPC board includes a land region in a primary surface to be attached to a host board, a wiring patter in a secondary surface opposite to the primary surface, and a via hole that is configured to be drawn from the land region and to connect the land region electrically to the wiring pattern. The FPC board according to the present invention, when the FPC board is bent such that the primary surface becomes outside, the wiring pattern on the secondary surface may cross a virtual plane reflecting the boundary of the land region and at least a portion of the wiring pattern crossing the virtual plane may be covered by a cover layer. 
   Another aspect of the present invention relates to an optical module that includes; an optical subassembly (OSA) installing a semiconductor optical device, an electronic circuit communicating with the semiconductor optical device, a host board installing the electronic circuit, and a flexible printed circuit (FPC) board connecting the OSA with the host board. The FPC board includes a land region on a primary surface that faces and comes in contact to the host board, a wiring pattern on a secondary surface opposite to the primary surface, and a via hole that is drawn from the land region and connects the land region electrically to the wiring pattern. The FPC board is bent such that the primary surface thereof becomes outside. The wiring pattern on the secondary surface may cross a virtual plane reflecting the boundary of the land region and at least a portion crossing the virtual plane may be covered by a cover layer. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a cross section of a flexible printed circuit board according to an embodiment of the present invention; 
       FIG. 2  shows the flexible printed circuit board shown in  FIG. 1  and a host board to be connected with the flexible printed circuit board; 
       FIG. 3  shows a cross section when the flexible printed circuit board connected to the host board and bent upward; 
       FIG. 4  is a plan view of the flexible printed circuit board according to the embodiment of the present invention; 
       FIG. 5  is a perspective view of the flexible printed circuit board connecting the optical subassembly with the host board; 
       FIG. 6  is a side view when the flexible printed circuit board connects the optical subassembly with the host board; 
       FIG. 7  is a schematic view of an optical module installing the optical subassembly electrically connected to the host board with the flexible printed circuit board of the present invention; and 
       FIG. 8  shows a conventional arrangement of the flexible printed circuit board to prevent the snapping of the wiring patterns on the flexible printed circuit board. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Next, preferred embodiments of the present invention will be described as referring to accompanying drawings.  FIG. 1  is a cross section of the FPC board  100 ; in particular,  FIG. 1  shows a coupling portion of the FPC board  100  with the circuit board  200 . The FPC board  100  provides a base board  101 , a copper foil  102  on a primary surface of the FPC board  100 , a nickel coating  103  on the primary surface, a gold coating  104  on the primary surface, a copper foil  105  on a secondary surface, a nickel coating  106  on the secondary surface, a gold coating  107  on the secondary surface, an adhesive layer  108 , a cover layer  109 , a via hole  110 , and a land region  111 . 
   The FPC board  100  provides the land region  111  to be connected with the host board. In the explanation, a side of the FPC board  100  where the land region  111  is provided is defined as the primary surface, while the other side is defined as the secondary surface. Moreover, a left side of the land region in  FIG. 1  is defined as an outer side, while, a right hand side in  FIG. 1  is defined as an inner side. 
   The land region  111  stacks, on the base board  101 , the copper foil  102 , the nickel coating  103 , and the gold coating  104 . The land region  101  does not provide the cover layer  109  to electrically come in contact with the host board. The gold coating  104  with the nickel coating  103  protects the copper foil  102  from oxidizing. The nickel coating enhances the adhesiveness of the gold coating  104  with the copper foil  102 . 
   Other arrangements may be available to protect the copper foil  102  from oxidizing, for instance, a solder is provided on the copper foil  102 , which is called as a solder leveler, or some flux coats the surface of the copper foil  102 . The gold coating  104  may be superior form viewpoints of the stableness and the reliability. Thickness of the gold coating  104  may be smaller than 0.1 μm, while, that of the nickel coating  103  may be between 3.0 to 8.0 μm. The copper foil  102  has a thickness of about 18 μm. 
   The land region  111  provides a via hole  110 . On the secondary surface of the land region  111  is provided with another copper foil  105  immediately on the base board  101 . The copper foil  105  forms interconnections that is electrically connected with the gold coating  104  on the primary surface thorough the via hole  110  and finally connected with the host board  200 , which is described later. Within the via hole  110  is filled with or coated with stacked metals of the copper, the nickel, and the gold similar to those on the primary and the secondary surface of the FPC board  100 . 
   In an inner side of the gold coating  107  and the nickel coating  106  in the secondary surface is provided with the cover layer  109  to protect the copper foil  105 . This cover layer  109  is stacked with the copper foil  105  with the adhesive  108 . 
   When an edge of the stacked metal of the copper foil  102 , the nickel coating  103 , and the gold coating  104  on the primary surface is denoted as q, and a virtual plane including the edge q and perpendicular to the base board  101  is denoted as Q, the interconnections on the secondary surface crosses this virtual plane Q. The cover layer  109  covers at least a portion of the land region  111  on the secondary surface, that is, the cover layer  109  extends the land region  111  on the secondary surface by crossing the virtual plane Q. 
     FIG. 2  shows an arrangement of the FPC board  100  and the host board  200  to be connected with the FPC board  100 . The host board  200  provides an interconnection  202  made of copper foil on a base board  201  that is generally made of glass epoxy material. A resist film  205  covers the host board  200  except a land region  206  thereof, where a stacked metal of the nickel coating  203  and the gold coating  204  is exposed similar to the arrangement of the FPC board  100 . A size of the lad region  111  of the FPC board  100  is comparable to or smaller than the land region  206  of the host board  200 , and two land regions,  111  and  206 , are fixed and electrically connected to each other. 
     FIG. 3  shows an arrangement when the FPC board  100  is connected with the host board  200  in land regions,  111  and  206 , with a solder  20 . In this arrangement, the FPC board  100  is bent about right angle to be electrically connected with an optical subassembly. In the present embodiment, the FPC board  100  is bent at the virtual plane Q. Because this virtual plane Q locates at the edge of the nickel coating  103  of the primary surface and the other nickel coating  106  of the secondary surface is outside the virtual plane Q, both nickel coatings,  103  and  106 , may be prevented from bending. Further, the copper foil  105  is covered with the cover layer  109  in the secondary surface; accordingly, the copper foil  105  for the interconnection may be prevented from snapping. 
     FIGS. 4A and 4B  are plan views of the FPC board  100  according to the present embodiment.  FIG. 4A  is a plan view of the secondary surface, while,  FIG. 4B  is a plan view of the primary surface viewed from the land region  111 . As shown in  FIG. 4 , the FPC board  100  provides a plurality of land regions  111  and each land region  111  configures the same stack that shown in  FIGS. from 1 to 3 . 
   On the primary surface of the FPC board  100  is formed with four interconnections  105  made of copper foil with the cover layer  109  on the top thereof. An end portion of each interconnection  105  is exposed from the cover layer  109  and forms a via hole  110  thereat. The via hole  110  is connected to the gold coating  104  of the land region  111  in the primary surface as shown in  FIG. 1 . On the via hole  110  in the secondary surface is exposed from the cover layer  109 . The other end portion of each interconnection  105  also forms another via hole  112  that is guided to the primary surface. 
   The primary surface of the FPC board  100  provides a copper foil  113  in almost whole portion thereof except the land region  111  and portions corresponding to the other via holes  112  provided in the end portion of the interconnection  105 . The cover layer  114  covers the copper foil  113 . 
     FIG. 5  is a perspective view showing the FPC board of  FIG. 4  that connects the OSA  10  to the host board  200 . The OSA  10  includes a laser diode (LD) and a photodiode (PD) and has four lead pins  11  each connected to the cathode of the LD, the anode of the PD, the OSA package, and commonly connected to the anode of the LD and the cathode of the PD. Four via holes  112  formed in the end portion of the interconnection  105  receive these lead pins  11 . That is, the lead pin  11  passes through the via hole  112 . 
   Interconnections connected to the lead pins except for the ground pin have specific width and gaps to the neighbor interconnections, and the base board  101  of the FPC board also has a specific thickness to satisfy the impedance matching condition for the interconnection  105  with the copper foil  13 , as shown in  FIG. 4 . That is, by adjusting the width of the interconnection  105  and the gaps to the neighbor interconnection, the characteristic impedance of the interconnection  105  on the secondary surface may be controlled because the primary surface of the FPC board  100  has the copper foil covering the substantial whole area of the base board  101 , which forms a microstrip line. 
     FIG. 6  is a side view of the OSA  10  connected to the host board  200  with the FPC board  100 . The FPC board  100  is bent at the virtual plane Q extending form the edge of the land region when the OSA  10  with the lead pins  11  and the FPC board  100  provides the via holes  112  into which the lead pin  11  is inserted. The virtual plane Q, as shown in  FIG. 3 , crosses inner side of the nickel coating at the secondary surface, accordingly, the nickel coatings of the primary and secondary surfaces does not bend and may be prevented from snapping. Moreover, the cover layer  109  of the secondary surface covers the interconnections  105  thereon; accordingly, the interconnections  105  may be prevented from snapping. 
   The arrangement of the FPC board  100  of the present embodiment may shorten the length thereof from the OSA  10  to the host board  200 , which may not only match the characteristic impedance of the interconnection but also suppress the loss of the high frequency signal. Thus, according to the present invention, the high frequency performance of the optical module and the optical module may be improved. 
     FIG. 7  shows a schematic drawing that explains an arrangement of an optical module installing the OSA  10 , the host board  200 , and the FPC board  100 . The optical module  300  includes a housing  301  and an electronic circuit  302  on the host board  200 . The optical module  300  includes, as the OSA  10 , a transmitter optical subassembly (TOSA) and a receiver optical subassembly (ROSA) to perform the full-duplex optical communication. 
   The ROSA includes a semiconductor light-receiving device such as avalanche photodiode (APD) and a PIN-photodiode (PIN-PD), and a pre-amplifier to amplify a faint electrical signal converted by such PD. The TOSA includes a laser diode (LD) to emit signal light and a photodiode to monitor the amplitude of the signal light. The TOSA occasionally installs a thermo-electric device such as Peltier element to adjust a temperature of the LD, and sometimes provides a driver circuit to drive the LD when the operational speed of the LD reaches and exceeds 10 GHz. These OSAs are built with an optical receptacle into which an external optical connected is mated to optically couple optical fibers configured within the optical connector with the LD and the PD within the OSAs. 
   The housing  301  covers the host board  200  on which the electronic circuit  302  is installed. The electronic circuit  302  includes a driver for driving the LD within the TOSA and a signal processor that extracts a clock and regenerates a data from the signal output from the ROSA. When the ROSA installs the APD, the electronic circuit  302  may include a bias supplying circuit for the APD. 
   When the thermo-electronic device is installed within the TOSA to adjust the temperature of the LD, a driving circuit for the thermo-electric device may be also installed on the host board  302 . In addition, when a processor and a memory device to comprehensively control such circuits are also provided, additional circuit board may be installed within the housing, which mounts circuits not processing high frequency signals such as the bias supplying circuit for the APD and the driver for the thermo-electronic device. The rear end of the host board  200  forms an electronic plug that mates with an external electronic connector. 
   The electronic connection between the circuit  302  on the host board  200  and the OSA  10  may be performed with the FPC board  100  as shown in  FIG. 4 . Thus, according to the present arrangement of the FPC board, even the FPC board is bent to receive the lead pin of the OSA  10  in one end thereof and to solder with the host board  200  in the other end thereof, the interconnection on the FPC board may be prevented from snapping, which enhances the reliability of the interconnection. Moreover, according to the present invention, the interconnection on the FPC board may be shortened, which secures the impedance matching condition and suppress the signal loss in the high frequency regions.