Patent Publication Number: US-2006001133-A1

Title: Optical sub-assembly for a transceiver

Description:
INTRODUCTION  
      1. Field of the Invention  
      The invention relates to electro-optical devices such as transceivers, transmitters, or receivers, particularly for high frequency operation.  
      2. Prior Art Discussion  
      FIG. A shows cross-sectional front and side views of a prior art device, and FIG. B shows a side view cross-section of an assembly incorporating the device and a fibre termination. FIG. C shows a strip of metallic lead frames (LF) used during production of the device. Components such as a photo diode (PD) are placed on the lead frame (LF). These ends of the lead frame are placed in a mould in which bodies (MB) of transparent epoxy material are moulded around the ends of the lead frame (LF) and the component (PD).  
      After moulding, the individual parts are cut from the lead frame strip, leaving portions of the lead frame protruding from the moulded body, so as to act as electrical leads for the device. It is also known: 
          to incorporate a lens (L) into such moulded encapsulation;     to place a component so made into a housing (H) which forms a socket allowing a plug (P) attached to the end of an optical fibre (F) to be engaged so that the fibre is aligned with the lens; and     to incorporate a guide feature (GF) in the moulded body to assist in the alignment of the plug and the fibre with the lens.        

      While this approach is satisfactory in some situations, for high frequency applications it is very difficult to achieve sufficient alignment between the optical component and the moulded lens and ultimately the optical fibre.  
      Another aspect of high frequency electro-optical devices is that signal integrity problems arise because of discontinuity of a ground plane between the drive circuit into which the device is connected and the device itself. These problems lead to bit errors. It is known that the transmission of high frequency signals along an electrical conductor will be most effective if that electrical conductor is a constant small distance from, but insulated from, another conductor, this second conductor being connected to electrical ground. This second conductor is commonly known as the “ground plane”. In printed circuit boards, this is commonly achieved by constructing the board with multiple layers of conductors separated by insulating material. One of these layers, known as a “ground plane”, is connected to electrical ground.  
      In existing electro-optical device circuits there is a discontinuity in the “ground plane” effect at the point where the signal leaves a printed circuit board and enters the leads of the device. This discontinuity, though the dimensions may only be in the order of a few millimetres, can cause serious problems in the transmission of high frequency electronic signals.  
      The invention is therefore directed towards providing improved electro-optical devices for high-frequency operation, and methods of manufacturing them.  
     SUMMARY OF THE INVENTION  
      According to the invention, there is provided a method of manufacturing an electro-optical device having a lead frame supporting a component and a moulded body around the component and part of the lead frame, the method comprising the steps of: 
          providing a locating feature in the lead frame; and     engaging a feature of the mould with the lead pin locating feature during moulding.        

      In one embodiment, the lead frame locating feature is used as a reference by a vision system when placing the component on the lead frame.  
      In another embodiment, the lead frame locating feature is within an area of the lead frame surrounded by the moulded body.  
      In a further embodiment, the lead frame locating feature is an aperture in the lead frame, and a mould feature is a pin.  
      In one embodiment, the aperture extends fully through the lead frame.  
      In a further embodiment, the mould pin is removed, leaving a recess in the moulded body.  
      In one embodiment, the recess is used subsequently as a device locating feature for connection of a fibre termination to the moulded body.  
      In another embodiment, a part of the fibre termination extends into the recess.  
      In a further embodiment, the recess is at least partially filled to prevent ingress of contaminants.  
      In one embodiment, the lead frame includes a spring element, allowing the part of the lead frame in which the locating feature is located to move for guiding of the mould feature into engagement.  
      In a further embodiment, the lead frame spring element comprises a cranked portion of a lead frame leg.  
      In one embodiment, the method comprises the further step of providing a ground plane in the device.  
      In a further embodiment, the ground plane is adhered to the moulded body.  
      In one embodiment, the ground plane also extends to lie alongside the exposed part of the lead frame.  
      In another embodiment, the ground plane is incorporated in the lead frame.  
      In a further embodiment, the lead frame is multi-layered, comprising: 
          a lead frame conducting layer,     an insulating layer, and     a ground plane layer separated from the conducting layer by the insulating layer.        

      In another aspect, the invention provides an electro-optical device comprising a lead frame on which is mounted a component, wherein the device further comprises a ground plane separated from a lead frame conductor.  
      In one embodiment, the ground plane is adhered to a moulded body encapsulated around part of the lead frame and the component.  
      In another embodiment, the ground plane is a layer of the lead frame.  
      In a further embodiment, the ground plane extends alongside an exposed part of the lead frame.  
      In one embodiment, the ground plane comprises legs for insertion in apertures of a circuit board.  
      In another embodiment, the lead frame legs are cranked to allow surface mounting of the device with the lead frame lying in the general plane of a circuit board.  
      The invention also provides an assembly comprising a device as defined above and a waveguide connected to the device.  
      The invention also provides a circuit comprising a circuit board having a ground plane and a device as defined above, wherein the ground plane of the device is electrically connected to the ground plane of the circuit board. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:  
       FIG. 1 ( a ) is a set of views of an electro-optical device, and  FIG. 1 ( b ) is a diagrammatic view of an alternative lead frame;  
       FIG. 2  is a diagrammatic side view of a device after application of a ground plane;  
       FIG. 3  is a diagrammatic perspective view of another device of the invention when mounted on a circuit board;  
       FIG. 4  is a perspective view of an alterative lead frame;  
       FIG. 5  is a diagrammatic side view of a further device of the invention; and  
       FIG. 6  is a side view of a still further device. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      FIG. ( 1 ) shows an electro-optical device  1  of the invention, having a photo-diode  2  mounted on a lead frame  3  and around which a transparent epoxy body  4  is moulded. The body  4  incorporates a lens  5 .  
      The lead frame  3  comprises a locating hole  10  close to the component  2  and within the moulded body  4 . Section A-A shows registry of the component  2  with the lens  5 . Very accurate registry is achieved because the locating hole  10  provides a reference during moulding because a moulding pin is inserted in it, thus allowing highly accurate moulding and hence location of the lens  5  with respect to the component  2 . The lead frame locating hole  10  provides a rigid link between the lead frame in close proximity to the component  2  and the mould body. The dimensions of the locating hole are typically 1 mm in diameter and the distance from the component  2  to the centre of the locating hole is typically 1.5 mm.  
      As shown in the B-B Section, after removal of the mould pin from the hole  10  there is a deep recess  35 . This is partially filled in a subsequent operation with polymer  36 , leaving a more shallow recess for use in alignment of the end of a waveguide.  
      The locating feature on the lead frame is also used as the reference by the machine placing the electro-optical component  2 , thus allowing even greater accuracy. Thus, the locating hole  10  provides a single reference for both component placement and moulding. This is particularly advantageous for the level of accuracy required for high frequency use of the device.  
      Turning again to the moulding, as the lead frame is inserted into the mould the tapered end of the mould pin will engage in the hole  10 , and guide it to the correct location relative to the mould. To facilitate this guiding action, it may be advantageous to incorporate resilience in a part of the lead frame to allow the portion of the lead frame carrying the electro-optical component and the locating hole to move relative to the rest of the lead frame, so as to allow it to be guided by the pin. This is illustrated in  FIG. 1 ( b ), which is similar in function to the lead frame shown in  FIG. 1 ( a ), except that the member carrying the locating hole has a curved support portion  20 , providing a spring effect between the lower main strip with the tooling holes and the point where the locating pin accurately positions the portion of the lead frame carrying the electro-optical component.  
      As mentioned above the effect of the pin will be to leave a hole in the finished moulded component. After completion of the moulding operation, this is partially filled to prevent the ingress of moisture to the lead frame at a point dose to the location of the electro-optical component.  
      The shape of the pin is such that this hole, (or as much of it as remains after part of it is filled) acts as a locating element for the light guide, or for a carrier which locates the light guide or optical fibre in the correct location relative to the lens or other appropriate feature on the device.  
      In an alternative embodiment, the locating feature may be located just outside the portion of the lead frame that is encapsulated in the mould. In this case the pin or other feature of the mould that locates the locating feature will not leave an impression in the finished moulded part, but the provision of such a locating pin or other locating mechanism close to the electro-optical component in each cavity of the mould will serve to precisely locate the electro-optical components relative to the moulded features.  
      Referring to  FIG. 2 , in a further method step for producing a device  50  having a lead frame  51 , a component  52 , and a moulded body  53 , a ground plane  55  is bonded at the rear surface of the body  53 . The ground plane  55  may be connected by a soldered link to the circuit board&#39;s ground plane. This provides continuity between the circuit board&#39;s ground plane, and that of the device.  
      Referring to  FIG. 3 a  device  60  is shown mounted in a circuit board  61  having a ground plane  62 . A ground plane  63  is adhered to the back of a moulded body  64 , and it extends down at the back of the lead frame  64 . At the board level it is located in holes  66  for electrical connection with the circuit&#39;s ground plane  62 .  
      Thus the signals flowing in the tracks parallel to the ground plane  62  continue to be a similar distance from the ground plane  63  as they flow up the leads formed by the lead frame  64 .  
      However, other arrangements are possible. The leads could be bent to facilitate surface mounting, or the component may be inserted in an appropriate socket or connected to wires or other discrete conductors.  
      In another embodiment the lead frame material is assembled with the ground plane material and a thin layer of insulating material, which is between them. This is then used as a basis on which to mount the electronic or electro-optical components. This construction is illustrated in  FIGS. 4 and 5 .  FIG. 4  shows a lead frame  70  prepared with insulating material  71 , ground plane material  72 , and conducting material  73 . An additional member  74  on the bottom serves to hold the elements of the lead frame together until the moulding operation has been completed. This element  74  will be cut away after moulding. This drawing shows a locating hole  75  close to where a component is placed.  
      In  FIG. 5  the whole device is shown, including a moulded body  81  and an electro-optical component  85  which is mounted on the signal-carrying layer  73  of the lead frame  70 . The ground plane layer of the lead frame,  72 , is separated from the signal-carrying layer  73  by the insulating layer  71 .  
      Parts made according to this construction may be particularly suitable for surface mounting, by arranging for the lead or leads corresponding to the ground plane to be longer than, shorter than, or shaped differently from the other leads and for one or both sets of leads to be bent to bring them to the same plane. This is illustrated in  FIG. 6 , showing a device  100  with a body  101  and a lens  102  moulded on a composite lead frame consisting of a signal carrying layer  105  and a ground plane layer  106 , both of metal, with an intermediate insulating layer  107 . The leads formed by the portions of the lead frame which protrude from the body of the component are bent so that when the component is laid flat on a printed circuit board, the ground plane leads and the signal leads will all come in contact with different portions of the printed circuit board, to which they may be attached by solder.  
      The distance between the conductor carrying the high frequency signal and the ground plane should be altered if the dielectric constant of the material between them changes. Thus, for example, if the dielectric constant of the material used between the lead frame and the ground plane in the device is different from the dielectric constant of the material used between the signal conductor and the ground plane conductor in the printed circuit board, it may be appropriate that the distance between the first pair be correspondingly greater or less than the distance between the latter pair. For example, fr4 PCB material has a dielectric constant of 4.6 and the gap between the active layer and ground plane is 0.30 mm. Optical plastic package material has a typical dielectric constant of 3.8. So the distance between the data leads and ground plane within the package should be set to 0.25 mm provided the data leads are the same width inside the package as they are outside. The impedance may also be matched by adjusting the data lead width in cases where the thickness of package cannot be adjusted.  
      It will be appreciated that the invention provides for elimination of the manufacturing error in lead frame dimensions between tooling hole and reference. Location of the moulded body is made by the same reference as is used for the vision system of the component placing machine. Errors arising from relative thermal expansion of the lead frame and the moulded body are reduced by a large factor because of the short distance between the component placement position and the locating hole. The invention also reduces errors in mould dimensions and registry between the lens and the fibre if the mould pin recess is used for registration of the fibre termination. It will also be appreciated that the invention provides for significantly enhanced electro-optical performance because of the ground plane arrangement.  
      The invention is not limited to the embodiments described but may be varied in construction and detail.