Patent Publication Number: US-6982521-B2

Title: Visual display with cathode plate connected to a separate backplate

Description:
This application claims the benefit of U.S. Provisional Application No. 60/208,710 filed Jun. 1, 2000. 

   TECHNICAL FIELD 
   The present invention relates to a visual display, particularly though not exclusively for use with data processing apparatus. 
   BACKGROUND OF THE INVENTION 
   In prior International patent application, No. PCT/US98/20813, published on 8 th  Apr. 1999 under No. WO 99/17330 (“The Earlier International Application”) we described and claimed. 
   a field effect emission device for a visual display comprising:
         a substrate and   an emission layer on one face of the substrate, the emission layer having:
           a multiplicity of emitters and gates, arranged as an array of emission pixels and   conductive connections in the emission layer to the emitters and the gates,   
           the substrate having:
           conductive vias provided through the substrate or at least a front layer thereof to at least some of the said conductive connections in the emission layer for electrical connection to their emitters and gates.   
               

   In this specification, we refer to the type of field emission device described in The Earlier International Application as the Front-Layer-Via FED Device. 
   SUMMARY OF THE INVENTION 
   We have now developed farther both the device and the display incorporating the device. 
   According to the present invention a visual display comprising:
         a cathode plate in the form of a field effect emission device including
           a substrate and   an emission layer on one face of the substrate, the emission layer having;
               a multiplicity of emitters and gates, arranged as an array of emission pixels and   conductive connections in the emission layer to the emitters and the gates;   
               the substrate having:
               conductive vias provided through the substrate or at least a front layer thereof to at least some of the said conductive connections in the emission layer for electrical connection to their emitters and gates and   
               
           an anode plate;
 
wherein it includes:
   a back plate, the cathode plate being carried on the front side of the back plate; and   a frame connecting the back plate to the anode plate.       

   Usually the frame will extend peripherally around the cathode plate. 
   Whilst it is envisaged that the frame may be constructed as a separate member and then jointed to the back plate, as by frit sealing; in the preferred embodiment, the back plate and the frame are a single structure. In this case, the frame is essentially distinguishable from the back plate as that part of the structure extending further towards the anode plate from the main body of the structure which constitutes the back plate. The frame and the back plate can be laminated from multiple layers of ceramic material. The layers are laminated together in the green state and fired together to unify them into a single structure. 
   Where as is preferred, the back plate is pressure tight to atmospheric pressure, this arrangement has the advantage that atmospheric pressure does not act on the back of the cathode plate, but acts only on the back of the back plate and on the front of the anode to place the frame joint(s) under compression. Further, the back of the cathode plate is isolated from atmospheric pressure and the joint between the cathode plate and the back plate is not subject to tension due to the interior of the display being evacuated. 
   For electrical connection of the anode, the frame also preferably incorporates a network of vias extending from one layer to the next and interconnection tracks at interfaces between the layers. 
   The substrate of the cathode plate may be a multilayer substrate having a front substrate layer and at least one additional substrate layer, with conductive vias provided through the front layer and the or each additional layer and with electrical interconnection tracks at at least some of the interface(s) between adjacent layers so arranged that a front layer via is offset from a via in a back one of the additional layer(s) to which it is electrically connected by the interconnection tracks. Nevertheless, it is conceivable that the substrate would be comprised of or include two layers with vias in one aligned with vias in the next. It is also envisaged that the substrate may have only a single layer, with the emission layer built up on it. 
   We envisage that, in accordance with a feature described in the co-pending U.S. provisional patent application Ser. No. 60/208776, dated 1 st  Jun. 2000, (“The Co-pending Application”) the cathode plate will usually include a thick ceramic foundation layer usually with one or more additional thinner ceramic layers laminated to one or other side of the thicker, foundation layer. The last two features of the preceding paragraph are conveniently effected in combination with a foundation layer. 
   The back plate also will usually include a thick foundation layer with additional layer(s) laminated to either or both sides thereof. Again the layers will have vias whereby their pitches fans out towards the back layer; with vias in the front layer of the back plate being offset from those in the back layer thereof. Normally, a greater degree of fan out will occur in the back plate than in the front plate. 
   For connection to the cathode plate, the back plate preferably has vias in a front layer positioned to connect with vias in the back layer of the cathode plate, possibly with the interposition of connection tracks on either or both of the back plate front layer or the cathode plate back layer. 
   The vias or the tracks on either or both of these layers may be provided with reflowable solder deposits or with a ball grid array. The solder deposits can be reflowed to provide electrical contact between the two components. Alternatively, the ball grid array comprises an array of balls welded to the vias on one side and pressed on assembly of the cathode plate to the back plate into the vias or connection tracks on the other side. 
   To isolate the thin gap between the face plate and the back plate from the front side of the cathode plate, a flowable connection is made around the back edge of the cathode plate to the back plate. The connection can be by solder or frit. 
   Preferably the solder for electrical connection and the solder or the frit for edge sealing has a melting point above 300° C. and preferably above 320° C. This permits heat soaking of the cathode and back plate assembly in vacuum at 300° C. for out-gassing, i.e. removal of gases after connection of the cathode plate and prior to sealing of the anode plate to the carrier. Preferably this latter sealing is carried out at a temperature close to 300° C., typically 290° C., to enable further soldering operations, such as connection of driver and power supply components to the back face of the back plate, to be carried out subsequently with a lower melting point solder. 
   For assembly of the cathode plate to the back plate in correct position for electrical connection, bearing in mind that the cathode plate is unlikely to be provided with margins that can be gripped, at least when an array of cathode plates is abutted together in a sizeable display, the back plate can be provided with apertures for handling pins. The back side of the cathode plate has recesses in register with the apertures, whereby the pins extend through the apertures to engage in the recesses. The cathode plate can then be lowered onto the back plate. After, or on reflow of the cathode securing solder/frit, the apertures can be plugged. The operations including the lowering of the cathode plate and plugging of the apertures can be carried out under vacuum to avoid the necessity of having to evacuate the inter-plate space through the small apertures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To help understanding of the invention, 
       FIG. 1  is a cross-sectional side view of a visual display of the invention; 
       FIG. 2  is a scrap cross-sectional view on a larger scale showing the layers of the cathode plate and of the back plate, 
       FIG. 3  is a scrap view on an even larger scale showing inter-via connection between the cathode plate and the back plate; 
       FIG. 4  is a similar view of a pin arrangement for lowering the cathode plate onto the back plate; 
       FIG. 5  is a similar view of a frit plug in a lowering aperture. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   Referring to the drawings, the visual display there shown has a cathode plate  101 , which is a Front-Layer-Via FED Device having an emission layer  102  built up on a ceramic front layer. It has a thicker foundation layer  103 , as described in The Co-pending Application. Vias from the front layer have their pitch fanned out to that of the cathode-plate back-layer vias  104 . 
   Other main components of the visual display are a frame  111 , a back plate  112  and an anode  114  plate, which latter will not be described in detail in this application, since its structure forms no part of this invention, The back plate and the frame are integrally formed of a number of layers of tape cast ceramic material. The back plate has a via and interconnect fan-out arrangement described in more detail below. The frame also has a via and interconnect arrangement for making electrical connection to the anode plate. 
   The front layer  121  of the back plate is a foundation layer, which is thicker than the other layers, typically 0.030″ thick as against 0,006″. There are four thinner back layers  122 ,  123 ,  124 ,  125 . The front layer has a pattern of vias  126 , which corresponds in layout to that of the cathode plate vias  104 . The sequence of manufacture of the back plate is that the thick layer is tape cast and fired. Its via apertures are laser cut and filled with via material. This can be resistive, also as described in The Co-pending Application. The back layers are tape cast onto a mylar layer. The via apertures are filled to form vias  119  and interconnection tracks  118  are screen printed onto the layers, typically of silver paste. The back layers are then assembled to the foundation layer, with the assembly being pressed together temporarily hold it together. Again, the method is analogous to that described in The Co-pending Application in respect of the cathode plate. 
   Similarly the layers  116  constituting the frame are built up and laid on the periphery of the margin of the back plate. They have vias  115 , which are conveniently aligned from one layer to the next. The entire back plate/frame structure is then fired. Although on firing, it will shrink in the Z direction, its X/Y dimensions will remain those of the pre-fired foundation layer. Thus the pitch of the front layer vias  126  remains that at which they were formed, whereby they align with the cathode plate vias  104 . 
   Again as described in The Co-pending Application, the vias in the back face of the back plate are provided—after firing—with a ball grid array  127  for assembly of driver chips  128 . Also provided on the back face are screen printed contact pads  129  for surface mount power supply components  130  and a video input connection  1301 . 
   Around each via, a contact pad  1041  of screen printed silver may have been provided prior to lamination and firing of the cathode plate. Also around the margin of the back face of the cathode plate, a continuous peripheral strip of screen printed silver  140  is similarly provided. The front face of the front layer of the back plate is provided with complementary pads  1261  and continuous marginal strip  141 . For electrical connection of the cathode, immediately prior to introduction of the cathode plate into a sealing machine, or at an earlier stage in its preparation, flux is screen printed onto its back face around each via  104  and along the strip  140 . The plate is dipped in solder powder, which adheres to the flux. The solder is heated to fuse into balls on each pad and on the strip. Conveniently this can be done by diffuse infra red radiation or by laser irradiation to the exact positions of the adhered solder. It is chosen to have a melting point in excess of 320° C. The solder forms balls  134  on the vias and their contact pads and a continuous ridge  135  along the strip  140 . 
   Alignment of the cathode plate and the back plate is critical for proper via alignment and connection. A set of alignment pin apertures  131  is provided in the back plate, with corresponding blind recesses  132  in the back of the cathode plate. On assembly, pins  133  arranged in the sealing machine are introduced through the apertures  131 , the cathode plate is placed on the pins with their ends engaging in the recesses, The cathode is then lowered into position, by withdrawal of the pins. Final positioning of the cathode plate is determined by abutment of its edges  105  with the inside of the frame  111 . 
   After the cathode has been lowered onto the back plate, and the two components are heated to say 325° C., where the solder melts at 320° C. The solder balls wet the opposite via pads and connect respective vias  104 ,  126 . Similarly, the solder ridge  135  melt and wet the opposite strip  141 . Thus this peripheral solder joint seals the thin space  136  between the components from the space to be evacuated in front of the cathode plate. This soldering of the vias and the peripheral margin is anticipated to be feasible without fluxing the pads  1261  and strip  141  on the back plate, since they will be kept clean since formation. This is of advantage, since fluxing of the strip  141 , in the corner between the frame and the front of the back plate is awkward to access. However, should fluxing be necessary the pads and strip can be fluxed and coated with powdered solder to further enhance the solder joints. 
   The positioning and soldering of the cathode plate can be carried out in atmospheric conditions. However, it is preferably carried out in a vacuum chamber of a sealing machine. In either case, the next operation—performed under vacuum—is the sealing of the apertures  131 , for instance by introduction of frit plugs  137  into them and fusing of the frit by means of a laser. 
   After evacuation and sealing of the space  136  between the cathode plate and the back plate, the vacuum level in the processing chamber is increased and the assembly is heat soaked at 300° C. to remove as much gaseous material as possible. With the space  136  already sealed, difficulties in evacuation of this confined space are avoided. The step of high level evacuation and heat soak is carried out with the anode plate already introduced into the chamber. 
   Previously, a margin of frit  151  has been deposited on the front edge  152  of the frame, and solder has been deposited onto vias  115  to ITO/phosphor lines on the inside surface of the anode plate  114 . This solder is such as to melt at just below 300° C., typically Indium 290, which melts at 290° C. The anode plate is lowered onto the frame, with the solder wetting the phosphor line contacts. The frit is fused by laser traverse as described in The Earlier International Patent Application (and claimed in its sister application No. WO 99/17329), sealing the anode plate to the frame. 
   The thus assembled display is ready for use when its drivers, power supply components and video input connectors are connected to it The latter two  130 ,  1301  are soldered onto the contact pads  129 . This is by means of a solder melting below 290° C. to avoid disturbance of the electrical connection to the phosphor lines and indeed connection of the cathode plate to the back plate. The drivers are connected by ball grid array techniques, which are within the capabilities of the man skilled in the art and will not be described in detail here. 
   The invention is not intended to be restricted to the details of the above described embodiment, for instance, in place of the peripheral solder seal, a margin of frit is screen printed onto the back face of the cathode plate or the front face of the back plate. The frit is chosen to melt at the same temperature as the inter-via solder, whereby the two materials fuse at the same time, Further, where the vias are of a material readily wetted by the solder, the contact pads at the vias can be dispensed with. Furthermore, where there is a possibility of flux resulting in unwanted vapours inside the sealed display, its use is now preferred to be dispensed with and fluxless solder used instead, particularly between the back plate and the cathode plate. Again, in place of the use of frit between the anode plate and the frame, solder, such as indium solder, can be used. This can be fused by heat soaking or by laser traverse, as described in The Earlier International Patent Application.