Patent Publication Number: US-2013242243-A1

Title: Methods and apparatus for connecting electrically conductive glass to a substrate in a liquid crystal panel

Description:
FIELD OF THE INVENTION 
     Embodiments of the present invention are generally related to the field of liquid crystal displays, and, more particularly, to the optical performance of such displays. 
     BACKGROUND 
     A liquid crystal (LC) display cell can have an electrically conductive glass layer over a liquid crystal layer which can be supported by a silicon backplane substrate. The LC display cell can be die-attached to a printed circuit board or other substrate to produce an LC panel. The printed circuit board can be used to make electrical connections to the cell for power and data purposes. A conventional LC panel can have one or more electrical connections directly between the electrically conductive glass and the printed circuit board. These electrical connections can be made from a conductive adhesive which can be formed into one or more pillars to connect a conductive layer of the conductive glass to a conductive trace on the circuit board. Power can be supplied from the printed circuit board to the conductive glass through the conductive adhesive pillar without passing through the silicon substrate. 
     A diagrammatic elevational view of a conventional LC panel is shown in  FIG. 1  and is generally designated using reference number  10 . Panel  10  can have a display cell  12  die-attached to a printed circuit board substrate  14 , such as FR4. Display cell  12  can include an electrically conductive glass layer  16 , a liquid crystal layer  18  and a silicon backplane substrate  20 . Other layers can be included but are not shown in this simplified example for purposes of clarity. The electrically conductive glass can have an overhang where the glass overhangs the LC and silicon substrate layers. A pillar  24  of conductive adhesive can be formed between the printed circuit board and overhang of the conductive glass to electrically connect the printed circuit board substrate to the conductive glass. In this arrangement, the pillar does not contact the LC or silicon backplane substrate layers, but instead extends directly from the printed circuit board to the conductive glass. 
     During operation, the display cell applies electrical field signals across the liquid crystal layer between pixel electrodes of the silicon backplane substrate and the electrically conductive glass to change a characteristic of the liquid crystal to modulate light for creating an image. If the electrical connection through the pillar is broken, then the display cell is unable to create the electrical fields and the display cell becomes non-functional. 
     The pillar can be formed after display cell  12  is die-attached to the printed circuit board substrate using carefully controlled dispense methods and custom made dispensing equipment. The formation of the pillar is not a typical manufacturing process and the need to form the pillar in a LC panel limits the number of available manufacturing vendors. 
     The pillar can be a source of failure in the LC panel. Since the pillar has to span at least the thickness of the silicon substrate and the LC layer, the pillar can be on the order of 0.7 mm high to span the distance between the conductive glass layer and the circuit board substrate. The thickness/height of the pillar can exceed the recommended maximum thickness of the conductive adhesive to form the posts. As a result of the required thickness/height, the pillar can be subject to handling related mechanical failure. The pillar can also be subject to failure caused by adverse environmental conditions, such as high temperature and high humidity. 
     In some circumstances the pillar can be broken because the conductive adhesive from which the pillar is made can have a coefficient of thermal expansion CTE which is different than a CTE of the silicon backplane substrate of the display cell. Because of this CTE difference, the silicon backplane substrate and the pillar can expand and contract at different rates in response to temperature fluctuations. When this happens, mechanical stresses are created on the pillar and the pillar can break. Once the pillar is broken, the LC panel ceases to function properly. 
     The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic side view of certain layers of a conventional liquid crystal panel. 
         FIG. 2  is a diagrammatic side view of an embodiment of a liquid crystal panel having an electrical connection according to the present disclosure. 
         FIG. 3  is a diagrammatic top view of an embodiment of a liquid crystal panel having an electrical connection according to the present disclosure. 
         FIG. 4  is a diagrammatic side view of another embodiment of a liquid crystal panel having an electrical connection according to the present disclosure. 
         FIG. 5  is a diagrammatic side view illustrating the assembly of an embodiment of a liquid crystal panel having an electrical connection according to the present disclosure. 
         FIG. 6  is a flow diagram illustrating an embodiment of a method involving the assembly of a liquid crystal panel according to the present disclosure. 
         FIG. 7  is a diagrammatic side view illustrating the assembly of another embodiment of a liquid crystal panel having an electrical connection according to the present disclosure. 
         FIG. 8  is a flow diagram illustrating another embodiment of a method involving the assembly of a liquid crystal panel according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable one of ordinary skill in the art to make and use embodiments of the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles taught herein may be applied to other embodiments. Thus, embodiments of the present invention are not intended to be limited to the embodiments shown, but are to be accorded the widest scope consistent with the principles and features described herein including modifications and equivalents, as defined within the scope of the appended claims. It is noted that the drawings are not to scale and are diagrammatic in nature in a way that is thought to best illustrate features of interest. Descriptive terminology may be adopted for purposes of enhancing the reader&#39;s understanding, with respect to the various views provided in the figures, and is in no way intended as being limiting. 
     Attention is now directed to the remaining figures wherein like reference numbers may refer to like components throughout the various views.  FIG. 2  is a diagrammatic representation of an embodiment of a liquid crystal on silicon (LCOS) panel in a side view, generally indicated by reference number  100 . LCOS panel  100  includes a liquid crystal (LC) cell  102  having a silicon backplane die  104 , an LC layer  106  and a transparent electrically conductive glass  108 . The transparent electrically conductive glass can include a glass layer  110  and a transparent electrical conductor layer  112  formed from a transparent electrically conductive material such as, indium-tin-oxide (ITO). As can be seen from the diagrammatic side view of  FIG. 2 , transparent electrically conductive glass  108  can be offset from silicon backplane die  104  and LC layer  106  to create an overhang  114  on one end of the LC cell and a shelf  116  on the opposite end of the LC cell. 
     LCOS panel  100  can include a substrate  118 , such as a flexible printed circuit board, an FR4 printed circuit board or other substrate which includes electrically conductive traces  120  or other conductors for carrying electrical signals to and/or from the LC cell. The LC cell can be die-attached to the substrate using conventional die-attach methods to bond the silicon backplane die to substrate  118 . The silicon backplane die can have one or more bond pads  122  positioned on shelf  116  for electrically connecting to traces  120  using wire bonds  124  to transfer the electrical signals between substrate  118  and LC cell  102 . 
     Referring now to  FIG. 3  in conjunction with  FIG. 2 , the former is a diagrammatic top view of LCOS panel  100 . LC layer  106  can include a liquid crystal material  132  and a liquid crystal perimeter seal  134 . Perimeter seal  134 , electrically conductive glass  108  and silicon backplane die  104  can form the boundaries of a liquid crystal reservoir  130  which contains the liquid crystal material. During the assembly of the display cell, the liquid crystal perimeter seal can be formed on either the electrically conductive glass or the silicon backplane die. The liquid crystal perimeter seal can be made from an adhesive and can be applied using a syringe needle or can be printed using offset printing, an ink jet printer, or other suitable printing or application methods. The perimeter seal can bond the electrically conductive glass to the silicon backplane die to create laminated display cell  102 . 
     The liquid crystal layer can have spacers  136  which can be located in the perimeter seal and/or in the reservoir to maintain a gap  138  ( FIG. 2 ) between the electrically conductive glass and the silicon backplane die. The spacers can be particles of silica or polymer or another material having a specific dimension that is substantially the same as gap  138 . During manufacture, the perimeter seal can be formed with an opening  140  so that the reservoir can be filled with liquid crystal material  134  after the perimeter seal has cured. After the reservoir is filled through the opening, the reservoir can be sealed with a plug  142  which can be formed using an adhesive such as the adhesive used for the perimeter seal. 
     LCOS panel  100  can include an electrically conductive gap filler  150 . Gap filler  150  can be positioned in an overhang gap  152  between electrically conductive glass  108  and substrate  118  when the LC cell is attached to substrate  118 . The gap filler is electrically conductive and a first side of the gap filler can be electrically connected to a bond pad  154  of substrate  118  using an electrically conductive bonding material  156 . Bond pad  154  can be connected to a signal source for powering electrically conductive glass  108  through a circuit trace  120   a.  Circuit trace  120   a  and bond pad  154  can be part of substrate  118  and circuit trace  120   a  that can extend under the LC cell. An opposite side of gap filler  150  can be electrically connected to transparent electrically conductive layer  112  of electrically conductive glass  108  using an electrically conductive bonding material  158 . The gap filler can span a majority of the overhang gap or substantially the entire overhang gap depending on a thickness of the electrically conductive bonding material used. Electrically conductive bonding material  158  can be used to span any distance in the overhang gap that is not spanned by the gap filler. Electrically conductive glass  108  can be powered through circuit trace  120   a,  bond pad  154 , gap filler  150  and conductive bonding material  158 . 
     Gap filler  150  can be made, by way of non-limiting example, from an electrically conductive material, such as for example, gold plated bronze, gold plated copper, metal plated ceramic, such as for example a surface mount resistor, or other material such as a solid conductive metal or other conductive material. The gap filler can be formed in different shapes, such as for example, those having rectangular surface areas as show in  FIGS. 2 and 3  or in a cylindrical shape as is discussed below, or any other suitable shape that can be used for conducting electricity and which has a dimension that is suitable for spanning the majority of overhang gap  152 . The shape can be customized in view of the contact resistance needed as well as the particular types of attachments/electrical connections that are used in an embodiment. In an embodiment, overhang gap  152  can be on the order of approximately 0.7 mm, such that the gap filler can be approximately 0.6 mm and conductive bonding material  158  can be approximately 0.1 mm in height in view of  FIG. 2 . 
     Referring now to  FIG. 4 , a diagrammatic side view of an LCOS panel is generally indicated by reference number  170 . LCOS panel  170  includes an at least generally cylindrically shaped gap filler  172  that is positioned between bond pad  154  and transparent electrically conductive glass  108 . Gap filler  172  can be electrically and physically attached to substrate  118  using electrically conductive bonding material  156  and can be electrically and physically attached to electrically conductive glass  108  using electrically conductive bonding material  158 . The cylindrically shaped gap filler can be made using an elongated stock material such as a length of wire having a suitable diameter and gap filler  172  can be oriented such that electricity can travel across the diameter or cross-sectional width of the cylindrically shaped gap filler in a direction that is at least generally transverse to the elongation axis to electrically connect the bond pad of the substrate to the electrically conductive glass. The length of the cylindrically shaped gap filler can be based on available space in view of contact resistance requirements. The cross-sectional shape of the elongated gap filler can be any suitable shape including, but not limited to elliptical, trapezoidal, a parallelogram, an asymmetrical shape, and a rectangular shape. Moreover, the elongation axis is not required to be straight but can be curved to suit a particular application. Further, the elongation axis is not required to be parallel to other features of the assembled panel such as, for example, the near edges of the die or transparent glass. 
     Irrespective of any particular shape, the gap filler can be a material that has a coefficient of thermal expansion (CTE) that is similar to a CTE of silicon backplane die  104 , such as by way of non-limiting example, a metal clad ceramic. During temperature fluctuations, overhang gap  152  between electrically conductive glass  108  and substrate  118  can increase and decrease in size due at least partially to thermal expansion and contraction of the silicon backplane die. By selecting the gap filler material having a CTE that is similar to the CTE of the silicon backplane die, the gap filler can expand and contract at a similar rate to match related behavior of the silicon backplane die. For example, when the silicon backplane die expands, the overhang gap increases in height and the gap filler expands to continue to span the increased size of the overhang gap. In this situation, if the gap filler has a CTE that is substantially different from the CTE of the silicon backplane die, the gap filler may expand more than the silicon backplane die, in which case the gap filler may de-laminate display cell  102  or break one of the layer of the LCOS panel. On the other hand, the gap filler may not expand as much as the silicon backplane die and the overhang gap, in which case the electrical connection across the overhang gap may be broken. In either case, the LCOS panel may be rendered non-functional. Damage to LCOS panel  100  can be reduced or eliminated if the CTE of the gap filler is close enough to the CTE of the silicon backplane die. Conventional pillars made entirely from conductive adhesive can fail due to thermally induced expansion and contraction because the CTE of the conductive adhesive can be dissimilar to the CTE of the silicon backplane die. For increased reliability over a conventional LCOS panel having a pillar made from a conductive adhesive, the CTE of the gap filler can be closer to the CTE of the silicon backplane die than to a CTE of the conductive adhesive used to electrically connect the gap filler to the electrically conductive glass. 
     In one embodiment, electrically conductive bonding material  158  can be a conductive adhesive, such as an ultra-violet curing acrylic adhesive, epoxy adhesive or other optical adhesive. In another embodiment, the gap filler is soldered to the electrically conductive layer, for example, by using an indium-tin solder when the electrical conductor layer is indium-tin-oxide. Electrically conductive bonding material  156  can also be a conductive adhesive, or the gap filler may be soldered to substrate  118  using a soldering method such as a conventional reflow soldering technique. The gap filler can be soldered to the substrate while being electrically connected to the electrically conductive glass by conductive adhesive. The gap filler can also be electrically connected to both the electrically conductive glass and the substrate using conductive adhesive. The gap filler can also be electrically connected to both the electrically conductive glass and the substrate using solder. 
     Turning now to  FIG. 5 , a diagrammatic side view of LC cell  102  and substrate  118  is shown prior to the assembly into LCOS panel  100  by die-attaching the silicon backplane die  104  to substrate  118 . Gap filler  150  can be electrically and physically attached to electrically conductive glass  108  using electrically conductive bonding material  158  prior to the die-attach. The gap filler can be attached to the electrically conductive glass by applying the bonding material to the gap filler and/or the electrically conductive glass after which the gap filler can be scrubbed into electrically conductive layer  112  by moving the gap filler while the gap filler is in contact with the conductive glass. Scrubbing the gap filler against the electrically conductive glass abrades the polyimide layer of the electrically conductive glass and can result in a low resistance contact between electrical conductor layer  112  and gap filler  158 . Electrically conductive bonding material  156  can be applied to bond pad  154  on substrate  118  and the LC cell, with the gap filler electrically connected, can be die-attached to substrate  118  to produce LCOS panel  100 . The gap filler can be electrically connected to substrate  118  at substantially the same time that the LC cell is die-attached to the substrate by contacting bonding material  156  on the substrate. Wire bonds  124  ( FIG. 2  and  FIG. 4 ) can be connected to bond pads  122  and traces  120  following the die-attach. 
     Turning now to  FIG. 6 , a flow diagram illustrating an embodiment of a method in a liquid crystal display panel is generally indicated by the reference number  180 . Method  180  begins at  182  and proceeds to  184  where a first side of the gap filler is electrically connected to the electrically conductive glass using a conductive adhesive. Method  180  then proceeds to  186  where an opposite side of the gap filler is electrically connected to the substrate to electrically connect the electrically conductive glass to the substrate through the gap filler and the conductive adhesive. Method  180  then proceeds to  188  where the method ends. 
     Turning now to  FIG. 7 , another diagrammatic side view of LC cell  102  and substrate  118  is shown prior to the assembly into LCOS panel  100  by die-attaching silicon backplane die  104  to substrate  118 . Gap filler  150  can be electrically connected to substrate  118  prior to die-attaching silicon backplane die  104  to substrate  118  using bonding material  156  such as a conductive adhesive or solder. Once the gap filler is connected to substrate  118 , bonding material  158  can be applied to the opposite side of the gap filler from the attachment to substrate  118  and/or to the electrically conductive glass. LC cell  102  can then be die-attached to substrate  118 , using conventional die-attach methods, at which time the electrically conductive glass is moved into position to electrically connect the electrically conductive glass to the gap filler through electrically conductive bonding material  158  to complete the LCOS panel. Wire bonds  124  ( FIG. 2  and  FIG. 4 ) can be connected to bond pads  122  and traces  120  following the die-attach. 
     Turning now to  FIG. 8 , a flow diagram illustrating an embodiment of a method in a liquid crystal display panel is generally indicated by the reference number  200 . Method  200  begins at  202  and proceeds to  204  where a first side of the gap filler is electrically connected to the substrate, for example, by soldering. Method  200  then proceeds to  206  where an electrically conductive adhesive is applied to an opposite side of the gap filler from the substrate. Method  200  then proceeds to  208  where silicon backplane die  104  is die-attached to the substrate substantially simultaneously to the electrically conductive glass being electrically connected to the substrate through the gap filler and electrically conductive adhesive. Method  200  then proceeds to  210  where the method ends. 
     The foregoing descriptions of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or forms disclosed, and other modifications and variations may be possible in light of the above teachings wherein those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof.