Abstract:
The present invention relates to a display assembly having a liquid crystal on silicon display device disposed on a first substrate and having first electrical contact pads. The display assembly also has a second substrate having second electrical contact pads coupled to the first electrical contact pads and having an array of ball contact elements. Various embodiments and features are disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to packaging liquid crystal display panels and more particularly to mounting liquid crystal display panels onto a substrate using ball grid array.  
           [0003]    2. Background Information  
           [0004]    Conventional flat-panel displays use electroluminescent materials or liquid crystals in conjunction with incident light to produce high quality images in products such as digital wristwatches, calculators, panel meters, thermometers, and industrial products. Liquid crystals are a state of matter that mixes the droplet or pouring property of a liquid and the long-range order property of a solid. This combination allows an optical activity having a magnitude without parallel in either solids or liquids. Further, when a magnetic or electrical field is applied normal to the liquid crystal material, the liquid crystal material forms a localized monocrystal that is polar in character. This localized polarization of the liquid crystal material affects the travel path of light incident to the liquid crystal material. By controlling the electrical field applied across the liquid crystal material, the travel path of light incident to the liquid crystal material can be controlled to help produce high quality images.  
           [0005]    Modern approaches for developing high quality liquid crystal displays (LCDs) utilize the active-matrix approach in which thin-film transistors (TFTs) are co-located with the LCD pixels. Micro liquid crystal display panels may be those displays having a two inches or less diagonal viewing screen. Micro-displays are an emerging, enabling technology; that is, micro-displays will enable many people to design and develop countless new products to the betterment of humankind. To get a sense of the compactness of a micro-display, a Super Video Graphics Array (SVGA) display may have a 600 by 800 pixel matrix (480,000 pixels) within a 0.9 inch diagonal viewing screen.  
           [0006]    Reflective micro-displays sandwich a liquid crystal material between a reflecting material such as aluminum and a glass cover that permits light to enter the liquid crystal material. Due to small size of micro-displays, micro-displays require the drive circuitry of an integrated circuit to be integrated into the display panel along with the pixel transistors. Typically, the drive circuitry is integrated into the display substrate that is located below the reflecting material. Because the drive circuitry must be integrated with the display substrate, microdisplays are generally limited to high quality transistor technology such as single crystal (x-Si) and polysilicon (p-Si).  
           [0007]    Thus, in general, reflective micro-displays are usually based on single-crystal silicon integrated circuit substrates with a reflective aluminum pixel forming a pixel mirror over the pixel transistors and addressing lines. Buses or leads are used to communicate power, ground and other signals between these transistors and addressing lines and devices external to the micro LCD panels.  
           [0008]    The pattern of the electrical leads of a micro-display may be as small as fifty-two leads within a distance of 12.1 millimeters (i.e., lead pitch is 0.22 millimeters between each lead). To package the micro-display for use in other products, the micro-sized pattern of the electrical leads of the micro-display needs to be rearranged into a pattern that is usable by existing connectors. One known technique is to first mechanically attach the LCD panel to a rigid printed circuit board that extends in one direction into a flexible printed circuit board. At the end of the flexible printed circuit board is a male connector of a conventional pattern that fits into conventional female sockets mounted to a driver board. To form electrical paths between the micro LCD panel and the rigid printed circuit board, micro-wire bonds are sonic welded between the electrical leads of the micro LCD panel and the rigid printed circuit board.  
           [0009]    The use of a flexible printed circuit has several advantages. It is a low profile method of mounting that is relatively inexpensive that is readily available. However, because the flex circuit extends from only one side of the LCD panel, the driver devices on the driver board must be located so as to account for the orientation of the flex circuit. The driver devices also must be located away from the viewing screen of the LCD panel by the length of the flex circuit, resulting in longer buses that increase the chance of picking up noise as well as diminishing the signal power. Since the socket is a function of the flex circuit, it would seem that original equipment manufacturer (OEM) designers are limited in their choice of female sockets. Even worse, most custom designers request a custom flex circuit lead to fit their choice of female socket. The lack of a universal package requires maintaining several product lines for essentially the same product. This increases the unit cost of each micro LCD panel.  
           [0010]    Since real estate on a driver board is at a premium, there is a need to shrink the packaging footprint of existing micro LCD panels. There is also a need to employ a universal connector that allows OEM designers to arrange the micro LCD panel in any position they choose while eliminating the requirement for a female socket mounted to the driver board.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention relates to a display assembly having a liquid crystal on silicon display device disposed on a first substrate and having first electrical contact pads. The display assembly also has a second substrate having second electrical contact pads coupled to the first electrical contact pads and having an array of ball contact elements. Various embodiments and features are disclosed.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 show a side view of liquid crystal display  100  mounted to flexible printed circuit board  120 .  
         [0013]    [0013]FIG. 2 shows bond pads  122  as extended by traces  124  into male connector  130 .  
         [0014]    [0014]FIG. 3 is an isometric exploded view of an embodiment of the invention.  
         [0015]    [0015]FIG. 4 illustrates BGA fabrication  310  with mount side  312  facing up.  
         [0016]    [0016]FIG. 5 shows an array of ball contact elements  370  populating BGA fabrication  310 .  
         [0017]    [0017]FIG. 6 is a schematic wiring diagram of BGA fabrication  310 .  
         [0018]    [0018]FIG. 7 highlights the use of capacitors  376  in schematic wiring diagram  374  of FIG. 6.  
         [0019]    [0019]FIG. 8 illustrates a preferred arrangement of capacitors  376  on BGA substrate  300 .  
         [0020]    [0020]FIG. 9 is a detailed view of height beads  340  taken from detail line  9 - 9 .  
         [0021]    [0021]FIG. 10 is a section view of height beads  340  taken off line  10 - 10 .  
         [0022]    [0022]FIG. 11 is an exploded isometric view of LCOS device  210 .  
         [0023]    [0023]FIG. 12 illustrates LCOS device being mounted to BGA substrate  300 .  
         [0024]    [0024]FIG. 13 shows an isometric top view of display cover  500 .  
         [0025]    [0025]FIG. 14 shows an isometric plan view of display cover  500 .  
         [0026]    [0026]FIG. 15 is an isometric view of the back side of display cover  500 .  
         [0027]    [0027]FIG. 16 is an isometric view of display assembly  200  in its ready to ship configuration.  
         [0028]    [0028]FIG. 17 is section view of display assembly  200  of FIG. 16 taken off of line  17 - 17 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Referring to the figures, exemplary embodiments of the invention will now be described. The exemplary embodiments are provided to illustrate aspects of the invention and should not be construed as limiting the scope of the terms of the invention. The exemplary embodiments are primarily described with reference to block diagrams or flowcharts. As to the flowcharts, each block within the flowcharts represents both a method operation and an apparatus element for performing the method operation. Depending upon the implementation, the corresponding apparatus element may be configured in hardware, software, firmware or combinations thereof.  
         [0030]    [0030]FIG. 1 show a side view of liquid crystal display  100  mounted to flexible printed circuit board  120 . Liquid crystal display  100  is made of glass cover  102  maintained a distance away from circuit substrate  104  by spacers  106 . This distance forms gap  108  into which liquid crystal material  110  is placed. Between spacers  106  is image viewing area  107  (FIG. 2) on which color images may be presented. Bond pads  114  may be provided on exposed portion  112  (FIG. 2) of display  100  as electrical contact points to permit devices external to display  100  to communicate with display  100 , such as communicate with the circuitry within circuit substrate  104 .  
         [0031]    As shown in FIG. 1, underside  116  of liquid crystal display  100  mechanically is attached to flexible printed circuit board  120 , preferably using a solvent-free, thermal-set adhesive. Flexible printed circuit board  120  consists of bond pads  122  that permit board  120  to be electrically couple to liquid crystal display  100 . FIG. 2 shows bond pads  122  as extended by traces  124  into male connector  130 . Male connector  130  has a flat, conventional profile that can fit into any existing female flex circuit socket. For example, connector  130  may fit into a Zero Insertion Force (ZIF) socket mounted on a printed circuit board containing the device driver that runs liquid crystal display  100 .  
         [0032]    Flexible printed circuit board  120  of FIG. 1 may be electrically connected to display  100  using wire bonds  140 . Wire bonds  140  may be aluminum-wedge wire bonds. The bonding process may be performed by sonic welding at room temperature. Bonding wires  140  then may be encapsulated with encapsulant  150 . Encapsulant  150  may be a solvent-free, ultra-violet curable adhesive to protect the delicate wire bonds  140  from damage.  
         [0033]    To further protect the electrical connection at wire bonds  140 , stiffener  145  may be added under a portion of flexible printed circuit board  120  to complete LCD package  150 . In one sense, stiffener  145  provides support to display panel  300  and yet allows flexibility at flex portion  148  of connector  130 .  
         [0034]    [0034]FIG. 3 is an isometric exploded view of an embodiment of the invention. Display assembly package  200  may be formed first by electrically assembling liquid crystal on silicon (LCOS) display cell or device  210  onto ball grid array (BGA) substrate  300 . Wire bonds  270  are sonic welded between LCOS device  210  and BGA substrate  300  to form assembly  400 . To complete display assembly package  200 , display cover  500  is placed over LCOS display device  210  and snapped into place on BGA substrate  300 .  
         [0035]    [0035]FIG. 4 illustrates BGA fabrication  310  with mount side  312  facing up. BGA fabrication  310  preferably is a six layer rigid printed circuit board wherein each layer consists of Fire Resistant 4 (FR4) board material onto which one half ounce copper traces are disposed. The copper traces are connected by through holes formed into the FR4 boards. Preferably manufactured in accordance with IPC-A-600E, each of the exposed areas are gold plated (120 mlN NI, 8 mlN AU).  
         [0036]    The features of fabrication  310  include carrier board  314 , capacitor contact pads  316 , resistor contact pads  320 , electrical contact pads  330 , height beads  340 , surface  350 , and cover holes  360 . Carrier board  314  preferably is a multi layered FR4 board. Capacitor contact pads  316 , resistor contact pads  320 , electrical contact pads  330  serve as pads for capacitors, resistors, and sire bonds, respectively. Cover holes  362  are used to couple display cover  500  to BGA substrate  300 .  
         [0037]    Height beads  340  may be view as three or more preferably hemispherical beads disposed on surface  350  to a set height. The set, uniform height of height beads  340  gage the distance in which LCOS device  210  resides above BGA substrate  300 . This permits metering the amount of epoxy necessary to mechanically and thermally bond LCOS device  210  to BGA substrate  300 .  
         [0038]    [0038]FIG. 5 shows an array of ball contact elements  370  populating BGA fabrication  310 . Balls  370  populate carrier board  314  on contact side  372  as arranged in an array matrix. Primarily, balls  370  serve as a conduit that routes signals to and from LCOS device  210 .  
         [0039]    Of the preferable configuration of 64 balls of BGA fabrication  310 , LCOS device makes electrical signal use of 52 balls in a preferred embodiment. Twelve ball have been adapted in the invention as a thermal area to help conduct heat out the back of BGA substrate  300  into the circuit board on which it is mounted.  
         [0040]    [0040]FIG. 6 is a schematic wiring diagram of BGA fabrication  310 . FIG. 7 highlights the use of capacitors  376  in schematic wiring diagram  374  of FIG. 6. In a micro display, variations in the supplied power of greater than one percent are visible to the human eye. Display assembly package  200  uniquely takes advantage of close couple capacitating in a package having a LCOS display mounted to a ball grid array. Close couple capacitating aids in keeping the display signal clean without interference.  
         [0041]    Capacitors  376  preferably are used as bypass capacitors for the power disbursed within BGA substrate  300 . Each capacitor  376  works to filter noise off the power and ground coming into LCOS display  210  by holding power so that LCOS display  210  can quickly draw off this power when needed.  
         [0042]    Capacitors  376  may reside within a 18.0 millimeter (mm) by 18 mm perimeter where LCOS display  210  is disposed within this perimeter. Preferably, there are eleven capacitors, wherein each capacitor resides as close as possible to LCOS device  210  to filter the power and ground. In one embodiment, capacitors  376  reside within a 9.0 mm by 18 mm perimeter where LCOS display  210  is disposed within this perimeter.  
         [0043]    [0043]FIG. 8 illustrates a preferred arrangement of capacitors  376  on BGA substrate  300 . FIG. 9 is a detailed view of height beads  340  taken from detail line  9 - 9 . FIG. 10 is a section view of height beads  340  taken off line  10 - 10 . As seen in FIG. 10, height bead  340  preferably is spherical in shape and resides approximately 0.127 mm above surface  350 .  
         [0044]    [0044]FIG. 11 is an exploded isometric view of LCOS device  210 . As seen, glass cover  102  is disposed adjacent to circuit substrate  104  by spacer  106  over viewing area  107 . This alignment leaves bond pads  114  exposed for subsequent electrical connection.  
         [0045]    [0045]FIG. 12 illustrates LCOS device being mounted to BGA substrate  300 . First, adhesive  390  is disposed onto surface  350  of BGA substrate  300 . Preferably, adhesive  390  is a thermally conductive epoxy. LCOS device  210  is then picked and placed onto BGA substrate  300  and pressed until LCOS device  210  comes into contact with height beads  340 . As noted above in connection with FIG. 4, the uniform height of height beads  340  ensure that the measured amount of adhesive  390  does not ooze beyond the footprint of LCOS device  210 . Wire bonds  270  are then sonic welded into place as shown in FIG. 12. To complete assembly  400 , wire bonds  270  are covered with encapsulant  392  to protect the wire bonds.  
         [0046]    [0046]FIG. 13 shows an isometric top view of display cover  500 . FIG. 14 shows an isometric plan view of display cover  500 . As seen in FIG. 14, display cover  500  has aperture  510  and press fit pins  520 . Aperture  510  serves as a framed opening for viewing area  107  of LCOS device  210 . As a dark, non-reflective surface, bevel  512  directs stray light away from the view&#39;s eyes so that the viewer does not pick up on any stray light reflection. Bevel  512  preferably is at a 45 degree angle. Preferably black General Electric PBT resin such as Vailox™ or some other light absorbing color, aperture  510  serves to block out any extraneous or stray or reflected light. In another embodiment, display cover  500  is formed of encapsulant material molded in place over assembly  400 .  
         [0047]    Press fit pins  520  preferably are cylinder in shape and have jog  525  raidally extending there from to provide a press fit into cover holes  360  of BGA fabrication  310 . With real estate being a premium on all electronics, the embodiment of display cover  500  for a press fit on to BGA substrate  300  creates a smaller foot print for display assembly package  200 , thereby permitting more room on the device driver board for other electronic components.  
         [0048]    Display cover  500  may be thought of as a controllable interface gauging surface. When display assembly package  200  is use, magnifying optical devices will be coupled to package  200 . Maintaining a proper optical focal length is critical. Display cover  500  has registration features that account for this critical focal length. As seen in FIG. 13, a variety of registration features  530  are provided on the outer surface of display cover  500 .  
         [0049]    [0049]FIG. 15 is an isometric view of the back side of display cover  500 . Shown are registers  540 . Registers  540  contact cover glass  102  of LCOS device  210  to provide a registration that accounts for the critical optical focal length.  
         [0050]    [0050]FIG. 16 is an isometric view of display assembly  200  in its ready to ship configuration. Prior to shipping, display cover  500  is covered by protective film material  570 . Protective film  570  serves as a safe surface for picking and placing display assembly package  200 . Protective film  570  may be, for example, a blue protective film manufactured by Semiconductor Equipment Corporation as part number 118733-11.0. FIG. 17 is section view of display assembly  200  of FIG. 16 taken off of line  17 - 17 .  
         [0051]    Several benefits are derived from the invention. For example, since the contacts of the LCD panel are brought from the viewing side of the LCD panel to the bottom of the packaging, the footprint of the package may be maintained within the BGA substrate  300 . The exemplary embodiments described herein are provided merely to illustrate the principles of the invention and should not be construed as limiting the scope of the terms of the invention. Rather, the principles of the invention may be applied toward a wide range of systems to achieve the advantages described herein and to achieve other advantages or to satisfy other objectives, as well.