Patent Document

BACKGROUND 
     1. Technical Field 
     The described embodiments relate generally to a method and apparatus for attaching a flexible PCB to a number of positions on an LCD assembly. More specifically the disclosure describes a method for precisely attaching the flexible PCB around an obstruction 
     2. Related Art 
     Compact computing devices such as laptop computers, smart phones, etc. have become ever smaller, lighter and more powerful. One factor contributing to this reduction in size can be attributed to the manufacturer&#39;s ability to fabricate various components of these devices in smaller and smaller sizes, assembling the components in ever more dense configurations, and in most cases increasing the power and or operating speed of such components. In many cases the close packing of components can result in obstructions making routing of cables and connectors particularly challenging. While flexible printed circuit boards (PCBs) can assist in easing such routing problems unfortunately the overall flexibility of the flexible printed circuit boards themselves can make precise placement of those flexible PCBs more challenging. Precise placement becomes even more challenging when a routing path for the PCB needs to lie up along a very specific path. 
     Therefore, a method for precise placement of a flexible PCB within a larger electronic device is desired. 
     SUMMARY 
     The embodiments relate to a method, system, and computer readable medium for accurately and efficiently attaching a flexible circuit to a display assembly. 
     In one embodiment, a method for aligning and bonding a flexible circuit having a number of attaching surfaces to a number of mounting areas is disclosed. The method including at least the following steps: (1) securing the flexible circuit to a securing mechanism, the securing mechanism having at least as many securing areas as there are attaching surfaces; (2) concurrently lifting a first attaching surface and a second attaching surface using the securing mechanism; (3) using an optical guidance system to direct the securing mechanism in aligning the first attaching surface with a first mounting area and independently aligning the second attaching surface with a second mounting area; and (4) bonding the first attaching surface to the first mounting area and the second attaching surface to the second mounting area. 
     In another embodiment a system for electrically coupling a flexible circuit to a plurality of electrical contacts arranged on a display assembly is disclosed, including at least the following: (1) an optical guidance system comprising a plurality of CCD cameras arranged to observe fiducials disposed on a first surface of the display assembly and corresponding fiducials disposed on the flexible circuit; (2) a number of vacuum chucks configured to independently maneuver a number of attaching surfaces of the flexible circuit until a number of fiducial marks on the flexible circuit are in alignment with corresponding fiducial marks on the first surface of the display assembly; and (3) a plurality of hot bars configured to adhesively and electrically couple the number of attaching surfaces to associated electrical contacts arranged on the display assembly by a conductive, pressure sensitive adhesive. 
     In yet another embodiment a non-transitory computer readable medium for storing computer instructions executed by a processor in a computing device is disclosed. The non-transitory computer readable medium includes at least the following: (1) computer code for pre-bending a flexible circuit, the flexible circuit comprising a first and second attachment surface; (2) computer code for concurrently aligning the first and second attachment surfaces to alignment indicia arranged on a first surface of an electrical component by independently maneuvering the first and second attachment surfaces with a first vacuum chuck attached to the first attachment surface and a second vacuum chuck attached to the second attachment surfaces; (3) computer code for pressing the first and second attachment surfaces onto the first surface of the electrical component, wherein the pressing of the first and second attachment surfaces onto the first surface of the electrical component pre-bonds the first and second attachment surfaces onto the first surface of the electrical component together; and (4) computer code for adhesively and electrically coupling the first and second attachment surfaces to the first surface of the electrical component by a plurality of hot bars. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The described embodiments will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG. 1A  illustrates a perspective view of a flexible circuit having a number of pre-bent portions designed to conform around an intervening structure; 
         FIG. 1B  illustrates a perspective view of a flexible circuit disposed across a precision stage for precisely arranging the flexible circuit prior to a bonding operation; 
         FIG. 1C  illustrates a pair of vacuum chucks coupled by vacuum suction to a pair of attaching surfaces; 
         FIG. 1D  illustrates a perspective view of a lower surface of a vacuum chuck; 
         FIG. 2  illustrates a flexible circuit supported by vacuum chucks prior to a pre-bonding operation; 
         FIG. 3A  illustrates a number of CCDs used for determining a correct position for a pair of attaching surfaces on a Thin Film Transistor (TFT) ledge; 
         FIG. 3B  illustrates an alternate configuration for positioning a number of CCDs; 
         FIG. 4  illustrates a perspective view of a landing position for a hot bar on a flexible circuit; 
         FIG. 5  illustrates a flexible circuit undergoing a final bonding operation; 
         FIG. 6A  illustrates a perspective view of a flexible circuit attached to a display assembly; 
         FIG. 6B  illustrates a cross-sectional side view of a flexible circuit and display assembly as defined by cross-section A-A from  FIG. 6A ; 
         FIG. 7  shows a flowchart detailing an assembly process  700  for combining touch sensor input signals from disparate portions of a touch sensor; and 
         FIG. 8  is a block diagram of an electronic device suitable for controlling some of the processes in the described embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to selected embodiments an example of which is illustrated in the accompanying drawings. While the invention will be described in conjunction with a preferred embodiment, it will be understood that it is not intended to limit the invention to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the invention as defined by the appended claims. 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the present invention. 
     As devices grow in complexity and functionality signal routing complexities can grow exponentially. Increasingly larger numbers of sensors and standalone component devices are required to communicate back and forth in a networked fashion, each communication channel sometimes requiring its own set of wires. While flexible printed circuits can allow for denser configurations, such flexible circuits can be difficult to place with the high precision standards demanded by modern electronic device manufacturers. One way to ensure that a flexible printed circuit is both predictably shaped and precisely positioned is to apply a preforming or and/or pre-bending operation in which a desired finished geometry is achieved by placing the flexible printed circuit on a stage device that can maneuver the flexible printed circuit into at least a shape very closely matching its final geometry. Once the geometry is achieved the flexible circuit can be moved to the electronic device itself for precise placement. In one embodiment the moving device is a set of vacuum chucks. By constraining attachment points of the flexible circuit in strict relation to one another a correct final geometry can be achieved once the flexible circuit is attached inside an electronic device. Precise positioning of the attachment points can be achieved by a robotic vision system including at least one Coupled Charged Device (CCD) cameras. In embodiments in which at least one of the attaching surfaces are transparent the at least one CCD can function as a form of feedback control to direct precise positioning of the flexible circuit prior to a pre-bonding step. By looking through the at least one transparent surface the CCD can give feedback about whether the fiducial marks on the flexible circuit properly align with corresponding fiducial marks located on the surface to which it will be attached. After the pre-bonding step is complete a final bonding operation can be applied involving both heat and pressure to be applied to maximize conductivity across an adhesive layer and to firmly set the adhesive between the flexible circuit and its targeted attachment points. 
     In one more specific embodiment a flexible circuit can be utilized to combine signals from various regions of a touch sensor matrix. The flexible circuit can be required when intervening components or limiting geometry prevents a solid printed circuit board (PCB) from being utilized to connect the various touch sensor outputs. In one embodiment attachment points for the flexible circuit have tight tolerances that need to be achieved so that in one extreme delicate parts are not crushed and in another extreme an insufficiently strong adhesive bond results. The vision system previously described can be utilized to achieve these strict tolerances; however, in certain cases CCD viewing positions can be altered by the use of optics to reposition the CCDs into positions in which there is sufficient space. 
     Various embodiments of flexible PCB attachment methods are discussed below with reference to  FIGS. 1A-8 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the full extent of the embodiments goes beyond these limited descriptions. 
       FIG. 1A  illustrates flexible circuit  100  with a number of pre-bent portions  102 . Pre-bent portions  102  are designed to allow flexible circuit  100  to be routed over an intervening obstruction, in one embodiment embodied as an integrated circuit. Flexible circuit  100  can be made completely from flexible PCB material. The flexible PCB material can be a flexible plastic substrate such as for example polyimide, PEEK (polyester ether ketone), or transparent conductive polyester film. In another embodiment flexible circuit  100  can be made from a combination of rigid and flexible materials, allowing only specific portions of flexible circuit  100  to bend and flex. Flexible circuit  100  can includes two attaching surfaces  104 . Attaching surfaces  104  can include touch sensor connectors (not shown) disposed on a bottom surface of connection branches  104 . In some embodiments of an electronic device with a touch sensor different inputs from different regions of the touch sensor are routed through different channels. The depicted pair of touch sensor connectors can merge signals from at least two different regions, thereby providing a processor or logic unit a combined set of inputs to be processed. Merged connector  106  can be configured to carry signals routed from attaching surfaces  104  to another component such as a main logic board for further processing. In  FIG. 1B  a precision stage  110  is illustrated for precisely arranging flexible circuit  100  prior to a bonding operation. Precision stage  110  can have a central element  112  flanked by two adjustable elements  114 . Adjustable elements  114  can be precisely translated in the X-axis with respect to central element  112 . In this way flexible circuit  100  can span a precision distance from one end to another. 
       FIG. 1C  illustrates a pair of vacuum chucks  116  vacuum attached to attaching surfaces  104 . Vacuum chucks  116  allow connection branches  104  to be maneuvered; each vacuum chuck can be independently maneuverable in at least a Y and θ axes. In such an embodiment a mating surface can maneuver in X and Z axes to facilitate contact between flexible circuit  100  and targeted touch sensor contacts. In some embodiments vacuum chucks  116  can be maneuverable in X, Y, Z and θ axes, thereby allowing vacuum chucks  116  to be responsible for all maneuvering of flexible circuit  100  with respect to target touch sensor contacts in a subsequent attachment procedure.  FIG. 1D  shows a perspective view of a bottom surface of vacuum chuck  116 . For vacuum chuck  116  to attach effectively to flex circuit  100  the vacuum attachment should be sturdy enough to allow the attachment to withstand sheering forces caused by maneuvering of vacuum chuck  116  with respect to flex circuit  100 . Attachment strength can be directly related to configuration of vacuum holes  118  and material composition of bottom surface  120 . Vacuum chuck  116  can also include mechanical features  122  to help prevent slipping of flexible circuit  100  with respect to vacuum chuck  116  during a maneuvering operation. 
       FIG. 2  illustrates flexible circuit  100  supported by vacuum chucks  116  just prior to a pre-bonding operation. Flexible circuit  100  reaches this position by being lifted off of precision stage  110  and moved over Thin Film Transistor (TFT) ledge  202  of display assembly  200 . Vacuum chucks  116  can be configured to maintain a specific spacing established by precision stage  110  as it moves flexible circuit  100  from precision stage  110  to a position above display assembly  200 . Display assembly  200  as depicted includes color filter glass  204  and TFT glass  206 . Positioned on TFT ledge  202  of display assembly  200  are bonding pads  208 . Bonding pads  208  include conductive adhesive strips arranged on top of touch sensor contacts (not shown). In one embodiment conductive adhesive strips can be pressure sensitive anisotropic conductive film (ACF). Conductivity through ACF is increased when pressure is applied as a result of conductive particles within the adhesive substrate being compressed together. In one embodiment ACF must be thermally activated to form a finalized bond. It should be noted that while the embodiments describe a TFT type LCD component display assembly any other display type such as for example AMOLED, or even more generally any electrical component benefiting from precise placement of flexible circuits to join a number of signals together is also within the scope of this disclosure. 
     Also depicted in  FIG. 2  is display driver integrated circuit (IC)  210 . Display driver IC  210  can be configured to power functionality within display assembly  200 . Display driver IC  210  is the reason flexible circuit  100  is used since a flat and rigid circuit would not be able to conform with the physical obstruction caused by display driver IC  210 . Once vacuum chucks  116  have established flexible circuit  100  above display assembly  200 , vacuum chucks  116  can be maneuvered with reference to feedback provided by charged coupled device (CCD)  212 . CCD  212  is a manufacturing camera that can be positioned about  10 mm from a bottom surface of TFT glass  206 . In this way CCD  212  can provide precise information regarding alignment of attaching surfaces  104  with bonding pads  208 . 
       FIG. 3A  illustrates a number of CCDs  212 . CCDs  212  are able to determine a correct position for attaching surfaces  104  on TFT ledge  202  by way of a number of fiducials  302  located on both TFT ledge  202  and attaching surfaces  104 . In some embodiments where flexible circuit  100  is transparent CCDs  212  can be alternatively positioned above flexible circuit  100 . Each attaching surface  104  can have at least two fiducials  302  which correspond to at least two fiducials  302  on TFT ledge  202 . Although not depicted vacuum chucks  116  are used to maneuver attaching surfaces  104  until fiducials  302  are in alignment, at which point vacuum chuck  116  lowers its attaching surface  104  until it is in contact with bonding pad  208 . This alignment and placement process can be configured to run in an automated fashion or manually. Once attaching surface  104  is in contact with bonding pad  208  vacuum chuck  116  can apply pressure to attaching surface  104  to establish a pre-bond between attaching surface  104  and bonding pad  208 . 
     As a result of squeezing components into tighter and tighter spaces in some embodiments there may not be enough space beneath TFT glass  206  for CCDs  212 . In such an eventuality  FIG. 3B  shows a work around.  FIG. 3B  shows how a series of optics can be arranged to position CCDs  212  in a position that allows viewing of fiducials  302 . In one embodiment a CCD  212  can be offset laterally from underneath TFT glass  206 . Mirror  304  can be configured at a  45  degree angle allowing a field of view of CCD  212  to see alignment of fiducials  302 . In another embodiment one CCD  212  can be configured to view two pairs of fiducials  302 . A series of mirrors can split light collected by CCD  212  into two halves one half showing alignment of a first set of fiducials  302  and another half of the field of view configured to view a second set of fiducials  302 . Finally, in yet another embodiment a single wide field of view CCD  306  can be configured to view two sets of fiducials  302 . 
       FIG. 4  illustrates a landing position  402  for a hot bar to complete a final bonding operation between attaching surface  104  and TFT ledge  202 . Landing portion  402  is a target position for a hot bar (not depicted) to come into contact with, thereby resulting in heating and compression of ACF layer and electrical connection of flexible circuit  100  with TFT touch sensor contacts arranged on TFT ledge  202 . Landing portion  402  of attaching surface  104  is designed to be thinner than the rest of flexible circuit  100  as a layer of coverlay has been removed from it. As a result heat from a hot bar can be transmitted more easily through landing portion  402 , thereby reducing total heat expulsion and limiting duration and/or total heat imparted to display assembly  200 . In one embodiment landing portion  402  can be marked out by visual guides on an upper surface of flexible circuit  100  to provide a visual target to indicate exactly what position the hot bar should be positioned in. 
       FIG. 5  illustrates flexible circuit  100  undergoing a final bonding operation. Hot bars  502  are in contact with landing portions  402  of flexible circuit  100 . By providing heat to a thin portion of flexible circuit  100  ACF disposed on a surface portion of TFT ledge  202  can be activated by both heat and pressure provided by hot bars  502 . Hot bars  502  can assert pressure independently of one another, allowing pressure to be applied equally to both surfaces, thereby achieving a more consistent bond than would otherwise be achieved by a single hot bar with two contact positions. In some embodiments the positioning of hot bars  502  can also be influenced by CCDs  212  which can track the position of hot bars  502  in relation to fiducials  302 , thereby providing another indication of proper placement of hot bars  502 . 
       FIG. 6A  illustrates a perspective view of flexible circuit  100  attached to display assembly  200 . It should be noted that in this depicted embodiment landing position  402  does not extend all the way to pre-bent portion  102 , thereby preventing deformation of pre-bent portion  102  or even worse damage to display driver IC  210  from inadvertent contact with a hot bar. Of further interest, cross-section A-A cuts through a portion of flexible circuit  100  and TFT ledge  202 .  FIG. 6B  shows a cross-sectional side view of flexible circuit  100  and display assembly  200  as defined by cross-section A-A from  FIG. 6A . Attaching surface  104  is shown having various thicknesses in this view. This is due in part to removal of coverlay from a portion of a lower surface of attaching surface  104 , thereby making flexible circuit  100  thinner and in some embodiments exposing electrical connector  602  of attaching surface  104 . Arranged underneath attaching surface  104  is ACF layer  604 . ACF layer  604  acts as a bonding agent between attaching surface  104  and bond pad  610 . Bond pad  610  is configured to route signals touch inputs from trace layer  612 , through crush portion  606  of ACF layer  604  into electrical connector  602 . ACF layer  604  is adhesively and conductively activated by a combination of pressure and heat. Hot pad  502  can be lowered to exert pressure and heat on a thin portion of attaching surface  104 . In this way hot pad  502  can conductively transfer heat and exert pressure on ACF layer  604 , thereby finalizing a bond between the two substrates. 
     It should be noted that coverlay can be removed from attaching surface  104  to make heat transfer between hot bar  502  and ACF layer  604  more efficient. Furthermore, this thinning of attaching surface  104  makes it more imperative that positioning of hot bar  502  in the X-axis is quite precise. If hot bar  502  exerts pressure upon a thicker portion of attaching surface  104  an excessive amount of force can be transferred through portion  610  of ACF layer  604  due to the increased thickness of attaching surface  104 , thereby resulting in possible puncture of trace layer  612 . Puncture of trace layer  612  can result in an undesirable higher likelihood of electrical shorts. 
       FIG. 7  shows a flowchart detailing an assembly process  700  for combining touch sensor inputs from disparate portions of a touch sensor. In a first step  702  a flexible circuit is loaded onto a precision stage. The flexible circuit includes two lateral connectors capable of merging two signals into one receiving channel. The precision stage can be adjusted to set an initial geometry of the flexible circuit. At step  704  at least two vacuum chucks are used to pick up a left and right side of the flexible circuit. The left and right vacuum chucks can be configured to maintain an initial spacing relationship established when they pick up each side of the flexible stage. In this way the flexible circuit can be transferred from the precision stage to just above a TFT glass substrate. The TFT glass substrate can include a series of bonding pads on a TFT ledge of a display assembly to adhesively attach to the flexible circuit in at least two positions on the TFT ledge. In this way touch sensor inputs can be combined into one signal conduit in order to allow combination of touch sensor inputs from two regions of the touch sensor. 
     Once vacuum chucks are positioned above the TFT ledge step  706  begins in which vacuum chucks are maneuvered to properly align electrical connectors on the flexible circuit with TFT connectors on disposed on the TFT ledge. This alignment can be accomplished manually or in an automated fashion. Alignment is facilitated by inclusion of at least two fiducials on each side of the flexible circuit and four corresponding fiducials disposed on the TFT ledge. CCDs can be arranged to direct movement of the vacuum chucks with respect to the TFT ledge. Once each of the fiducials is aligned the vacuum chucks can deposit the flexible circuit onto the TFT ledge at step  708 . The electrical connectors disposed on the TFT ledge can be covered with an anisotropic conductive film (ACF) layer. Consequently, when the vacuum chucks deposit the flexible circuit on the TFT ledge they can do so while exerting force between the flexible circuit and the ACF layer resulting in a pre-bond being formed that keeps the flexible circuit safely in place until a final bond can be accomplished. After the vacuum chucks disengage from the flexible circuit at step  710  hot bars can be introduced on a specific portion of the flexible circuit where a coverlay portion has been removed. Removal of the coverlay portion increases heat transfer efficiency between the hot bars and the ACF layer, thereby allowing for more quickly achieving a final bond. Once the ACF layer has been heated and compressed so that internal particles within the layer are squeezed closer together, yielding increased conductivity, the final bonding of the flexible circuit to the TFT ledge is complete. 
       FIG. 8  is a block diagram of an electronic device suitable for controlling some of the processes in the described embodiment. Electronic device  800  can illustrate circuitry of a representative computing device. Electronic device  800  can include a processor  802  that pertains to a microprocessor or controller for controlling the overall operation of electronic device  800 . Electronic device  800  can include instruction data pertaining to manufacturing instructions in a file system  804  and a cache  806 . File system  804  can be a storage disk or a plurality of disks. In some embodiments, file system  804  can be flash memory, semiconductor (solid state) memory or the like. The file system  804  can typically provide high capacity storage capability for the electronic device  800 . However, since the access time to the file system  804  can be relatively slow (especially if file system  804  includes a mechanical disk drive), the electronic device  800  can also include cache  806 . The cache  806  can include, for example, Random-Access Memory (RAM) provided by semiconductor memory. The relative access time to the cache  806  can substantially shorter than for the file system  804 . However, cache  806  may not have the large storage capacity of file system  804 . Further, file system  804 , when active, can consume more power than cache  806 . Power consumption often can be a concern when the electronic device  800  is a portable device that is powered by battery  824 . The electronic device  800  can also include a RAM  820  and a Read-Only Memory (ROM)  822 . The ROM  822  can store programs, utilities or processes to be executed in a non-volatile manner. The RAM  820  can provide volatile data storage, such as for cache  806   
     Electronic device  800  can also include user input device  808  that allows a user of the electronic device  800  to interact with the electronic device  800 . For example, user input device  808  can take a variety of forms, such as a button, keypad, dial, touch screen, audio input interface, visual/image capture input interface, input in the form of sensor data, etc. Still further, electronic device  800  can include a display  810  (screen display) that can be controlled by processor  802  to display information to the user. Data bus  816  can facilitate data transfer between at least file system  804 , cache  806 , processor  802 , and controller  1013 . Controller  1013  can be used to interface with and control different manufacturing equipment through equipment control bus  814 . For example, control bus  814  can be used to control a computer numerical control (CNC) mill, a press, an injection molding machine or other such equipment. For example, processor  802 , upon a certain manufacturing event occurring, can supply instructions to control manufacturing equipment through controller  1013  and control bus  814 . Such instructions can be stored in file system  804 , RAM  820 , ROM  822  or cache  806 . 
     Electronic device  800  can also include a network/bus interface  1011  that couples to data link  812 . Data link  812  can allow electronic device  800  to couple to a host computer or to accessory devices. The data link  812  can be provided over a wired connection or a wireless connection. In the case of a wireless connection, network/bus interface  1011  can include a wireless transceiver. Sensor  1026  can take the form of circuitry for detecting any number of stimuli. For example, sensor  1026  can include any number of sensors for monitoring a manufacturing operation such as for example a Hall Effect sensor responsive to external magnetic field, an audio sensor, a light sensor such as a photometer, computer vision sensor to detect clarity, a temperature sensor to monitor a molding process and so on. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. 
     The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.

Technology Category: h