Patent Publication Number: US-2010118091-A1

Title: Enhanced traces of flexible tab circuit for attachment on bond pads of inkjet printhead chip in printhead cartridge assembly

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to flexible tape automated bonding (TAB) circuits used in inkjet printers and, more particularly, to enhanced traces on a flexible TAB circuit for attachment on bond pads of an inkjet printhead chip in a printhead assembly. 
     2. Description of the Related Art 
     Typically, an inkjet printer cartridge assembly includes an ink-filled polymeric container, an inkjet printhead chip adhesively secured to the container, and a flexible TAB circuit attached to the container. The printhead chip is mounted to the container through and within a window on the TAB circuit. The TAB circuit has a substrate, for example made of polyimide, and multiple electrical contact pads at one end of the substrate for connecting to corresponding contacts in the inkjet printer. The TAB circuit also has multiple closely-spaced electrically-conductive traces, for example made of metal, formed on a bottom side of the substrate that connect the printhead chip to the contact pads. The trace pattern is created with a photolithography process. The metal traces are either electroplated on the polyimide substrate or a thin sheet of metal is attached to the polyimide substrate with adhesive. 
     As illustrated and described in U.S. Pat. No. 5,953,032, assigned to the assignee of the present invention and the disclosure of which patent is hereby incorporated herein by reference, the traces have main body sections and end sections extending from the main body sections and defining beam leads. The main body sections are formed on the substrate with the end sections extending from the main body sections past an edge of the substrate. The first side of the substrate faces the container such that the main body sections are positioned between the substrate and the container. A protective overcoat material is applied to the backside of the main body sections so as to provide corrosion protection by preventing ink from contacting those sections. The extending end sections of the traces are attached to the bond pads via the conventional tape automated bonding (TAB) bonding process. Preferably, the bonding process is performed before either the printhead or the flexible circuit is secured to the container. 
     Reliability of tape automated bonding of flexible circuits is a concern. The choice of metallurgy and geometry of the traces as well as the bond pads are important in forming a reliable bond. In addition, the tape automated bonding parameters of force, time, and thermosonic energy and scrubbing direction are equally important. In the case of TAB circuit assemblies for inkjet applications, the TAB circuit provides the electrical interconnection between the printer and the printhead chip. The trace end sections of the TAB circuit are in constant contact with ink and moisture during printing and maintenance wiping. In this especially corrosive environment, it is important to minimize corrosion and dendridic growth between the TAB circuit trace end sections to ensure reliability of the printhead. 
     In the case of inkjet TAB circuit assemblies of Lexmark International Inc. all established products are considered “East-West” bonding configuration. This means the TAB circuit trace end sections connect to the bond pads on the East and West sides of the printhead chip. All the TAB tools are configured so the thermosonic scrubbing energy is directed in the East-West direction as well. In this configuration bond pull values are very robust. Newer products have adopted the “North-South” bonding configuration where the TAB circuit trace end sections connect to bond pads on the North and South ends of the printhead chip, while the thermosonic scrubbing energy still must be directed in the East-West direction due to tooling constraints. TAB evaluations have shown that with all circuit and chip metallurgy and geometry remaining constant, rotating the thermosonic scrubbing direction from parallel to perpendicular to the trace end sections can cause reduction of 30% or more in bond pull strength such that bonding pull values then do not always meet the process capability guidelines established for tape automated bonding. 
     Thus, there is still a need for an innovation that will compensate for the situation where the orientation of the thermosonic scrubbing energy is perpendicular to the direction of the trace end sections that extend to the bond pad on the printhead chip and the orientation cannot be rotated easily due to tooling constraints. 
     SUMMARY OF THE INVENTION 
     The present invention meets some or all of the foregoing described needs by providing an innovation that creates a more robust interconnect for a situation where the thermosonic scrubbing energy is perpendicular to the direction of the trace end sections that extend to the bond pad on the chip. With such innovation the reliability of tape automated bonding of flexible circuits is improved and enhanced. The innovation proposes to create a wider end portion of the trace end section, that is, only the area where it is bonded to the chip. The provision of a larger and more stable bonding platform has provided the opportunity to reduce Au thickness back to earlier levels at significant cost savings from where Au thickness had been increased at significant increase in cost in a recent effort to increase the process capability for North-South bonding. Furthermore, the innovation of a wider end portion of the trace end section only in the bond pad area avoids adoption of a superficially attractive, but in actuality less practical, solution of creating wider overall leads or trace end sections in order to create more area and a more stable platform for scrubbing. The negative side effect of wider overall trace end sections is that they would now be closer together which would accelerate electrical shorting of traces due to dendrite growth during service of the device. Since inkjet TAB circuit assemblies are very susceptible to corrosion and dendrite growth, this is not a practical solution. Overall widening of the trace end sections would only accelerate a failure mode caused by dendrite growth between traces. 
     Accordingly, in an aspect of the present invention, an enhanced electrically-conductive trace on a flexible tape automated bonding circuit includes a main body section supported on a substrate and an end section integrally connected with the main body section and extending in a predetermined first direction from the main body section. The end section has an end portion spaced from the main body section and attachable on a bond pad of an inkjet printhead chip. The end portion of the end section has a width greater than the width of the remainder of the end section between the end portion and the main body section as measured in a second direction transverse to the predetermined first direction. 
     In another aspect of the present invention, a flexible tape automated bonding circuit includes an elongated substrate and a plurality of elongated electrically-conductive traces on the substrate spaced apart from one another and extending side-by-side in a predetermined first direction. Each of the traces includes a main body section supported on the substrate and an end section integrally connected with the main body section and extending in the predetermined first direction from the main body section. The end section has an end portion spaced from the main body section and attachable on one of a plurality of bond pads of an inkjet printhead chip. The end portion of the end section has a width greater than the width of the remainder of the end section between the end portion and the main body section as measured in a second direction transverse to the predetermined first direction. 
     In still another aspect of the present invention, an inkjet printhead cartridge assembly includes an inkjet printhead chip having a plurality of bond pads, and a flexible tape automated bonding circuit having an elongated substrate and a plurality of elongated electrically-conductive traces on the substrate spaced apart from one another and extending side-by-side in a predetermined first direction. Each of the traces includes a main body section supported on the substrate and an end section integrally connected with the main body section and extending in the predetermined first direction from the main body section. The end section has an end portion spaced from the main body section and attached on one of the bond pads of the inkjet printhead chip. The end portion of the end section has a width greater than the width of the remainder of the end section between the end portion and the main body section as measured in a second direction transverse to the predetermined first direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a simplified representation of a prior art inkjet printhead cartridge assembly having a flexible tape automated bonding circuit connected to an inkjet printhead chip of the assembly by a plurality of electrically-conductive traces on the flexible circuit. 
         FIG. 2  is a flow chart with accompanying schematic representations, not to scale, of a sequence of stages in a prior art process for creating the end sections of the prior art traces of  FIG. 1 . 
         FIG. 3  is a simplified representation of an inkjet printhead cartridge assembly having a flexible tape automated bonding circuit connected to an inkjet printhead chip of the assembly by a plurality of enhanced electrically-conductive traces on the flexible circuit in accordance with the present invention. 
         FIG. 4  is a flow chart with accompanying schematic representations, not to scale, of a sequence of stages for creating the end sections of the enhanced traces of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numerals refer to like elements throughout the views. 
     Referring now to  FIG. 1 , there is schematically represented, in a simplified form, a prior art flexible TAB circuit  10  and a printhead chip  12  of an inkjet printhead cartridge  14 . The flexible TAB circuit  10  generally includes a flexible substrate  16 , for example made of polyimide, and multiple closely-spaced, side-by-side, electrically-conductive traces  18  made, for example, of metal formed on the flexible substrate  16 . The metal traces  18  are either electroplated on the polyimide substrate  16  or a thin sheet of metal is attached to the polyimide substrate  16  with adhesive. 
     The multiple traces  18  each has a main body section  20  and an end section  22  which is an extension forming an electrical lead from the main body section  20 . The multiple trace end sections  22  extend from the edge  16   a  of the flexible substrate  16  to the printhead chip  12  to where they are electrically and mechanically connected to multiple bond pads  24  on the chip  12 . The electrical traces  18  at their other ends (not shown) are connected to corresponding multiple contact pads (not shown) which, in turn, are interconnected to multiple electrical terminals (not shown) on a movable carriage (not shown) within an inkjet printer. The trace pattern is created with a photolithography process, as described below. 
       FIG. 2  illustrates a block diagram accompanied by schematic representations of a sequence of stages in a prior art process for creating the end sections  22  of the prior art traces  18  of  FIG. 1 . As per block  26 , the flexible base substrate  16 , such as of polyimide, is prepared on which the remaining stages of the process are carried out Next, as per block  28 , the substrate  16  is coated with a layer  30  of a negative resist material. Then, as per block  32 , a mask  34  to pattern the end sections  22  of the traces  16  is applied on the negative resist layer  30  and thereafter the negative resist layer  30  is exposed through the mask  34  with high-intensity ultraviolet (UV) light. Next, as per block  36 , a developer is applied removing the mask  34  and the material of the negative resist layer  30  under the mask  34 . The negative resist material that was exposed to the UV light remains on the substrate  16 . Following next, as per block  38 , the end sections  22  of the traces  18  are electroplated on the substrate  16  within the gaps  40  left in the layer  30  of the negative resist material. Finally, as per block  42 , the negative resist material is stripped away, leaving the metal trace end sections  22 . 
     Turning now to  FIG. 3 , there is schematically represented, in a simplified form, a flexible TAB circuit  50  and a printhead chip  52  of an inkjet printhead cartridge  54  wherein the circuit  50  contains enhanced metal traces  56  in accordance with one embodiment of the present invention. Each metal trace  56  has a main body section  58  and an end section  60  integrally connected to and extending from the main body section  58  in a predetermined first direction as indicated by arrow  62 . The end section  60  of the trace  56  has been enhanced by the provision of a wider end portion  64  thereon in the area where it is bonded to the chip  52 . The end portion  64  is attached to one of the bond pads  24  on the inkjet printhead chip  12  and spaced from the main body section  58  by an interconnect section  66  of the end section  60 . The end portion  64  having a width greater than the width of the remainder of end section  60 , that is, greater than the width of the interconnect portion  66  spanning between the end portion  64  and the main body section  58 , as measured in a second direction, as indicated by arrow  68 , that is transverse to the predetermined first direction  62 . The interconnect portion  66  extends from the main body section  58  past the edge  16   a  of the substrate  16  and is not supported by the substrate  16 . Although shown as square-shaped in  FIG. 3 , the end portion  64  can also be other shapes, such as, for example, circular-shaped or diamond-shaped. 
       FIG. 4  illustrates a block diagram accompanied by schematic representations of a sequence of stages in a process for creating the enhanced end sections  60  of the traces  56  of  FIG. 3 . As per block  70 , the flexible base substrate  16 , such as of polyimide, is prepared on which the remaining stages of the process are carried out. Next, as per block  72 , the substrate  16  is coated with a layer  74  of a negative resist material. Then, as per block  76 , a mask  78  to pattern the enhanced end sections  60  of the traces  56  is applied on the negative resist layer  74  and thereafter the negative resist layer  74  is exposed through the mask  78  with high-intensity ultraviolet (UV) light. Next, as per block  80 , a developer is applied removing the mask  78  and the material of the negative resist layer  74  under the mask  78 . The negative resist material that was exposed to the UV light remains on the substrate  16 . Following next, as per block  82 , the end sections  60  of the traces  56  are electroplated on the substrate  16  within the gaps  84  left in the layer  74  of the negative resist material. Finally, as per block  86 , the negative resist material is stripped away, leaving the enhanced metal trace end sections  60  with the end portions  64  wider than the interconnect portions  66 . 
     In summary, the present invention creates a wider end portion  64  for the end section  60  of the trace  56  only in the area where it is bonded to the chip  12 , in other words, on the bond pad  24 . This is done easily with a photolithography mask change, as see in  FIG. 4  compared to  FIG. 2 , to create wider trace end portions  64  only in the area of the bond pads  24  while the remaining or interconnect portions  66  of the traces  56  would be left at their current widths, as seen in  FIG. 3 . Also,  FIG. 3  shows a square shape defined in the bond pad area; however, other shapes, such as circular or diamond, could also be used, as seen in dashed outline forms  88 ,  90 . This would create the necessary metal area to ensure better thermosonic scrubbing while not affecting the distance between the traces  56  where they are most susceptible to dendrite growth and corrosion. Creating increased width traces  56  in the TAB area has the potential to provide a more robust TAB process while saving money on thicker Au plating of traces. 
     The foregoing description of several embodiments of the invention has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.