Abstract:
Disclosed is an inkjet printhead including a substrate having a plurality of individual ink ejection elements formed on a first surface of said substrate, said ink ejection elements electrically connected to bond pads on said substrate, a barrier layer formed on said first surface of said substrate, said barrier layer defining a plurality of individual ink ejection chambers, said barrier layer further providing isolation of the bond pads on the substrate and a flexible circuit having electrical traces formed thereon, said electrical traces having leads attached to said bond pads; said flexible circuit overlaying and affixed to said barrier layer such that a plurality of nozzles formed in a nozzle member portion of said flexible circuit, such that said nozzles align with said ink ejection chambers and said ink ejection elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application relates to the subject matter disclosed in the following U.S. Patent Application and U.S. Patents: 
     U.S. patent application Ser. No. 09/302,837, filed Apr. 30, 1999, entitled “Inkjet Print Cartridge Design to Decrease Ink Shorts Due to Ink Penetration of the Printhead;” 
     U.S. patent application Ser. No. 09/303,246, filed Apr. 30, 1999, entitled “Inkjet Print Cartridge Design for Decreasing Ink Shorts By Using an Elevated Substrate Support Surface to Increase Adhesive Sealing of the Printhead from Ink Penetration;” 
     U.S. Pat. No. 5,442,384, entitled “Integrated Nozzle Member and TAB Circuit for Inkjet Printhead;” 
     U.S. Pat. No. 5,278,584 to Keefe, et al., entitled “Ink Delivery System for an Inkjet et Printhead;” 
     U.S. Pat. No. 5,291,226, entitled “Nozzle Member Including Ink Flow Channels.” 
     The above patents are assigned to the present assignee and are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to inkjet printers and, more particularly, to the printhead portion of an inkjet print cartridge. 
     BACKGROUND OF THE INVENTION 
     Inkjet printers have gained wide acceptance. These printers are described by W. J. Lloyd and H. T. Taub in “Ink Jet Devices,” Chapter 13 of  Output Hardcopy Devices  (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Inkjet printers produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper. 
     An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes “dot locations”, “dot positions”, or “pixels”. Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink. 
     Inkjet printers print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed. 
     The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed nozzles attached to a printhead substrate that incorporates an array of firing chambers which receive liquid ink from the ink reservoir. Each chamber has a thin-film resistor, known as a inkjet firing chamber resistor, located opposite the nozzle so ink can collect between it and the nozzle. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper. 
     The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium. 
     In U.S. Pat. No. 5,442,384, entitled “Integrated Nozzle Member and TAB Circuit for Inkjet Printhead,” a novel nozzle member for an inkjet print cartridge and method of forming the nozzle member are disclosed. A flexible circuit tape having conductive traces formed thereon has formed in it nozzles or orifices by Excimer laser ablation. The resulting flexible circuit having orifices and conductive traces may then have mounted on it a substrate containing heating elements associated with each of the orifices. The conductive traces formed on the back surface of the flexible circuit are then connected to the electrodes on the substrate and provide energization signals for the heating elements. A barrier layer, which may be a separate layer or formed in the nozzle member itself, includes vaporization chambers, surrounding each orifice, and ink flow channels which provide fluid communication between a ink reservoir and the vaporization chambers. 
     In U.S. Pat. No. 5,648,805, entitled “Adhesive Seal for an Inkjet Printhead,” a procedure for sealing an integrated nozzle and flexible or tape circuit to a print cartridge is disclosed, A nozzle member containing an array of orifices has a substrate, having heater elements formed thereon, affixed to a back surface of the flexible circuit. Each orifice in the flexible circuit is associated with a single heating element formed on the substrate. The back surface of the flexible circuit extends beyond the outer edges of the substrate. Ink is supplied from an ink reservoir to the orifices by a fluid channel within a barrier layer between the flexible circuit and the substrate. In either embodiment, the flexible circuit is adhesively sealed with respect to the print cartridge body by forming an ink seal, circumscribing the substrate, between the back surface of the flexible circuit and the body. This method and structure of providing a seal directly between a flexible circuit and an ink reservoir body has many advantages. Also, in U.S. Pat. No. 5,736,998, entitled “Inkjet Cartridge Design for Facilitating the Adhesive Sealing of a Printhead to an Ink Reservoir,” and U.S. Pat. No. 5,852,460, entitled “Inkjet Print Cartridge Design to Decrease Deformation of the Printhead When Adhesively Sealing The Printhead to the Print Cartridge;” improved headland designs are disclosed which alleviate some of the above-mentioned problems. 
     Flexible circuit leads are bonded to pads or electrodes on the outer edges of the substrate. To enable this bonding, a window is created in the flexible circuit to allow a bonder thermode to apply force and temperature to the flexible circuit leads that are resting on the bond pads. After the leads have been bonded, an encapsulant is dispensed across the window to protect the exposed bond pad region from intrusion of ink or contamination. 
     On most flexible circuits these leads are also protected on the back side by a laminated cover layer. In addition, the leads are further protected by the structural adhesive that is used to adhere the flexible circuit to the print cartridge body. However, there is a region at both ends of the substrate where the flexible circuit traces cannot be protected by the cover layer. In this region, the traces are only protected by the structural adhesive, and are therefore susceptible to corrosion and electrical shorting if ink penetrates the structural adhesive to flexible tape interface. This penetration of ink is increased due to the fact that the flexible tape to structural interface provides a wicking surface for the ink. This can lead to corrosion and electrical shorting behind the substrate. In addition, the encapsulant and the structural adhesive are cured at different stages in the manufacturing process and this creates a weak “cold joint” between the adhesive and encapsulant that can fail and permit ink intrusion. Finally, air pockets may be created on the underside of the flexible tape near the ends of the substrate when the structural adhesive does not squish uniformly against the flexible circuit during attachment of the flexible circuit to the print cartridge body. These air pockets can provide a path for ink to the flexible circuit traces or the bond pad region and thus lead to corrosion and electrical shorting of the leads or traces. 
     In inkjet printheads, adjacent electrodes or bond pads located on the printhead substrate can act as an anode and cathode and with ink serving as an electrolytic fluid. In this situation, bond pad electrochemical corrosion will occur due to the migration of metal ions under the driving force of the electric field established by a voltage bias. 
     In prior printheads this problem was minimal because they employed center-feed/side-connect designs. The older center-feed/side-connect designs were more robust because the bond pad regions were further away from the ink channel. Newer designs employ edge-feed/end connect designs to enable space on the die for active logic and an increased number of nozzles. This means the bond pads are closer to the ink channel. 
     Accordingly, there is a need for an improved method of isolating adjacent bond pads on the substrate to reduce ink corrosion due to ink penetration into the bond pad region of the substrate. 
     SUMMARY OF THE INVENTION 
     The present invention provides for the isolation of adjacent bond pads on the substrate to reduce ink corrosion due to ink penetration into the bond pad region of the substrate. The inkjet printhead includes a substrate having a plurality of individual ink ejection elements formed on a first surface of said substrate, said ink ejection elements electrically connected to bond pads on said substrate, a barrier layer formed on said first surface of said substrate, said barrier layer defining a plurality of individual ink ejection chambers, said barrier layer further providing isolation of the bond pads on the substrate and a flexible circuit having electrical traces formed thereon, said electrical traces having leads attached to said bond pads; said flexible circuit overlaying and affixed to said barrier layer such that a plurality of nozzles formed in a nozzle member portion of said flexible circuit, such that said nozzles align with said ink ejection chambers and said ink ejection elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an inkjet print cartridge. 
     FIG. 2 is a plan view of the front surface of a printhead assembly removed from a print cartridge. 
     FIG. 3 is a highly simplified perspective view of the back surface of the printhead assembly of FIG. 2 with a silicon substrate mounted thereon and the conductive leads attached to the substrate. 
     FIG. 4 is a side elevational view in cross-section taken along line A—A in FIG. 3 illustrating the attachment of conductive leads to electrodes on the silicon substrate. 
     FIG. 5 is a perspective view of the headland area of the inkjet print cartridge of FIG. 1 with the printhead assembly removed. 
     FIG. 6 is a schematic cross-sectional view taken along line B—B of FIG. 1 showing the adhesive seal between the printhead assembly and the print cartridge. 
     FIG. 7 is an elevational cross-sectional view of the bond pad region of the present invention. 
     FIG. 8 is a top plan view of the bond pad region showing the interlocking barrier pattern of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, reference numeral  10  generally indicates an inkjet print cartridge incorporating a printhead according to one embodiment of the present invention. The inkjet print cartridge  10  includes an internal ink reservoir (not shown) and a printhead formed using Tape Automated Bonding (TAB). The printhead or TAB head assembly  14  includes a nozzle member  16  comprising two parallel columns of offset holes or orifices  17  formed in a flexible polymer flexible circuit  18  by, for example, laser ablation. The flexible circuit  18  provides for the routing of conductive traces  36  which are connected at one end to electrodes on a substrate and on the other end to contact pads  20 . The print cartridge  10  is designed to be installed in a printer so that the contact pads  20  on the front surface of the flexible circuit  18 , contact printer electrodes providing externally generated energization signals to the printhead. 
     FIG. 2 shows a front view of a TAB head assembly  14  removed from a print cartridge  10 . TAB head assembly  14  has affixed to the back of the flexible circuit  18  a silicon substrate  28  containing a plurality of individually energizable thin film resistors. Each resistor is located generally behind a single orifice  17  and acts as an ohmic heater when selectively energized by one or more pulses applied sequentially or simultaneously to one or more of the contact pads  20 . 
     Flexible circuit leads are bonded to pads or electrodes  40  on the outer edges of the substrate  28 . To enable this bonding, a windows  22 ,  24  which extend through the flexible circuit  18  are created in the flexible circuit  18  to allow a bonder thermode to apply force and temperature to the flexible circuit leads  37  that are resting on the bond pads  40 . The windows  22 ,  24  in the TAB head assembly  14  are chemically milled in the flexible tape  18 . Earlier during intermediate assembly of the TAB head assembly  14  after the leads  37  have been bonded to the bond pads  40 , an encapsulant  34  is dispensed across the windows  22 ,  24  from the top to protect the exposed bond pad region from intrusion of ink or contamination. 
     The portion of the windows  22 ,  24  which are off the substrate extend back approximately to the location on the flexible circuit  18  where the laminated cover layer  38  of the flex circuit  18  terminates. Thus, the openings in windows  22 ,  24  must be large enough to be open near the end of the cover layer  38  so that the leads  37  without any cover layer  38  are fully encapsulated by adhesive  90  and encapsulant  34 . For additional details on intermediate assembly, see U.S. Pat. No. 5,442,384, entitled “Integrated Nozzle Member and TAB Circuit for Inkjet Printhead;” and U.S. Pat. No. 5,278,584 to Keefe, et al., entitled “Ink Delivery System for an Inkjet Printhead;” which are herein incorporated by reference. 
     The orifices  17  and conductive traces  36  may be of any size, number, and pattern, and the various figures are designed to simply and clearly show the features of the invention. The relative dimensions of the various features have been greatly adjusted for the sake of clarity. 
     FIG. 3 shows a highly simplified view of the back surface of a TAB head assembly  14 . The back surface of the flexible circuit  18  includes conductive traces  36  formed thereon using a conventional photolithographic etching and/or plating process. The silicon die or substrate  28  is mounted to the back of the flexible circuit  18  with the ink vaporization chambers  32  aligned with the nozzles or orifices  17 . The conductive traces  36  are terminated by leads  37  that are bonded to bond pads or electrodes  40  on the substrate  28  and on the other end by contact pads  20  as discussed above. Also shown is one edge of the barrier layer  30  containing vaporization chambers  32  formed on the substrate  28 . Shown along the edge of the barrier layer  30  are the entrances to the vaporization chambers  32  which receive ink from an internal ink reservoir within the print cartridge  10 . 
     FIG. 4 shows a side view cross-section taken along line A—A in FIG. 3 illustrating the connection of the leads  37  of the conductive traces  36  to the electrodes  40  formed on the substrate  28 . A portion  42  of the barrier layer  30  is used to insulate the conductive traces  36  from the substrate  28 . Also shown is the flexible circuit  18 , the barrier layer  30 , the windows  22  and  24  and the entrances to the ink vaporization chambers  32 . Also shown is the encapsulant  34  that is dispensed into the windows  22 ,  24  after bonding of the leads  37  to the bond pads  40  to insulate the leads  37  and conductive traces  36 . Droplets of ink  100  are shown being ejected from orifices  17  associated with each of the ink vaporization chambers  32 . 
     FIG. 5 shows the headland area  50  of print cartridge  10  of FIG. 1 in a perspective view and with the TAB head assembly  14  removed to reveal the headland design used in providing a seal between the TAB head assembly  14  and the body of the print cartridge  10 . Shown are an inner raised wall  54 , an adhesive support surface  53  on the inner raised wall, openings  55  in the inner raised wall  54 , a substrate support surface  58 , a flat top surface  59  and a gutter  61 . Also shown are adhesive ridges  57  and the area  56  on the substrate support surface  58  between the adhesive ridges  57 . Adhesive  90  is dispensed along the adhesive support surface  53  of inner raised wall  54  and across substrate support surface  58  in the wall openings  55  of the inner raised wall  54  and adjacent to and suspended off adhesive ridges  57 . 
     As the TAB head assembly  14  is pressed down onto the headland  50 , the adhesive  90  is squished down. The adhesive squishes through the wall openings  55  in the inner raised wall to encapsulate the traces leading to electrodes on the substrate. The adhesive  90  also squishes both inwardly and upwardly through the windows  22 ,  24  and flush with the bottom surface of the encapsulant and partially encapsulates the exposed leads  37 . 
     This seal formed by the adhesive  90  circumscribing the substrate  28  allows ink to flow around the sides of the substrate  28  to the vaporization chambers  32  formed in the barrier layer  30 , but prevents ink from seeping out from under the TAB head assembly  14 . Thus, this adhesive seal  90  provides a strong mechanical coupling of the TAB head assembly  14  to the print cartridge  10 , a fluidic seal and flexible circuit lead encapsulation. 
     FIG. 6 is a cross-sectional view taken along line B—B of FIG. 1 showing vaporization chambers  32 , thin film resistors  70 , and orifices  17  after the barrier layer  30  and substrate  28  are secured to the back of the flexible circuit  18  at location  84  and the flexible circuit is secured to the body of the print cartridge  10  by adhesive  90 . A side edge of the substrate  28  is shown as  86 . In operation, ink flows from reservoir  12  around the side edge  86  of the substrate  28 , and into vaporization chamber  32 , as shown by the arrow  88 . Upon energization of the thin film resistor  70 , a thin layer of the adjacent ink is superheated, causing a droplet of ink  100  to be ejected through the orifice  17 . The vaporization chamber  32  is then refilled with ink by capillary action. Also shown is a portion of the adhesive seal  90 , applied to the inner raised wall  54  surrounding the substrate  28 . 
     The adhesive  90  and the encapsulant  34  are effective at preventing shorting between the leads, but are not effective at isolating the gold/tantalum bond pads because the adhesive  90  and encapsulant  34  do not adhere well to gold. The situation is worsened by the close proximity of adjacent bond pads on the substrate  28 . A typical separation distance between bond pads on a printhead is 20-40 μm. Ink shorts and bond pad corrosion cause malfunctioning of the printhead and premature failure of the print cartridge. 
     Prior printhead designs have not adequately addressed the problem of corrosion occurring near the bond pads  40  on the substrate and leads  37  of the flexible circuit  18  of TAB head assembly  14  due to ink penetration. The adjacent electrodes or bond pads  40  located on the printhead substrate  28  act as an anode and cathode with the ink serving as an electrolytic fluid. In this situation, bond pad electrochemical corrosion will occur due to the migration of metal ions under the driving force of the electric field established by a voltage bias. This causes the formation of dendrites between adjacent bond pads and malfunctioning and failure of the printhead. 
     FIG. 7 is a cross-sectional view of the bond pad  40  region of the present invention. Referring to FIG. 7, the present invention solves the bond pad electrochemical corrosion problem by isolating the individual bond pads  40  with barrier  30  material. Barrier  30  material is placed between adjacent bond pads  40  (shown in FIG.  8 ). The barrier  30  material also extends along the length of the substrate  28  up to bond pads  40  from the center of the substrate  28  and also extends from the bond pads  40  to the end of the substrate  28 . Accordingly, the bond pads  40  are completely circumscribed by barrier  30  material. This barrier  30  material also lies under the under the traces  36  to the end of the substrate  28 . The barrier material  30  isolates the bond pads  40  and the leads  37  and prevents corrosion and electrical shorting by eliminating the electrolytic path between adjacent leads  37  and bond pads  40 . Also shown is the encapsulant  34  placed in the windows  22 ,  24  after bonding of the leads  37  to the bond pads  40 . The adhesive  90  discussed above will help insulate the conductive traces  36  between the end of the substrate and the end of the cover layer  38  when the printhead is mounted to the headland  50  of the print cartridge body  12 . The encapsulant  34  and the adhesive  90  may be the same or different materials. 
     FIG. 8 is a top plan view of the bond pad region showing the interlocking barrier pattern of the present invention. Barrier  30  material is placed between adjacent bond pads  40 . The barrier  30  material also extends along the length of the substrate  28  up to bond pads  40  from the center of the substrate  28  and also extends from the bond pads  40  to the end of the substrate  28 . The is a narrow gap between the barrier material  30  and the bond pads  40  due to manufacturing tolerances. Accordingly, the bond pads  40  are completely circumscribed by barrier  30  material. Also shown are conductive traces  36  of the flexible circuit  18  and leads  37  bonded to the bond pads  40 . While the bond pads are shown in FIG. 8 as being the same size, the bond pads can be of different size depending on the current load to the bond pad  40 . As shown in FIG. 8, one of the bond pads  40  is shown with a lead  37  attached and the other with no lead attached. 
     In a typical inkjet printhead assembly  14 , during the process of bonding the leads  37  to the bond pads  40 , the thermode applies a force sufficient to compress typical leads  37  approximately 10 to 50 percent. If the nominal barrier thickness is such that the barrier height is the same as the height of the compressed leads  37  during bonding, it is evident that the bonder thermode would come in contact with barrier layer  30  placed between the pads  40 . If this occurs the thermode would melt the barrier  30  material and become contaminated with the barrier material. Therefore, if barrier material is placed between the bond pads  40  either the traces  36  and leads  37  need to be thick enough so that compression of the leads  37  does not cause contact with the barrier material. Making the copper traces  36  and leads  37  thicker prevents the bonder thermode from contacting the barrier layer  30  between the bond pads  40  during intermediate assembly of the TAB head assembly  14 . Moreover, making the traces  36  thicker enables a reduction in the width of traces  36  and leads  37  and this allows the traces to be moved closer together and thus farther away from the ink channels  32  without increasing the overall width of the printhead assembly  14 . Alternatively, the thermode bonder needs to be redesigned, for example, in a notched like manner so that the thermode only contacts the leads  37  and not the barrier material  30 . Redesigning the thermode bonder like this would require better alignment between the thermode and the leads  37 . 
     The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. As an example, the above-described inventions can be used in conjunction with inkjet printers that are not of the thermal type, as well as inkjet printers that are of the thermal type. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.