Abstract:
An adhesive joint with an ink trap is provided. The joint may be employed in a cartridge for an inkjet printer. The cartridge includes a headland region attached to a printhead assembly by an adhesive layer. The adhesive joint between the headland region and the printhead assembly include notches for retaining additional adhesive in order to reduce degradation of adhesive due to ink penetration. A method of assembling the printer cartridge to include an ink trap in the adhesive joint is also provided.

Description:
BACKGROUND  
         [0001]    The present invention generally relates to adhesive joints and, more particularly, to adhesive joints configured to resist degradation in a chemically-hostile environment.  
           [0002]    Adhesive joints are widely used in industry to join components. In some applications, an additional requirement placed upon an adhesive joint is that it be resistant to degradation in a chemically-hostile environment. An example of a chemically-hostile environment is the ink storage and delivery system of an inkjet printer.  
           [0003]    Inkjet printers have gained wide acceptance. 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 referred to as “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.  
           [0004]    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 and position for the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.  
           [0005]    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 an 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.  
           [0006]    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.  
           [0007]    The printhead may include a flexible circuit tape having conductive traces formed thereon and have nozzles or orifices formed by Excimer laser ablation, for example. 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 an ink reservoir and the vaporization chambers.  
           [0008]    Typically, the integrated nozzle and flexible circuit or tape circuit is sealed to a print cartridge. 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.  
           [0009]    However, it has been determined that adhesive loses its adhesive qualities due to exposure to the ink. Over time ink concentration in the adhesive increases. Degradation in joint strength has been found to occur in direct proportion to the concentration of ink absorbed by the adhesive. Prior solutions to protecting adhesive joints from the effects of the ink include: providing protecting coatings that cover the joint; using adhesives that are more resistant to the effects of the ink; providing designs that lengthen the diffusion distance of the ink into the adhesive by modifying the joint design; and modifying the joint design to reduce stress. All of these solutions are expensive to implement and/or provide less than satisfactory results.  
           [0010]    Thus, there remains a need to increase the life of adhesive joints in ink jet cartridges, and other applications, that may be implemented simply and cost effectively without requiring additional materials or changes in the existing materials.  
         SUMMARY OF THE INVENTION  
         [0011]    In one embodiment of the present invention an adhesive joint is provided. The joint has improved resistance to degradation resulting from ink penetration and may include an adhesive layer located between two opposing surfaces. The adhesive layer includes an ink trap for diluting the concentration of ink penetrated into the adhesive. Preferably, the ink trap is formed by providing a notch in at least one of the two opposing surfaces.  
           [0012]    The adhesive joint may be employed, for example, in a cartridge for an inkjet printer. The cartridge may include a headland region attached to a printhead assembly by an adhesive layer. The adhesive joint between the headland region and the printhead assembly may include notches for retaining additional adhesive in order to reduce degradation of adhesive due to ink penetration. A method of assembling components, such as printer cartridges, to include an ink trap in the adhesive joint is also provided.  
           [0013]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    These and other features, aspects, and advantages of the present invention will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below.  
         [0015]    [0015]FIG. 1 is a perspective view of an inkjet print cartridge according to one embodiment of the present invention.  
         [0016]    [0016]FIG. 2 is a plan view of the front surface of a Tape Automated Bonding (TAB) printhead assembly removed from a print cartridge according to one embodiment of the present invention.  
         [0017]    [0017]FIG. 3 is a highly simplified perspective view of the back surface of the TAB head assembly of FIG. 2 with a silicon substrate mounted thereon and the conductive leads attached to the substrate according to one embodiment of the present invention.  
         [0018]    [0018]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 according to one embodiment of the present invention.  
         [0019]    [0019]FIG. 5 is a perspective view of the headland area of the inkjet print cartridge of FIG. 1 according to one embodiment of the present invention.  
         [0020]    [0020]FIG. 6 is a top plan view of the headland area of the inkjet print cartridge of FIG. 1 according to one embodiment of the present invention.  
         [0021]    [0021]FIG. 7 is a side elevational view in cross-section taken along line C-C in FIG. 6 illustrating the configuration of the adhesive support surface, inner wall, gutter and of the headland design according to one embodiment of the present invention.  
         [0022]    [0022]FIG. 8 is a top plan view of the headland area showing generally the location of the adhesive bead prior to placing the TAB head assembly on the headland area according to one embodiment of the present invention.  
         [0023]    [0023]FIG. 9 is a partial schematic cross-sectional schematic view taken along line B-B of FIG. 1 showing portion of the print cartridge in the proximity to the TAB head assembly according to one embodiment of the present invention.  
         [0024]    [0024]FIG. 10 is a cross-sectional, perspective view along line B-B of FIG. 1 with the TAB head assembly removed illustrating the internal structure of a inkjet print cartridge and the headland  50  area according to one embodiment of the present invention.  
         [0025]    [0025]FIG. 11 is a top plan view of the headland area of an alternative embodiment of the present invention showing generally the location of the adhesive bead prior to placing the TAB head assembly on the headland area.  
         [0026]    [0026]FIG. 12 is a side elevational view in cross-section taken along line C-C in FIG. 6 illustrating the configuration of the adhesive support surface, inner wall, gutter and of the headland design of an alternative embodiment of the present invention.  
         [0027]    [0027]FIG. 13 is a side elevational view in cross-section taken along line A-A in FIG. 8 illustrating the configuration of the adhesive support surface, inner wall, gutter and of the headland design of an alternative embodiment of the present invention.  
         [0028]    [0028]FIG. 14 is a side elevational view in cross-section taken along line A-A in FIG. 11 illustrating the configuration of the adhesive support surface, inner wall, gutter and of the headland design of an alternative embodiment of the present invention.  
         [0029]    [0029]FIG. 15( a ) is a cross-sectional view of an adhesive joint according to an embodiment of the present invention.  
         [0030]    [0030]FIG. 15( b ) is a cross-sectional view of an adhesive joint according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0031]    An embodiment of the present invention is disclosed in FIG. 15. Two components  1 ,  2  are connected together by an adhesive  9 . An adhesive joint is formed between two opposing surfaces  3 ,  4  of the components. When exposed to ink, the adhesive joint provides improved resistance to degradation of the adhesive due to ink penetration. As shown in FIG. 15, the joint preferably includes notch  5  in one of the opposing surfaces. The notch  5  allows a larger volume of adhesive to serve as an ink trap and dilute the concentration of ink in the adhesive. The ink may penetrate the joint from the side as shown by the arrow A in FIG. 15( a ). Thus, the distance between the surfaces is smallest adjacent the ink.  
         [0032]    While FIG. 15( a ) discloses a notch in one of the two opposing surfaces the present invention includes many alternatives such as the structure shown in FIG. ( 15   b ), for example. As shown in FIG. 15( b ) each opposing surface includes a notch to thereby form an ink trap  8 . The shape of notch as shown in the Figures includes squared corners however any surface indentation increasing the volume of adhesive between the joined surfaces is within the scope of the present invention.  
         [0033]    An inkjet printer cartridge  10  incorporating a printhead according to a particular exemplary embodiment of the present invention is shown in FIG. 1. The inkjet print cartridge  10  includes an internal ink reservoir and a printhead  14 , where the printhead  14  is formed using Tape Automated Bonding (TAB). The printhead  14  includes a nozzle member  16  comprising two parallel columns of offset holes or orifices  17  formed in a flexible polymer circuit  18  by, for example, laser ablation. The circuit  18  typically may be formed in Kapton tape.  
         [0034]    As shown in FIG. 2, the flexible circuit  18  provides for the routing of conductive traces  36  which are connected at one end to electrodes on a substrate (described below) and on the other end to contact pads  20  in the exemplary embodiment. 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.  
         [0035]    [0035]FIG. 2 shows a front view of a printhead assembly  14  removed from a print cartridge  10 . Printhead 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 . Windows  22  and  24  extend through the flexible circuit  18  and are used to facilitate bonding of the other ends of the conductive traces  36  to electrodes on the silicon substrate.  
         [0036]    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.  
         [0037]    [0037]FIG. 3 shows a simplified view of the back surface of a printhead assembly  14  according to the exemplary embodiment. The back surface of the flexible circuit  18  includes conductive traces  36  formed thereon using a conventional photolithographic etching and/or plating process, for example. The silicon die or substrate  28  is mounted to the back of the flexible circuit  18  with the nozzles or orifices  17  aligned with an ink vaporization chamber  32 . The conductive traces  36  are terminated by leads  37  that are bonded to electrodes  40  on the substrate  28  and by contact pads  20  designed to interconnect with a printer.  
         [0038]    One edge of a barrier layer  30  containing vaporization chambers  32  formed on the substrate  28  is shown in FIG. 3. 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 . The windows  22  and  24  allow access to the leads  37  of the conductive traces  36  and the substrate electrodes  40  to facilitate bonding of the leads to the electrodes.  
         [0039]    [0039]FIG. 4 illustrates the connection of the ends 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 leads  37  of the conductive traces  36  from the substrate  28 . Also shown is a side view of the flexible circuit  18 , the barrier layer  30 , the windows  22  and  24 , and the entrances of the ink vaporization chambers  32 . As shown in FIG. 4, during operation droplets of ink  100  are ejected from orifice holes (not shown in the figure) associated with each of the ink vaporization chambers  32 .  
         [0040]    As shown in FIGS.  5 - 7 , a headland area  50  of print cartridge  10  of the exemplary embodiment includes an inner raised wall  54 , an adhesive support surface  53  on the inner raised wall, openings  55  in the inner raised wall  54 , a surface  58 , a raised substrate support surface  58 A, a flat top surface  59  and a gutter  61 . A pair of walls  62  are provided to define the ink flow path  88  (shown in FIG. 9) to the back of the substrate  28 .  
         [0041]    [0041]FIG. 8 is top plan view showing generally the location of the dispensed adhesive  90  (shown in FIGS. 8 and 9) along the adhesive support surface  53  of inner raised wall  54 , on elevated substrate support surface  58 A and across surface  58  in the wall openings  55  of the inner raised wall  54 . As an alternative to the generally rectangular substrate support surface  58 A shown in FIG. 8, the cartridge  10  may include a tongue shaped support surface  58 B as shown in FIG. 11. As shown in FIG. 5, the present invention includes a headland portion  50  in which the substrate support surfaces are coplanar or elevated. An example of coplanar surfaces is shown in FIG. 11, where the tongue shaped portion  58 B is coplanar with the support surface  58 . It should be noted that the embodiment shown in FIG. 8 may be modified to in this same manner.  
         [0042]    As shown in FIG. 8, the adhesive  90  circumscribes the substrate  28  when the printhead assembly  14  is properly positioned and pressed down on the headland  50 . The adhesive  90  forms a structural attachment between the printhead assembly  14  and the inner raised wall  54  and the support surface  58  of the print cartridge  10 . The adhesive also provides a liquid seal between the above-described circumscribed location and the back of the printhead assembly  14  when printhead assembly  14  is affixed to the headland portion  50  of the cartridge.  
         [0043]    [0043]FIG. 9 shows the vaporization chambers  32 , thin film resistors  70 , and orifices  17  after the barrier layer  30  and substrate  28  have been secured to the back of the flexible circuit  18  and the flexible circuit  18  is secured to the inner raised wall  54  of the print cartridge  10  by adhesive  90 . In operation, ink flows from reservoir  12  around the edge of the substrate  28 , and into vaporization chamber  32 , as shown by the arrow  88 . A barrier layer  30 , the flexible circuit  18  and the substrate  28  define the ink vaporization chambers  32 . 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. As shown in FIG. 9, an adhesive seal  90  is provided for attaching the inner raised wall  54  to the flexible tape or circuit  18 .  
         [0044]    The plastic print cartridge  10  of the exemplary embodiment includes a body formed such that an ink conduit directs the flow of ink  88  from a reservoir  12  within the print cartridge  10  towards the back of the substrate and through a narrow gap that exists between the back surface of substrate  28  and the walls  62 . The flow of ink  88  is along the back surface of substrate  28 , around the edge of substrate  28  and into the vaporization chambers  32 . The filter carrier  63  and the walls  62  direct the flow of ink  88 .  
         [0045]    Referring to FIG. 10, the internal structure of the headland area  50  of the printer cartridge  10  is shown in FIG. 10. The cartridge  10  includes an ink reservoir region  12  for containing ink, a filter carrier  63  with its filter screen  65  removed, walls  62 , the ink flow path  88  defined by the filter carrier  63  and walls  62  leading to the back surface of the substrate  28 . Also shown is a portion of the headland area  50  including inner raised wall  54 , adhesive support surface  53  on the inner raised wall, flat top surface  59  and gutter  61 .  
         [0046]    As described above, traditional adhesive connections or joints between the headland portion  50  and the printhead assembly  14  are subject to reduced lifetimes due to ink penetration into the adhesive. The present invention addresses this problem by providing an ink trap  52 ,  56 ,  57  in the area of the joint. The provision of an ink trap reduces the rate of increase of ink concentration in the adhesive and, therefore, reduces the amount of degradation of adhesion between the components.  
         [0047]    According to the present invention the adhesive joint includes two regions, a thin adhesive region and a thick adhesive region or trap. The thin adhesive region is located immediately adjacent to the ink is a thin as possible in order to reduce the area of adhesive exposed to the ink and thereby limit the amount of ink that may diffuse into the adhesive. At the same time, the ink trap is provided to increase the amount of adhesive available to absorb diffused ink and thereby reduce the concentration of ink in the adhesive. The ink trap is a region of increased thickness in the adhesive layer between the headland portion  50  and printhead assembly  14 .  
         [0048]    As described above, the headland region  50  of the cartridge is connected to the printhead assembly  14  with at least two joints. First, the tape circuit  18  is connected to the support surface  53  on the inner raised wall. Second, the ends of the substrate  28  are connected to the support surfaces  58 A,  58 B. The substrate is also connected to the support surface  58 . The support surfaces  58 A,  58 B may be elevated relative to the adjacent support surface  58  as shown in FIGS. 8 and 12 or coplanar with the adjacent support surface  58  as shown in FIGS. 11 and 13.  
         [0049]    As shown in FIG. 12, the support surface  53  may include a notch or ink trap  52  for containing adhesive. The trap  52  serves to create the thick adhesive region discussed above, while the adjacent areas of the support surface  53 A serve to create the thin adhesive region. Similarly, as shown in FIG. 13, the tongue shaped support surface  58 B may include a notch or ink trap  57 . The areas of the support surface  58 B adjacent to the ink trap  57  are coated with a thin adhesive layer while the notch or trap  57  contains a thick adhesive layer. The generally rectangular shaped support surface  58 A, may also include a notch or ink trap  56  and adjacent areas  58 A for creating the thick adhesive region bounded by thin adhesive regions. The ink trap and support surfaces are formed so that the thickness of the adhesive near the ink is at a minimum.  
         [0050]    Modeling suggests that the provision of an ink trap may increase the life of the adhesive joint by a factor of approximately eight. The modeling was based on the following assumptions: that adhesive strength is a linear function of ink or ink components that have diffused into the adhesive; that the ink concentration in the adhesive is at saturation when failure occurs; that the increase in contact area between the adhesive and the headland portion due to the presence of the ink trap is of no significant benefit; that bulk diffusion and not interface diffusion of ink or ink components is the primary mechanism of ink penetration into the adhesive; and, that residual stresses play no role in the failure. If one or more of these assumptions are not correct, the ink trap may still be of benefit, however, the magnitude of the benefit may be different than the factor of eight mentioned above.  
         [0051]    The adhesive joint of the present invention may also be effectively applied in assembly of other ink storage and delivery components, and in other chemically-hostile environments. The present invention may be utilized in any application where exposure to chemicals causes degradation of adhesive joints.  
         [0052]    Given the disclosure of the present invention, one versed in the art would appreciate that there may be other embodiments and modifications within the scope and spirit of the invention. Accordingly, all modifications attainable by one versed in the art from the present disclosure within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is to be defined as set forth in the following claims.