Abstract:
An inkjet printer printhead has a layering of a flexible polymer tape, a patterned barrier material that is acting as an adhesive as well as ink channels, and a substrate that has a plurality of ink expulsion devices. Each of the ink propulsion devices is aligned with an orifice hole ablated in the flexible polymer tape where the ink expels and patterns the medium beyond. To keep adhesives and encapsulants required in the assembly of the inkjet printer printhead out of the critical ink channel area near the orifice holes, fluid accumulation channels are ablated into the flexible polymer tape in a strategic location between the adhesive bead and the ink channel. These accumulation channels function as both a diversion a containment point for the excess flow of adhesive.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to the control of the flow of an adhesive along a substrate and more particularly to the control of the flow of viscosity varying adhesives required in the manufacture of an inkjet printer printhead.  
           [0002]    Inkjet printers operate by expelling a small volume of ink through a plurality of small nozzles or orifices in a flexible polymer tape held in proximity to a medium upon which marks or printing is to be placed. This orificed flexible polymer tape is referred to as oriflex. The orifices are arranged in the oriflex such that the expulsion of a droplet of ink from a determined number of orifices relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the substrate or the medium and another expulsion of ink droplets continues the production of more pixels of the desired character or image. Inks of selected colors may be coupled to individual arrangements of orifices so that selected firing of the orifices can produce a multicolored image by the inkjet printer.  
           [0003]    Each orifice in the oriflex is coupled to an associated small unique ink firing chamber filled with ink and having an individually addressable ink propulsion device, mounted on a substrate, and coupled to the ink. The ink is forced out of the orifice by the ink propulsion device, and deposited on the medium. The displaced volume of ink is replenished from a larger ink reservoir by way of ink feed channels that are patterned into a layer, commonly called barrier, that is interposed between the oriflex and the substrate.  
           [0004]    The back surface, that which is opposite the surface facing the media, of the oriflex includes electrically conductive traces which are terminated at the one end by large contact pads designed to interconnect with a printer. The print cartridge is designed to be installed in a printer so that the contact pads on the front surface of the oriflex contact printer electrodes which provide externally generated energization signals to the printhead. To access these traces from the front surface of the oriflex, holes, or vias, must be formed through the front surface of the oriflex to expose the ends of the traces. The exposed ends of the traces are then plated with, for example, gold to form the contact pads on the front surface, that which is facing the media, of the oriflex.  
           [0005]    Apertures are excised through the oriflex and are used to facilitate bonding of the second ends of the conductive traces to electrodes on a substrate containing ink propulsion devices. The apertures, after bonding is complete, are filled with a bead of encapsulating adhesive to protect any exposed portion of the traces and substrate. This encapsulating adhesive is referred to as encapsulant. The encapsulant is a liquid system until cross-linking takes place, creating a solid matrix when fully cured. Initially, the encapsulant decreases in its viscosity as it is being cured, further causing it to flow before curing is complete. With no control of the flow of this encapsulant, it is possible for the encapsulant to flow along the substrate, into the ink channel, and ultimately into the ink dispersion orifices. During the low viscosity state of the encapsulant, it is possible for the adhesive to be drawn between the layer of oriflex and the substrate, and into the ink channels formed by the barrier by a capillary force created at the exterior edge of these layers. This phenomenon is commonly referred to as wicking. Wicking takes place at the oriflex to barrier interface with the encapsulant wicking along the oriflex.  
           [0006]    Every orifice in an inkjet printer printhead has a function. It is critical that every orifice is free from obstructions in order to eject a droplet of ink. A single orifice which does not fire an ink droplet when it is commanded to do so will leave a portion out of a printed character and will leave an unprinted band on the medium when a solid image is expected. This results in a poorer quality of printed matter, highly undesirable for an inkjet printer.  
           [0007]    Other attempts have been made to divert this flow of encapsulant away from the ink dispersion orifices. Experiments have been performed involving heat cycling to control the encapsulant curing process, and adding holes in the oriflex prior to the ink channels to allow the encapsulant to escape prior to reaching the ink dispersion orifices. These experiments have met with minimal success and have, therefore, been unable to consistently control this wicking problem. Prior to the present invention, the wicking was noted as one of the largest contributors to inkjet pen failure. With the invention as described hereinafter, wicking is no longer an issue.  
         SUMMARY OF THE INVENTION  
         [0008]    An inkjet printer printhead utilizes a barrier layer with an ink channel, a first substrate disposed on a first side of the barrier layer, and a second substrate disposed on a second side of the barrier layer opposite the first substrate. A bonding aperture extends through the second substrate and the barrier layer to expose the first substrate without entering the ink channel. An adhesive is disposed on a first side of the second substrate opposite the barrier layer, and engulfing the bonding aperture. At least one fluid accumulation channel is excavated into a second side of the second substrate between the bonding aperture and the ink channel with a portion of the accumulation channel extending over the first substrate thereby reducing the flow of the adhesive from the bonding aperture into the ink channel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The present invention can be further understood by reference to the following description and attached drawings which illustrate the preferred embodiment.  
         [0010]    [0010]FIG. 1 is a perspective view of an inkjet printer print cartridge according to one embodiment of the present invention.  
         [0011]    [0011]FIG. 2 is a plan view of the top surface of the Tape Automated Bonded (TAB) printhead assembly (hereinafter “TAB head assembly”) removed from the print cartridge of FIG. 1.  
         [0012]    [0012]FIG. 3 is view A from FIG. 2, expanded for clarity and a better perspective of the points of cross-sectioning for FIG. 4 and FIG. 5.  
         [0013]    [0013]FIG. 4 is a side elevation view in a cross-section taken along line B-B in FIG. 3 illustrating the relationship of the fluid accumulation channels with respect to the layered components of a substrate on a TAB head assembly.  
         [0014]    [0014]FIG. 5 is a side elevation view in a cross-section taken along line C-C in FIG. 3 illustrating the outermost edge of the substrate and the wicking path of an encapsulant bead originating in a bonding aperture, flowing into the first fluid accumulation channel and wicking toward the ink channel.  
         [0015]    [0015]FIG. 6 is a rear view of FIG. 3, illustrating the wicking path of the encapsulant bead with respect to the TAB bond aperture and the fluid accumulation channels along the bottom and exterior edges of the substrate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    Referring to FIG. 1, reference number  101  generally indicates an inkjet printer print cartridge incorporating a printhead according to one embodiment of the present invention. The inkjet printer print cartridge  101  includes an ink reservoir  102  and a printhead  109 , where the printhead  109  is formed using Tape Automated Bonding (TAB). One conventional technique is described in U.S. Pat. No. 4,917,286 (Pollacek). The printhead  109  (hereinafter “TAB head assembly  109 ”) includes a nozzle member  108  comprising two parallel columns of offset holes or orifices  107  formed in a flexible polymer tape  104  (hereinafter “oriflex  104 ”) by, for example, laser ablation. The oriflex  104  may be purchased commercially as KAPTON tape, available from 3M Corporation. Other suitable tapes may be formed of UPILEX or its equivalent.  
         [0017]    A back surface of the oriflex  104  includes conductive traces  207  (shown from the top surface in FIG. 2) formed thereon, for example, using a conventional photolithographic etching and/or plating process. These conductive traces are terminated by large contact pads  103  designed to interconnect with a printer. The print cartridge  101  is designed to be installed in a printer so that the contact pads  103 , on the front surface of the oriflex  104 , contact printer electrodes providing externally generated energization signals to the TAB head assembly  109 .  
         [0018]    In the various embodiments shown, the traces are formed on the back surface of the oriflex  104 , opposite the surface which faces the recording medium. To access these traces from the front surface of the oriflex  104 , holes, or vias are formed through the front surface of the oriflex  104  to expose the exterior trace ends  211  (FIG. 2). The exposed trace ends are then plated with, for example, gold to form the contact pads  103  shown on the front surface of the oriflex  104 .  
         [0019]    [0019]FIG. 2 shows a front view of the TAB head assembly  109  of FIG. 1 removed from the inkjet printer print cartridge  101 . Bonding apertures  105  and  106  extend through the oriflex  104  and are used to facilitate bonding of the interior trace ends  205  and  206  of the conductive traces  207  to electrodes  410  (FIG. 4) on the substrate  210 . The bonding apertures  105  and  106  are filled with a bead of encapsulating adhesive  201  and  202  (hereinafter “encapsulant bead  201  and  202 ”) to protect any underlying portion of the conductive traces  207  and substrate  210  that otherwise may be exposed through the bonding apertures  105  and  106 . It is a feature of the present invention that fluid accumulation channels  208  and  209  are excavated into the oriflex  104 .  
         [0020]    Affixed to the back of the TAB head assembly  109  is a substrate  210  containing a plurality of individually energizable ink propulsion devices. Each ink propulsion device is located generally behind a single orifice  107  and expels a droplet of ink  407  (FIG. 4) when selectively energized by one or more pulses applied to one or more of the contact pads  103 . The ink is supplied from the ink reservoir  102  (FIG. 1) via the ink channel  203  which is defined in the barrier layer  204 . In the preferred embodiment, the individually energizable ink propulsion devices are thin film resistors that are contained on a silicon substrate  210 . Each resistor acts as an ohmic heater when selectively energized, boils the ink, thereby ejecting the ink through the orifices  107  and onto the medium beyond. The orifices  107  and conductive traces  207  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.  
         [0021]    The cross-sectional view at line B-B of FIG. 3 is shown in FIG. 4. This illustrates the substrate  210  mounted to the back of the oriflex  104  and also shows one edge of the patterned barrier layer  204  formed on the substrate  210  containing ink channels  203 . The patterned barrier  204  is the center layer between the substrate  210  and the oriflex  104 . Shown along the edge  417  of the barrier layer  204  are the entrances of the ink channels  203  which receive ink from the ink reservoir  102  (FIG. 1). The conductive traces  207  formed on the back of the oriflex  104  terminate at the interior trace ends  206  and are bonded to the electrodes  410  located on the substrate  210  on the opposite side of the oriflex  104  from the conductive traces  207 . The bonding aperture  105  (FIG. 3) allows access to the ends of the conductive traces  207  and the substrate electrodes  410  (FIG. 4) from the other side of the oriflex  104  to facilitate bonding.  
         [0022]    In FIG. 4, showing a preferred embodiment of the present invention, fluid accumulation channels  208  are excavated or laser ablated into the oriflex  104  by a series of 2 microns wide by 140 microns long micro-channels separated by 2 microns wide non-ablated spaces to a width of approximately 55 microns. An alternate embodiment is contemplated where the fluid accumulation channel  208  would be excavated or laser ablated in the oriflex  104  by a series of 2 microns wide by 55 microns long micro-channels separated by 2 microns wide non-ablated spaces to a width of approximately 140 microns. The fluid accumulation channel  208  patterns are included in the laser mask used for the ablation of the orifices  107 . The spaces in the mask are necessary to attenuate the laser beam for control of the channel depth and to avoid ablating through the oriflex  104 . The ablation in the preferred embodiment is done with an Eximer laser at a wavelength of  248  nanometers, an energy of 350-400 mJ/cm2, and takes approximately 2 seconds to complete. As a result of this method of ablating, the fluid accumulation channels  208  have sloped sides  415 . The finished dimensions of the fluid accumulation channels  208  are approximately 25 microns wide and 110 microns long at the channel base  411 , and approximately 55 microns wide by 140 microns long at the surface of the oriflex  413 . The fluid accumulation channels  208  are spaced between the TAB bond aperture  105  (FIG. 3) and the ink channel  203  (FIG. 3). The preferred embodiment utilizes the fluid accumulation channels  208  in the same orientation as the TAB bond aperture  105  but other orientations such as angular shapes, curved shapes, etc. will perform the same function. Approximately half of the length of the fluid accumulation channels  208  extend beyond the edge of the substrate  210  (FIG. 3). In the present invention, there are three fluid accumulation channels  208  constructed adjacent to each comer of the substrate  210 , located at the end of each of the two parallel columns of orifices  107  and  108  (FIG. 1). The number of fluid accumulation channels  208  can be from one to eight and is necessitated by the properties of the adhesive. The fluid accumulation channels  208  can be constructed in a number of sizes, shapes and quantities. The critical criteria for the fluid accumulation channels  208  is that they cross the substrate to barrier mating edge  301 .  
         [0023]    Also shown in FIG. 4 is a side view of the oriflex  104 , the barrier layer  204 , fluid accumulation channels  208 , the top side bead of encapsulant  201 , the under-flow bead of encapsulant  409  which is formed when the encapsulant bead  201  flows through the bonding aperture  105  (FIG. 1) and between the conductive traces  207  prior to cure, and advances toward the ink channels  203 . A droplet of ink  407  is shown being ejected from orifice  107  associated with each of the ink channels  203 .  
         [0024]    The parallel lines created in the channel base  411  of the fluid accumulation channels  208  create a capillary effect, further drawing the encapsulant from beads  201  and under-flow encapsulant bead  409  into the fluid accumulation channels  208 , and keeping the encapsulant away from the orifices  107 .  
         [0025]    The cross-sectional view at line C-C of FIG. 3 is shown in FIG. 5. FIG. 5 cuts through the oriflex  104 . This view shows the side edge of the substrate  210 , the substrate to barrier mating edge  301 , and the entrance to the ink channel  203 . FIG. 5 is an illustration of the under-flow encapsulant bead  409  flowing around the comer of substrate  210  at encapsulant edge  501 , filling the first fluid accumulation channel  208  and continuing the wicking path toward the next accumulation channel.  
         [0026]    The preferred encapsulating adhesive, GRACE, is a liquid system until cross-linking takes place, creating a solid matrix when fully cured. At the onset of the curing process, the encapsulant decreases in viscosity, further causing it to flow before the curing is complete. The amount of flow varies between encapsulants. The more potential flow, the greater number of fluid accumulation channels  208  required to collect the excess uncured encapsulant  501  (FIG. 5) prior to the ink channel  203 . The fluid accumulation channel  208  and  209  (FIG. 2) in the preferred embodiment are ablated in all four comers of the oriflex  104  as illustrated in FIG. 2.  
         [0027]    [0027]FIG. 6 is a bottom view of the TAB head assembly of FIG. 3. It illustrates the flow path  501  of the under-flow encapsulant bead  409 . The under-flow encapsulant bead  409  wicks along the substrate edge  601  toward the center of the substrate  204 . Without the fluid accumulation channels  208 , the under-flow encapsulant bead  409  often flows into the ink channel  203  (FIG. 5), and blocks an orifice  107  (FIG. 4). Each orifice  107  is essential for superior print quality in the inkjet printer print cartridge system.  
         [0028]    In the preferred embodiment shown in FIG. 3, the addition of the fluid accumulation channels  208  to divert the encapsulant  201  that wicks along the substrate to barrier mating edge  301  coupled with the extension of the barrier layer  204  at the substrate to barrier mating edge  301  to reduce the capillary effect previously described, the blocking of orifices  107  due to wicking encapsulant  201  has been effectively eliminated.