Abstract:
Disclosed is a process for preparing an ink jet printhead which comprises: (a) providing a diaphragm plate having a plurality of piezoelectric transducers bonded thereto; (b) aligning an electrical circuit board having a fill joint with the piezoelectric transducers and temporarily attaching the electrical circuit board to the piezoelectric transducers, thereby creating a layered structure having interstitial spaces between the diaphragm plate, the piezoelectric transducers, and the electrical circuit board; (c) applying a thermoset polymer through the fill joint and allowing it to fill the interstitial spaces via capillary action; and (d) curing the thermoset polymer to form an interstitial polymer layer.

Description:
BACKGROUND 
     Disclosed herein are piezoelectric ink jet printheads and methods for making them. 
     Ink jet systems include one or more printheads having a plurality of jets from which drops of fluid are ejected towards a recording medium. The jets of a printhead receive ink from an ink supply chamber or manifold in the printhead which, in turn, receives ink from a source, such as an ink reservoir or an ink cartridge. Each jet includes a channel having one end in fluid communication with the ink supply manifold. The other end of the ink channel has an orifice or nozzle for ejecting drops of ink. The nozzles of the jets can be formed in an aperture or nozzle plate having openings corresponding to the nozzles of the jets. During operation, drop ejecting signals activate actuators in the jets to expel drops of fluid from the jet nozzles onto the recording medium. By selectively activating the actuators of the jets to eject drops as the recording medium and/or printhead assembly are moved relative to one another, the deposited drops can be precisely patterned to form particular text and graphic images on the recording medium. An example of a full width array printhead is described in U.S. Pat. No. 7,591,535, the disclosure of which is totally incorporated herein by reference. Additional examples of ink jet printheads are disclosed in U.S. Pat. Nos. 7,934,815, 7,862,678, and 7,862,160, and in U.S. Patent Publications 2011/0175971, 2011/0141203, 2011/0141204, 2011/0141205, and 2010/0294545, the disclosures of each of which are totally incorporated herein by reference. 
     Piezoelectric ink jet printheads typically include a flexible diaphragm and a piezoelectric transducer attached to the diaphragm. When a voltage is applied to the piezoelectric transducer, typically through electrical connection with an electrode electrically coupled to a voltage source, the piezoelectric transducer vibrates, causing the diaphragm to flex which expels a quantity of ink from a chamber through a nozzle. The flexing further draws ink into the chamber from a main ink reservoir through an opening to replace the expelled ink. 
     One goal of printhead design is to provide increasing numbers of ink jet ejectors in a printhead. The more ink jet ejectors in a printhead, the greater the density of the dot matrix and the higher the perceived quality of the image. One approach to increasing ink jet ejector density in a printhead is to locate the manifold external to the ink jet ejector. One way of implementing this approach includes providing an inlet in the diaphragm layer for each ejector. Coupling the inlet to the manifold to receive ink for ejection from the ejector, however, requires an opening in the piezoelectric transducer layer to enable ink flow from the manifold to the inlet and then into the pressure chamber in the ink jet body plate. Each opening in the piezoelectric transducer layer is located in a polymer portion in the interstices between the piezoelectric transducers. 
     To facilitate manufacture of an ink jet array printhead, an array of ink jet ejectors can be formed from multiple laminated plates or sheets. These sheets are configured with a plurality of pressure chambers, outlets, and apertures and then stacked in a superimposed relationship. These sheets or plates include a diaphragm plate, an ink jet body plate, an inlet plate, an outlet plate, and an aperture plate. The piezoelectric-transducer is bonded to the diaphragm, which is a region of the diaphragm plate that overlies the ink pressure chamber. 
     Conventional approaches to assembling a high density ink jet printhead stack array include the use of a thermoset polymer to be used as an interstitial fill between the piezoelectric transducers. The polymer is planarized flat with the piezoelectric transducer array (within 5 microns) and excess polymer on top of the piezoelectric transducer array is etched away to expose clean piezoelectric transducer material for electrical connection. Further, an additional film adhesive layer, the standoff, is used to bond the top electrical connect circuitry to the array. Upon laser drilling, the thermoset polymer becomes a channel for ink flow. Potential quality issues with this method include the possibility of polymer on the top of the piezoelectric transducers, which could cause electrical connectivity issues, and potential bond degradation at the electrical connection from lack of potting material. 
     Accordingly, a need remains for improved methods for forming high density ink jet printhead stack arrays. In addition, a need remains for methods for making ink jet printheads with fewer layers. Further, a need remains for methods for making ink jet printheads with improved electrical connections. Additionally, a need remains for methods for making ink jet printheads in which there is no need to planarize the polymer with the top of the piezoelectric transducer layer and no need for a post-planarization etching process, thereby eliminating extra processing equipment and steps. 
     SUMMARY 
     Disclosed herein is a process for preparing an ink jet printhead which comprises: (a) providing a diaphragm plate having a plurality of piezoelectric transducers bonded thereto; (b) aligning an electrical circuit board having a fill joint with the piezoelectric transducers and temporarily attaching the electrical circuit board to the piezoelectric transducers, thereby creating a layered structure having interstitial spaces between the diaphragm plate, the piezoelectric transducers, and the electrical circuit board; (c) applying a thermoset polymer through the fill joint and allowing it to fill the interstitial spaces via capillary action; and (d) curing the thermoset polymer to form an interstitial polymer layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross sectional side view of an embodiment of an ink jet printer. 
         FIG. 2  is a schematic view of the embodiment of the ink jet printer of  FIG. 1 . 
         FIG. 3  is a profile view of a partially completed ink jet printhead, including a diaphragm layer, body layer, and a polymer layer. 
         FIG. 4  is a profile view of the same partial ink jet printhead of  FIG. 3  additionally including piezoelectric transducers bonded to the diaphragm layer. 
         FIG. 5  is a profile view of the same partial ink jet printhead of  FIG. 4  further including thermoset polymer filling an interstitial area between the piezoelectric transducers. 
         FIG. 6  is a schematic view in profile of an assembly process wherein the underfill layer is applied. 
         FIG. 7  is a graph depicting the effect of temperature on fill speed for the assembly process of  FIG. 6 . 
         FIG. 8  is a profile view of the completed assembly of  FIG. 5  after the assembly is bonded to an electrical circuit board and ink channels have been ablated. 
         FIG. 9  is a profile view of a complete ink jet head including an outlet plate attached to the body layer and an ink manifold attached to a rigid or flexible electrical circuit layer. 
     
    
    
     Drawings are not to scale. 
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the word “printer” encompasses any apparatus that performs a print outputting function for any purpose, such as a digital copier, bookmaking machine, facsimile machine, multi-function machine, or the like. Devices of this type can also be used in bioassays, masking for lithography, printing electronic components such as printed organic electronics, and making 3D models among other applications. The word “polymer” encompasses any one of a broad range of carbon-based compounds formed from long-chain molecules, including thermoset polyimides, thermoplastics, resins, polycarbonates, epoxies, or related compounds known to the art, as well as mixtures thereof. The word “ink” can refer to wax-based inks or gel-based inks known in the art and can also refer to any fluid that can be driven from the jets, including water-based solutions, solvents and solvent-based solutions, or UV-curable polymers, as well as mixtures thereof. The word “metal” encompasses single metallic elements, including those such as copper, aluminum, titanium, or the like, or metallic alloys, including those such as stainless steel alloys, aluminum-manganese alloys, or the like, as well as mixtures thereof. A “transducer” as used herein is a component that reacts to an electrical signal by generating a moving force that acts on an adjacent surface or substance. The moving force may push against or retract the adjacent surface or substance. 
       FIGS. 1 and 2  illustrate one example of a single ink jet ejector  10  suitable for use in an ink jet array of a printhead. The ink jet ejector  10  has a body  48  coupled to an ink manifold  264  through which ink is delivered to multiple ink jet bodies. The body also includes an ink drop-forming orifice or nozzle  274  through which ink is ejected. In general, the ink jet printhead includes an array of closely spaced ink jet ejectors  10  that eject drops of ink onto an image receiving member (not shown), such as a sheet of paper or an intermediate imaging member. 
     Ink flows from the manifold to nozzle in a continuous path. Ink leaves the manifold  264  and travels through a port  116 , an inlet  262 , and a pressure chamber opening  120  into the ink pressure chamber  122 . Ink pressure chamber  122  is bounded on one side by a flexible diaphragm  30 . A piezoelectric transducer  132  is rigidly secured to diaphragm  30  by any suitable technique and overlays ink pressure chamber  122 . Metal film layers  34  that can be coupled to an electronic transducer driver  36  in an electronic circuit can also be positioned on both sides of the piezoelectric transducer  132 . 
     Ejection of an ink droplet is commenced with a firing signal. The firing signal is applied across metal film layers  34  to excite the piezoelectric transducer  132 , which causes the transducer to bend. Upon actuation of the piezoelectric transducer, the diaphragm  30  deforms to force ink from the ink pressure chamber  122  through the outlet port  124 , outlet channel  270 , and nozzle  274 . The expelled ink forms a drop of ink that lands onto an image receiving member. Refill of ink pressure chamber  122  following the ejection of an ink drop is augmented by reverse bending of piezoelectric transducer  132  and the concomitant movement of diaphragm  30  that draws ink from manifold  264  into pressure chamber  122 . 
     To facilitate manufacture of an ink jet array printhead, an array of ink jet ejectors  10  can be formed from multiple laminated plates or sheets. These sheets are configured with a plurality of pressure chambers, outlets, and apertures and then stacked in a superimposed relationship. 
     Referring once again to  FIGS. 1 and 2  for construction of a single ink jet ejector, these sheets or plates include a diaphragm plate or layer  104 , an ink jet body plate  111 , an inlet plate  46 , an outlet plate  112 , and an aperture plate  272 . The piezoelectric transducer  132  is bonded to diaphragm  30 , which is a region of the diaphragm plate  104  that overlies ink pressure chamber  122 . 
       FIG. 3  is a profile view of a partially completed ink jet printhead including a diaphragm plate or layer  104 , body layer  111 , and a thermoplastic polymer layer  108 . The diaphragm plate  104  may be formed from a metal, ceramic, glass, or plastic sheet that has one or more ink ports  116  that extend through the layer, with one ink port corresponding to each pressure chamber  122  in the body layer  111 . The diaphragm plate should be thin enough to be able to flex easily, but also resilient enough to return to its original shape after it has been deformed. The diaphragm layer is bonded to a polymer layer, which is bonded as an unbroken sheet. DuPont ELJ-100® is an example of a material that is suitable to form the polymer layer. The polymer layer may also be formed from a polyimide material or other polymers including polyetherether ketone, polysulfone, polyester, polyethersulfone, polyimideamide, polyamide, polyethylenenaphthalene, etc. The polymer layer can be a self-adhesive thermoplastic or have a thin layer of adhesive deposited on the side of the polymer layer that is placed in contact with the body layer  111 . Alternatively, another thermoplastic or thermoset adhesive could be used to bond the polymer layer to the diaphragm. In yet further alternatives the adhesive could be a dispensed or transfer film of liquid adhesive. 
     The body layer is bonded to the opposite side of the polymer layer. The fluid path layer may be formed from one or multiple metal sheets that are joined via brazing as shown here as the body plate  111  and the outlet plate  112 . The fluid path layer can also be made from a single structure molded, etched, or otherwise produced. The fluid path layer contains openings or channels through the various layers that form paths and cavities for the flow of ink through the finished printhead. A pressure chamber is structured with diaphragm layer  104  and polymer layer  108  forming the top portion, the body plate  111  and the outlet plate  112  forming the fluid body layer and providing the lateral walls and base for the pressure chamber. The chamber base has an outlet port  124  that allows ink held in the pressure chamber to exit the body layer when the diaphragm is deformed by a piezoelectric transducer (not shown). 
       FIG. 4  is a profile view of the same partial ink jet printhead of  FIG. 3  additionally including bonded piezoelectric transducers. In this view, a piezoelectric transducer  132  has been bonded to the diaphragm plate  104  in alignment with the pressure chamber  122 . In order to bond the piezoelectric transducers to the appropriate locations, they are first arranged on a carrier plate (not shown) with the sides opposite the diaphragm plate temporarily affixed to the carrier plate. Then, a thermoset polymer, typically an epoxy, is deposited on the surface of the diaphragm sheet. The carrier plate is aligned with the diaphragm plate, and pressure and heat are applied until the thermoset polymer has bonded the piezoelectric transducers to the diaphragm plate. The carrier plate is then released using known techniques from the piezoelectric transducers. The pressure from the bonding process squeezes excess thermoset polymer  128  from under the piezoelectric elements, leaving residual adhesive on the exposed diaphragm, some of which may flow into the ink ports  116 . Flow of the bonding adhesive is stopped at the polymer bonding layer  108 . The piezoelectric transducers are now rigidly bonded to the diaphragm plate so that when one of the piezoelectric transducers deforms, the diaphragm plate deforms in the same direction. 
       FIG. 5  is a profile view of the same partial ink jet printhead of  FIG. 4  further including an interstitial polymer layer  136  formed between the piezoelectric transducers. This layer fills in the spaces between piezoelectric transducers including the pre-existing openings in the diaphragm layer. 
     Interstitial polymer layer  136  is formed from a thermoset polymer. Examples of suitable thermoset polymers include epoxies, acrylics, and the like, as well as mixtures thereof. One example of a suitable thermoset polymer is a combination of EPON™ 828 epoxy resin (100 parts by weight) available from Miller-Stephenson Chemical Co., Danbury, Conn. and EPIKURE™ 3277 curing agent (49 parts by weight) available from Hexion Specialty Chemicals, Columbus, Ohio. The thermoset polymer in one specific embodiment has a modulus of less than one gigaPascal (GPa). The thermoset polymer is dispensed in a quantity sufficient to cover exposed portions of an upper surface of the diaphragm and to encapsulate the piezoelectric transducers subsequent to curing. 
     In prior known methods of forming piezoelectric printheads, a standoff layer was used to bond the electrical connect circuitry to the array. In contrast, in the embodiments disclosed herein, the interstitial polymer is used to bond the electrical connect circuitry to the array directly, thereby eliminating the need for a standoff layer. The method disclosed herein entails first temporarily securing the electrical connect circuitry to the array, followed by injecting the thermoset polymer (prior to curing) into the spaces formed between the electrical connect circuitry, the diaphragm plate, and the piezoelectric transducers, relying on capillary action of the fluidic thermoset polymer to fill the open cavities of the transducer array. 
     In one embodiment, an electrical connect circuit, such as a flex circuit, used hereinbelow for illustration purposes, is attached by first stencilling onto the piezoelectric transducers an adhesive, such as an epoxy such as EPO-TEK® E2101, available from Epoxy Technology, Billerica, Mass., or the like, by known methods, such as those described in, for example, U.S. Patent Publication 2010/0294545, the disclosure of which is totally incorporated herein by reference. Formation of depressions or “bumps” in the flex circuit enable alignment of the flex circuit with the piezoelectric transducers. Snap-curing (i.e., curing for periods of about 10 minutes) of the adhesive provides a temporary connection of the flex circuit to the piezoelectric transducers. 
     The process of adding interstitial polymer  136  is further illustrated in  FIG. 6 . As shown in  FIG. 6 , diaphragm plate  104  having piezoelectric transducers  132  situated thereon is temporarily attached to electrical circuit board (ECB)  252 , a flex circuit in this instance, via temporary adhesive  302 . Depressions or bumps  304  in flex circuit  252  facilitate alignment of flex circuit  252  with piezoelectric transducers  132 . When this assembly has been completed as described in the previous paragraph, the assembly is placed on a heat source  306 , such as a hot plate, at a temperature suitable for reducing the viscosity of the thermoset polymer and enhancing the capillary action. In one specific embodiment, this temperature is at least about 25° C., in another embodiment at least about 50° C., and in yet another embodiment at least about 70° C., and in one embodiment no more than about 200° C., in another embodiment no more than about 150° C., and in yet another embodiment no more than about 110° C., although the temperature can be outside of these ranges. Fluid thermoset polymer  308  is then dispensed by any suitable or desired method, such as via a dispense needle  310 , through fill hole or joint  312  in flex cable  252 . Interstitial spaces between piezoelectric transducers  132 , diaphragm plate  104 , and flex circuit  252  are filled with interstitial polymer  136  via capillary action. 
     Filling can take place from the approximate center of an array, which will halve the number of spaces to be filled per unit of time since the interstitial polymer will be flowing outwards in two directions, or from one end of an array. Other options are also possible, such as filling from two directions at once, filling from an off-center site asymmetrically, or the like.  FIG. 7  illustrates the effect of temperature on fill speed when the interstitial polymer is EPON™ 828 epoxy resin (100 parts by weight) and EPIKURE™ 3277 curing agent (49 parts by weight) and the array is 3 inches long and 0.5 inch wide. 
     The temperature and viscosity of the thermoset polymer affect the speed of flow. If temperature and viscosity are too high, the polymer may cure before it has flowed across the entire array and before all of the interstitial spaces have been filled. If the temperature and viscosity are too low, the polymer will not flow across the entire array. Desirable viscosities depend on the dimensions of the array being filled. 
     A ridge or bump of excess thermoset polymer  314  will in many instances remain at fill joint  312  subsequent to filling. If desired, this excess can be removed prior to curing by any desired method, such as mechanical wiping. In another embodiment, this excess can be planarized by any desired or suitable method, such as by application of pressure with, for example, a plate or other relatively flat object, in which embodiment the excess remains in place but does not form a ridge or bump. 
     Final curing can be at any desired or effective temperature, which will, of course, depend on the thermoset polymer selected. Final curing can be by any desired method, such as oven heating or the like. In one specific embodiment, the final curing temperature is at least about 20° C., in another embodiment at least about 50° C., and in yet another embodiment at least about 100° C., and in one embodiment no more than about 300° C., in another embodiment no more than about 250° C., in yet another embodiment no more than about 200° C., and in still another embodiment no more than about 190° C., although the temperature can be outside of these ranges. Final curing can be for any desired or effective amount of time, in one embodiment at least about 2 minutes, in another embodiment at least about 30 minutes, and in yet another embodiment at least about 1 hour, and in one embodiment no more than about 24 hours, in another embodiment no more than about 4 hours, and in yet another embodiment no more than about 2 hours, although the time can be outside of these ranges. 
     In some embodiments, a thin sheet of non-stick polymer, such as polytetrafluoroethylene (commonly referred to as PTFE and sold commercially as TEFLON®), may be applied to the upper surface of the thermoset polymer before curing to planarize the surface. This PTFE layer is then removed after curing. Alternatively, a UV curable polymer could be used for the interstitial fill and then a UV light used to cure the polymer. The ink inlet holes are then drilled through the multiple polymer layers and through the pre-existing openings in the diaphragm. 
       FIG. 8  is a profile view of the completed assembly of  FIG. 5  after the ink jet ejector is bonded to an electrical circuit board (ECB)  252  and the ink inlets have been ablated. In one embodiment, a laser is used to drill the ink passages  262  through the polymer layer  108  and the interstitial polymer  136 . Many laser drilling processes can be used to form the ink passages through these layers. In one process an excimer laser illuminates a lithography mask with transparent regions corresponding to one or several of the ink passages that are to be drilled through the polymers. The laser illuminated mask openings are positioned on an exposed layer on the printhead in alignment with the locations for the desired openings in the layer. The mask is then imaged onto the exposed surface. The substrate is then moved under a laser imaging system in a step-and-repeat process. Excimer lasers at 248 nm or 308 nm with laser fluence of 250 mJ/cm 2  to 800 mJ/cm 2  are suitable parameters though other laser wavelengths and fluencies may be used. Alternatively, a scanned laser beam may be used to drill individual ink passages. In this alternative process, the laser can be scanned with galvanometer-driven mirrors and focused onto the substrate with a scan lens. The ink passages can be generated with a beam at a fixed position to produce each hole or it can be scanned in a circle or other shape to form each ink passage through the polymer layers. Suitable lasers for the scanned laser drilling include a solid state laser or a fiber laser at 355 nm or a CO 2  laser having a 9.4 to 10.6 μm wavelength. 
     Pre-existing holes  263  in the ECB  252  are larger than the ink passages  262  and aligned with the ink passages so that the ink path is not interrupted by the circuit board  252 . In another embodiment, the circuit board can be replaced by a flexible circuit having electrical pads aligned to the array of piezoelectric elements similar to the ECB. For the flexible circuit pre-existing holes for ink passages can exist, or in one embodiment, the ink passages are formed in the laser drilling process that forms the ink passage  262 . As further described below, the full printhead assembly and order of layer processing can happen in many different orders so long as the polymer layer  108  is attached to the diaphragm  104  prior to the piezoelectric elements  132  and interstitial polymer  136  being added to the assembly. 
       FIG. 9  is a profile view of a complete ink jet head including an aperture plate  272  attached to the outlet plate  112  by aperture plate adhesive  268 . The manifold  264  acts as an ink reservoir supplying ink to the inlets of one or more pressure chambers, and each pressure chamber has a dedicated ink inlet connected to the manifold. The body layer  111  is attached to an outlet layer  212  to form a portion of each pressure chamber. The aperture plate adhesive  268  includes an outlet channel  270  corresponding to each pressure chamber. The aperture plate  272  may be formed from metal or a polymer and has apertures or nozzles  274  extending through the plate to allow ink to exit the printhead as droplets. 
     Other embodiments may have different numbers of layers or combine several functions into a single layer. Other assembly and processing orders are also possible. 
     In operation, ink flows from the manifold through ECB channel  263  and the inlet port  262  into the pressure chamber  122 . An electrical firing signal sent to the piezoelectric transducer  132  in piezoelectric layer  210  via conductive traces  256  and conducting epoxy  248  or other means of producing the electrical connection  248  causes the piezoelectric transducer to bend, deforming the diaphragm  104  and polymer layer  108  into the pressure chamber. This deformation urges ink out the outlet port  124 , into the outlet channel  270 , and through the nozzle  274  where the ink exits the printhead as a droplet. After the ink droplet is ejected, the chamber is refilled with ink supplied from the manifold with the piezoelectric transducer aiding the process by deforming in the opposite direction to cause the concomitant movement of the diaphragm and polymer layers that draw ink from the manifold into the pressure chamber. 
     Other embodiments and modifications of the present invention may occur to those of ordinary skill in the art subsequent to a review of the information presented herein; these embodiments and modifications, as well as equivalents thereof, are also included within the scope of this invention. 
     The recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefor, is not intended to limit a claimed process to any order except as specified in the claim itself.