Abstract:
A method and apparatus for assembling an inkjet printhead is disclosed. The apparatus comprises one or more microheater resistors and a material, which can be reflowed by use of the heat generated by the microheater, disposed proximate to the heater resistor. The microheater resistor comprises a heater resistor with an input port and an output port. A first electrical conductor, coupled to the input port of the heater resistor, receives a first voltage and a second electrical conductor, coupled to the output port of the heater resistor, receives a second voltage. A curing effect in the adhesive is produced by heat generated in the heater resistor when electrical current flows through the resistor in response to an application of a first voltage to the first electrical conductor and a second voltage to the second electrical conductor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to inkjet printing devices, and more particularly to the use of patterned thin film microheaters for assembly of inkjet printhead assemblies. 
     BACKGROUND OF THE INVENTION 
     Thermal inkjet (TIJ) technology is widely used in computer printers. Very generally, a TIJ includes a printhead typically comprising several tiny controllable inkjets, which are selectively activated to release a jet or spray of ink from an ink reservoir onto the print media (such as paper) in order to create an image or portion of an image. TIJ printers are described, for example, in the Hewlett-Packard Journal, Volume 36, Number 5, May, 1985, and Volume 39, Number 4, August, 1988. 
     Thermal inkjet print cartridges operate by rapidly heating a small volume of ink to cause the ink to vaporize and be ejected through one of a plurality of orifices so as to print a dot of ink on the print medium. Typically, the orifices are arranged in one or more linear arrays in a nozzle member. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the print medium as the printhead is moved relative to the print medium. 
     In one known design, the inkjet printhead generally includes ink channels to supply ink from an ink reservoir to each vaporization chamber proximate to an orifice, a metal orifice plate or nozzle member in which the orifices are formed in the required pattern, and a silicon substrate containing a series of thin film resistors, one resistor per vaporization chamber. 
     To print a single dot of ink, an electrical current from an external power supply is passed through a selected thin film resistor. The resistor is then heated, in turn superheating a thin layer of the adjacent ink within a vaporization chamber, causing explosive vaporization, and consequently, causing a droplet of ink to be ejected through an associated orifice onto the print medium. 
     An exemplary inkjet cartridge is described in U.S. Pat. No. 4,500,895, entitled “Disposable Inkjet Head”, and is assigned to the present assignee. 
     Another inkjet printhead is described in U.S. Pat. No. 4,683,481, entitled “Thermal Ink Jet Common-Slotted Ink Feed Printhead”, wherein ink is fed from an ink reservoir to the various vaporization chambers through an elongated hole formed in the substrate. The ink then flows to a manifold area, formed in a barrier layer between the substrate and a nozzle member, then into a plurality of ink channels, and finally into the various vaporization chambers. This design is known as a center feed design, whereby ink is fed to the vaporization chambers from a central location and then distributed outwardly into the vaporization chambers. 
     In TIJ pens it is necessary to connect the ink reservoir to the printhead. The size of this connection affects the design of the printer that the pens are used in. An ideal reservoir-to-printhead coupler, from a print design point of view, would be no longer than the TIJ head is long, and would be high or tall enough to allow the drive and pinch wheels to get as close to the printhead as possible. Any increase in the size of this coupler will compromise the paper handling ability, which may affect the print quality, and increase the size of the printer. 
     In one exemplary spring bag pen design, the pen frame of a first molded material is lined with a second molded material, such as polyethylene, on the inside to produce a surface suitable for staking the films of the spring bag. The first molded material from which the frame is made could be, for example, an engineering plastic, and provides the necessary structure for the pen which could not be accomplished with the second molded material. In this design, the fluid connection of the first and second molded materials needs to designed in such a way as to provide a space-efficient, leak-resistant connection. 
     Conventional methods of connecting plastic materials, including those used in TIJ pens, include the use of glue, seals, such as gaskets or  0 -rings, or mechanical press fits. In these cases two or more separate parts are fabricated and assembled together to form a single unit. Each part must be designed and sized with respect to its needs in manufacturing, structural integrity, and with the tolerance of the mating part in mind. Such joints as these take up space, and their reliability can be affected by the part tolerances, surface finishes, and the assembly operation and the material properties of the adhesive. 
     Commonly assigned application Ser. No. 07/853,372 describes a leak-resistant joint between the first and second molded materials, wherein the first and second molded materials have different shrink rates. As the first material cools from a molten state, the second molded material is molded about a standpipe formed of the first molded material thereby creating a tight joint between the two molded materials. In each of these exemplary devices, the TIJ printheads are assembled using polymer substrates and thermoset adhesives which are cured by heating the entire printhead/pen assembly. This procedure results in large deformations due to coefficient of thermal expansion mismatches and substrate processing defects. In addition, this heating of the entire assembly frequently effects performance of other parts or limits the kinds of assembly sequences and automation strategies that can be pursued. Furthermore, some materials which might be beneficial can not be used in the assemblies since the entire assembly is subjected to this curing process. 
     SUMMARY OF THE INVENTION 
     The invention makes use of patterned thin film resistors (microheaters) for assembly of the inkjet printhead assemblies wherein once activated, the microheaters cure the interfaces between different parts of the assembly thereby alleviating the need for curing the entire printhead assembly. 
     According to one embodiment of the invention, an apparatus for assembling an inkjet printhead is disclosed. Other embodiments discussed hereinafter include, but are not limited to: reflowing thermoplastic or thermoset materials such as thermoset polymers, all of which are referred to hereinafter as temperature curable adhesive. Still other embodiments contemplate reflowing a solder or brazing material. The heating apparatus comprises a microheater and a temperature curable adhesive disposed proximate to the heater resistor. 
     The microheater comprises a heater resistor with electrical contacts referred to as an input port and an output port. A first electrical conductor, coupled to the input port of the heater resistor, receives a first voltage and a second electrical conductor, coupled to the output port of the heater resistor, receives a second voltage. A curing effect in the adhesive is produced by heat generated by current flow caused by the application of a first voltage to the first electrical conductor and a second voltage to the second electrical conductor. A voltage differential across the input and output ports causes a current to flow through the heater resistor, elevating its temperature. 
     According to another embodiment of the invention, an inkjet printer cartridge is disclosed. The inkjet cartridge comprises a printhead comprising a top side and a bottom side, the bottom side having a microheater, and a printhead attachment. The microheater comprises a heater resistor comprising an input port and an output port. A first electrical conductor, coupled to the input port of the heater resistor, receives a first voltage and a second electrical conductor, coupled to the output port of the heater resistor, receives a second voltage. The printhead attachment is disposed below the bottom side of the printhead, wherein the printhead attachment is affixed to the printhead by an application of a first voltage to the first electrical conductor and a second voltage to the second electrical conductor. 
     According to another embodiment of the invention, a method for assembling an inkjet printhead assembly is disclosed. A heater resistor is patterned onto a bottom surface of a printhead. A printhead attachment is aligned in a desired position wherein a surface of the printhead attachment is in contact with the heater resistor. A voltage drop is applied across a heater resistor which produces an electrical current in the heater resistor. A temperature increase in the heater resistor and an area proximate to the heater resistor is generated in response to the production of an electrical current thereby causing the printhead and printhead attachment to attach to the heater resistor. 
     According to another embodiment of the invention, a method for attaching pieces of a manifold using localized heating is disclosed. A thin film resistor is patterned onto a surface of a first half of a manifold with a first fluid passage. A second half of the manifold is aligned with a second fluid passage so as to provide a third fluid passage through the manifold, wherein a surface of the second half of the manifold is in contact with the thin film resistor. A voltage drop is applied across the thin film resistor to produce an electrical current in the thin film resistor. A temperature increase in the thin film resistor and an area proximate to the thin film resistor is generated in response to the production of the electrical current. At least portions of the first half and the second half of the manifold proximate to the thin film resistor are cured by the temperature increase thereby attaching and sealing the first half and the second half of the manifold to the thin film resistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings, in which: 
     FIGS.  1 ( a )- 1 ( b ) illustrate an exemplary thermal inkjet pen assembly; 
     FIG. 2 illustrates using a thin film resistor to attach a printhead to a printhead assembly according to one embodiment of the invention; 
     FIG. 3 illustrates using a thin film resistor to attach a printhead to a fluid interconnect device according to one embodiment of the invention; 
     FIG. 4 illustrates using a thin film resistor and a temperature curable adhesive to attach a printhead to the printhead attachment according to one embodiment of the invention; 
     FIG. 5 illustrates using a thin film resistor and a seal material to attach a printhead to a pen assembly; and 
     FIG. 6 illustrates using a thin film resistor to attach pieces of a manifold according to one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     An exemplary thermal inkjet pen assembly is illustrated in FIGS.  1 ( a )- 1 ( b ). The pen includes a pen body  12  defining a reservoir  28 . The reservoir  28  is configured to hold a quantity of ink. A printhead  20  with an orifice plate  33  is fit into the bottom of the pen body  12  and controlled for ejection of ink droplets. The printhead includes minute nozzles  25  through which ink is expelled in a controlled pattern during printing. 
     Each nozzle  25  is in fluid communication with a firing chamber  44  defined in the printhead  20  adjacent to the nozzle. Each firing chamber  44  is constructed adjacent to a part of the printhead substructure  16  that includes an electrical energy-to-thermal energy transducer, preferably a resistor component  30 , not shown in FIG.  1 ( a ) or  1 ( b ). The resistor  30  is selectively driven (heated) with sufficient electrical current to instantly vaporize some of the ink in chamber  44 , thereby forcing an ink droplet through the nozzle  25 . 
     Conductive drive lines for each resistor component  30  are carried upon a flexible circuit  24  mounted to the exterior of the pen body  12 . Circuit contact pads  23  at the ends of the resistor drive lines engage similar pads carried on a matching circuit attached to the printer carriage (not shown). A signal for firing the resistors is generated by a microprocessor and associated drivers that apply the signals to the drive lines. 
     According to one embodiment of the invention, thin film resistors (microheaters) can be used to provide localized heating, to attach and seal a silicon printhead to a substrate of a printhead attachment or assembly as illustrated in FIG.  2 . In this embodiment, the printhead  210  is initially manufactured in a known manner, for example as described in U.S. Pat. Nos. 4,862,197 and 5,883,650. A thin film resistor or heater resistor  212  is then patterned onto the bottom side of the printhead  210 . 
     Resistive heaters can be fabricated in a number of ways and on a number of substrate materials. For the resistive heater to work correctly, the resistive heater material is usually electrically insulated from the substrate or the substrate material is nonconducting. Therefore if the substrate material is nonconducting the resistive heater material can be fabricated onto the substrate or substrate material by means of selective plating (either additive or subtractive), vacuum deposition of the resistive material or by other means of deposition. If the substrate material is a moldable, and non-conductive material, the thin resistive heating element may be molded into the substrate material itself. If the substrate material is conductive, then an insulating layer must be installed between the substrate and the resistive heater elements. The insulating layer can be installed by plating means or deposition. Once the insulating layer is available the resistive heater can be fabricated by selective plating (additive or subtractive), vacuum deposition of the resistive material or by other means of deposition. The specific dimensions of the resistive element can be defined in many ways. Among these are patterning by means of photolithographic processes, laser removal of unwanted material, or selective etching of unwanted material. 
     The thin film resistor  212  has an input port  213  and an output port  215 . A first electrical conductor  214  is coupled to the input port  213  of the thin film resistor  212  and receives a first voltage from a first voltage source  229 . A second electrical conductor  216  is coupled to the output port  215  of the thin film resistor  212  and receives a second voltage. The second voltage can comprise a circuit ground  220 . A switching device  219  such as one or more bipolar or field effect transistors or functional equivalents thereto, can be coupled between the circuit ground  220  and the second electrical conductor  216 , wherein when the switching device  219  is enabled an electrical current is produced in the thin film resistor  212 . In addition, the thin film resistor  212  can be electrically insulated from areas of the substrate by means of a thin film insulator  222 , such as silicon oxide or other dielectric materials with sufficient insulating properties. 
     When the printhead  210  is to be connected to the substrate  224  of the printhead assembly, the substrate  224  is placed in an appropriate position so as to be in contact with the thin film resistor  212 . In this embodiment, the substrate  224  is of an inorganic material which for purposes of this disclosure is considered to be ceramic. The inorganic material can comprise silicon, glass, ceramic, or a low thermal expansion metal but is not limited thereto. Once the substrate  224  and the silicon printhead  212  have been properly aligned, the first and second voltages are applied to the conductors of the thin film resistor  212  causing electrical current to flow through the resistor  212 . The current flowing through the thin film resistor  212  provides localized heating to the substrate  224  so as to attach and seal the substrate  224  to the thin film resistor  212 . In addition, mechanical pressure can be applied to both the printhead  210  and the substrate  224  during the heating and curing process to help seal the printhead  210  and substrate  224  together. 
     It will be understood by one skilled in the art that the thin film resistor  212  can alternatively be patterned on the substrate  224  rather than the printhead  210 . In such an embodiment, when the voltages are applied to the thin film resistor  212 , the thin film resistor  212  provides localized heating to the printhead  210  so as to attach and seal the printhead  210  to the thin film resistor  212 . In addition, it will be understood that the thin film resistor can be thermally isolated from areas of the printhead and/or substrate by means of a thin film insulator (not shown). 
     It will also be understood that the printhead attachment can be to a substrate, fluid interconnect device or pen assembly, but the invention is not limited thereto. For example, the thin film resistor can be used to connect or “solder” the printhead to a fluid interconnect device as illustrated in FIG.  3 . The thin film resistor  312  can be patterned onto either the bottom surface of the printhead  310  or a top surface of a fluid interconnect device  314 . When the printhead  310  and the fluid interconnect device  314  are aligned in a desired position, the first and second voltages are applied to the thin film resistor  312 . The localized heating of the thin film resistor  312  then causes the printhead  310  and the fluid interconnect device  314  to attach and seal to the thin film resistor  312  so long as the bottom surface of the printhead  310  and the top surface of the fluid interconnect device  314  have substantially similar coefficient of thermal expansion characteristics. Furthermore, the thin film resistor can be positioned so as to allow the printhead  310  to be in fluidic connection with the fluid interconnect device  314 . 
     In another embodiment of the invention, the microheaters can be used in combination with a polymer seal material to attach the printhead to a substrate of a printhead assembly as illustrated in FIG.  4 . In this embodiment, a temperature curable adhesive  412  can be patterned proximate to the thin film resistor  414  on the printhead  416  and/or the printhead attachment  418 . When the first and second voltages are applied to the thin film resistor  414 , the thin film resistor provides localized heating, which cures the adhesive  412 . The curing of the adhesive  412  thereby attaches and seals the printhead  416  to the printhead attachment  418 . It will be understood that the printhead attachment can be to a substrate, fluid interconnect device or pen assembly, but the invention is not limited thereto. For example, the printhead attachment can be a plastic pen assembly  518  with an insert molded metal rim  520  as illustrated in FIG.  5 . In this example, a second shot seal material  512  can be patterned proximate to the thin film resistor  514  on the printhead  516  and/or the pen assembly  518  and/or the molded metal rim  520 . When the first and second voltages are applied to the thin film resistor  514 , the thin film resistor provides localized heating, which cures the seal material  512 . The curing of the seal material  512  thereby attaches and seals the printhead  516  to the pen assembly  518  (molded metal rim  520 ). 
     According to another embodiment of the invention, thin film resistors can be used to assemble pieces required to make a manifold with internal passages for ink delivery as illustrated in FIG.  6 . In the embodiment illustrated in FIG. 6, the manifold  610  has two halves: a top half  612  with a fluid passage  614 , and a bottom half  616  with a fluid passage  618 . A thin film resistor  620  can be patterned onto either the top half  612  or the bottom half  616  of the manifold  610 . When the top half  612  and the bottom half  616  have been properly aligned so as to provide a fluid passage through the manifold, first and second voltages are applied to the thin film resistor  620  to provide localized heating. The heating causes the top half  612  and the bottom half  616  to attach and seal to the thin film resistor  620  so long as the surfaces of the two halves of the manifold in contact with the thin film resistor  620  have similar coefficient of thermal expansion characteristics. In addition, it will be understood that a temperature curable adhesive can be patterned proximate to the thin film resistor  620  to help secure the two halves of the manifold. Furthermore, a thin film insulator can be patterned proximate to the thin film resistor  620  to insulate portions of the two halves of the manifold from the heating effects of the thin film resistor  620 . 
     In yet other embodiments of the invention, the microheater resistors can be used to reflow thermoplastic material from which a pen body, or parts thereof, are assembled. Thermoset polymers can also be cured as well. In yet other embodiments, solder or brazing material can be heated to reflow as well. 
     For purposes of claim construction, thermoplastic and thermoset materials, solder and brazing materials are considered to be equivalent temperature-curable adhesives. When such temperature-curable adhesives are disposed proximate to the aforementioned microheaters, localized temperature-sensitive curing of adhesives used in the assembly of a variety of structures such as substrates or even compete pen devices is enabled. 
     While the present invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention.