Patent Publication Number: US-6213587-B1

Title: Ink jet printhead having improved reliability

Description:
TECHNICAL FIELD 
     The present invention relates to an ink jet printhead with improved transducer life, and, more specifically, to an ink jet printhead having a reduced nozzle plate thickness, a reduced barrier height, and a reduced power density applied to the heaters of the printhead. 
     BACKGROUND OF THE INVENTION 
     Ink jet printers typically include recording heads, referred to hereinafter as printheads, that employ transducers which utilize kinetic energy to eject ink droplets. For example, thermal printheads rapidly heat thin film resistors (or heaters) to boil ink, thereby ejecting an ink droplet onto a print receiving medium, such as paper. According to this ink jet method, upon firing a resistor, a current is passed through the resistor to rapidly generate heat. The heat generated by the resistor rapidly boils or nucleates a layer of ink in contact with or in proximity to a surface of the resistor. 
     The nucleation causes a rapid vaporization of the ink vehicle, creating a vapor bubble in the layer of ink. The expanding vapor bubble pushes a portion of the remaining ink through an aperture or orifice in a plate, so as to deposit one or more drops of the ink on a print receiving medium, such as a sheet of paper. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the print receiving medium as the printhead is moved relative to the print receiving medium. 
     Typically, the orifices provided on such a plate are arranged in one or more linear arrays. Moreover, the paper is typically shifted each time the printhead moves across the paper. The thermal ink jet printer is generally fast and quiet, as only the ink droplet is in contact with the paper. Such printers produce high quality printing and can be made both compact and economical. 
     In general, the reliability of a printhead can be dependent on the reliability of the energy-generating elements or transducers it utilizes. Accordingly, and as can be understood, increasing the expected lifespan of the transducers would improve the reliability of the printheads in which they are used. Thus, it would be advantageous to have a printhead that has increased transducer life. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to improve the reliability of inkjet printheads. 
     It is another object of the present invention to provide an inkjet printhead including a transducer having an increased life span. 
     According to one embodiment of the present invention, an inkjet printhead comprises a transducer (such as a heater resistor), a chamber, and a plate. At least a portion of the transducer is arranged within the chamber, and the plate is provided with at least one aperture capable of cooperating with the chamber to allow ink to be ejected therefrom. The plate has a thickness of less than 62 microns and the transducer can be selectively energized with a power density less than 2.159 GW/m 2  to cause droplets of the ink to be ejected. 
     Preferably, the plate is separated from the transducer by a distance of less than 28 microns. More preferably, the plate is so separated by about 8 to about 27 microns. In preferred inkjet printheads according to this embodiment, the transducer comprises a heater having a heater area of less than about 2800 microns 2 , and/or the inkjet printhead comprises a mono ink. 
     According to another preferred embodiment of the present invention, the plate thickness is less than about 60 microns and, more preferably, is about 35 to about 55 microns. In a further preferred embodiment, the transducer is capable of being selectively energized with a power density less than about 2 GW/m 2  to cause droplets of ink to be ejected from the chamber. With mono-ink printheads, this transducer is capable of being selectively energized with a power density preferably less than about 1.3 GW/m 2  to cause droplets of ink to be ejected from the chamber and, more preferably, from about 0.7 to about 1 GW/m 2 . Meanwhile, with multi-color ink printheads, this transducer is capable of being selectively energized with a power density preferably from about 0.7 to about 1.5 GW/m 2 . 
     In a preferred embodiment, the printhead comprises a mono ink. This embodiment can be particularly preferred when utilizing a transducer capable of being selectively energized with a power density greater than 1 GW/M 2  to cause droplets of ink to be ejected from the chamber or when the plate is separated from the transducer by a distance of less than 28 microns. When using mono ink and a heater as a transducer, the heater area is preferably greater than about 1900 microns 2 . 
     According to an alternative embodiment, the printhead comprises a multi-color non-phosphate ink. This alternative can be particularly preferred when utilizing a transducer capable of being selectively energized with a power density less than 2 GW/r 2  to cause droplets of ink to be ejected from the chamber or when the plate thickness is greater than 40 microns. As with mono ink, when using a multi-color non-phosphate ink and a heater as a transducer, the heater area is preferably greater than about 1900 microns 2 . By comparison, when using an ink containing phosphates and a heater as a transducer, the heater area is preferably less than about 2800 microns 2 . 
     In another embodiment of the present invention, an inkjet printhead comprises a plurality of transducers and chambers, with at least a portion of each transducer being arranged within a respective chamber. A plate having a plurality of apertures is also provided. Each aperture cooperates with a respective chamber to allow ink to be ejected therefrom. 
     According to this embodiment of the present invention, the plate has a thickness of less than 62 microns. Moreover, each transducer can be selectively energized with a power density less than 2.159 GW/m 2  to cause the ejection of the ink. Preferably, the plate is separated from the transducer by a distance of less than 28 microns. 
     In yet another embodiment of the present invention, an inkjet printer comprises a printhead and power source. The printhead includes a transducer, a chamber, and a plate. At least a portion of the transducer is arranged within the chamber. 
     The plate is provided with at least one aperture capable of cooperating with the chamber to allow ink to be ejected therefrom. The plate also has a thickness of less than 62 microns. In addition, the power source is capable of selectively energizing the transducer with a power density less than 2.159 GW/m 2  to cause the ejection of the ink from the chamber. In a preferred form, the plate can be separated from the transducer by a distance of less than 28 microns. 
     Still other aspects of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described various embodiments of this invention, simply by way of illustration. As will be realized, the invention is capable of other different aspects and embodiments without departing from the scope of the invention. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not as restrictive in nature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the invention, it is believed the same will be better understood from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a schematic plan view of a thermal ink jet printer for receiving a printhead to which the novel method and apparatus of the present invention pertains; 
     FIG. 2 is a schematic and fragmentary view of a portion of the apparatus illustrated in FIG. 1, showing printhead and print receiving medium relative motion; 
     FIG. 3 is an enlarged, partially exploded, fragmentary cross-sectional view of a portion of the apparatus shown in FIG. 1, taken along line  3 — 3  of FIG. 1; 
     FIG. 4 is a partial perspective view of an ink jet printhead; 
     FIG. 5 is an enlarged cross-sectional detail of an ink jet printhead; 
     FIG. 6 is a selectively sectioned cross-sectional detail of an ink jet printhead; 
     FIGS. 6A through 6E are selectively sectioned cross-sectional details of alternative ink jet printheads according to the present invention; 
     FIG. 7 is a selectively sectioned perspective view of the ink jet printhead of FIG. 5; 
     FIG. 8 is a top view of the selectively sectioned perspective view shown in FIG. 7; 
     FIG. 9 is an enlarged schematic view in plan of a printhead chip showing the relative positions of electrical components positioned thereon; 
     FIG. 10 is a top view of a multi-color printhead chip according to one embodiment of the present invention; 
     FIG. 11 is a top view of a nozzle plate for the printhead chip shown in FIG. 10; 
     FIG. 12 is a top view of a mono-ink printhead chip according to another embodiment of the present invention; 
     FIG. 13 is a top view of a nozzle plate corresponding to the printhead chip shown in FIG. 12; 
     FIG. 14 is a contour plot of the log of life as a function of nozzle plate thickness and power density for a multi-color printhead using a phosphate containing color ink with a barrier height of 30 microns (prior to attachment of the nozzle plate); 
     FIG. 15 is a contour plot of the log of life as a function of nozzle plate thickness and power density for a multi-color printhead using a color ink containing no phosphates with a barrier height of 30 microns (prior to attachment of the nozzle plate); 
     FIG. 16 is a contour plot of the log of life as a function of nozzle plate thickness and power density for a multi-color printhead using a color ink containing no phosphates with a barrier height of 26 microns (prior to attachment of the nozzle plate); and 
     FIG. 17 is a contour plot of the log of life as a function of nozzle plate thickness and power density for a mono-ink printhead using a mono ink with a barrier height of 27 microns (prior to attachment of the nozzle plate). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. 
     Referring now to the drawings in detail, wherein like numerals indicate the same elements throughout the views, FIG. 1 illustrates an embodiment of an ink jet printer  30  to which the present invention can be applicable. A print receiving medium  32 , which can be a recording medium made from paper, thin film plastic or the like, can be moved in the direction of an arrow  34 , being guided by super-imposed pairs  36 ,  38  of sheet feed rollers and under the control of a medium drive mechanism, such as a drive motor  39 , for example. 
     As shown in FIGS. 1 and 2, a printhead  10  can be mounted on a carrier  40 , which can be carried in close proximity to a print receiving medium  32 , which in turn can be transported by roller pairs  36 ,  38 . As shown by the arrow  42 , the printhead  10  (and thus the printhead carrier  40 ) can be mounted for orthogonal, reciprocatory motion relative to the print receiving medium  32 . To this end, and as shown in FIG. 1, the carrier  40  can be mounted for reciprocation along a pair of guide shafts  44  and  46 . 
     The reciprocatory or side-to-side motion of the carrier  40  can be established by a carrier drive, such as one having a transmission mechanism including a cable or drive belt  50  and pulleys  52 ,  54  which carry the belt  50  driven by a motor  56 . In this manner, the printhead  10  may be moved and positioned at designated positions along a path defined by and under the control of the carrier drive and machine electronics  58 . The carrier  40  and the printhead  10  are connected electrically by a flexible printed circuit cable  60  for supplying power from the power supply  62  to printhead  10 , and to supply control and data signals to printhead  10  from the machine electronics  58 , which includes the printer control logic (PCL). 
     According to one embodiment of the present invention, printhead  10  includes a printhead chip  11  attached, preferably by way of an adhesive bond, to a plate  12  having a plurality of individually selectable and actuable nozzle orifices or apertures  22 . The printhead  10  can also include a supply of ink in, for example, an ink-holding reservoir  48 , such as a tank or bottle. As illustrated in FIG. 3, the nozzle plate  12  and chip  11  can be bonded to the reservoir  48 . 
     Chip  11  can be one of many cut from, in a conventional manner, a silicon wafer which, for example, has been coated with photoresist, photolithographically exposed through a mask, subjected to an etch bath and doped by processes well known in the art of semiconductor manufacturing. This process can be repeated through several layers, including metalization for interconnects  70 . Usually, multiple integrated circuit chips  11  are made on a single wafer, which is then cut or diced, into individual chips, or dies. 
     As shown in FIG. 4, the input and output of the chip  11 , including control signals and power, can be applied through a TAB (tape automated bonding) circuit  64  and spaced apart integrated beams or lands (not shown) therein for making input and output (including electrical) connection to the chip, preferably at interconnects  70 . The TAB circuit  64  typically surrounds the chip  11  and can be fastened to a circuit platform (not shown) on the reservoir  48  using a pressure sensitive adhesive, also known as a pre-form adhesive. After the printhead chip  11  is placed on the circuit platform and the TAB circuit  64  is attached to the interconnects  70 , an ultraviolet (UV) photosensitive adhesive can be applied along the sides of the chip and over the beams, as an encapsulant and protectant. A light source can then be applied to the UV adhesive to cure the same. 
     In the illustrated instance, the tape  64  extends along one surface  29  of the reservoir  48 , with electrical contact or terminal pads  28  therein for mating engagement with terminal protrusions or projections  68  on the flexible printed circuit cable  60 . For ease of illustration and understanding, the portion of the carrier  40  carrying the flexible printed circuit cable  60  and its protruding electrical connections  68  is shown in FIG. 3 as being spaced from the pads  28  of the TAB circuit or tape  64 . Upon insertion of the printhead  10  into the carrier  40 , however, electrical mating engagement occurs between the pads  28  of tape  64  and the protrusions or projections  68  of the flexible printed circuit cable  60 . There are numerous techniques for engagement between the contacts  68  and the pads  28 , including sliding frictional engagement, and any such technique is acceptable as long as static discharge between the two connections is minimized or avoided during mating engagement or interconnection. 
     As depicted in FIG. 5, a printhead  10  comprises at least one energy-generating element or transducer, such as an electro-thermal converting element (e.g., a heater  24 ). In a preferred form, the transducer comprises a thin film resistor formed on the chip  11 . The thin film resistor (referred to hereinafter as a heater) can generate thermal energy by applying a voltage difference across electrodes (not shown) connected to the resistive material. 
     Referring to FIG. 6, according to a preferred embodiment of the present invention, the heater  24  can be formed from a resistance layer  19  that is deposited on a surface  15  of a substrate base  14 . Preferably, a thermal barrier (not shown) is provided between the resistance layer  19  and the surface  15  of the base  14 . Although the resistance layer can comprise materials such as tantalum oxide (TaO) or hafnium diboride (HfB 2 ), it preferably comprises tantalum aluminum (TaAl). Meanwhile, the substrate  14  can comprise materials such as quartz and glass, but preferably comprises silicon. 
     A conductive layer  21 , preferably comprising an aluminum-copper alloy (AlCu), can be formed over or under the resistance layer  19 . Conventionally, the conductive layer  21  is approximately 0.5 microns thick. Portions of the respective upper layer can be removed by techniques known in the art, such as chemical etching. With the selected portions removed, the remaining portions of the conductive layer  21  form electrodes and the remaining portion of the resistance layer  19  forms the heater  24 . 
     The printhead  10  also has an ink supply labyrinth comprising, for example, an ink vaporization chamber  18 . According to a preferred embodiment of the present invention, the ink supply labyrinth can be preferably formed between the chip  11  and the plate  12 , and also comprises a channel  16  and a conduit lateral  20  for connecting the channel  16  and the chamber  18 . The channel  16  (also referred to as a via) can be preferably disposed through the base  14  of the chip  11  and allows ink to pass from the ink reservoir  48  (typically behind the chip) into the conduit lateral  20  and into chamber  18 . According to a preferred form of the present invention, the channel  16  can be cut into the base  14  by means of grit blasting or laser cutting, or can exist between an edge of the chip  11  and the ink reservoir  48 . 
     As illustrated in FIGS. 6-8, at least a portion of the heater  24  is arranged within the chamber  18 . For example, a surface area (A) of the heater  24  can be arranged within the chamber  18 . Although the various figures illustrate a preferred embodiment wherein the entire heater  24  is arranged within the chamber  18 , the heater could also be only partially arranged within the chamber. 
     The chamber  18  has a wall or barrier  27  that extends for a height (H) above the heater  24 , including any layers over the heater, such as a protective layer  17  (e.g., passivation or anti-cavitation layer), for example. As with the conductive layer  21 , a layer such as protective layer  17  is conventionally also approximately 0.5 microns thick. The barrier  27  can be operative to help separate the heater  24  a separation distance (S) from the nozzle plate  12 , also including any layers over the heater, and can serve to define a part of the ink labyrinth. 
     Although the barrier  27  is shown in the illustrated embodiments as being an integral wall that generally rises from the surface  15  of the base  14  to the nozzle plate  12 , the present invention is also directed towards embodiments where the barrier  27  may not be integral, such as where it may include apertures for example, as well as towards embodiments where the barrier does not generally rise from the surface of the base and/or rise to the nozzle plate. For example, although FIGS.  6 A and  6 C- 6 E illustrate alternate embodiments where the barrier height (H) is substantially equal to the nozzle plate separation distance (S) (given the tolerances associated with the barrier  27  and the relative thickness of any existing layers, such as protective layer  17  and/or conductive layer  21 , for example), the barrier height (H) need not necessarily be equal to the separation distance (S), as depicted in FIG.  6 B. For simplification, however, barrier height (H) and nozzle plate separation distance (S) will hereinafter be assumed to be substantially equal. 
     Preferably, the chamber  18  can be formed in a thick spacer or insulating film  26 , referred to hereinafter as the thick film layer. Although the thick film layer  26  can comprise a number of materials, such as dry resist, spun-on, or wet process type films, it preferably comprises a photo-developable polymer, such as the dry film resist marketed by Tokyo Ohka Kogyo of Kawasaki, Japan as Ordyl. Typically, the thick film layer  26  is deposited over the resistance layer (and any additional layers such as protective layer  17  and conductive layer  21 ) on a printhead chip  11 . Conventionally, the thickness of the thick film layer  26  can be determined within a tolerance range of 10%. The chamber  18  can be formed, for example, by chemically etching away at least a portion of the thick film layer  26 , as is also known in the art. 
     A plate  12  having a thickness (T) and provided with at least one aperture  22 , cooperates with the chamber  18  to allow the heater  24  to eject ink from the chamber through the aperture  22 . Although the plate  12  can, for example, be integral with the reservoir  48 , it is preferably separable to allow for the attachment of a chip  11 . Likewise, in an alternative embodiment, the plate  12  could also be formed from TAB circuit  64  or the like. 
     According to one embodiment, a separable plate can be attached to the thick film layer  26  throught the application of heat and compression. An adhesive can also be used in this process. Conventionally, the use of heat and compression reduces the height of the thick film layer  26  by approximately 2 microns. 
     The aperture  22 , also referred to as an ink ejection orifice or nozzle, in the plate  12  of the printhead  10  confronts the print receiving medium  32 . Accordingly, ink may be ejected by applying kinetic energy to the ink in the chamber  18  to effect printing on the print receiving medium  32 . In operation, the ink can flow from the channel  16 , into the channel  20 , into the chamber  18 , and out through the nozzle  22 . It should be noted that the nozzles  22  shown in the figures are not to scale, and while a plurality are shown, the number is only by way of example. 
     The plate  12  (referred to hereinafter as the nozzle plate) can preferably be made of stainless steel (sometimes coated on opposite sides with gold and/or tantalum for attachment to the thick film  26 ) or a hard, thin and high wear-resistant polymer layer. Alternatively, the chamber  18  and nozzle  22  can be created from, for example, a single polymer material, as is known in the art. Such a polymer nozzle plate  12  might include, for example, slots or openings to expose interconnects  70 . 
     According to a preferred embodiment of the present invention, the printhead  10  comprises a plurality of heaters  24 . Although the plurality of heaters  24  can be arranged within one chamber  18 , and portions of an individual heater can be arranged within a plurality of chambers, each of the heaters is preferably arranged in a respective one of a plurality of chambers. One advantage of arranging each heater  24  in a respective chamber  18  is that this tends to reduce “cross talk” between the heaters, as can be understood by one of ordinary skill in the art. 
     As depicted in FIG. 9, in a further preferred embodiment, a printhead chip  11  can be formed with an array of heaters  24 , as well as active elements  72  (such as semi-conductor devices capable of being formed in silicon), on the substrate base  14 . Each heater  24  can be connected to an active circuit  72  comprising, for example, a field effect transistor (FET), arranged on opposite sides of the arrays of heaters. The heaters  24  and active elements  72  are preferably arranged on the surface  15  of the base  14  in longitudinally extending arrays, wherein one heater is associated with each nozzle  22 . The chip  11  can also include data and address lines (not shown) connecting the active devices to the interconnects  70 , which are typically located along the periphery of the chip  11 . 
     Depending upon the physical orientation of the nozzle plate  12  relative to the print receiving medium  32 , the vertical height or extent, the diameter of the nozzles  22  and the spacings between nozzles determine the vertical size of the print swath, and the horizontal width and spacing determine the packing density and firing rate of the printhead  10 . As printing speeds and resolution density increase, larger and larger arrays of elements are required. 
     In the above structure, when printing occurs, simultaneously with the movement of the carrier  40  in the direction of the arrow  42  in FIG. 1, each heater  24  can be selectively driven with a power density in accordance with recording data so that the heater nucleates the ink and ejects a droplet from the nozzles  22  in the nozzle plate  12 . The ink droplets impinge upon the surface of the print receiving medium  32 , wherein they form the recording information on the print receiving medium. For example, a computer controlled switching program and apparatus can selectively connect an appropriate energy source to the pads  28  as required to “fire” the heaters  24  in a sequence necessary to meet the computer directed graphic requirements of the recording data. 
     Referring to FIGS. 10-13, in general, multi-color (color) printheads  210  separately and selectively eject inks of at least two different colors, typically through associated dedicated apertures  22 . In contrast, mono-ink (mono) printheads  110  generally eject ink of a single color through each aperture  22 . Typically, multi-color (color) ink (i.e., an ink capable of taking on a number of different colors—e.g., through the addition of dyes or pigments) is utilized with color printheads  210 , while mono ink (i.e., ink specifically created for a particular color, such as black) is utilized with mono printheads  110 . 
     According to the present invention, an improved printhead  10  preferably has a nozzle plate thickness (T) less than the nominal value (e.g., 62 microns) and a power density less than the nominal value (e.g., 2.159 GW/m 2 ). For example, a thickness (T) less than about 60 microns is preferred, with a thickness (T) of from about 35 microns to about 55 microns being more preferred. In particular, a thickness (T) of about 40 microns appears to be especially beneficial when using phosphate-containing multi-color inks, and a thickness (T) of about 51 microns appears to be especially beneficial when using non-phosphate multi-color inks, particularly when using heaters having a surface area (A) of about 1850 microns 2 . 
     Preferably, a power density less than about 2 GW/m 2  and, more preferably, from about 0.7 GW/m 2  to about 1.5 GW/m 2 , should be selectively applied when firing a transducer, such as a heater  24 . In particular, using a power density of about 1 GW/m 2  appears to be especially beneficial for transducer life. In addition, using a non-phosphate multi-color ink instead of a phosphate-containing multi-color ink is also preferred, particularly when using low power densities (e.g., less than 2 GW/m 2 ) or when using thicker nozzle plates  12  (e.g., where T is greater that 40 microns). 
     The separation distance (S) between the nozzle plate  12  and the transducer is also preferably reduced to less than the nominal value (e.g., 28 microns). For example, a separation distance (S) of from about 8 microns to about 27 microns would be preferred. In particular, a separation distance (S) of about 24 microns appears to be especially beneficial. 
     As a further example, for a printhead  10  having a nominal power density of 1.424 GW/m 2 , a preferred embodiment of the present invention might utilize, for example, power densities less than about 1.3 GW/m 2  and, more preferably, from about 0.7 GW/m 2  to about 1 GW/m 2 . In particular, a power density of about 0.77 GW/m 2  appears to be especially beneficial for transducer life. 
     In yet another preferred embodiment of the present invention, an improved printhead  10  utilizing a mono ink or a non-phosphate multi-color ink with heaters  24 , includes heaters having a surface area (A) greater than the nominal value (e.g., about 1,900 microns 2 ). For example, such printheads  10  tested with heaters  24  having surface areas (A) of about 2,900 microns 2  appear to have an increased life. In contrast, printheads  10  utilizing a phosphate-containing multi-color ink with heaters  24  preferably use heaters having surface areas (A) less than the nominal value (e.g., about 2,800 microns 2 ). For example, such printheads  10  tested with heaters  24  having surface areas (A) of about 1,850 microns 2  appear to have an increased life. 
     In addition, according to yet a further preferred embodiment, using a mono ink instead of a multi-color ink also appears to increase transducer life. This embodiment proves especially beneficial, for example, when utilized with printheads  10  using higher power densities (e.g., greater than 1 GW/m 2 ) or with printheads  10  having shorter nozzle plate separation distances (S) (e.g., less than 28 microns). Similarly, it appears that printheads  10  with shorter nozzle plate separation distances (S) (e.g., less than 28 microns) are more beneficial when utilized with mono inks or with heaters having smaller heater areas (A) (e.g., less than about 2,800 microns 2 ). 
     The following examples demonstrate various embodiments of the invention, and have been provided for purposes of illustration and description. The examples are not intended to be exhaustive or to limit the invention to the precise forms disclosed. 
     EXAMPLE 1 
     Color printheads  210  and mono printheads  110 , similar to those shown in FIG. 10-13, were manufactured according to various embodiments of the present invention. Although utilizing heaters with different areas (A) (color=1,849 microns 2 ; mono=2,888 microns 2 ), the manufactured printheads had a comparable number of heaters. The printheads were then tested with different inks and power densities, with the results being shown in Table 1, wherein “wafer batch” merely refers to the production batch in which the wafer for the respective printhead was manufactured. In this and the remaining examples, barrier height (H) is given prior to attachment of the respective nozzle plate  12 . Typically, once the respective plate  12  has been attached, the nozzle plate separation distance (S) is about 2 microns less than the barrier height (H) prior to attachment of the nozzle plate. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 (H) Barrier 
                   
                 Average 
               
               
                   
                   
                 (A) Heater 
                 Power Density 
                 (T) Nozzle Plate 
                 Heights 1   
                   
                 Observed MTTF 3   
               
               
                 Printhead 
                 Wafer Batch 
                 Area (μm 2 ) 
                 (GW/m 2 ) 
                 Thickness (μm) 
                 (μm) 
                 Ink 2   
                 (M) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1-1 
                 2 
                 1,849 
                 1 
                 40 
                 26 
                 Color - NP 
                 224 [125, 401] 
               
               
                 1-2 
                 2 
                 2,888 
                 0.77 
                 51 
                 30 
                 Color - NP (D) 
                 328 [247, 434] 
               
               
                 1-3 
                 2 
                 2,888 
                 1.8 
                 40 
                 26 
                 Color - NP 
                  17 [15, 19] 
               
               
                 1-4 
                 1 
                 2,888 
                 1.8 
                 40 
                 30 
                 Color - NP (D) 
                  81 [74, 88] 
               
               
                 1-5 
                 1 
                 2,888 
                 1.8 
                 51 
                 26 
                 Mono 
                  16 [13, 20] 
               
               
                 1-6 
                 2 
                 2,888 
                 1.8 
                 51 
                 30 
                 Color - P 
                  15 [12, 20] 
               
               
                 1-7 
                 1 
                 1,849 
                 1 
                 40 
                 30 
                 Color - NP (D) 
                  50 [27, 95] 
               
               
                 1-8 
                 2 
                 1,849 
                 1.9 
                 40 
                 30 
                 Color - P 
                  30 [20, 46] 
               
               
                 1-9 
                 1 
                 1,849 
                 1.9 
                 40 
                 26 
                 Mono 
                 206 [140, 301] 
               
               
                  1-10 
                 1 
                 2,888 
                 0.77 
                 51 
                 26 
                 Color - NP 
                 335 [284, 396] 
               
               
                  1-11 
                 2 
                 2,888 
                 0.77 
                 40 
                 30 
                 Mono 
                 259 [190, 354] 
               
               
                  1-12 
                 1 
                 2,888 
                 0.77 
                 40 
                 26 
                 Color - P 
                 142 [92, 221] 
               
               
                  1-13 
                 1 
                 1,849 
                 1 
                 51 
                 30 
                 Mono 
                  75 [50, 113] 
               
               
                  1-14 
                 1 
                 2,888 
                 1.3 
                 40 
                 30 
                 Color - NP 
                 125 [90, 175] 
               
               
                  1-15 
                 2 
                 2,888 
                 1.3 
                 40 
                 26 
                 Color - NP (D) 
                  25 [22, 30] 
               
               
                  1-16 
                 2 
                 1,849 
                 1 
                 51 
                 26 
                 Color - P 
                 173 [124, 239] 
               
               
                   
               
               
                   1 Prior to attachment of nozzle plate  
               
               
                   2 Color - NP (D) dyeless non-phosphate multi-color ink  
               
               
                     Color - NP = non-phosphate multi-color ink  
               
               
                     Color - P = phosphate-containing multi-color ink  
               
               
                     Mono = monocolor ink.  
               
               
                   3 MTTFs (median time to failures) represent the number of fires before failure, in millions (M), and the bracketed values represent the 95% confidence intervals  
               
            
           
         
       
     
     The median time to failure (MTTF) is a common measure of the average life of a heater. Generally, the higher the MTTF, the more reliable the printhead. The MTTFs discussed herein are given in terms of numbers of fires before failure (in millions). 
     A printhead was considered to have failed after the first heater failure. In this experiment, failure was considered to have occurred when the resistance of at least one heater increased by approximately 1.5 times its nominal value. All failures were confirmed optically. 
     After completion of this experiment, a regression equation was produced to further model and test the present invention. Table 1A shows a comparison of the model predictions to the observed values for the printheads tested in Table 1. The model was also tested by running up to three printheads at each set of conditions. The observed MTTF and the model predictions for these printheads are shown in Table 2. 
     
       
         
           
               
               
               
             
               
                 TABLE 1A 
               
               
                   
               
               
                   
                 Average Observed MTTF +   
                   
               
               
                 Printhead 
                 (M) 
                 Predicted MTTF + (M) 
               
               
                   
               
             
            
               
                 1-1 
                 224 [125, 401] 
                 242 [162, 362] 
               
               
                 1-2 
                 328 [247, 434] 
                 236 [131, 424] 
               
               
                 1-3 
                  17 [15, 19] 
                  14 [8, 22] 
               
               
                 1-4 
                  81 [74, 88] 
                  81 [38, 171] 
               
               
                 1-5 
                  16 [13, 20] 
                  19 [13, 29] 
               
               
                 1-6 
                  15 [12, 20] 
                  15 [8, 27] 
               
               
                 1-7 
                  50 [27, 95] 
                  47 [31, 72] 
               
               
                 1-8 
                  30 [20, 46] 
                  29 [18, 46] 
               
               
                 1-9 
                 206 [140, 301] 
                 177 [120, 263] 
               
               
                  1-10 
                 335 [284, 396] 
                 369 [203, 671] 
               
               
                  1-11 
                 259 [190, 354] 
                 271 [181, 405] 
               
               
                  1-12 
                 142 [92, 221] 
                 123 [78, 195] 
               
               
                  1-13 
                  75 [50, 113] 
                  86 [53, 142] 
               
               
                  1-14 
                 125 [90, 175] 
                 116 [56, 239] 
               
               
                  1-15 
                  25 [22, 30] 
                  32 [20, 53] 
               
               
                  1-16 
                 173 [124, 239] 
                 130 [82, 206] 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 (T) Nozzle 
                   
                   
                   
                 Average 
                   
               
               
                   
                 Wafer 
                 (A) Heater 
                 Power Density 
                 Plate Thickness 
                 (H) Barrier 
                   
                 No. of 
                 Observed 
                 Predicted* 
               
               
                 Printhead 
                 Batch 
                 Area (μm 2 ) 
                 (GW/m 2 ) 
                 (μm) 
                 Height (μm) 
                 Ink 
                 Obs. 
                 MTTF (M) 
                 Range (M) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 2-1 
                 2 
                 2,888 
                 1.424 
                 62 
                 hi 
                 Mono 
                 3 
                  41 
                 25-60 
               
               
                 2-2 
                 2 
                 2,888 
                 1.424 
                 40 
                 low 
                 Mono 
                 3 
                  73 
                  68-207 
               
               
                 2-3 
                 2 
                 2,888 
                 0.77 
                 40 
                 low 
                 Mono 
                 3 
                 189 
                 203-629 
               
               
                 2-4 
                 2 
                 1,849 
                 2.159 
                 40 
                 low 
                 Color - NP 
                 2 
                  50 
                 10-28 
               
               
                 2-5 
                 2 
                 1,849 
                 0.99 
                 40 
                 low 
                 Color - NP 
                 2 
                 221 
                 162-362 
               
               
                 2-6 
                 1 
                 1,849 
                 0.99 
                 40 
                 low 
                 Color - P 
                 1 
                 120 
                 103-308 
               
               
                 2-7 
                 2 
                 1,849 
                 0.99 
                 40 
                 low 
                 Color - P 
                 1 
                 214 
                 133-410 
               
               
                 2-8 
                 2 
                 1,849 
                 2.159 
                 40 
                 low 
                 Color - P 
                 2 
                  28 
                 16-36 
               
               
                 2-9 
                 1 
                 1,849 
                 2.159 
                 62 
                 hi 
                 Color - NP 
                 2 
                 6.7 
                 10-23 
               
               
                  2-10 
                 1 
                 1,849 
                 2.159 
                 62 
                 hi 
                 Color - P 
                 2 
                 4.7 
                  6-15 
               
               
                   
               
               
                 *range is 95% confidence interval for MTTF.  
               
            
           
         
       
     
     The discrepancies between the predicted and observed failure times may be due to the use of a different batch to test the model. However, some batch to batch variation is unavoidable. In all cases, the 95% confidence bounds of the model predictions are expressed as a long term average of a large sample. With only a few printheads tested at each condition, perfect agreement between model and observation is not expected, as can be understood by one of ordinary skill in the art. Still, an empirical model appears to be an effective predictor of printhead failure, and was used to develop the subsequent experiments. 
     EXAMPLE 2 
     The color printheads used for these experiments have a nominal barrier height (H) of about 30 microns (prior to attachment of the nozzle plate) and a nominal nozzle plate thickness (T) of about 62 microns, and are fired using a nominal power density of about 2.159 GW/m 2 . Generally, once attached, the nozzle plate separation distance (S) is about 2 microns less than the barrier height (H) prior to attachment. As shown in Table 3, when using a phosphate-containing color ink, such as a dye-based magenta ink, such a printhead has a predicted life of about 9.7M, where M signifies the number of fires in millions, with 95% confidence bounds of [6M, 15M]. Reducing the nozzle plate thickness (T) to about 40 microns increases the predicted MTTF from about 9.7M to about 30M [19, 48], thereby tripling the expected life of the printhead. Meanwhile, also reducing the barrier height (H) to about 26 microns (prior to attachment) increases the predicted MTTF of the printhead to about 31M [21, 47]. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 (A) Heater 
                 Power Density 
                 (T) Nozzle Plate 
                 (H) Barrier 
                   
                 Predicted 
                 Predicted+ 
               
               
                 Printhead 
                 Wafer Batch 
                 Area (μm 2 ) 
                 (GW/m 2 ) 
                 Thickness (μm) 
                 Heights 1  (μm) 
                 Ink 
                 MTTF (M) 
                 Range (M) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 3-1 
                 1 
                 1,849 
                 2.159* 
                 62* 
                 30* 
                 Color - P 
                 9.7 
                  6-15 
               
               
                 3-2 
                 1 
                 1,849 
                 2.159* 
                 40  
                 30* 
                 Color - P 
                 30 
                 19-48 
               
               
                 3-3 
                 1 
                 1,849 
                 2.159* 
                 40  
                 26  
                 Color - P 
                 31 
                 21-47 
               
               
                 3-4 
                 1 
                 1,849 
                 2.159* 
                 62* 
                 30* 
                 Color - NP 
                 15 
                 9.7-48  
               
               
                 3-5 
                 1 
                 1,849 
                 2.159* 
                 40  
                 26  
                 Color - NP 
                 22 
                 13-36 
               
               
                 3-6 
                 1 
                 1,849 
                 2.159* 
                 55  
                 26  
                 Color - NP 
                 26 
                 17-36 
               
               
                 3-7 
                 1 
                 1,849 
                 1.5 
                 62* 
                 30* 
                 Color - P 
                 18 
                 10-30 
               
               
                 3-8 
                 1 
                 1,849 
                 1.5 
                 40  
                 26  
                 Color - P 
                 82 
                  54-125 
               
               
                 3-9 
                 1 
                 1,849 
                 1.5 
                 62* 
                 30* 
                 Color - NP 
                 39 
                 26-60 
               
               
                  3-10 
                 1 
                 1,849 
                 1.5 
                 51  
                 26  
                 Color - NP 
                 95 
                  62-144 
               
               
                  3-11 
                 1 
                 1,849 
                 1 
                 62* 
                 30* 
                 Color - P 
                 37 
                 19-74 
               
               
                  3-12 
                 1 
                 1,849 
                 1 
                 40  
                 26  
                 Color - P 
                 234  
                 133-410 
               
               
                  3-13 
                 1 
                 1,849 
                 1 
                 62* 
                 30* 
                 Color - NP 
                 110  
                  70-174 
               
               
                  3-14 
                 1 
                 1,849 
                 1 
                 51  
                 26  
                 Color - NP 
                 348  
                 224-537 
               
               
                   
               
               
                   1 Prior to attachment of nozzle plate  
               
               
                 *nominal dimension  
               
               
                 +range is 95% confidence interval for MTTF  
               
            
           
         
       
     
     As can be understood from Table 3, when a lower power density is used with the nominal color printheads, the life of the printheads also increase. For example, when a power density of about 1.5 GW/m 2  is applied to the nominal color printhead, the predicted life of the nominal printhead increases to about 18M [10, 30]. Moreover, applying a power density of about 1 GW/m 2  increases the predicted life of the nominal color printhead  210  to about 37M [19, 74]. 
     Accordingly, it appears that reducing the nozzle plate thickness (T) and barrier height (H) can produce an improvement in printhead life. Moreover, reducing the power density also increases printhead life. However, as shown below, applying a reduced power density in combination with the aforementioned reduced dimensions leads to an unexpectedly large increase in printhead life. For example, decreasing the power density to 1 GW/m 2  and reducing the nozzle plate thickness (T) and barrier height (H) (prior to attachment) to 40 microns and 26 microns respectively, increases the predicted MTTF of the printhead to about 234M [133, 410], about six times greater than the predicted life of a nominal color printhead operated under nominal conditions. 
     FIG. 14 is a contour plot of the natural logarithm of life of a heater as a function of nozzle plate thickness (T) and power density for the color ink jet printhead using a dye-based, phosphate-containing magenta ink. For this plot, the barrier was set to the nominal height (H) of 30 microns (prior to attachment). The curved contours of the plot indicate that power density and nozzle plate thickness (T) interact. 
     From the plot, it can thus be understood that lower power densities and thinner nozzle plates produce the longest life. The behavior is essentially the same for a barrier height (H) of 26 microns (prior to attachment). Therefore, the MTTF of a printhead can be greatly improved by decreasing the power density, the nozzle plate thickness (T) and barrier height (H). 
     As shown in FIGS. 15-16, when using a color ink containing no phosphates, such as a dye-based, non-phosphate magenta ink, the interaction between power and nozzle plate thickness (T) appears to be weaker. For example, the MTTF of a printhead using a power density of 2.159 GW/m 2  and non-phosphate color ink, and having a nozzle plate thickness (T) and barrier height (H) (prior to attachment) of 40 and 26 microns, respectively, is 22M [13, 36], which is shorter than that seen with a phosphate-containing color ink. A shorter life using a non-phosphate color ink was unexpected, since previous tests had shown that the MTTFs of printheads using a non-phosphate color ink should have been at least as long as the MTTFs of printheads using a phosphate-containing color ink. 
     For lower power densities, the life of a printhead using a non-phosphate color ink appears to be slightly longer for a nozzle plate thickness (T) of about 50 microns, than for the minimum tested thickness (T) of 40 microns. For example, by increasing the nozzle plate thickness (T) to 55 microns, the MTTF can be slightly improved to 26 M [17, 36]. Thus, the optimum value for the nozzle plate thickness (T) may not always be the minimum. 
     Using a power density of 1 GW/m 2  with the minimum tested values for nozzle plate thickness (T) and barrier height (H), the predicted MTTF when using a non-phosphate color ink rises to 309 M [202, 471]. However, an additional improvement can be obtained by increasing nozzle plate thickness (T) from 40 to 51 microns. In this case, the predicted MTTF is 348 M [224, 537]. Accordingly, at lower power densities, non-phosphate color ink appears to give a longer MTTF than phosphate-containing color ink. Moreover, it appears that increasing the phosphate content of an ink to be used with a printhead will adversely affect the reliability of such a printhead. 
     Accordingly, the interactions between the variables must be known in order to choose the optimum operating conditions. For example, the best nozzle plate thickness (T) tested with a phosphate-containing color ink is 40 microns, but for a non-phosphate color ink, a higher MTTF was achieved with a nozzle plate thickness (T) of about 50 microns, depending on power level, etc. Moreover, although the longest observed life was attained by reducing the power density to 1 GW/m 2 , such a power density can be unacceptable with conventional printheads due to diminished print quality. However, the model can be used to reach a compromise by predicting the MTTF for a desired power density. 
     The trends shown by this model and the tested data should continue outside the tested ranges. For example, the generally unexpectedly large increase in predicted printhead life should continue for printheads with nozzle plate thicknesses (T), barrier heights (H), and power densities below the minimum tested values of 40 microns, 26 microns (prior to attachment), and 0.77 GW/m 2  respectively. Accordingly, these arbitrarily chosen test values should not be viewed as limits with respect to the present invention. 
     However, under the current state of the art, the minimum practical values for the nozzle plate thickness (T), barrier height (H), and power density are approximately 35 microns, 10 microns (prior to attachment), and 0.7 GW/m 2  respectively. As can be understood, these practical values reflect the current state of the art and not the present invention. For example, although a power density of about 0.7 GW/m 2  is currently needed to nucleate ink above a particular heater, this practical limitation in the art could be overcome with new technology that might enable the use of thinner protective layers over the heater, thereby requiring the application of less power to the heater. 
     EXAMPLE 3 
     Table 4 gives a summary of model predictions for a mono printhead under various conditions. The behavior of the mono printhead was much the same as the color printhead. For example, from FIG. 17, it can be understood that lowering the power density and thinning the nozzle plate can improve heater life. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Print- 
                 Wafer 
                 (A) Heater Area 
                 Power Density 
                 (T) Nozzle Plate 
                 (H) Barrier 
                   
                 Predicted 
                 Predicted+ 
               
               
                 head 
                 Batch 
                 (μm 2 ) 
                 (GW/m 2 ) 
                 Thickness (μm) 
                 Heights 1  (μm) 
                 Ink 
                 MTTF (M) 
                 Range (M) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 4-1 
                 1 
                 2,888 
                 1.424* 
                  62* 
                 30* 
                 Mono 
                  51 
                 33-79 
               
               
                 4-2 
                 1 
                 2,888 
                 1.424* 
                 40 
                 30* 
                 Mono 
                 153 
                  96-245 
               
               
                 4-3 
                 1 
                 2,888 
                 1.424* 
                 40 
                 27  
                 Mono 
                 156 
                  88-275 
               
               
                 4-4 
                 1 
                 2,888 
                 0.77 
                  62* 
                 30* 
                 Mono 
                 107 
                  70-163 
               
               
                 4-5 
                 I 
                 2,888 
                 0.77 
                 40 
                 30* 
                 Mono 
                 355 
                 234-539 
               
               
                 4-6 
                 1 
                 2,888 
                 0.77 
                 40 
                 27  
                 Mono 
                 468 
                 266-823 
               
               
                   
               
               
                   1 Prior to attachment of nozzle plate  
               
               
                 *nominal dimension  
               
               
                 +range is 95% confidence interval for MTTF  
               
            
           
         
       
     
     The nominal power density for the mono printheads was about 1.424 GW/m 2 . With nominal nozzle plate thicknesses (T) and barrier heights (H) (prior to attachment) of 62 microns and 30 microns, respectively, the predicted MTTF for the nominal mono printheads using a mono ink, such as a dye-based black ink, was 51 M [33, 79]. At nominal power with mono ink, the optimum tested values for the barrier height (H) (prior to attachment) and the nozzle plate thickness (T) were 27 and 40 microns respectively. Under these circumstances, the MTTF of the mono printhead was predicted to be about 156 M [88, 275], which is three times higher than the nominal configuration. If the power density is further reduced to 0.77 GW/m 2 , (with all other variables constant) the predicted MTTF goes up to 468 M [266, 823]. 
     EXAMPLE 4 
     Table 5 gives a summary of predicted printhead life with two different heater areas under different conditions. From past experiments, it was believed that printhead life decreased as heater area (A) was reduced. This belief is only partially validated by the present invention. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Wafer 
                 (A) Heater 
                 Power Density 
                 (T) Nozzle Plate 
                 (H) Barrier 
                   
                 Predicted 
                 Predicted* 
               
               
                 Printhead 
                 Batch 
                 Area (μm 2 ) 
                 (GW/m 2 ) 
                 Thickness (μm) 
                 Heights 1  (μm) 
                 Ink 
                 MTTF (M) 
                 Range (M) 
               
               
                   
               
             
            
               
                  5-1 
                 1 
                 1,849 
                 2 
                 62 
                 30 
                 Mono 
                 23 
                 13-41 
               
               
                  5-2 
                 1 
                 2,888 
                 2 
                 62 
                 30 
                 Mono 
                 38 
                 21-69 
               
               
                  5-3 
                 1 
                 1,849 
                 2 
                 62 
                 30 
                 Color - NP 
                 18 
                 12-27 
               
               
                  5-4 
                 1 
                 2,888 
                 2 
                 62 
                 30 
                 Color - NP 
                 24 
                 13-42 
               
               
                  5-5 
                 1 
                 1,849 
                 2 
                 62 
                 30 
                 Color - P 
                 11 
                  7-17 
               
               
                  5-6 
                 1 
                 2,888 
                 2 
                 62 
                 30 
                 Color - P 
                 9 
                  5-15 
               
               
                  5-7 
                 1 
                 1,849 
                 2 
                 40 
                 30 
                 Mono 
                 75 
                  49-116 
               
               
                  5-8 
                 1 
                 2,888 
                 2 
                 40 
                 30 
                 Mono 
                 104  
                  54-204 
               
               
                  5-9 
                 1 
                 1,849 
                 2 
                 40 
                 30 
                 Color - NP 
                 26 
                 12-53 
               
               
                 5-10 
                 1 
                 2,888 
                 2 
                 40 
                 30 
                 Color - NP 
                 28 
                 14-56 
               
               
                 5-11 
                 1 
                 1,849 
                 2 
                 40 
                 30 
                 Color - P 
                 35 
                 22-55 
               
               
                 5-12 
                 1 
                 2,888 
                 2 
                 40 
                 30 
                 Color - P 
                 23 
                 10-51 
               
               
                 5-13 
                 1 
                 1,849 
                 2 
                 40 
                 30 
                 Mono 
                 149  
                  86-260 
               
               
                 5-14 
                 1 
                 2,888 
                 2 
                 40 
                 30 
                 Mono 
                 251  
                 167-378 
               
               
                 5-15 
                 1 
                 1,849 
                 1 
                 40 
                 30 
                 Color - NP 
                 186  
                  90-384 
               
               
                 5-16 
                 1 
                 2,888 
                 1 
                 40 
                 30 
                 Color - NP 
                 249  
                 115-535 
               
               
                 5-17 
                 1 
                 1,849 
                 1 
                 40 
                 30 
                 Color - P 
                 140  
                 188-285 
               
               
                 5-18 
                 1 
                 2,888 
                 1 
                 40 
                 30 
                 Color - P 
                 113  
                  66-196 
               
               
                 5-19 
                 1 
                 1,849 
                 1 
                 40 
                 26 
                 Mono 
                 490  
                 254-945 
               
               
                 5-20 
                 1 
                 2,888 
                 1 
                 40 
                 27 
                 Mono 
                 303  
                 178-513 
               
               
                 5-21 
                 1 
                 1,849 
                 1 
                 40 
                 26 
                 Color - NP 
                 309  
                 202-471 
               
               
                 5-22 
                 1 
                 2,888 
                 1 
                 40 
                 27 
                 Color - NP 
                 151  
                  93-244 
               
               
                 5-23 
                 1 
                 1,849 
                 1 
                 40 
                 26 
                 Color - P 
                 234  
                 133-410 
               
               
                 5-24 
                 1 
                 2,888 
                 1 
                 40 
                 27 
                 Color - P 
                 68 
                  44-104 
               
               
                   
               
               
                   1 Prior to attachment of nozzle plate  
               
               
                 *predictions with 95% confidence bounds  
               
            
           
         
       
     
     While printheads with smaller heater areas (A) may exhibit lower reliability than those with larger heater areas (A) (depending on the power density, ink, and nozzle plate and barrier dimensions), it appears to be evident from Table 5 that, when using phosphate-containing color inks, printheads featuring smaller heater areas (A) tend to last longer than those featuring larger heater areas (A). On the other hand, the presence of mono ink causes printheads featuring smaller heater areas (A) to fail earlier, except under conditions of power density=1 GW/m 2 , nozzle plate thickness (T)=40 microns, and barrier height (H) (prior to attachment)=26 microns (or 27 microns for mono). Therefore, Table 5 shows that heater area (A) can also play a role in reliability, depending on power density, nozzle plate thickness (T), barrier height (H), and ink type. 
     While the invention directly applies to the printheads tested, its implications are broader. For example, reducing the power density while simultaneously reducing nozzle plate thickness (T) and barrier height (H) should greatly improve printhead reliability. Moreover, at low power densities and with reduced chamber dimensions, printheads  10  featuring smaller heater areas (A) tend to last longer than those featuring larger heater areas (A). In addition, although these trends should be observed for any ink type, the choice of a non-phosphate containing color ink, can further improve reliability at lower powers. 
     Under nominal power density, an improvement in MTTF can be obtained by lowering the nozzle plate thickness (T) and barrier height (H). Reducing the power density while keeping nozzle plate thickness (T) and barrier height (H) nominal also increases the MTTF. By reducing all three factors, a very large improvement in life can be achieved. Moreover, choice of heater area (A) depends on how the previous three factors are set, as does the choice of ink. In a preferred embodiment, the optimum conditions would be derived from the empirical model, which takes interactions between these variables into account. 
     The foregoing description of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings. For example, although a number of materials and shapes have been described or shown for use in the preferred embodiments of the present invention, it is to be understood that other materials and shapes could be used as alternatives to those described or shown without departing from the scope of the invention. 
     In particular, although the chamber  18  has been shown as having a generally square-shaped conformation, it could have a variety of shapes such as, for example, any other generally polygonal, circular, or similar shaped conformation. Similarly, although the barrier  27  is depicted in the several figures as being formed from a thick film layer  26  extending above the heater  24  a generally uniform height (H), the barrier need not necessarily be formed from the thick film or any other layer, and the height (H) could be variable. Further examples of modifications and variations within the scope of the present invention may include using other varieties of transducers, such as piezo-electric elements for example, providing the chamber  18  and transducer within a printhead  10  without using a chip  11 , providing the ink to the chamber  18  according to alternative arrangements not shown by the various figures, such as by using an edge-feed arrangement, eliminating the conduit laterals  20 , and/or eliminating the channel  16  altogether, and utilizing a configuration other than a configuration known in the art as a roof shooter, such as side shooter configuration for example. 
     Similarly, the various figures have been provided in order to illustrate various features of the present invention. They should not be viewed as restrictive in nature. For example, the various figures are not always depicted in scale nor should they be so interpreted. 
     Thus, it should be understood that the embodiments were chosen and described in order to best illustrate the principals of the invention and its practical application. This illustration was provided to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited for the particular use contemplated. Accordingly, it is intended that the scope of the invention be defined by the claims appended hereto.