Abstract:
A printhead ( 30 ) for a printer cartridge comprises a die including a plurality of nozzles ( 32 ) and channels for admitting ink to said nozzles. A resistor network comprises a resistor ( 29 ) at each nozzle, which when energised heats and expels ink from the respective nozzle. The printhead includes a set of contact pads ( 33 ), each resistor being connected by a trace ( 34 ) to a respective contact apd of the set of contact pads. One or more of the set of contact pads (R 50 , ID 2 ) is adapted to receive more than one connection ( 38 ) from a circuit ( 35 ) external to said printhead.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a printhead for a printer cartridge, a printer cartridge including such a printhead and a method of fabricating a printer cartridge.  
         BACKGROUND OF THE INVENTION  
         [0002]    [0002]FIG. 1 shows a known form of ink jet cartridge, for example, a HP51649G from Hewlett Packard. In this case, the cartridge  10  comprises a body  20  including three chambers (not shown) individually isolated from one another for containing different colours of ink. A plug  22  seals the body  20  and the individual chambers.  
           [0003]    Ink from the chambers is emitted from a printhead  30  which carries three sets of nozzles  32 . Each set of nozzles is arranged to spray one colour of ink and comprises 16 individual nozzles, carried in two parallel rows.  
           [0004]    The operation of the nozzles is well known to those skilled in the art and will be explained further below. Suffice it to say that the nozzles are controlled by resistors located in the printhead, and each resistor is under the control of external printer control circuitry. This circuitry is connected to the cartridge via a series of contacts  40 . The contacts  40  are located on a flexible circuit  35 , and flexible circuit  35  carries traces (not visible) from the flexible circuit contacts  40  to a plurality of wire bonds (not visible), also known as beams, which in turn connect the circuit traces to printhead contact pads. In other words, the flexible circuit  35  with its contact pads  40 , circuit traces and wire bonds, acts to make a connection between the contact pads of the printhead  30  and the external printer control circuitry.  
           [0005]    It is a problem with such cartridges that the wire bonds connecting the flexible circuit to the printhead can fail due to corrosion, manufacturing defect or mishandling.  
         DISCLOSURE OF THE INVENTION  
         [0006]    According to the present invention there is provided a printhead for a printer cartridge comprising a die including a plurality of nozzles and channels for admitting ink to said nozzles, a resistor network having a resistor at each nozzle, which resistor when energised heats and expels ink from the respective nozzle, said printhead including a set of contact pads, each resistor being connected by a trace to a respective contact pad of said set of contact pads, characterised in that one or more of said set of contact pads is adapted to receive more than one connection from a circuit external to said printhead and in that said one or more contacts comprises one contact directly connected to another otherwise redundant one of said set of contact pads within said printhead to receive two contacts from said external circuit.  
           [0007]    In a further aspect of the present invention there is provided a printer cartridge comprising a body incorporating one or more ink chambers in fluid communication with a printhead comprising a die including a plurality of nozzles and channels for admitting ink to said nozzles, a resistor network having a resistor at each nozzle, which resistor when energised heats and expels ink from the respective nozzle, said printhead including a set of contact pads, each resistor being connected by a trace to a respective contact pad of said set of contact pads, and a circuit affixed to said printer cartridge comprising a plurality of circuit traces, one end of at least some of said circuit traces being connected to respective contacts of said set of contact pads of said printhead and the other end of at least some of said circuit traces being arranged to, in use, connect to printer control circuitry characterised in that one or more of said set of contact pads is connected to more than one of said circuit traces and in that:said one or more contacts comprises one contact directly connected to another otherwise redundant one of said set of contact pads within said printhead.  
           [0008]    In a still further aspect of the present invention there is provided a method of fabricating a printer cartridge comprising the steps of: fabricating a body incorporating one or more ink chambers; affixing a printhead to said body, said-printhead comprising a die including a plurality of nozzles and channels for admitting ink from said chambers to said nozzles, a resistor network having a resistor at each nozzle, which resistor when energised heats and expels ink from the respective nozzle, said printhead including a set of contact pads, each resistor being connected by a trace to a respective contact pad of said set of contact pads; affixing a circuit to said printer cartridge, said circuit comprising a plurality of circuit traces, one end of at least some of said circuit traces being arranged to, in use, connect to printer control circuitry; and connecting the other end of at least some of said circuit traces to respective contacts of said set of contact pads characterised in that one or more of said set of contact pads is connected to more than one of said circuit traces and in that said one or more contacts comprises one contact directly connected to another otherwise redundant one of said set of contact pads within said printhead.  
           [0009]    Using the present invention, circuitry connected across contacts that are prone to failure can continue to operate in spite of the failure of a contact. The invention can be implemented without any changes to software controlling the printer cartridge and with relatively modest re-engineering of the printer cartridge. Also printer cartridges employing the invention can be compatible with those not employing the invention.  
           [0010]    Preferably, the printhead includes printer cartridge identification circuitry associated with one or more contact pads and one or more of these contact pads is arranged to receive more than one trace.  
           [0011]    The operation of printer cartridge identification circuitry is critical to the functionality of a printer cartridge. As distinct from a single nozzle resistor circuit, if this circuitry fails and printer controller circuitry cannot identify a printer cartridge, then the printer cartridge may not be operable at all, in spite of the cartridge retaining much of its ink supply. The preferred embodiment allows such identification circuitry to partially fail without causing the complete failure of the printer cartridge.  
           [0012]    Further preferably, one of the contacts arranged to receive more than one trace comprises one contact directly connected to another otherwise redundant one of said set of contact pads within said printhead.  
           [0013]    In this case, the minimum amount of re-engineering is required to implement and provide the benefits of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1 is a perspective view of a conventional ink jet cartridge;  
         [0016]    [0016]FIG. 2 is a schematic cross-section through the front face of the cartridge of FIG. 1;  
         [0017]    [0017]FIG. 3( a ) is a plan view of a portion of a printhead for a HP51649G printer cartridge with wire bonds affixed;  
         [0018]    [0018]FIG. 3( b ) is a schematic view of the interconnections between the printhead of FIG. 3(a) and the flexible circuit for the printer cartridge;  
         [0019]    FIGS.  4  to  7  are schematic circuits illustrating the identification circuitry employed for various models of printer cartridge;  
         [0020]    [0020]FIG. 8( a ) is a plan view of a portion of a printhead with wire bonds affixed according to a first embodiment of the invention;  
         [0021]    [0021]FIG. 8( b ) is a schematic view of the interconnections between the printhead of FIG. 8( a ) and the flexible circuit for the printer cartridge; 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Referring now to FIG. 2 which shows in cross section a portion of the body  20  of a printer cartridge  10  of FIG. 1 in more detail. Channels lead from the chambers though openings in the base of a cavity  31  located in the front of the body  20 . The printhead  30  sits on a sealing gasket  27  comprising dispensed adhesive.  
         [0023]    Each of the three channels leads via respective openings in gasket  27 , to a channel reservoir (not shown) in the printhead  30 . The gasket  27  seals the printhead  30  to the body  20  and provides isolated communication for each chamber with a selected one of the channel reservoirs in the printhead. The individual nozzles of the nozzle groups  32  (FIG. 1) are in communication with the coloured ink of the respective channel reservoir and ejection of the ink from the reservoir via an individual nozzle is under the control of the relevant resistor.  
         [0024]    Referring now to FIG. 3( a ), a corner of the printhead is shown. The corner shown is designated as the top right hand corner, in the sense that the complete printhead will extend further to the left hand side, and further down the page in the view shown, as indicated by the dashed lines which show the continuation of the top edge  30   a  and the right-hand edge  30   b.    
         [0025]    A series of contact pads  33  are provided down the right-hand edge  30   b . Corresponding pads are on the left-hand edge (not shown). These contact pads  33  comprise: (a) resistor contact pads R 1 -R 48  (of which R 48 , R 46 , R 44  and R 42  can be seen; R 40 , R 38 , R 36 , . . . , R 2  extend further down the right-hand side, while R 47 , R 45 , . . . , R 1  similarly extend down the left-hand side); (b) identification circuitry contact pads ID 1 , ID 2  and R 50  and possibly R 49  (ID 2  and R 50  can be seen; ID 1  and R 49  are on the left-hand side); and (c) voltage supply contact pads C 1 , C 2 , C 3 , C 4  (referred to also as COM 1 -COM 4 ; only pad C 4  is visible in FIG. 3 a )).  
         [0026]    [0026]FIG. 3 a  shows printhead traces  34  connecting each of the resistor contact pads R 1 -R 48  with a corresponding resistor  29  which lies below a respective nozzle  32 . During printing operations, the printer control circuitry selectively connects pads R 1  to R 48  to ground thus allowing current to flow from a common voltage supply (in this case C 4 ) through the resistors  29 . This causes the resistor to heat and form a gas bubble in the ink surrounding the resistor within the printhead, so causing ink to be dispensed through the nozzle  32  associated with the resistor  29 .  
         [0027]    As explained above, there are 3×16=48 individual nozzles, and each of these is controlled by one of the resistors  29 , to which connection is made by one of the contact pads R 1 -R 48 . The printer control circuitry in turn is connected to each of the  48  resistor contact pads by individual contacts  40 , traces and wire bonds on the flexible circuit  35 .  
         [0028]    Referring back to FIG. 1, the generally rectangular flexible circuit  35  includes an aperture  37  conforming to the outline of the printhead. The flexible circuit  35  is affixed to the cartridge by, for example, hot-melt or pressure sensitive adhesive, with the aperture surrounding the printhead  30 . The flexible circuit traces traverse the flexible circuit to the side edges of the aperture  37  and extend from the flexible circuit  35  to the contact pads  33  on the printhead as wire bonds  38 , which are shown in FIG. 2. The wire bonds including the gap between the side wall of the cavity  31  and the side of the printhead which the wire bonds must traverse are covered with an encapsulant  39 , such as an ultra-violet cured adhesive, to prevent corrosion of the exposed wire bonds for example from chemicals in the ink being dispensed. The encapsulant also offers mechanical protection for the fragile wire bonds.  
         [0029]    The contact pads  40  at the ends of the circuit traces on the flexible circuit  35  are also labelled in the same way as the corresponding contact pad on the printhead (i.e. R 1 -R 50 , ID 1 , ID 2 , C 1 -C 4 ). These pads provide an electrical connection between the printer cartridge  10  when located on a print carriage (not shown) and printer control circuitry located off the carriage.  
         [0030]    As mentioned above, some of the contacts (ID 1 , ID 2 , R 49 , R 50 ) provided on the printhead and their corresponding contacts on the flexible circuit are employed to enable the printer control circuitry to identify the printer cartridge being employed within the printer.  
         [0031]    In one scheme described in European Patent No. 315,417, contact pads ID 1  and ID 2  (designated I 1  and I 2  in the specification) may be connected to pads R 47  or R 49 ; and R 49  or R 50  respectively. The various interconnections allow  9  possible combinations and so  9  different printer cartridges can be identified using this scheme.  
         [0032]    Of course as printer cartridges are continually developed and where there is a requirement for cartridges to be interoperable with more than one printer, there arises a requirement to extend the printer cartridge identification scheme.  
         [0033]    Thus, FIG. 4 shows the interconnections the printer control circuitry expects to see between the flexible circuit contact pads  40  through to the printhead for the HP51649G printer cartridge. (This circuit is also applicable to the interchangeable HP51649A printer cartridge.) In this case, contacts ID 2 , R 46  and R 50  are employed to identify the printer cartridge. The contacts R 46  and R 50  are connected within the printhead via a circuit trace  41 , also shown in FIG. 3( a ).  
         [0034]    In order to identify the printer cartridge, the control circuitry checks to determine if contact ID 2  is open circuited. The printer control circuitry also open circuits R 46  and measures the voltage/current at contact R 50 . If this corresponds with the expected voltage drop or current across the trace  41  connecting contact R 46  to R 50 , then the control circuitry knows the printer cartridge is indeed a model HP51649G. As an aside a further trace TSR links contact pads ID 1  and R 49  within the printhead. The resistance of this trace varies with temperature and by monitoring a voltage drop/current across these contacts at any time, the printer control circuitry can determine the operating temperature of the printhead.  
         [0035]    [0035]FIG. 5 shows the circuitry the printer control circuitry expects to see between the flexible circuit contact pads  40  through to the printhead for a model HP51625A printer cartridge. In this case, the printer control circuitry expects to see that the contacts ID 2  and R 50  are open circuited and that there is a predetermined voltage drop across the link  42  between contacts ID 1  and R 49 . There is no printhead temperature monitoring circuitry.  
         [0036]    [0036]FIG. 6 shows the circuitry the printer control circuitry expects to see between the flexible circuit contact pads  40  through to the printhead for a model HP51626A printer cartridge. In this case, the printer control circuitry expects to see that there is a predetermined voltage drop across the link  43  between contacts ID 2  and R 50  and that the contact ID 1  is open circuited. Again, there is no printhead temperature monitoring circuitry.  
         [0037]    [0037]FIG. 7 shows the circuitry the printer control circuitry expects to see between the flexible circuit contact pads  40  through to the printhead for a model HP51629A printer cartridge. In this case, the printer control circuitry expects to see that there are predetermined voltage drops across the links  44  and  45  between contacts R 46  and ID 2 ; and between contacts R 48  and R 50  respectively. Again, a further TSR trace links contact pads ID 1  and R 49  within the printhead.  
         [0038]    If other cartridges are installed, then the control circuitry could for example test other combinations of contacts to determine the model of printer cartridge.  
         [0039]    In any case, once identified the printer control circuitry can drive the contact pads appropriately to time the dispensing of ink properly.  
         [0040]    It will therefore be seen that the correct identification of the printer cartridge is vital in enabling a printer to function. If the identification circuitry fails, in spite of the remainder of the cartridge being functional and possibly only a fraction of the cartridge ink being used, then the printer control circuitry may not be able to operate the cartridge properly, if at all. For example, in the case of a model HP51649G, it may not be desirable, if the printer control circuitry fails to identify the model correctly, for the control circuitry simply to assume the printer cartridge is a black ink model cartridge, as the operation of such a cartridge may not be compatible with the colour cartridge.  
         [0041]    A common cause of such a failure is due to the seal provided by the encapsulant  39  eroding and in turn exposing the wire bonds  38  to corrosion or short-circuits, FIG. 2. Because the identification circuitry is located towards the extreme ends of the printheads (i.e. in the corner as shown in FIG. 3( a )) and thus also towards the ends of the body of encapsulant, these wire bonds are most prone to attack.  
         [0042]    However, changing the order of the contact pads or many other possible solutions to this problem would result in huge re-engineering problems requiring changes to the printhead, the flex circuitry and to printer control circuitry and software, and in any case, this may result in incompatibility between a re-designed printer cartridge and existing printer cartridges.  
         [0043]    Referring now to FIG. 3( a ), as explained above, in the case of model HP51649G printer cartridge, contact pad ID 2  adjacent pad R 50  is not actively used in the printer identification circuitry. In spite of the fact that a wire bond extends from the flexible circuit  35  to the contact pad  33 , no circuit trace extends from the ID 2  contact pad on the printhead. (It can be seen that a gap  46  is defined between the contact pad ID 2  on the printhead so isolating the contact pad from any circuitry on the printhead.)  
         [0044]    On the other hand, and referring to FIG. 3( b ), the corresponding contact pad ID 2  on the flexible circuit  35  is also isolated. In fact a circuit trace from the wire trace leading to the printhead contact pad ID 2  is linked to the flexible circuit contact pad for R 50  as indicated within the circle  47 . This is because, while it was seen as beneficial to isolate the flexible circuit contact pad ID 2  to mitigate potential problems caused by corrosion of the print head, this would have left the trace for ID 2  on the flexible circuit open ended in the region of the contact pad for ID 2 . The end of the trace could then become prone to lifting and possibly causing the flexible circuit to fail. Thus, the trace for ID 2  was linked to the trace for R 50  to prevent such failure.  
         [0045]    In a first embodiment of the present invention, the problems of the prior art are mitigated (e.g. in the case of model HP51649G printer cartridge) by joining the contact pads ID 2  and R 50  with a bridge portion  49  on the printhead to form one large ‘L shaped’ pad, FIG. 8( a ).  
         [0046]    As shown in the schematic diagram of FIG. 8( b ), this means that the wire bonds linking the flexible circuit traces for contact pads ID 2  and R 50  form a parallel circuit linked back to the flexible sheet contact pad R 50 .  
         [0047]    Ink will still penetrate encapsulant  39  as before, however, when the R 50  wire bond corrodes, rather than the printer cartridge failing immediately, the adjoining pad adds a back-up wire bond to the identification circuitry.  
         [0048]    Because the flexible circuit contact pad for ID 2  is isolated there is no impact on this aspect of the printer identification circuitry.  
         [0049]    The parallel circuit for R 50  results in a change in resistance of approximately 0.15 ohm. Using a parallel circuit reduces current through each wire bond and it is thought that this in turn results in slower corrosion of the wire bonds.  
         [0050]    It will be seen that the invention is applicable not alone to mitigating the problems of the particular cartridge of the first embodiment but also of other cartridges, in that if a redundant wire bond is available, and a contact pad for a portion of a circuit whose operation is critical to the operation of the circuit is prone to corrosion, then the contact pad connected to the redundant wire bond can be connected to the contact pad for the critical circuit to effectively form a single contact pad receiving two wire bonds. Thus, controlling circuitry can use traces extending from both wire bonds or link these traces (as in the preferred embodiment) in order to mitigate the problems of corrosion.  
         [0051]    So in a second embodiment of the invention, which can be applied to the circuit shown in FIG. 6, the printhead contact for R 50  is connected to the otherwise redundant (open circuit) printhead contact for ID 1 . The flexible circuit traces  36  are then designed to isolate the flexible circuit contact pad for ID 1  (in the same manner as for the flexible circuit contact pad ID 2  in FIG. 8( b )) and to connect the wire bond for ID 1  via a trace to the flexible circuit contact pad for R 50 . Alternatively, the wire bond for ID 1  could be used as a redundant circuit for the wire bond for the printhead contact pad R 49 . These solutions, however, may require re-engineering of both the printhead and the flexible circuit unlike in the first embodiment where only the printhead needs to be changed to implement the invention.  
         [0052]    In the case of the printhead and flexible circuit designs of FIGS. 5 and 7 there are no immediately available redundant contact pads and so to implement the invention some more significant redesign of both the printhead and flexible circuit may be required to implement the invention. This may be done either by adding otherwise redundant contact pads to the printhead, each having an associated wire bond, or increasing the number of wire bonds for the given number of contact pads.  
         [0053]    It will therefore be seen that the invention is applicable to the provision of additional wire bonds connecting a circuit (flexible or otherwise) to a die such as the printhead  30 . This can be done either by employing improved processing capability to deploy an increased number of wire bonds within the same length of die or by increasing the size of a die so that it can accommodate more wire bonds and contact pads which can be used as redundant circuits to mitigate the problems of wire bond corrosion or even manufacturing defects or other post manufacturing faults.  
         [0054]    It will be seen that the invention has a particular advantage in the field of printheads as it does not need to eliminate wire bond corrosion, rather it helps to delay the results of such corrosion at least until the printer cartridge is empty after which it usually becomes redundant anyway.