Abstract:
An actuator assembly ( 81 ) for ink jet printheads, both monochromatic and color, with a large number of nozzles ( 62 ), consists of a die ( 58 ) stuck on a rigid substrate ( 166 ) and divided into two parts lengthwise to permit the flow of ink from the tank to the nozzles ( 62 ), and a flat cable ( 130 ) with nozzles ( 62 ) stuck on the die ( 58 ); the actuator assembly ( 81 ) is produced by means of the operations of sticking the die ( 58 ) on the rigid substrate ( 166 ), making a through cut ( 173 ) along the entire length of the die ( 58 ), sticking the flat cable ( 130 ) with nozzles ( 62 ) on the die ( 58 ) and sealing the ends of the longitudinal cut ( 173 ) with glue. The object of the actuator assembly ( 81 ) and the relative manufacturing process is to prevent particularly long dice from breaking during manufacture of the head.

Description:
TEXT OF THE DESCRIPTION  
         [0001]    1. Background of the Invention  
           [0002]    This invention relates to a printhead used in equipment for forming black and colour images, by way of successive scanning passes, on a print medium, normally though not exclusively a sheet of paper, using the thermal type ink jet technology, and more particularly to the actuator assembly of the head, and to the relative manufacturing process.  
           [0003]    2. Prior Art  
           [0004]    The composition and general mode of operation of a printhead according to the thermal type technology, and of the “top-shooter” type in particular, i.e. those that emit the ink droplets in a direction perpendicular to the actuator assembly, are already widely known in the sector art, and will not therefore be discussed in detail herein, this description instead dwelling more fully on some only of the features of the heads and their manufacturing process, of relevance for the purposes of understanding this invention.  
           [0005]    [0005]FIG. 1 shows an enlarged perspective view of an actuator assembly  80  of a monochromatic ink jet printhead, consisting of a die  51  of a semiconductor material (usually Silicon) on the upper face of which resistors  52  have been made for the emission of the ink droplets, driving circuits  53  for controlling the resistors  52 , pads  54  for connecting the head to an electronic controller, not depicted in the figures, a resistive temperature sensor  65 , reference marks  69 , and which has a pass-through slot  55  along which the ink flows from a tank not shown in the figure. Attached to the upper face of the die is a layer  60  of photopolymer having a thickness less than or equal to 25 mm wherein are made, using known photolithographic techniques, a plurality of ducts  57  and a plurality of chambers  64  positioned in correspondence with the resistors  52 . Stuck above the photopolymer  60  is a nozzles plate  61 , usually made from a sheet of gold-plated Nickel or of Kapton, of thickness 50 mm or less, bearing a plurality of nozzles  62 , each nozzle  62  being in correspondence with a chamber  64 . In the current art, diameter of the nozzles is usually between 10 and 60 mm, while their centres are usually set apart by a step A of {fraction (1/150)} or {fraction (1/300)} of an inch (169 mm or 84.5 mm). Usually, though not always, the nozzles  62  are disposed in two parallel rows, staggered by a distance B=A/2, in order to double the resolution of the image in the head scanning direction, which accordingly becomes {fraction (1/300)} or {fraction (1/600)} of an inch.  
           [0006]    Also in FIG. 1 the axes x, y and z giving the three-dimensional references of the die  51  are defined.  
           [0007]    The traditional process for manufacture of the actuator assembly will now be described below in brief, with reference to the flow diagram of FIG. 3, starting from a first step  70  in which a wafer  66  is made available whereupon the dice  51  are made (FIG. 2). In a subsequent step  71 , the wafer  66  is tested. In a step  72 , the wafer  66  is coated with a layer of photopolymer, generally of the dry film type.  
           [0008]    In a step  73  the photopolymer is exposed and, in a subsequent step  74 , the chambers  64 , in line with the resistors  52 , and the ducts  57  are made in the layer of photopolymer  60  (FIG. 1), through development using known techniques. In a step  75  a protection is applied to the entire wafer and, in a subsequent step  76 , the slots  55 , which bring the ink to the ducts  57 , are cut by way of a sandblasting operation. In a step  77 , the protection is washed off and a sight check is made that the component is still whole.  
           [0009]    In a subsequent step  100 , the nozzles plates  61  are positioned in such a way that the nozzles  62  are aligned with the chambers  64 , and stuck on the dice  51  belonging to the wafer  66 . Subsequently (step  101 ) the wafer  66  is applied to an adhesive tape  113  (FIG. 4), mounted on a frame  114 . The individual dice  51  are separated in a step  102  by cutting with a diamond wheel  115 , 50÷100 mm thick (FIG. 5), but are kept fast in their original positions by way of the adhesive tape  113  to which they adhere. Washing and drying are then performed (step  103 ), using an Ultratech machine for example.  
           [0010]    In a step  105 , a pick and place device of known technology, picks each die  51  off the adhesive tape  113  and places it with precision (error less than ±10 mm on the x axis) on an alignment base. In a step  104 , in the form of a continuous reel, a multiplicity of flat cables  117  (FIG. 1) is supplied separately, each having a window  122  with fingers  123  that will be soldered to the connecting pads  54  of the dice  51 , machine contacts pads  121  and interconnecting tracks  120  which connect the pads  121  to the fingers  123 . In a step  107  the flat cable  117  is aligned with the die  51 , with a tolerance of ±5 mm on the x and y axes.  
           [0011]    In a step  110  an ultrasound soldering head comes into position above the connecting pads  54  of the die  51 , whereto it solders one by one all the fingers  123  of the flat cable  117  (point-to-point TAB). The operations involved in the steps  105 ,  107  and  110  are effected using the technique known as Tape Automatic Bonding (TAB).  
           [0012]    In a subsequent step  111  the individual flat cables  117  are separated into distinct actuator assemblies  80 .  
           [0013]    A variant of the known art consists in making the nozzles directly on the flat cable (U.S. Pat. No. 5,278,584), which accordingly also has the function of nozzles plate, and is illustrated in FIG. 6. The flat cable  180  with nozzles is applied on a die  183  in which the feeding of the ink is effected from both sides. As a result, the windows  181  containing the fingers  123  are disposed perpendicularly to the ends of the rows of nozzles.  
           [0014]    As the technology evolves, so the demand grows for heads with an ever greater number of nozzles, in order to reduce the number of scanning passes the head needs to complete a page and improve the printer&#39;s productivity. To increase the number of nozzles, dice must be produced that are longer and longer and have the minimum possible width (4÷5 mm, where the mechanical requirements permit) so as to better exploit the wafer  66 .  
           [0015]    Accordingly the slots  55  are particularly long (typically though not exclusively greater than 12.5 mm) and are an open invitation for the dice  51  to break. When the nozzles plates (step  100 ) are assembled conventionally, the risk of the entire wafer  66  breaking when under pressure during soldering is high, with considerable economic damage.  
           [0016]    Even when the step  100  is completed without damage, there is still a high risk of the individual dice  51  breaking in the subsequent machining operations, with serious economic damage on account of the notable dimensions of the dice  51  themselves. With a step A (see FIG. 1) of less than {fraction (1/300)} of an inch, in practice the nozzles plate have to be produced in kapton. This further increases the risk of the dice  51  breaking.  
         SUMMARY OF THE INVENTION  
         [0017]    The object of this invention is to solve the problem represented by the risk of the dice breaking during the different machining stages of the nozzles assembly of an ink jet printhead, whether monochromatic or colour, by sticking the wafer on a rigid substrate and, instead of cutting the slot in a sandblasting operation, by effecting instead a through cut over the entire length of the dice.  
           [0018]    Another object is to handle the individual dice, rendered fragile by the slot, with safety and not expose them to the risks of breaking, keeping them stuck upon a portion of the said base.  
           [0019]    A further object is to make resistors underneath said substrate such that the operation of soldering the nozzles plates on the dice may be effected more rapidly, with local heating and a soldering temperature controlled by a sensor.  
           [0020]    A further object is to improve the thermal dissipation of said actuator, by using the contribution to heat conduction made by said substrate.  
           [0021]    A further object is to lower the time to refill the chamber following emission of the droplet of ink, since the edge of the through cut made with a diamond wheel, is more precise than the edge of the slot made by sandblasting, and can therefore be made at a lesser distance from the resistors.  
           [0022]    The above objects are obtained by means of an ink jet printhead with a large-size Silicon wafer and relative manufacturing process, characterized as defined in the main claims. 
       
    
    
       [0023]    These and other objects, characteristics and advantages of this invention will be apparent from the description that follows of the preferred embodiment, provided purely by way of an illustrative, non-restrictive example, and with reference to the accompanying drawings, where:  
         [0024]    [0024]FIG. 1—represents an enlarged view of an actuator assembly made according to the known art;  
         [0025]    [0025]FIG. 2—represents a wafer of semiconductor material, containing dice not yet separated;  
         [0026]    [0026]FIG. 3 a —illustrates the flow of the first part of the conventional manufacturing process of the actuator assembly of FIG. 1;  
         [0027]    [0027]FIG. 3 b —illustrates the flow of the second part of the conventional manufacturing process of the actuator assembly of FIG. 1;  
         [0028]    [0028]FIG. 4—represents the wafer of FIG. 2 mounted on an adhesive tape;  
         [0029]    [0029]FIG. 5—represents schematically the operation of separating the dice of FIG. 2 using a diamond wheel;  
         [0030]    [0030]FIG. 6—represents a known type flat cable provided with nozzles;  
         [0031]    [0031]FIG. 7—represents an actuator assembly according to the invention;  
         [0032]    [0032]FIG. 8—represents a resistor screen-printed on one face of a substrate belonging to the actuator assembly of FIG. 7;  
         [0033]    [0033]FIG. 9 a —illustrates the flow of the first part of the manufacturing process, according to the invention, of the actuator assembly of FIG. 7;  
         [0034]    [0034]FIG. 9 b —illustrates the flow of the second part of the manufacturing process, according to the invention, of the actuator assembly of  7 ;  
         [0035]    [0035]FIG. 10—represents a substrate provided with a pre-incision and slots;  
         [0036]    [0036]FIG. 11—represents the plurality of resistors screen-printed on the second face of the substrate of FIG. 10;  
         [0037]    [0037]FIG. 12—represents schematically the operation of spreading the glue on the first face of the substrate of FIG. 10;  
         [0038]    [0038]FIG. 13—represents a wafer, according to the invention, on which the dice have been separated;  
         [0039]    [0039]FIG. 14—represents the dice partially mounted on the substrate of FIG. 10;  
         [0040]    [0040]FIG. 15—represents schematically the operation of sticking the base of FIG. 10 on a adhesive tape;  
         [0041]    [0041]FIG. 16—represents schematically the operation of making a through cut on the dice with a diamond wheel;  
         [0042]    [0042]FIG. 17 a  represents a subassembly consisting of the die stuck on a support wafer produced by fragmenting the substrate of FIG. 10;  
         [0043]    [0043]FIG. 17 b —is the plan view of the same subassembly of FIG. 17 a , illustrating the areas destined to receive the glue that will seal the ends of the through cut;  
         [0044]    [0044]FIG. 18—represents a flat cable with nozzles according to the invention;  
         [0045]    [0045]FIG. 19—illustrates the flow of the manufacturing process of the actuator assembly of FIG. 7 a , in accordance with a second embodiment;  
         [0046]    [0046]FIG. 20—represents nozzles plates, in accordance with the second embodiment, that are stuck on the dice; and  
         [0047]    [0047]FIG. 21—represents an actuator assembly of a colour printhead, according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0048]    [0048]FIG. 7 represents the enlarged view of an actuator assembly  81  of a monochromatic ink jet printhead, according to this invention. Being already known and not directly concerning the invention, the other parts of the head have been omitted for simplicity&#39;s sake. In particular, the actuator assembly  81  comprises:  
         [0049]    a support plate  166 ;  
         [0050]    a die  58 ;  
         [0051]    a layer of photopolymer  60 ′;  
         [0052]    a flat cable with nozzles  130 .  
         [0053]    The support plate  166 , of a thickness preferably between  0 . 6  and  1  mm, is made preferably though not exclusively of ceramic; it contains a pass-through slot  162 , and a first face  168 .  
         [0054]    The die  58  is divided into two semidice  174 ′ and  174 ″, specularly substantially identical, between which there is a through cut  173  replacing the slot  55 . The die  58 , like the die  51  of FIG. 1, contains the resistors  52 , the driving circuits  53 , the pads  54 , and the resistive temperature sensor  65 .  
         [0055]    The layer  60 ′ of photopolymer is also divided into two parts, and is laid over the die  58 . Like the layer of photopolymer  60  in FIG. 1, it contains a plurality of ducts  57  and a plurality of chambers  64  located in correspondence with the resistors  52 .  
         [0056]    The flat cable with nozzles  130 , usually though not exclusively, consists of a kapton plate of thickness less than or equal to 50 mm, bears the plurality of nozzles  62 , and is stuck on top of the photopolymer  60 ′.  
         [0057]    Also defined in FIG. 7 are the x, y and z axes representing the three-dimensional references of the die  58 .  
         [0058]    Visible in FIG. 8 is a second face  169  of the plate  166 , upon which a resistor  164  of Rutenium Oxide or similar, placed all around the slot  162 , and two pads  163  of Ag Pd or similar, connected to the ends of the resistor  164 , have been deposited, for example by screen printing or by evaporation in a vacuum.  
         [0059]    [0059]FIG. 9 a  illustrates the first part of the flow diagram of the process used for manufacturing the head of the invention according to one embodiment thereof. Steps  101 ′,  102 ′,  103 ′ are effected, similar to the steps  101 ,  102 ,  103  of the known process. In the step  101 ′ a wafer  68 , containing the dice  58 , is applied to the adhesive tape  113 .  
         [0060]    The individual dice  58  are separated in the step  102 ′ by means of the cut made with the diamond wheel  115 , and are kept fast in their original positions by means of the adhesive tape  113  to which they adhere. Washing and drying are then effected in the step  103 ′.  
         [0061]    [0061]FIG. 13 represents a wafer  68 , upon which the dice  58  are made, stuck to the adhesive tape  113  borne by the frame  114 . Depicted in the enlargement is the single die  58 , before it is divided into the two semidice  174 ′ and  174 ″, where the area  167  that must be left completely free of components, tracks, resistors, ducts, etc. is illustrated in dash lines.  
         [0062]    In parallel (step  133  of FIG. 9 a ), and using known technologies, a substrate  160  (FIG. 10) is made available, preferably though not exclusively made of ceramic, between 0.6 and 1 mm thick and having a first face  168 ′ bearing an incision of an orthogonal grating, referred to in the following as pre-incision  161 , having steps in the x and y directions preferably 0.2÷0.5 mm greater than the corresponding steps of the dice  58  on the wafer  68 .  
         [0063]    The base  160  also has a plurality of slots  162 , made using known techniques, each slot  162  being substantially in the centre of each corresponding rectangle  166  delimited by the pre-incision  161 . Each slot  162  has a substantially rectangular shape, with a first dimension L1 approximately 0.2 mm greater than the width of a cut in the silicon die that will be illustrated in more detail later, and a second dimension L2 obtained from the following expression:  
           L 2 =A ( N− 1)+ B+D+C    
         [0064]    where, with reference to FIG. 1, A represents the step between the nozzles, N the number of nozzles in a row, B the stagger between the rows, D the diameter of a nozzle, and where the term C, of a value preferably between 0.2 and 0.5 mm, is added to guarantee a greater flow of the ink to the nozzles located at the ends of the rows.  
         [0065]    In a subsequent step  144 , on a second face  169 ′ of the substrate  160  (FIG. 11), the plurality of pads  163  and the plurality of resistors  164  are screen-printed around each slot  162 .  
         [0066]    In the step  136 , a continuous bead  165  of epoxy glue (FIG. 12) is dispensed on the first face  168 ′ of the base  160  by means of known technologies, such as for example screen-printing, use of a needle actuated off-line, use of a preform syringe with screen-extruded glue, stopping-out. The bead  165  must be continuous to prevent ink from seeping out during operating, and must be distributed with constant thickness in order to create uniform mechanical support and heat conduction between the die  58  and the base  160 .  
         [0067]    In the step  137 , using a known type automatic pick-and-place machine, a die  58  is picked off the adhesive tape  113 .  
         [0068]    In the step  141  the pick-up moves above the base  160 , aligns itself and deposits the die  58 ; the die  58  is then pressed against the bead of glue  165 . The first die  58  picked and placed on the base  160  is aligned with the slot  162  with a tolerance of ±50 mm on the x and y axes  162 , and is taken as the reference. The reference marks  69  of the dice  58  deposited subsequently are aligned with the marks  69  of the first die  58  with a tolerance of ±10 mm on the x axis. Shown in FIG. 14 is the base  160  on which part of the dice  58  have been stuck. In a step  142 , attachment of the die  58  is effected to the base  160  by hardening of the bead of glue  165 , using known technologies.  
         [0069]    In a subsequent step  145  the base  160  is stuck on an adhesive tape  170  (FIG. 15) borne by a frame  171 . In the step  143  the through cuts  173  (FIG. 16) are made on the dice  58  with a diamond wheel  172  of a thickness preferably between 100 and 300 mm, which effects a single cut of the whole column of dice  58  in the y axis direction, at a low feed rate. The precision alignment along the x axis, effected in the step  141 , ensures that the cuts  173  of all the dice  58  of a column are made at the right distance from the resistors  52  The semidice  174 ′ and  174 ″ remain aligned because they are stuck to the support  160 . In a subsequent step  146  the base  160  is broken along the incisions of the pre-incision  161 , and the individual subassemblies  175  are obtained (FIG. 17 a ), consisting of the individual support plates  166  to which the semidice  174 ′ and  174 ″, separated by the through cut  173 , are stuck. In the plan view of the subassembly  175  (FIG. 17 b ), the areas  178  destined to receive the glue for end sealing of the through cut  173  are illustrated in dash lines.  
         [0070]    The subsequent operations will now be described with reference to FIG. 9 b . In the step  147  the adhesive tape  170  is expanded, after which the subassemblies  175  are still adhering to the adhesive tape  170 , but are at a distance of 0.2÷0.5 mm from each other.  
         [0071]    In a step  149 , a multiplicity of flat cables with nozzles  130  in the form of a continuous reel is supplied separately (FIG. 18). The flat cable  130  has nozzles  62 , and in this way also performs the function of nozzles plate. It also has the fingers  123  accommodated inside appropriate windows  132 , and slots  131  destined to accommodate the glue that will seal the ends of the through cut  173 . For usage of the flat cable  130  integrating the function of nozzles plate, a technique for attachment to the subassembly  175  is required that will be described in the steps that follow.  
         [0072]    On the TAB machine, the subassembly  175  is picked off the adhesive tape  113  and placed on an alignment base (step  150 ); the flat cable with nozzles  130  is aligned with the subassembly  175  (step  151 ) and the fingers  123  are soldered on the pads of the die  154  (step  110 ′). In a subsequent step  152 , the flat cable  130  is stuck on the subassembly  175 . This is done by applying pressure on the flat cable using an isostatic press of known technology, while at the same time the subassembly  175  is heated using the resistor  164  located on the face  169  of the support plate  166 , while the temperature of the soldering cycle is detected by means of the sensor  65  already present on the die  58  for effecting the known function of temperature control during operation of the head. This enables the sticking operation to be performed much faster and under better controlled temperature conditions, as the heating is dosed using the sensor  65  for feedback, at no extra cost.  
         [0073]    In the step  153  the ends of the through cut  173  are sealed, by dispensing a high viscosity, epoxy glue or similar, on the areas  178  (see FIG. 17 b ) through the slots  131  of the flat cable  130 . In the step  154  said glue is UV prepolimerized, and in the step  155  it is thermally polimerized.  
         [0074]    In a subsequent step  111 ′ the individual flat cables  130  are separated into distinct actuator assemblies  81 .  
         [0075]    A first variant of the preferred embodiment consists of the fact that the pads  163  and the resistors  164  are made before the slots  162  are drilled. In the step  133  a substrate still minus the slots is made available. The step  144  follows, in which the pads  163  and the resistors  164  are made. Next the slot holes  162  are drilled by way of a CO 2  laser cut and the pre-incision  161  is made.  
         [0076]    In a second variant of the preferred embodiment, after the dice  58  have been stuck on the base  160 , the through cut  173  is not made, but instead the slots  55  are drilled by sandblasting through the slots  162  already made in the base  160 . With this system, each slot  55  can be made very close to the end edges of the die  58  without any danger of breaking as the flow of sand is guided by the slot  162  in the base  160 . This allows a better feeding of ink to the end nozzles during operation.  
         [0077]    A third variant of the preferred embodiment consists of the fact that the entire wafer  68  is stuck on the base  160  for reference, while the separation of the dice  58  along the y axis made with the grinding wheel  115  and the through cut  173  made with the grinding wheel  172  are effected subsequently in a single machining operation.  
         [0078]    2 nd  Embodiment  
         [0079]    This embodiment of the actuator of the printhead according to the invention differs from the preferred embodiment in that the flat cable with nozzles  130  is replaced by the nozzles plate  125 , which comprises the nozzles  62  and two slots  126  (see FIG. 20), and by the flat cable  117  (see FIG. 1). In addition, the resistor  164  and the pads  163  are not made on the face  169  of the support plate  166 . This embodiment follows the steps of the preferred embodiment, with the exception of step  144 , through to the step  143  (FIG. 9 a ), in which the through cut  173  is made in the centre of the dice  58 . Then the nozzles plates  125  are stuck on the dice  58  by means of the heated isostatic press of known technology (step  176 , FIG. 19). Following this, the substrate  160  (step  146 ) is fragmented, and the adhesive tape  170  expanded (step  147 ). In the step  104 , the reel of flat cable  117 , including the window  122 , is supplied. The steps  150 ,  151  and  110 , already described in the preferred embodiment, are effected on the TAB line. The method continues with the steps  153  and following, as described in the preferred embodiment (FIG. 9 b ).  
         [0080]    In a first variant of this embodiment, the entire wafer  68  is stuck on the base  160 , while the separation of the dice  58  along the y axis made with the grinding wheel  115  and the through cut  173  made with the grinding wheel  172  are effected subsequently in a single machining operation.  
         [0081]    Naturally, the principles of this invention are also applicable to the manufacture of a colour head, using three or more monochromatic inks to compose a wide range of perceptible colours. To describe the production of the colour head, reference is made, though not exclusively, to the preferred embodiment of the monochromatic head. The actuator assembly  210  of a colour head comprises the following parts (FIG. 21):  
         [0082]    a wafer  211 , in which three distinct slots  212  are made;  
         [0083]    a die  213 , divided into two semidice  218 ′ and  218 ″, in each of which three groups of resistors  214  are made;  
         [0084]    a flat cable  215 , bearing three groups of nozzles  217 , two end slots  216  into which the glue that will seal the ends of the through cut  173  is introduced and two intermediate slots  216 ′ into which the glue that separates the different colour inks is introduced.  
         [0085]    The colour head manufacturing process corresponds to the one described in the preferred embodiment and illustrated with the flow diagram of FIGS. 9 a  and  9   b , where the support plate  166 , the die  58  and the flat cable with nozzles  130 , i.e. those of the monochromatic head, are replaced by the support plate  211 , the die  213  and the flat cable  215 . In the step  153 , the end slots  216  and the colour separation slots  216 ′ are sealed with glue.  
         [0086]    In general, if M is the number of different inks used by the head, the number of intermediate slots  216 ′ will be M−1.  
         [0087]    If two inks are used (for example, graphic black and character black), a single intermediate slot  216 ′ is needed;  
         [0088]    if four inks are used (for example, yellow, magenta, cyan and character black), three intermediate slots  216 ′ are needed;  
         [0089]    if five inks are used (for example, yellow, magenta, cyan, graphic black and character black), four intermediate slots  216 ′ are needed;  
         [0090]    if six inks are used (for example, three full colours and three light colours), five intermediate slots  216 ′ are needed;  
         [0091]    Here again, the actuator assembly of the colour head can be made according to variants and embodiments similar to those described previously for the actuator assembly of the monochromatic head.  
         [0092]    In short, while fully maintaining the principle of this invention, the construction details and the embodiments may be abundantly varied with respect to what has been described and illustrated, without departing from the scope of the invention.