Patent Publication Number: US-7222942-B2

Title: Wiring board for inkjet head, method of polarizing piezoelectric elements, and inkjet recording apparatus

Description:
This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 2003-332468 filed in Japan on Sep. 24, 2003, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wiring board for an inkjet head, a method of polarizing piezoelectric elements, and an inkjet recording apparatus, and more particularly to a wiring board for an inkjet head, a method of polarizing piezoelectric elements, and an inkjet recording apparatus, which are suitable for polarizing piezoelectric elements. 
     2. Description of the Related Art 
       FIGS. 9A and 9B  are respectively a side view and a plan view of the principal part of a head block comprising a conventional wiring board for an inkjet head. 
     As shown in  FIG. 9B , a wiring pattern for installing upper surface electrodes (individual electrodes) of piezoelectric elements  2  and a drive IC  3 , which is a drive element for applying a drive voltage between the electrodes of the piezoelectric elements  2 , are formed on a wiring board  1 , such as a flexible wiring board. 
     The lower surface of the wiring board  1  which includes lower surface electrodes of the piezoelectric elements  2  is bonded to a conductive plate (vibration plate)  4 , by inserting a thermosetting adhesive, an anisotropic conductive adhesive, or an anisotropic conductive film, between the respective bonding surfaces and then bonding them together by applying heat and pressure. Furthermore, reflow soldering is used to bond the wiring board  1  with the upper surface electrodes (individual electrodes) of the piezoelectric elements  2  and to install the drive IC  3  onto the wiring board  1 . 
     A plate  4  forming a common electrode for the respective piezoelectric elements  2  is electrically connected by soldering to a ground wire  1 A of the wiring board  1 , via a conductive material or wiring (not illustrated) leading from the lower surface to the upper surface of the piezoelectric element, or the like. Furthermore, in  FIG. 9B , numeral  1 B denotes a ground wire of the drive IC  3  and GND denotes a common ground terminal of the ground wires  1 A and  1 B. 
     The piezoelectric elements have a Curie temperature at which they lose their polarized state. This Curie temperature is different for each type of piezoelectric element. 
     If heat exceeding the Curie temperature is applied to a piezoelectric element  2 , when bonding the piezoelectric elements  2  with the plate  4 , bonding the wiring board  1  with the piezoelectric elements  2 , or installing the drive IC  3  on the wiring board  1 , then the piezoelectric element  2  becomes depolarized and is no longer able to function as an actuator. 
     Possible solutions for this include a method using piezoelectric elements having a high Curie temperature, or a method where the piezoelectric elements are polarized or re-polarized after being installed on the wiring board. 
     The former method is described in paragraph “0002” of Japanese Patent Application Publication No. 2003-55045. More specifically, paragraph “0002” states that “If the Curie temperature is high, then the piezoelectric elements have high heat tolerance, and degradation of piezoelectric characteristics due to heat can be restricted, to a relative degree, even if the piezoelectric elements are heated to a high temperature of approximately 200° C. when passed through a solder reflow oven during installation, for example.” 
     On the other hand, if the latter method is adopted, it is necessary to polarize the piezoelectric elements by applying a voltage exceeding the normal operating voltage range to the piezoelectric elements. Therefore, a high voltage is applied to the piezoelectric elements after they have been installed on the wiring board and before the drive IC is installed, in such a manner that a voltage exceeding the voltage tolerance is not applied to the drive IC. 
     More specifically, as shown in  FIG. 10A , after installing a piezoelectric element  2  on the wiring board  1 , the piezoelectric element  2  is polarized or re-polarized by applying a polarizing voltage E 1  between the terminal IC on the wiring board  1 , which is electrically connected to the individual electrode  2 A of the piezoelectric element  2 , and the ground terminal GND of the wiring board  1  which is electrically connected to the common electrode  2 B of the piezoelectric element  2 . 
     A drive IC is then installed on the wiring board  1 , as shown in  FIG. 10B . In order to drive the piezoelectric element  2 , a signal from a drive signal source S is input to the signal input terminal Is of the wiring board  1 . 
     If piezoelectric elements having a high Curie temperature (for example, a Curie temperature of 600° C.–700° C.) as described in Japanese Patent Application Publication No. 2003-55045 are used in order to prevent loss of polarization of the piezoelectric elements during installation on the wiring board, then this reduces the possible range of selection of the piezoelectric elements. 
     On the other hand, if the piezoelectric elements are polarized or re-polarized after being installed on the wiring board and the drive IC is then installed on the wiring board, then two installation processes are required. Moreover, as shown in  FIG. 9A  and  FIG. 9B , it is desirable that the piezoelectric elements  2  and the drive IC  3  are installed as closely together as possible, but in this case, the heat used during installation of the drive IC  3  also affects the polarized or re-polarized piezoelectric elements  2 . 
     SUMMARY OF THE INVENTION 
     The present invention is devised with the foregoing in view, an object thereof being to provide a wiring board for an inkjet head, a method of polarizing a piezoelectric element, and an inkjet recording apparatus, which allow the installation steps of installing piezoelectric elements and a drive IC on a wiring board to be completed in one step, and which allow the piezoelectric elements to be polarized or re-polarized after installation, without harming the installed drive IC. 
     In order to achieve the aforementioned object, the present invention provides a wiring board for an inkjet head electrically connected to a piezoelectric element for discharging ink from the inkjet head and installed with a drive element for applying a drive voltage between an individual electrode of the piezoelectric element and a common electrode opposing the individual electrode, comprising a first ground wire which is electrically connected to a ground of the drive element, and a second ground wire which is electrically connected to the common electrode of the piezoelectric element, wherein the first ground wire and the second ground wire are electrically separated. 
     In other words, since the first ground wire connected to the ground of the drive element and the second ground wire connected to the common electrode of the piezoelectric element are separated electrically, then it is possible to apply a voltage for polarizing the piezoelectric element between the first and second ground wires. Since this polarizing voltage is not applied to the drive element, no harm is caused to the drive element. 
     Preferably, in the wiring board for an inkjet head according to the present invention, the drive element has diodes for preventing a reverse bias from being applied to the semiconductor switching elements in the drive element. Thereby, not only is it possible to supply a normal current for driving the piezoelectric element, but furthermore, a polarizing current can also be supplied to the piezoelectric element in order to polarize the piezoelectric element, without applying a reverse bias to the semiconductor switching elements. 
     Preferably, the wiring board for an inkjet head according to the present invention further comprises a first terminal connected to the first ground wire and a second terminal connected to the second ground wire. 
     Preferably, the wiring board for an inkjet head according to the present invention is a flexible wiring board. 
     In order to achieve the aforementioned object, the present invention provides a method of polarizing a piezoelectric elements, comprising a step of polarizing the piezoelectric element by applying a polarizing voltage between the first terminal and second terminal of the wiring board for an inkjet head according to the present invention. 
     In order to achieve the aforementioned object, the present invention provides a method of polarizing a piezoelectric element, comprising the steps of polarizing the piezoelectric element by applying a polarizing voltage between the first terminal and second terminal of the wiring board for an inkjet head according to the present invention, and shorting the first terminal and second terminal after polarizing the piezoelectric element. 
     Consequently, the device side connectors, which connect with the connection terminals of the wiring board, and the like, can be used without any modifications. 
     In order to achieve the aforementioned object, the present invention provides an inkjet recording head manufactured by using the above-described wiring board for an inkjet head. 
     According to the present invention having the composition described above, the installation of a piezoelectric element and a drive IC on a wiring board can be completed in one installation operation, and a piezoelectric element that has lost its polarization due to the application of a temperature exceeding the Curie temperature in this installation process can be re-polarized, or a piezoelectric element that has not been polarized previously can be polarized, after installation. A voltage equal to or exceeding the voltage tolerance of the drive element is applied to the piezoelectric element during this polarization process, but since it is not applied to the drive element, no harm is caused to the drive element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general compositional view showing an inkjet recording apparatus to which an inkjet head according to the present invention is applied; 
         FIG. 2  is a principal cross-sectional diagram showing an example of the composition of a head; 
         FIGS. 3A and 3B  are a side view and a plan view of the principal part of a head block including a wiring board for an inkjet head according to the present invention; 
         FIGS. 4A and 4B  are schematic diagrams of a wiring board after installation of a piezoelectric element and a drive IC; 
         FIGS. 5A ,  5 B and  5 C are principal cross-sectional views of a head block after installation of a piezoelectric element and a drive IC on a wiring board; 
         FIG. 6  is a circuit diagram of a principal section, including a drive IC; 
         FIG. 7  is a circuit diagram of a principal section, including a drive IC; 
         FIGS. 8A and 8B  are a side view and a plan view of the principal part of a head block including the wiring board for an inkjet head according to the present invention; 
         FIGS. 9A and 9B  are a side view and a plan view of the principal part of a head block including a conventional wiring board for an inkjet head; and 
         FIGS. 10A and 10B  are schematic diagrams of a wiring board for the purpose of describing a conventional method of polarizing a piezoelectric element. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, detailed description will be made below on the preferred embodiments of the wiring board for inkjet head, method of polarizing piezoelectric elements, and inkjet recording apparatus in the present invention with reference to the accompanying drawings. 
     General Configuration of an Inkjet Recording Apparatus 
       FIG. 1  is a general schematic drawing of an inkjet recording apparatus according to an embodiment of the present invention. As shown in  FIG. 1 , the inkjet recording apparatus  10  comprises: a printing unit  12  having a plurality of print heads  12 K,  12 C,  12 M, and  12 Y for ink colors of black (K), cyan (C), magenta (M), and yellow (Y), respectively; an ink storing/loading unit  14  for storing inks to be supplied to the print heads  12 K,  12 C,  12 M, and  12 Y; a paper supply unit  18  for supplying recording paper  16 ; a decurling unit  20  for removing curl in the recording paper  16 ; a suction belt conveyance unit  22  disposed facing the nozzle face (ink-droplet ejection face) of the print unit  12 , for conveying the recording paper  16  while keeping the recording paper  16  flat; a print determination unit  24  for reading the printed result produced by the printing unit  12 ; and a paper output unit  26  for outputting image-printed recording paper (printed matter) to the exterior. 
     The recording paper  16  delivered from the paper supply unit  18  retains curl due to having been loaded in the magazine. In order to remove the curl, heat is applied to the recording paper  16  in the decurling unit  20  by a heating drum  30  in the direction opposite from the curl direction in the magazine. 
     In the case of the configuration in which roll paper is used, a cutter (first cutter)  28  is provided as shown in  FIG. 1 , and the continuous paper is cut into a desired size by the cutter  28 . The cutter  28  has a stationary blade  28 A, whose length is equal to or greater than the width of the conveyor pathway of the recording paper  16 , and a round blade  28 B, which moves along the stationary blade  28 A. The stationary blade  28 A is disposed on the reverse side of the printed surface of the recording paper  16 , and the round blade  28 B is disposed on the printed surface side across the conveyor pathway. When cut paper is used, the cutter  28  is not required. 
     The decurled and cut recording paper  16  is delivered to the suction belt conveyance unit  22 . The suction belt conveyance unit  22  has a configuration in which an endless belt  33  is set around rollers  31  and  32  so that the portion of the endless belt  33  facing at least the nozzle face of the printing unit  12  and the sensor face of the print determination unit  24  forms a horizontal plane (flat plane). 
     The belt  33  has a width that is greater than the width of the recording paper  16 , and a plurality of suction apertures (not shown) are formed on the belt surface. A suction chamber  34  is disposed in a position facing the sensor surface of the print determination unit  24  and the nozzle surface of the printing unit  12  on the interior side of the belt  33 , which is set around the rollers  31  and  32 , as shown in  FIG. 1 ; and the suction chamber  34  provides suction with a fan  35  to generate a negative pressure, and the recording paper  16  is held on the belt  33  by suction. 
     The belt  33  is driven in the clockwise direction in  FIG. 1  by the motive force of a motor (not shown) being transmitted to at least one of the rollers  31  and  32 , which the belt  33  is set around, and the recording paper  16  held on the belt  33  is conveyed from left to right in  FIG. 1 . 
     Since ink adheres to the belt  33  when a marginless print job or the like is performed, a belt-cleaning unit  36  is disposed in a predetermined position (a suitable position outside the printing area) on the exterior side of the belt  33 . 
     A heating fan  40  is disposed on the upstream side of the printing unit  12  in the conveyance pathway formed by the suction belt conveyance unit  22 . The heating fan  40  blows heated air onto the recording paper  16  to heat the recording paper  16  immediately before printing so that the ink deposited on the recording paper  16  dries more easily. 
     The printing unit  12  forms a so-called full-line head in which a line head having a length that corresponds to the maximum paper width is disposed perpendicular to a paper conveyance direction (a main scanning direction). Each of the print heads  12 K,  12 C,  12 M, and  12 Y is composed of a line head, in which a plurality of ink-droplet ejection apertures (nozzles) are arranged along a length that exceeds at least one side of the maximum-size recording paper  16  intended for use in the inkjet recording apparatus  10 . 
     The print heads  12 K,  12 C,  12 M, and  12 Y are arranged in this order from the upstream side along the paper conveyance direction of the recording paper  16  (hereinafter referred to as the paper conveyance direction). A color print can be formed on the recording paper  16  by ejecting the inks from the print heads  12 K,  12 C,  12 M, and  12 Y, respectively, onto the recording paper  16  while conveying the recording paper  16 . 
     The print determination unit  24  has an image sensor for capturing an image of the ink-droplet deposition result of the print unit  12 , and functions as a device to check for ejection defects such as clogs of the nozzles in the print unit  12  from the ink-droplet deposition results evaluated by the image sensor. 
     A post-drying unit  42  is disposed following the print determination unit  24 . The post-drying unit  42  is a device to dry the printed image surface, and includes a heating fan, for example. It is preferable to avoid contact with the printed surface until the printed ink dries, and a device that blows heated air onto the printed surface is preferable. 
     A heating/pressurizing unit  44  is disposed following the post-drying unit  42 . The heating/pressurizing unit  44  is a device to control the glossiness of the image surface, and the image surface is pressed with a pressure roller  45  having a predetermined uneven surface shape while the image surface is heated, and the uneven shape is transferred to the image surface. 
     The printed matter generated in this manner is outputted from the paper output unit  26  after cut into a desired size by the cutter  28 . Preferably, the target print intended to be printed (in which the desired image is printed) and the test print are output separately. In the inkjet recording apparatus  10 , a selection device (not shown) is provided and the selection device switches an output route so that the target print and the test print are sorted and fed to an output unit  26 A,  26 B, respectively. When the target image and the test print are printed on a large size paper, a cutter (a second cutter)  48  cuts off the test print portion. 
     Next, the structure of the print heads is described. The print heads  12 K,  12 C,  12 M, and  12 Y provided for the ink colors have the same structure, and a reference numeral  50  is hereinafter designated to any of the print heads  12 K,  12 C,  12 M, and  12 Y. 
     As shown in  FIG. 2 , the print head  50  in the present embodiment has a structure in which a plurality of ink chamber units including nozzles  51  for ejecting ink-droplets and pressure chambers  52  corresponding to the nozzles  51  are disposed in the form of a staggered matrix, and the effective nozzle pitch is thereby made small. 
     The planar shape of the pressure chamber  52  provided for each nozzle  51  is substantially a square, and the nozzle  51  and an inlet of supplied ink (supply port)  54  are disposed in both corners on a diagonal line of the square. Each pressure chamber  52  is connected to a ink supply passage  55  through the supply port  54 . 
     A piezoelectric element  58  having an upper surface electrode (a discrete electrode)  57  and a lower surface electrode (not shown) is joined to a plate  56 , which forms the ceiling of the pressure chamber  52 . The plate  56  is conductive, and it is bonded with the lower surface electrode of the piezoelectric element  58  so as to form an electrical connection between them by a thermosetting adhesive, an anisotropic conductive adhesive, an anisotropic conductive film, or the like, and thereby acts as a common electrode of each piezoelectric element  58  and as a vibration plate to eject ink. The piezoelectric element  58  and the plate  56  are deformed by applying drive voltage between the discrete electrode  57  and the plate  56  used as the common electrode, thereby ink is ejected from the nozzle  51 . When ink is ejected, new ink is delivered from the ink supply passage  55  through the supply port  54  to the pressure chamber  52 . 
     (First Embodiment of Wiring Board for Inkjet Head) 
     Next, a first embodiment of an inkjet head according to the present invention will be described. 
       FIGS. 3A and 3B  are respectively a side view and a plan view of the principal part of a head block comprising a wiring board for an inkjet head according to the present invention. 
     In  FIGS. 3A and 3B , numeral  100  denotes a wiring board, such as a flexible wiring board. A wiring pattern for installing a drive IC  110 , which is a drive element for applying a drive voltage between the individual electrodes of the piezoelectric elements  58 , and the common electrode of the piezoelectric elements  58 , is formed on the wiring board  100 . A plurality of piezoelectric elements  58  are installed on the wiring board  100  for each head block unit, but in order to simplify the description, a single piezoelectric element  58  is taken as a representative example. Furthermore, a full line type head as described with reference to  FIG. 1 , and the like, may be constituted by arranging a plurality of head blocks. 
     As shown in  FIG. 3B , the wiring pattern in the wiring board  100  includes a ground wire  102 A connected electrically to the ground of the drive IC  110 , and a ground wire  102 B connected electrically to a plate  56  forming a common electrode of the piezoelectric element  58 . The connection terminal section  104  of the wiring board  100  is provided with ground terminals GND 1  and GND 2  connected to the aforementioned ground wires  102 A and  102 B, and signal input terminals Is to which drive signals are input. 
     As  FIG. 3B  reveals, the ground wire  102 A and ground terminal GND 1  are electrically separated from the ground wire  102 B and ground terminal GND 2  on the wiring board  100 . 
     Before installing the piezoelectric element  58  on the wiring board  100 , firstly, the piezoelectric element  58  and the plate  56  are joined together. The lower surface of the piezoelectric element  58  including the lower surface electrode is bonded with the plate  56  so as to form an electrical connection between them, by introducing a thermosetting adhesive, an anisotropic conductive adhesive, an anisotropic conductive film, or the like, between the piezoelectric element  58  and the plate  56 , and then applying heat or heat and pressure. 
     The piezoelectric element  58  joined to the plate  56 , and the drive IC  110 , are then installed respectively on the wiring board  100 . 
     In this case, the wiring board  100  and the drive IC  110  are aligned in position and the wiring board  100  and the individual electrode of the piezoelectric element  58  are aligned in position, whereupon the elements are inserted into a reflow oven and heated. Cream solder is previously printed onto the wiring board  100  by means of screen printing, or the like, and when heated in the reflow oven, the cream solder melts and solders the drive IC  110  and the individual electrode of the piezoelectric element  58  to the wiring board  100 . 
     In the reflow oven, the wiring board  100  and other members are heated to a temperature in excess of 200° C. Therefore, if the piezoelectric element  58  has been polarized, these polarization characteristics will be lost. Furthermore, the plate  56  forming the common electrode and the ground wire  102 B of the wiring board  100  are electrically connected by soldering, using a conducting material  120 , for example, or wiring from the lower surface or the upper surface of the piezoelectric element (not illustrated). 
     (Method of Polarizing Piezoelectric Element) 
     Next, the method of polarizing the piezoelectric element  58  will be described. 
       FIGS. 4A and 4B  are schematic diagrams of a wiring board  100  after installation of the piezoelectric element  58  and the drive IC  110  shown in  FIG. 3A  and  FIG. 3B . 
       FIG. 4A  shows the state of the connections and the applied voltage when polarizing or re-polarizing the piezoelectric element  58 . As  FIG. 4A  shows, when polarizing or re-polarizing the piezoelectric element  58 , a DC power source generating a voltage El for polarizing the piezoelectric element  58  (for example, 70V) is connected between the ground terminal GND  1  connected to the ground of the drive IC  110  in the connection terminal section of the wiring board  100 , and the ground terminal GND 2  connected to the common electrode of the piezoelectric element  58 . By this means, a voltage El is applied between the individual electrode  57  of the piezoelectric element  58 , and the plate (common electrode)  56 , thereby polarizing the piezoelectric element  58 . 
     In this polarization process, a voltage within the voltage tolerance is applied to the drive IC  110  (a ground level in  FIG. 4A ), and by implementing protective measures as described hereinafter, the drive IC  110  is prevented from being harmed. 
       FIG. 4B  shows the state of the connections during normal operation. As shown in  FIG. 4B , when the piezoelectric element  58  is operating normally, the ground terminals GND 1  and GND 2  of the wiring board  100  are shorted and the drive signal source S is connected between the signal input terminal Is and the ground terminal GND 1 . Thereby, a drive signal is input to the drive IC  110 , and the piezoelectric element  58  is driven via the drive IC  110 . 
     The ground terminals GND 1  and GND 2  are shorted by means of a connector which is connected to the connection terminal section  104  of the wiring board  100  (see  FIG. 3B ), or a circuit board having a connector of this kind. 
       FIGS. 5A ,  5 B and  5 C are principal cross-sectional views of a head block after installation of a piezoelectric element and a drive IC on the wiring board.  FIG. 5A  corresponds to  FIG. 4A , and more specifically, it shows the state of the piezoelectric element  58 . The piezoelectric element  58  is polarized by making the connections shown in  FIG. 5A  and applying a voltage E 1  between the individual electrode  57  of the piezoelectric element  58  and the lower surface electrode  59 , via the plate  56 . In  FIG. 5A , arrow P indicates the direction of polarization of the piezoelectric element  58  and the “+” symbol indicates the polarity. 
       FIGS. 5B and 5C  correspond respectively to  FIG. 4B , and show the connections and the state of voltage application to the piezoelectric element, during normal operation after polarization or re-polarization. More specifically,  FIG. 5B  shows a state where 0V is applied to the piezoelectric element  58 , and  FIG. 5C  shows a state where +V is applied to the piezoelectric element  58 . 
     When 0V is applied to the piezoelectric element  58 , no electrical field is generated and there is no change in the piezoelectric element  58 . On the other hand, when V is applied to the piezoelectric element  58 , an electrical field is generated in the direction of polarization P and due to piezoelectric effects, an extension is generated in the piezoelectric element  58  ( FIG. 5C ). Therefore, the plate  56  deforms together with the piezoelectric element  58  and ink is discharged from the nozzle  51 . 
     Next, the protective measures for the drive IC  110  during polarization of the piezoelectric element  58  will be described. 
       FIG. 6  shows a circuit diagram of the principal section including the drive IC  110 .  FIG. 6  relates to a case where the semiconductor switching elements are transistors. 
     The drive IC  110  has a buffer  112 , transistors Q 1  and Q 2 , diodes D 1  and D 2 , and a resistance R.  FIG. 6  shows a circuit for driving one piezoelectric element  58 . 
     In  FIG. 6 , when a high-level signal is applied from the buffer  112  to the bases of the transistors Q 1  and Q 2 , the transistor Q 1  switches on, the transistor Q 2  switches off, and a charging current flows to the piezoelectric element  58  via the transistor Q 1  and the resistance R. On the other hand, when a low-level signal is applied to the bases of the transistors Q 1  and Q 2  from the buffer  112 , the transistor Q 1  switches off, the transistor Q 2  switches on, and the electrical charge collected in the piezoelectric element  58 , and the like, is discharged to ground via the resistance R and the transistor Q 2 . 
     Therefore, in a piezoelectric element  58  which has been polarized and is functioning as an actuator, when a high-level signal is output from the buffer  112 , the piezoelectric element  58  deforms, and when a low-level signal is output, it reverts to its original state. 
     Furthermore, the anode side of the diode D 1  and the cathode side of the diode D 2  are connected to a junction point a, which connects the transistors Q 1  and Q 2  with the resistance R. The cathode side of the diode D 1  is connected to a power supply voltage Vcc of the drive IC  110 , and the anode side of the diode D 2  is connected to the ground of the drive IC  110 . 
     Therefore, the diode D 1  acts in such a manner that the reverse bias applied to the transistor Q 1  escapes into the power supply voltage Vcc, and the diode D 2  acts in such a manner that the reverse bias applied to the transistor Q 2  escapes as a charging current. In other words, the diodes D 1  and D 2  respectively act as protective circuits which prevent the transistors Q 1  and Q 2  from being harmed by the action of the reverse biases. 
     Here, if a DC power supply having a voltage of E 1  is connected between the ground of the drive IC  110  (in other words, the ground terminal GND 1  of the wiring board  100 ) and the ground connected to the common electrode of the piezoelectric element  58  (in other words, the ground terminal GND 2  of the wiring board  100 ), then a charge current flows from the DC power supply and through the piezoelectric element  58  in the direction indicated by the dotted line (in other words, the direction from the ground of the drive IC  110 , through the diode D 2  and the resistance R, to the piezoelectric element  58 ). By this means, it is possible to polarize the piezoelectric element  58  by applying a high voltage exceeding the voltage tolerance of the transistors Q 1  and Q 2  to the piezoelectric element  58 , without applying a reverse bias voltage to the transistors Q 1  and Q 2 . 
       FIG. 7  shows a further circuit diagram of the principal section including the drive IC  110 .  FIG. 7  relates to a case where the semiconductor switching elements are field effect transistors (FET). The FETs Q 1  and Q 2  illustrated in  FIG. 7  differ in type from the transistors (junction transistors) illustrated in  FIG. 6 , but their action is the same and therefore detailed description thereof is omitted. 
     (Second Embodiment of Wiring Board for Inkjet Head) 
     Next, a second embodiment of an inkjet head according to the present invention will be described. 
       FIGS. 8A and 8B  are respectively a side view and a plan view of the principal part of a head block comprising a wiring board for an inkjet head according to the present invention. Parts that are common to those in the first embodiment illustrated in  FIGS. 3A and 3B  are labeled with the same reference numerals, and detailed description thereof is omitted here. 
     In  FIG. 8A  and  FIG. 8B , numeral  200  denotes a wiring board, such as a flexible wiring board, and only the wiring board  200  is different from the first embodiment. 
     The wiring pattern in the wiring board  200  includes a ground wire  202 A connected electrically to the ground of the drive IC  110 , and a ground wire  202 B connected electrically to a plate  56  forming a common electrode of the piezoelectric element  58 . Furthermore, a ground terminal GND 1  connected electrically to the ground wire  202 A and a ground terminal GND 2  connected electrically to the ground wire  202 B are provided in a separate position to the connection terminal section  204  of the wiring board  200 . 
     The ground terminal GND 1  is connected electrically to the ground terminal GND provided in the connection terminal section  204 . Furthermore, the ground terminal GND 2  is provided in the vicinity of the ground terminal GND 1 , but it is electrically separated from the ground terminal GND  1 . 
     The piezoelectric element  58  is polarized after installation of the piezoelectric element  58  and the drive IC  110  onto the wiring board  200 , by connecting a DC voltage supply between the ground terminals GND 1  and GND 2 , similarly to the first embodiment. After polarization, a zero ohm resistance, for example, is connected electrically between the adjacent ground terminals GND 1  and GND 2  by soldering, or the like. 
     Thereby, the connection terminal section  204  of the wiring board  200  can be formed similar to the conventional shape illustrated in  FIG. 9A  and  FIG. 9B , and hence there is no requirement to change the connectors, board, or the like, to which this connection terminal section  204  is connected. 
     The wiring board for an inkjet head according to the present invention is not limited to a full line type head as illustrated in  FIG. 1 , and it may also be applied to a shuttle type head where the head is moved back and forth in a direction orthogonal to the feed direction of the print medium.