Patent Publication Number: US-6663232-B2

Title: Configuration of ink jet print head capable of reliably maintaining its continuity

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet print head, and more specifically to a continuity configuration of a laminated piezoelectric element of the head. 
     2. Related Art 
     A conventional ink jet print head includes a ceramic plate serving as a stationary plate, individual front electrodes mounted on a front surface of the ceramic plate, laminated piezoelectric elements electrically connected to the corresponding front electrodes by an electrically conductive adhesive, individual rear electrodes mounted on a rear surface of the ceramic plate, and flexible printed cable (FPC) electrically connected to the corresponding rear electrodes by soldering. The ceramic plate is formed from alumina with a plurality of through holes having a diameter of 0.25 mm. The through holes are filled with silver paste as filler, which electrically connecting the front electrode with the corresponding rear electrode. That is, the laminated piezoelectric element is electrically connected to the flexible printed cable via the front and rear electrodes and the filler. 
     This type of ink jet print head is formed in the following process. A green sheet is prepared and subject to calcinations to provide the ceramic plate. The plurality of through holes are formed in the ceramic plate by laser beam technique. The silver paste is applied into the through holes and also over the entire front surface of the ceramic plate, and is subject to sintering. The electrically conductive adhesive is coated over the front surface, and a block of piezoelectric element is placed thereon. The resultant product is subject to a dicing process to cut the block into the plurality of piezoelectric elements. At the same time, the silver paste on the ceramic sheet is also cut into a plurality of pieces, thereby providing the plurality of individual front electrodes. 
     However, the conventional ceramic plate with the above configuration has the following problems. 
     Firstly, when the above ceramic plate is placed in a high-temperature high-humid environment or get wet with ink, migration occurs in the front and rear electrodes. This results in continuity failure. 
     Secondly, a heat cycle that repeats between a room temperature and a higher temperature of 130° C. weakens the strength of the solder joint between the flexible printed cable and the rear electrodes, thereby causing disconnection. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to overcome the above problems and also to provide an ink jet print head in which continuity failure is effectively prevented and a method for producing the same. 
     In order to achieve the above and other objects, there is provided an ink jet print head including a stationary plate having a first surface and a second surface, a first electrode provided on the first surface, and a laminated piezoelectric element mounted on the first surface. The laminated piezoelectric element is electrically connected to the first electrode via an electrically conductive adhesive. The first electrode is formed of one of tungsten and molybdenum-manganese alloy. 
     There is also provided a producing method of producing an ink jet print head. The method includes the steps of a) forming a plurality of through holes in a green sheet, the through holes having first ends opened to a first surface of the green sheet and second ends opened to a second surface of the green sheet, b) filling in the through holes with filler, wherein the filler is one of tungsten and molybdenum-manganese alloy, c) forming a plurality of first electrodes from one of tungsten and molybdenum-manganese alloy on the first ends, d) forming a plurality of second electrodes from one of tungsten and molybdenum-manganese alloy on the second ends, e) plating the first and second electrodes with gold, f) placing a laminated-piezoelectric block on the first surface, g) applying an electrically conductive adhesive over the first surface and the first electrodes, and h) dicing the laminated-piezoelectric block into a plurality of laminated piezoelectric elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a cross-sectional view of an ink jet print head according to an embodiment of the present invention; 
     FIG. 2 is an enlarged partial view of FIG. 1; 
     FIG.  3 ( a ) is a plan view of a stationary plate with electrodes formed thereon in a manufacturing process of the head; 
     FIG.  3 ( b ) is a plan view of the stationary plate in a manufacturing process of the head, where piezoelectric-element blocks are placed on the stationary plate; 
     FIG.  3 ( c ) is a plan view of the stationary plate in the manufacturing process of the head, where an electrically conductive adhesive is coated over the stationary plate; and 
     FIG.  3 ( d ) is a plan view of the stationary plate after a dicing process is performed to cut the piezoelectric-element blocks into a plurality of piezoelectric elements in the manufacturing process of the head. 
    
    
     PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
     Next, an ink jet print head  1  according to an embodiment of the present invention will be described while referring to the attached drawings. 
     As shown in FIG. 1, an ink jet print head  1  of the present embodiment includes an orifice plate  11 , a chamber plate  12 , a restrictor plate  13 , a diaphragm plate  19 , a support plate  14 , and a stationary ceramic plate  15 . All are attached one on the other in this order to form a print head unit. The stationary plate  15  is formed from alumina (A1203) with high rigidity. 
     The orifice plate  11  is formed with an orifice  21 , and the chamber plate  12  is formed with an ink chamber  22 . The restrictor plate  13  is formed with a pressure chamber  23 . The orifice  21 , the ink chamber  22 , and the pressure chamber  23  together define a nozzle. 
     The ink jet print head  1  further includes a laminated piezoelectric element  31  and a flexible printed cable  16 . One end of the piezoelectric element  31  is attached to the diaphragm plate  19 , and the other end is fixedly attached to a front surface  15   a  of the stationary plate  15  by epoxy adhesive (not shown). The flexible printed cable  16  is provided to a rear surface  15   b  of the stationary plate  15  for transmitting electrical signal to the piezoelectric element  31 . The stationary plate  15  is formed with through holes  36 , which are filled with an electrically conductive filler  34 . 
     As shown in FIGS. 2 and 3, electrodes  33 ,  35  are formed on the front and rear surfaces  15   a,    15   b  at positions corresponding to the through holes  36 . The flexible printed cable  16  is connected to the electrode  35  by soldering. 
     Although not shown in the drawings, the piezoelectric element  31  is provided with a pair of side electrodes, one on a side of the piezoelectric element  31  for positive polarity and the other on opposite side for negative polarity. The electrode  33  is coated with the electrically conductive adhesive  32  for the continuity with the side electrodes of the piezoelectric element  31 . 
     The electrodes  33 ,  35  are both plated with gold. It has been confirmed through an experiment that if the electrode  33  is not plated with gold, the contact resistance between the electrode  33  made from the tungsten and the electrically conductive adhesive  32  will be undesirably large. Plating the electrode  33  with gold solves this problem. 
     Also, plating the electrode  35  with gold well secures the solder joint between the flexible printed cable  16  and the electrode  35 . In addition, a high-ink-resistant solder joint is provided. 
     Next, the method for providing the stationary plate  15  attached with the piezoelectric element  31  will be described. First, a green sheet, which will be the stationary plate  15 , is prepared, and the plurality of through holes  36  are formed thereto. In the present embodiment, a diameter d 2  of the through holes  36  is set to 0.2 mm. Then, tungsten is filled as the filler  34  in the through holes  36  and is also applied on the front and rear surfaces  15   a,    15   b  over the through holes  36  for the electrodes  33 ,  35 . The resultant product is subject to calcinations at a temperature of about 2000° C. Because the tungsten has a melting point of 3387° C., the tungsten will not be melted down during the calcinations at the temperature of about 2000° C. As a result, the stationary plate  15  with the electrodes  33 ,  35  formed thereon is provided (FIG. 3 ( a )). In this embodiment, the stationary plate  15  has a thickness of 1 mm. A diameter d 1  of the electrodes  33  is preferably 0.20 mm to 0.45 mm, and is set to 0.45 mm in this embodiment. Then, the surface of the electrodes  33  and  35  is plated with gold, and a pair of piezoelectric-element blocks  31   a  are placed and fixed by an epoxy adhesive (not shown) on the stationary plate  15  at predetermined positions and orientation as shown in FIG.  3 ( b ). 
     Next, as shown in FIG.  3 ( c ), the adhesive  32  are coated on the front surface  15   a  and on the electrodes  33 , but not on the piezoelectric-element block  31   a.  Then, as shown in FIG.  3 ( d ), the piezoelectric-element blocks  31   a  are cut into a plurality of piezoelectric elements  31  with dicing process. As shown, a dicing width is H 2 , and the resultant piezoelectric element  31  has a width of H 1 . In this embodiment, the dicing width H 2 =0.18 mm, and the element width H 1 =0.33 mm. Because the element width H 1  is smaller than the diameter d 1  of the electrode  33 , the dicing process cuts away portions of the electrodes  33 , resulting in oval-shaped electrodes  33 . Finally, the electrode  35  is soldered with the flexible printed cable  16 . 
     As described above, according to the present embodiment, the diameter d 1  of the electrode  33  is set to 0.45 mm. This dimension of the electrode  33  is small enough for providing a sufficient adhering area between the adhesive  32  and the stationary plate  15 , and is large enough for securing sufficient conductivity. That is, the adhering strength between the adhesive  32  and gold plated on the electrodes  33  is relatively weak, and so there is a danger that the adhesive  32  is peeled off the stationary plate  15  during the dicing process. However, because the adhesive  32  is securely adhered to the stationary plate  15  with relatively large area, the peelings will be avoided during the dicing process and even in the heat cycle. At the same time, the electrode  33  has the sufficient dimension to secure the conductivity between the adhesive  32 . 
     Although the diameter d 2  of the through holes  36  could be as large as 0.51 mm, within which the interference between the neighboring electrodes  33  would be theoretically prevented, the diameter d 2  is set to equal to or less than the dicing width H 1  (d 2 ≦H 1 ) in the present embodiment. This is because that if the diameter d 2  is set greater than the dicing width H 1 , the dicing process will whittle a portion of the filler  34 . This is a waste of the filler  34 , and there is no reason for setting the diameter d 2  greater than the dicing width H 1 . 
     As described above, because the tungsten is used as the filler  34  rather than the conventional silver paste, the filler  34  can be filled into the through holes  36  before the green sheet is subject to calcinations. Therefore, the sintering process for the tungsten is unnecessary after the calcinations. Accordingly, compared with the conventional process where the filler is sintered after the calcinations, the process of the present embodiment is simplified and economical. 
     Further, when the filler is sintered after the calcinations, the volume of the filler will be reduced at the time of sintering, resulting in air bubbles generated within the filler. Such air bubbles weaken the strength of the filler and causes breakage of the filler in the worse case. However, the calcinations for the stationary plate  15  and the sintering for the filler are performed at the same time, air bubbles will be not generated within the filler because the shrinkage percentage of the green sheet is larger than that of the filler. Accordingly, such problems can be avoided. 
     Because the individual electrodes  33  are formed on the stationary plate  15  before the dicing process, the adhesive  32  is not peeled off the electrode  33  during the dicing process, so the continuity can be well maintained. That is, if the tungsten plated with gold is first coated all over the front surface  15   a  and then cut into individual electrodes  33  by the dicing process as in the conventional manner, the conductive adhesive  32  would be easily peeled off the electrode  33 . This results in undesirable electrical disconnection between the piezoelectric element and the front electrodes. However, the present embodiment prevents such problems. 
     Moreover, migration has not occurred with the tungsten electrodes  33 ,  35  even when the resultant product has been placed in high-temperature high-humidity environment. Accordingly, the continuity failure can be avoided, and the head  1  will less likely have continuity failure. This contrast to the conventional head with electrodes formed of silver. 
     Because electrode  35  is formed of tungsten plated with gold, the electrode  35  is easily soldered with the flexible printed cable  16  with a soldering temperature, which is nearly 30° C. lower than a soldering temperature for a conventional silver electrode. Also, the soldering strength between the gold-plated electrode  35  and the flexible printed cable  16  is sufficiently strong even under the heat cycle. 
     It is easier to form the through holes  36  as small as 0.2 mm before the calcinations compared with after the calcinations. This further improves integration of nozzles. 
     It should be noted that although the tungsten is used as the filler  34  in the above described embodiment, molybdenum-manganese alloy can be used instead. In this case also, the head  1  can be formed in the same manner, and the above effects can be obtained. 
     While some exemplary embodiments of this invention have been described in detail, those skilled in the art will recognize that there are many possible modifications and variations which may be made in these exemplary embodiments while yet retaining many of the novel features and advantages of the invention.