Abstract:
A bonding apparatus and method thereof to bond the bonding portions of an FPC cable to pads of a print head die in order to electrically connect the print head die of an ink jet print head assembly include a stage and a bonding tool. The print head die includes resistive heaters, signal lines connected to the resistive heaters, and electrical pads to connect the signal lines to an outside of the print head die to the FPC cable having conductors having bonding portions facing the pads. The print head die is supported on the stage with the pads facing upward. The bonding tool includes a tip that press bonding portions of the FPC cable against corresponding pads of the print head die placed on the stage, and heats the bonding portions in contact with the pads to bond the bonding portion to the pads. The bonding tool includes a protrusion positioned to align with a position beyond the boundary of the print head die, and that extends lower than the tip, such that, during the bonding operation, the protrusion bends the FPC cable downward. In addition, during the bonding operation, the upper protective film of the FPC cable around the bonding portions are left intact to provide protective layer over the resulting bond, and thus results in less amount of encapsulation over the bond.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Korean Application No. 2002-27867 filed May 20, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for bonding a flexible printed circuit (FPC) cable to an ink jet print head assembly, and an apparatus adopting the method. 
   2. Description of the Related Art 
   In an ink jet head adopting an electro-thermal transducing technique to discharge ink droplet using bubbles obtained by instantaneously heating ink, a print head die having heaters and nozzles thereon is included. The print head die also includes a plurality of signal lines electrically connected to the heaters, and a plurality of electric contact pads arranged on the peripheral edges of the print head die in an inline configuration in order to allow the signal lines to be coupled to an external circuit. 
   The pads are connected to FPC cable on which a plurality of conductors corresponding to the pads are arranged side by side. The pads are bonded to the conductors by thermo-sonic bonding. 
   U.S. Pat. Nos. 4,635,073 and 6,126,271 each disclose a method of bonding the pads to conductors using a thermo-sonic bonding technique. Due to the high vibration energy required for the bonding of each conductor/pad pair, simultaneous bonding of all conductor/pad pairs may have detrimental effect on the integrity of the print head die and any other parts of the ink jet head. Thus, the thermo-sonic bonding technique is a single point bonding method to bond a single conductor to a single pad at a time. In a conventional method shown in  FIG. 1 , a conductor  3  is brought into contact with a pad  2  installed on a print head die  1 , and then a thermo-sonic bonding tool  4  applies pressure and high frequency (ultrasonic) vibration to the conductor  3  one conductor at a time to permanently bond to the pad  2 . The remaining pads and conductors are sequentially bonded together one at a time. According to the conventional sequential bonding method, depending on the number of conductor/pad pairs, the bonding process takes a significant amount of time. 
   In addition, the vibration energy generated by ultrasonic frequency may deform the portion of the conductor  3  where the energy is focused, decreasing the rigidity of the bond, and, as a result, the conductor  3  may easily become separated from the pad. 
   Moreover, after a conventional bonding process described above, a considerable length of the FPC cable is left to extend above the print head die, which blocks the travel path of a wiper in the maintenance station of the ink jet printing apparatus. Also, when the wiper travels beyond the peripheral boundary of the print head die, the wiper continues to press on the extended portion of the FPC cable, resulting in an unnecessary shear force being applied to the bond between the print head die and the FPC cable. 
   Furthermore, during a conventional bonding process, the conductor of the FPC cable is completely exposed, i.e., the protective and/or insulation film covering the conductor is completely removed near the bond, requiring an excessive amount of encapsulation over the bond to insulate the exposed upper surface of the conductor. The excessive encapsulation adds height over the bond, and thus also reduces the clearance for a wiper to travel, and increases the distance the ink droplets must travel before landing on the surface of the print medium, e.g., a sheet of paper. 
   SUMMARY OF THE INVENTION 
   To solve the above-described problems, it is an aspect of the present invention to provide an ink jet print head assembly and a method and apparatus for bonding a flexible printed circuit (FPC) cable to an ink jet print head assembly, in which the time spent to bond a conductor is decreased. 
   In accordance with another aspect of the present invention, there is provided an ink jet print head assembly and a method and apparatus for bonding an FPC cable to an ink jet print head assembly, in which the portion where a conductor and a pad are joined together provides an increased structural intensity. 
   In accordance with still another aspect of the present invention, there is provided an ink jet print head assembly and a method and apparatus for bonding an FPC cable to an ink jet print head assembly, in which a printing quality is increased by a reduction in an flying distance of droplet. 
   In accordance with an aspect of the present invention, there is provided a method of bonding an FPC cable to an ink jet print head assembly. The method includes: preparing a print head die of the ink jet print head assembly, the print head die having resistive heaters, signal lines connected to the resistive heaters, and electrical pads for connecting the signal lines to the outside of the print head die; preparing the FPC cable having conductors having bonding portions, wherein each of the conductors corresponds to each of the pads, respectively; mounting the print head die on a stage of the bonding apparatus to combine the print head die with the FPC cable; arranging the FPC cable on the print head die so that the bonding portions of the conductors face corresponding one of the pads; pressing all of the conductors down on the corresponding pads at a time; bending the FPC cable when the bonding apparatus bonds the conductors to the pads, to place the bent portion of the FPC cable at a rear part of the print head die; and heating all of the conductors to bond the conductors to the pads. 
   In accordance with an aspect of the present invention, there is provided another method of bonding an FPC cable to an ink jet print head assembly. The method includes: preparing a print head die of the ink jet print head assembly, the print head die having resistive heaters, signal lines connected to the resistive heaters, and electrical pads to connect the signal lines to the outside; preparing the FPC cable having conductors with bonding portions facing the pads; completely covering an upper surface of the conductors using an upper protect film; covering a lower surface of the conductors using a lower protect film except for a bonding portion facing the pads; mounting the print head die on a stage of a bonding apparatus to combine the print head die with the FPC cable; arranging the FPC cable on the print head die so that the bonding portions of the conductors face corresponding one of the pads; pressing all the conductors down on the corresponding pads at a time; and heating all the conductors to bond the conductors to the pads. 
   According to an aspect of the present invention, in the above method, there is further provided bringing the bonding portions of the conductors in contact with the corresponding pads at both edges of the bonding apparatus. 
   In accordance with an aspect of the present invention, in the above method, there is further provided bending the FPC cable when the bonding apparatus bonds the conductors to the pads, to place the bent portion of the FPC cable at a location proximate to the bonded portion where a bent portion thereof is placed at a rear of the print head die. 
   In accordance with an aspect of the bonding method, the bonding is performed with a bump interposed between a bonding portion of the pads and the corresponding conductors. The lower protect film may have an aperture facing each of the pads. A bump is inserted into the aperture, and the bonding portion of a conductor is electrically connected to a corresponding pad. 
   In accordance with an aspect of the present invention, there is provided a bonding apparatus to bond the bonding portions of an FPC cable to pads of a print head die in order to electrically connect the print head die of an ink jet print head assembly, the print head die having resistive heaters, signal lines connected to the resistive heaters, and electrical pads to connect the signal lines to an outside of the print head die to the FPC cable having conductors having bonding portions facing the pads. The bonding apparatus includes: a stage on which the print head die is mounted comprising the pads facing upward; and a bonding tool moving up and down over the stage to press all the bonding portions down on corresponding pads and to heat all the bonding portions in contact with the pads to bond all the bonding portion to the pads using a hot pressure welding technique at one time, wherein the bonding tool has a protruding extension to bend the FPC cable beside each edge of the print head die so that the FPC cable is placed from an upper surface level of the print head die to a position at a rear of the print head die. 
   In the bonding apparatus, the bonding tool has a tip to bring the bonding portions of the conductors of the FPC cable in contact with corresponding pads. The tip protrudes from both edges of the bonding tool to bond opposite conductors to corresponding opposite pads from the pads formed on both edges of the print head die. 
   In accordance with an aspect of the present invention, the bonding tool has a dam-shaped tip that covers the pads formed on the print head die. Also, the dam-shaped tip is symmetrically formed at both ends of the bonding tool so that the dam-shaped tip covers the pads formed along both edges of the print head die. Furthermore, the bonding tool has a partial depressing protrusion corresponding to the pads. 
   In accordance with an aspect of the present invention, there is provided an ink jet print head assembly including an ink jet print head chip and an FPC cable. The inkjet print head chip has a plurality of nozzles, from each of which an ink droplet is ejected when a corresponding respective one of a plurality of heating elements disposed within said ink jet print head assembly chip is active. The inkjet print head chip further has a plurality of bonding pads disposed thereon to provide electrical connections to said plurality of heating elements from an external electrical signal source. The FPC cable has a plurality of conductors and upper and lower films to protect the conductors. Each of the conductors has a bonding portion to be bonded to corresponding one of the pads. At least one of the conductors is hot pressure welded to at least one of the binding pads to form a bonded connection. The upper film completely covers the upper surface of the conductors, and the lower film covers the lower surface of the conductors except for the bonding portion facing the pads. 
   In the above-described ink jet print head assembly, the FPC cable is bent downward at a location proximate to the bonded portion where a bent portion thereof is placed at a rear of the print head die. The lower film has a window formed directly over the corresponding pads to allow the corresponding conductors to contact the pad. 
   According to an aspect of the present invention, a conductive bump is interposed between the bonding portion of the conductors and the corresponding pads. 
   In accordance with an aspect of the present invention, there is provided another ink jet print head assembly including an inkjet print head chip and an FPC cable. The inkjet print head chip has a plurality of nozzle, from each of which an ink droplet is ejected when a corresponding respective one of a plurality of heating elements disposed within said ink jet print head assembly chip is active. The inkjet print head chip further includes a plurality of bonding pads disposed thereon to provide electrical connections to said plurality of heating elements from an external electrical signal source. The FPC cable has a plurality of conductors, among which at least one conductor is hot pressure welded to at least one of said plurality of binding pads to form a bonded connection. The FPC cable is bent downward at a location proximate to the bonded portion where a bent portion thereof is placed at a rear of the print head die. 
   These together with other aspects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part thereof, wherein like numerals refer to like parts throughout. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a schematic diagram illustrating a method of bonding a flexible printed circuit (FPC) cable of an ink jet print head assembly using a conventional thermo-sonic welding technique; 
       FIG. 2  is a flowchart illustrating an exemplary embodiment of a method of bonding the FPC cable of the ink jet print head assembly according to the principles of the present invention; 
       FIG. 3  is a schematic structure diagram of an exemplary embodiment of a bonding apparatus according to the principles of the present invention, illustrating a first exemplary embodiment of a method of bonding the FPC cable of the ink jet print head assembly according to the principles of the present invention; 
       FIG. 4  is a plan view of the FPC cable used for the ink jet print head assembly; 
       FIG. 5  is a perspective view showing a bonding manner in which two pads at one time are bonded to the FPC cable by the FPC cable bonding method of  FIG. 3 ; 
       FIG. 6  is a perspective view showing a second exemplary embodiment of a method of bonding the FPC cable of the ink jet print head assembly according to the principles of the present invention; 
       FIG. 7  is a schematic diagram illustrating a third exemplary embodiment of a method of bonding the FPC cable of the ink jet print head assembly according to the principles of the present invention; 
       FIG. 8  is a perspective view of an exemplary embodiment of a bonding tool of the bonding apparatus, according to the principles of the present invention, illustrating a fourth exemplary embodiment of a method of bonding the FPC cable of the ink jet print head assembly according to the principles of the present invention; 
       FIG. 9A  is a cross-sectional diagram illustrating a fifth exemplary embodiment of a method of bonding an FPC cable to an ink jet print head assembly according to the principles of the present invention; 
       FIGS. 9B and 9C  are plan views of embodiments of the partial depressing protrusion  42   a  of  FIG. 9A ; 
       FIG. 10  is a partial cross-section of a print head die to which the FPC cable is bonded by the method of  FIG. 9A ; 
       FIG. 11  is a partial cross-section of an ink jet print head assembly cartridge on which a print head die to which the FPC cable is bonded by the method of  FIG. 9A  is installed; and 
       FIG. 12  is a cross-sectional view illustrating a sixth exemplary embodiment of a method of bonding the FPC cable of the ink jet print head assembly according to the principles of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. An ink jet print head assembly is not described but will be understood through the description of bonding methods and apparatuses according to the present invention. 
   According to the principles of one aspect of the present invention, a hot pressure welding (thermo-pressure bonding) using local compressive energy may be adopted. In the hot pressure welding, a fragile print head die is not subjected to vibrational impact, so that simultaneous bonding with respect to several pads is possible in contrast with thermo-sonic bonding. However, there are other aspects of the present invention that may be implemented in ink jet head assembly regardless of the particular bonding method, i.e., the thermo-pressure and/or the thermo-sonic bonding. 
     FIG. 2  is a flowchart illustrating an exemplary embodiment of a bonding method according to the principles of the present invention. Referring to  FIG. 2 , at operation  91 , a print head die for an ink jet print head assembly is prepared. Then, at operation  92 , a flexible printed circuit (FPC) cable to be bonded to the print head die is prepared. Next, at operation  93 , a bonding apparatus to bond the conductors of the FPC cable to pads on the print head die using the hot pressure welding (i.e., thermo-pressure bonding) technique is prepared. Thereafter, at operation  94 , the print head die is mounted on a stage (i.e., a stage  50  of  FIG. 3 ) of the bonding apparatus. Subsequently, at operation  95 , the FPC cable is arranged on the print head die. Next, at operation  96 , a bonding tool of the bonding apparatus bonds the conductors of the FPC cable to the pads on the print head die using the hot pressure welding technique. At this time, at least two conductors are bonded to at least two pads. The bonding method repeats and returns to operation  96 , until all of the pads on the print head die are bonded to the corresponding conductors. However, all of the pads can be simultaneously bonded to the corresponding conductors at one time. 
     FIG. 3  is a schematic structure diagram of the bonding apparatus according to an aspect of the present invention, in which the bonding tool bonds the FPC cable to the print head die using a first exemplary embodiment of a bonding method according to the principles of the present invention. Referring to  FIG. 3 , a print head die  10  to be bonded, is mounted on a stage  50  of the bonding apparatus adopting the hot pressure welding technique. While, for brevity,  FIG. 3  shows only the electric pads  20 , the print head die  10  may also include other elements, e.g., one or more nozzles through which ink droplets may be ejected, a plurality of resistive heaters, a plurality of signal lines connected to the heaters 
   Conductors  61  of a FPC cable  60  are placed on the pads  20  and depressed by a tip  41  of a bonding tool  40 . Although not shown in  FIG. 3 , a solder (or other conductive material, e.g., gold or the like) bump or ball may be disposed on the pad  20 , which melts due to the heat applied through the bonding apparatus  40 , permanently bonding the conductor  61  to the pad  20 . The unexposed portions of the conductors  61  are protected by upper and lower insulation/protect films  62 . 
     FIG. 4  is a plan view of the FPC cable  60 . The FPC cable  60  is largely divided into a pad area A having a plurality of contact pads (indicated by alphabets), which provides electrical communication access to circuits external to the print head assembly (or the cartridge), e.g., the control mechanism (not shown) of a printer, and a bonding area B where the conductors  61  to be bonded to the pads  20  of the print head die  10  of the inkjet print head are arranged. Reference numeral  11  denotes a nozzle area formed on the print head die  10 . In the FPC cable  60  of  FIG. 3 , the bonding portions (i.e., the bonding ends) of the conductors  61  are arranged in areas C. 
   Referring back to  FIG. 3 , the bonding portions of the conductors  61  are bonded to corresponding pads  20  installed on the print head die  10 . In the example shown in  FIG. 3 , two of the conductors  61  are shown to be bonded to two corresponding pads  20  installed on ends of the print head die  10  at a time. However, it should be apparent to and understood by those skilled in the art that any number of conductors and pads may be bonded simultaneously according to the principles of the present invention. Bonding energy is created by heat and/or pressure. 
   The FPC cable  60  is arranged on the print head die  10  mounted on the stage  50 , such that the bonding portions of the FPC cable  60  is aligned over the pads  20  of the print head die  10 . Next, the bonding tool  40  heated to a predetermined temperature performs the hot pressure welding by depressing the conductors  61  down on the pads  20  of the print head die  10  At this time, heat may also come from the stage  50 , applying heat to the print head die  10  and the pads  20 . This hot pressure welding is achieved within a very short period of time, in which solder (or gold) bumps disposed on the pads  20  are locally melted, and the conductor  61  and the pads  20  are thus joined together. In  FIG. 3 , reference numeral  42  denotes a recess formed to prevent any surface other than the tips  41  from contacting the surface of the print head die  10 . 
   In the embodiment shown in FIG.  3  and in  FIG. 5 , two conductors  61  are bonded to two pads installed on opposite ends of the print head die  10 . However, as shown in  FIG. 6 , two adjacent pads  20  installed on the same side of a print head die may be bonded to corresponding conductors  61  by a bonding tool  40   a . In the exemplary embodiment illustrated in  FIG. 6 , the bonding tool  40   a  includes a tip  41   a , which may be made to traverse across the surface of the stage  50 , so that the other pairs of pads  20  can also be bonded. A bonding method, according to the embodiments thus far described, provides at least two pads being bonded to corresponding conductors  61  at a time, where any at least two pads  20  can be selected. However, in a preferred embodiment, at least two pads on opposite ends of the print head die  10  are selected and bonded, as shown in FIG.  3 . 
   As previously described, a conductive bump  70  is placed between the pad  20  and the conductor  61  as shown in FIG.  7 . The bump  70  is made of conductive material, e.g., lead, gold, or the like. The type of conductive material of the bump  70  does not restrict the technical scope of the present invention. The bump  70  installed on the pad  20  is formed by a general method well known in the art, so the formation method will not be described. 
   In a preferred embodiment of the present invention as shown in  FIG. 8 , all of the pads  20  on the print head die  10  are bonded to all of the corresponding conductors  61  simultaneously. To perform the simultaneous bonding of all pads  20  (often referred to as a gang bonding process), a bonding tool  40   b  has a size that covers all of the pads  20  on the print head die  10 , and two opposite tips  41   b  are formed on the bottom part of the bonding tool  40   b , extending along both ends of the print head die  10 . That is, the tips  41   b  simultaneously bond all of the pads arranged on opposite edges of the print head die  10  to corresponding conductors  61  using heat and/or pressure. 
   According to the principles of another aspect of the present invention, the FPC cable may be properly shaped during the bonding process. As shown in  FIG. 9 , the print head die  10  is mounted on the stage  50 . The pad  20  is installed on the edge of the print head die  10 , and the bump  70  is placed on the pad  20 . An FPC cable  60   a  includes upper and lower protective films  62   a  and  62   b  formed on the upper and lower sides of a conductor  61 , respectively. According to the principles of yet another aspect of the present invention, as shown in  FIG. 9A , the FPC cable  60   a  has the upper protect film  62   a  left intact to completely cover the upper surface of the conductor  61 . 
   The bonding apparatus shown in  FIG. 9A  includes a bonding tool  40   c  that has a tip  41   c  and a protrusion  41   d  extending to beyond the peripheral edge of the print head die  10 . Because the protrusion  41   d  extends lower than the tip  41   c , the FPC cable  60   a  is bent downward along the outer edge of the print head die  10  during the bonding process. After the bonding process is completed, the bent portion of the FPC cable  60   a  remains bent, and the FPC cable  60   a  is thus reshaped as illustrated in FIG.  10 . 
   Referring again to  FIG. 9A , a depressing protrusion  42   a  is formed on the tip  41   c  so as to be located directly over the pad  20 . The depressing protrusion  42   a  increases its adhesive strength of the resulting bond. The contacting surface of the depressing protrusion  42   a  can have various shapes to increase the shear strength of the bond. For example, the contacting surface of the depressing protrusion  42   a  may have a cross (+) shape as shown in  FIG. 9B , or  42   b  may be a circular shape outlining a cross “+”-shape as shown in FIG.  9 C. It should be understood that the contacting surface of the depressing protrusion  42   a  may take any shape, which makes an impression through the conductor  61 , the bump  70  and/or the pad  20  so as to increase the shear force there between. 
   As shown in  FIG. 10 , exposed portions of the conductor  61  and pad  20  are encapsulated by an insulating material, e.g., resin, forming an encapsulation  80 . According to the principles of another aspect of the present invention, the upper protective film  62   a  remains intact, completely covering the conductor  61 . A smaller amount of encapsulating (insulating) material may be used in comparison with the case where the upper surface of the conductor  61  is completely exposed. The height of the encapsulation  80  is thus shorter, which presents less of interference when the ink jet head is being wiped by a wiper of a maintenance station during a service operation of the printer. 
   With the FPC cable  60   a  bent as shown in  FIG. 10 , the print head die  10  is mounted on an ink cartridge  90  as shown in  FIG. 11 , the FPC cable  60   a  is affixed to the upper surface of the cartridge  90 , providing sufficient clearance for a wiper that wipes the nozzle area time-to-time during the operation of the printer. 
   In  FIG. 11 , reference numeral  91  denotes resin, such as epoxy, to attach the print head die  10  to the ink reservoir, and to prevent ink leakage, and reference numeral  92  denotes a double-sided tape to attach the FPC cable  60   a  to the cartridge  90 . It can be seen that the upper surface of the conductor  61  is covered by the protective film  62   a , requiring a small amount of resin to form the encapsulation  80  over only the exposed portions of the conductor  61  and the pad  20 . Consequently, an encapsulation  80  is small, particularly the height thereof may have a relatively low profile over the surface of the print head die  10 . Hence, the distance between the print head die  10  and a medium, such as paper, can be significantly minimized. In the conventional ink jet head, because the upper surface of a conductor is exposed, much resin enough to cover the exposed portion must be supplied, so that a relatively taller encapsulation is created, which leads to a large distance between the medium to be printed on and the surface of the print head die  10 . The application of the bending of the FPC cable  60   a  and the low profile encapsulation  80  according to the aspects of the present invention is not limited to the thermo-pressure and/or gang bonding generally described herein, but extends to implementation during the thermo-sonic and/or sequential single bonding as well. 
     FIG. 12  shows an exemplary embodiment of a structure of an FPC cable  60   b  capable of being applied to any of the embodiments described herein. In  FIG. 12 , which shows a modification to the FPC cable  60   a  shown in  FIGS. 9 through 11 , the bonding portion of the conductor  61  is covered by the upper and lower protective films  62   a  and  62   b . An aperture  62   c  is formed in the portion of the lower protective film  62   b  facing the pad  20 . The FPC cable  60   b  is electrically connected to the print head die  10  via the aperture  62   c  by bonding the conductor  61  to the pad  20 . 
   Preferably, the size of the aperture  62   c  is chosen such that the bump  70  fits completely within the aperture  62   c , further reducing the amount of the encapsulation  80  required. 
   In one aspect of the above-described present invention, an electrical defect due to pad peel-off phenomenon caused by the ultrasonic energy in a conventional tape automated bonding (TAB) method is avoided to increase the reliability of bonding. In another aspect of the present invention, the bonding process time is minimized by bonding at least two pairs of conductors and pads at a time, and more preferably, by bonding all of the pads on the print head die at the same time, i.e., gang bonding. In an yet another aspect of the present invention, the distance between the print head die and a printing medium is reduced by reducing the distance a droplet discharged from the print head die has to travel. Thus, more stable printing quality with higher density is provided. 
   The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention that fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, and all such modifications and equivalents would fall within the scope of the invention.