Abstract:
The present invention provides a radio frequency identification (RFID) tag which exchanges information with an external device in a noncontact manner, in which a paste is used as a material for an antenna, and which is designed to avoid the problem due to swelling of the paste caused by a pressing force from bumps of an IC chip. A paste escape recess is provided in which part of the paste provided to form the antenna is caused to flow therein by the pressure received from the bumps when the IC chip with the bumps is connected to the antenna.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a radio frequency identification (RFID) tag which exchanges information with an external device in a noncontact manner and a method of manufacturing the same. In some cases, among those skilled in the technical field corresponding to the present invention, the “RFID tag” referred to in this specification is called “RFID tag inlay” as an internal constituent member (inlay) for an “RFID tag”. In some other cases, this “RFID tag” is called “wireless IC tag”. Also, this “RFID” tag comprises a noncontact-type ID card.  
         [0003]     2. Description of the Related Art  
         [0004]     In recent years, various types of RFID tags capable of noncontact information exchange by means of radio waves with external devices typified by reader/writers have been proposed. One of the various types of RFID tags proposed has an antenna pattern for radio wave communication and an IC chip mounted on a base sheet made of a plastic or paper. One of possible applications of such type of RFID tag is that the RFID tag is attached to an article and information about the article is exchanged with an external device for identification or the like of the article.  
         [0005]      FIG. 1  (A) and (B) are a front view and a sectional side view, respectively, of an example of an RFID tag.  
         [0006]     The RFID tag  1  shown in  FIG. 1  is constituted by an antenna  12  provided on a base  13  formed of a material in the form of a sheet such as a polyethylene telephthalate (PET) film, an IC chip  11  connected to the antenna  12  through bumps  16 , and a cover sheet  14  bonded to the base  13  by an adhesive  15  so as to cover the antenna  12  and the IC chip  11 .  
         [0007]     The IC chip  11  constituting the RFID tag  1  is capable of exchanging information with an external device by performing wireless communication through the antenna  12 .  
         [0008]     Various applications including the above-mentioned one have been presented with respect to this type of RFID tag, which however has been hampered by high manufacturing cost. Therefore various attempts have been made to reduce the manufacturing cost of the RFID tag.  
         [0009]     As one of the attempts to reduce the manufacturing cost, there has been pursued the idea of forming an antenna by using a paste material which is made conductive by blending a metallic filler (Ag in ordinary cases) with a resin material such as an epoxy resin (Japanese Patent Laid-Open No. 2000-311226 (paragraph [0066])). If such a paste material can be used to replace a thin sheet of a metallic material such as Cu, Al or Au, it can largely contribute to a reduction in the manufacturing cost of the RFID tag.  
         [0010]     In manufacture of the RFID tag shown in  FIG. 1 , the IC chip  11  is connected to the antenna  12  formed on the surface of the base  13  formed of a PET member in the form of a sheet or the like, through the bumps (metal protrusions)  16  formed on electrodes of the IC chip  11 . However, in a case where the antenna  12  is formed by printing a case material, a problem described below may arise with the connection between the IC chip  11  and the antenna  12 .  
         [0011]      FIG. 2  (A) shows a case where a metal is used as an antenna material and  FIG. 2  (B) shows a case where a paste is used as an antenna material to be compared with each other.  
         [0012]     An antenna  121  (in  FIG. 2  (A)) formed of a thin sheet of a metal or an antenna  122  formed of a paste material (in  FIG. 2  (B)) is formed on the base  13  formed of a PET member. The bumps  16  are formed on electrodes  111  on the IC chip  11  in each of the cases shown in FIGS.  2 (A) and (B).  
         [0013]     FIGS.  2  (A) and (B) show a state in which the IC chip  11  with bumps  16  is placed on the base  13  on which the antenna  121  or  122  is formed, the bumps  16  facing the base  13 , and in which the IC chip  11  is connected to the antenna  121  or  122  through the bumps  16 .  
         [0014]     In  FIG. 2 , illustration of the adhesive  15  and the cover sheet  14  shown in  FIG. 1  (B) is omitted. At the time of connection, the IC chip  11  is pressed from above as viewed in the figure with a jig (not shown). A pressing force is thereby applied from the bumps  16  to the antenna  121  or  122 . In the case of the antenna  121  made of a metallic material as shown in  FIG. 2 (A), there is no problem with this pressing, since the hardness of the antenna  121  is high. In the case of the antenna  122  made of a paste material as shown in  FIG. 2  (B), there is a problem described below. A swelling  122   a  of paste material forming the antenna  122  around the bump  16  is produced. In this case, the necessary insulation distance cannot be maintained between the IC chip  11  and the antenna  122 . If this distance is changed, characteristics of the RFID tag including a wireless communication characteristic (hereinafter referred to as “tag characteristics”) are changed. This causes variations in tag characteristics when the RFID tags are mass-manufactured.  
         [0015]     The method of mounting various types of IC chips on a circuit board apart from the RFID tag is being widely practiced. In ordinary cases, a multiplicity of bumps are formed on an IC chip and the pressing force per bump is small even when a paste material is used as a wiring material on a circuit board and, therefore, there is no serious problem due to protrusion of the paste material.  
         [0016]     In contrast, in the case of the RFID tag, the number of bumps provided in one IC chip for connection to the antenna is about two or four, the pressing force per bump is extremely large presenting the above-mentioned problem of a swelling in paste material. In order to reduce the pressing force, it is necessary to reduce the pressing force applied when the IC chip is placed, by an apparatus for placing the IC chip on the base, to an extremely small value in comparison with the case of placing an ordinary IC chip on which a multiplicity of bumps are formed. However, considering the presence of an adhesive between the base and the IC chip, it is extremely difficult to reduce the pressing force to an extremely small value while enabling a reliable connection to be made in a short time.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention has been made in view of the above circumstances and provides an RFID tag using a paste as a material for an antenna and capable of avoiding the problem that the tag characteristics are changed by the above-mentioned rise of the paste, and a method of manufacturing the RFID tag.  
         [0018]     According to a first aspect of the present invention, there is provided an RFID tag having  
         [0019]     a base,  
         [0020]     an antenna for communication provided on the base,  
         [0021]     a circuit chip connected to the antenna through bumps, the circuit chip performing wireless communication through the antenna,  
         [0022]     wherein the antenna is formed of a paste in which a metallic filler is blended with a resin material, and  
         [0023]     a paste escape portion, in which part of the paste provided to form the antenna is caused to flow therein by a pressure received from the bumps when the circuit chip with the bumps is connected to the antenna, is provided.  
         [0024]     In the RFID tag in the first aspect of the present invention, the paste escape portion is provided to avoid the problem due to swelling of the paste described above with reference to  FIG. 2  (B).  
         [0025]     In the RFID tag in the first aspect of the present invention, the paste escape portion may be recesses provided at positions adjacent to portions of the base which receive the pressure from the bumps when the circuit chip with the bumps is connected to the antenna, or may be plural grooves between projections provided on portions of the antenna with which the bumps contact. Or it also may be plural grooves provided in the surface of each of the bumps which contacts with the antenna.  
         [0026]     Further, in the RFID tag in the first aspect of the present invention, each of the bumps may have a projection formed at the central portion on its surface on the antenna side, and the paste escape portion may include gaps formed between the antenna surface and the bump along a portion of the bump encircling the projection.  
         [0027]     According to a second aspect of the present invention, there is provided an RFID tag having  
         [0028]     a base,  
         [0029]     an antenna for communication provided on the base,  
         [0030]     a circuit chip connected to the antenna through bumps, the circuit chip performing wireless communication through the antenna,  
         [0031]     wherein the antenna is formed of a paste in which a metallic filler is blended with a resin material, and  
         [0032]     a coating film which is capable of suppressing swelling of the paste forming the antenna against a pressure received from the bumps when the circuit chip with the bumps is connected to the antenna, and through which the bumps are passed to be connected to the antenna, is provided on an upper surface of the antenna.  
         [0033]     In the RFID tag in the second aspect of the present invention, the coating film is provided to avoid the problem due to swelling of the paste described above with reference to  FIG. 2  (B), thus achieving the same effect as that of the RFID tag in the first aspect.  
         [0034]     According to the first aspect of the present invention, there is provided a method of manufacturing an RFID tag, including the steps of  
         [0035]     printing an antenna for communication on a base by using a paste in which a metallic filler is blended with a resin material,  
         [0036]     mounting a circuit chip with bumps capable of performing wireless communication through the antenna, the circuit chip and the antenna being connected to each other through the bumps, and  
         [0037]     forming a paste escape portion in which part of the paste provided to form the antenna is caused to flow therein by a pressure received from the bumps when the circuit chip with the bumps is connected to the antenna.  
         [0038]     According to the second aspect of the present invention, there is provided a method of manufacturing an RFID tag, including the steps of  
         [0039]     printing an antenna for communication on a base by using a paste in which a metallic filler is blended with a resin material,  
         [0040]     mounting a circuit chip with bumps capable of performing wireless communication through the antenna, the circuit chip and the antenna being connected to each other through the bumps, and  
         [0041]     forming on an upper surface of the antenna a coating film which is capable of suppressing swelling of the paste forming the antenna against a pressure received from the bumps when the circuit chip with the bumps is connected to the antenna, and through which the bumps are passed to be connected to the antenna.  
         [0042]     According to the present invention, as described above, a paste is used as the material of an antenna thereby solving the problem related to the tag characteristic variation due to swelling of the paste material caused by the pressing force received from the bumps of the circuit chip. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]      FIG. 1  (A) is a front view and  FIG. 1  (B) is a sectional side view of an example of an RFID tag;  
         [0044]      FIG. 2  (A) shows the case of using a metal as the material of an antenna and  FIG. 2  (B) shows the case using a paste as the material of the antenna to be compared with  FIG. 2  (A);  
         [0045]      FIG. 3  is a sectional view of an RFID tag according to a first embodiment of the present invention;  
         [0046]      FIG. 4  is a sectional view of an RFID tag according to a second embodiment of the present invention;  
         [0047]      FIG. 5  is a sectional view of an RFID tag according to a third embodiment of the present invention;  
         [0048]      FIG. 6  is a sectional view of an RFID tag according to a fourth embodiment of the present invention;  
         [0049]      FIG. 7  is a sectional view of an RFID tag according to a fifth embodiment of the present invention;  
         [0050]      FIG. 8  is a diagram showing a method of forming bumps on electrodes of an IC chip;  
         [0051]      FIG. 9  is a diagram showing a method of leveling the bump;  
         [0052]      FIG. 10  is a diagram showing the bump after leveling;  
         [0053]      FIG. 11  is a diagram showing a method of forming paste escape recesses in the RFID tag shown in  FIG. 3 ;  
         [0054]      FIG. 12  is a diagram showing another example of a method of forming paste escape recesses in the RFID tag shown in  FIG. 3 ;  
         [0055]      FIG. 13  is a diagram showing a method of forming projections in the RFID tag shown in  FIG. 4 ;  
         [0056]      FIG. 14  is a diagram showing a method of forming grooves in the bumps  16  in the RFID tag shown in  FIG. 5 ; and  
         [0057]      FIG. 15  is a diagram showing the bump in which grooves are formed by the method shown in  FIG. 14 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0058]     The present invention will be described below with respect to embodiments thereof.  
         [0059]      FIG. 3  is a sectional view of an RFID tag according to the first embodiment of the present invention.  
         [0060]     In  FIG. 3  and other figures referred to below, constituents corresponding to those of the RFID tag described above with reference to  FIG. 2  are indicated by the same reference numerals and the description for them will not be repeated. Description will be made only of points of difference from the described RFID tag. In  FIG. 3  and the other figures referred to below, illustration of the adhesive  15  between the base  13  and the IC chip  11  and the cover sheet  14  (see  FIG. 1  (B)) that covers upper portions of the RFID tag is omitted, as is that in  FIG. 2 . Also, in the embodiments described below, the base  13  is formed of a PET member and the antenna  122  is formed by using a paste prepared by blending an Ag filler with a resin material such as an epoxy resin unless otherwise specified.  
         [0061]     In the RFID tag  1 A shown in  FIG. 3 , paste escape recesses  131  are formed at positions adjacent to portions of the base  13  which receive a pressing force from bumps  16  of the IC chip  11  when the IC chip  11  with bumps  16  is connected to the antenna  122 . Receiving the pressing force from the bumps  16 , part of the paste is caused to flow into the paste escape recesses  131 . The desired insulation distance between the IC chip  11  and the antenna  122  is thereby maintained in the RFID tag  1 A. The RFID tag  1 A is free from a change in tag characteristics due to insufficiency of insulation distance.  
         [0062]      FIG. 4  is a sectional view of an RFID tag in a second embodiment of the present invention.  
         [0063]     In the RFID tag  1 B shown in  FIG. 4 , a plurality of projections  122   a  are formed on portions of the antenna  122  which contact with bumps  16  of the IC chip  11 . When the IC chip  11  with bumps  16  is connected to the antenna  122 , part of the paste, which is caused to flow by receiving a pressing force from the bumps  16 , flows into gaps  122   b  between the projections  122   a . The desired insulation distance between the IC chip  11  and the antenna  122  is thereby maintained in the RFID tag  1 B as well as in the RFID tag in the first embodiment shown in  FIG. 3 , thus stabilizing the tag characteristics.  
         [0064]      FIG. 5  is a sectional view of an RFID tag in a third embodiment of the present invention.  
         [0065]     In the RFID tag  1 C shown in  FIG. 5 , a plurality of grooves  161  are provided in the surface of bumps  16  which contact with the antenna  122 . When the IC chip  11  with bumps  16  is connected to the antenna  122 , part of the paste, which is caused to flow by receiving the pressing force from the bumps  16 , flows into the grooves  161  of the bumps  16 . The desired insulation distance between the IC chip  11  and the antenna  122  is thereby maintained in the RFID tag  1 C as well as in the RFID tags in the first and second embodiments shown in  FIGS. 3 and 4 , thus stabilizing the tag characteristics.  
         [0066]      FIG. 6  is a sectional view of an RFID tag in a fourth embodiment of the present invention.  
         [0067]     In the RFID tag  1 D shown in  FIG. 6 , a projection  162  is provided on a central portion of the surface of each bump  16  on the antenna  122  side. When the IC chip  11  with bumps  16  is connected to the antenna  122 , part of the paste which is caused to flow by receiving the pressing force from the bumps  16  flows into a gap  163  formed along a portion of the bumps  16  encircling the central projection. The desired insulation distance between the IC chip  11  and the antenna  122  is thereby maintained in this RFID tag well as in the RFID tags in the above-described embodiments, thus obtaining stabilized tag characteristics.  
         [0068]      FIG. 7  is a sectional view of an RFID tag in a fifth embodiment of the present invention.  
         [0069]     In the RFID tag  1 E shown in  FIG. 7 , a coating film  17  is formed on the upper surface of the antenna  122 . The coating film  17  is formed of an insulating material such as polyethylene, epoxy, polyester or saponified ethylene vinyl acetate (EVA) and has such strength that when the IC chip  11  with bumps  16  is connected to the antenna  122 , the tip of each bump  16  passes through the film to be connected to the antenna  122 , and that the film suppresses swelling of the paste against the pressing force received from the bump  16  at the time of connection. The desired insulation distance between the IC chip  11  and the antenna  122  is maintained thanks to the existence of this coating film  17  in this RFID tag as well as in the RFID tags in the above-described embodiments, thus obtaining stabilized tag characteristics.  
         [0070]     Methods of manufacturing the various RFID tags  1 A to  1 E described above will now be described.  
         [0071]      FIG. 8  is a diagram showing a method of forming the bumps on the electrodes of the IC chip.  
         [0072]     First, a thin metal wire  30  to be formed as bumps is caused to project from the tip of a jig  20  with a hole, as shown in  FIG. 8 (A), thereby causing electric between the thin metal wire  30  and a discharge electrode  40 . A portion of the thin metal wire  30  at the tip is molten by the discharge energy to form a metal ball  31 .  
         [0073]     Subsequently, the metal ball  31  is pressed against the electrode  111  of the IC chip  11  and ultrasonic waves are applied to the metal ball  31  through the jig  20 , as shown in  FIG. 8 (B). The metal ball  31  is joined to the electrode  111  of the IC chip  11  by the ultrasonic waves. When the jig  20  is removed, the metal ball  31  and the thin metal wire  30  at the foot are torn off to form the bump in original form  32  on the electrode  111  of the IC chip  11 , as shown in  FIG. 8 (C).  
         [0074]      FIG. 9  is a diagram showing a method of leveling the bump, and  FIG. 10  is a diagram showing the bump after leveling.  
         [0075]     After being formed on the electrode  111  of the IC chip  11  as shown in  FIG. 8 , the bump in original form  32  is pressed on a flat surface of a glass plate  50 , as shown in  FIG. 9 . The load for this pressing and the pressing height are selected to change the shape of the bump. That is, the bump  16  having the shape shown in  FIG. 10  (A) is formed in the case of low-load high-position pressing; the bump  16  having the shape shown in  FIG. 10  (B) is formed in the case of medium-load medium-position pressing; and the bump  16  having the shape shown in  FIG. 10  is formed in the case of high-load low-position pressing.  
         [0076]      FIG. 11  is a diagram showing a method of forming paste escape recesses  131  of the RFID tag  1 A shown in  FIG. 3 .  
         [0077]     A PET film  61  for supporting and a PET film  62  having holes bored therein in correspondence with the paste escape recesses  131  shown in  FIG. 3  are prepared ( FIG. 11 (A)). The two PET films  61  and  62  are stuck on each other to form the base  13  with paste escape recesses  131  in the RFID tag  1 A shown in  FIG. 3  ( FIG. 11  (B)), and the antenna  122  is formed by printing a paste on the base  13 . As the material of the antenna, a paste should have such a viscosity that the paste does not flow into the paste escape recesses  131  and does not fill the voids in the paste escape recesses  131  when no particular external force is applied.  
         [0078]     When the IC chip with bumps is placed and pressed on the paste, the paste which receives the pressing force from the bumps  16  flows into the paste escape recesses  131 , as shown in  FIG. 3 .  
         [0079]      FIG. 12  is a diagram showing an example of another method of forming the paste escape recesses  131  in the RFID tag  1 A shown in  FIG. 3 .  
         [0080]     In the example shown in  FIG. 11  two PET films  61  and  62  are prepared and stuck on each other. In  FIG. 12  (A), a PET film  63  having a thickness corresponding to the thickness of the two PET films  61  and  62  after sticking is prepared and portions where paste escape recesses  131  are to be formed are bored with a boring tool  70  ( FIG. 12  (B)), thereby forming the base  13  in which paste escape recesses  131  are formed. The same step as in  FIG. 11  is performed to form the antenna  122  by printing the paste on the base  13 .  
         [0081]     While in  FIG. 12  (B) the formation of paste escape recesses  131  by boring is described, chemical etching may be performed instead of boring to form paste escape recesses  131 .  
         [0082]      FIG. 13  is a diagram showing a method of forming projections  122   a  in the RFID tag  1 B shown in  FIG. 4 .  
         [0083]     The antenna  122  is formed on the base  13  by printing and dried to be hardened ( FIG. 13 (A)). A printing master  80  having holes formed in the portions which correspond to the projections shown in  FIG. 4  is thereafter placed on the antenna and the paste  83  of the same material as that of the antenna  122  is printed by being forced into the holes of the printing master  80  with a squeegee  81  ( FIG. 13 (B)).  
         [0084]     The printing master  80  for forming the projections is removed and the paste is dried to be hardened, thereby forming projections  122   a  on the antenna  122  ( FIG. 13 (C)).  
         [0085]     As the printing master  80 , a thin plate of Al, SUS or the like having holes formed at the desired positions by etching can be used. Although it is not shown in the drawings and not particularly described, a printing master is also used for printing an antenna and printing of the paste is performed by using a squeegee.  
         [0086]      FIG. 14  is a diagram showing a method of forming grooves  161  in the bumps  16  of the RFID tag  1 C shown in  FIG. 5 , and  FIG. 15  is a diagram showing the bumps in which the grooves are formed by the method.  
         [0087]     After the bumps in original form  32  have been formed on the electrodes  111  of the IC chip  11  as described above with reference to  FIG. 8 , they are pressed against a glass plate  90  having grooves formed in its surface. Bumps  16  having grooves  161  formed therein are thereby formed, as shown in  FIG. 15 .  
         [0088]     The grooves in the glass plate  90  can be formed, for example, by cutting the surface of the glass plate  90  with a diamond cutter such as one used for cutting a semiconductor ware. Etching may alternatively be performed on the surface of the glass plate  90  to form the grooves in the glass plate surface. The depth of the grooves  161  in the bumps  16  shown in  FIG. 15  is adjusted by changing the depth of the grooves formed in the glass surface and the pressure at which the bumps in the original form is pressed against the glass surface in which the grooves are formed.  
         [0089]     The RFID tag  1 D shown in  FIG. 6  is characterized in that each bump  16  has the projection  162  at a center of the surface on the antenna  122  side. This can be realized in such a manner that the bumps  16  shown in  FIG. 10  (A) is formed by adjusting the load and height when the bump in original form  32  is pressed against the surface of the glass plate  50  in leveling described above with reference to  FIG. 9 .  
         [0090]     The RFID tag  1 E shown in  FIG. 7  is realized in such a manner that after the antenna  122  has been formed by printing and drying of a paste, coating with the above-mentioned insulating material (for example, spraying of the material) is performed to form coating film  17 . In this case, however, the bumps  16  are formed by the above-described leveling (see  FIGS. 9 and 10 ) so as to have a projection capable of passing through the coating film.