Abstract:
An RFID tag, including: a base; an antenna pattern that is provided on the base to form a communication antenna and has a tapered connection end portion which provides a connection terminal to the antenna; an electric conductor that is attached onto the connection end portion of the antenna pattern and is smaller than the connection end portion; and a circuit chip that is electrically connected to the antenna pattern via the electric conductor and performs radio communication by use of the antenna.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an RFID (Radio Frequency Identification) tag that performs information exchange with external equipment in a noncontact manner. Incidentally, among those skilled in the art related to the technical field of the present application, the “RFID tag” used in the specification of the present application may sometimes be called an “inlay for RFID tag” by regarding the “RFID tag” as an internal component member (inlay) for “RFID tag.” Or alternatively, in some cases, this “RFID tag” may be referred to as “a radio IC tag.” Also, a noncontact type IC card is included in this “RFID tag.” 
   2. Description of the Related Art 
   In recent years, there have been proposed various RFID tags that perform information exchange with external equipment represented by a reader/writer in a noncontact manner by use of radio waves (refer to U.S. Pat. No. 6,239,703B1, for example). As one kind of this RFID tag, there has been proposed an RFID tag in which an antenna pattern for radio communication and an IC chip are mounted on a base sheet made of plastics or paper. A conceived mode of using an RFID tag of this type is such that the RFID tag is stuck to an article and the like and performs the identification of the article by exchanging information on the article with external equipment. 
   In such an RFID tag, a circuit chip and an antenna pattern are electrically connected together by pinching a micro electric conductor called a bump between the circuit chip and an end portion of the antenna pattern. The circuit chip is fixed to the above-described base sheet and on this occasion, the area between the circuit chip and the end portion of the antenna pattern except the part occupied by the bump is filled with an adhesive. As a result, it follows that a micro capacitor is formed, with the circuit chip and the end portion of the antenna pattern serving as electrodes. The capacity of such a capacitor that is unnecessary in terms of design is called a parasitic capacity, and a parasitic capacity in an RFID tag has an adverse effect as described below. 
   There is a type of RFID tag that obtains operating power of an internal circuit chip from external equipment, and in this type of RFID tag the power from external equipment is supplied to the circuit chip via an antenna pattern. When at this time, a parasitic capacity exists between the mutually electrically connected circuit chip and end portion of the antenna pattern, the input of operating power from the antenna pattern to the circuit chip is interfered with. As a result, there is a possibility that troubles such as the deterioration of the communication distance of the RFID tag due to insufficient operating power might be caused. 
   Therefore, there have been proposed techniques which involve providing, on the antenna pattern side, an adjustment pattern capable of changing an impedance by working such as etching, and adjusting an impedance including the above-described parasitic capacity between the antenna pattern and the circuit chip by working this adjustment pattern (refer to U.S. Pat. No. 6,535,175B2, for example). 
   However, such adjustment work has the problem that productivity is lowered thereby. 
   Hence, measures against the above-described parasitic capacity in an RFID tag often involve predicting the parasitic capacity and providing, on the antenna pattern side, a pattern which generates an inductance which cancels out the parasitic capacity by a circuital resonance. 
   The above-described prediction of a parasitic capacity in an RFID tag is performed on the basis of the area of a portion in which the circuit chip and the end portion of the antenna pattern overlap each other, with the above-described bump pinched between the two, (an overlapping portion). 
   However, there are variations in the positional relationship between the above-described circuit chip and the antenna pattern during the manufacture of an RFID tag, and it is difficult to predict a parasitic capacity in a stable manner. Furthermore, even when a predicted parasitic capacity is determined beyond the limits of reason and an inductance is found on the basis of the parasitic capacity, in a case where an actual parasitic capacity should differ from the predicted parasitic capacity, it becomes impossible to ensure matching between the actual parasitic capacity and the impedance and the above-described resonance does not occur any more. As a result, the parasitic capacity is not canceled and it follows that the above-described problem of deterioration of the communication distance of the RFID tag or the like occurs. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstances and provides an RFID tag which can be manufactured with high productivity while avoiding the deterioration of communication distance. 
   An RFID tag of the present invention is provided with a base, an antenna pattern that is provided on the base to form a communication antenna and has a tapered connection end portion which provides a connection terminal to the antenna, an electric conductor that is attached onto the connection end portion of the antenna pattern and is smaller than the connection end portion, and a circuit chip that is electrically connected to the antenna pattern via the electric conductor and performs radio communication by use of the antenna. 
   In an RFID tag of the present invention, a form in which “the antenna pattern has an inductance generation portion that generates an inductance suited to a capacitance between the connection end portion and the circuit chip and prevents performance deterioration of the antenna due to the capacitance” is a typical form. 
   In an RFID tag of the present invention, the circuit chip overhangs the connection end part of the antenna pattern, with the electric conductor pinched. As a result, it follows that the circuit chip and the connection end portion of the antenna pattern overlap each other with a gap for the electric conductor. In the part of this portion where the two overlap each other (the overlapping portion) except the conductor, there is formed a capacitor which is unnecessary in terms of design, with the circuit chip and the connection end portion which are opposed each other serving as electrodes. The capacity of this capacitor (the parasitic capacity) is proportional to the area of the overlapping portion and inversely proportional to the gap between the circuit chip and the connection end portion of the antenna pattern in this overlapping portion. 
   Incidentally, in manufacturing an RFID tag of the present invention, it is a general practice to adopt a method by which the electric conductor is attached to the circuit chip and after that, the circuit chip is heated while the circuit chip is being pressed against the base to ensure that the electric conductor is attached to the connection end portion of the antenna pattern. When this method is adopted, the electric conductor, along with part of the connection end portion, sinks into the base. 
   In a case where the circuit chip is mounted in such a manner as to be shifted toward the leading end side of the connection end portion, the area of the overlapping portion becomes narrow and a decrease proportional to this narrowing of the are occurs in the parasitic capacity. In this case, the position where the electric conductor is attached to the connection end portion is shifted toward the leading end side of the connection end portion. And because the connection end portion is tapered and its strength decreases toward the leading end, the electric conductor sinks deep into the base, with the result that the gap between the circuit chip and the connection end portion of the antenna pattern in the overlapping portion becomes narrow and that an increase inversely proportional to this narrowing of the gap occurs in the parasitic capacity. As a result, the above-described decrease and increase cancel each other out and, therefore, it follows that a change scarcely occurs in the parasitic capacity even when the mounting position of the circuit chip is shifted toward the leading end side of the connection end portion. 
   On the other hand, in a case where the circuit chip is mounted in such a manner as to be shifted toward the base side of the connection end portion, contrary to the above-described case, the area of the overlapping portion widens and the gap between the circuit chip and the connection end portion of the antenna pattern in the overlapping portion widens. As a result, an increase proportional to the widening of the area of the overlapping portion and a decrease inversely proportional to the widening of the gap between the circuit chip and the connection end portion occur in the parasitic capacity and the two cancel each other out and, therefore, it follows that a change scarcely occurs in the parasitic capacity even when the mounting position of the circuit chip is shifted toward the base side of the connection end portion. 
   As described above, in an RFID tag of the present invention, the parasitic capacity is stable for manufacturing reasons and, therefore, the parasitic capacity can be predicted in a stable manner. As a result of this, it is possible to take, as a measure related to a parasitic capacity, a measure capable of obtaining high productivity without imposing burden on manufacturing, for example, providing an antenna pattern having an inductance generation portion which causes an inductance determined on the basis of a predicted parasitic capacity to be generated. In other words, according to the present invention, it is possible to obtain an RFID tag capable of being manufactured with high productivity while avoiding the deterioration of communication distance. 
   As described above, according to the present invention, it is possible to provide an RFID tag capable of being manufactured with high productivity while avoiding the deterioration of communication distance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail based on the following figures, wherein: 
       FIGS. 1(A) and 1(B)  are a front view and a side view, respectively, of the first embodiment of the present invention; 
       FIG. 2  is an enlarged view which shows the shape of a pad of the antenna pattern of  FIG. 1 ; 
       FIGS. 3(A) and 3(B)  are an enlarged front view and an enlarged side view, respectively, of the area near a pad in a case where a circuit chip is mounted in such a manner as to be shifted toward the leading end side of the triangular pad from a standard position; 
       FIGS. 4(A) and 4(B)  are an enlarged front view and an enlarged side view, respectively, of the area near a pad in a case where a circuit chip is mounted in such a manner as to be shifted toward the base side of the triangular pad from a standard position; 
       FIG. 5  is an enlarged view which shows the shape of an antenna pattern in an RFID tag of the second embodiment of the present invention and; 
       FIG. 6  is an enlarged view which shows the shape of an antenna pattern in an RFID tag of the third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1(A) and 1(B)  are a front view and a side view, respectively, of the first embodiment of the present invention. However, in the front view shown here, the internal structure is seen through from the front of an RFID tag and in the side view shown here, the internal structure is seen through from the side of an RFID tag. 
   An RFID tag  1  shown in  FIGS. 1(A) and 1(B)  is constituted by an antenna pattern  3  provided on a base sheet  2 , a circuit chip  4  which is bonded onto the base sheet  2  with an epoxy adhesive  8  and is electrically connected to the antenna pattern via a bump  6 , an inductance pattern  5  which is electrically connected to the antenna pattern  3  and generates an inductance, and a cover sheet  7  which covers these antenna pattern  3 , circuit chip  4  and inductance pattern  5  and bonded to the base sheet  2 . The cover sheet  7  is usually formed from a material selected from among PET materials, polyester materials, polyolefin materials, polycarbonate materials, acrylic materials, etc. 
   In this first embodiment, the above-described base sheet  2 , circuit chip  4  and bump  6  correspond, respectively, to an example of the base, circuit chip and electric conductor according to the present invention. And a combination of the antenna pattern  3  and the inductance pattern  5  corresponds to an example of the antenna pattern according to the present invention, and inductance pattern  5  corresponds to an example of the inductance generation portion according to the present invention. 
   This RFID tag  1  receives the energy of an electromagnetic field released by a reader/writer as electric energy by use of the antenna pattern  3  and the circuit chip  4  is driven by the electric energy, whereby the communication action is realized. 
   This RFID tag  1  is made by performing the following procedure. First, the bump  6  is attached to the circuit chip  4 . The end portion of the antenna pattern  3  on the circuit chip  4  side provides a pad  3   a  to electrically connect this antenna pattern  3  to other parts. The circuit chip  4  to which the bump  6  is attached is pressed against the base sheet  2  to ensure that the bump  6  is attached to the pad  3   a  and at the same time, the circuit chip  4  is fixed with an adhesive  8  and heated. 
   The end portion of the antenna pattern  3  on the circuit chip  4  side provides the pad  3   a  to electrically connect this antenna pattern  3  to the circuit chip  4  via the bump  6 , and this pad  3   a  and the circuit chip  4  partially overlap each other in such a manner as to pinch the bump  6  between the two, with a gap for the bump  6 . In the portion where the two overlap each other (the overlapping portion), the adhesive  8  is filled in the area except the bump  6 , with the result that a capacitor which is unnecessary in terms of design is formed in the above-described overlapping portion, with the pad  3   a  and the circuit chip  4  serving as electrodes. The capacity of this capacitor (the parasitic capacity) prevents the input of power from the antenna pattern  3  to the circuit chip  4 , causing troubles such as the deterioration of the communication distance of the RFID tag  1 . Therefore, the present embodiment is constituted in such a manner that by causing a resonance to occur between the inductance generated by the inductance pattern  5  provided on the antenna pattern  3  side and the above-described parasitic capacity, this parasitic capacity is canceled out. 
   In the present embodiment, the inductance is found using Eq. (1) below on the basis of a parasitic capacity (standard capacity) which is considered to be generated in the overlapping portion of the circuit chip  4  and the pad  3   a  on the assumption that the circuit chip  4  is mounted in a standard position determined in performing designing. In Eq. (1), “L” is an inductance, “C” is the standard capacity, and “f” is resonance frequency.
 
(2π f ) 2 =1/( L×C )  (1)
 
   On the assumption that the circuit chip  4  is mounted in the standard position, the standard capacity C is found using Eq. (2) below, where the area of the overlapping portion of the circuit chip  4  and of the pad  3   a  (the standard area) is denoted by “S,” the gap between the circuit chip  4  and the pad  3   a  (the standard gap) is denoted by “D,” and the dielectric constant of the adhesive  8  is denoted by “ε.”
 
 C=ε×S/D   (2)
 
   Variations ascribed to the accuracy of manufacture etc. exist in the mounting position of the circuit chip  4 , and hence it follows that the area of an actual overlapping portion of the circuit chip  4  and of the pad  3   a  has some error with respect to the above-described standard area S. If the area of an actual overlapping portion differs from the standard area S, also a parasitic capacity which actually occurs in an RFID tag  1  differs from the standard capacity C. Therefore, even when the inductance pattern  5  which generates an inductance L is found on the basis of the standard capacity C, it becomes impossible to ensure the matching given by Eq. (1) between an actual parasitic capacity and the inductance L and hence there is a possibility that the parasitic capacity may not be canceled out. In the present embodiment, therefore, even when variations occur in the mounting position of the circuit chip  4 , in order to ensure that an actual parasitic capacity is substantially equally to the standard capacity C, that is, the parasitic capacity is stabilized at the standard capacity C for manufacturing reasons, the shape of the pad  3   a  of the antenna pattern  3  is contrived as described below. 
     FIG. 2  is an enlarged view which shows the shape of a pad of the antenna pattern of  FIG. 1 . 
   In  FIG. 2  is shown an enlarged view of the area near the pad  3   a  of the antenna pattern  3  in the RFID tag  1  in the present embodiment. As shown in  FIG. 2 , that the pad  3   a  has the shape of a triangle the leading end of which faces the circuit chip  4 . Part  3   a - 1  of this pad  3   a  overlaps the circuit chip  4 . The above-described parasitic capacity is generated in this overlapping portion. This triangular shape of the pad  3   a  corresponds to an example of the “tapered shape” according to the present invention. 
   Even in a case where variations occur in the mounting position of the circuit chip  4 , the triangular pad  3   a  shown in  FIG. 2  is provided as a contrivance to stabilize the parasitic capacity at the standard capacity C, and the effect obtained from this pad  3   a  will be described below. 
     FIGS. 3(A) and 3(B)  are an enlarged front view and an enlarged side view, respectively, of the area near a pad in a case where a circuit chip is mounted in such a manner as to be shifted toward the leading end side of the triangular pad from a standard position. In this enlarged side view, the internal structure near the pad in an RFID tag  1  is seen through from the side. In the present specification, all drawings called enlarged side views are similar ones. 
   In the case where a circuit chip  4  is mounted in such a manner as to be shifted toward the leading end side of the triangular pad  3   a  from a standard position P, as shown in the enlarged front view of  FIG. 3(A) , the area of an overlapping portion of the circuit chip  4  and of the pad  3   a  is narrower than the area of an overlapping portion in a case where the circuit chip  4  is disposed in the standard position P, i.e., the above-described standard area S. 
   As described above, the circuit chip  4  is pressed against a base sheet  2 , with a bump  6  pinched between the two. As a result, the bump  6  receives a depressing force from the circuit chip  4 , and as shown in the enlarged side view of  FIG. 3(B) , the bump  6 , along with part of the pad  3   a , sinks into the base sheet  2 . 
   Also, the bump  6  is first attached to the circuit chip  4 . For this reason, when the circuit chip  4  is mounted in such a manner as to be shifted toward the leading end side of the triangular pad  3   a  from a standard position P, as in the example of  FIGS. 3(A) and 3(B) , the bump  6  is attached to the part near the leading end on the pad  3   a  compared to the case where the circuit chip  4  is disposed in the standard position P. Because at this time the pad  3   a  is in the shape of a triangle, the width W 1  of the pad  3   a  in a part where the bump is disposed, becomes narrower than the width W of the pad  3   a  in a part where the bump is disposed when the circuit chip  4  is disposed in the standard position P. As a result of this, the strength of the part of the pad  3   a  which supports the bump  6  becomes weaker than the strength obtained when the circuit chip  4  is disposed in the standard position P, and the bump  6  sinks into the base sheet  2  deeper than in the case where the circuit chip  4  is disposed in the standard position P. As a result, the gap D 1  between the circuit chip  4  and the pad  3   a  in the overlapping portion of the circuit chip  4  and of the pad  3   a  becomes narrower than the gap between the circuit chip  4  and the pad  3   a  when the circuit chip  4  is disposed in the standard position P, that is, the gap D. 
   When the circuit chip  4  is mounted in such a manner as to be shifted toward the leading end side of the triangular pad  3   a  from the standard position P, as in the example of  FIGS. 3(A) and 3(B) , as described above, the area of the overlapping portion becomes narrower than the standard area S. As is apparent from Eq. (2) above, the parasitic capacity of the overlapping portion is proportional to the area of the overlapping portion and, therefore, the above-described narrowing of the overlapping portion reduces the parasitic capacity to a level lower than the standard capacity C. On the other hand, as is apparent from Eq. (2), the parasitic capacity is inversely proportional to the gap between the circuit chip  4  and the pad  3   a  and, therefore, the shortening of the gap between the circuit chip  4  and the pad  3   a  increases the parasitic capacity to a level higher than the standard capacity C. Eventually, a decrease in the parasitic capacity due to the narrowing of the area of the overlapping portion is compensated for by an increase in the parasitic capacity due to the shortening of the gap between the circuit chip  4  and the pad  3   a . That is, even when the circuit chip  4  is mounted in such a manner as to be shifted toward the leading end side of the triangular pad  3   a  from the standard position P, as in the example of  FIGS. 3(A) and 3(B) , the parasitic capacity in the overlapping portion becomes almost equal to the standard capacity C. 
   Next, a description will be given of a case where, contrary to the example of  FIGS. 3(A) and 3(B) , a circuit chip  4  is mounted in such a manner as to be shifted toward the base side of a triangular pad  3   a.    
     FIGS. 4(A) and 4(B)  are an enlarged front view and an enlarged side view, respectively, of the area near a pad in a case where a circuit chip is mounted in such a manner as to be shifted toward the base side of the triangular pad from a standard position. 
   In the case of  FIGS. 4(A) and 4(B) , as shown in the enlarged front view of  FIG. 4(A) , the area of the overlapping portion of the circuit chip  4  and of the pad  3   a  becomes wider than the area of the overlapping portion when the circuit chip  4  is disposed in the standard position P, i.e., the above-described area standard area S. On the other hand, in this case, the width W 2  of the part where the bump  6  is disposed, becomes wider than the width W of the part where the bump  6  is disposed when the circuit chip  4  is disposed in the standard position P. As a result of this, the strength of the part of the pad  3   a  which supports the bump  6  becomes stronger than the strength obtained when the circuit chip  4  is disposed in the standard position P, and the bump  6  sinks into the base sheet  2  less deep than in the case where the circuit chip  4  is disposed in the standard position P. As a result, the gap D 2  between the circuit chip  4  and the pad  3   a  in the overlapping portion of the circuit chip  4  and of the pad  3   a  becomes wider than the above-described standard gap D. The widening of the area of the overlapping portion increases the parasitic capacity to a level higher than the standard capacity C, and the widening of the gap between the circuit chip  4  and the pad  3   a  reduces the parasitic capacity to a level lower than the standard capacity C. Eventually, an increase in the parasitic capacity due to the widening of the area of the overlapping portion is canceled out by a decrease in the parasitic capacity due to the widening of the gap between the circuit chip  4  and the pad  3   a . That is, even when the circuit chip  4  is mounted in such a manner as to be shifted toward the base side of the triangular pad  3   a  from the standard position P, as in the example of  FIGS. 4(A) and 4(B) , the parasitic capacity becomes almost equal to the standard capacity C. 
   As described above by referring to  FIGS. 3(A) and 3(B)  and  FIGS. 4(A) and 4(B) , in an RFID tag  1  of the present embodiment, even when the mounting position of the circuit chip  4  is shifted from the above-described standard position P toward the lead end side or the base side of the triangular pad  3   a , the parasitic capacity becomes almost equal to the standard capacity C. That is, in an RFID tag of the present embodiment, the parasitic capacity is stable at the standard capacity C for manufacturing reasons and, therefore, the inductance L found on the basis of the standard capacity C works effectively. In this manner, in an RFID tag  1  of the present embodiment, it is possible to find an effective inductance L in the design stage and hence during manufacturing, troublesome work such as the adjustment of inductance is unnecessary and it is necessary only that an inductance pattern as designed be made. Therefore, it is possible to manufacture an RFID tag  1  of the present embodiment with high productivity while avoiding the deterioration of communication distance ascribed to the parasitic capacity. 
   Incidentally, the “tapered shape” in the present invention is not limited to the triangular shape in the above-described first embodiment and may be shapes as described below. 
   In the following, two examples in which the shape of the pad is different from that of the first embodiment will be described as the second embodiment and the third embodiment. However, these embodiments differ from the first embodiment only in the shape of the pad, and hence in the following, descriptions will be made by paying attention to the differences from the first embodiment. 
     FIG. 5  is an enlarged view which shows the shape of an antenna pattern in an RFID tag of the second embodiment of the present invention. Incidentally, in  FIG. 5 , like reference numerals refer to elements similar to the component elements of the above-described first embodiment. 
   In an RFID tag  9  shown in  FIG. 5 , a pad  10   a  of an antenna pattern  10  on which a bump  6  has a tapered shape with a rounded leading end. In the present embodiment, this shape of the pad  10   a  produces the same effect as the triangular shape of the pad  3   a  of the first embodiment, and variations in the parasitic capacity ascribed to variations in the mounting position of the circuit chip  4  are suppressed, with the result that this parasitic capacity becomes stable for manufacturing reasons. 
     FIG. 6  is an enlarged view which shows the shape of an antenna pattern in an RFID tag of the third embodiment of the present invention. Also in  FIG. 6 , like reference numerals refer to elements similar to the component elements of the above-described first embodiment. 
   In An RFID tag  11  shown in  FIG. 6 , a pad  12   a  of an antenna pattern  12  on which a bump  6  is mounted has a tapered shape the width of which becomes narrower by stages toward the circuit chip  4  side. In the present embodiment, this shape of the pad  12   a  produces the same effect as the triangular shape of the pad  3   a  of the first embodiment and the tapered shape of the pad  10   a  of the second embodiment with a rounded leading end and variations in the parasitic capacity ascribed to variations in the mounting position of the circuit chip  4  are suppressed, with the result that this parasitic capacity becomes stable for manufacturing reasons. 
   As described above, according to the RFID tags of the first to third embodiments, the parasitic capacity is stable regardless of variations in the mounting of the circuit chip, and hence an inductance effective for canceling out the parasitic capacity is found in the design stage. As a result of this, unnecessary troublesome work such as the adjustment of inductance during manufacturing can be saved and high-productivity manufacturing becomes possible. That is, it is possible to manufacture the RFID tags of the first to third embodiments with high productivity while avoiding the deterioration of communication distance ascribed to the parasitic capacity. 
   In the foregoing, as examples of the antenna pattern according to the present invention, the descriptions were given of three examples of the antenna pattern  3  having the triangular pad  3   a , the antenna pattern  10  having the tapered pad  10   a  with a rounded leading end, and the antenna pattern  12  having the tapered pad  12   a  in which the width decreases by stages to ward the circuit chip side. However, the present invention is not limited to them. As an antenna pattern of the present invention, any antenna pattern having a pad which becomes narrow toward the leading end can be used regardless of the shape of the pad.