Abstract:
The present invention provides an RFID tag excellent in the diffusion of heat. The RFID tag includes a base, an antenna pattern that is provided on the base and forms a communication antenna, a circuit chip that is electrically connected to the antenna pattern and performs radio communication via the antenna, a cover that is provided in close contact with the base in such a manner as to cover the antenna pattern except a prescribed region including the circuit chip, and an insulating thermal diffusion material that covers the prescribed region and is in thermal contact with the circuit chip. The insulating thermal diffusion material has thermal conductivity higher than the thermal conductivity of the cover.

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. 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 RFID tags, it has been proposed that a cover that covers a base sheet be provided in order to protect an antenna pattern or an IC chip. 
       FIGS. 1(A) and 1(B)  are a front view and a side view, respectively, of a conventional RFID tag. The side view shown here is a drawing in which the internal structure is seen through from the side-surface side of the RFID tag. In this specification, drawings hereinafter called a side view are all similar drawings. 
     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 , an IC chip  4  that is bonded to the base sheet  2  with an epoxy adhesive  7  and electrically connected to the antenna pattern  3  via a bump  5 , and a cover sheet  6  bonded to the base sheet  2  in such a manner as to cover the antenna pattern  3  and the IC chip  4 . The cover sheet  6  is usually formed from a material selected from among PET materials, polyester materials, polyolefin materials, polycarbonate materials, acrylic materials, etc. 
     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 IC chip  4  is driven by the electric energy, whereby the communication action is realized. 
     In the RFID tag  1  constructed as described above, the height of the IC chip  4  portion is larger than the height of other portions. Therefore, when something rubs the cover sheet  6  of the RFID tag  1  and when the RFID tag  1  is used in such a manner as to be sandwiched between books, impacts and loads are concentrated on the IC chip  4  and this might cause troubles in the IC chip  4  and faults such as the exfoliation of the IC chip  4 . Furthermore, there is also a possibility that the stretch or sag of the cover sheet  6  occurs near the IC chip  4 , generating residual stresses, with the result that the cover sheet  6  might come off due to the residual stresses. 
     Compared to such RFID tags of typical structure, there have also been proposed RFID tags in which ideas for protecting IC chips are incorporated (refer to, for example, U.S. Pat. Nos. 6,100,804, 6,265,977, 6,147,604, 6,215,401 and 6,294,998). In RFID tags disclosed in these patent documents, an IC chip is embedded in a sealing member or an intermediate layer and the surface of the RFID tag is made flush, whereby the concentration of impacts and loads on the IC chip are avoided. 
     However, in such conventional RFID tags, the heat generated by the IC chip is captured and confined in due to the presence of the sealing member and the intermediate layer and this might cause malfunctions of the IC chip. Also, in the case of the RFID tag  1  of a typical construction as shown in  FIGS. 1(A) and 1(B) , the thermal conductivity of the cover sheet  6  is low, the thermal capacity of the IC chip  4  itself is also low and, besides, also between the IC chip  4  and the antenna pattern  3 , the greater part except the portion of the bump  5  is embedded with an epoxy adhesive  7  and the thermal resistance by connection is large. For this reason, heat is apt to be captured and confined in the IC chip  4 . Therefore, in a case where the RFID tag  1  is present near a reader/writer and subjected to a strong electromagnetic field, it might be thought that the temperature rises abruptly due to the heat generated in the IC chip  4 . Such an abrupt temperature rise might cause malfunctions of the IC chip  4 . Also, in a case where the temperature of an article to which the IC chip  4  is stuck is constantly a high temperature of 50° C. to 70° C., even when the RFID tag  1  is placed at a distance from a reader/writer, there is a possibility that a critical temperature for the stable operation in a transistor within the IC chip  4  and a critical temperature for long-term memory holding might be exceeded due to the heat generation from the IC chip  4 . 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above circumstances and provides an RFID tag excellent in the diffusion of heat. 
     An RFID tag of the present invention includes: a base; an antenna pattern that is provided on the base and forms a communication antenna; a circuit chip that is electrically connected to the antenna pattern and performs radio communication via the antenna; a cover that is provided in close contact with the base in such a manner as to cover the antenna pattern except a prescribed region including the circuit chip; and an insulating thermal diffusion material that covers the prescribed region and is in thermal contact with the circuit chip, the insulating thermal diffusion material having thermal conductivity higher than the thermal conductivity of the cover. 
     According to an RFID tag of the invention, because the thermal diffusion material is provided, the RFID tag is excellent in the diffusion of heat and malfunctions by heat and the like are prevented. Also, because the cover does not cover the circuit chip, the coming off of the cover due to residual stresses as described above is also prevented. 
     It is preferred that in the RFID tag, the thermal diffusion material be provided with a center part on the circuit chip and a protective part that encloses the center part, the height of the protective part being larger than the height of the center part. 
     By providing the thermal diffusion material having this protective part, the concentration of impacts and loads on the circuit chip can be avoided. 
     The thermal diffusion material in the RFID tag of the invention may be formed by bonding a seat-like member to the prescribed region or may be a liquid material that is applied to the prescribed region and has solidified. 
     Such a thermal diffusion material as described above can be provided only in an accepted product after an operation test in the process of manufacturing RFID tags and it becomes easy to distinguish between accepted products and rejected products. When a liquid material is used, the manufacturing process becomes simple and cost reduction is expected. 
     The thermal diffusion material in the RFID tag of the invention may be an insulating material into which a thermally conductive granule having thermal conductivity higher than the thermal conductivity of the insulating material is mixed or the thermal diffusion material may have a lamellar structure including a first layer and a second layer having thermal conductivity higher than the thermal conductivity of the first layer. 
     The case of the thermal diffusion material into which the thermally conductive granule is mixed, is excellent in that an RFID tag of high thermal conductivity can be easily obtained. The case of the thermal diffusion material having the lamellar structure, is excellent in that an RFID tag excellent in both strength and the diffusion of heat can be easily obtained. 
     As described above, the RFID tag of the invention is excellent in the diffusion of heat and malfunctions of the circuit chip by heat and the like can be prevented. 
    
    
     
       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 a conventional RFID tag; 
         FIGS. 2(A) and 2(B)  are a front view and a side view, respectively, of the first embodiment of the present invention; 
         FIG. 3  is a conceptual diagram of the process of manufacturing an RFID tag; 
         FIG. 4  is a detail drawing of the perforation step shown in  FIG. 3 ; 
         FIGS. 5(A) and 5(B)  are a front view and a side view, respectively, of a semifinished product; 
         FIGS. 6(A) and 6(B)  are a front view and a side view, respectively, of an intermediate product after the laminating step; 
         FIGS. 7(A) and 7(B)  are each a detail drawing of a thermal diffusion material; 
         FIG. 8  is an explanatory drawing of the thermal diffusion material addition step shown in  FIG. 3 ; 
         FIG. 9  is a drawing that shows the second embodiment of the invention; 
         FIG. 10  is a drawing that shows another manufacturing method in the second embodiment of the invention; 
         FIG. 11  is a drawing that shows the third embodiment of the invention; 
         FIG. 12  is a drawing that shows the fourth embodiment of the invention; and 
         FIG. 13  is a drawing that shows the fifth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below by referring to the drawings. 
       FIGS. 2(A) and 2(B)  are a front view and a side view, respectively, of the first embodiment of the present invention. 
     An RFID tag  10  shown in  FIGS. 2(A) and 2(B)  is constituted by an antenna pattern  12  provided on a base sheet  11 , an IC chip  13  that is bonded to the base sheet  11  with an epoxy adhesive  17  and electrically connected to the antenna pattern  12  via a bump  16 , a cover sheet  14  bonded to the base sheet  11  in such a manner as to cover the antenna pattern  12  except an area near the IC chip  13 , and a thermal diffusion material  15  bonded to the cover sheet  14  and the IC chip  13  from above the IC chip  13 . 
     Also this RFID tag  10  receives the energy of an electromagnetic field released by a reader/writer as electric energy by use of the antenna pattern  12  and the IC chip  13  is driven by the electric energy, whereby the communication action is realized. 
     In this first embodiment, the base sheet  11  corresponds to an example of the base of the invention, the antenna pattern  12  corresponds to an example of the antenna pattern of the invention, the IC chip  13  corresponds to an example of the circuit chip of the invention, the cover sheet  14  corresponds to an example of the cover of the invention, and the thermal diffusion material  15  corresponds to an example of the thermal diffusion material of the invention. 
     Although the cover sheet  14  is formed from a PET material, the cover sheet  14  may also be formed from a material selected from among polyester materials, polyolefin materials, polycarbonate materials, acrylic materials, etc. The bonding of the IC chip  13  is possible by using an epoxy film in place of the epoxy adhesive  17 . 
     Although details of the construction of the thermal diffusion material  15  will be given layer, this thermal diffusion material  15  has thermal conductivity higher than the thermal conductivity of the cover sheet  14  and holds the temperature of the IC chip  13  at low levels by efficiently diffusing the heat generated by the IC chip  13  into the air and the like, thereby making it possible to realize stable operation and the like. Furthermore, because the structure in which this thermal diffusion material  15  is bonded from above the IC chip  13  does not cause the stretch or sag of the cover sheet  15 , there is no possibility that the thermal diffusion material  15  and the cover sheet  14  might come off due to residual stresses. 
     The method of manufacturing the RFID tag  10  will be described below. 
       FIG. 3  is a conceptual diagram of the process of manufacturing an RFID tag. 
     The RFID tag  10  shown in  FIGS. 2(A) and 2(B)  is manufactured from a semifinished product  21  of an RFID tag and a laminate film  22  by undergoing a perforation step  20 , a laminating step  30 , a test step  40  and a thermal diffusion material addition step  50 . 
     In the perforation step  20 , holes are made in the laminate film  22  by use of a perforating jig  23 . In the laminating step  30 , a sheet in which a large number of semifinished products  21  are linked together and the laminate film  22  are transferred by transfer rolls  31  and superposed on each other, and the semifinished product  21  and the laminate film  22  are bonded together by being heated and pressurized by a thermocompression device  32 . In the test step  40 , an operation test of an IC chip  13  in a region enclosed with electromagnetic shields  42  by use of a reader/writer  41  is performed to ascertain whether the product has a capacity necessary for an RFID tag. In the thermal diffusion material addition step  50 , a silicone grease, which is a kind of thermally conductive grease, is applied by use of a dispenser  51  and the thermal diffusion material is stuck by use of a sticking jig  52 . 
       FIG. 4  is a detail drawing of the perforation step  20  shown in  FIG. 3 . 
     In this perforation step  20 , holes are made by the perforating jigs  23  in places corresponding to the IC chip  13  of  FIGS. 2(A) and 2(B)  in the laminate film  22  of a PET material that constitutes the cover sheet  14  shown in  FIGS. 2(A) and 2(B) . The laminate film  22  in which holes have been made is transferred in the direction indicated by an arrow A in the figure and superposed on a sheet in which a large number of semifinished products  21  are linked together. And the laminate film  22  superposed on the sheet containing the semifinished products  21  is delivered to the laminating step  30  shown in  FIG. 3 . 
       FIGS. 5(A) and 5(B)  are a front view and a side view, respectively, of a semifinished product. 
     In  FIGS. 5(A) and 5(B) , one of many semifinished products  21  that are linked together in sheet form is illustrated, and in this semifinished product  21 , the antenna pattern  12  and the IC chip  13  are provided in an exposed condition on the base sheet  11 . The method of manufacturing this semifinished product  21  itself is the same as used in the manufacture of conventional RFID tags and is not directly related to the present invention. Therefore, a description of the method of manufacturing this semifinished product  21  is omitted here. 
       FIGS. 6(A) and 6(B)  are a front view and a side view, respectively, of an intermediate product after the laminating step. 
     Also in  FIGS. 6(A) and 6(B) , one of semifinished products  21  that are essentially linked together in a large quantity in sheet form is illustrated. 
     As shown in  FIGS. 6(A) and 6(B) , the cover sheet  14  is formed in the laminating step and in this cover sheet  14  a hole  14   a  is made in a part corresponding to the IC chip  13 . 
     This intermediate product is delivered to the test step  40  shown in  FIG. 3  to ascertain whether the intermediate product has a capacity necessary for functioning as an RFID tag. For a rejected product that has been judged to have an insufficient capacity in this test step  40 , its position in the sheet in which many intermediate products are linked together is recorded, and this rejected product is delivered as it is to the thermal diffusion material addition step  50  shown in  FIG. 3 . 
       FIGS. 7(A) and 7(B)  are each a detail drawing of a thermal diffusion material. 
     Details of the lamellar structure of the thermal diffusion material  15  are shown in  FIG. 7(A) . The thermal diffusion material  15  has a lamellar structure constituted by an insulating silicone rubber sheet  15   a , an electrically conductive graphite sheet  15   b  and an insulating, sticky polyimide tape  15   c . For the thickness of each layer, thicknesses of 20 μm to 100 μm are suitable for the silicone rubber sheet  15   a , thicknesses of 10 μm to 100 μm are suitable for the graphite sheet  15   b , and thicknesses of 20 μm to 50 μm are suitable for the polyimide tape  15   c . The silicone rubber sheet  15   a  has strength higher than the strength of the graphite sheet  15   b , while the graphite sheet  15   b  has thermal conductivity higher than the thermal conductivity of the silicone rubber sheet  15   a . For this reason, the thermal diffusion material  15  is tough and excellent in thermal diffusion. The silicone rubber sheet  15   a  corresponds to an example of the first layer of the invention, and the graphite sheet  15   b  corresponds to an example of the second layer of the invention. The polyimide tape  15   c  is a kind of sticky material and the thermal diffusion material  15  is bonded to the intermediate product with the polyimide tape  15   c.    
     As shown in  FIG. 7(B) , this thermal diffusion material  15  as described above is obtained by punching a sheet  18  and shaped like a patch, and this patch-like thermal diffusion material  15  is stuck in the thermal diffusion material addition step in such a manner as to block the hole  14 a of the intermediate product shown in  FIGS. 6(A) and 6(B) . 
       FIG. 8  is an explanatory drawing of the thermal diffusion material addition step  50  shown in  FIG. 3 . In this figure, however, only part of the step at which the thermal diffusion material is stuck by use of the sticking jig is shown. 
     For an intermediate product that has been accepted in the operation test in the test step, a silicone grease  19  to improve adhesion to the thermal diffusion material  15  is applied to the IC chip  13  by use of the dispenser  51  shown in  FIG. 3 , and the thermal diffusion material  15  is stuck on the silicone grease  19  by use of the stacking jig  52 . On the other hand, for an intermediate product that has been rejected in the operation test and its position has been recorded as a rejected product, neither the application of the silicone grease  19  nor the sticking of the thermal diffusion material  15  is performed, and this rejected product is left as an intermediate product. As a result of this, accepted products can be distinguished at a glance from rejected products and the mixing of rejected products is prevented. Incidentally, although wax-based phase-changing materials that liquefy by the application of heat can be adopted in place of thermally conductive greases represented by this silicone grease  19 , in this embodiment the silicone grease  19  is to be used. 
     The sticking jig  52  has a center projection  52   a  that pushes a center part  15   e  of the thermal diffusion material  15  against the IC chip  13 , and a ring  52   b  that pushes an edge part  15   d  of the thermal diffusion material  15  against the cover sheet  14 , and the height of the center projection  52   a  is larger than the height of the ring  52   b . For this reason, the height of the edge part  15   d  of the thermal diffusion material  15  bonded to the cover sheet  14  by the ring  52   b  is larger than the height of the center part  15   e  of the thermal diffusion material  15  pushed against the IC chip  13  by the center projection  52   a . This center part  15   e  corresponds to an example of the center part of the invention and the edge part  15   d  corresponds to an example of the protective part of the invention. 
     Because the height of the edge part  15   d  is larger than the height of the center part  15   e  in this manner, impacts and loads on the RFID tag are applied to the edge part  15   d  and impacts and loads on the center part  15   e  and the IC chip  13  are relieved, with the result that the breakage and exfoliation of the IC chip  13  are prevented. 
     With this the description of the first embodiment of the present invention is finished, and other embodiments of the invention will be described below. Incidentally, each of the embodiments described below is the same as the first embodiment described above, with the exception that different thermal diffusion materials are used. Therefore, the following descriptions will be given by paying attention to only differences from the first embodiment and overlaps in descriptions will be avoided. 
       FIG. 9  is a drawing that shows the second embodiment of the invention. 
     An RFID tag  60  of the second embodiment shown in  FIG. 9  is provided with a thermal diffusion material  61  formed from a ceramic (Al 2 O 3 : alumina, SiO 2 : silica) powder. This thermal diffusion material  61  is formed by blowing a liquid ceramic paint (for example, the ceramic α made by Oki Electric Industry Co., Ltd.) from a nozzle  54  through an opening of a mask  55  and causing the liquid ceramic paint to solidify by drying or the like. 
       FIG. 10  is a drawing that shows another manufacturing method in the second embodiment of the invention. 
     A thermal diffusion material  61  in the RFID tag  60  of the second embodiment can also be formed by applying the above-described liquid ceramic paint by use of a dispenser  56  and causing the liquid ceramic paint to solidify by drying or the like. 
     Thus, in the RFID tag  60  of the second embodiment the manufacture of the thermal diffusion material  61  is easy and cost reduction is expected. 
       FIG. 11  is a drawing that shows the third embodiment of the invention. 
     An RFID tag  70  of the third embodiment is provided with a thermal diffusion material  71  formed from a silicone rubber sheet and this thermal diffusion material  71  is bonded directly to the IC chip  13 . Because this thermal diffusion material  71  has a very simple construction, its manufacturing cost is thought to be held down. Incidentally, although it might be thought that the capacity for thermal diffusion in the third embodiment is inferior to the capacity for thermal diffusion in the first embodiment mentioned above, it is expected that the third embodiment is superior to conventional techniques and, therefore, sufficient usability is expected under some service conditions. 
       FIG. 12  is a drawing that shows the fourth embodiment of the invention. 
     An RFID tag  80  of the fourth embodiment is also provided with the same thermal diffusion material  71  formed from a silicone rubber sheet as with the RFID tag of the third embodiment and the area around the IC chip  13  is embedded with a silicone grease  81  to improve thermal diffusion properties. In this RFID tag  80 , both cost reduction and an improvement in thermal diffusion properties are achieved and its applications are thought to be wide. 
       FIG. 13  is a drawing that shows the fifth embodiment of the invention. 
     An RFID tag  90  of the fifth embodiment is provided with a thermal diffusion material  91  that is formed by mixing a ceramic (Al 2 O 3 : alumina, SiO 2 : silica) granule  92  of high thermal conductivity into an insulating silicone sheet. As with the thermal diffusion material  15  in the first embodiment, this thermal diffusion material  91  is bonded with an adhesive. 
     Because in this thermal diffusion material  91 , thermal conductivity can be easily adjusted by adjusting the kind and mixed amount of granule  92 , the thermal diffusion material  91  of high thermal conductivity can be easily obtained. 
     Although in the above description, a cover sheet in which holes are made beforehand in the perforation step is shown as an example of the cover of the present invention, the cover of the invention may be such that the cover once covers the whole base and after that, part of the cover in an area near the circuit chip is stripped off. 
     Also, in the above description, the base sheet or the cover sheet is shown as an example of the base or cover of the invention. However, the shape of the base or cover of the invention is not limited to the sheet. 
     Also, in the above description, a protective part obtained by sticking a thermal diffusion material is shown as an example of the protective part of the invention. However, the protective part of the invention may be a part shaped like a projection formed in the thermal diffusion material before affixing the thermal diffusion material to the circuit chip.