Abstract:
Radio-frequency identification (RFID) devices are used in a variety of applications to facilitate the identification and tracking of people, objects, and animals. One problem with RFID devices or tags concerns manufacturing cost. Specifically, some tag designs use an integrated-circuit chip requiring three external connections, instead of two as do many other designs. Accordingly, the present inventor devised a unique RFID module which uses a three-terminal integrated circuit and two capacitors, but only requires two external leads. One exemplary embodiment of the module includes two external terminals for connection to an antenna coil and an internal terminal for connection to a center-tap of a dual (center-tapped) capacitor. Other aspects of the invention include subcomponents of the module and methods of tag assembly using the module.

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. Provisional Application No. 60/223,071, which was filed on Aug. 4, 2000. This application is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention concerns radio-frequency identification devices, particularly related assembly methods and structures for modules in such devices. 
     BACKGROUND 
     Radio-frequency identification (RFID) devices are used in a variety of applications to facilitate the identification and tracking of people, objects, and animals. Each device, also known as a tag or smart card, can be attached to a person, object, or animal. The devices typically include an antenna coil, radio circuitry, and a memory, enabling them not only to receive radio-frequency signals from a reader or interrogation device, but also to send back signals representing the data in their memories. The data, generally a unique identification number, can then be used with a computerized database to associate the tagged person, object, or animal with other data, such as price, shipping, inventory, or owner information. 
     One problem with RFID devices or tags concerns manufacturing cost. In particular, the inventor recognized that some tag designs use a module requiring three external connections, instead of two as do many other designs. For example, one such design uses a module that has three external terminals A, B, and C, and requires connection of an antenna coil between terminals A and C and connection of one external capacitor between terminals A and B and another external capacitor between terminals B and C. Although the two capacitors in the design provide an increased communication range, the additional complexity of connecting to the extra terminal restricted commercial application for the design. 
     Accordingly, the inventor identified a need for a better way of manufacturing tags that require three external terminals and two capacitors. 
     SUMMARY 
     To address these and other needs, the present inventor devised a unique radio-frequency-identification (RFID) module which uses two capacitors, but only requires two external connections. In one exemplary embodiment, the module includes two external terminals for connection to an antenna coil and an internal terminal for connection to a tap of a dual (center-tapped) capacitor. Other aspects of the invention include components of the module and methods of assembling tags using the module. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of an exemplary RFID module  100  incorporating teachings of the present invention. 
     FIG. 2 is a conceptual view of an exemplary RFID device  200  incorporating RFID module  100  of FIG.  1 . 
     FIG. 3 is a flow chart illustrating an exemplary assembly method based on the RFID module  100  and RFID device  200 . 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following detailed description, which references and incorporates the above-identified figures, describes and illustrates one or more specific embodiments of the invention. These embodiments, offered not to limit but only to exemplify and teach, are shown and described in sufficient detail to enable those skilled in the art to implement or practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art. 
     FIG. 1 shows an exemplary two-terminal RFID module  100  incorporating teachings of the present invention. Module  100  includes insulative structure  110 , lead frame  120 , integrated-circuit chips (or dies)  130  and  140 , and conductor set  150 . 
     Insulative structure  110  includes a number of rivet-like posts  112 , which project through holes or openings  121  in lead frame  120 , securing the relative position and electrical isolation of various portions of lead frame  120 . In the exemplary embodiment, insulative structure  110 , which is generally rectangular, is formed of an insulative mold material, such as a semiconductor transfer-molding compound. One commercially available compound is MG46F-AM compound from Dexter Hysol. 
     In addition to openings  121 , lead frame  120  includes contact structures  122  and  124  and an intermediate contact structure  126 . Contact structure  122  includes an extended portion  122 . 1  and a body portion  122 . 2 . Extended portion  122 . 1 , which has an exemplary thickness of 0.006 inches, extends beyond insulative structure  110  by about 0.057 inches. Body portion  122 . 2  includes a rectangular recessed region  122 . 21 . Recessed region  122 . 21 , which has an exemplary thickness that is 50-80% of the thickness of the remainder of the body portion, is formed, for example, by etching or stamping. 
     Similarly, contact structure  124  which lies opposite contact structure  122 , includes an extended portion  124 . 1  and a body portion  124 . 2 . Extended portion  124 . 1 , which has an exemplary thickness of 0.006 inches, extends beyond insulative structure  110  by about 0.057 inches. Body portion  124 . 2  includes a rectangular recessed region  124 . 21 . Recessed region  124 . 21  has a thickness in the range of 50-80% of its surrounding regions. 
     Inclusion of the recessed regions in body portions  122 . 2  and  124 . 2  reduces the overall height of module  100 . However, other embodiments can reduce module height by removing all or part of material within the recessed regions, defining holes or slots. 
     Lying between contact structures  122  and  124  is intermediate contact structure  126 . Intermediate contact structure  126  includes a central portion  126 . 1  and end portions  126 . 2  and  126 . 3 . Central portion  126 . 1  is not only centered between end portions  126 . 2  and  126 . 3 , but also approximately centered within lead frame  120 . 
     The exemplary embodiment forms lead frame  120  from conductive material, such as OLIN 194 copper using conventional techniques, and then silver-plates the lead frame to enhance its electrical conductivity. Other embodiments, however, plate the lead frame with other conductive materials, such as gold, or omit plating altogether. Some embodiments spot-plate highly-conductive runs on the contact structures to reduce the amount of plated material, whereas others form the entire lead frame from gold or silver. Still other embodiments for the conductive structure through deposition of material on 2 substrates. The invention is not limited to any particular material composition or method of forming lead frames. 
     In addition to insulative structure  110  and lead frame  120 , RFID module  100  includes application-specific integrated-circuit (ASIC) chips  130  and  140 , and conductor set  150 . Integrated-circuit chip (or module)  130 , which is attached to rectangular recess region  122 . 21  using a conventional chip-on-board technique, includes wireless-communications circuitry  132  and a memory circuit  134  to implement a passive RFID read-only tagging function. In the exemplary embodiment, the memory circuit is pre-programmed with data, such as a unique identification number; however, in other embodiments, for example, those that use read-write tagging circuitry, it need not be pre-programmed. Chip  130  also includes three external test terminals  136  for testing the RFID circuitry, and three external-connection terminals (or contact pads)  138 . 1 ,  138 . 2 , and  138 . 3  for connection to external components, such as integrated-circuit chip  140 . 
     Although the invention is not limited to any particular form of integrated-circuit chip  130 , the exemplary embodiments uses the 13.56 MHz MCRF355 RFID chip from Microchip Technology, Incorporated of Chandler, Ariz. In this case, external-connection terminals  138 . 1 ,  138 . 2 , and  138 . 3  correspond respectively to terminals A, B, and Vss. 
     Integrated-circuit module  140 , which is attached to rectangular recess region  124 . 21  using a conventional chip-on-board or epoxy-globbing technique, includes an integrated dual or center-tapped capacitor  142  and has external-connection terminals (or contact pads)  144 . 1 - 144 . 3 . Capacitor  142  includes a pair of end contacts and a center tap (not shown) which are coupled to terminals  144 . 1 - 144 . 3 . 
     In the exemplary embodiment, each half of center-tapped capacitor  142  has a nominal capacitance of 68 picofarads, with a tolerance of ±3%. However, in other embodiments, the capacitor is not center tapped, meaning that each “half” has a different nominal capacitance. Some other embodiments use two distinct capacitors, with each having one of its terminals (or nodes) connected to a common one of the external-connection terminals. Other embodiments may provide two pairs of external-connection terminals, with one pair coupled to one capacitor and the other pair coupled to the other capacitor. 
     Conductor set  150  includes wire-bond connections  152 ,  154 , and  156  and double-wire-bond connections  153 ,  155 , and  157 , which electrically connect integrated-circuit chips  130  and  140  to each other through lead frame  120 . 
     Specifically, wire-bond connection  152  electrically connects intermediate contact structure  126  to terminal  138 . 1 ; wire-bond connection  154  electrically connects contact structure  124  to terminal  138 . 2 ; and wire-bond connection  156  connects contact structure  122  to terminal  138 . 3 . Double-wire-bond connection  153  connects terminal  144 . 1  to contact structure  122 ; double-wire-bond connection  155  connects terminal  144 . 2  to intermediate contact structure  126 ; and double-wire-bond connection  157  connects terminal  144 . 3  to contact structure  124 . 
     The exemplary embodiment forms these connections using a conventional gold-ball bonder. However, other embodiments use an aluminum-edge bonder. Additionally, the exemplary embodiment uses double-wire bonding for connections  153 ,  155 , and  157  to facilitate automatic testing of the connections. Double-wire bonding can also be used for connections  152 ,  154 , and  156 . The invention is not limited to any particular connection structure or technique. 
     These connections not only couple each of the three terminals of center-tapped capacitor  142  in chip  140  to one of the three external-connection terminals of chip  130 , but also couple chip  140  to contact structures  122  and  124 . As FIG. 2 shows, this exemplary embodiment enables one to install a three-terminal RFID chip, such as the MCRF355 RFID chip from Microchip Technology, Incorporated, in a conventional antenna substrate assembly using only two connections. 
     More particularly, FIG. 2 shows an unlaminated RFID card (or tag) assembly  200  incorporating RFID module  100 . In addition to module  100 , assembly  200  includes a conventional card substrate  210  and an antenna coil  220 . Card substrate  210  includes a cutout  212  which has opposing notches  212 . 1  and  212 . 2  that expose corresponding conductive portions  222  and  224  of antenna coil  220 . Conductive portions  222  and  224  are coupled to extended portions  122 . 1  and  124 . 1  of extended contact structures  122  and  124  using conventional techniques. 
     FIG. 3 shows a flow chart  300  which illustrates an exemplary fabrication or assembly method based on module  100  and assembly  200  in FIGS. 1 and 2. The flow chart includes process blocks  310 - 350 . The present invention is not limited to the order of the blocks in flow chart  300 . 
     The exemplary method begins at block  310  with provision of a lead frame in accord with lead frame  120  in FIG.  1 . Execution then proceeds to block  320 , which entails attaching chips  130  and  140  to the lead frame. Block  330  entails completing the electrical connections of the chips to the three contact structures of the lead frame, and block  340  entails encapsulating the resultant lead frame and chip assembly in an insulative material to yield module  100  as shown in FIG.  1 . The exemplary method concludes at block  350  with attachment or connection of two of the contact structures, for example,  122  and  124 , of module  100  to an antenna coil on a card substrate, as shown in FIG.  2 . 
     CONCLUSION 
     In furtherance of the art, the inventors have presented an exemplary RFID module which uses a three-terminal integrated circuit and two capacitors, but only requires two external leads or connections. One exemplary embodiment of the module includes two external terminals for connection to an antenna coil and an internal terminal for connection to a center-tap of a center-tapped capacitor. Other aspects of the invention include the structure of various components of the module and methods of tag assembly using the module. 
     The embodiments described above are intended only to illustrate and teach one or more ways of practicing or implementing the present invention, not to restrict its breadth or scope. The actual scope of the invention, which embraces all ways of practicing or implementing the teachings of the invention, is defined only by the following claims and their equivalents.