CONTACTLESS ELECTRONIC TRANSPONDER WITH PRINTED LOOP ANTENNA CIRCUIT

Abstract: A hand-held electronic transponder (10) with a printed loop antenna circuit. The antenna circuit is manufactured by depositing on a flat substrate (12) a mixture of discrete conductive particles suspended in liquid, according to a predetermined design. When the suspension medium evaporates, the conductive particles randomly touch one another to establish a conductive pathway. The process can also be used to produce three-dimensional conductor patterns by printing layers (22, 24) of electronically conductive pathways interlaminated with dielectric films. When the conductive pathways on two adjacent layers overlap, a capacitor (14) is created, whose capacitance is function of the surface area of the overlapping conductive pathways (18) and the thickness of the dielectric medium (28) between the pathways. The capacitor can be used to tune the antenna circuit to a predetermined frequency. A resistor can also be introduced in the antenna circuit by adding to the suspension of conductive particles a quantity of particles having comparatively high resistivity.

Description: TITLE; CONTACTLEB8 ELECTRONIC TRANSPONDER WITH PRINTED LOOP ANTENNA CIRCUIT FIELD OF THE INVENTION The invention relates to a transponder capable of transmitting information to an interrogator station through electromagnetic radiation, inductive coupling or capacitive coupling. More specifically, the invention provides a hand-held transponder with a novel antenna circuit including conductive pathways formed of discrete conductive particles in physical contact with one another. The invention also relates to a method for manufacturing the transponder. BACKGROUND OF THE INVENTION Hand-held electronic cards capable of exchanging data with an interrogator station through modulated electromagnetic radiation are used in a variety of applications. For example, such cards can be used to open a lock, perform financial transactions or as storage devices to hold vital data, such as the medical record of the user. Typically, the card contains an integrated electronic circuit mounted on a flexible substrate made of plastic material. The electronic circuit includes a memory for storing information, a processing unit and a modulator/demodulator section connected to an antenna circuit. In use, the card is brought close to the interrogator station that generates a burst of electromagnetic radiation that is converted by the antenna on the card into electrical energy sufficient to power up the electronic circuit. Once activated, the processing unit retrieves the information present in the memory and directs the data to the modulator that transmits the information back to the interrogator station via the antenna. The data transmission process toward the interrogator station may be active or passive. The active transmission requires the emission of a modulated carrier wave. This mode of communication is particularly useful when the transponder is at some distance from the interrogator station. The passive transmission is much simpler and it is effected by modulating the energy transfer from the interrogator station to the transponder to communicate a message. For instance, by varying the impedance of the antenna circuit of the transponder the coupling with the interrogator station changes which translates in a variation of the energy transfer rate between the two devices. Such variation can be made by switching on and off a resistor in the antenna circuit in accordance to a binary coded sequence. The sequence is detected at the interrogator station by monitoring the changes in the energy transfer rate. In more sophisticated interrogator/transponder units, the contents of the memory can be modified by the interrogator. An example is an automated toll collection system. The card that is normally carried by the user contains a memory in which is stored a numerical value representing a credit amount allocated to the user. Λt each toll passage, the interrogator station overwrites the data with a value corresponding to the credit amount less the toll charge. Access is denied if insufficient credit is available on the card. In another possible application, medical data can be stored in the card memory to enable a doctor to gain access to the treatment and medication history of the user quickly. The medical data is updated when significant events occur, such as a new treatment prescribed to the user. The antenna circuit on the card includes a coil- shaped conductor to which is incorporated a capacitor forming a resonant RLC circuit. Various methods have been developed in the past recent years to embed or otherwise affix the coil-shaped conductor to the card. One example is the U.S. patent 5,339,847 issued to Francois Droz on March 21, 1995 that suggests to wind a conductor wire in a loop pattern and then embed the wire in a substrate that also encapsulates the electronic circuitry of the card. This approach has not been particularly successful because the method of encapsulation is complex and expensive to carry out. An alternative approach is disclosed in the U.S. patent 5,108,822 granted to Tokai Electronics Co., Ltd. on April 28, 1992. This reference suggests etching selected portions of a layer of conductive material bonded to an insulating base to produce a loop antenna. The layer of conductive material is metallic foil, such as aluminum. The insulating base preferably made of polyethylene, is coated on both its faces with foil. A pattern of conductors is printed on the foil with acid resistant ink. The substrate is then immersed in an acid solution that erodes the non protected areas. It has been noted, however, that the foil conductor may develop cracks due to bending of the insulating base that may result in an open antenna circuit that is partially or totally inoperative. Such bending strains are particularly common in hand-held electronic cards carried in one's wallet or purse. Thus, unless the card is handled delicately and with great care, which is atypical of this sort of devices, the risks of damaging the antenna circuit are high. OBJECTB AND STATEMENT OF THE INVENTION An object of the invention is a portable electronic transponder, such as a hand-held electronic card with a conducting pathway that is highly resistant to flexing and can be produced easily and at low cost. An ancillary object of the invention is a portable electronic transponder with an antenna circuit incorporating a capacitor component and a resistor component. Another object of the invention is a method for manufacturing the aforementioned electronic transponder. For the purpose of this specification "transponder" means a device capable of accepting the challenge of an interrogator station and issuing a response. This implies that the transponder has both a reception and a transmission capability. Note that "reception" is used in a very general sense and it is not limited to the conversion of modulated signals into desired useful information. For instance, the ability to recognize a trigger signal issued from the interrogator station, which conveys no particular intelligence but merely causes the transponder to emit a response, will be considered to meet the requirement for reception capability. Similarly, "transmission" should be interpreted to cover the remote communication of information, irrespective of the particular method chosen to do so. Possible methods are the active transmission and the passive transmission, both discussed in the introductory portion of this specification. Aε embodied and broadly described herein, the invention provides a hand-held electronic transponder, comprising: - a substrate; - at least one electronic component mounted to said substrate; and - a multitude of discrete conductive particles fixed to said substrate, said discrete conductive particles being distributed over a predetermined surface area of said substrate and being in physical contact one with another to form a conductive pathway admitting passage of electrical current. In a most preferred embodiment, the substrate is an integrally formed plate of synthetic material that is relatively flexible. The plate is milled on one of its main surfaces to provide a recess for holding the electronic circuit that combines the memory, processing unit, modulator and demodulator functions, among others. The surface of the card containing the recess is imprinted with a pattern of coil-shaped conductors by using a mixture of discrete conductive particles suspended in binder (organic binder, epoxy binder, thermoplastic binder, etc) . The pattern can be printed by using any suitable technique, such as gravure or screen printing. The binder shrinks when it sets, thus causing the conductive particles to touch in a random pattern and produce an electrically conductive coating. The fact that the coating is an agglomeration of particles permits the conductive pattern to better survive mechanical stresses applied on the substrate. This process can be used to create three-dimensional conductor patterns that are useful to allow one conductor to cross over another conductor on the substrate, or create capacitors. The three-dimensional conductor array requires a three-step print. First, the conducting pathways on the lowermost level are deposited and the binder is allowed to set. An intermediate layer of dielectric material is laid on part of the conducting pathway on the lowermost level that will eventually be overlaid by the uppermost conducting pathway formation. When the dielectric material is cured, the final printing step deposits a mixture of discrete conductive particles on the dielectric zone. The superposed conducting pathways, separated by the dielectric layer form a capacitor structure that is fully integrated to the antenna circuit. The surface area of conductive material overlap, and the thickness of the dielectric layer, determine the capacitance. The above described procedure is extremely advantageous in that it allows to integrate capacitance in the printed circuit at very low cost without the necessity of soldering or otherwise joining discrete components. To control the resistance of the conducting pathways, discrete particles having low electrical conductivity can be admixed with the high conductivity particles. This creates a distributed resistor structure. When a discrete resistor is required, a two-step printing procedure can be used, first to lay a pattern of high conductivity pathway that contains a blank, i.e., an unconnected area designed to receive the resistor. At the second printing step, a mixture containing solely low electrical conductivity particles is deposited to fill the blank and thus complete the electrical path of the circuit. As embodied and broadly described herein, the invention also provides a portable electronic transponder, comprising: - an insulating substrate: - at least one electronic component mounted to said substrate; and - an antenna circuit electrically coupled to said electronic component, said antenna circuit including a multitude of discrete conductive particles fixed to said substrate, said discrete conductive particles being distributed on a predetermined surface area of said substrate in the form of a coil and being in physical contact one with another over said surface area to form a conductive pathway. As embodied and broadly described herein, the invention further provides a method for manufacturing an antenna circuit on a substrate, said method comprising the steps of: - providing a mixture of discrete conductive particles suspended in liquid; - depositing said mixture on said substrate according to a predetermined pattern; and - fixing said discrete conductive particles to said substrate and causing said discrete conductive particles to contact each other for establishing a conductive pathway, said conductive pathway forming at least a component part of the antenna circuit. As embodied and broadly described herein the invention also provides a method for providing a conductive path on a substrate, said method comprising the steps of: - providing a mixture of discrete conductive particles within a binder, said binder manifesting a reduction in volume when setting; - depositing said mixture on said substrate according to a pattern; and - causing said binder to set, whereby allowing said discrete conductive particles to establish an electrically conductive pathway over said pattern by touching one another. BRIEF DESCRIPTION OF THE DRAWINGS - Figure 1 is a top plan view of an electronic card according to the invention showing a coil-shaped conductor that forms an antenna circuit; and - Figure 2 is the equivalent electric circuit of the lay out shown in Figure 1. DESCRIPTION OF A PREFERRED EMBODIMENT A partially assembled electronic card incorporating an antenna printed according to the present invention is shown in Figure 1 and comprehensively designated by the reference numeral 10. The card 10 comprises a substrate 12 made of non-conductive synthetic material such as polyvinyl chloride, PETG polyester or polycarbonate, polysulfonal or any other suitable rigid plastics material, reinforced plastics material or ceramic material. In a most preferred embodiment the substrate 12 is in the form of a flat rectangular plate having a thickness of about 0.030 inches. The length and the width dimensions of the substrate approximate those of a credit card, but may greatly vary without departing from the spirit of the invention. The substrate is machined near the upper right corner to provide a recess 14 of a circular configuration that receives an integrated circuit combining memory, processing unit, and modulator and demodulator functions. An integrated circuit available under the brand name Myfare from Mikron, Austria that is currently produced by the Atmel silicon foundry in the United-States has been found satisfactory. The same circuit is also available from Siemens Electric G Bh. The type of integrated circuit that will be used depends upon the intended application. Thus, the present invention should not be limited to the type of integrated circuitry provided on the substrate 12. The recessed area 14 is formed by machining with a rotary end mill a central circular cavity 16 having a depth of approximately 0.0150 inches. The cavity 16 is surrounded by an annular tapered zone 18 that forms the transition between the main surface of the substrate 12 and the edge of the circular cavity 16. During the manufacturing process, the integrated circuit is deposited in the circular cavity 36 and the connections with the antenna circuit that will be described below made by the intermediary of connection pads formed on the tapered area 18. To enable the integrated circuit to communicate with an interrogator station (not shown in the drawings) , an antenna circuit 20 is formed on the main surface of the substrate 12. The antenna circuit 20 is a single conductive pathway arranged as a multi turn coil which runs near the peripheral edges of the substrate 12. The leading extremity 22 of the conductive pathway, the one that initiates the spiral formation from the inside terminates with an enlarged contact pad area 24 extending over an angular sector of about 120° on the tapered zone 18. The terminal extremity 24 of the conductive pathway crosses over the coil formation to join with a connection pad 26 that is similar in construction to the connection pad 24. To prevent the terminal segment 24 from shorting the coil in the crossover region, a layer 28 of the dielectric material is applied between them. The three-dimensional conductive pathway configuration in the crossover region constitutes a capacitor used for tuning the antenna to a desired resonant frequency. The capacitance of the structure can be established by the following formula. C=kε°A d C = Capacitance in farads, F k = relative dielectric constant of the insulator (layer 28) . The relative dielectric constant compares the dielectric layer 28 to vacuum which is a constant of 1. e° = permittivity of free space = 8.85 x IO "12 Farads/meter A = the overlapping areas of the conductive pathways in square meters d = is the separation of the conductors (thickness of layer 28) in meters. The formula shows that the capacitance is primarily function of two factors: a) the surface area of the conductors on either side of the dielectric film 28; b) the thickness of the dielectric film. Therefore, an increase in the surface area of the overlapping conductive pathways (parameter A) increases the capacitance. Increasing the thickness of the dielectric layer 28 (parameter d) decreases the capacitance. By varying these factors, one may adjust the capacitance value within a comparatively wide range to tune the antenna to the desired resonance freguency precisely. This feature allows the manufacturer of the transponder 10 quickly to effect changes of the resonance frequency simply by modifying the deposition pattern of the conductive pathways or varying the thickness of the dielectric layer 28. The segment of the conductive pathway between the leading portion 22 and the terminal portion 24 is in the form of a flat coil and has an inductance function in the overall antenna circuit 20. The value of the inductance can be approximated by the following formula: L = 2-7 x 1..0-10 x , .5/3 (,1+,r-1) L = Inductance in henries D = Dimension as shown in Figure l in cm p = spiral conductor width in cm q = spacing between the conductor coil in cm r p/q The equivalent circuit of the antenna 20 is shown in Figure 2. The circuit comprises an inductor 30 formed by the coil shaped conductive pathway in parallel with a capacitor 32 resulting from the conductors overlap in the crossover area. The capacitor 32 is connected to the integrated circuit 34 mounted in the cavity 16, by the intermediary of connection pad areas 26 and 44. The antenna circuit 20 is produced by a printing technique that consists of depositing at a precise location on the surface of the substrate 12 a mixture of discrete conductive particles suspended in binder. In a most preferred embodiment the composition is a mechanical mixture of silver powders and an organic solvent based binder. The silver particles are a combination of 80% flake particles and 20% amorphous shaped particles. By "amorphous" is meant particles having random geometrical shapes. In a variant, particles plated with a conductive coating can be used, such as silver or gold plated carbon, silver or gold plated nickel or silver or gold plated copper particles. The preferred binder base is poly methyl methacrylate dissolved in toluene, xylene, naphtha or εec-butanol. Epoxy binders and thermoplastic binders can also be used. Epoxy binders require a catalyst to effect curing. The catalyst may be of chemical nature or physical nature such as ultra-violet radiation. The thermoplastic binders are plastics which when elevated to high temperature convert to a liquid state. While in this state the conductive particles are added in the correct proportions to form the conductive composition. The conductive composition so formed is applied hot and sets upon cooling. In a specific example, a conductive composition including 80% by weight of silver powder and 20% by weight of poly methyl methacrylate binder has been found satisfactory. The screen printing technique is the method of choice for creating the conductive pathways. First, a mask with areas cut out according to the desired conductor pattern (a multi turn coil in the present example) is placed against the surface of the substrate 12. The suspension of discrete conductive particles is then spread over the screen so it contacts the surface of the substrate 12 through the screen apertures to form a deposit matching precisely the desired conductive pathway design. The screen is then carefully removed and the suspension deposit allowed to set. The curing procedure depends upon the nature of the binder. Organic binders cure by release of solvents. Epoxy binders require a chemical reaction initiated by a suitable catalyst which is applied at the appropriate step during the manufacturing procedure. Thermoplastic binders require cooling to set. In all three cases, the mixture, while in the liquid state, is substantially non conductive because the binder is a dielectric medium and the conductive particles are separated from one another. However, when the binder sets it shrinks which causes the discrete metallic particles to contact one another in a random fashion and thus donate conductivity to the agglomeration of particles. Also the binder fixes the conductive particles to the substrate 12 so they are permanently retained to it. The fact that the deposit is an agglomeration of conductive particles, rather than a continuous conductive medium such as metallic foil permits the conductive pathway to survive mechanical bending stresses encountered in normal use of the transponder 10. To create the three-dimensional conductive pathway configuration depicted in Figure 1, a three-step printing process is employed. During the first step, the entire coil-shaped conductive pathway is produced except for the terminal segment 24 at the crossover area. When the suspension of conductive particles is cured, a εecond printing step deposits the dielectric layer 28 of suitable material to electrically insulate the εegments of the conductive pathway in the crossover area. After the dielectric film is completely dry, the final printing step lays the terminal portion 24 to connect the conductor 20 with the connection pad 26. Each printing step uses a separate screen with apertures patterned to create the desired deposition pattern. For instance, the first screen corresponds to the multi turn coil-shaped conductive pathway less the terminal portion 24. The second screen has a rectangular aperture registering with the previously laid conductive pathway in the crossover area. The third and final screen has an elongated slot to electrically connect the coil- shaped conductive pathway with the connection pad area 26. If desired to introduce a resistor in the antenna circuit 20, it is possible to add to the liquid suspension of metallic particles some particles of carbon that have high resistivity and therefore contribute to elevate the global resistance of the antenna circuit 20. In this embodiment, the equivalent resistor is distributed over the entire conductive pathway. In a variant, a localized resistor component may be produced by first depositing mixture containing solely low conductivity particles. The deposition pattern is such that a "blank", i.e., an empty area is left in the conductive pathway. During a second printing step a low conductivity solution is deposited on the blank to complete the electrical circuit. A resiεtive mixture formulation that could be used has 80% carbon particles and 20% organic binder such as poly methyl methacrylate in toluene. The manufacturing process of the transponder 10 begins by machining the substrate 12 using a shaped vertical plunge mill to form the recess 14 that holds the integrated circuit. The primary antenna circuit 20 is imprinted on the milled substrate followed by the deposition of the dielectric film at the crossover and the capacitance areas. A solution of conductive particles is deposited last on the substrate to form the contact pads for the integrated circuit and over the previously laid dielectric film to complete the crossover area and the capacitance. Before the solution of conductive particles sets, the integrated circuit is placed in the recess 14 to bring its terminals in contact with the solution still in the liquid phase. Adhesive is used to bond the body of the integrated circuit to the substrate 12. As the solution of conductive particles sets a permanent electrical contact is created with the terminals of the integrated circuit. It is also possible to install the integrated circuit in the substrate 12 immediately after the deposition of the dielectric film and then print the contact pads to effect the electrical connection. If desired the antenna pattern and the integrated circuit may be covered by a protective polymer coating. Polyurethane monomer with an ultra violet polymerizing agent is a suitable covering material. In a variant, acrylics or other plastic monomers can be used. While the electronic transponder has been described above in the form a hand-held card it may take various other configurations. For example the transponder may be designed as a keyfob or any other suitable εtructure that can be conveniently carried in one's pocket, purse or wallet. The above description of the invention should not be interpreted in any limiting manner as variations and refinements are possible without departing from the spirit of the invention. The scope of the invention is defined in the appended claims and their equivalents.

Claims: I CLAIM; 1) An electronic transponder, comprising: - a substrate; - at least one electronic component mounted to said substrate; and - a multitude of discrete conductive particles fixed on said substrate, said discrete conductive particles being distributed over a predetermined surface area of said substrate and being in physical contact one with another to form a conductive path admitting passage of electrical current. 2) A hand-held electronic transponder as defined in claim 1, wherein said conductive path is electrically coupled to said electronic component. 3) A hand-held electronic transponder as defined in claim 2, wherein said conductive path forms at least a component of an antenna circuit. 4) A hand-held electronic transponder as defined in claim 3, wherein said conductive path is in the form of a coil and forms an inductor. 5) A hand-held electronic transponder as defined in claim 1, wherein said conductive path includes a pair of segments in an overlapping relationship forming a capacitor. 6) A hand-held electronic transponder as defined in claim 5, wherein said segments are separated by a dielectric medium. 7) A hand-held electronic transponder as defined in claim 1, wherein said conductive path includes a plurality of first conductive particles and a plurality of second conductive particles, said second conductive particles having a higher electrical resistivity than said first conductive particles, whereby said second conductive particles contribute to increase a resistance of said conductive path. 8) A hand-held electronic transponder as defined in claim 7, wherein a segment of said conductive path contains first and second conductive particles in physical admixture. 9) A hand-held electronic transponder as defined in claim 7, wherein said second conductive particles are concentrated on a predetermined site of said surface area to form a localized resistor incorporated to said conductive path. 10) A hand-held electronic transponder as defined in claim 7, wherein said first conductive particles include particles selected from the group consisting of metallic particles and particles including a metallic coating. 11) A hand-held electronic transponder as defined in claim 7, wherein said second conductive particles include carbon particles. 12) A hand-held electronic transponder as defined in claim 1, wherein said substrate is integrally formed of material that is an insulator. 13) A hand-held electronic transponder as defined in claim 12, wherein said substrate is substantially flat and includes a recessed area receiving said electronic component. 14) A hand-held electronic transponder as defined in claim 13, wherein said conductive path is formed as a coil around said recess, said conductive path includes terminal portions extending in said recess that electrically connect with εaid electronic component. 15) A hand-held electronic transponder as defined in claim 14, wherein said conductive path includes a first segment forming said coil and a second segment overlying a portion of said coil. 16) A hand-held electronic transponder as defined in claim 15, wherein said first and second segment are separated by a dielectric medium. 17) A hand-held electronic transponder as defined in claim 1, comprising a binder substance fixing said discrete conductive particles to said substrate. 18) A hand-held electronic transponder as defined in claim 1, wherein said binder substance is εelected from the group consisting of organic binder, epoxy binder and thermoplastic binder. 19) A hand-held electronic transponder as defined in claim 17, wherein εaid binder is capable of shrinking upon setting. 20) A hand-held electronic transponder as defined in claim 1, wherein said discrete conductive particles include flake-shaped particles and amorphous particles. 21) A portable electronic transponder, comprising: - an insulating substrate: - at least one electronic component mounted to said substrate; and - an antenna circuit electrically coupled to said electronic component, said antenna circuit including a multitude of discrete conductive particles fixed on said substrate, said discrete conductive particles being distributed on a predetermined surface area of said substrate in the form of a coil and being in physical contact one with another over said surface area to form a conductive path. 22) A portable electronic transponder as defined in claim 21, wherein said conductive path includes two segments in an overlapping relationship, said segments forming a capacitor. 23) A portable electronic transponder as defined in claim 22, comprising a dielectric medium between said segments. 24) A portable electronic transponder as defined in claim 21, wherein said conductive path includes discrete particles having different electrical resistivity for controlling a resistance of said conductive path. 25) A portable electronic transponder as defined in claim 24, wherein the discrete particles having different electrical resistivity are in physical admixture and spread over a substantial portion of said conductive path. 26) A portable electronic transponder as defined in claim 24, wherein the discrete particles having different electrical resistivity are substantially segregated, thereby providing a localized resiεtor in εaid conductive path. 27) A portable electronic transponder as defined in claim 21, wherein said conductive path forms a loop antenna. 28) A portable electronic transponder as defined in claim 27, wherein said conductive path is in the form of a multi-turn coil. 29) A portable electronic transponder as defined in claim 21, wherein said discrete conductive particles are fixed to said substrate with binder. 30) A method for manufacturing an antenna circuit on a substrate, said method comprising the steps of: - providing a suspension of discrete conductive particles; depositing said suspension on said substrate according to a predetermined pattern; and - fixing εaid diεcrete conductive particles to said substrate and causing said discrete conductive particles to contact each other for establishing a conductive pathway, said conductive pathway forming at least a component part of the antenna circuit. 31) A method as defined in claim 30, wherein said predetermined pattern is in the form of a coil. 32) A method as defined in claim 30, wherein said suspension includes binder, when setting said binder fixing said diεcrete conductive particles to said substrate. 33) A method as defined in claim 30, further comprising the stepε of: - depositing over a segment of said conductive path a layer of dielectric medium; and - depositing over said dielectric medium a suspension of discrete conductive particles according to a predetermined pattern to form a conductive area, said conductive area and said segment forming a capacitor. 34) A method as defined in claim 33 compriεing the step of establishing an electrical path between said conductive area and said conductive path. 35) A method for providing a conductive path on a substrate, said method comprising the steps of: - providing a mixture of diεcrete conductive particleε within a binder, said binder manifesting a reduction in volume when setting; - depositing said mixture on said substrate according to a pattern; and - causing said binder to set, whereby allowing said discrete conductive particles to establish an electrically conductive pathway over said pattern by touching one another.

Inventors: BISSON MICHEL, BISSON, MICHEL
Applicants: BISSON MICHEL, BISSON, MICHEL
Application year: 1996
Main classification: G01V15
IPC codes: G01V15, G06K19