Method for producing a contactless chip card

A method of producing a contactless chip card (10) includes first the step of holding a chip (14), which is provided with terminal pads (16a, 16b) on a surface thereof, in a mould defining a chip-card substrate (12), in such a way that the chip surface provided with the terminal pads is located substantially in the same plane as a chip-card substrate surface defined by the mould. In addition, a chip-card substrate material is introduced into the mould, whereupon a coil structure (18) is applied by screen-printing a conductive paste onto the chip-card substrate (12) in such a way that the coil structure (18) extends up to the terminal pads (16a, 16b) on the chip.

FIELD OF THE INVENTION
 The present invention refers to a method of producing a contactless chip
 card which is supplied with energy and which communicates by means of
 electromagnetic waves.
 1. Description of Background Art
 Lately, it has become much more common to use contactless chip cards e.g.
 as car keys or as door keys. Such contactless chip cards permit a fast and
 contact-free opening of doors or also a fast identification of a specific
 person.
 Contactless chip cards normally comprise on a chip-card substrate a
 microchip and an antenna structure. Normally, the chip card is not
 provided with an internal power supply so as to prevent the chip card from
 becoming ineffective when the battery has run down. The chip card is
 supplied with electric power in the form of electromagnetic waves from a
 base station, this being one reason for the fact that induction antennae
 or coil antennae are normally used for chip cards. In contrast to dipole
 antennae, loop antennae permit a more effective coupling in of
 electromagnetic power in the antenna on the chip. This power coupled into
 the antenna is rectified for supplying the microchip, which is provided on
 the chip card as well, with electric power so that said microchip can
 transmit a special code e.g. to the door lock of a car or to an
 identification means so as to accomplish an act coded by said code.
 The operating principles of coupling electric power into the chip card and
 of communication between the microchip and a stationary or mobile station
 are known to those skilled in the art and described e.g. in U.S. Pat. No.
 5,473,323. This patent describes a method for contactless data and power
 transmission between a stationary microstation comprising a pair of coils
 and a means for providing an oscillating clock signal and a mobile
 microunit comprising one or two coils.
 In addition, U.S. Pat. No. 5,449,894 describes a method for operating a
 contactless chip card which is not provided with a battery and which is
 supplied with power via a power-transmitting alternating field from a
 write/read unit. The bidirectional data transmission is accomplished by
 modulation of the power-transmitting alternating field by means of an
 antenna coil, said power-transmitting field producing a coil current in
 the coil and a coil voltage at the coil; an input circuit is coupled to
 ends of the coil for generating a supply voltage from the current in the
 coil and a voltage detector is coupled to the ends of the coil for
 detecting specific properties of the power-transmitting alternating field.
 U.S. Pat. No. 5,440,302 describes a device for contactless data and power
 transmission, which comprises a stationary part and a mobile part.
 2. Description of Prior Art
 For producing contactless chip cards, an insulating chip-card substrate is
 normally used on which a microchip that is capable of carrying out the
 desired functions is placed. At a point of the chip-card substrate which
 is remote from the microchip or on a separate substrate, the necessary
 antenna structures, which are substantially implemented as coils, are
 additionally applied e.g. by means of known photolithograhic techniques.
 For connecting the coil structures to the microchip, bonding wires can be
 used in the case of known contactless chip cards. The bonding wires extend
 from a connection point of a coil to a connection point on the microchip,
 said bonding wires being produced in the manner known by means of a
 bonding device so as to interconnect the coil structure and the microchip
 in an electrically conductive manner.
 This method of producing known contactless chip cards is disadvantageous
 insofar as expensive bonding devices are required for connecting the
 microchip to the antenna structure. In the case of a mass product, and a
 contactless chip card is such a mass product beyond all doubt, even small
 cost advantages of the product decide whether said product will gain
 acceptance in comparison with a more expensive competing product.
 EP 0756244 A2, which has been published after the priority date of the
 present application, discloses a circuit module and a method of producing
 a circuit module. For embedding an integrated circuit into a supporting
 substrate, a suitable recess can either be provided or the integrated
 circuit can be pressed into the supporting substrate by applying heat and
 pressure.
 DE 4416697 A1 discloses a data carrier provided with an integrated circuit.
 The integrated circuit is connected to at least one coil via contact
 elements, said integrated circuit and said contact elements forming a
 module and the coil being arranged on a card body. The coil is arranged on
 an inner layer of the card body, said inner layer being also provided with
 an opening for receiving the module therein. The module, which comprises
 at least one integrated circuit with two connections, is introduced in a
 previously produced recess of the inner layer.
 DE 4431606 A1 discloses a chip-card module for a contactless chip card and
 a method of producing the same. A semiconductor chip is arranged on a
 flexible, non-conductive supporting chip, two ends of an enamelled-wire
 coil being electrically connected to the semiconductor chip so as to
 establish a direct connection between said semiconductor chip and the coil
 ends. The chip is placed on the carrier body and is then contacted.
 DE 44 10 732 A1 discloses both a method of producing a chip card and a chip
 card. The chip card comprises a chip and a coil, said chip and said coil
 being arranged on a common substrate and the coil being formed by laying a
 coil wire. The chip is incorporated into the surface of the substrate by
 thermocompression, i.e. by making the surface of the substrate soft by
 means of heat and by pressing the chip into said surface, or by "rubbing
 in" by means of ultrasonic action.
 SUMMARY OF THE INVENTION
 It is the object of the present invention to provide an economy-priced and
 reliable method of producing a contactless chip card.
 This object is achieved by a method of producing a contactless chip card,
 comprising the steps of: holding a chip, which is provided with terminal
 pads on a surface thereof, in a mould defining a chip-card substrate, in
 such a way that the chip surface provided with said terminal pads is
 located substantially in the same plane as a chip-card substrate surface
 defined by the mould; introducing a chip-card substrate material into the
 mould; and applying a coil structure by screen-printing a conductive paste
 onto the chip-card substrate in such a way that the coil structure extends
 up to the terminal pads on the chip.
 The present invention is based on the finding that the connection of the
 coil structure to the microchip, which has to be carried out in a separate
 step, can be dispensed with, when a coil structure as well as a chip are
 arranged on a common insulating chip-card substrate. By realizing the coil
 structure by the application of a conductive paste, after arranging the
 chip on the chip-card substrate, in such a way that the coil structure
 extends up to the terminal pads on the chip, it is no longer necessary to
 carry out a separate bonding step, since the coil structure is already
 connected to the chip.

DETAILED DESCRIPTION OF THE PRESENT INVENTION
 FIG. 1 shows a schematic top view of a contactless chip card 10 which has
 been produced by the method according to the present invention. The
 contactless chip card 10 comprises an insulating chip-card substrate 12
 having arranged therein a chip 14 which is provided with two terminal pads
 16a and 16b. The terminal pads 16a and 16b, respectively, have connected
 thereto the ends of a conductive coil structure 18 having three turns in
 the case of the embodiment shown in FIG. 1. The surface of the microchip
 14 is normally implemented as an insulating surface with the exception of
 the terminal pads 16a, 16b. The conductor sections of the coil structure
 18 extending over the chip are therefore insulated from said chip.
 FIG. 2A to 2D show a cross-sectional view of the contactless chip card 10
 in a sequence of production steps for said chip card. In a mould having a
 recess defining the shape of the chip-card substrate, the chip 14 is held
 in the manner known. In the next step, the chip is fixedly placed in the
 chip-card substrate 12 by casting a material suitable for the chip-card
 substrate 12 into the substrate mould and by curing said material.
 Subsequently, an electrically conductive paste 22 is applied by means of a
 mask to the chip-card substrate in which the chip 14 is fixedly
 accommodated. As can be seen in FIG. 1, the electrically conductive paste
 22 forms, in one operation, the coil structure 18 as well as the
 connections 24 to the terminal pads 16a, 16b, which have to be carried out
 in a separate step in the prior art. The application of the electrically
 conductive paste, which can preferably be an electrically conductive
 adhesive, is carried out e.g. by means of the known screen printing
 process. The mask defines the coil structure 18 as well as the two ends of
 the coil structure which establish the conductive connections 24 with the
 terminal pads 16a, 16b of the chip 14. After curing of the conductive
 paste 22, a cover layer 26 is cast analogously to the chip-card substrate
 12, whereupon the chip-card substrate 12 and the cover layer 26 are
 laminated so as to protect the contactless chip card against external
 undesired influences and so as to secure the coil structure 18 in
 position.
 As can be seen from FIG. 2D, separate fastening of the microchip 14 or of
 the coil structure 18 is therefore not necessary due to the lamination of
 the cover layer 26 and of the chip-card substrate 12, since said microchip
 14 and said coil structure 18 are fixedly and reliably held at their
 correct positions relative to one another by the connections 24 as well as
 by the cover layer 26.
 Due to the fact that the chip 14 is cast into the chip-card substrate 12,
 as can be seen in FIGS. 2A to 2D, it is guaranteed that the chip is
 arranged essentially without any gap towards the chip-card substrate. In
 addition, this is a simple way of guaranteeing that a main surface of the
 chip, which is provided with the terminal pads, is arranged such that the
 surface thereof is substantially flush with the surface of the chip-card
 substrate, whereby the screen printing process can be used for applying
 the conductive paste so as to produce in one step the coil structure as
 well as the connection between said coil structure and the chip in a
 reliable manner.
 If, notwithstanding the above, the main surfaces of the chip 14 and of the
 chip-card substrate 12 are not exactly flush with one another due to
 deviations caused by the production process, certain tolerances exist in
 this respect, since the height of the conductive paste 22 can compensate
 errors of alignment within certain limits for reliably establishing the
 connections 24 with the terminal pads 16a, 16b of the chip 14.
 Although the drawings of FIGS. 1 and 2A to 2D do not show any gap between
 the chip 14 and the chip-card substrate 12, this exact production
 precision for casting the chip-card substrate material is not absolutely
 necessary either, since with regard to the function of the contactless
 chip card it is of no importance when a small amount of the conductive
 paste 22 penetrates into an existing gap as long as the individual turns
 of the coil structure 18 are not short-circuited and as long as possible
 high-frequency effects caused by parasitic stray capacitances of the coil
 structure do not make the coil antenna ineffective.