Contactless IC card

The pattern line width of a coil is partially widened in a predetermined dimension and wiring patterns of a coil on the surface side and a coil on the rear side are arranged so as not to be in overlapped positions. Consequently, a direct current resistance of the coil can be reduced and an adverse influence by a self resonance of the coil on a communication can be reduced.

BACKGROUND OF THE INVENTION 
The present invention relates to a contactless IC card for receiving power 
and for receiving/transmitting signals from/to a reader/writer as an upper 
terminal by electromagnetic coupling. More particularly, the invention 
relates to the structure of an antenna coil in the contactless IC card. 
In recent years, attention has been paid to an IC card having therein an IC 
such as a processor as a storage medium of electronic money or the like. 
There are two types of IC cards according to different signal transmitting 
methods. One is a contact IC card which receives power and clocks from a 
reader/writer as an upper terminal by using a contact point and processes 
a command from the reader/writer. The other is a contactless IC card 32 in 
which, as shown in FIG. 3, a coil 321 receives radio waves 
(electromagnetic waves) generated from a reader/writer 31 through a coil 
311, transmitting and receiving data signals are generated, and a command 
transmitted from the reader/writer 31 is processed. 
The contactless IC card includes a contact type having the communication 
distance of about few mm to 10 mm, a proximity type having the 
communication distance of tens mm, and a remote type having the 
communication distance of tens cm to few m. Among them, the contact IC 
card has the international standard ISO10536-2 in which the location and 
shape of the coils and the electric characteristics are defined. 
According to the standard ISO10536-2, as shown in FIG. 3, page 2, ISO/IEC 
10536-2:1995(E), two coils are arranged at an interval of 22 mm in a card 
and the inner size of the coils is defined as (11.6 mm.times.4 mm). 
In order to reduce thickness, an IC card in which antenna coils are formed 
by printing a conductive paste (for example, silver paste) or the like on 
a PET film is used. As conventional techniques of the IC card obtained by 
printing the conductive paste, there are techniques described in Japanese 
Patent Application Laid-Open No. 8-216570, Japanese Patent Application No. 
7-120237, and the like. 
The method of forming the antenna coil of the IC card by printing the 
conductive paste (for example, a silver paste) or the like on a PET 
(polyethylene terephthalate) film of the conventional techniques have a 
problem that the conductance, that is, the value of resistance of the coil 
is larger than that of a coil made of a general copper wire and that of an 
etching coil on a printed board by one digit or more. 
Specifically, in a coil having about 20 turns, although a direct current 
resistance of a coil formed by copper foil etching is 10 .OMEGA. or less, 
the resistance of a thick film printed coil formed by printing the silver 
paste or the like is 100 to 200 .OMEGA.. When power is received and a 
current flows, if the direct current resistance of the coil is large, the 
coil itself consumes the power. Consequently, a loss occurs and power 
efficiency deteriorates. It is therefore important to reduce the 
resistance of the thick film printed coil by widening the pattern line 
width of the coil as much as possible. 
The coil is printed on the PET film in order to reduce the thickness of the 
card in the conventional techniques. When coils are printed on both of the 
surface and rear surface in order to increase the number of turns of the 
coils, there is a problem such that the antenna coils themselves have a 
self resonant frequency by a floating capacity (capacitor) occurring 
between the coils on the surface and rear surface and L (reactance) of the 
coils and an adverse influence may be exerted on a transmission frequency 
band. Specifically, the self resonant frequency by the floating capacity 
when the silver paste coils each having about 20 turns are formed on the 
PET film having the thickness of 50 .mu.m is few MHz to tens MHz. Since 
the power and the communication frequency of the contact IC card 
conforming to ISO10536 are defined as 4.91 MHz, the self resonant 
frequency is a problem. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a contact IC card in which a 
low-resistance antenna coil is formed by a conductive paste. 
It is another object of the invention to reduce the direct current 
resistance and the floating capacity when an antenna coil in a card is 
formed by using a conductive paste in a contact IC card, especially, 
according to ISO10536 standard. 
In order to achieve the objects, according to the first feature of the 
invention, as means for reducing the direct current resistance of the 
coils, means for partially widening the pattern line width of the coil in 
parts having dimensional room when the coils are formed is provided. 
The second feature of the invention is realized in such a manner that when 
the coils are formed in two layers on the surface and the rear surface, as 
means for reducing the capacity between the coils, the coil on the surface 
and the coil on the rear surface are arranged so as not to be in 
overlapped positions and the wiring pattern of the coil on the rear side 
is arranged between the patterns of the coil on the surface. 
The features of the invention will be more clearly described as follows. 
According to a fundamental feature of the invention, as means for reducing 
the direct current resistance of the coil in a predetermined dimension, 
coupling areas are formed in the central parts on the surface and the rear 
surface of coil forming areas having predetermined dimensions on an 
insulating film, and antenna coils consisting of a pair of coils on the 
surface and the rear surface which have the turns in the same direction 
when seen from the outside and are connected via a though hole in an end 
part of the coupling areas are formed by a thick film printing method. 
According to the feature, the low-resistant antenna coils each having the 
large number of turns can be formed in the coil forming areas of the 
predetermined dimensions. Since leads of the surface and rear surface 
coils formed so as to face each other on the surface and the rear surface 
of the insulating film are located on the outside of the coils, no leads 
traversing the coils are formed. Both of the pair of surface and rear 
surface coils can be formed in almost the same pattern and in the same 
winding direction on the insulating film, so that it is advantageous also 
from the viewpoint of manufacturing. 
According to another feature of the invention, by forming two or more pairs 
of the surface and rear coils on the insulating film, connection between 
the pairs of coils (antenna coils) and external circuits and connection 
between the antenna coils can be performed on the outside of the coils. 
According to another feature of the invention, in the fundamental feature, 
the coil forming area is a rectangle and the line width of the coil 
pattern in the longitudinal direction of the coil forming area is formed 
so as to be wider. According to the feature, the direct current resistance 
of the coil can be further reduced. 
According to further another feature of the invention, in the fundamental 
feature, the patterns of the surface and rear surface coils of the pair of 
coils are formed so as to be deviated from each other at a half pitch. 
According to the feature, the floating capacity between both of the coils 
can be remarkably reduced. 
According to the invention, in a contactless IC card in which coils are 
printed by a conductive paste, the direct current resistance of the 
antenna coils can be reduced and the adverse influence on the 
communication by the self resonance of the coils can be reduced, so that 
there is an effect that the transmission efficiency of power and signals 
is improved. 
The above and other objects and features of the invention will become more 
apparent from the following description of the embodiments with reference 
to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a diagram showing the shape of coils of the invention. FIG. 2 is 
a cross section of the coil shape of FIG. 1. 
The circuit construction of a contactless IC card will be first described. 
FIG. 3 is a block diagram showing the circuit construction of the 
contactless IC card. In the contactless IC card, power and signals are 
transmitted and received between a coil 311 in a read/write control 
circuit 31 in a reader/writer (R/W) and a coil 321 in an IC card 32 having 
a communication control circuit by electromagnetic coupling. The IC card 
32 comprises: a power source circuit 322 for generating a power voltage 
from an electromagnetic wave signal received by the coil 321; a 
transmitting and receiving control circuit 323 for controlling 
modulation/demodulation and transfer of the transmitted and received 
signals; a central processing unit (CPU) 324; and a memory 325. 
An embodiment of a data transmitting/receiving method between the 
read/write control circuit 31 and the IC card 32 will be described with 
reference to FIG. 6. 
For example, with respect to a transmitting data signal, an RS232 
transmitting data signal 610 is supplied from a CPU or the like to the 
read/write control circuit 31 and becomes a transmitting data signal 601. 
The transmitting data signal 601 is a serial signal and a logical signal 
of 0 and 1 and performs phase modulation (PSK modulation) of 180.degree. 
to a carrier (4.91 MHz) signal. A PSK modulated data signal 602 passes 
through a filter circuit and the like, becomes a transmitting coil drive 
signal 603, and drives the transmitting coil 311. 
On the other hand, on the IC card side, a received data signal 604 is 
obtained by the reception coil 321 electromagnetically connected to the 
coil on the transmitting side. The received data signal is converted to a 
binary form data signal 605 and processes such as demodulation are 
performed by the digital signal. A clock signal 606 is generated by using 
the binary form data signal 605 as a reference. By sampling the binary 
form data signal 605 at the leading edges of the clock signal 606, a 
demodulated signal 607 is obtained. 
The layout of the IC card according to the invention is as shown in FIG. 4 
so as to follow ISO10536. The IC card 32 is constructed by an LSI 3 and 
coils 1 and 2. The LSI 3 has circuits 322 to 325 in FIG. 3. The coils 1 
and 2 correspond to the coil 321 in FIG. 3. According to ISO10536, an area 
for transmitting and receiving signals is determined by two coils (coils 1 
and 2) arranged at an interval of 22 mm (in the outer size of the coils) 
as shown in FIG. 4. The dimension of a coupling area in the coil is 
defined as about 11.6 mm.times.4 mm from the relation with the 
reader/writer. Therefore, when the coils have to be formed in limited coil 
forming areas by printing means on a card, if the number of turns of the 
coil is large, it is necessary to narrow the line width of the printing 
pattern. 
FIG. 5 is a detailed diagram showing an example of two coils formed by 
silver paste printing. Each of the coils 1 and 2 is constructed by total 
10 turns of five turns each on the surface and rear face. A solid line A 
shows the pattern on the surface and a broken line B shows the pattern on 
the rear face. The patterns on the surface and the rear face are connected 
via a through hole at a point C on the inner side of the coil. The silver 
paste coil has the pattern width of 150 .mu.m, the thickness 20 of .mu.m, 
and the pattern interval of 300 .mu.m. 
Generally, since the conductance of the silver paste coil is smaller than 
that of an etching coil or a winding coil by 10 to 20 times, there may be 
a problem of an impedance (internal resistance). 
That is, when the pattern width of the printed coil is narrowed, the 
impedance (resistance) of the coil increases and the power is not easily 
obtained. Especially, when a conductive paste such as silver paste is 
used, the value of resistance in case of a coil having 20 turns and the 
line width of 150 .mu.m is around 200 .OMEGA. and large, which is much 
larger than that of the etching coil. The internal resistance of the coil 
is a loss when the power is obtained. When the consuming current of the IC 
card on which a CPU is mounted is, for example, about 3 V/10 mA, the power 
is 30 mW, and the loss by the coil is 20 mW. It is an order which cannot 
be ignored. 
FIG. 1 is a diagram showing a preferred embodiment of the invention. In 
FIG. 1, wiring t1 in the vertical direction of the antenna coils 1 and 2 
having the coupling areas in the center has the line width of 150 .mu.m 
same as that of FIG. 5 and the wiring line width t2 in the lateral 
direction is 300 .mu.m which is wider than t1. When the pattern in the 
vertical direction along the short side of the rectangular coupling area 
cannot be widened, because if it is widened, the coils 1 and 2 are 
short-circuited. The pattern in the lateral direction has dimensional room 
in the ISO standard and can be widened. The widths of the patterns in the 
vertical and lateral directions are varied as mentioned above, thereby 
enabling the impedance of the whole antenna coils to be further reduced. 
FIG. 2 is a cross sectional view taken on line X-Y of the antenna coil 2 in 
FIG. 1. Reference numeral 201 denotes a PET film having the thickness of 
50 .mu.m; 202 a silver paste printed coil on the surface; and 203 a silver 
paste printed coil on the rear face. In case of the printed coil using the 
PET film, the thickness of the film is thinner than that in case of 
performing an etching to a general substrate, so that a large floating 
capacity occurs. When the coil wiring on the surface and rear face are 
arranged in facing positions, the floating capacity occurs between the 
wiring on the surface and that on the rear face, and the floating capacity 
and the coil form a resonant circuit. When the resonant frequency is 
almost equal to the communication frequency, an adverse influence is 
exerted. 
According to experiments, the self resonant frequency in case of the above 
coil construction lies in a range from few MHz to tens MHz The 
communication frequency of the power and signals of the contact IC card 
according to ISO10536 is defined as 4.91 MHz, so that the self resonant 
frequency is a problem. 
FIG. 2 shows the antenna coil constructed so as to reduce the influence of 
the floating capacity. As shown in the diagram, in order to reduce the 
floating capacity occurring between the coils on the surface and the rear 
surface, the coil on the surface and the coil on the rear face are 
arranged to deviate from each other at a 1/2 pitch so as not to be in 
overlapped positions. The wiring pattern 203 of the coil on the rear side 
is arranged between the wiring of the coil pattern 202 on the surface. 
FIG. 7 is a diagram showing the whole substrate (PET film substrate) of the 
IC card employing the invention. The dimension in the lateral direction of 
the antenna coils 1 and 2 is larger than that in the vertical direction. 
An integrated circuit is mounted in a position shown by 3. Further, a 
terminal 701 is a test terminal for a circuit debug and is cut when the IC 
card is completed.