Note: Descriptions are shown in the official language in which they were submitted.
CA 02432701 2003-06-20
Electrically conductive connection between a chip and a
coupling element, and security element, security paper
and document of value with such a connection
The invention relates to an electrically conductive
connection between an integrated circuit, formed as a
chip, and a coupling element for transmitting data
and/or energy from and/or to the chip. The invention
likewise relates to documents of value, in particular
banknotes, and security papers for the production of
these documents of value, and security elements to be
applied to or incorporated in the security paper, which
in each case have such an electrically conductive
connection between chip and associated coupling
element. Finally, the invention also relates to a
method for the capacitive transmission of data and/or
energy between connected chip and coupling element.
The documents of value in the sense of the present
invention can be banknotes, certificates or else credit
cards and the like. The invention is particularly
suitable for use in thin, papery documents of value
such as banknotes, but can also be used in conjunction
with other documents of value.
The application of integrated circuits in form of chips
in or on a banknote is proposed, for example, in EP 0
905 657 Al, in order to increase the security of
banknotes against forgeries by means of information
stored in the chip. The chip is a transponder chip
with which, therefore, contact is not made directly by
external data processing devices, but in which the data
interchange is carried out via a coupling element
formed as an open or closed dipole antenna, for which
purpose, for example, the security thread contained in
the banknote or a closed coil provided separately in
the banknote can be used.
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The production of an electrically conductive connection
between the chip and the coupling element is subject to
certain restrictions, because of the low paper layer
thickness and because of the low thickness of the chip.
A connection by means of final (gold) wires in the wire
bonding technique is ruled out because of these
limiting conditions. Instead, the flip-chip technique,
known from smart card technology, is suitable, in which
the chip with its contact elements is immersed in an
electrically conductive compound applied to the
coupling element. The use of an electrically
conductive adhesive is mentioned, for example, in DE 44
16 997 Al.
In order to ensure that the adhesive used does not lead
to a short-circuit link, in particular in the case of
closely adjacent chip contact elements, in connection
with smart cards it is proposed in EP-A-512 546 and DE
199 57 609 Al to use anisotropic conductive adhesives,
which are therefore conductive in only one direction,
specifically between chip and contact element. Such an
anisotropic adhesive layer can cover all the contact
elements together without this leading to the formation
of a short circuit. For this purpose, DE 199 57 609 Al
proposes the provision of spherical, possibly
resilient, conductive bodies beside one another and
spaced apart from one another in a compressible heat-
seal adhesive layer which can be handled. In the case
in which the chip is applied to the coupling element
using the hot punching technique, the application of
pressure and temperature means that the spherical
conductive bodies form an electrically conductive
connection between the contact elements of the chip and
the coupling element, but the conductive bodies not
being connected to one another, so that the formation
of a short circuit is avoided. Practical statements
relating to the resilient conductive bodies are not
contained in DE 199 57 609 Al.
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The aforementioned proposals cannot readily be
transferred from smart card technology to the
technology of documents of value, which is attributed
in particular to the extremely low material thickness
of the documents of value and chips. The risk of chip
fracture during the application of the extremely thin
chip to the coupling element has proven to be a
particular problem.
It is therefore an object of the present invention to
propose an electrically conductive connection with
which, in particular in conjunction with documents of
value, chips and associated coupling elements can be
connected reliably to one another.
Further objects of the invention consist in proposing
corresponding security elements for document of value,
documents of value as such and security papers for the
production of such documents of value, and also a
method for the transmission of data and/or energy.
According to the invention, these objects are in each
case achieved by the features of the dependent
secondary claims. In claims dependent on these,
advantageous refinements and developments of the
invention are specified.
The solutions according to the invention are based on
the finding that the electrically conductive connection
between the chip and coupling element is to produce as
few mechanical stresses as possible in the frangible
chip, in particular at the time of chip application.
According to a first embodiment of the invention, this
finding is implemented, in order to achieve the
aforementioned object, by the adhesive that connects
the chip and coupling element being an anisotropic
conductive adhesive which contains electrically
conductive particles whose maximum size is less than
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the distance between the chip and the coupling element,
preferably less than one m. These particles are preferably
oriented in the adhesive, for example in a magnetic field,
such that they conduct only in the direction between chip and
coupling element and are otherwise insulated from one another
by the adhesive material. Given an adhesive layer thickness
of about 2 m, it is thus ensured that the particles can exert
no mechanical stresses on the chip, because of their small
size.
According to a second embodiment of the invention, an
anisotropic adhesive with electrically conductive particles is
likewise used, but consists of a solder material which is soft
at an elevated temperature at which the chip is connected to
the coupling element. After it has been cooled down, the
solder material solidifies in its form assumed during the
application of the chip. It is thus ensured that, in
particular during the application of the chip, no mechanical
stresses are exerted on the chip by the electrically
conductive particles. The solder material particles are
arranged beside one another and spaced apart from one another
in a suitable adhesive matrix in such a way that they are also
still spaced apart from one another following application of
the chip. Examples for the manner in which the chip and the
anisotropic adhesive are applied to the coupling element, and
also the possible alternatives for the configuration of the
specific anisotropic adhesive layer comprising particles or
conductive bodies spaced apart from one another, can be found
in the prior art literature, for example, in DE 199 57 609 Al.
Instead of the use of an electrically conductive adhesive for
making non-reactive contact between chip and contact element,
a third embodiment of the invention provides capacitive
coupling. This embodiment is suitable for use with high
frequency
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automating currents of, for example, about 2 GHz. Any
conductive particles in the adhesive layer which
promote mechanical stresses can be dispensed with.
Instead, the adhesive layer forms a thin, dielectric
layer between the chip contact surfaces and the
coupling element. The supply of energy for the chip is
provided by an alternating field coupled capacitively
into the chip, and the chip comprises a rectifier and a
capacitor for storing energy.
In the following text, the invention will be explained
by way of example using the appended figures, in which:
Figure 1 shows an electrically conductive connection
with anisotropic adhesive according to the first
embodiment of the invention,
Figure 2 shows an electrically conductive connection
with anisotropic adhesive according to the second
embodiment of the invention, and
Figure 3 shows an electrically conductive connection
with capacitive coupling according to the third
embodiment of the invention.
Figure 1 shows a substrate 1 comprising a coupling
element 2 and an integrated circuit 3 electrically
conductively connected to the coupling element 2 via an
adhesive 4.
The substrate 1 can be a document of value, in
particular a banknote, but also a preproduct of such a
document of value, specifically a security paper which
is only later further processed to form a document of
value. However, the substrate 1 can also be other
documents of value, for example a smart card. In
particular, the substrate 1 can also be a security
element, for example an optically variable element
(OVD) or a security thread which is only later
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connected to a document of value or security paper. In
the case of an OVD, the security element has preferably
been applied to the document of value in such a way
that the object of the variable side of the OVD points
outward and the OVD hides the view of the integrated
circuit 3. If the optically variable element is
designed as a hologram, the coupling element 2 can at
the same time serve as a reflective layer in order to
reflect light passing through the optically variable
element.
The invention is preferably used in a document of value
such as banknotes. The substrate 1 and the coupling
element 2 preferably form a security thread therein,
the substrate 1 being, for example, a transparent
plastic thread, on which the coupling element 2 is
present as an aluminum coating. The coupling element 2
then represents an open dipole, via which data and/or
energy can be transmitted without contact to the
integrated circuit 3 from an external device. Instead
of a security thread, the chip 3 can also be applied to
another substrate or security element and connected to
the banknote, it then being possible, depending on the
available area, to form the coupling element 2 as a
closed dipole or as a coil. Furthermore, a laminating
film, which is not illustrated in the figure, can also
be arranged, at least in the region of the integrated
circuit 3.
Before its application, the integrated circuit is
present as a chip which can be handled. This can be,
for example, a pure memory chip (ROM), a rewritable
chip (EPROM, EEPROM) or else a microprocessor chip.
The chips used have a thickness of 5 m to 50 m,
preferably 10 m, and an edge length of about 0.1 mm to
3 mm, preferably 0.6 mm. On the chip 3 there are
preferably 2 to 4 contact surfaces 5.
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The adhesive 4, by means of which the chip 3 and the
coupling element 2 are firmly connected to each other,
contains electrically conductive particles 6.
According to the embodiment according to Figure 1, the
maximum size of these particles 6 is substantially less
than the distance between the contact surfaces five and
the coupling element 2. This ensures that, when the
chip 3 is applied to the coupling element 2, no
mechanical stresses occur in the chip 3 on account of
particles 6 lying in between. The distance between the
contact surfaces 5 and the coupling element 2 is
preferably about 2 gm and the maximum particle size is
therefore less than about 1 m, preferably about 0.5
gm.
In order to achieve the situation in which the
particles 6 within the adhesive 4 are electrically
conductive only in the direction from and to the
contact surfaces 5 lying closest to the coupling
element 2, the particles 6 are oriented before the
adhesive 4 has been cross-linked to such an extent that
the particles 6 are largely fixed in their oriented
position. The orientation of the particles 6 can be
carried out, for example, in a magnetic field, so the
adjacent particles 6 repel one another and form lines
in the preferential electrical direction.
A material that has proven to be suitable for the
conductive particles 6 is the conductive powder
marketed by DuPont under the trade name "ZELEC ECP-
3010XC". Likewise suitable is silver nanopowder with a
particle size of preferably about 0.5 m.
The selection of the adhesive is not unimportant
either. It must be protected against mechanical
overstressing and therefore be flexible within limits.
With a layer thickness of 2 gm, the non-reactive volume
resistance should be about 5 ohms. Furthermore, it
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must be ensured that adequate contact is made with the
small contact surfaces 5 of the chip 3, with a size of
in each case about 70 m x 70 m. In particular,
making contact with aluminum layers with an oxide
covering must be ensured, since the contact between the
coupling element 2 and air causes natural oxidation of
the aluminum. Finally, it must be ensured that the
electrical conductivity of the adhesive in the high
frequency field in the direction opposite to the
preferential direction reaches no more than 100 m.
100 m is approximately the distance between adjacent
contact regions of the coupling element 2.
According to a second embodiment, which is illustrated
in figure 2, the electrically conductive particles 6 of
the anisotropic adhesive 4 consist of a solder
material, for example tin-bismuth. The chip 3 is
normally placed on the coupling element 2 using the hot
punching technique, the temperature introduced leading
to softening and cross-linking of the adhesive 4 and
the slight pressure leading to contact between the chip
3 and the connecting regions of the coupling element 2.
The temperature of the hot punching and the softening
temperature of the solder material are matched to each
other in such a way that the solder material also
softens during the application of the chip 3 and, in
accordance with the pressure of the hot punching, is
matched faithfully to the contact surfaces 5. During
the application of the chip 3 to the coupling element
2, the electrically conductive solder material
particles thus do not exert any substantial stresses on
the chip 3, instead are deformed plastically, but
ensure an electrically conductive connection between
the contact surfaces 5 of the chip 3 and the coupling
element 2. The solder particles 6 are distributed in
the adhesive 4 in the manner of a matrix, so that they
are not transversely connected to one another, even
after the application of the chip, as described in DE
199 57 509 Al.
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In conjunction with the solder material particles, the
adhesive marketed under the designation "Loctite 3440"
has proven to be a suitable adhesive.
According to a third embodiment, which is illustrated
in Figure 3, use is made of an adhesive 4 which does
not necessarily contain electrically conductive
particles. The transmission of data and/or energy is
carried out capacitively between the coupling element 2
and the contact surfaces 5 of the chip 3. This type of
transmission is particularly suitable at high
frequencies of, for example, 2 GHz for the transmission
of energy, the data stream being modulated on, for
example at a frequency of about 400 MHz. In this case,
the slight distance between the contact surfaces 5 and
the respectively immediately adjacent sections of the
coupling element 2 acts as a short circuit. It is
possible to dispense with making non-reactive contact.
The chip has a capacitor and a voltage rectifier, in
order to charge up the capacitor with energy via an
alternating field coupled in capacitively. The
adhesive 4 acts as a dielectric between the coupling
element 2 and the contact terminals 5. For the
capacitive coupling, it is also necessary to. enlarge
the connecting areas of the integrated circuit.
