WO2007059797A1

WO2007059797A1 - Method to weld conductive tracks together

Info

Publication number: WO2007059797A1
Application number: PCT/EP2005/014193
Authority: WO
Inventors: Jean-Jacques Mischler
Original assignee: FCI SA; Framatome Connectors International SAS
Current assignee: FCI SA
Priority date: 2005-11-25
Filing date: 2005-11-25
Publication date: 2007-05-31
Anticipated expiration: 2008-05-25

Abstract

The invention relates to a method for welding a first conductive track (2, 2’) to a second conductive track (12), one of said first and second conductive tracks carrying on at least one side thereof an insulating material (30), said method comprising the steps of pressing said first and second conductive tracks on top of each other in the region of welding so as to form a joint, and welding said first conductive track with said second conductive track by supplying a current in the vicinity of said joint. The invention also relates to a method of manufacturing an antenna circuit constructed with the welding method and to an antenna circuit.

Description

METHOD TO WELD CONDUCTIVE TRACKS TOGETHER

The present invention generally relates to welding methods, and more specifically to methods to weld together conductive tracks extending on insulating material.

Merely for an illustrating purpose, such a method applied to RFID field is disclosed hereunder.

The use of a tag having an RFID antenna to identify and monitor objects is well known in the art. With no self source of energy, the tag comprises an antenna and an integrated circuit (IC) chip that includes a memory. It uses electromagnetic field produced by an RFID reader. If such a tag enters the magnetic field, the RFlD antenna will become energized and the electronic circuit can transmit a coded signal towards the reader or a separate receiving antenna.

An example of RFID layout is shown on Figs. 1a to 1c. This layout includes an antenna 1. An integrated circuit (IC) chip (not shown on Figs. 1a to 1c) is connected to this antenna. The antenna loop 1 , also called later on antenna tracks, generally comprises a plurality of turns that spiral around on a planar dielectric substrate. The antenna tracks 1 may be either formed before being laid on the substrate or formed directly upon it (by printing and/or etching techniques for example), among known techniques. When the antenna pattern requires more than one turn, the antenna terminals 2 and 2' as seen in Figs. 1a to 1c are separated by the plurality of turns, and an electrical bridge must be formed over these spiral turns to close the antenna loop. Different techniques are known to form and connect this bridge, also called later on a conductive strap or strap, to the antenna, while keeping this bridge electrically isolated from the tracks.

Generally speaking, all these techniques necessitate making an electric connection between conductive tracks while isolated from these tracks.

The applicant looked for a simple, straightforward, efficient and improved method for making such connections. Accordingly, the present invention provides an assembly method according to claim 1. The present invention further provides an antenna circuit according to claim 13.

The invention takes advantage of the local electric discharge and/or heating generated by the passage of the current in the vicinity of said joint, i.e. through the contact or joint between the tracks and/or between the conductive bridge and at least one track and/or through the resistance of the tracks and /or the conductive bridge, in the joint region. Consequently, the metals of the tracks and/or of the conductive bridge melt in the region of welding so that they locally fuse and a weld is formed between them. Thus a good electrical connection is established between the tracks. Such a welding technique is also known as resistive welding. The method according to the invention is effective thought simple and straightforward.

In an additional implementation of the method according to the invention, the first and second conductive tracks are arranged so that the joint comprises the insulating material placed between the conductive tracks, and the current is then supplied in one of the conductive tracks in the region of welding so that the heat generated in this conductive track melts the insulating material. The invention takes advantage of the applicant's observation that when using a current supplied in the joint and/or in at least one track and/or in the conductive bridge, the heat generated from the resistance of the conductor is sufficient to melt and/or destroy locally the insulating material between the tracks and fuse at least one of the conductive tracks and/or bridge. With this implementation, a simplified welding method is thus provided, as a selective printing or deposition of the insulating material is no longer needed. With the method according to the invention, it is no longer necessary to selectively apply the insulating material. A good electrical connection can be obtained between the conductive tracks, even with insulating material in between. It is no longer necessary to keep the region of welding clean of any added material, contrary to the teachings of known welding technologies.

The method according to the invention appears particularly well suited for the welding of a conductive strap on an antenna circuit. Other embodiments and/or implementation of the invention correspond to claims 2-12 and 14-17, considered either independently or as twofold or manifold combined.

Other features and advantages of this invention will further appear in the hereafter description when considered in connection to the accompanying drawings, wherein:

FIG. 1a - 1c illustrate different configurations of antenna terminals when a bridge or strap is required,

FIG. 2 illustrates a 3D view of a bridge that is welded to antenna terminals in an RFID antenna according to a the first exemplary implementation of the method according to the present invention;

FIG. 3 illustrates a cross section view of the RFID antenna from FIG. 2;

FIG. 4 illustrates a view of a bridge or strap with its welded pads that is soldered to the antenna circuit in the first exemplary implementation of the method according to the present invention;

FIG. 5 illustrates a view of a carrier with straps used in the antenna circuit according to the invention;

FIG. 6 illustrates a cross section view of an RFID antenna, after welding of a strap underneath the dielectric substrate carrying the tracks; FIG. 7 illustrates the example of a double sided antenna, with antenna terminals provided on opposite side of the dielectric substrate carrying the antenna tracks; and

FIG. 8 illustrates another example of implementation of the method according to the invention. As an example, the invention is an antenna circuit. Such a circuit generally comprises a dielectric substrate (not shown in FIGs. 1a to 1c) that carries at least one turn 1 forming the antenna loop in the example of a spiral antenna, and first and second antenna terminals 2 and 2'. A chip (not shown in FIGs. 1a to 1c) is generally attached to the antenna tracks, so as to form a RFID antenna. The chip is either attached in the region inside the turns 1 to the chip terminals 3 and 3' (FIG. 1b), or outside the turns 1 to the chip terminals 3 and 3' (FIG. 1c). The chip may also be provided on the bridge (not shown in FIGs. 1a to 1c) that closes the antenna loop (FIG. 1a). In all three instances, a bridge or conductive strap has to be attached to the antenna terminals 2 and 2' to close the antenna loop. The antenna circuit may be provided without a chip, e.g. in the case of a capacitive antenna which may be similar to the antenna of FIG. 1a. As an example of the method according to the invention, one of the conductive tracks carries an insulating material on at least one side thereof.

In the example of an antenna circuit, the bridge or strap on the one hand, as well as the antenna terminals on the other hand, are conductive tracks that are to be welded to each others. The antenna terminals are carried by the dielectric substrate of the antenna. This dielectric substrate may be made for example from a polyester film, such as a PET film (polyethylene terephthalate) or a PEN film (polyethylene naphthalate), from polyimides, from epoxy-glass, from PVC. Substrates such as paper may also be used. The substrate may be rigid as well as flexible. For instance, the substrate is less than 200 microns thick.

The antenna turns may be printed and/or etched over the dielectric substrate. The turns may be made of copper, gold, platinum, aluminum, brass, tin, silver or any other suitable conductive metal or alloy for a printed circuit. The turns may also be laminated over the dielectric substrate using an adhesive laid over the substrate beforehand if necessary. The thickness of the tracks is generally in the 9 to 35 microns range, but may also be thinner or thicker depending on the antenna circuit specifications.

The bridge or strap may also be formed using the same manufacturing techniques used for the antenna, i.e. formed on a dielectric reel tape as seen in FIG. 5. It may also be formed through stamping of a thin metal sheet that is rolled as a reel for example. The strap may also carry or comprise an insulating material as seen later on.

In a first step of the method according to the invention, the conductive tracks of the antenna and the strap are pressed on top of each other in the region of welding so as to form a joint.

FIG. 2 illustrates a 3D view of the antenna terminal regions of an antenna, here an RFID antenna, according to the invention. The antenna circuit corresponds to the antenna of Fig. 1a with the chip 20 provided on the conductive strap 10 formed of two strap conductors 12 and 12'. Antenna turns

1 are formed over a dielectric material 5. As described before for FIG. 1a, antenna terminals 2 and 2' are spaced apart from each other with turns 1 in between, so that a bridge or conductive strap 10 is necessary to electrically link these terminals together.

The two strap conductors 12 and 12' are attached to antenna terminals

2 and 2' respectively. Were the chip provided on the inner section or outer section of the antenna, the strap 10 would be formed of one conductor. The man skilled in the art can easily transpose the teaching described for the antenna of FIG. 1a to the other types of antenna configurations that require a bridge (FIG. 1b and 1c among others),

FIG. 3 is a sectional view of the conductive strap 10 and the antenna circuit 1 after welding of the strap 10 with the method according to the invention. In an implementation example of the method according to the invention, the conductive tracks are arranged so that the resistive joint comprises an insulating material 30 placed between the conductive tracks.

The insulating layer 30 (see FIGs. 2 and 3) is laid over the bridge 10 on one side so that the strap conductors 12 and 12' do not come into contact with the antenna turns 1 when the strap 10 is laid over the antenna terminals 2 and

2'. In the embodiment here disclosed, the insulating layer 30 is laid over the strap 10, regardless of the areas which are to come into contact with the antenna terminals 2 and 2'.

In a further step of the method according to the invention, the conductive tracks are welded to each other or fused together by supplying a current in the resistive joint formed in the previous step. In resistance welding, the conductive tracks, here the antenna terminals and the conductor to be welded, are held together to form a joint, through which a current, e.g. generated through a capacitor, passes and generates heat beyond the fusion point of either the terminals and/or the conductor to fuse them together. The current may be supplied through electrodes that are pressed against the conductive strap laid on top of the antenna terminals. As can be seen on FIG. 3, as a result of the method, the conductive strap 10, here formed with strap conductors 12 and 12', is pressed respectively over the antenna terminals 2 and 2' in the region of welding. A local bending of strap 10 can be seen in the welded region of the strap over the antenna terminals 2 and 2'.

When the insulating material is placed between the conductive tracks, the current is supplied in one of the conductive tracks in the region of welding so that the heat generated melts at least the insulating material in the joint.

In a preferred implementation of the method according to the invention, the current comprises a first charge, preferably of low intensity, and a second charge of higher intensity. The first charge is to melt the insulating layer, while the second high intensity charge is the welding charge, i.e. an intensity charge high enough to fuse and weld locally the conductive tracks with each others.

In a preferred implementation of the method according to the invention, parallel gap welding is used. To that effect, a pair of electrodes is placed in contact with one of the conductive tracks. In the example of the circuit antenna, the electrodes are pressed on the conductive strap 10, on locations 25 and 26 for the welding region over antenna terminal 2, and on locations 25' and 26' for the welding region over antenna terminal 2', as seen in FIG. 3. Parallel gap welding is a specific form of resistance welding, and consists in pressing a pair of parallel electrodes against one of the elements of a joint to weld, here a conductive track, and supplying this pair of electrodes with a high intensity current. The resulting local heat from the resistance of the conductive track will result in welding the conductive tracks forming the joint, as can be seen in FIG. 3. Indeed, the insulating layer 30, after the welding process, is left between the main body of the strap overlaying the antenna turns 1 , while the insulating layer in the welding region has melted or fused away. Locations 25, 26 on a first end of the strap 10, and locations 25', 26' on the other end, are thus welded to the antenna terminals 2 and 2' respectively. As mentioned before, applying a pair of electrodes in the region of welding has caused a local bending of the strap 10. This results actually from the combination of the pressing in the first step of the method, as well as the local melting of the insulating layer in the region of welding in the second step. In an alternative implementation of the method according to the invention, as illustrated in FIG. 6, the strap 10 is made of a mere conductive strip that is applied directly opposite the antenna terminals 2 and 2' on the face of the dielectric substrate free of the antenna terminals and, the dielectric substrate 5 forming thus the insulating layer of said antenna terminals. In this case, applying an insulating layer on the strap is no longer needed. When parallel gap welding is used, local bending of the strap occurs around welding locations 25, 26, 25' and 26' as seen in FIG. 6.

In another implementation of the method according to the invention, the insulating material is spread on the conductive antenna turns and the strap is a mere conductive tape without any insulating layer. Then, the strap is welded on the terminal portions of the antenna, with the insulating material in between. The welding is implemented through the insulating material applied on the antenna. As mentioned before, the conductive strap may be stamped on a metal foil, or printed or etched on a dielectric substrate. An example of embodiment of the strap is shown in FIG. 4. The strap 10 comprises a first and a second enlarged terminal portions 115 and 115', which are to be welded to the antenna terminals 2 and 2' respectively. The strap 10 further comprises a central narrow portion 110 which helps reduce the stray capacity of the strap when welded to the antenna circuit. The visual aspect of the antenna circuit, for instance when embedded in a contactless card is also improved.

The welding region on the strap 10 is represented by the doted lines defined regions 116 and 116', respectively on terminal portions 115 and 115'. The welding regions are preferably of a few mm² in surface, depending on the size of the antenna and the strap, e.g. 1.2 x 1 mm². Each trace resulting from the parallel gap welding is preferably of a few mm² as well, preferably higher than 1 x 0.5 mm², so that a solid and strain resistant weld can be formed. The current intensity passing between the electrodes and through the strap terminal portions is of a few dozens to a few hundreds of amperes, e.g. 500 A. More specifically the first charge is of a lesser intensity, from a few amperes to a few dozens, while the second charge is a few to 50 fold. The two successive charges, as well as their respective durations, depend upon various paremeters. The first charge intensity depends notably upon the nature and the thickness of the insulating layer, as well as the nature of the track upon which the electrodes are applied. The second charge intensity notably depends upon the nature of the metal or alloys of tracks as well as their respective thicknesses.

For example, for 35 micron conductives tracks, one of them tin plated, the first charge to melt the insulating layer is of about 100 A applied for 60 to 100 miliseconds. The second charge for the welding itself is about 400 A applied for 6 miliseconds. These numbers are for Tungsten electrodes of 0.5 x 1.0 mm, spaced 0.5 mm apart. The same conductive tracks, with no tin layer, require a second intensity charge of approximately 700 A applied for 8 ms.

In a preferred embodiment, the strap is tin plated on at least one side with a few μm of tin; e.g. 5 μm, as less heat is needed to weld the strap to the antenna terminals, which results in a better resistance of the dielectric material of the antenna.

As seen in FIG. 5, the strap may be formed on a carrier 200, whether it is stamped or else. If the straps are printed or etched, or laminated, the carrier may comprise a dielectric band 60 carrying the straps. If straps 10 on carrier 200 are to be welded on top of antenna terminals 2 and 2', the straps may be cut out with a portion of the band 60 to be used as the insulating layer that is welded through.

An insulating layer is needed when the straps of FIG. 5 are flipped over the antenna turns so as to isolate said turns from the strap. The insulating layer may be laid, e.g. printed, over the central portion 110 of the straps, in a continuous manner, e.g. in the form of a continuous band 70 also laid over dielectric band 60. A mask may be used to leave the terminal portions 115 and 115' free of insulating material. When the strap is welded through its insulating layer, a mask is no longer needed and the insulating layer is laid over the whole surface of the straps, without distinction between the terminals portions 115, 115' and the central portion 110. In the method according to the invention, the two enlarged terminal portions 115 and 115' of strap 10 may be welded either simultaneously or successively.

The insulating material laid over the strap may be similar to the dielectric substrate over which the antenna is formed. The insulating material may also be a resin, a non conductive ink, or any suitable non conductive material that can be applied in a thin layer on the strap. Examples are polyester, polyurethane or acrylic resins. As mentioned before, in some instances, the dielectric substrate may act as the insulating material. The insulating material may be a non plastics material such as paper.

In this case, the insulating material is burnt at the welding location, during the welding process.

FIG. 7 is an illustration of another implementation of the method according to the invention, with the example of a double face or sided antenna. Antenna terminals 2 and 2' are provided on opposite side of dielectric substrate 5. Electrodes may be applied on locations 25 and 26 of antenna terminal 2. A first intensity charge may be supplied to first melt the insulating layer formed by dielectric substrate 5 followed by the second charge to weld antenna terminals 2 and 2' together. The method according to the invention may be used for providing electrical paths through one or several insulating layers. For instance, several insulating layers each one of which having a conductive track on it may be stacked one on the other. A conductive path may be created between at least two of them using the welding method according to the invention. As shown on FIG.8, the method according to the invention may be also used for welding flex interconnection systems 200, 201 , also called flat flex cables or Flexible Laminated Cables, together. Each one of these flex cables 200, 201 comprises a metallic conductive track 12 embedded in an insulating cover 30 or sandwiched between two insulating layers. In the embodiment seen on FIG. 8, one 200 of the flex cables 200, 201 has an end with the insulating cover 30 removed. The conductive tracks 12 of this flex cable 200, has been welded to the underlying flex cable 201 with the method according to the invention. An electric path connects the conductive tracks 12 for the three joint locations shown on FIG.8.

Claims

1. A method for welding a first conductive track (2, 2', 10, 12, 12') to a second conductive track (2, 2', 10, 12, 12'), one of said first and second conductive tracks carrying on at least one side thereof an insulating material (30), said method comprising the steps of: a) pressing said first and second conductive tracks on top of each other in the region of welding so as to form a joint, b) welding said first conductive track with said second conductive track by supplying current in the vicinity of said joint.

2. A method according to the previous claim, wherein in step a), the first and second conductive tracks are arranged so that the joint comprises the insulating material placed between said first and second conductive tracks,

3. A method according to claim 1 or 2, wherein in step b), the current is supplied in one of said first and second conductive tracks in the region of joint so that the heat generated in this region fuses at least said insulating material.

4. A method according to the previous claim, wherein in step b) the current comprises a first charge to melt the insulating layer, and a second charge of higher intensity to fuse the conductive tracks together.

5. A method according to one or more of the previous claims, wherein in step b) the current is supplied through a pair of electrodes placed in contact with one of the first or second conductive tracks.

6. A method according to one or more of the previous claims, wherein the first conductive track comprises first (2) and second (2') antenna terminals of an antenna circuit (1) formed on a dielectric substrate (5), said antenna circuit comprising at least one turn (15), the second conductive track comprising a conductive strap adapted to electrically connect said first terminal with said second terminal.

7. A method according to the previous claim, wherein in step a) the strap is applied directly opposite the antenna terminals on the face of the dielectric substrate (5) free of the first conductive track, said dielectric substrate (5) forming thus the insulating layer of said first conductive track.

8. A method according to the claim 6 or 7, wherein the insulating material (30) is provided on the conductive strap to form an insulating layer between said conductive strap and the antenna.

9. A method according to one of the claims 6 to 8, wherein the strap comprises first and second enlarged terminal portions (115, 115') that are to be welded respectively to said first and second antenna terminals, and a central narrow portion (110).

10. A method according to claim 9, wherein electrodes are held in contact with the first and second enlarged terminal portions to weld either simultaneously or successively said enlarged terminal portions with their respective antenna terminals.

11. A method according to one of the claims 6 to 10, wherein the strap is tin plated at least on one side.

12. A method according to one or more of the previous claims, wherein the insulating material (30) is less than 200 microns thick.

13. An antenna circuit comprising a dielectric substrate (5), and an antenna circuit (1) formed thereon, said antenna circuit comprising more than one turn

(1) and a first (2) and second (2') antenna terminals, said antenna further comprising a conductive strap (10, 12, 12') separated from said more than one turn through an insulating layer (5, 30), said conductive strap being welded to said first and second terminals through said insulating layer.

14. An antenna circuit according to the previous claim, wherein the strap is laid against the dielectric substrate (5) opposite the side of said dielectric substrate carrying the antenna terminals, said dielectric substrate forming thus said insulating layer.

15. An antenna circuit according to claim 13, wherein the insulating layer (30) is provided on the strap to form an insulating layer between said conductive strap and the antenna turns.

16. An antenna circuit according to claim 13 or 15, wherein the strap comprises first (115) and second (115') enlarged terminal portions welded respectively to said first and second antenna terminals, and a central narrow portion (110).

17. An antenna circuit according to one of the claims 13 to 16, wherein the strap is tin plated at least on one side.