RFID TRANSPONDER STRUCTURE OPTIMIZED FOR IN-LINE LABEL CONSTRUCTION
The present invention relates to the manufacture of RFID transponders such as might be used with medicinal packages containing tablets, when compliance monitoring is required, or in other situations in which data pertaining to a shipped article or package is to be collected during transit for later downloading or transmission.
The present invention provides a method for manufacturing such transponders such that they can be readily adhered to a package, as a label, in a condition ready for use therewith.
SUMMARY OF THE INVENTION
In accordance with the present invention RFID transponders are manufactured using a continuous roll of pressure sensitive tape or other such material. The tape is removed from the roll, exposing the adhesive face thereof. A portion of the adhesive surface is coated with an anti-adhesive material and an antenna is printed, using electrically conductive ink, on the tape on top of the anti-adhesive coating, providing a stable foundation for the antenna. Typically, two spaced apart antenna pads will be printed on the tape. An RFID chip strap, carrying the required RFID chip, with suitable electrical connections, is adhered to the antenna pads so that the chip is located between the pads and electrical connection with the antenna pads is achieved through the strap.
For greater adhesion, or to promote curing, the strap and pads can be subjected to ultrasonic vibrations.
The method of the present invention is suited to high speed automated equipment and represents an inexpensive procedure for creating transponders for subsequent assembly to packages or other articles. The completed transponders can be severed from tape carrying a plurality of the transponders and then adhered to the package by way of adhesive on the tape portion thereof surrounding the transponders BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a sketch showing the basic principles of the present invention.
Figure 2 is an enlarged sketch showing the conductive antenna pads on the adhesive tape.
Figure 3 is an enlarged sketch showing the RFID strap in position on the antenna pads.
Figure 4 is a view showing ultrasonic welding of the strap to the pads.
Figure 5 is a sketch showing the RFID chip strap.
Figure 6 shows textured surface of the printing.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following sets forth the primary embodiment of the present invention as well as several alternative or complementary aspects of the invention.
1. Pressure sensitive label stock adhesive is used for the strap bonding The transponder (Fig 1) consists of an antenna 4 which is printed on the adhesive side 1 of delaminated label stock 5, such as adhesive tape. The label stock is provided on a roll thereof and stock is progressively removed from the roll as required. If the label stock is provided with a liner, as illustrated, the liner is continuously separated from the adhesive surface of the tape, so as to expose the adhesive surface. An RFID chip strap 3 is then mounted to the antenna 4.
Antenna 4, in the form of a conductive pad, is printed using electrically conductive ink on top of the pressure sensitive adhesive tape after delaminating of the stock liner.
The area of the tape on which the antenna is to be printed is initially covered with a printable anti-adhesive coating which promotes a stable foundation for the conductive ink to be transferred to.
The pressure sensitive adhesive is deactivated by the anti-adhesive coating, so that it no longer performs as an adhesive. The anti-adhesive coating may be applied in line on a printing press as a first colour and the conductive ink is printed as a second colour.
The printed conductive pads 3 and 4 (Fig 2) are antenna areas designed for the RFID
chip strap to be attached thereto. Conductive pads 3 and 4 have multiple adhesion windows 1. Each adhesion window has a central area not covered with any coating or ink, so that the pressure sensitive adhesive from the label stock is exposed to the chip strap coming on top of the pad.
The strap is bonded to the antenna through the adhesion windows and through the area between and around the conductive pads which is not covered with anti-adhesive coating (Fig 3). The strap needs to have an extended width to the sides, so that there is enough area for bonding outside the perimeter of the conductive pads. The special shape of the conductive antenna pads and of the strap will increase the probability of a good electrical connection and strap-to-antenna bonding during registration variations in the in-line process.
When the chip strap (which includes the substrate 6 (Fig 4) and its own printed conductive pads 5) is pushed against the conductive antenna pads (printed on substrate 1 with anti-adhesive layer 3 and conductive ink layer 4), a stable electrical connection is established.
2. Ultrasonic welding is used to establish electrical connection between RFID
chip strap and printed antenna When welding plastics, a temperature rise in the bonding area is produced by the absorption of mechanical vibrations, the reflection of the vibrations in the connecting area, and the friction of the surfaces of the parts. The vibrations are introduced vertically. In the contact area, frictional heat is produced so that material plasticizes locally, forging an insoluble connection between both parts within a very short period of time.
The prerequisite is that both working pieces have a near equivalent melting point.
This process could be used to bond printed conductive pads 5 of the RFID chip strap to printed conductive pads 4 of the RFID antenna (Fig 4). The conductive ink which is used to print antenna pads and chip strap pads has the ability to be bonded ultrasonically.
The pad design and location is very important in ultrasonic welding, so that it will be accomplished in such a way that there is no interference between welding of joints (the joint between the strap pad and antenna pad) which are getting welded.
Ultrasonic welding of the strap to the antenna pads can be used on its own or it can be used in conjunction with the first-described method of adhesive bonding, to improve or speed up adhesive curing. Ultrasonic welding also enhances electrical conductivity at the junction of the conductive pads of the strap and the antenna.
3. Linear array design of antenna with capacitive coupled fingers, one of which has the RFID
chip strap on it In the case of RFID UHF antennas, characteristics such as high gain, beam scanning or steering capability are possible when discrete fingers are combined to form arrays. The center points of the elements (fingers) are adjusted to coincide with the voltage maxima of the transmission line feeder so that each element appears as a small load.
A single finger with strap on it works as an on/off switch for the antenna.
The RFID strap could be attached to a printed antenna array structure without having any electrical connection to any antenna element. Such RFID transponder structure has the advantage of simplified strap attachment in-line with antenna printing.
The bandwidth and coupling parameters could be adjusted by tapering the width of rectangular elements such that it forms a diamond shaped patch.
4. Forming of 3D textured surface of antenna during of printing process There is a significant improvement of RFID antenna performance if the antenna surface has 3D textured surface. The sizes and shapes of 3D surface elements need to be tuned to the antenna resonance frequency and bandwidth.
Flexographic and gravure printing processes give all required means to control size and shape of antenna surface texture elements. For example plate screening could be used to control depth and shape of antenna surface texture. Ink viscosity, softness of plate material and of sticky back, as well as printing speed are other variables affecting 3D
antenna surface structure. The antenna could be printed on a textured substrate which has surface parameters tuned to the antenna resonance. See for example Fig. 6.
Printing on a substrate with variable ink absorption of the surface gives the same effect to the textured substrate. Ink absorption could be controlled by a non-absorbing material mask printed as an under layer.