A New Flexible Polymer Silver from ESL ElectroScience
Introduction
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Fig. 1 Smart label from the Fraunhofer Institut Zuverl?ssigkeit und Mikrointegration (IZM) in Munich, Germany |
There is an undoubted awareness of security, identity theft and fraud issues in the modern world. Smart cards and smart labels are seen as the primary weapons in the battle to make the world more secure. The use of biometric recognition (iris and fingerprint scanning) is also being tested.
The amount of information that can be stored on smart cards is up to 100 times greater than that which can be stored on regular magnetic stripe cards. It takes a long time to read the greater level of information on contact smart cards. Contactless smart cards have been developed to speed up this process. Using a standard 13.56 MHz frequency, information can be read and updated via radio waves coupling with an antenna joined to the processing/memory chip and embedded in the card. Advances in the technology have meant that these devices can be made smaller and flexible enough to be used as smart labels for goods identification as a means of controlling and distributing stock (see figure 1). It has even found its way onto shop shelves in trials made by the large supermarket chains as a potential replacement for the ubiquitous barcode. Contactless cards are the ideal solution when transactions have to be processed very quickly, as in mass transit or toll collection activities. Many of the transport systems in the major cities in the world are using smart card technology.
By the end of 2003 there were 2 billion smart cards in circulation. Ten per cent of these utilize contactless (radio frequency identification or RFID) technology. The majority of these use copper wire antennas.
Printed Antennas
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Fig.2 Fraunhofer IZM reel to reel process |
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Fig. 3 1901-S microstructure showing the silver particles – courtesy of Fraunhofer IZM |
The rapid growth in the use of these devices has necessitated the search for a fast, cost-efficient, seamless manufacturing route. Reel to reel technology, such as is used in the newspaper industry, is considered to be faster than traditional methods of handling substrate material. Institutes like Fraunhofer IZM are preparing smart cards/labels using continuous production lines.
Central to the success of this manufacturing route is the use of polymer based thick-film pastes that can be screen-printed using specially adapted printers that accept a reel to reel process (see figure 2).
ESL 1901-S screen-printable, polymer silver paste may be used in large volume manufacturing routes such as the reel to reel process.
The choice of substrate material allows for relatively high processing temperatures (up to 150 °C for the few minutes it takes to cure the paste). Line/space resolutions of 200 μm have been achieved quite easily and the spread in resistance values of tracks printed at this thickness are good. The resistivity of ESL1901-S is 15-20 mW/
at a thickness of 25 μm when cured at 80 °C for two hours. The substrate chosen is the plastic that is used for credit cards. It may well be that this resistivity is too high for some applications and ESL is working on a lower-resistivity polymer silver. While silver is the metal that has been chosen to produce the pioneer product, other metals are being considered for inclusion in a polymer matrix to make a screen-printable conductor for antennas (see figure 3).
Figure 4 shows data collected in thermal cycling tests between –40 and 85 °C up to 1000 times (x axis) The results show little or no change to resistivity (y axis). Extended periods of time at temperatures at up to 85 °C have shown changes to resistance of tracks due to further curing. These results are taken from work done at Fraunhofer IZM on tracks that have been cured for two minutes at 125 °C.
Temperature Cycling [-40 °C/+85 °C]
Fig. 4 Thermal Cycling Tests
The polymer paste can be printed onto a variety of substrates including paper, fabric, and many plastics. The silver film remains flexible after curing.
The advantages of using the polymer paste are:
1. Increased flexibility over copper wire antennas enables polymer pastes to be used as the antenna in devices for tagging clothes and can be printed directly onto the cloth. The flexibility of polymer pastes is also useful for tagging devices that have curved surfaces.
2. Reel to reel processing. Other techniques (pad printing, photogravure, etc.) may also be used at a future date.
3. Cost-effective manufacturing.
Flexible Electronics
It is evident that electronics are becoming less remote and are being incorporated into our every day lives (mobile phones, portable audio, etc.). More and more devices will be developed that will bring electronics to hand. Flexible displays will mean that electronic newspapers that can be folded up after reading them and updated the next day will be with us sooner or later. Chips are thin (30 μm) and flexible enough to be used as security devices in bank notes and sensitive documentation. Transistors are being printed and we await the day when they are produced in large volumes. Smart cards and, in particular, smart labels are part of this electronics revolution.
ESL is very conscious of the developments that are taking place and is in communication with companies involved in flexible electronics. We see the day approaching when electronics will be embedded in the clothes that we wear. ESL is developing flexible polymer based resistors for embedded circuitry.
