Thin inlay technology for ultra flat 0.3mm RFID cards

Introduction

Our wallets and purses are bulging with more and more plastic cards. Some are standard bank cards with magnetic stripes, smart card chips and possibly a RFID interface, others are service, loyalty and membership cards. They are popularly known in the industry as “ISO cards” and must conform amongst other things to the thickness standard of 0.8mm +/-0.04mm. The obvious downside to all this plastic is that your wallet is too thick. Now in the RFID market there is a growing trend to reduce the thickness of cards for some applications, especially transportation and multi-use ticketing and even a move to 0.3mm cards. To meet this trend SES RFID Solutions GmbH has developed a thin inlay (TIN) technology.

 

RFID ISO cards

A RFID ISO card contains an antenna and a chip module. There are typically three main frequency bands using RFID ISO cards today; 125kHz (LF), 13.56MHz (HF) and 868-900MHz (UHF). The antenna technology used can be either copper wire for 125kHz and 13.56MHz, or printed tracks on a substrate for HF and UHF or etched tracks on a substrate also for HF and UHF. See the table below for an overview.

Frequency	125kHz	13.56MHz	868-900MHz
Antenna	Copper wire	Copper wire	Printed
		Printed	Etched
		Etched

The RFID chip is either bonded directly to the chip for some 125kHz applications or more generally the chip is mounted on a module for the vast majority of LF and HF ISO cards. The module is typically between 340 and 400 microns thick and the antenna is either soldered or bonded onto its’ connection pads. Flip chip technology is used to mount HF and UHF chips either onto a strap or directly onto the antenna connection for UHF applications.

RFID ISO Card construction

For copper wire wound antennas, the chip mounted on the module and antenna are placed onto a PVC or PETG substrate in the desired card position. A top cover sheet is placed over the substrate and the two sheets are laminated together on a flat bed laminator to form a “pre-lamination sheet”. Typically the thickness of the sheet is between 400 and 480microns, with some suppliers offering less than 400 microns.

White or printed sheets together with optional magnetic stripes or overlays are then placed on the top and bottom surfaces of the pre-lamination sheet. A second lamination is then carried out for the total construction. Finally the sheets are punched out to reveal a 0.8mm ISO card.

 

0.5mm RFID cards

For high frequency, the advantage of RFID ISO cards for mass transit applications has long been recognised with major world cities adopting the technology for season tickets that are rechargeable and useable for weeks, months or even years. However, the vast majority of tickets purchased today on many mass transport systems are still for single journeys or short multiple use; such as a day pass or 10 journeys. The transport authorities are now turning to thinner RFID tickets for ways to reduce fraud, cut waste by recycling tickets and saving cost on ticket reading machine maintenance.  

The challenge for a 0.5mm multiple use RFID ticket made out of PVC rather than paper is to reduce the overall thickness, but also maintaining an even and flat surface so that the antenna and chip module are not visible or detectable by touch. SES RFID Solutions achieves this by using patented technology to produce a flat inlay instead of a pre-lamination sheet. The first step of the process is to wind the antenna coil with each turn positioned directly on top of the neighbouring turn in a single plane. The resulting complete antenna is then only as thick as a single wire, typically tens of microns and is then placed on a PVC or PETG substrate. The chip module is replaced by a thinner commercially available 280 micron flip chip strap which is soldered to the antenna. Heat and pressure are used to sink the antenna and chip module into the PVC substrate producing a flat 0.3mm inlay. By adding printed sheets and carrying out a further lamination a 0.5mm card is produced. 

Further applications for the 0.5mm RFID cards are to be found in re-useable retail labels and small size tickets.

 

0.3mm RFID Cards

In response to market demands for yet thinner cards, SES has successfully developed F.I.T. a flat inlay technology that can be used to reduce the thickness of a RFID card down to 0.3mm, or as an inlay for 0.5mm or even ISO cards. The F.I.T. is the subject of a world wide patent application and it uses a unique strap for the chip bonding. The chips are mounted on the strap using a flip chip process and it is useable for all standard HF chip products.

The antenna coil winding technology is the same as used on the 0.5mm cards and is once again only 10’s of microns in thickness and placed on a PVC or PETG core. The strap is added and overlays of the same material are laminated on the top and bottom to form a flat finished sheet that is less than 0.3mm in thickness. The thin profile of the strap ensures the finished sheets are flexible enough to be printed on some industrial printers, thereby allowing direct printing on the inlay surface using professional techniques. The major advantage of this step is that card printers no longer need to laminate printed sheets to the RFID inlay, so potentially reducing costs and time to market. 

One of the major concerns for such a thin card is the stress placed on the critical connection between the antenna and the flexible strap as the card is bent and twisted in use. Mechanical tests were carried out on the F.I.T. including full ISO bending, torsion and twisting and extensive 3 wheel tests directly on the chip and strap. Accelerated lifetime and heat shock tests were also performed and the results demonstrate the suitability of the SES strap for reliable, flexible thin cards.

The technology is in production with sheet sizes up to 640mm x 500mm possible and customised card layouts. 

 

Conclusion

With the introduction of the 0.3mm flat inlay technology F.I.T., SES is now able to address new market areas with significant applications where previously, RFID was considered impossible due to the thickness of the electronics, mechanical stress or electrical restrictions.