In recent years, contactless smart cards have increasingly been used in the field of payment and identification. In addition to the bus industry, which is currently the most widely used smart card, more and more countries are considering extending non-contact applications to other national projects. Given the global growth of contactless smart card applications, and considering the technical requirements of different products and the different needs of end customers, designing smart card antennas that meet the needs of different applications has become a challenging task. This article will discuss the various factors that need to be considered in the design of smart card antennas, as well as the challenges in different application areas.

Smart card antenna design considerations

A smart card antenna is an electrical component that supplies power to a smart card integrated circuit (IC) through electromagnetic induction of a radio frequency (RF) magnetic field generated by the card reader. It is also the communication medium between the smart card IC and the card reader. Improperly designed antennas can greatly reduce the performance of the IC card, and a properly designed antenna will help the IC card achieve the best performance of its design, achieving the following features:

Work field and load modulation requirements in accordance with ISO/IEC 14443/10373-6

Compliant with PayPass-ISO/IEC 14443 implementation specification - V1.1 and EMV contactless communication protocol specification V2.0, compatible with existing certified readers to optimize working distance: to bring the best working distance for a given application, Do not affect the smart card function to support multiple cards, even if these cards are stacked on top of each other

Accurate positioning of the antenna in the card: In order to ensure that the smart card is used in conjunction with a card reader with a small antenna, the antenna must be designed in a specific area on the card. Because only then, the antenna of the smart card and the card reader can achieve the predetermined magnetic coupling.

Smart card antenna design considerations and application scenarios
Figure title: Typical construction of a dual-interface contactless smart card

addiTIonal overlay-coaTIng foil, thickness 50-100um: additional cover, thickness 50-100 microns

Printed overlay foil, thickness 100-150um: printed overlay, thickness 100-150 microns

Basic foil with coil: 200-300um, PVC, surface glueless: base layer with coil: 200-300 microns, PVC material, degummed surface

Module: 540um total thickness: module: total thickness 540 microns

In the smart card antenna design, three main components that affect the resonant frequency of the card need to be considered. In order to optimize the performance of smart card working distance and RF communication stability, the impact of these components must be fully considered.

Integrated circuit (IC)

This is the core part. The input capacitance and minimum operating voltage of the chip will determine the maximum working distance of the smart card and the multi-card simultaneous operation.

IC module

The smart card IC is placed inside the module. The module makes the IC easy to handle while protecting the IC from external stresses (such as excessive bending, etc.) and UV damage. In addition, the module design expands the antenna connection area, which provides convenience for different antenna connection methods. In the smart card packaging process, modules are used more often than bare ICs. From an electrical point of view, the module adds extra capacitance to the resonant circuit of the IC card.

Smart card packaging material

Due to its dielectric properties, the packaging material also adds extra capacitance to the resonant circuit of the final IC card. Is the smart card antenna design and its impact on specific application areas a smart card antenna that is suitable for all applications without any glitch? This is not the case. Carefully designed antennas play an extremely important role in the overall performance of non-contact applications, but the technical requirements for different applications are completely different. Therefore, designing a universal antenna is a challenging task. The following sections briefly describe the challenges faced in some typical applications.

Payment application

Critical coupling between card and card reader systems RF communication is a challenge when readers are smaller than smart cards. For the sake of simplicity and ease of design, the current popular standard is to design contactless readers as small as possible and as compact as possible. This means that the reader's antenna is smaller than the usual ID1 size. However, most payment cards (such as Visawave, Paywave, JCB), which are generally accepted in the industry, still implement the ISO/IEC 7810 standard (ID1, 85mm*54mm), and use a smaller-sized card reader for RF communication. The challenge.

This situation creates a critical coupling effect between the card and the reader system, which typically results in extremely unstable RF communication between the card and the reader. Although seemingly unreasonable, this coupling effect does violate the basic logic, that is, the closer the card is to the reader, the stronger the coupling effect!

However, the following methods can be used to minimize the impact of this problem:

In order to overcome the negative effects caused by the mismatch of the size of the card antenna and the reader antenna, one method is that the designer can adjust the size of the card antenna and the reader antenna so that the size of the reader antenna is larger than that of the card antenna. . Depending on the limitations of the payment system, the reader antenna can be adjusted or the smart card antenna design changed. In fact, payment cards that are only half the size of ID1 are becoming more and more popular on the market. Although this method solves the above problems, it also brings other problems. These cards, which are only half the size of ID1, are difficult to meet the modulation requirements for minimum load as specified in ISO 14443. Despite this, the industry has found designs that use smaller form factors (ID1/2 and ID1/3) and meet the load modulation limits specified in ISO14443.

Change the design of the card antenna (such as inductance, coil material, etc.) to achieve the purpose of adjusting the Q value or resonant frequency. If the Q value of the coil is low, the energy coupling it transmits to the card is relatively small, and detuning the card to obtain a higher resonant frequency will achieve the same effect. Both of these methods can reduce the interaction between the card antenna and the reader antenna, thereby reducing the coupling effect between them. The advantage of this approach is that there is no need to change the design of the card reader to avoid the high cost of upgrading the card reader system. Of course, the drawback of this method is that it does not fully meet the requirements of working distance for some projects. Although the problem cannot be completely solved, this method can still greatly reduce the negative effects of the coupling effect.

Electromagnetic interference (EMD)

Another problem faced by designers is electromagnetic interference (EMD). As a passive device, the contactless smart card captures all of the energy from the RF field generated by the reader. During internal operation of the IC, such as cryptographic calculations, EEPROM programming, etc., electromagnetic interference (EMD) is generated to the RF field to which the IC supplies energy. This interference causes the reader's receiving circuit to detect "false." "Communication information, causing communication problems between the card and the reader system. The closer the card is to the card reader, the greater the impact. Although the interference can be partially mitigated by fine-tuning the card antenna system (eg, adjusting the tuning inductance of the coil), the problem cannot be completely solved. This problem has now been largely solved by improvements to IC clock technology, including built-in hardware EMD suppression mechanisms.

Bus application

The bus industry was one of the first industries to adopt contactless technology, but since most of its card reading facilities were installed six or seven years ago, some were even installed before the ISO14443 standard was developed, so the facilities were quite old. The main challenge in this area is not meeting the relevant standards. The modulation parameters generated by some of the obsolete readers in the bus industry do not fully comply with the ISO 14443 standard, resulting in a so-called communication "vulnerability" between the card and the reader. The ISO14443 standard specifies the corresponding RF parameters for cards and readers. These parameters give the working range of the specified RF signal, ensuring that the card and reader can achieve interoperability when meeting these parameters. Therefore, if the modulation RF parameters generated by the reader exceed the limits specified by the ISO standard, it is difficult to achieve interoperability between the card reader and the card. The problem that the parameters discussed above do not match the ISO standard is usually related to the generation of the “suspension pattern” defined by the ISO14443 standard, which is generally expressed as the rise time, fall time, overshoot signal and residual carrier of the reader waveform. did not qualified. Optimizing the design of the card antenna does not completely solve these problems, so a more reliable solution is to replace those outdated readers with new ISO-compliant devices, but this option may not be possible because all replacements are made. The facilities being used are costly and may not be feasible in some cases.

Therefore, a viable solution is to improve the design of contactless smart card ICs with superior fault tolerance to accommodate these card reader systems that do not conform to ISO standards.

Identification application

In recent years, government-implemented identification projects have become the main driving force for the development of contactless technology, and have also prompted the industry to pay more attention to the implementation of ISO standards, emphasizing the interoperability of card and reader systems.

The issues discussed above regarding payment applications also exist in identity applications. The difference between a government identity system and other systems is that the government has developed standards based on the application, such as ICAO LDS, with key organizations in the industry. RF protocol testing, etc., and is strictly followed and promoted throughout the industry chain. The overall framework for the mosaic design of ePassports is regulated by ICAO “Electronic Passport RF Protocol and Application Testing Standards – Part 2” (Class 1 Antenna).

These standards, together with the mandatory US e-passport regulations, effectively ensure interoperability and consistency between the card and reader systems, forcing those participating countries to accelerate their e-passport projects. For many years, most countries participating in the US Visa Waiver Program have been actively involved in ICAO e-passport interoperability testing and cross-border pilot projects, which are contactless readers, Inlay and chip manufacturers provide a platform that allows them to work together to develop common standards and address interoperability issues in this particular area.