Producing card bodies containing integrated coils
Contactless smart cards need coils for transferring energy and data. At high frequencies, the coil can be made so small that it can be integrated into the chip module.With such a contactless card, the production process is thus nearly the same as for cards with contacts. The chip module with the coil is simply laminated between two or more plastic foils, or set into a cavity. However, relatively low frequencies are used for most present-day contactless smart cards, which means that larger-diameter coils are necessary. These coils are usually rectangular with rounded corners and measure approximately 75 mm by 45 mm. This means they are only slightly smaller than an ID-1 format card body. They normally have four turns, an inductance of around 4 μH and an electrical resistance of a few ohms. Printed coils are an exception, since they have a resistance of around 300 ohms. Producing special card bodies with integrated coils requires the standard production process to be adapted to meet the modified requirements. However, some general principles remain the same. For example, contactless smart cards are also mostly made using multiple copes for 48 cards printed on large sheets, rather than individually, since this is significantly more economical.

Connecting the chip to the coil
In the currently standard technology, the die to be connected to the coil is attached to a lead frame and electrically connected to two contacts on the lead frame, which are used to make electrical connections to the two ends of the coil. The reason for using a lead frame is that the electrical connections to the coil do not have to be positioned as precisely as they would have to be if the coil were connected directly to the die. Although this method is more expensive than connecting the coil directly to the die, the latter method requires very precise positioning and high-quality bonding technology. If the coil is integrated into the card body, two different methods are used to make the connections between the chip and the lead frame or coil. In the widely used wire bonding method, the chip is connected using fine bonding wires. A significantly more elegant and less expensive solution is die bonding, in which the chip is pressed and glued against the lead frame or coil to make a direct electrical contact. This requires the chip to be flipped over (relative to the wire bonding technique), so this method (and sometimes the chip itself) is called ‘flip-chip’. With both of these methods, the chip is covered with a protective layer of plastic resin after the connections are made.

Etched coils
There are severalways to integrate a coil into a card body. In the first process thatwas developed for producing contactless smart cards, plastic foils coated with a 35-μm film of copper are exposed and then etched to produce a coil in the copper film. The etched track is around 100 μm wide. After the etching, a chip is placed at the terminals of the coil and electrically connected to them. This assembly is then treated as an internal foil, to which overlay foils are laminated on the front and rear sides. The smart card is then finished. This type of card is referred to as a contactless smart card with an etched coil.

Wound coils
The wound coil is a further development of the etched coil. Copper wire with a diameter of 150 μm is wound on a tapered coil form and then slid from the form onto a thin foil, to which it is attached by thermoplastic welding using heat and pressure. Following this, the chip or chip carrier is put in place and the foils are laminated. This method is distinctly less expensive than etched coils, so it is more suitable for large quantities.

Embedded coils
Another method for producing wire coils is called the embedded coil method. This works in a relatively simple manner. A coil of 150 μm-diameter copper wire is laid out directly on a plastic foil and simultaneously bonded to the foil using ultrasonic energy. A device called a ‘sonotrode’ is used to make the coil; it mechanically guides the wire and at the same time ultrasonically welds the wire to the plastic foil. After the coil has been made, the chip or chip carrier is connected to the coil and a cover foil is laminated on top.

Printed coils
Of all of the possible methods, the printed coil method is the most highly developed and the least expensive for mass production. The windings of the coil are printed on an internal foil by silk screening using conductive ink. Silk screening must obtain an ink thickness of approximately 50 μm, so that the electrical resistance of the coil is not excessively high. After the coil has been printed, the chip is connected by die bonding and encapsulated using epoxy resin. The final production step is laminating a protective cover foil on top of this assembly. The main advantage of this method is that it permits high throughput due to the technical simplicity of the printing process. It is thus exceptionally well suited to producing large numbers of cards. However, it takes a considerable amount of expertise to achieve the necessary level of quality in printing and die bonding.