Transponder Construction Formats
Disks and Coins
The most common construction format is the so-called disk (coin), a transponder in a round (ABS) injection moulded housing, with a diameter ranging from a few millimetres to 10 cm (Figure 2.2). There is usually a hole for a fastening screw in the centre. As an alternative to (ABS) injection moulding, polystyrol or even epoxy resin may be used to achieve a wider operating temperature range.

Glass Housing
Glass transponders have been developed that can be injected under the skin of an animal for identification purposes (see Chapter 13).

Glass tubes of length just 12–32mm contain a microchip mounted upon a carrier (PCB) and a chip capacitor to smooth the supply current obtained. The transponder coil incorporates wire of just 0.03 mm thickness wound onto a ferrite core. The internal components are embedded in a soft adhesive to achieve mechanical stability.

Plastic Housing
The plastic housing (plastic package, PP) was developed for applications involving particularly high mechanical demands. This housing can easily be integrated into other products, for example into car keys for electronic immobilisation systems.

The wedge made of moulding substance (IC casting compound) contains almost the same components as the glass transponder, but its longer coil gives it a greater functional range . Further advantages are its ability to accept larger microchips and its greater tolerance to mechanical vibrations, which is required by the automotive industry, for example. The PP transponder has proved completely satisfactory with regard to other quality requirements, such as temperature cycles or fall tests (Bruhnke, 1996).

2.2.4 Tool and Gas Bottle Identification
Special construction formats have been developed to install inductively coupled transponders into metal surfaces. The transponder coil is wound in a ferrite pot core. The transponder chip is mounted on the reverse of the ferrite pot core and contacted with the transponder coil.

In order to obtain sufficient mechanical stability, vibration and heat tolerance, transponder chip and ferrite pot core are cast into a PPS shell using epoxy resin (Link, 1996, 1997).

The external dimensions of the transponder and their fitting area have been standardised in DIN/ISO 69873 for incorporation into a retention knob or quick-release taper for tool identification. Different designs are used for the identification of gas bottles.

Keys and Key Fobs
Transponders are also integrated into mechanical keys for immobilisers or door locking applications with particularly high security requirements. These are generally based upon a transponder in a plastic housing, which is cast or injected into the key fob.

The keyring transponder design has proved very popular for systems providing access to office and work areas.

Clocks
This construction format was developed at the beginning of the 1990s by the Austrian company Ski-Data and was first used in ski passes. These contactless clocks were also able to gain ground in access control systems (Figure 2.10). The clock contains a frame antenna with a small number of windings printed onto a thin printed circuit board, which follows the clock housing as closely as possible to maximise the area enclosed by the antenna coil – and thus the range.

ID-1 Format, Contactless Smart Cards
The ID-1 format familiar from credit cards and telephone cards (85.72 × 54.03 × 0.76 mm ± tolerances) is becoming increasingly important for contactless smart cards in RFID systems (Figure 2.11). One advantage of this format for inductively coupled RFID systems is the large coil area, which increases the range of the smart cards.

Contactless smart cards are produced by the lamination of a transponder between four PVC foils. The individual foils are baked at high pressure and temperatures above 100 ◦C to produce a permanent bond (the manufacture of contactless smart cards is described in detail in Chapter 12).

Contactless smart cards of the design ID-1 are excellently suited for carrying adverts and often have artistic overprints, like those on telephone cards, for example.

However, it is not always possible to adhere to the maximum thickness of 0.8 mm specified for ID-1 cards in ISO 7810. Microwave transponders in particular require a thicker design, because in
this design the transponder is usually inserted between two PVC shells or packed using an (ABS) injection moulding procedure.

Smart Label
The term smart label refers to a paper-thin transponder format. In transponders of this format the transponder coil is applied to a plastic foil of just 0.1 mm thickness by screen printing or etching. This foil is often laminated using a layer of paper and its back coated with adhesive. The transponders are supplied in the form of self-adhesive stickers on an endless roll and are thin and flexible enough to be stuck to luggage, packages and goods of all types. Since the sticky labels can easily be overprinted, it is a simple matter to link the stored data to an additional barcode on the front of the label.

Coil-on-Chip
In the construction formats mentioned previously the transponders consist of a separate transponder coil that functions as an antenna and a transponder chip (hybrid technology). The transponder coil is bonded to the transponder chip in the conventional manner.

An obvious step down the route of miniaturisation is the integration of the coil onto the chip (coil-on-chip, Figure 2.16). This is made possible by a special microgalvanic process that can take place on a normal CMOS wafer. The coil is placed directly onto the isolator of the silicon chip in the form of a planar (single layer) spiral arrangement and contacted to the circuit below by means of conventional openings in the passivation layer (Jurisch, 1995, 1998). The conductor track widths achieved lie in the range of 5–10 µm with a layer thickness of 15–30 µm. A final passivation onto a polyamide base is performed to guarantee the mechanical loading capacity of the contactless memory module based upon coil-on-chip technology.

The size of the silicon chip, and thus the entire transponder, is just 3 × 3 mm. The transponders are frequently embedded in a plastic shell for convenience and at 6 × 1.5 mm are among the smallest RFID transponders available on the market.

Other Formats
In addition to these main designs, several application-specific special designs are also manufactured. Examples are the ‘racing pigeon transponder’ or the ‘champion chip’ for sports timing. Transponders can be incorporated into any design required by the customer. The preferred options are glass or PP transponders, which are then processed further to obtain the ultimate form.