Chip modules
The most important component of a smart card is naturally the chip. Of course, this very fragile component cannot be simply laminated to the surface of the card like a magnetic stripe. Instead, it needs a sort of enclosure to protect it from the rough everyday life of the card. This enclosure is the called the chip module. In addition to protection from ambient conditions, chips for contact-type smart cards need six or eight contacts, which provide power to the chip and allow data communications with the terminal. A portion of the module’s surface serves to provide these electrical contacts to the outside world. Naturally, the chip module should be as inexpensive as possible. A wide variety of module designs have been devised in the course of the development of smart cards in order to meet these two technical requirements – protection of the fragile
semiconductor chip and provision of contact surfaces. The most important of these are shown in Figures 3.14 and 3.15.

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Figure 3.14 Classification of the various types of chip modules

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Figure 3.15 These examples illustrate the evolution of the chip-on-flex process, starting with one of the first eight-contact chip-on-flex modules at the upper left and proceeding to contemporary modules with six or eight contacts

Electrical connections between the chip and the module
Electrical connections are required between the chip inside the module and the contacts on the outside of the module. Presently, two processes are primarily used for this. In the wire-bonding process, an automatic bonding machine attaches gold wires with a diameter of only a few micrometers between the chip and the rear surfaces of the contacts. The wires are electrically attached to the chip and the module using ultrasonic welding. With this process, the contact arrangement on the top surface of the chip is always opposite that of module. This has been a standard process in the semiconductor industry for some time, and it can be readily used for mass-producing chip modules. However, each chip must be electrically connected to the module by five wires, which naturally costs time and money. The die-bonding process was developed to further reduce the cost of fitting chips into modules. In this process, the electrical connections between the chip and module are not made with wires. Instead, the connections are made by mechanically attaching the chip to the rear surface of the module.

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Figure 3.16 Photograph of the contact zone between a bonding wire and a bonding pad of a smart card microcontroller, magnified 1000 times (Source: Giesecke & Devrient)

TAB modules
Tape-automated bonding (TAB) was a standard process for large-volume chip packaging at the beginning of the 1990s, but it is presently not commonly used, since it has become technically obsolescent and too expensive. It is described here primarily for the sake of completeness.

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Figure 3.17 View of the electrical connections between a smart card microcontroller (bottom) and the chip module (top), magnified 400 times (Source: Giesecke & Devrient)

A chip module produced using the TAB process is shown in Figure 3.18. The special feature of this process is that metallic bumps are first electrically attached to the pads of the chip, and the leads of the carrier film are then soldered to these bumps. The solder connections are so sturdy that no additional support is required for the chip, which hangs from its leads. The active surface of the chip is protected against ambient conditions by an encapsulation material. The advantages of the TAB process are the mechanical strength of the connections to the chip and the low profile of the module. However, these advantages come at the price of higher costs compared with other module preparation processes.

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Figure 3.18 Cross-section of a chip module using the TAB process

Fitting a TAB module into a smart card is not easy, since the module must be taken into account in preparing the lamination foils for the card. Before the layers are laminated, suitable openings are punched in them, and the chip module is then inserted. The chip module is subsequently welded to the body of the card during the lamination process. This process provides a highly reliable bond between the chip module and the card body. It is nearly impossible to remove the chip from the card without destroying the card.

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Figure 3.19 A TAB module ready for embedding in a smart card (left), and a TAB module fitted in a smart card (right)

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Figure 3.20 Inserting a TAB module during the lamination process

Chip-on-flex modules
Currently, the chip-on-flex module with wire-bonded contacts is the most widely used type of module. The construction of such a module is shown in cross-section in Figure 3.21.With this process, an opening into which the chip module can be glued is milled into the finished card body. The carrier material is a flexible circuit board made of fiberglass-reinforced epoxy resin with a thickness of 120 μm. The contacts are formed from a layer of copper laminated onto the carrier, with a thickness of 35 or 75 μm. The contact surfaces are electroplated with gold in a later process step to protect them against processes that could adversely affect their electrical conductivity, such as oxidation. Holes are punched into the carrier to receive the chips and wire bonds. The chips, which are around 200 μm thick, are taken from the sawn wafer by a pick-and-place robot and fitted into the openings in the circuit board. Next, the chip contacts are connected to the rear surfaces of the contacts using bonding wires a few micrometers in
diameter. Finally, the chip and the bonding wires are encapsulated in a blob of synthetic resin to protect them against ambient conditions. The total thickness of the finished module is typically around 600 μm.

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Figure 3.21 Cross-section of a chip-on-flex chip module

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Figure 3.22 The four main process steps in the production of chip-on-flex modules

The advantage of this process is that it is largely based on a standard process used in the semiconductor industry for fitting chips in standard packages. It does not require as much specialized experience as the TAB process, so it less expensive. This process also lends itself well to producing very complex card bodies with many active components. This is because defective card bodies can be separated from the rest before the expensive chip modules have been fitted. The disadvantage of this process is that the thickness and the surface dimensions of the chip module are significantly greater that those of a TAB module, since not only the chip but also the bonding wires must be covered by the protective encapsulation. This is particularly disadvantageous, in that the standard smart card thickness of 0.76 mm does not leave a lot of room for overly thick modules.

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Figure 3.23 Inserting the chip module in a milled opening in the card body

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Figure 3.24 Front and rear views of chip-on-flex modules on 35-mm tape. The five openings in the carrier circuit board, for the bonding wires that make the electrical connections to the chip, can be clearly seen in the rear view

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Figure 3.25 Front and rear views of a chip-on-flex module for a dual-interface card

Lead-frame modules
Technically, the TAB and chip-on-flex processes leave something to be desired, since they both provide little scope for reducing production costs. In the TAB process, producing the card body is very costly due to the characteristics of the module, while in the chip-on-flex process, the complexity of the module and the use of wire bonding lead to unfavorable production costs. These problems led to the development of a new type of module, the lead-frame module, which is mechanically just as robust as TAB and chip-on-flex modules but has lower production costs. The structure of a lead-frame module is relatively simple. The contacts, which are stamped from a gold-plated copper alloy, are held together by a plastic mold body. The chip is placed onto the lead frame by a pick-and-place robot and then connected to the backs of the contacts using wire bonding. Next, the chip is covered by a protective blob of opaque epoxy resin, usually black. The lead-frame process is currently one of the least expensive processes for making chip modules, without any accompanying reduction in the mechanical robustness of the modules.

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Figure 3.26 Cross-section through a lead-frame chip module

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Figure 3.27 Stamped-out lead-frame module with the two coil connections for a contactless smart card, with a match for comparison

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Figure 3.28 Lead-frame modules for contactless smart cards, arranged in pairs on a 35-mm tape. The two empty locations for modules that have been stamped out can be seen at the top

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Figure 3.29 Lead-frame modules for smart cards with contacts, arranged in pairs on a 35-mm tape

The chip-on-surface process
For chips with relatively small surface areas, a process available since the mid-1990s offers a technically very interesting alternative to the usual process of fitting chips into modules.With the MOSAIC (Microchip on Surface and in Card) process, developed by Soliac [Sligos], no module is needed for the chip, since it is located directly in the card body. The MOSAIC process is suitable for chips whose surface area is around 1 mm2. This presently limits its application to pure memory chips, since microcontrollers are still too large for this process. The process works as follows: first, a laser is used to remove material from the location where the chip is to be placed, and then the chip is glued into this recess. In the next step, a conductive silver paste is silk-screened onto the surface of the chip and the card body, thus forming contact surfaces and connecting them to the chip at the same time. In the final step, the chip and the leads to the contacts are covered with a non-conductive lacquer. This provides electrical insulation and protects them against external ambient conditions.

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Figure 3.30 The four stages in the production of a smart card using the chip-on-surface process

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Figure 3.31 A memory chip with an edge length of 0.5 mm (0.25 mm2 area) and ISO/IEC 7816-3 contact surfaces, fitted to a telephone card along with its contacts using the chip-on-surface process

As can clearly be seen from the figure, the chip-on-surface process is highly suitable for mass production of large numbers of cards, since it essentially consists of only a brief laser milling of the card body and two printing processes. However, this process requires an extremely precise printing process to ensure that the contacts for the chip are located correctly. Up to now, the card body has been primarily made of polycarbonate, which is especially suitable for the chip-on-surface process. The production capacity for finished cards lies in the region of 5000 pieces per hour per machine. Another process is the flip-chip process in which the chip is placed with its face against the
rear surface of the module and electrically bonded, after which the assembled module is filled with a casting resin. This type of low-cost module is usually referred to as FCOS (flip-chip on substrate).

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Figure 3.32 Cross-section of a chip module made using the flip-chip process