Smart Cards

Similar in size to today's plastic payment card, the smart card contains a microprocessor or memory chip that, when coupled with a reader, provides a means for users to purchase goods or services (i.e. parking and tolls) or exchange money — basically an electronic "purse." They are also used as access-control devices, making personal and business data available only to the appropriate users.

Smart cards come in two varieties: memory and microprocessor. A memory card simply stores data and can be viewed as a small floppy disk with optional security. A microprocessor card, on the other hand, can add, delete and manipulate information in its memory on the card. Similar to a miniature computer, a microprocessor card has an input/output port operating system and hard disk with built-in security features.

Smart cards have two different types of interfaces: contact and contactless. A contact smart card is inserted into a smart card reader, making physical contact with the reader. It is easy to identify because of its gold connector plate (Figure). A contactless smart card has an antenna embedded inside the card that enables communication with the reader without physical contact. A combi card combines the two features with a very high level of security.

A contact smart card is easy to identify because of its gold connector plate. (Source: Philips)

Because the primary use of the smart card is for the portable storage and retrieval of data, memory is key. Although smart cards can employ many different types of memory — ROM, PROM, EPROM, EEPROM, RAM or flash — EEPROM is the most popular, with flash just now coming onto the scene. STMicroelectronics, for example, recently introduced what it calls "page flash" in a smart card that combines a 32-bit microcontroller with 1 MB of flash memory technology. The device offers a large (768 KB) user flash memory for storing program code and 256 KB of page flash memory that replaces the traditional EEPROM. A flash loader enables application code to be loaded or updated, while an additional 128 KB of user ROM is available to hold fixed routine libraries. Storing application code in flash rather than ROM increases the flexibility of the card by allowing personalization of generic products, software upgrades and post-issuance downloads.

Standards are designed to limit the silicon die size to ~25 mm², although it is possible to handle 35 mm² or more. This is mostly a reliability issue because the larger die will be more prone to mechanical fracture. The fabrication of the card involves a number of processes. The first part of the process is to manufacture the substrate that contains the chip. This is often called a COB (chip on board) and consists of a glass epoxy connector board on which the chip is bonded to the connectors.

There are three technologies available for this process: wire bonding, flip-chip processing and tape automated bonding (TAB). Wire bonding is the most commonly used technique in the manufacture of smart cards. This approach is acceptable because generally only five or six wires are bonded for smart card applications.

However, in the semiconductor industry, flip-chip and tape automated bonding generally are used. In both cases, gold bumps are formed on the die. In flip-chip processing the die are placed face down on the substrate and bonding is accomplished by solder reflow. With tape automated bonding, the die are attached by thermocompression to copper leads supported on a flexible tape similar to a 35 mm film. The finished substrate is hermetically sealed with an inert material such as epoxy resin. The complete micromodule is then glued into the card, which contains the appropriately sized hole. The fabrication of a contactless card is somewhat different since it always involves a laminated card. The ICs and their interconnections, as well as the aerial circuits, are prepared on a flexible polyimide substrate.

Editor's note: This information could not have been compiled without the help of David Everett's "Smart Card Technology: Introduction to Smart Cards," "Smart Card Basics" produced by the Smart Card Forum and Gemplus, and "Smart Card Tutorial" by the Smart Card Group.