MECHANICAL PROPERTIES
When a smart card is inserted into a terminal, two things happen in a mechanical sense. First, the card’s contacts must be electrically connected to the terminal computer. This is the task of the contact unit. Second, the terminal must detect the fact that a card has been inserted. This can be handled by a microswitch or an optical sensor (light barrier). One drawback of the latter is that its reliability can be affected by dirt or cards with transparent bodies. A mechanical switch is generally the most effective solution. Terminals differ very greatly in terms of the contact units and contacts that are used. The GSM 11.11 specification imposes certain limits on the insertion force and the shape of the contacts, and almost all terminals use these values. According to this specification, the tips of the contact elements in the terminal should be rounded rather than pointed, with a radius of curvature of at least 0.8 mm. This largely prevents scratching the contact surfaces of the card. In addition, the force required to insert the card into the contact unit is significantly lower if the contact elements have rounded leading edges than if they are pointed. According to the GSM specification, the maximum force exerted on a single contact must not exceed 0.5 N under any circumstances (the EMV specification allows 0.6 N). This is intended to protect the chip located beneath the contacts, since this piece of silicon crystal could break under greater stress. Although the location of the contacts on the card is internationally standardized by ISO and
should thus be the same everywhere, a French national standard (AFNOR) has the chip nearer the top edge of the card. Consequently, there are terminals that have two contact heads. This allows both ISO and AFNOR contact locations to be supported. This technically complicated solution is of interest in systems in which smart cards with ISO and AFNOR contact positions are used together. This is only a transitional situation, since ISO specifies that the AFNOR location should no longer be used. Several French banking applications, for example, employ terminals with dual contact heads. This allows both the old AFNOR cards and the newer ISO cards to be used during the transition period. Problems can occur with the electrical contacts between the terminal and the smart card, especially with portable terminals and terminals installed in vehicles. Such terminals, in particular those in vehicles, are often subjected to high accelerations, which can cause the contacts to briefly separate from the card’s contact surfaces. You can imagine that a vehicle traveling over cobblestones at a certain speed can cause the spring-loaded contacts to oscillate at their resonant frequency. If the card is electrically activated at the time, it is simply impossible to predict what will happen.

In the extreme case, when all contacts simultaneously lift free and then reconnect with the card, the card would probably execute an activation sequence and then send an ATR. However, in this situation it is certain that the electrical activation sequence will not comply with the ISO standard, which means that this can eventually lead to chip failure if it is frequently repeated. In any case, this brief power interruption will naturally result in the loss of all states that have been achieved in the card during the current session. Depending on the application, it may thus be necessary to enter the PIN again or re-authenticate the user. If only one contact lifts free, the consequences strongly depend on which contact it is. If it is the I/O contact, the only consequence is a temporary disturbance to the communications link. This disturbance can be handled using standard error recovery mechanisms. If a different contact lifts free, the card will be reset. In this case, the communications link must be reestablished from the very beginning. In order to prevent the contacts from lifting free due to acceleration forces, the contact force can be increased, but the upper limit is still 0.5 N per contact. There is no simple satisfactory technical solution to this problem, but the probability of contact separation can be minimized by sensible placement of the terminal. For example, the terminal can be mounted so that the contacts are perpendicular to the main axis of acceleration. In any case, the terminal software must be able to independently re-establish communications if the contacts have briefly lifted free of the card. The millions of GSM telephones in daily use demonstrate that smart cards can be used in portable equipment without any problems. The service life of the contacts and the technical construction of the terminals vary immensely. The service life is also strongly affected by environmental conditions, such as temperature, humidity and the like. An MTBF (mean time between failures) of 150,000 insertion cycles, however, is considered to be a normal value for a terminal.

Contact units with wiping contacts
The technically simplest terminals, which are thus the least expensive, have only wiping contacts in the form of leaf or disc springs. No other mechanical contact elements are present in these simple terminals. However, with such a simple spring-based contact unit, the contact surfaces and part of the card are always dragged across the contacts when the card is inserted and withdrawn, which produces scratch marks. These are undesirable for both aesthetic and technical reasons. Repeated scratching of the gold-plated contact surfaces of the card gradually wears away the protective gold layer, and the exposed metal underneath this plating will then oxidize. This adversely affects the electrical connection. The user may have to insert and remove the card several times in order to rub off the oxide layer so that a satisfactory electrical connection can be made.

Mechanically driven contact units
The next higher class of terminals does not have fixed sliding contacts, but instead a mechanism that presses the contact unit against the contact surfaces of the card when the card is inserted in the terminal. A lever mechanism converts the force used to insert the card into a force perpendicular to the contact surfaces. An optimally designed mechanism also produces a very small amount of movement of the contact unit along the length of the card while the contacts are being applied to the card. This ensures reliable electrical contact with the card, since the sliding motion rubs away any light soiling on the contact surfaces. The contact pins are also individually spring-loaded, in order to ensure a well-defined contact pressure for each contact surface.

Electrically driven contact units
The technically most complex solution, which is also the best mechanical solution, is a terminal with an electrically driven contact unit. Here a set of parallel contact pins is driven by a motor or solenoid to make perpendicular contact with the card from above, with a slight lateral motion. Due to the complexity of this electromechanical construction, the terminal is relatively large. However, this type of terminal is quite suitable for use in professional applications, in which many millions of contact cycles must be made without maintenance. It is therefore typically used in automated teller machines (ATMs) and personalization machines employed in smart card manufacturing.

Card ejection
The smart card is normally inserted manually, which means without any assistance from the terminal. Only ATMs have self-feeding card readers, which use a conveyor mechanism to feed the card to the contact unit within the machine. Ordinary terminals do not have such mechanisms, and they differ only in the manner in which the card is ejected. Simple terminals do not automatically eject the card, which means that the card must be manually removed from the reader. Two different techniques are used for this, called ‘push–push’ and ‘push–pull’. With a push–push contact unit, the card is inserted by hand as usual, and it must removed by again pushing it and then pulling it. This is not ergonomically desirable, since this sequence of motions is unnatural, so people often forcibly pull the card out of the terminal. This causes the contact pins to be scraped over the contact surfaces and the body of the card, since the contacts have not yet been released by the mechanism. Push–pull contact units better match normal motion sequences, since the card is simply pushed into the terminal to insert it and pulled out of the terminal to remove it. Terminals that automatically eject the card have a spring that is tensioned by inserting the card. This can be released by the terminal computer via a solenoid. This causes the card to be partially extended from the terminal, rather than fully ejected, so that the user can grasp it and pull it out completely. Card-ejecting readers have one major advantage relative to other types. Ejection of the card very clearly signals the end of the session to the user, while also reminding the user not to forget the card in the terminal. This reminder is often emphasized by an audible beep. This practical argument is the main reason for using card-ejecting readers. Cash dispensers in particular are usually able to retain smart cards if necessary. Since they routinely have self-feeding card readers, it is naturally technically feasible to route the card to a special retention bin in the machine if necessary, rather than to the exit slot. From a technical viewpoint, retaining cards presents no major problems, as long as the terminal is large enough to hold the extra mechanism and the retention bin. In certain circumstances, however, there can be legal problems if the card user is also the legal owner of the card.

Ease of card withdrawal
The reliability of a system based on smart cards can suffer severely if users can withdraw their cards from the terminal at any time during a session. For one thing, this causes the card to be disconnected from the power supply without following the prescribed deactivation sequence. It could also interrupt EEPROM read or write operations, causing the content of a file to be undefined. This could cause the card to fail completely. For these reasons, it is advantageous to use terminals with card-ejecting readers that are designed such that it is impossible to manually pull the card out of the terminal. A hidden mechanical emergency ejector can be provided to remove a smart card from the terminal in case of a power failure. However, under normal circumstances the terminal can determine when to return the card to the user, thus preventing the user from interfering with ongoing processes.