Smartcards Require Smart Testing
Although behind other countries in starting, the UK is
starting to embrace the smartcard world in a big way. Trials for electronic
purse applications have been running in Swindon for over a year (see Smartcard Applications) and have extended to UK
universities, and now BT has launched their long awaited replacement for
the phonecard [Electronics Times, 18/7/96]. The likely scale of the change
in technology brought about by the smartcards will result in a corresponding
requirement for new test capability for card manufacturers and issuers.
Background
There are two main types of smartcards currently available.
Memory cards are the simplest and least expensive cards. They combine intelligent
EEPROMs with security logic and high security authentication. The second
kind of card, the processor card, is more expensive than the memory card
and features processor based file and directory structures, making it ideal
for data storage applications.
The new BT phonecard is of the memory card variety, and
is based on the Siemens SLE4438 or 'Eurochip' technology. These third generation
cards, containing a 221-bit EEPROM and a 16-bit masked programmed ROM, are
once programmable or 'throw-away'. This compares to the French banking Smartcard
which has a 1K EEPROM, 3K ROM and 128 bytes of RAM. BT now have 60,000 public
phones capable of accepting the new phonecard to be available.
Smartcard Standard Definition
The standard smartcard electrical interface is defined
in ISO 7816. The memory cards use a simpler protocol than the one defined
in the standard and are relatively easy to access. The waveforms are TTL
with a minimum timing requirement of a 10mS clock period.
Processor cards, on the other hand, conform fully to ISO
7816 which defines the communications protocol, and electrical and mechanical
interfaces. The protocol can be described as RS232 with TTL levels with
a shared (half-duplex) transmit / receive line. A stable 3.579545 MHz clock
is applied and this is internally divided by 372 to give a 9600 bit per
second communications rate.
Smartcard Production
There are three main stages at which testing will be required
in the smartcard life story. First, the smartchips are tested on the wafer
after fabrication. Failing chips are marked and discarded as soon as the
wafer is split. Passing chips are bonded up into modules ready for embedding
into plastic cards, or as a semiconductor die for customer packaging. Many
smartcard manufacturers buy the smartchips rather than fabricate them themselves.
The remaining two stages of test are therefor usually carried
out outside the specialist fabrication houses. (See Figure : Two test stages).
Smartchips are fixed to the carrier modules on 35mm tape. The modules contain
the smartcard contact pads, and the leads from the chips are bonded to them.
The leads are also all connected together at this stage for ESD protection.
Typically chips are mounted on the tape in pairs with three common bond
points between each pair of chips.
The smartchip modules are tested at this stage before they
are mounted on to the cards. The testing is needed to verify that the chips
are still functioning after the mechanical processing. The value of the
assembled card will vary according to the card types, from a few tens of
cents for simple memory cards, to a few tens of dollars for processor cards.
With expected yields of smartchips of the order of 95% and quantities for
any particular customer of the order of tens of millions, a simple calculation
gives the value of the 'dead' cards to be of the order of millions of dollars.
If the faulty chips are found before mounting, the cost of 'dead' chips
can be reduced to the order of tens of thousands of dollars.
Figure : Two test stages
The test requirements are electrically simple at the pre-mounting
stage. The smartchip modules are available on a 35mm reel sandwiched between
the 35mm film and a protective layer. The film has sprocket holes along
each edge for locating the tape in the handling equipment. Before any testing
can take place the protective layer has to be stripped away, and once testing
has been completed this layer can be reapplied. (See figure : Smartchip
Test Station.) Also, before testing can take place, the test station has
to punch out the bonding points connecting all the smartchip leads together.
Then the chips can be tested to ensure that they are still alive.
At this stage the test is typically a simple one which
verifies the manufacturers identification while in issuer mode. (In this
mode, the chip is protected against fraud with a transport locking code
known only to the manufacturer and issuer. This code must be correctly presented
to the card before personalisation can take place, otherwise the card is
locked forever.) By performing this test all five of the device pins of
the smartchip are tested for functionality. This gives the manufacturer
a reasonable level of confidence that the smartchip is alive and well and
has been correctly mounted on to the contact pads.
Figure : Smartchip Test Station
The tests are performed on 20 devices at a time to boost
throughput. Contacts with the smartchips modules are made through a bed
of nails fixture. This uses dome-headed nails to minimise any damage to
the smartchips gold plated contacts. The smartchips on the 35mm tape are
fed through the test head by a motorised, dual continuous band transport
system. The tape position is optically sensed through the sprocket holes
to align the contact pad centres with an accuracy of +/- 0.25mm. Once the
20 devices have been tested, any faulty devices detected are marked by punching
a hole in one of the device contact pads. This hole can be detected by an
optical sensor at the mounting stage and the chip discarded.
The complete test station consists of six main elements,
four of which operate independently of each other, with tape buffers being
used to connect the four elements together. The level of tape in each buffer
is topped up when it falls below a predetermined limit. The six elements
are the de-reeler, the disconnect punch, the testhead, the M505 functional
test system, the failure punch and the re-spooler.
Assembled Cards
The second major test stage takes place on fully assembled
cards. The cards carriers are already prepared with the relevant artwork
before the smartchips are mounted on to them. The test at this stage is
a conformance or interoperability test, performed on a batch test basis
and is used to characterise each batch of smartcards. A characterisation
test is essential because the value of each card does not simply consist
of the cost of manufacture. Since the card will also be used as virtual
money it will contain a value of, say, $50. The eventual card owner and
user would want to be able to take his card to any reader in the country,
at any time of day or night, summer or winter and expect to be able to get
access to the full value of his stored money.
Figure - Interoperability Tests ensure that each
Smartcard can be used anywhere in the country, summer or winter, day or
night
The effects of interoperability can be simulated by varying
the key operating parameters of the card, i.e. the supply logic, the threshold
levels and the access timing conditions. This requirement puts the assembled
card tester in a class above the simple testers using standard card readers.
These simple testers are unable to vary these parameters. By interfacing
the tester to an environmental chamber, there is also the possibility of
testing the cards under a range of temperature and humidity conditions.
The test hardware at this stage is effectively imitating
the functions of the card reader which would normally be found on a payphone
or cash machine, but the tests which can be performed at this stage need
not be limited to the normal transaction cycles. The tester could be customised
to allow the card manufacturer to construct more complex test sequences.
For example, a test sequence could consist of a number of subtests or subtasks,
including presenting the card with valid or invalid transport locking codes,
authentication and personalisation of the card, setting the counter start
value representing the monitory value of the card. Since different cards
will contain different information depending on their use, the card contents
require a thorough checking for programming integrity and functionality.
Most smartcards will have an authentication process to
ensure that the card is a valid one. Normally this would consist of a cryptographic
challenge / response process. For security reasons, any test solution would
not normally contain the challenge / response algorithm in software, but
instead would use a processor smartcard inside the test fixture.
Other Test Needs
One of the next steps in the evolution of the smartcard
will be the widespread use of contactless smartcards (which are accessed
using induction or RF techniques) for transport and road tolling applications.
The reported effective operating ranges of these cards will vary from 1m
for cards operating at a frequency of 125kHz to 10cm for an operating frequency
of 13.5MHz. Possible test requirements for contactless cards include power,
range and susceptibility tests.
Because of the nature of the physical abuse most cards
will experience during their lifetime (e.g. being kept in a wallet in your
back pocket) cards will also have to be tested to ensure that they are rugged
enough to withstand flexing in both vertical and horizontal planes and to
a lesser extent, torsion.
Conclusions
Because of their complexity, interoperability and usage
volumes, Smartcards require specialised testing to minimise the cost of
faulty cards to the manufacturer and to the end user. Not only should the
cards themselves be tested at several key stages, but the equipment which
will be used with the smartcards has to be thoroughly tested too. MICAS,
Marconi Instruments solutions group, can deliver electrical, RF and mechanical
test solutions for standard and RF contactless smartcards, customised to
customers' specific requirements.
Acknowledgements
I would like to thank Alan Whyte, former Marconi Instruments
Applications Engineer, and Duncan Askew, Product Specialist for some of
the background information for this article.
| Articles
| Web Authoring
| Sales Aids
| Papers
| Cartoons
| Contact
|
| 