Since their introduction in the 1970s, smartcards have become integral to a large proportion of financial transactions. Solution designers are on the verge of the next wave of evolution where active-matrix display technology is being integrated into a smarter new generation of smartcard to further improve security and usability for us humans.
Electrophoretic displays (EPDs) enable crystal-clear visibility of complex images, whether they’re barcodes, card numbers, ticket IDs, or a mixture of graphics and characters. In this article, I’m going to outline some of the design considerations needed to accommodate a display in something so tightly defined by industry standards. Hopefully some of the information will also prove useful to other design challenges involving the addition of displays to smart portable devices.
Is This Really Happening?
The simple answer is yes. My examples in this article relate to completed development projects for customers. Smartcards with active-matrix displays are allowing assets and packages to be labeled in more intelligent ways. They enable special active sensors to be included for applications like produce monitoring. And perhaps more relevant to you or me, they allow us to carry one solitary smartcard that can adopt the identity of the entire contents of your wallet. Or have ticketing cards that let you see the last 10 locations swiped.
What a Designer Must Consider
There’s a well-defined standard that any designer needs to adopt to fulfill ISO10373/IEC 7816 for mechanical dimensions, and more critically, the “bending” requirements for the smartcard in your pocket. It’s challenging to meet these with any electronics product, but if you apply a display technology that is itself flexible, the outcomes are generally excellent. So, it doesn’t necessarily present additional problems, just more of the same!
And Size is Important
The application defines what size and geometry of display is required. We provide displays with an active area from 1.1 in. diagonal with a resolution of 150 ppi equivalent to 148 ×70 pixels, up to a 3.1-in. display with 105 ppi or 312 × 74 pixels. While the former can show a logo or multiple lines of a transaction, the latter can show your full credit-card number in its original size.
Display thickness is typically 480 µm, which is rarely problematic. However, it’s possible to reduce this to as low as 300 µm to meet application demands (this is typically only needed where a smartcard is already very complex and crammed with other features). Some additional external components are required to update the display; a simple voltage booster (see figure) can be located near the edge of the card because it’s relatively small and easy to track.
Smartcards can incorporate many thin technologies, from displays to buttons.
Will it Bend?
The all-important bend radius for smartcards isn’t typically an issue even if you want a display to extend the full width of the card. This is because the bend radius for our EPD display is 30 mm for standard thickness and even lower for reduced thickness production units. More crucial is the IC display driver that’s needed.
Much like the other critical processing elements of any smartcard, standard silicon devices are used with a package size of up to 10 × 1 mm based on the required number of outputs to drive a high-resolution display. The orientation and position within card should be carefully considered so that it’s as close to the neutral axis as possible. A stiffener will typically be required to support the driver IC within the layers of the card.
The stiffener simply spreads the bending force around any components that can’t flex, so that the overall bend radius of the card can still be achieved. There are various approaches to the way vulnerable components are positioned and grouped, and how stiffeners are used. Typically, stainless steel with a thickness of 0.1 to 0.2 mm is a tried and trusted material.
Furthermore, the stiffener doesn’t have to go the full length of the card or even the display—it just needs to protect the driver IC so that it can’t break or delaminate when subjected to bending forces. It doesn’t add thickness to the display, since it’s typically placed outside the active area where the display is much thinner.
A standard serial peripheral interface (SPI) is used to drive the display, so it’s easy to integrate with almost any smartcard controller. Proximity to the display isn’t particularly critical because EPD displays are deployed in applications that don’t typically involve many display refresh cycles, making data corruption uncommon. An electrophoretic display only needs the transfer of a single image, not a continuous video stream.
Image data formats are 2-bit/pixel encoded bitmaps. Waveforms are already pre-programmed into the display to allow for different update modes. The user can select between a four grey-level update of about 0.7 seconds or a faster monochrome update of 0.3 seconds.
End of Life and Power
It’s easy to construct an NFC-powered smartcard that updates a built-in display only while being read or scanned—maybe to show that last transaction number or refresh date/barcode. An EPD display will then hold and display that image indefinitely without power, or until it’s read or scanned again.
In more advanced applications, the use of a micro-battery, potentially rechargeable, gives the smartcard standalone interactivity. In other words, the user can interact with the card and select different information to be displayed as functions are accessed via buttons or a touch sensor. The typical power consumption for every display update is 5 mA (for less than one second), which needs to be factored in to any end-of-life projections for the application of the card.
Indeed, battery life is the biggest influence on end of life, but it need not be a major contributor. In many applications, operational life is projected at less than two years due to normal wear and tear and routine physical degradation.
Rechargeable battery options are readily available, so if longevity is required, such as when display updates are quite frequent in normal operation, wireless recharging solutions can be integrated. An example application for this would be dynamic tracking tags, used by logistics systems to track packages, parts, or assemblies; where human/legacy-machine readability is key, but tag information (barcodes and text) might be updated several times an hour.
Pushing Those Buttons
Interactivity in smartcards needs something fingers can push (see figure, again). Mechanical buttons blended into the card layers are relatively easy to accommodate physically, typically enabling scrolling up and down a pre-programmed list of options to change the mode of operation of the card, or show a variety of recent transactions. There are limits to the mechanical durability, but they’re well-known and understood by manufacturers.
An obvious requirement that we’ve deployed for customers is a touch sensor layered on top of the display surface. This requires an additional film to be applied using an Optically Clear Adhesive, which adds at least another 50 µm to the overall thickness, plus 25 µm for the adhesive. It may be necessary to further protect the sensor in high-durability situations with additional hard coat or cover lens layers.
In such cases, total thickness may become a critical issue as the add up may be in the range of 200 to 300 µm, so total stack thickness (EPD display + touch solution) will already approach the ISO spec thickness limit for card applications of 0.84 mm max. Besides, the touch sensor will also have design impact on the power, as the power requirement and management will need more careful consideration. A battery will be critical, but the ability to, say, enter a PIN number or other numeric value might be hugely beneficial to the overall solution.
Smartcards literally changed the way we all make millions of transactions. Adding displays to show complex images is already extending smartcard deployment in real applications and I hope the tips above are useful if you’re considering integrating EPDs in yours. Good luck with your next design.
Robert Nitsche is Director of Application Engineering at Plastic Logic.