Six Critical Design Choices that can Make or Break Your Touch Display Performance

November 18, 2019 Mats Sjöbrand - Avnet Integrated

Some of the most innovative, disruptive, and profitable products today such as phones, tablets, computers, kiosks, navigation systems, and industrial equipment feature a touch-screen. No interface provides a more direct, immersive, and intuitive way to engage with the digital world.

Though easy to use, touch-screen displays can be challenging to develop. They involve a host of critical decisions that directly affect product quality, speed to market, user experience, and ultimately -- business results.

Despite the challenges, consumer expectations continue to soar: Users demand impeccable performance in sleek, design-forward styles. That in turn is forcing developers to find more ways to pack more functions into slimmer form factors, with absolutely no compromises.

The following are six critical design challenges in touch displays. 

Design Challenge No. 1: Cover Lens Appearance and Durability

The cover lens of a display is where the rubber meets the road or rather, fingertip meets functionality. That makes the cover lens a uniquely important selling factor, and you have a wide array of plastics and glass to choose from. Glass is the predominant choice largely because of its hardness. Plain soda-lime glass, which is as common window glass, can be used.

Durability and safety considerations typically require the glass to be strengthened, or hardened, using either chemical or thermal processes. Chemical hardening is possible with all glass, while thermal hardening is typically only available for thicknesses over about three millimeters. Chemical hardening can make surfaces more scratch-resistant, but it only affects the outer surfaces. Thermal hardening, on the other hand, hardens the material throughout, including below the surface. For a particularly robust lens, ion-exchange strengthened (alumino-silicate) glass is the ideal product to use. It’s created using a chemical process that exchanges the native sodium ions in soda-lime glass for larger potassium ions. The result is a denser and more durable glass, enabling lightweight, slim device form factors, especially with bezel-less designs.

Finally, when selecting a cover glass, know that there are a variety of treatments that mitigate glare, reflections, fingerprints and microbial contamination, all of which can make a big difference with the user experience.

Design Challenge No. 2: Mechanical Robustness

Even in non-rugged settings, displays must be designed to hold up to everyday use. Device durability is more than a function of your cover lens choice; one must also consider suspension and bonding techniques. The two major choices are air gap bonding, which is economical, and optical bonding, which is superior but expensive.

Air gap bonding involves bonding the sensor to the display with spacers and double-sided tape such as the very high bond (VHB) line of products from 3M. Optical bonding is based on optical clear resin, a silicone or acrylic-based adhesive that mounts the touch sensor directly to the display, without an air gap. Using this approach, you can create a more durable, rigid assembly that helps reduce reflections in bright light.

Another factor in mechanical robustness is how you intend to anticipate and mitigate the negative effects of ultraviolet (UV) infrared (IR) light on your chosen set of components. Finally, give some thought about how easily your lens will break. The impact class (IK class) for a given material doesn’t determine the impact resistance of the final assembly; that depends on the design as a whole.

 

Design Challenge No. 3: Space Constraints and Unique Shapes

The more devices evolve, take new shapes, and in many cases shrink, the more likely you’ll benefit from flexible printed circuits (FPCs), which are more versatile than circuit boards. There are two types of FPC: Passive FPC, which doesn’t incorporate controller components, and active FPC which does.

Custom FPCs can be valuable in cases where off-the shelf FPCs are not suitable for specific dimensions, shapes, or interfaces. Custom FPCs can also combine the functionality of multiple existing FPCs, which may simplify a design, reduce space requirements, or improve thermal characteristics of the finished device. The cost of a custom FPC may be significantly lower than the cost to develop a completely new sensor design, making a new FPC a low-cost means of “customizing” a sensor.

Design Challenge No. 4: Grounding Design

Touch displays can be temperamental in terms of what components are grounded and how. One common malady is poor noise immunity, where electromagnetic interference (EMI) from wireless networks or power lines disrupt the display’s operation. Another problem is long-term drift where output voltage from power-up shifts over time. For good grounding, keep ground connections (tracks or wires) as short as possible and keep impedance to a minimum, especially in ground connections between display housing and touch controllers.

Remember that each connector adds impedance and also introduces stray or parasitic capacitance effects that can make circuits behave in unintended ways. Avoid ground loops, which can create interference from unintended current flows. Try star topologies instead.

Design Challenge No. 5: False Touch

Touch sensors act as antenna arrays, drawing strong levels of EMI from any number of sources. As a result, a screen can respond to touches the user isn’t making or ignore the touches actually being attempted. A related problem is a noticeable offset between where on the screen the user touches and where on the display the touch is manifested. False touches infuriate users and leave a black eye on your product’s reputation.

Your system’s tolerance for interference depends on the filter integrated in the controller, and quality varies widely. Power supplies, also variable in quality, can also generate false touch effects. Dirty power abnormalities such as low power, varying voltage, frequency variations, and energy spikes are typical root causes. The display can be another significant source of noise in a projected capacitive (PCAP) display, the kind typically used in phones and tablets. Thus, it’s important to maintain an adequate distance between the touch sensor and thin-film transistor (TFT) polarizer. You’ll typically need to conduct trial-and-error testing.

Design Challenge No. 6: EMI Testing

Passing EMI testing can be a big obstacle in bringing an industrial or higher-rated product to market. Planning is crucial here, because the earlier in the design and development process that you can tackle problems, the less expensive and complex they will be. Consult with engineers at the touch display hardware provider if you can.

To establish design requirements for a device, begin with external factors related to how the device will be used. Will users be wearing heavy gloves? Will there be highly conductive liquids on the sensor? Will there be unusual electromagnetic fields nearby? To address this, focus on smart device design, including sound grounding, and on choosing components with proven noise immunity. That said, comparing the EMI immunity among products from various vendors can be difficult, and noise levels from certain components can be spiky.  

Pre-testing is a smart tactic in completing CE certification. One test to consider is IEC EN 61000-4-6,   EMC Part 4-6: Testing and measurement techniques — immunity to conducted disturbances, induced by radio-frequency fields. This test is often the most difficult one to pass, but by pre-testing with a prototype, you can increase your chances of passing CE certification on the first try.

To dig deeper on these challenges, download these white papers on designing sleek custom displays and overcoming touch display challenges

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