Practical aspects of touchscreen systems

As consumer mobile communication devices increasingly adopt digital and functional convergence, the need for intuitive and innovative user interface solutions becomes more essential to the device design. Projected-capacitance touchscreens, as part of a user-interface design, can help respond to this challenge.

Designing a successful projected-capacitance touchscreen system needs careful consideration of the following aspects: device mechanical design, substrate selection and user interface design. Cost and technology trade-offs are also useful to consider at all stages of the assessment process.

Unlike resistive touchscreen technologies, projected-capacitance touchscreens are better designed to handle finger gesturing, in particular, multi-touch user input. Resistive technologies need finger pressure to enable the mechanical layers of the touchscreen to make electrical contact, making fluid finger sliding and gesture operations cumbersome. In addition, the multilayer mechanical assembly of a resistive touchscreen is prone to early wear-and-tear from repeated usage.

Multi-touch gestures enabled by projected-capacitance touchscreens include common variants such as pinching, zooming, two-finger scrolling and rotating. They hasten manipulating of data, content and user preferences. Portable gaming and text/email applications can also take advantage of multi-touch technology. Multi-touch all-points-addressable (APA) can precisely determine the coordinate location of each finger press in a multi-finger touch event.

Typing shift characters is simply singular multi-touch event operation instead of having first to shift the character set and then typing the actual shift character. Multi-touch also has wide applications in GPS navigation. Instead of entering starting and destination addresses, APA enables the selection of end-points right on the screen, making it much faster for people to get to where they want to be. Figure 1 shows some of the possibilities with multi-touch.

Design factors
There are several factors to consider when evaluating a device's mechanical design:

1. Is the cover lens or touch surface flat or curved? It is generally recommended that capacitive touchscreen applications should be mounted on flat touch surfaces. Having a curved surface introduces some complications. To achieve a robust capacitive-sensing design, the transparent touch sensor must be laminated flush along the underside of the cover lens. Any air pockets or "bubbles" caused by lamination unevenness can result in reduced touch performance and affect the overall product aesthetics.

A curved cover lens restricts the choice of touch sensor substrates to polyethylene terephthalate (PET). The plastic sensor will be able to bend to fit the form-factor of the curved cover lens. If a curved cover lens must be used, it is recommended that the curvature does not exceed 45° from the point of inflection. Having a much steeper curvature makes lamination much more challenging, and can damage the indium tin oxide (ITO) conductive patterning. As a result, production yield may be jeopardized.

Cheaper methods of lamination involving the use of pressure sensitive adhesives may not be possible with a curved overlay. Instead, more costly UV-curing liquid polymer adhesives may need to be used to ensure greater lamination integrity. UV-curing adhesives are expensive because they are easy to use, thin and have very high optical qualities (>95 percent transparency).

Figure 1: Multi-touch touchscreens can be used in multiple ways by the user and fingers.

2. What are the border widths of the cover lens' opaque inactive areas? For 4-inch (10cm) touchscreen, the border widths of the cover lens that are adjacent to the side with the touch sensor tail should be no thinner than 3mm and the side of the touch sensor tail should be no thinner than 10mm. The needed border space is used to hide the nontransparent silver traces linking the transparent ITO pattern to the control circuitry and the control circuitry itself. It may be possible to achieve thinner borders using glass-based substrates, but the given guidelines are still recommended. Figure 2 summarizes the guidelines.

Figure 2: This shows non-active border requirements for touchscreens.

3. What is the overlay/cover lens material? The lens/overlay material and any decorative artwork within the touchscreen active area must not use any conductive materials. The use of a conductive material will shield the e-field of the capacitive sensors and drastically reduce the sensing performance. Cover lens should be 1mm or less in thickness.

4. What is the distance between the bottom of the cover lens and the liquid crystal module (LCM)? As portable communication devices strive for slimmer profiles, it is important to consider the gap between the LCM and the cover lens. There must be enough space to fit a thin touchscreen sensor, as well as an air gap to protect the touch sensor from unwanted radiated EMI interference from the LCM. At least a 0.5mm gap between the underside of the touch sensor substrate and the LCM is recommended.

5. How am I going to handle ESD? To offer protection against ESD events on the touch surface, a low impedance path to ground must exist through the device. The touch sensor should be protected using a ground ring placed in the non-active border area of the cover lens. The ground ring could be a simple metal foil. It is necessary to ensure that there is a firm connection between the ground ring and device system ground.

Figure 3: This shows typical projected-capacitance indium tin oxide patterning (red and blue indicate two different layers).

Upon completion of the mechanical evaluation, it is necessary to select an appropriate substrate for the touchscreen design. Figure 3 shows a typical ITO patterning design for a projected-capacitance touchscreen.

There are two mainstream substrates used for projected-capacitance touchscreens: glass and PET. Assuming the device's mechanical design does not drive substrate selection, there are benefits and advantages to both, so choosing the best substrate that fits your device and your marketing strategy is very important. Table 1 provides a comparison between the two substrates.

Collaborating glass substrates
Glass substrates are typically used in applications that need superior optical performance and environmental endurance. In most applications, glass-substrate touch sensors are paired with tempered-glass cover lens with matching or similar index of refraction. In addition, the cover lens is usually treated with antiglare, antireflection and scratch-resistant coatings to further increase optical performance by decreasing the amount of reflectance. Transparency is used to define the amount of light that will pass through a material. Reflectance is a measure of the amount of light reflected.

All projected-capacitance touchscreens consist of patterned transparent ITO conductors. Ideally, the reflectance of the ITO patterns should be equal to that of the trace gaps, where there is no ITO patterning. This will ensure that the conductive ITO traces will remain invisible. Glass touch sensors and cover lens can also be chemically tempered to provide drop and affect resistance. Unfortunately, one of the downsides to using glass projected-capacitance systems is that it is typically more expensive than film.

Table 1: This compares the characteristics of glass and PET substrates.

Film substrates are used primarily for the benefits of thinness, improved lamination yield and its lighter construction. Film touchscreen systems cost significantly less than similar glass substrate solutions. Film substrates are typically paired with relatively inexpensive polymethyl methacrylate acrylic overlays, which are prone to surface scratches.

To understand the cost differences between film and glass-based touch sensors, it is useful to look at differences in yield of various manufacturing stages. In the manufacture of touch sensors, ITO is sputtered onto a glass/film substrate, resulting in a thin, microscopic deposition of ITO on the substrate surface. A photomask of the required projected-capacitance patterning is created to prepare for the next processing stage.

The photomask is used in a process known as etching to remove areas of unwanted ITO, creating the needed ITO patterning. For a dual-substrate projected-capacitance design, the worst-case manufacturing yield is given by the following equation.

Production experience has shown the following relationships:

In addition, if the glass touch sensor will undergo chemical tempering to increase its mechanical strength, it is another potential yield-loss procedure to which film-based solutions are not subjected.

Another very important consideration for projected-capacitance touchscreen design is the user-interface. Capacitive touchscreens are not well suited for stylus-based applications. Two variants of capacitive stylus are currently being researched: conductive-rubber-based stylus and conductive-electret devices, but these have not proven to be promising in delivering a compact, easy to use and cheap stylus.

In addition, capacitive sensing will be affected by the area of contact between the finger and the touchscreen. A fine-tipped stylus will not be able to generate the capacitive signatures of a finger. Capacitive touchscreens are designed mostly for realizing finger gestures. The integration of capacitive touch for Web browsing can fully take advantage of accurate finger selection. In Websites, where there are many link content closely spaced together, it may be difficult to accurately select the desired link.

One approach to interface design is to enable users to sequentially scroll through the link options. While scrolling, each link option is magnified for easy touch selection. Once the desired link has been magnified, the user can easily touch the link to access it.

Gauging finger's touch
Another approach is to magnify all links in proximity to the finger touch, so that the user can then select his/her desired link from the second set of enlarged links. Consequently, user interface design should consider the inaccuracies associated with finger size, movement and positioning.

The variation in accuracy of a repeating human press is typically 3mm to 4mm, partly due to eye-finger parallax and differences in dexterity. Selection icons/buttons should optimally be greater than 5mm in diameter. Buttons should also be amply spaced apart to improve usability (5mm to10mm at a minimum). If a thumb is intended to operate the button, they should be even larger and spaced further apart. In addition, some form of visual, aural or haptics feedback should be promptly provided to the user to indicate accurate selection. Delayed feedback will cause increased user input inaccuracies.

Haptics provides the tactile feedback to finger press events on a solid-state touchscreen. It simulates user feedback using vibration motors (actuators). In mobile devices, the most common actuators are eccentric rotating mass actuators and linear resonant actuators. Upon a touch event, the surface of the projected-capacitance touchscreen will vibrate to indicate an input has been detected. The strength and duration of the vibration can be adjusted depending on the type of input feedback.

The foundation of a good user interface design is to keep things simple. Users should be able to perform common tasks within only a few touches of the screen. Not only does this make for a more pleasant user experience, it also lessens the learning curve.

Mechanical evaluation, substrate selection and user interface design are all very essential considerations to a projected-capacitance touchscreen system. Understanding the mechanical constraints will help in substrate selection and ensuring the best possible performance from the touchscreen. Substrate selection is a cost-feature trade-off among cost, robustness and optical performance. Keeping the user interface simple and intuitive, with properly sized finger-selection elements will ensure a high degree of usability. Careful consideration of touchscreen design ensures the success of the end-product and significantly reduces the development risk.

- Yi Hang Wang
Product Manager, CapSense Solution Group
Cypress Semiconductor Corp.