To achieve a more cost-efficient implementation, system designers working with capacitive technology must understand how component integration affects system cost and performance, both of which can be optimized with intelligent design choices.
Cover lens: The cover lens and touchscreen sensor are complex structures that constitute a touchscreen’s stack-up. The cover lens, the topmost layer, can be made of a variety of materials. Choosing a lens made with polymethyl methacrylate (PMMA) instead of glass can reduce cover-lens cost by up to 50%. PMMA is shatter-resistant but may lower signal sensitivity.
3. “Stack-up†refers to the complexity of component assembly. The choice of layers affects cost and user satisfaction.
Touchscreen sensor: Figure 3 shows several touchscreen sensor stack-up options. Each layer has custom patterns and structures etched in either ITO on glass (better optical clarity) or a PET substrate (better noise immunity). Cost can be reduced by integrating layers. For example, a single-layer sensor can cost up to 50% less, making it attractive to applications that traditionally use resistive touchscreens or haven’t yet moved to a touchscreen-based interface.
Flexible printed circuit (FPC): The FPC interconnects the touchscreen panel, touchscreen controller, and host processor. More efficient FPC routing facilitates its integration with the rest of the system. Routing on a single layer also keeps cost to a minimum while increasing signal integrity.
Display: Displays couple noise to touchscreen sensors, diminishing sensitivity and increasing the potential for false touches. To mitigate noise, an additional ITO shielding layer can be placed between the display and touchscreen sensor. However, this adds cost and thickness to the module. Alternatively, an air gap of 0.2 to 0.5 mm can be used for separation. This helps reduce cost, but still requires the additional thickness.
Touchscreen controller: The touchscreen controller impacts performance, functionality, and the user experience by how well it handles processing of noise-sensitive signals. At a minimum, a controller needs high-quality analog front ends, built-in noise handling capabilities, and sophisticated processing algorithms. By providing a high signal-to-noise ratio (SNR) and effective noise handling, a controller can compensate for the signal-strength degradation that comes from noise sources, such as a cheaper cover lens or a noisy display. The controller also needs algorithms compatible with the sensors being used. To benefit from a single-layer FPC, the controller pin-out must support flexible routing. The controller also determines which advanced features, such as water tolerance or hover, the system can support.
Resistive touchscreens still reign in cost-sensitive applications that require large touchscreens. They also prevail in point-of-sale terminals, industrial, automotive, and medical applications. Overall, though, projected capacitance has become the dominant touchscreen technology in the market. It’s replaced resistive touchscreens in high-volume consumer electronics applications, such as mobile phones, tablets, GPS, digital still cameras, and MP3 players, by innovating to reduce solution costs as well as enhance features to make for more intuitive, yet exciting, user-interface options.
Understanding the capacitive touchscreen system and its key components gives developers a strong hand in significantly lower costs through different stack-up and component choices. In the end, it will bring capacitive technology to a wide range of mid- and low-end applications.
