FEATURE DISPLAYS & UIS Resistive or capacitive touch?
Chitiz Mathema, Product Manager, Cypress Semiconductor Corp. explores the differences between resistive and capacitive touchscreens
M
ost touchscreens use either resistive or projected capacitance touch technology.
Resistive touchscreens generally support single-finger touch and basic gestures and are cheaper to produce. On the other hand, projected capacitance touchscreens, having superior multi-touch performance, durability, and optical clarity are usually adopted into smartphones and tablets. However, projected capacitance touchscreens are now displacing resistive touch in most small and medium- sized touchscreen devices as well. Moreover, increasing innovation in projected capacitance touch such as integrated stack-ups has allowed it to be more price competitive while surpassing resistive touch in performance.
RESISTIVE TOUCH PERFORMANCE The primary value proposition of resistive touchscreens is that they are low cost to manufacture. Although resistive touch performance is usually limited to basic single finger touches and gestures, it still serves a wide user base. Resistive touchscreens also continue to dominate applications that require touchscreens greater than 10 inches since costs for projected capacitance technologies rise exponentially with screen size. In contrast, projected capacitance touchscreens offer high performance in accuracy, power consumption, and refresh rate. They also feature excellent optical transmissivity (> 90%), resulting in brighter, clearer displays. Unlike resistive touchscreens, projected capacitance is durable, scratch-resistant, free of aging symptoms, and needs no calibration. Projected capacitance can also support multi-finger touch input and gestures. A major difference between resistive and projected capacitance touch technology is touchscreen composition, which significantly impacts technical functionality and cost. The high transparency and high resistivity properties of indium-tin oxide (ITO) actually were first realised and taken advantage of by resistive touch technology. Resistive touchscreens are constructed of two layers of polyethylene terephthalate (PET) with ITO coated on each layer. The two layers of PET are separated by an air gap and spacer dots. The bottom PET layer is placed on top of
an insulating substrate usually made of glass. A protective layer of hard coating is placed on top of the other PET layer. When a
8 JUNE 2015 | ELECTRONICS
finger presses onto the touchscreen, the action causes the top and bottom ITO layers to physically come in contact, which signifies a finger touchdown.
A standard capacitive touchscreen system
(Figure 1) comprises a projected capacitive touchscreen sensor laminated to a protective cover lens, a bonded Flexible Printed Circuit (FPC) with the touchscreen controller mounted to it, and a display. The FPC connects the touchscreen controller to the host processor. The display sits under the touchscreen sensor and is usually separated by an air gap or is directly laminated.
PROJECTED CAPACITANCE Projected capacitance does not use pressure for touch detection and can detect even the lightest of touches. Rather, projected capacitance reads finger touches based on the differential change in capacitance when a finger is placed on the touchscreen. Without pressure-based sensing, this means that a bendable protective cover is no longer required. Thicker plastic or a glass cover lens that is strong and scratch resistant can be deployed. Contrary to resistive touchscreens, projected capacitance
touchscreens can use glass or PET substrates and can be single or dual- layered. OEMs have multiple stack-up options for projected capacitance touchscreens. Note that a single- layer sensor with ITO deposited on a glass substrate will greatly enhance the optical clarity of the touchscreen. Despite the widespread use of resistive touchscreens, there are considerable
Figure 1:
Standard touchscreen system
drawbacks. Users consistently encounter frustration with resistive touchscreens that inaccurately report button activation on a different part of the touchscreen or not activating presses as intended. Since resistive touchscreens depend on pressure-activated touch, this requires movement and flexing of the different layers in its stack-up. Moreover, the top hardcoat layer needs to be thin enough to maintain the flexibility of the touchscreen panel. The combination of moving layers and a thin protective layer leads to reduced durability and a vulnerability to scratches. System designers working with capacitive technology must carefully understand how the integration of each component affects system cost and performance, both of which can be optimised with intelligent design choices: Cover lens: The cover lens and touchscreen sensor are complex structures that make up a touchscreen’s stack-up. The cover lens is the topmost layer and can be made of a variety of materials. By choosing a lens made with polymethyl methacrylate (PMMA) instead of glass, cover lens cost can be reduced by up to 50 percent. PMMA is shatter-resistant but may lower signal sensitivity.
“A major difference between resistive and
projected capacitance touch technology is touchscreen composition, which has a large impact on technical functionality and cost ...”
TOUCHSCREEN SENSOR OPTIONS There are several touchscreen sensor stack-up options. Each layer has custom patterns and structures etched in either indium tin oxide (ITO) on glass (better optical clarity) or a polyethylene terephthalate (PET) substrate (better noise immunity). Cost can be reduced by integrating layers. For example, a single-layer sensor can cost up to 50 percent less, making it attractive to applications that traditionally use resistive touchscreens or have not yet moved to a touchscreen-based interface. The Flexible Printed Circuit (FPC) interconnects the touchscreen panel, touchscreen controller, and host processor. The more efficient the routing of the FPC, the easier it is to integrate with the rest of the system. Routing on a single layer also keeps cost to a minimum while increasing signal integrity.
Projected capacitance technology
has replaced resistive touchscreens by reducing costs as well as
enhancing features to make for more
intuitive yet novel and exciting user interface options.
Cypress Semiconductor Corp.
www.cypress.com 01279 873160
Enter 200 / ELECTRONICS
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