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 realized and taken advantage of by resistive touch technology.
1. When a finger presses onto a resistive touchscreen sensor, it makes an actual electrical contact. The screen consists of a glass substrate topped by two layers of polyethylene terephthalate (PET), each coated with indium-tin oxide (ITO). These are separated by an air gap and held apart with spacer dots. Below that is an insulating substrate, usually made of glass. Touching the screen causes the top and bottom ITO layers to physically come in contact.
Resistive touchscreens consist of two layers of polyethylene terephthalate (PET) with ITO coated on each layer (Fig. 1). 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 finger presses onto the touchscreen, the action causes the top and bottom ITO layers to physically come in contact, which signifies a finger touchdown. Resistive touchscreens come in 4-, 5-, 6-, and 8-wire variants, which offer different degrees of durability and noise suppression.
2. In a capacitive touchscreen with an integrated display, the entire assembly is laminated together with a display below the screen itself.
A standard capacitive touchscreen system (Fig. 2) 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 directly laminated.
Projected capacitance does not use pressure for touch detection—it can detect even the lightest of touches. The technology reads finger touches based on the differential change in capacitance when a finger is placed on the touchscreen.
Without pressure-based sensing, projected capacitance does away with bendable protective covers. Instead, thicker plastic or a glass cover lens that’s strong and scratch resistant can be deployed. Contrary to resistive touchscreens, projected-capacitance touchscreens is able to exploit glass or PET substrates, and they can be single or dual-layered. OEMs have multiple stack-up options for projected-capacitance touchscreens, too. Note that a single-layer sensor with ITO deposited on a glass substrate will greatly enhance the touchscreen’s optical clarity.
In addition, significant advances made by touchscreen controller suppliers like Cypress have led to projected-capacitance-supported passive styluses and gloves. Several years ago, such non-finger input support could only be provided by resistive touchscreens. Looking forward, further innovation in projected capacitance for novel features such as hover—detection of a finger hovering some distance above the touchscreen—opens up possibilities to even more revolutionary and enriching user experiences.
