Design A Lower-Cost Touchscreen System

Capacitive touchscreens have increasingly become mainstream and are no longer a novelty. Their adoption continues to grow as devices such as smart phones and tablets ship by the millions. But now that they are ubiquitous, consumers are more reluctant to pay a higher premium for the technology.

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To maintain profit margins in this competitive environment, OEMs must reduce device cost—and the touchscreen module is one of the most expensive components in touchscreen-powered devices. By using the right panel stack-ups and patterns, displays, materials, routing, and controller, designers can reduce their system costs. 

Cover Lens And Touchscreen Sensor

A standard touchscreen system 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 (Fig. 1). 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.

1. A standard capacitive touchscreen system comprises a projected capacitive touchscreen sensor laminated to a protective cover lens, a bonded FPC with the touchscreen controller mounted to it, and the display.

The cover lens is the topmost physical layer of the touchscreen system. Its cost can vary widely depending on material type (glass or polymethylmethacrylate, or PMMA), special coatings (oleophobic, hydrophobic), decorative ink, or number of drill holes for cameras or sensors. PMMA, a cheaper, lighter, and shatter-resistant alternative to the more durable and optically transmissive glass option, can reduce these costs by up to 50%.

However, PMMA sensors may suffer from lower signal sensitivity. Also, PMMA is more flexible than glass, so it can exhibit panel-bending issues when a finger or other object presses down with significant force. Panel bending can cause false and inaccurate touch reporting. Still, a glass substrate or an additional shield layer in the touchscreen sensor can prevent panel bending. Therefore, the cover lens material must be precisely considered for any touchscreen sensor stack-up (Fig. 2).

2. Modern touch products use multiple types of sensor stack-ups.

Touchscreen sensors are complex structures. They are built by sputtering indium tin oxide (ITO) onto a glass or polyethylene terephthalate(PET) substrate and then etching away a proprietary pattern into the ITO. The patterns and structures that compose each sensor layer are custom to the design needs of the system.

Standard touchscreen designs typically use two-layer ITO touchscreen sensors such as the MH3 and diamonds sensors in Figure 2 to achieve good accuracy, linearity, and multi-touch performance. Two-layer sensor designs use either glass or polyethylene terephthalate (PET) substrates. PET is cheaper and provides better display noise immunity but will suffer from slight optical clarity degradation. Ultimately, the most effective method of lowering touchscreen sensor costs is to reduce the number of stack-up layers.

By integrating single-layer sensors, system designers can decrease sensor costs by up to 50%. Fewer layers—substrate, ITO, optically clear adhesive (OCA)—help touch panel vendors trim material and tooling costs. Handling fewer layers also improves manufacturing yield. Low-cost single-layer touchscreens use a single PET substrate with a simplified proprietary pattern with good optical transmissivity.

From a performance standpoint, single-layer sensors feature lowered accuracy and linearity and limits the number of supported finger touches (usually one finger or two fingers only). These low-cost single-layer sensor solutions are ideal for low-end smart phones and feature phones.

System designers who previously used resistive touchscreens or no touchscreens should find this stack-up option well suited to their design and budget needs. Compared to resistive touchscreens, single-layer capacitive touchscreens offer distinctive advantages, including improved optical clarity, lower power, increased durability, and enhanced user experience.

Single-layer multi-touch solutions such as Cypress’s SLIM (Single-Layer Independent Multi-touch) can cut costs as much as 40% compared to dual-layer sensors. Single-layer sensors present slightly lowered performance but excel in supporting the thinnest form factors. Single-layer multi-touch sensors also support thin border or borderless touchscreen sensors, allowing the touchscreen active area to extend. Designers who are interested in reducing both cost and thickness can consider single-layer sensors as a viable option.

Smaller screen sizes are significantly more economical. The size of the active area will impact touchscreen costs. Clearly, system designers must consider all paths to optimize panel design and selection.

FPC Design

Another avenue for reducing device costs is through FPC design. The FPC connects individual sense input/output (I/Os) from the touchscreen panel to the touchscreen controller and from the touchscreen controller to the host processor.

FPCs can be active or passive. In active FPCs, the touchscreen controller and any other required external components such as resistors and capacitors are mounted onto the FPC itself. In passive FPCs, the FPC only includes routing traces and the touchscreen controller, and external components are mounted onto a printed-circuit board (PCB).

Whether active or passive, FPCs can be routed in several ways. The more efficient and versatile the routing, the easier it is for other hardware components to be integrated. Keep in mind, however, that costs do increase with the number of layers required for routing. Thoughtful routing on a single layer will help minimize FPC costs. Single-layer routing also has considerable advantages for both signal integrity and compact FPC design.

Displays

In a touchscreen system, the projected capacitive touchscreen sensor sits on top of the display. Displays are inherently noisy, which can cause display noise to directly couple with the touchscreen sensor (Fig. 3). This diminishes touch sensitivity and produces false touch activation. Good design choices can mitigate display noise and have a substantial impact on performance and cost.

3. Displays are inherently noisy. Noise from displays can capacitively couple with the touchscreen and diminish touch performance.

To block display noise, the industry traditionally implements an additional ITO “shield” layer between the display and touchscreen sensor. Though effective, the shield layer adds cost and increases the thickness of the touchscreen module. An alternative is using a tiny air gap, typically between 0.2 mm and 0.5 mm, to separate the display from the touchscreen sensor.

An air gap is more cost-effective than a shield layer, but it also increases the touchscreen module thickness, which is becoming undesirable to OEMs looking to build sleeker and thinner devices. A more important design choice will be the selection of the display itself.

Currently the most popular displays used for mobile phones and tablets are still thin-film transistor (TFT) LCDs, which are commonly available in two flavors: dc common voltage (DCVCOM) and ac common voltage (ACVCOM). The difference is the method used to drive the common electrode layer (VCOM). Another increasingly popular display in high-end devices is the active-matrix organic LED (AMOLED) with its wide viewing angles, improved brightness and contrast, lower power consumption, and reduced thickness.

AMOLEDs emit very little display noise and are among the quietest displays, but they are expensive. DCVCOM is also generally a quiet display and expensive. In contrast, ACVCOM is high-noise but relatively cheap. The choice of a display greatly relies on the device’s intent for end customers. The target application will deem the hardware and performance that is suitable for its customers.

Touchscreen Controller

Though not as expensive as the display or the touchscreen panel, the choice of the touchscreen controller has the most impact in terms of touchscreen system performance. The touchscreen controller incorporates capacitive sensing and processing technology to resolve finger touches and gestures by reporting their location and behavior to the host processor.

When a finger is placed on a projected capacitive touchscreen, the touchscreen controller detects a change in capacitance and converts that information into digital values. This digital conversion is further processed using sophisticated touch resolution algorithms within the touchscreen controller before passing on touch coordinates and other relevant data to the host processor.

Noise-sensitive signals are a major technical challenge for touchscreens. Controllers that employ high-quality analog front ends, built-in noise handling capabilities, and sophisticated processing algorithms is mandatory. With touch becoming the user interface of choice for many consumer electronic devices, the quality of touchscreen controllers will directly impact the end product user experience. Choosing the right touchscreen controller is integral to achieving performance and cost benefits.

A controller that provides high signal-to-noise ratio (SNR) and effective noise handling will be able to compensate for the signal strength degradation that comes from noise sources, such as a cheaper PMMA cover lens or a noisy ACVCOM display. To help optimize the performance of low-cost and multi-touch single-layer sensors, touchscreen controllers must supply compatible processing algorithms. In addition, the cost benefits of single-layer FPC routing can only be realized if the touchscreen controller pinout allows for flexible routing design.

Touchscreen controllers can also reduce system costs through some of the advanced features they support. For example, most touchscreen controllers interpret water on a touchscreen as a finger touch because the mutual capacitance signatures of water and a finger are similar. To solve this problem, touchscreen panel vendors can add an expensive layer of hydrophobic coating to the cover lens.

When drops of water land on the cover lens, the coating helps to break them apart into smaller droplets so they no longer register as touches. All the same, a touchscreen controller that enables water rejection natively through the use of its hardware and firmware features can detect and reject water on the touchscreen with built-in algorithms and can save the OEM additional coating costs.

Summary

Savvy designers who thoroughly understand the touchscreen system and its key components can significantly reduce costs through intelligent choices in the design and selection of the cover lens, sensor material and stack-up, display type, and FPC routing. An innovative and high-performance touchscreen controller can achieve cost reductions without compromising performance, which makes the end product sell in the first place.