Improving Touch Screen Performance by Good Design

As touch screens become more sophisticated, designing with them becomes more challenging. Fortunately the rules for good design have not changed, and there are a number of solutions out there to help you meet the challenge.

The human-machine interface (HMI) has undergone a revolution in the last several years. Mechanical QWERTY keyboards and their associated mice are falling out of favor to be replaced by sleek touch screens. One has only to consider the evolution of the mobile handset to see how manufacturers have been rushing to bring out products with bigger and better touch screens while ditching keyboards.

Touch control has been in use for less glamorous applications for many years. For example, touch-enabled keypads are popular for many household appliances such as microwaves and dishwashers, replacing expensive and unreliable mechanical keypads. Touch screens have been widely adopted in industrial control settings because they are ideally suited to the harsh operating environments typically encountered.

These advantages aside, one of the main motivations for touch screen adoption is that mechanical parts wear out and break down with extended use. While keyboards and mice, for example, have proven to be relatively durable HMIs for computer systems, they do eventually break. Dirty, dusty, or humid environments increase the likelihood of early failure because particles and moisture can get into mechanical parts and accelerate wear.

Finally, keyboards and switches are hardly the most hygienic devices. While this may not be such a big problem for industrial or domestic environments, it's certainly a major issue in medical applications.

This article will take a look at capacitive touch sensing, the most popular technology for touch screens in use today, and will describe design techniques that improve the user experience by improving response and minimizing false touches, whereby the device reacts to a finger in close proximity to a sensor button rather than a deliberate touch by the user.

Capacitive touch control basics

There are several technologies available to the designer for touch control, including inductive, resistive, and capacitive. Each has its advantages, particularly the elimination of wear-prone mechanical parts, but capacitive touch technology has proven to be the most popular in recent years. This is because it offers higher transmission of light from the display beneath the sensor — over 90 percent compared to resistive technology's 80 percent — which is particularly important for devices like smartphones that boast bright, high-resolution screens.

Other advantages include faster response, activation by a "low-pressure" touch (e.g., a finger rather than a stylus), the ability to handle multiple touches, a larger active area, and less susceptibility to wear and tear.

There are several types of capacitive touch sensor technology, including surface capacitance and projected capacitance, but all are based on the fact that the application of a finger changes the capacitance in a local region enabling the system's electronics to detect a touch and determine its position on the screen. Figures 1a and 1b illustrate the principle.




The choices of materials and PCB design have a major influence on the values of C_P and C_F. We'll look at how C_P is influenced later. The value of C_F can be determined by the formula:

C_F = (ε₀ ε_r A)/D

where:
ε₀ = Free space permittivity
ε_r = Dielectric constant of overlay
A = Area of finger and sensor pad overlap
D = Overlay thickness

It can be seen from the formula that choosing an overlay material with higher dielectric constant, decreasing the overlay thickness, and increasing the button diameter will raise the value of C_F and increase the sensitivity of the system.

Depending on the design, C_P will typically measure between 10 and 300 pF. In contrast, C_F is smaller and will likely measure between 0.1 and 10 pF. To make life more difficult for the designer, C_P (also known as the parasitic capacitance) varies with changes in the ambient conditions such as temperature and humidity. The challenge for the designer, therefore, is to establish as big a percentage increase as possible to the overall sensor capacitance when a finger is applied — in other words, maximize the signal-to-noise ratio (SNR) — to ensure the system can accurately discern a true touch from a false touch.

Designing touch screen PCBs

Fortunately, there are some hardware and software design techniques that help the design engineer increase the SNR of a touch screen control design.

Let's start by considering the PCB. Typical touch screen designs use a two-layer board (with board thickness ranging from 0.5 to 1.6 mm) with sensor pads and hatched ground plane on top and everything else on the bottom (see Figure 2). Four-layer boards can be used when the board area must be minimized.


Using short and narrow traces can minimize their parasitic capacitance. Traces should be less than 300 mm long and be between 0.17 to 0.20 mm in width. It's best to route sensor traces on the bottom layer of the PCB and connect each to its relevant sensor pad with a via. Vias should be positioned on the pad such that the sensor trace length is minimized (see Figure 3). With this approach to routing, when a finger is applied it will only interact with the sensor pad and not the trace.

The designer should not route traces directly under any sensor pad unless the trace is connected to that particular sensor. Moreover, the capacitive sensing traces should never be in close proximity with or parallel to high frequency communication lines. If it is impossible to avoid crossing communication lines with sensor traces, make sure the intersection is at right angles.


A touch sensor design will require ground fill to minimize EMI affects on the capacitive sensing system. However, there is a trade-off because when ground fill is adjacent to the sensor pad, it can increase the parasitic capacitance of the system, lowering its sensitivity. A good compromise is to use hatching for the ground fill of 15 percent on the top layer (for example, 0.18 mm line, 1.14 mm spacing) and 10 percent on the bottom layer (for example, 0.18 mm line, 1.78 mm spacing).

When using the hatched ground fill, the clearance between the sensor pad and ground plane influences the sensitivity of the associated button. Specifically, the magnitude of the parasitic capacitance of the sensor is related to the electric field generated between the button and ground plane (see Figure 1a).

It turns out the parasitic capacitance decreases as the clearance surrounding the button is increased. Figure 4 illustrates this relationship. In this plot, the board material is FR4 with a thickness of 1.57 mm, and the acrylic overlay has a thickness of 2 mm. Each plot contains data for three button sizes (5, 10, and 15 mm diameter).


Buttons are designed to sense the presence of a finger. The shape and size influence the sensitivity of the touch sensor. Shapes with angles of less than 90 degrees (such as triangles) do not work well, squares are better, and solid circles work best of all. Larger buttons are typically better than smaller ones. It's a good idea to try and make the button size match the area of the fingertip that will make contact with the sensor. Figure 5 plots finger capacitance (C_F) as a percentage of the sensor capacitance (C_SENSOR) for different sized buttons.


Eliminating false touches

The touch screen circuitry will have an inherent parasitic capacitance in the absence of a finger (starting capacitance). If the starting capacitance is large, the increase in capacitance from a finger touch instigates a relatively smaller change and is consequently more difficult to detect. In other words, lowering the starting capacitance increases the dynamic range of the capacitive touch sensor.

The PCB design techniques previously discussed will help to increase the dynamic range of the system, but there is more that can be done, especially when it comes to eliminating "false touches" — touches registered by the sensor but not intended by the user.

The smaller the screen size, the tougher the challenge. For example, the tips of the fingers are relatively large compared to features on a mobile handset touch screen, so it becomes more difficult to determine which button is actually being selected compared with, for instance, a computer monitor.

The problem is compounded when multiple keys are in close proximity because fingers that are near sensors, but not actually in contact, can still generate a measurable increase in capacitance and generate a false reading (see Figure 6).

Each sensor trace becomes an extension of the capacitive sensor to which it connects, so poor routing can couple capacitance from one sensor to another — especially if the trace is directed around nearby sensors — and increase the probability of false touches.

Closely grouped buttons are a particular problem because of the chance of a finger overlapping an adjacent button to the one intended to be pressed. Surrounding each sensor in a group with a ground-connected ring can help to isolate each. However, using ground rings is a trade-off because, as discussed previously, traces close to touch sensors increase parasitic capacitance and reduce sensor sensitivity.

Help from semiconductor vendors

In addition to PCB design changes, touch control technology providers can also provide some innovative technology to minimize the probability of false touches. For example, Cypress Semiconductor's CapSense® technology allows the design engineer to tune the circuit in order to achieve the best signal-to-noise ratio (SNR) to ensure touch detection and filter out false touches.

The system is based on the company's CY8C20XX6A technology that constantly measures the parasitic capacitance when a finger is not present. This measurement is converted into a digital count in order to set a noise baseline by establishing the average number of counts over a set time. This constant update of parasitic capacitance means the system can 'reset' the baseline if the capacitance changes due to environmental factors such as increases in heat and humidity.

When a finger is present, the system continues to frequently measure the capacitance in order to establish an average value for the "touch" signal. Figure 7 shows a sample of real sensor data from a CapSense-based system. Note that the noise baseline established from many measurements of parasitic capacitance is converted into digital counts. Similarly, many counts are performed when a finger is present to determine signal threshold. In this case, the SNR is 5:1.

In a manufacturing environment, 'identical' designs will exhibit a range of C_P, C_F, and C_X values that affect the system sensitivity. By analyzing the CapSense output, assembly technicians can pass or reject products based on an SNR threshold (for example, 4.75:1) set by the design engineer.

For its part, STMicroelectronics offers its S-Touch™ products, such as the STMPE16M31QTR for capacitive touch screens. The S-Touch technology is based on two RC networks, both driven by the same signal. One of the networks is the reference while the other is connected to the sensor. When a finger touches the pad, the capacitance increases, extending the time constant of the RC network compared with the reference. Depending on the length of the delay, S-Touch can determine if it is a deliberate touch, a false touch, or noise (see Figure 8).


The S-Touch technology utilizes a Data Detection Engine linked to a Calibration Unit that continuously runs a calibration routine to take account of changes in environmental influences such as temperature and humidity. Finally, a Data Filtering Block applies two filters, one to eliminate noise, and another to suppress signals from sensors adjacent to the one being touched to eliminate false touches.

Touch screen technology continues apace. Manufacturers are already promoting advanced technologies, including those that allow multiple finger touches to be detected simultaneously and projected capacitance, where the user does not even have to directly touch the screen. Simply moving a finger close is enough to trigger a response.

The basic design rules to ensure fast, accurate response without false touches remain the same. Aim for high sensitivity, such that the controlling electronics can easily discriminate between noise, false touches, and intended touches, and build in calibration so the system can cope with changes in ambient conditions.

References:

1. "CapSense Express," DigiKey/Cypress Semiconductor, Product Training Module.
2. "Capacitive Touch Sensing Solutions," DigiKey/STMicroelectronics, Product Training Module.