Touch Sensor PCB and Layout Guidelines Part 1

Introduction

This note provides guidelines for the construction and the layout of different types of printed circuit boards (PCBs) for the implementation of the Touch Sensor Controller (TSC) capacitive sensors on substrate materials such as FR4, flexible PCBs, or ITO panels. Various substrate materials are available for different PCB design construction. Among the substrate materials currently available on the market, the FR4 is the most common. FR4 is a glass fiber epoxy laminate and PCBs can have one or several layers. Given a limited size of the touch module, the one-layer PCB implementation is not always possible, whereas the fourlayer and the two-layer PCB are more common. For applications requiring a very compact form factor, a flexible PCB can be used. The capacitive touch module on top of the display unit requires a transparent sensor electrode and traces which can be implemented using an Indium Tin Oxide (ITO) layer on a glass/plastic panel.

PCB Design Tips

The Noise Influence by Other Chips

In the touch module, it is recommended that only the Controller be mounted without any other chips because other chips can cause noise signals when controlling components such as LCD or Buzzer, etc.

Cross Coupling Capacitance

Noise signals can be generated between sensor input lines. If sensor lines are both very close to each other and placed in parallel, they can become noise sources to one another. In order to avoid this, it is recommended to design sensor input lines as shown in Figure 2. Enlarge line spacing and make parallel portions as short as possible.

Disposition of Data Lines and Sensor Input Lines

Figure 3 on the following page shows a problem caused by overlapping sensor input lines with data lines. For example, the capacitance generated by power lines with characteristics of consistent voltage output will not deeply affect sensor input lines. However, the capacitance generated by data lines fluctuating high and low voltage output will make sensor input lines unstable. Thus, the data lines in the front panel application should be placed closer to the connector in order to avoid undesirable influence on sensor input lines. Another important aspect in layout design is that sensor input lines should be placed on the opposite side of data output lines. Finally, since overlapping data lines with sensor touch pads will be worse than overlapping data lines with sensor input lines, it is recommended that sensor pads should be apart from data lines.
Several Controller Sensor Line Noise

If two Controller chips are mounted on the same PCB, they can be noise sources to each other. Therefore, in applications that are using two Controller chips, you need to design the touch pad area as shown in Figure 4.

LCD Control Signal Noise Issue

If the PCB, which includes LCD control lines, is located near a touch PCB, it could be a noise source even if it is not on the same PCB. Therefore, you need to design the PCB like Figure 5, which is less affected by LCD control signals. Any kind of pulse-type signals should be as far from the Controller as possible.



Charge Sharing

The sensitivity of the Controller will be decreased if the GND pattern is located close to the sensor input pads and lines because an electrical field generated by GND patterns will attenuate the strength of the capacitance generated by finger touch. This will decrease the sensitivity of the sensor input as shown in (a) of Figure 6 on the next page. Although the GND pattern is used to reduce the interference of the lines, make sure to keep the GND pattern a distance from the sensor input pads.



Mismatch in Each Sensor Input

For a normal AIC function, each sensor input capacitance of the system should be within 6pF.

Large Sensor Input Pad

If a touch pad is larger than 10 x 10mm, it will become very sensitive to external environmental change. As a result, input impedance during no-touch could be unstable. In order to avoid this situation, it is recommended to use a pad layout as shown in example (b) of Figure 7, which is exactly the same pad size as shown in example (a), but it eliminates the problem by reducing real surface area.



Touch Sensor PCB and Layout Guidelines Part 2