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AN4312: DescriptionGuidelines for designing touch sensing applications with surface sensors; File Size927KB，36 Pages; ManufacturerSTMicroelectronics; Websitehttp://www.st.com/

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Guidelines for designing touch sensing applications with surface sensors

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AN4312

Application note

Guidelines for designing touch sensing applications

with surface sensors

Introduction

This application note describes the layout and mechanical design guidelines used for touch

sensing applications with surface sensors.

Capacitive sensing interfaces provide many advantages compared to mechanical user

interfaces. They:

•

offer a modern look and feel

•

are easy to clean

•

are waterproof

•

are robust

Capacitive sensing interfaces are more and more used in a wide range of applications. The

interface is based on surface sensors that are made of small copper foils. The sensor acts

as a capacitor that is alternatively charged and discharged. The capacitor value depends on

the presence of the user finger as well as the sensor design. This application note

introduces various sensor designs and recommendations to achieve optimum

performances, keeping in mind that none of the sensor elements should interfere with each

other.

This document provides simple guidelines covering three main aspects:

Printed circuit board (PCB)

Overlay and panel materials

All other items in the capacitive sensor environment

Table 1

lists the microcontrollers concerned by this application note.

Table 1. Applicable products

Type

Microcontrollers

Applicable products

STM32F0 series, STM32F3 series, STM32L1 series, STM8L series.

September 2013

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www.st.com

Contents

AN4312

Contents

Capacitive sensing technology in ST . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.1

1.2

Charge transfer acquisition principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Surface capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Main capacitive sensing guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1

2.2

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1

2.2.2

2.2.3

2.2.4

2.2.5

2.2.6

2.2.7

Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Electrode and interconnection materials . . . . . . . . . . . . . . . . . . . . . . . . . 9

Panel materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Mechanical construction and PCB to panel bonding . . . . . . . . . . . . . . . 12

Metal chassis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Air gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Transfer of an electrode from PCB to the front panel . . . . . . . . . . . . . . 14

2.3

2.4

Placing of LEDs close to sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Surface sensor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1

3.2

3.3

Touchkey sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Touchkey matrix sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Linear sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.3.1

3.3.2

Normal patterned linear sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Interlaced patterned linear sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Normal patterned rotary sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Interlaced patterned rotary sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Rotary sensor with central touchkey . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

LEDs and sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Driven shield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Using electrodes separated from the PCB . . . . . . . . . . . . . . . . . . . . . . 29

PCB and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.4

Rotary sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.4.1

3.4.2

3.4.3

3.5

Specific recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.5.1

3.5.2

3.5.3

3.5.4

3.5.5

2/36

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AN4312

List of figures

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Figure 12.

Figure 13.

Figure 14.

Figure 15.

Figure 16.

Figure 17.

Figure 18.

Figure 19.

Figure 20.

Figure 21.

Figure 22.

Figure 23.

Figure 24.

Figure 25.

Figure 26.

Figure 27.

Figure 28.

Figure 29.

Equivalent touch sensing capacitances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Example of capacitive sensor construction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Clear ITO on PET with silver connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Silver printing on PET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Flexible PCB (FPC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

FR4 (2-sided epoxy-fiberglass). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Typical panel stack-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Examples of cases where a LED bypass capacitor is required . . . . . . . . . . . . . . . . . . . . . 15

Typical power supply schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Sensor size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Recommended electrode size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Simple matrix implementation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Mono electrodes design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Half-Ended electrodes design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Dual electrodes design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Normal patterned linear sensor with 5 channels / 6 electrodes

(half-ended electrodes design) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Interlaced linear touch sensor with 3 channels / 4 electrodes

(half-ended electrodes design) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Normal patterned rotary sensor with 3 channels / 3 electrodes . . . . . . . . . . . . . . . . . . . . . 23

Interlaced patterned rotary sensor with 3 channels / 3 electrodes . . . . . . . . . . . . . . . . . . . 24

Back-lighting touchkey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

PCB 3D top view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

PCB bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

STM8L1xx driven shield example using the charge-transfer acquisition principle . . . . . . . 27

STM32L driven shield example using the charge-transfer acquisition principle . . . . . . . . . 28

Printed electrode method showing several connection methods . . . . . . . . . . . . . . . . . . . . 29

Spring and foam picture (both are not compressed) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Track routing recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Ground plane example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Hatched ground and signal tracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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