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:
1.
2.
3.
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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Contents
AN4312
Contents
1
Capacitive sensing technology in ST . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
1.2
Charge transfer acquisition principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Surface capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2
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
3
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
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3.5.6
3.5.7
Contents
Ground considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Rotary and linear sensor recommendations . . . . . . . . . . . . . . . . . . . . . 33
4
5
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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List of tables
AN4312
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Potential application problems with flex PCB placement . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Dielectric constants of common materials used in a panel construction. . . . . . . . . . . . . . . 11
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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List of figures
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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