The Ultimate Touch Sensor StackUp Guide

With capacitive sensing undergoing such a rapid growth and penetrating more and more industries, it attracts the interest of many engineers with little to none experience with it.

That’s why, we have created this guide and we would like to share it with the community, PCAP design engineers and everyone that will have to include capacitive touch sensor designs in their systems! It provides you with insight on the anatomy of a capacitive touch sensor and industry terms, including:

1) Common stackup components

2) Attributes of common materials in the stackup

Plus: A table of all the materials of a StackUp and their dielectric constants

The stackup is the part of the touch sensor that the electrodes are enclosed within. It consists of multiple layers and its functionality is to protect the electrodes and help achieve the desired sensitivity of the sensor.

 

Apart from the electrodes, the rest of the elements that make up a touch sensor include:

1) Front Panel Material
2) Dielectrics
3) Adhesives 
4) Air gaps

The front panel material is the material that is located at the top of the stackup. These materials provide protection to the sensor and should have a transparency of more than 90% if they are placed on top of screens.

Front Panel Material Required Properties:


1) Optical (e.g. transmittance, anti-haze, antiglare etc)

2) Mechanical (anti-scratch, drop resistance, anti-shatter)

The most common front panel materials are glass and plastic, though glass is mostly preferred.

The glass is a versatile material with multiple uses within the stackup. Its most common use is as a front panel material (also known as a cover glass), but it can also be used within the stackup. The main variables to consider when selecting a cover glass are its dielectric constant and its thickness.

The characteristics of the glass that make is such a good fit for many touch sensor applications include its high dielectric constant, its excellent optical properties and the mechanical structural integrity it offers. On the other hand, plastic has a worse dielectric constant and using it as a front cover material can result in a less sensitive sensor.

Dielectrics

Dielectrics can be used as substrates or as layers that help achieve the desired sensitivity of the sensor. A substrate is the base material that carries the electrodes.

Even though any insulating material could be used as a substrate for capacitive sensing, low loss ones such as PET are preferred. Glass can also be used as a dielectric.

The main thing to consider when selecting dielectrics is their dielectric constant.

The rule of thumb here is: dielectrics placed between the driving and receiving electrodes (when they are in different layers) should have a lower dielectric constant and a higher one when they’re placed on either side of them. This stems from the underlying science and it has to do with improving the sensor’s sensitivity.

You can read the rest of our guide here