[How to] Choose the Right Cover Glass for your touch sensor

[paschalis]

 

Something i found quite useful about touch panels and i would like to share with the NXP community. Designing a touch sensor is not an easy task. The touch sensor front panel you choose is more important than you think.

In touch sensor design there is a high level of complexity and many design choices to consider. Complex physical phenomena and cost decisions also play their part. So, as it always happens in engineering design, engineers make compromises in each step of the way. Compromises also happen on the touch sensor front panel material.

The amount of design choices a touch sensor designer has to make is overwhelming. Each design choice targets to fulfil the requirements of the customer, but they impact conflicting parts of the design.

Let’s assume that all the design aspects of the touch sensor are defined, apart from the cover glass selection. The touch sensor front panel is usually made of glass, hence the term “cover glass”.

This article addresses:

1. The steps during which the cover glass selection is under consideration.
2. The differences between different cover glasses.
3. The methods touch sensor designers use in order to achieve the most efficient design.
4. Finally, we will see a real-life example. Based on simulation data, we will examine how different cover glass thicknesses and glass permittivities affect the sensitivity of a touch sensor.

We start with the bigger picture, how the whole touch sensor is designed and then, narrow it down to the cover glass selection.

During which steps of the touch sensor design process is the cover glass considered?

The first time that cover glass is considered is when system requirements and specs are set. Depending on the sensor application, the touch sensor front panel can vary greatly. For example, in industrial settings a thicker and more robust material will be required. Ιn smartphones thin front panels are almost always a priority. Making the right design choice is very important because the cover glass plays a vital part in the touch sensor’s durability and mechanical integrity.

The touch sensor front panel thickness and material is decided during the mechanical design phase. The designer does not only have to keep in mind the requirements of the final design. Supply chains and relations with existing vendors also impact the type of front panel material. Mechanical properties and IC makers guidelines are also important.

With all these things to consider, it’s clear that the touch sensor designer will need material to refer to when making the design decisions.

Touch controller guidelines do not always provide enough guidance.

One of the most important aspects of the touch sensor design is compatibility with the selected controller for the application. The design engineer usually turns to the IC makers guideline documents to look for the best practices. However, more often than not, they will leave from those documents empty-handed.

Controllers are, by design, versatile and operate in applications as possible. When it comes to cover glass selection the guidelines are generic and broad. Suggestions for touch sensor front panel thickness ranging from 0.5 to 5 mm, which is too broad, is not uncommon for touch sensor designers. A 0.5 mm cover glass could be used in a smartphone, while a 5 mm cover glass could be used in an outdoor kiosk or a POS. So, this wide range can lead to no useful conclusions.

Let’s take a look at the main things a touch sensor designer should take into consideration when selecting a cover glass.

How the cover glass affects the sensitivity of the touch sensor?

In this article, we only take into account the factors that affect the sensitivity of the touch sensor. We omit the mechanical and optical factors, for the sake of simplicity.

There are 2 main variables to consider:

1. The material of the cover glass.
2. The cover glasses’ thickness.

Touch sensor front panel material effect.

Material is the one that has the least impact on the performance of the design between the two. That is of course for the sensitivity.. From a mechanical standpoint, there are many variables to consider, such as hardness, drop resistance etc. In this case, the only thing that changes from cover glass to cover glass is the relative permittivity. Cover glasses, typically, have a relative permittivity range of 7 to 8.

Rules of thumb, simulation data and measurement suggest all suggest the same. Even swapping a cover glass with a relative permittivity of 7 with one with a relative permittivity of 8 won’t have a drastic impact on the sensitivity of the touch sensor. However, even though the effect of this factor is minor, it still should be taken into account in applications where high sensitivity is important.

Touch sensor front panel thickness effect.

Cover glass thickness is the one that plays a vital role in all the aspects of the touch sensor’s performance. Selecting a thicker cover glass will result in a less sensitive sensor, that will be able to withstand more wear and tear, while a thinner one will result in a more sensitive but more damage prone sensor.

Signal to noise ration (SNR) and front panel material relationship.

These two factors are independent. Both play a role in the signal-to-noise ratio (SNR) of the sensor. SNR is one of the easiest ways to determine how stable a system is, or how much the system is affected by noise. It is a way of measuring how strong the signal is compared to unwanted disturbances of noise. In real applications, system SNRs should be at least 15 to provide a high level of reliability.

A thicker cover glass can lead to a worse SNR. A cover glass with higher dielectric constant is preferable since it can increase the SNR.

Interdependence of material and thickness of touch sensor front panel

To explain the relationship between these 2 factors let’s examine the sensitivity factor of each layer.

The sensitivity factor equals to the relative permittivity of a layer divided by its thickness, S=er/t. Again, this ratio is in compliance with what we discussed above.

Greater thickness lowers the sensitivity, while greater permittivity increases it.

So, the project specifications and the IC makers guidelines offer too much leeway on the front panel selection. How can touch sensor designers select the optimal front panel?

You can read more here.