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The first time that cover glass is considered is when system requirements and specs are set. Depending on the sensor application, the cover glass can vary greatly. For example, in industrial settings a thicker and more robust material will be required, while in smartphones being as thin as possible is 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 cover glass thickness and material will be decided during the mechanical design phase. It is important to keep in mind that the required specifications of the final design are not the only ones playing a role in this decision: supply chains and relations with existing vendors can impact the brand of the glass, narrowing down the designer’s choices, mechanical properties (eg hardness) of the material, and thickness suggested from the IC maker’s controller guidelines also have to be taken into account.
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.
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, the designer will leave from those documents confused or empty-handed. Controllers are designed to be versatile and operate under as many conditions and applications as possible. When it comes to cover glass selection the guidelines can be too generic and broad. Being suggested with a cover glass range thickness of 0.5 to 5 mm, which is too broad, is something touch sensor designers should be prepared for. Such a guideline could be used in two completely different applications: 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 and omit discussing the mechanical and optical factors, for the sake of simplicity.
So, there are 2 main variables to consider:
- The material of the cover glass.
- The cover glasses’ thickness.
Material is usually the one that has the least impact on the performance of the design between the two. This happens since we are examining just the sensitivity of the sensor. From a mechanical standpoint, there would be 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 their relative permittivity. Cover glasses, typically, have a relative permittivity range of 7 to 8.
Rules of thumb, simulation data and measurement suggest that 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 of major importance.
Cover glass thickness is the one that plays a vital role in all the aspects of the touch sensor’s performance. On the one hand, selecting a thicker cover glass will result in a less sensitive sensor, that will be able to withstand more wear and tear, while on the other hand, a thinner one will result in a more sensitive but more damage prone sensor.
Touch Sensor with Cover glass, modeled in Fieldscale simulation software
Interdependence of material and thickness
But it’s not like these two factors are independent. Both of these factors play a role in the signal-to-noise ratio (SNR) of the sensor. One of the easiest ways to determine how stable a system is, or how much the system is affected by noise, is to look at its Signal-to-Noise Ratio (SNR). Just as it sounds, this is a way of measuring how strong the signal is when compared to unwanted disturbances of noise. In real applications, system SNRs should ideally be at least 15 to provide a higher level of reliability.
A thicker cover glass can lead to a worse SNR, while a cover glass with higher dielectric constant is preferable since it can increase the SNR.
The relationship between these 2 factors and the impact they have on the performance of the sensor can further be explained by examining 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.