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:
- The steps during which the cover glass selection is under consideration.
- The differences between different cover glasses.
- The methods touch sensor designers use in order to achieve the most efficient design.
- 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.
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.
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.
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:
- The material of the cover glass.
- The cover glasses’ thickness.
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.
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.
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.
To explain the relationship between these 2 factors let’s exame 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?
Besides from intuition and rules of thumb, there are 3 main methods touch screen designers can use to reach to the optimal design.
- Analytical calculations.
- Extensive prototyping and measurements.
Analytical calculations is the simplest and fastest method. However, all is provides is estimates. Using in-house calculators and spreadsheets is also a choice. While these provide estimation, they can by no means be considered sufficient for optimization or for use in projects with strict requirements. Their use is limited to making educated “guesses”. Engineers can then proceed to selective prototyping of the designs showing the most potential. The main problem with this method is that since analytical tools offer limited accuracy, they may lead to excessive prototyping.
Prototyping and measurements can deliver the highest degree of product quality and verified functionality. However, if not done right, it leads to high development costs and delays in time-to-market. If one tries to “brute force” the optimal configuration, prototyping costs will be too high and too time consuming.
So, the trick here is not to eliminate prototyping completely, since it’s the only way to verify functionality. The trick is to minimize the number of prototypes you build and save engineering effort.
Finally, simulation can create countless digital prototypes of your design, each with different configuration. You can iterate your designs virtually. Then create prototypes for the best ones and finally manufacture your optimal design. Due to the complexity of the touch sensors, digital prototyping is your best bet. Small design changes can have a major impact. You need to be able to iterate countless similar designs, that have different performance.
These iterations traditionally happened with extensive prototyping. There is no denying that prototyping leads to a high quality final product. However, it’s proven that with simulation you can go-to-market faster and with less engineering time and costs.
This isn’t the only benefit: simulation also gives unprecedented insight on your designs. You can have an under-the-hood look on the underlying physical phenomena and see how each design change affects these phenomena. This can be extremely useful when optimizing designs or when performance is regulated by strict guidelines (for e.g. in automotive or aerospace applications).
Here’s an actual case study, with simulation results provided by our own touch sensor simulation tool, Fieldscale SENSE.
As mentioned earlier, relative permittivity has a slight impact on the designs’ performance. The same can’t be said for the touch sensor front panel thickness.
Should a designer check the IC maker’s guidelines, he’d find an acceptable cover glass thickness of 0.5 mm to 5 mm, or something similar, depending on the controller. You can see above that the difference between the 2 cases is huge: the sensor’s sensitivity changes almost 6x.
So, we’re going to consider these 2 cases and see that even narrowing down the cover glass thickness range between 0.5 mm and 2 mm still leaves us facing some hard design choices.
The sensor’s sensitivity is high, more than 27%. This means that touches will easily register. If the sensor needs to be sensitive (eg for smartphones), that’s a benefit. If the sensor is for an environment with water droplets or dust, these particles can trigger it. So, the high sensitivity is a drawback.
This design choice would also be good for a sensor that supports gestures. High sensitivity is important to such applications.
However, this cover glass is thin. It makes the sensor fragile and not durable enough for industrial applications.
In this case, the sensitivity is just below 15%. The sensor could still be able to be controlled with the same controller as the one above with proper controller tuning. But which configuration should the design engineer choose?
This design, even though it’s less sensitive, would perform better in industrial setting, since it will be harder to register false touched caused by dust or droplets and it will be more durable due to its thicker cover glass. But also, being thicker makes it less attractive for smart phone use, since usually being thin is of essence in those applications.
This article focused on selecting the most suitable touch sensor front panel. Cover glasses -another name for touch sensor front panel- made from different materials, aside from their mechanical properties, essentially differ in their relative permittivity. But this difference in the relative permittivity rarely has a significant impact on the sensitivity of the final design.
What should be thoroughly investigated before making the final selection is the cover glasses’ thickness, as it plays a major role on both the sensitivity and the mechanical properties of the sensor.
But isolating the material and thickness choice is also tricky: the two are interconnected and affect the SNR of the sensor and its sensitivity factor.
When facing these complex problems, touch sensor designers tend to seek guidance in IC maker’s design guidelines, but rarely find clear advice there. They have to find the most efficient solution themselves, using hand calculations, extensive prototyping or simulation.
Finally, we demonstrated in a simple use case the power of simulation, creating 20 virtual prototypes, iterating 2 designs with different cover glass materials, for 10 thicknesses each. This use case provided insight for each touch sensor configuration, allowing us to be able to figure out where 2 of the configurations we investigated could be used.