Designing a touch sensor is not an easy task.
There is a high level of complexity, many design choices to consider, complex physical phenomena taking place and cost decisions to be made. So, as it always happens in engineering design, compromises have to be made in each step of the way.
The amount of design choices a touch sensor designer has to make in order to fulfill the customer needs and meet the requirements, as well as the impact each one has is overwhelming.
Let’s assume that all the design aspects of the touch sensor are defined, apart from the cover glass selection.
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 can use in order to achieve the most efficient design.
- Finally, we will see a real-life example, based on simulation data, on 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 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 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.
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.
So, if the project specifications and the IC makers guidelines offer some leeway on the cover glass selection, how can touch sensor designers select the optimal cover glass?
Touch Sensor with Cover glass, modeled in simulation software Fieldscale Charge.
How can touch sensor designers optimize the cover glass selection?
Without taking into account their 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 are the simplest and fastest method, however, they provide no more than estimated values. Determining the effect of different cover glass configurations can be done with in-house calculations on spreadsheets. While calculations provide estimation, they can by no means be considered sufficient for optimization or for use in projects with strict requirements. So, they can just be used for making educated “guesses”. The configurations with the most potential will then be built into prototypes to be validated. 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 can lead 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) but to minimize the number of prototypes you build and also save engineering effort.
Finally, simulation can essentially create countless digital prototypes of your design, each with different configuration. This way, you can iterate your designs virtually, then create prototypes only for the few, best ones and finally manufacture your optimal design. Due to the complexity of the touch sensor system, digital prototyping is your best bet. Small design changes can have a major impact, thus you will be able to iterate countless similar designs, that will little to different performance.
These iterations were traditionally tested with extensive prototyping. However, even though prototyping can also lead to a high quality final product, 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).
How to choose a touch sensor cover glass: an actual case study
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 cover glass 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.
- Cover glass thickness: 0.5 mm
The sensor’s sensitivity is high, more than 27%. This means that the touches will easily be registered, a benefit if a sensitive sensor is needed (eg for smartphones) or a drawback if it’ll be a sensor used in an environment with water droplets or dust, these particles can trigger the sensor. This design choice would also be good for a sensor that supports gestures, as high sensitivity is needed in such applications.
However, this thin cover glass choice, will lead in a fragile sensor, not durable enough for industrial applications.
- Cover glass thickness: 2 mm
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.
Takeaways from this article
This article focused on selecting the most suitable cover glass for a touch sensor. Cover glasses 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.
Featured image courtesy of Corning.com.