FEATURE DISPLAYS
Displaying touchscreen technologies
Dr Andrew Morrison, technical director at
Zytronic, takes a look at how new technologies are widening the opportunities for touchscreens
T
ouchscreens are becoming more widely adopted. The most popular
technology, however, is Projected Capacitive (PCAP), which is not one but multiple technologies. While there are a growing number of ways to create a touch sensor capable of detecting the location of one or more contacts, the main difference is the conductive material used within them. The three main material technologies used currently are: Copper Micro Wires, Silver Metal Mesh and Silver Nanowires. So let’s compare these based on four key parameters.
ECONOMICS When considering the cost of touchscreens, some of the key issues are initial tooling costs and ongoing material requirements. Technologies that can be directly written to the substrate materials without a mask require little in the way of tooling and can be produced more cheaply in low volumes. If masks or other tooling are required, then this limits the ability to produce screens of different sizes flexibly in low volume, although it does have the potential to offer reductions in high volumes on standard sizes. In terms of tooling, copper micro wires
offer flexibility – the electrode can be written directly to the substrate, with no
Figure 1: 22” Zytronic Multi touch 10µm wire design
Figure 2: 22” Metal mesh 4µm design
Figure 3: 22” Metal mesh & Multi touch designs overlaid for comparison
Figure 3
mesh materials are patterned at source, so the size of sensor needs to be specified upfront. This leads to tooling charges which can range from around $10K to $20K per sensor design depending on screen size.
RESISTANCE
Touchscreen resistance is a key factor in determining sensitivity to touch, or ‘signal to noise’ ratio (SNR). Higher resistance materials limit the amount of current flowing through the conductors, making it harder to correctly pick out a touch event from surrounding ambient interference (EMI) coming from the display, power source or other surrounding electronics.
CUSTOMISED BI-STABLE DISPLAYS AVAILABLE
Custom displays using Bi-stable technology are now available from GTK. The display technology is similar to e-paper technology, in that it retains the image on the display screen without consuming power. However, while e-paper is limited to a black and white, Bi-stable technology uses printed fixed colours – blue, red and green are currently available as standard, with others on request. The typical configuration is for two colours to be used: one for text and the other for the contrasting background, but it is possible to add more colour layers to the display background if required. In addition, this display technology does not require a backlight, uses compact industry standard batteries and offers benefits such as sunlight readability, ultra-wide viewing angles and high resolution graphics. However, Bi-stable technology is limited to simple custom segment, graphic
or dot-matrix displays. It also has a marginally slower response time (image refresh rate) than more sophisticated displays, such as TFTs. It is also susceptible to UV light, although this can be alleviated by using protective covers.
GTK
www.gtk.co.uk
lasers, masks/chemicals/etching or tooling needed. Silver nanowires can be customised to a degree, via laser ablation, but then additional processing is required to link the conductors at the borders to the controller. By contrast, silver metal
Figure 2
to around 65”. Copper micro wires offer the lowest resistance at ~5Ω per square or less, and can be used to create touchscreens measuring over 100”. Furthermore, the extremely low resistance provides the best signal to noise ratio, resulting in touchscreens that are capable of detecting touch
through very thick overlaying glass and even gloves.
VISIBILITY All discrete overlay projected capacitive touch technologies involve introducing some material element between the user and the screen, which will make some optical difference, however small, to the image. With Copper micro wire based technologies the grid of 10µm conductors can be visible, particularly when the display is off. That said, light transmission is excellent and in the range of 90% before any anti reflective treatments are applied. In contrast, silver nanowire and metal mesh technologies enable the creation of slightly less visible conductive tracks (metal mesh in the 5-10µm range), however, nanowires can produce a slight colour cast or haze over the whole screen, and a base light transmission of around 85%.
THE BEST COMBINATION There remains no such thing as the ‘perfect’ conductive material for projected capacitive touchscreens – and designers should always look for the best combination of performance, optics, durability, scalability and reliability to suit their touchscreen application.
Zytronic
http://zytronic.co.uk 22 OCTOBER 2016 | INSTRUMENTATION / INSTRUMENTATION
Clearly, this resistance becomes more of an issue with larger touchscreens. ITO (indium tin oxide) has generally been limited to smaller touchscreens due to its relatively high resistivity of ~100Ω per square. As a result, most touchscreens using this material are smaller than around 22”, beyond which there are significant performance limitations. Silver nanowires have a better
resistance than ITO and can be used up to around 42”. Silver metal mesh has a lower resistance and has been used
Figure 1
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