Display


New technologies widen touchscreen opportunities


There is no such thing as the ideal touchscreen material – the best option depends on the requirements of the individual project. Dr. Andrew Morrison, technical director, Zytronic, reviews some of the different materials used to help specifiers make an educated decision on which option is best for them


T


ouchscreens are being more and more widely adopted, and easily the most popular touch sensing


technology is Projected Capacitive (PCAP) technology. PCAP encompasses two main methods of touch detection (self-capacitance and mutual- capacitance), with many different methods of manufacture and touch sensor construction. Another critical difference between these types of PCAP touchscreens are the conductive materials used within their construction. The most widely used material is presently Indium Tin Oxide (ITO), but alternatives include Copper Micro Wires, Silver Metal Mesh and Silver Nanowires. We will compare these based on four key parameters: economics, resistance, visibility and availability.


Economics


Some of the key issues when considering the cost of touchscreens 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 volume. If masks or other tooling is required, then this limits the ability to produce screens of different sizes flexibly in low volume, but has the potential to offer reductions in high volumes on standard sizes. In terms of tooling, copper micro wires


score in terms of flexibility. The electrode can be written directly to the substrate, with no 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, ITO and silver metal mesh constructions are patterned at source, so the size of sensor needs to be specified upfront. This leads to tooling charges which can range from a few $K to $20K per sensor design depending on screen size.


Another key factor in manufacturing 26 September 2016


cost is the number of layers required. Copper micro wires can be insulated, so that the x and y electrodes can be formed in a single layer. The encapsulating insulation also prevents oxidation of the material, which can seriously degrade touchscreen performance when exposed


22” Zytronic multi touch 10um wire design


to high heat and humidity in the field. ITO, silver nanowire and metal mesh constructions generally require two or more layers to insulate the (x and y) conductors, increasing the material content and processing over a single layer design.


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. Clearly, this resistance becomes more of an issue with larger touchscreens, especially if features like multi-touch, palm rejection and proximity detection (identifying a touch before the finger actually makes contact with the screen) are required.


Most touchscreens using this material are smaller than about 22”, beyond which there are significant performance limitations. Silver nanowires have a better resistance than ITO (~30 to ~50 Ω per square on PET film substrate). As a result, projected capacitive touch sensors using this technology are scalable up to around 42” (beyond which, again touch performance is hampered). Silver metal mesh has a lower resistance of around ~15 Ω to ~30 Ω per square, and as a result is capable of use in touchscreens up to around 65” in size. Copper micro wires offer the lowest resistance at ~5 Ω per square or less, and can be used to create enormous touchscreens, well over 100” in size. 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, without the need to drive the control electronics at high voltages or tile the screens using multiple linked controllers (both tricky methods used by alternative material technologies to achieve large size touchscreens).


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


22” metal mesh & multi touch designs overlaid for comparison


however, nanowires can produce a slight colour cast or haze over the whole screen, and a base light transmission of around 85 per cent.


Availability and longevity ITO and copper micro wire touch sensors have been in production for nearly 20 years and are a proven PCAP touchscreen materials. Silver metal mesh and nanowire based touch technologies have emerged into the mainstream quickly over the last few years, with many manufacturers installing the necessary printing and laser patterning equipment. The relative newness of these two technologies in the touchscreen world mean that their long term reliability is not yet proven – particularly in relation to how their resistance (and touch performance) changes as exposed to temperature and humidity in challenging applications, e.g. outdoors.


The bigger picture 22” metal mesh 4um design


The most popular touch technology for tablets and smartphones, ITO, is limited to smaller touchscreens due to its relatively high resistivity of ~100 Ω per square.


Components in Electronics


image. With copper micro wire based technologies the grid of 10um conductors can be visible, particularly when the display is off. That said, light transmission is excellent and in the range of 90 per cent 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 5-10um range),


Although the four technologies that we have noted above are currently the main ones available, there are others emerging. Carbon Nanobuds, Conductive Polymers and Graphene all in early stage development and likely to be commercialised in the coming years. Carbon Nanobuds are relatively expensive to manufacture at this time, because the procedure to deposit the NanoBuds is complex using a “NanoBud Reactor” then a laser patterning process to create electrodes. By contrast conductive polymers are also applied relatively easily with screen printing, however, they need to be patterned at either the silkscreen printing stage or after with etching or lasers. Graphene is potentially game- changing but also the furthest away from commercial production. Deposited as a one atom thick layer of Carbon, it combines a similar low resistance to copper micro wire, with the potential of “invisible” conductors.


www.zytronic.co.uk www.cieonline.co.uk


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