The choice of projected capacitive sensors (PCAP) also seems almost limitless in industrial systems such as HMI terminals, medical and outdoor applications. Following the historic success of mobile and smartphones in the consumer sector, capacitive sensor technology has also found favor and established itself in industrial product development. Like all professionally used controller and bonding technologies, PCAP technology has undergone various development cycles.

The general sensor structure has remained almost the same to this day. The basic structure consists of two levels (X and Y), usually with a conductive diamond pattern that extends over the entire sensor. The two planes are insulated from each other and are often covered by an adhesive layer (OCA = Optical Clear Adhesive). If a finger or a conductive medium is located at the intersection of the two levels, the capacitance is changed and a touch is detected. This basic functional principle makes it possible to operate an HMI system through a pane of glass, for example, as the field lines can spread out.

A central component of this technology is the transparent, conductive material ITO (indium thin oxide = indium tin oxide), which forms the diamond pattern described above. Despite changing technologies and manufacturing processes, it has not yet been possible to find an alternative that is both technically and optically (transparency) comparable to ITO. Essentially, the conductive material is applied to two different sensor substrates: glass or PET film.

No additional cover glass required

In the glass/glass sensor, the ITO is located between two glasses, which are held together by an OCA adhesive layer.

© Data module

One of the oldest PCAP technologies is the glass/glass sensor. The ITO is located between two glasses, which are held together by an OCA adhesive layer. ITO glass has a low surface resistance (50 Ω/square) and enables the integration of sensors in sizes from 7 to 32 inches without any problems thanks to its fast charging. The overall structure is approximately 2.4 mm (2 × 1.1 mm ITO glass + 0.2 mm OCA). Thanks to this robust structure and the fact that the ITO is located between the glasses, this sensor can be used without additional cover glass. Similar to a resistive touch, the sensor can be glued to a TFT - for example with double-sided adhesive tape - and installed in a housing from behind. Glass sensors can generally withstand temperatures from -20 °C to +70 °C, which is sufficient for all common industrial applications.

A significant disadvantage of this type of sensor, due to the manufacturing process, is the conductor path and the resulting external dimensions. All electrodes are contacted via a conductor track that leads to the flex tail at the edge of the sensor. With a line/space gap of 100/100 µm, i.e. the dimension/distance between two conductor tracks and the conductor track itself, the structure is quite wide. This is particularly evident with diagonals larger than 12.1 inches, as the resolution of such sensors is significantly higher. Due to the external dimensions and the robust overall structure, the glass/glass sensor can hardly assert itself in modern applications.

The design winners

Compared to glass/glass, ITO film sensors (film/film) are significantly thinner. With a sensor structure of around 0.5 mm to a maximum of 0.7 mm, their use in portable or mobile devices is particularly popular. The conductive ITO material is applied to a PET film. The separating layer is again an OCA adhesive layer.

In the case of a film/film sensor, the conductive ITO material is applied to a PET film. The separating layer is again an OCA adhesive layer.

© Data module

However, the flexible film cannot be used or loaded, so at least one thin carrier must be applied (glass/film/film). In this case, the glass has a purely protective function: it prevents the sensor from bending and the sensitive conductor paths from being destroyed. Due to the OCA layer already present on the film ITO sensors, these sensors are applied to an additional protective glass (also known as carrier glass or protective glass) using a lamination process. The carrier glass or cover glass as protective glass is available in different thicknesses, prints and/or shapes. This offers a high degree of flexibility in cover glass design, which is essential for PCAP sensors. ITO films are very easy to process, which allows even the most unconventional shapes.

Despite the PET film, the operating temperature range - with special films - is guaranteed between -20 and +60 °C or +70 °C. A line/space gap of 50/50 µm can be achieved using a combination of laser and etching processes. The external dimensions of the sensors are no larger than those of the TFT display, so that the sensor technology fits into almost any existing design during a redesign.
Film ITO sensors with a surface resistance of 100 to 150 Ω/square are usually limited to a diagonal of 23.8 inches, occasionally even 27 inches.

Like glass/glass sensors, ITO film sensors have been available on the industrial market for the longest time and can be found in numerous consumer applications. However, the availability of such sensors fluctuates due to the high purchase volumes in the consumer sector and the securing of stocks by smartphone and tablet manufacturers.

Film discontinuations are also not uncommon, as the raw material changes frequently in the consumer electronics cycle. Especially for professional HMI applications with a development period of up to one year and complex qualifications, such as medical devices, frequent changes of ITO foil sensors are quite unpleasant. Despite selected industrial touch suppliers, such a change can occur every three to five years. A touch manufacturer such as Data Modul is aware of these challenges and tries to recognize film changes early so that it can inform affected HMI customers in good time and offer replacement films in form/fit/function. In addition to the touch function and the resulting parameters, the visual appearance plays an important role.

Independence from market conditions

In order to be independent of such market circumstances, Data Modul introduced a further development of ITO sensors in 2014: the SITO (Single-Side ITO). Unlike film/film sensors, both ITO electrodes (X and Y direction) are applied to just one side of a constantly available glass substrate. The special feature here is that metal jumpers are used at the crossing points to prevent short circuits due to the single-layer touch structure.

Unlike film/film sensors, with the SITO sensor both ITO electrodes are applied to only one side of a constantly available glass substrate in both the X and Y directions. Metal jumpers prevent short circuits.

© Data module

Thanks to their glass construction, these sensors are particularly suitable for harsh conditions and outdoor use. They can withstand temperatures from -30 to +85 °C and are virtually insensitive to strong sunlight. A photo-lithography process can be used to create particularly narrow conductor paths (20/20 µm) and set new design standards.

All the advantages of a PCAP sensor, such as multi-touch operation with up to ten fingers, water and glove operation or EMC robustness, are retained thanks to the latest controller technologies.

SITO sensors must be bonded to a cover glass using a liquid adhesive process (LOCA) in order to protect the ITO structures on top. In addition, a robust laminated glass is created that easily protects against the effects of force, such as being hit with a tool. Usual diagonals are 7 to a maximum of 21.5 inches (maximum 27 inches). The size limit results from the mother glass size and the manufacturer's manufacturing process.

SITO technology has aroused great interest, particularly in modern machine control, due to its numerous advantages such as availability, robustness, material properties and slim design.

Diagonals up to 80 inches

However, despite its versatility, ITO has some limitations. In addition to its availability, it is very rigid and cannot be applied to curved surfaces/glass. Furthermore, surface resistance is a major drawback, especially for larger diagonals.

With metal mesh sensors, an alternative to these limitations now appears to have been found. This is a metal mesh, usually based on silver or copper, which forms the basis on the PET films. The essential structure is similar to an ITO film/film sensor with two conductive PET films held together by an OCA. Here too, a cover glass is connected to the film sensor by lamination. This time, the conductive structures are not ITO diamonds, but predominantly silver metal grids, applied using a special imprint lithography process. The structures are stamped onto the PET films and filled with conductive silver material. This enables surface resistances of 10 to 20 Ω/square, which makes integration in diagonals of up to 80 inches possible without any problems. The prerequisite for this is the correct choice of controller. The focus of high-resolution sensors in machine control systems has so far largely been on diagonals of up to 32 inches. Here, controllers from the manufacturer Ilitek are now a suitable addition to Atmel or Microchip.

With a metal mesh element width of 4 to 9 µm, metal mesh sensors - despite their high resolution - are barely larger than the matching TFT. And a real asset is the flexibility of the silver conductor tracks, without which there would be no curved touch displays. Metal mesh sensors have always been associated with the moire effect, which has so far delayed their introduction into the industry. This effect occurs when several grids are placed on top of each other, similar to the metal grid of the metal mesh sensor. However, it has been possible to counteract this effect with a special manufacturing process and metal mesh sensors have long been used in modern HMI applications.

PCAP sensor technology offers a variety of different modern technologies. Design, functionality and availability play a major role here. When developing an HMI product family, the desire is usually to have one sensor substrate and one controller generation for all diagonals. It is important to compare all the described advantages and disadvantages of different touch technologies.

Author:
Markus Hell is Head of Product Marketing for Touch Solutions at Data Modul.