Optical bonding of displays is a mature method for HMI display optimisation

25-05-2023 | Schurter | Industrial

Optical bonding has proved itself in industry and medicine and is a technically mature method of HMI display optimisation. Employing various bonding technologies, cover glasses with touch sensors and displays are bonded to create a single unit. Optical bonding of displays greatly improves and optimises the optical and mechanical performance characteristics of input systems for the application. SCHURTER Electronics explains the bonding possibilities and their advantages:

The correct selection of the most appropriate bonding technology for an application is based on the application's necessities. Deciding factors are the specified environmental influences, the display shape and diagonal, and the PCAP sensor technology definition.

Touch panels and display applications in outdoor areas are subject to strong temperature fluctuations. Extreme temperature changes can occur at night when temperatures are below zero and throughout the day due to sunlight. Condensation on the surfaces occurs when warm and humid air meets cold surfaces such as the display and front glass. Warm air can absorb more water vapour than cold air, and the water vapour condenses and forms condensation moisture on contact with cold surfaces.

If there is an air gap between the display and the front glass with the touch sensor, moisture can be deposited on both surfaces due to condensation. This condensation moisture leads to negative impairments of the screen display. Averting condensation would be possible if the internal temperature of the application is maintained. Filling the air gap with optical bonding material is a simpler and safer solution. Only UV-qualified bonding materials and components are used for outdoor applications with daily exposure to sunlight. This means that permanent UV radiation does not negatively impact during the lifetime.

The readability of a display depends not only on highly transparent PCAP sensors but also on the incident light of the environment. The light radiation is refracted and reflected by the air gap between the surfaces of the display and the cover glass with sensor. The filling with optical bonding material bridges this air gap. Thanks to optical bonding, the screen content is emitted to the front without reflection. The bonding material has a refractive index comparable to that of the glass. This means that no more reflection or refraction can occur. Also, bonding improves readability due to the increased contrast and allows a glare-free screen surface. Particularly for medical applications, high contrast is essential for visualisations in diagnostics. Optical bonding also improves robustness and resistance to shock, vibration and increased force. Another advantage is the heat dissipation from the LED backlights and the display surface via the bonding material to the cover glass. This heat dissipation increases the lifetime of the LED backlight due to the temperature reduction. Consequently, optical bonding is the optimal solution for all medical devices requiring perfect readability.

There are several technologies available for the optical bonding of displays. The optimal process depends on the choice of components, system integration and application requirements:

Dry Optical Bonding: In dry bonding, the bonding material is cut to the size of the visible display surface and the air gap is filled homogeneously with it. There is a choice of several bonding materials in various thicknesses and consistencies, which are selected based on the displays and the application's specifications. Displays have various heights of metal frames depending on the model and diagonal. The optimum material thickness is determined based on the displays. The front glass with touch and display are joined under a vacuum in the bonding machine.

Liquid Optical Bonding (LOCA): A UV-curable liquid material fills the air gap between the display surface and the back of the sensor or cover glass. The adhesive is silicone-free, ageing-resistant and UV-stable. The process is ideal for all TFT displays with or without frames. The liquid bonding material is dispensed onto the display surface in a special pattern, and the cover glass with sensor is bonded employing controlled surface pressure. Then, the bonding material is distributed evenly and bubble-free between the two components. The bonding material is then cured using UV light without further heat storage, i.e. there is no material stress due to the effect of temperature and thus no mechanical stress.

Air Gap Bonding: In this simple process, the display is firmly bonded with an adhesive frame directly behind the cover glass with sensor in the clean room. However, an air gap remains between the front glass with sensor and the display. This type of bonding is a simple, cost-effective way to connect displays with cover glasses or touch sensors without further mechanical fixation.

At SCHURTER Electronics, optical bonding is the optimal solution for applications with high demands and challenging fields of application. All bonding processes are carried out in special clean rooms with a high degree of automation and standardised processes. Benefit from the different bonding technologies and our know-how for the optimal solution for your customer-specific application.

By Seb Springall