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FEATURE
FASTENINGS & ADHESIVES OPTICAL BONDING
BENEFITS SCHURTER explains
why optical bonding is the ideal solution for HMI displays
environmental influences such as extremes of temperature, but need to offer readability whether used outdoors or in critical medical or industrial applications. But how would you go about specifying the right bonding solutions for HMI applications featuring a display? The first topic to think about before choosing a bonding solution is the application. Will the display be used outdoors or indoors? What environmental influences will it face? What shape is the display? These are just a few questions to think about.
H
PREVENTING CONDENSATION If the display or touch panel is to be used outdoors, it could experience extremes of temperature at different points during the day and night. If, for example, warm and humid air meets the cold surface of the display, condensation can form. If there is an air gap between the display and the front glass with the touch sensor, moisture can be deposited on both surfaces, negatively impairing the display. Although condensation can be prevented if
the internal temperature is kept constant, a simper solution would be to fill the air gap with optical bonding material. If the application is outdoors, UV-qualified bonding materials and components need to be used.
READABILITY The readability of the display depends both on highly transparent PCAP sensors as well as the incident light of the environment. Light radiation is retraced and reflected by the air gap between the surfaces of the display and the cover glass with the sensor. Filling with optical bonding material bridges this air gap. With 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, so no more reflection or refraction can occur. Furthermore, bonding increases readability
due to the increased contrast and enables a glare-free screen surface, a benefit for medical applications, for example, where high contrast is needed for visualisation during diagnostics. Optical bonding also increases robustness and
improves resistance to shock, vibration and increased force. Another benefit is the heat
22 DESIGN SOLUTIONS MAY 2023
uman Machine Interfaces (HMIs) are used across a wide range of industries, where they not only face different
dissipation from the LED backlights and the display surface via the bonding material to the cover glass, which increases the lifetime of the LED backlight due to the temperature reduction. Optical bonding is therefore the optimal solution for all medical devices that require perfect readability.
A ROBUST SOLUTION The use of optical bonding results in a more robust system, which can be beneficial for portable and automotive applications as it provides shatter protection in case of glass breakage. Furthermore, eliminating the air gap between the cover glass with touch sensor and the display prevents dew forming in outdoor applications, as well as preventing dirt or dust penetrating it. Added to this, the mechanical fixation of the displays by optical bonding eliminates the need for mounting brackets, reducing weight.
BONDING TECHNOLOGIES There are a number of technologies for bonding displays, but the optical process depends on the choice of components, system integration and application requirements. These are: • Dry optical bonding: In dry bonding, the bonding materials 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 different thicknesses and consistencies, which are selected on the basis of the displays and the specifications of the application. Displays have different metal frame heights depending on the model, and the optimum material thickness is determined on the basis of the displays. The front glass with touch and display are joined together under vacuum in the bonding machine. • Liquid Optical Bonding: With this, a UV- curable liquid material is used to fill 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 suitable 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 using
controlled surface pressure. The bonding material is then distributed evenly and bubble- free between the two components. The bonding material is then cured by means of UV light. With both technologies, a special pre-
treatment is carried out to increase the bonding strength of the bonding material on the surfaces of the joining partners. This ensures a strong bond between the different materials. The bond is so strong that the bonding material firmly fixes the display mechanically and no further mechanical screwing is necessary. • Air Gap Bonding: A simple process, with this the display is firmly bonded all around 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. Air Gap Bonding is a simple and cost-effective way to connect displays with cover glasses or touch sensors without further mechanical fixation.
E-PAPER DISPLAY REQUIREMENTS The properties of e-paper displays always require additional mechanical protection during integration. Stability and robustness can only be achieved through full-surface bonding via optical bonding with a cover glass. Here, too, the choice of bonding material is defined on the basis of the components and environmental conditions of the application. Different protective glasses with frame printing, touch sensors and plastic panes are used here. Compared to the optical bonding of TFT LC displays, the processes for extremely thin e-paper bonding are considerably more demanding and irreversible. E-paper displays require additional illumination in the dark, realised by a special structure with highly transparent adhesives and a light guide material. The light from LED strips is coupled in laterally and distributed homogeneously over the surface of the display via the build-up layers. This illumination option enables the readability of e-paper displays in the dark.
SCHURTER
schurter.com
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