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Exploring next generation optical bonding

Posted: 31 Mar 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Optical bonding? TFT display? touch screen? backlight? VacuBond?

Optical bonding is an enhancement procedure for TFT displays wherein a protective glass, a touch screen, or a combination of both is applied to a TFT display by means of an optical material.

In tape-bonded solutions, incidental light is reflected at the surface of the TFT display and cover glass. With increasing light intensity, this results in reduced contrast and limited readability of the information appearing on the display.

In addition, the emerging light from the TFT's backlight is scattered by the polarizer and reflected on the glass of each component depending on its surface texture and transparency, which causes additional readability issues for the display.

By eliminating air gaps with optical bonding, these reflections are reduced to 1% and the result is optimum contrast and exceptional colour brilliance even when the TFT display is used outdoors and under intense ambient light. The readability is improved significantly without increasing the brightness or power consumption.

Under certain environmental conditions, dirt can accumulate in the air gap between the polarizer and the protective glass of the tape-bonded solution. Weather conditions with rapid and significant temperature changes may cause condensation fogging between tape-bonded layers, additionally affecting operation and readability.

Optical bonding not only prevents these circumstances from affecting the display, but also improves heat dissipation. The heat generated by the operation of the TFT display is dissipated through optical material and is not blocked by insulating air layers. Additionally, as proved by ball-drop test, optical bonding increases the strength and resiliency of the display. Shaking, shock, and vibrations are absorbed and reduced by the optical material, enabling use in industrial applications that require higher resistance to vibration, shock and direct impact.

Bonding technologies
Two different bonding technologies are available for TFT display enhancement. The most common one is the wet bonding, which uses liquid adhesives and requires a time consuming curing process. The related material shrinkage can result in degradation of optical quality due to bubbles or moir effects.

These effects are avoided with the dry vacuum bonding process, which uses a non liquid optical bonding material. In addition, due to the consistent mechanical conditions when vacuum bonding, so-called 'yellow spots' are eliminated and the production process is much faster.

Vacuum optical bonding
With VacuBond technology, Distec currently uses an optical vacuum bonding technology and processes the silicone optoGel from the Japanese Taica company, a supplier of optical bonding materials, with a vacuum-pressing device. Clean room conditions ensure that no dust or dirt particles affect the result. The technology is touted to offer advantages compared to traditional bonding methods.

In particular, small TFT displays with an extremely compact design in sizes from 2.4" to 7" can be bonded reliably, precisely and cost-effectively with this new vacuum process. High-precision serial production is said to be guaranteed by the automated process and the homogeneous silicone material. The manufacturing equipment is capable of directly bonding TFT displays in custom bezels and front housings with integrated protective glass. Additionally, Vacubond technology allows for quick-turn prototyping.

VacuBond technology is also suitable for environments that require explosion protection. Free air spaces within the display itself, which lead to restrictions in protection certifications, can be avoided or eliminated by "free air exclusion" bonding. This is an alternative to the very expensive and often irreversible conventional methods such as filling the space with glass beads or injecting silicone into the gap between layers.

Production is streamlined through automated processes and the use of production jigs. These are durable moulding tools which guarantee the correct position of the individual components during the bonding process and provide consistent quality when producing large quantities.

The basis for any new combination of TFT display and glass is an exact measurement of the contact angles and the physical adhesion forces. This measurement is made with a modern instrument and software that decodes surface energy in polar and dispersion components by using special liquids. Such a measurement protocol is the basis for every new project. Moreover, the VacuBond process is reversible: individual components can be recovered, simplifying repairs and reducing repair costs.

About the author
Bernhard Staller is the Managing Director of Distec GmbH.

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