In recent years, a new technology trend has developed that requires an entirely new method for the assembly of TFT displays with touchscreen, protective glass or touchscreen and protective glass.
The previous TFT generations included extensive mechanics such as metal frames or sheet metal housings that could make good use of an assembly with double-sided adhesive industrial tape or of the necessary dam construction in the traditional bonding methods, such as wet bonding.
The latest TFT generation, especially the small TFTs in sizes from 2.2” to 7” (image 1: 3.2” TFT) and the very compact medium sized TFTs from 10.1” to 17.3” does not feature these mechanics anymore, and thus neither the tape assembly nor the wet bonding is a mounting option.
To meet these new demands the VacuBond Technology was developed and equal vacuum bonding systems have been installed at Distec in Germering/Germany and at Apollo Display Technologies, in Ronkonkoma/NY/USA. During the VacuBond process, a touch screen, a protective glass or a combination of both is applied to a TFT display. The assembly is made with the bonding/lamination material Opto-Gel over the complete surface, in pure vacuum and in vertical production direction. A CNC machined tool ensures the precise position of the components according to the product configuration. The process itself is fully automated and carried out in a clean room to avoid dirt inclusions. The production tool always guarantees consistent manufacturing tolerances and the careful handling of the fragile components. VacuBond enables faultless assembly and makes a cosmetic specification superfluous.
At first, both the surface of the TFT polariser and the touch screen or protective glass is measured with a laser-based measuring instrument to determine the contact angle and adhesive power. Precondition for a faultless bonding are thoroughly pre-cleaned components, the correct design of the tool, a perfect cut of the bonding material and a precise adjustment of the vacuum machine. If these conditions are met, no anomalies (opaque, translucent) occur and a “zero optical error” quality is produced.
The strength of the protective glass is freely selectable (image 2: 8.4 “with 6 mm glass) and all common touchscreen technologies can be used: resistive analog, glass/glass, AMR touch or projected capacitive.
The Opto-Gel for VacuBond is a specially developed high-performance material and available in 0.3 mm, 0.5 mm, 1 mm, 1.2 mm and 1.8 mm thickness. The selection is based on the technical and mechanical characteristics of the bonded components. Other gel thicknesses are available on a project basis.
Opto-Gel not only connects the individual components, but also contributes to the absorption of strong mechanical influences such as vibration or shock. The VacuBond technology already proved its worth in the essential drop-ball tests for public terminals, the very strict test runs for railway approval and also easily passed through the certification as e.g. to IEC 60721-3-7, MIL-STD 810F & G or DIN 75220-Z-OUT.
The VacuBond technology also offers advantages compared to the conventional laminating of a protective glass on a touch sensor in an autoclave. Depending on the used OCA laminating material, it often comes to temperatures exceeding the specified temperature range for the touchscreen Tst (storage temperature). The VacuBond process is carried out at room temperature.
Having the identical process installed on both sides of the Atlantic guarantees for a comprehensive risk management. This ensures a permanent supply, because production could be shifted to the other side of the Atlantic at any time in case of elementary problems such as water, fire or power failure.
