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Researchers working on CONTACT, a four-year EU-funded project, have developed a method of printing precise layers of water-based or pigmented inks onto a substrate material, such as glass or metal foil, for printed electronics.

Using the knowledge gained, industry partner Norbert Schlaefli Maschinen, based in Zofingen, Switzerland, is producing a gravure printing press called Labratester 2 for organic thin film transistors (TFT), which will be available at the end of this year.

‘The chief advantage of being able to print electronics is speed,’ explains Wolfgang Bock of Norbert Schlaefli. ‘The other advantage is the absence of satellite [warps]. Instead, you have narrow high-resolution patterns.’

Labratester 1, developed earlier in the project, is able to print with 25µm precision. The Labratester 2 is expected to print to 10µm.

‘The print cylinder has to be much more precise to print to 10µm, which is not trivial when you consider the cylinder has a diameter of 10-20cm,’ explains Bock. Another problem is printing TFT layers on top of one another. The Labratester 2 will use optical cameras to detect alignment marks.

Substrate materials are fed through the machine one sheet at a time, with room-temperature inks applied to it. Special inks have been made to withstand the aggressive solvents used in liquid crystal displays (LCD), allowing devices such as LCD watches to be created (the exact nature of these inks could not be revealed).

Glass success

One major success of the project has been gravure printing onto glass. Traditionally, flexible or elastic materials have been preferred for this type of printing, as the ink transfer can cause the ink cliché to routinely and sharply come into contact with the substrate. With its high brittle fracture behaviour, glass used in this capacity could easily crack or break.

‘For our printing tests, we used a cliché that consists of a rigid flat plate which was able to move horizontally,’ explains Dr Ulrike Brokmann of the Technical University of Ilmenau, Germany. The cliché was created from glass to make it gentler against the printing surface than chrome plated clichés, which can have a hardness of up to 1,100kp/mm2.

The ink is transferred to a chromium stamp before being printed onto a mechanical fixed flexible substrate – a 50µ thick glass film from Schott, based in Gruenenplan, Germany. Mechanical stability has been further improved by temporarily laminating the glass on an adhesive film.

‘The benefits of printing on glass are its chemical resistance against the organic solvents used in inks, its excellent barrier properties, transparency in a wide range of the electromagnetic spectrum and its thermal stability,’ adds Brokmann. It also allows gravure printing using electrically functionalised indium tin oxides inks, which achieve their optimal electrical properties following a thermal treatment above 300ºC. Her team is hoping to microstructure the glass for further uses in substrates and clichés.

Norbert Schlaefli, meanwhile, aims to have the mechanical development of the Labratester 2 completed this autumn, with the software components ready by the end of the year.

Further information:

CONTACT

Norbert Schlaefli Maschinen