Philips Research, The Netherlands, studies advanced technologies and production methods as a resource for Philips Business Units worldwide. One important current project is the area for plasma display panels. Researchers at Eindhoven have achieved valuable advances, enabled by DEK’s ProFlow DirEKt imaging technology.
Plasma display panels are constructed by depositing red, green and blue (RGB) phosphor pastes into an array of grooves made in a glass substrate. These grooves create the pixel array, and are created by sand blasting. Volume production presents a number of challenges. Overfilling any groove causes contamination of the phosphors in surrounding grooves, which would impair the colour fidelity of the display and cause it to be rejected. Phosphor inks are also relatively expensive and thus require a low-waste process. Also, even though plasma screens are filled in a controlled, clean room environment, the viscosity of the phosphors must be kept constant during processing.
Using small test strips 10-12 cm in length, the team analysed the performance of Philips' existing, squeegee-based screenprinting techniques. Sef Bosman of Philips Research explains, "In my view, there was no real alternative to a printing process. It's the only way to achieve the kind of throughput necessary. Therefore, we were constrained to look for a significant process enhancement that would answer the problems associated with manufacturing plasma displays."
At the SMT Nürnberg exhibition, a demonstration system was shown adapted for use with low viscosity polymer thick film inks in semiconductor component manufacture. Bosman spoke to DEK's Phil Lambert about using the same system to handle the phosphor inks.
Lambert: "We had adapted the standard ProFlow system, which was developed for use with solder paste, to address processes used by semiconductor manufacturers. Materials such as polymer thick film inks have a much lower viscosity than solder paste, and we adapted the ProFlow head to accommodate this by developing a fine mesh screen as an internal component of the transfer head."
This is what interested Bosman: to use a standard ProFlow system would require additives to increase the viscosity of the phosphor inks. "But the additives currently available for this purpose would also impair the efficiency of the phosphor and therefore compromise the performance of the display," he explained. "The modified ProFlow scheme offered a way of filling the glass grooves without altering the formulation of the phosphors." After making a number of minor modifications to the upper head design and also the start-up sequencing, ProFlow was tuned for its new application.
Enclosing the phosphor within the transfer head allowed greater process consistency - essential in the pursuit of a volume manufacturing solution. "Plasma screens are produced in a clean room environment with controlled temperature and humidity. We have to do this, because a relatively small rise in temperature, say from room temperature up to 30°C would create a 20% decrease in viscosity that would be enough to cripple the process. Stabilising the viscosity is vital, and the enclosed head enhances phospor stability.
"Stencil design came under scrutiny too," adds Lambert. "The resolution was not difficult; the grooves are 250-260 µm width, located on a 360 µm pitch and between 5-6 cm long in the test case. But in production, the stencil apertures would be up to 50 cm long."
First trials used a metal stencil, but achieving an adequate gasket seal against the glass surface was problematic. The resulting stencil was also fragile and required bridges throughout the slot length. These bridges prevented adequate filling of the grooves, creating an image in the resulting print. Moving to an emulsioned, woven stainless screen solved the gasketing and filling problems. The softer, emulsion coating was a better match against the hardness of the glass surface, and the mesh provided dependable rigidity while obviating the need for supporting bridges. Moving to the emulsion screen also necessitated a change of blades. "The standard metal wiper blades were not gentle on the surface of the screen," explains Lambert. "To raise the life expectancy of production screens, we recommended mylar wipers."
By combining the modified ProFlow head with the standard emulsion screen, Bosman was able to achieve satisfactory filling in a single stroke. This had proved impossible to achieve using traditional squeegees, which required multiple passes for an adequate fill. Placing three such machines in series would allow red, green and blue phosphors to be deposited rapidly for high-speed volume production.
ProFlow has helped many electronic assembly builders to save process waste by enclosing the transfer medium, thus preventing contamination and drying. Bosman noted the same result in the Philips plasma application. "After printing there is very little remnant phosphor on the screen," confirms Bosman. "It is still necessary to clean the surface after filling to remove traces of the material, because these would impair performance in the field. But ProFlow means there is little unnecessarily discarded phosphor. While the current market for plasma screens means production volumes are low, it is expected that millions of PDP screens are produced within the foreseeable future; at that kind of volume the kind of material savings made possible by ProFlow become significant."
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