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Ares Opinion: Mechanical Liftoff is Taking Off

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Panel manufacturers are increasingly looking for temporary bonding materials to release flexible displays after module completion – Mechanical Liftoff (MLO) seems to be the next generation go-to solution.

The advent of consumer-facing flexible displays is no longer in question: flexible phones are here. The manufacturing process by which they are made however, is still catching up. One area that is particularly sensitive to current limitations is the debonding process of the flexible panel from the carrier glass used to maintain dimensional stability during manufacturing. Currently, the industry standard methodology involves the use of excimer laser systems to ablate the interface between the flexible substrate and the carrier glass after module fabrication in a process known as laser lift-off (LLO). This leverages the experience manufacturers have with these excimer laser systems that are used for the laser annealing of amorphous silicon films into the high-performance low-temperature polysilicon (LTPS) semiconductors used in displays today. However, countless problems emerge from the use of this technology for the release step. The module and stage thickness sensitivity of the beam’s focal point, the power fluctuation of the beam from intra- and inter-sheet processes, and the necessity to consume the carrier glass due to beam width limitations all contribute to manufacturer’s desire to explore alternate debonding methodologies.

The concept of mechanical lift-off (MLO) is not unknown to display manufacturers, as even now the modules must be physically peeled away from the glass after the LLO process has finished. The challenge is to find a combination of materials and process technology compatible with the extreme temperatures, harsh solvents and high adhesion during fabrication, but low peel forces after module fabrication. Traction is growing for MLO solutions from many major display manufacturers such as BOE, AUO, Samsung and many others, however these solutions require significant process alterations, complex preparations of the motherglass carrier (Figure 1), or generate unacceptable forces during the final delamination.

Figure 1. Schematic of BOE’s proposed mechanical liftoff, involving porous carriers (11).

To solve these problems, Ares developed and commercialized Easybond: a material focused on providing a simple and easy-to-use temporary bonding solution and process, using existing commercial tooling for the module delamination process. The layer, which can be easily and quickly deposited on glass substrates with minimal preparation, can withstand temperatures in excess of 500 °C and is compatible with the current LTPS manufacturing process. This is achieved by depositing a thin (~500nm) Easybond layer atop the carrier glass and following up with the extrusion and conversion of varnish-based films such as the polyimides (PIs) currently used as the high-performance substrates for the flexible display modules. The Easybond material works on a principle of volumetric expansion at low temperature (< 100 C), where the peel force gradually decreases over time. After 120 minutes at ambient environmental conditions (23 °C, 50% R.H.), the release force will drop below manufacturer-specified limits of 5 cN/cm of applied force across the width of the module (Figure 2).

Figure 2. Mechanism of action for Easybond temporary bonding interlayer.

Ares has shown this concept with a multitude of varnish-based substrate materials involved in producing flexible displays and flexible display components including polyimides (PIs), colorless polyimides (CPIs) and Pylux-brand polysulfide thermosets (PSTs). The chemical compositions and methods of manufacture of all these films vary dramatically, yet the resulting films are all easily delaminated from carrier glass substrates with the low expected peel forces. This has been shown with bare films, substrates coated with conformal and patterned thin-film films (e.g. silicon nitride, patterned gold electrodes, etc.), and full devices (e.g. IGZO transistors, transparent touch panels, etc). Easybond material in display manufacturer’s existing production lines is currently being trialed at the pilot (Generation 2) and mass production (Generation 6) scales, with anticipated commercial use in early 2019.

Figure 3. Easybond-based MLO of (a) flexible pattered metal layers on PMDA-ODA polyimide and (b) IGZO TFTs fabricated on Pylux-MF flexible substrates.

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