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OCA for Direct Bonding

Optically clear adhesives (OCA) and liquid optically clear adhesives (LOCA or OCR for optically clear resins) have been used for more than for more than 15 years to bond rigid LCD and OLED displays for consumer electronics, industrial and automotive applications, offering optical, mechanical, and electrical performance benefits. The performance requirements of an OCA to bond cover glass, touch sensors, and circular polarizers in a plastic OLED display to bent cover glass or a flexible, foldable OLED display are drastically different from a flat, rigid device. For plastic OLED to bent cover glass bonding, the adhesive needs to be strong enough to resist spring back of the flat, plastic OLED devices. For flexible, foldable OLED displays, the neutral plane needs to be managed during folding keeping strain to a minimum in critical layers of the device (e.g., touch sensor, TFT), and the OCA cannot deform (or cause other layers to deform) during the folding process. Folding also brings challenges to touch sensors that can no longer use conventional passivation layers. As a result, the OCA will be responsible for preventing corrosion of touch sensor materials such as metal mesh, silver nanowire, carbon nanotube, and graphene.

OCA usage increased significantly since 2006. The advantage of using an OCA shows improvements in mechanical, optical and electrical performance of the display module and device. The initial application in OCAs in rigid OLED-based devices was similar to LCD devices, but as plastic OLED (pOLED) devices were introduced to the market, OCAs enabled new form factors like curved OLEDs.

There are two important mechanical considerations for optically clear adhesives – does it stick (adhesive strength) and how strong is it (cohesive strength). Film based OCAs are pressure sensitive adhesives, while LOCAs are liquids that are dispensed and cured. Pressure sensitive adhesives exhibit tack when the modulus is below 3 × 105 Pa or the so-called Dahlquist criterion at the application temperature.

Once the adhesive sticks, the cohesive strength of the material determines the mechanical properties of the bond. Conventional adhesives are functional to the point of bond failure. For optically clear adhesives, they are functional to the point of cavitation that can occur at a significantly lower strength than bond failure. The cohesive strength of an OCA can be designed to resist cavitation and bubble formation when it is warranted.

OCAs are often tested for reliability and durability at elevated temperatures and humidity (e.g. 65°C/90% RH). It is important to have sufficient adhesive strength to maintain performance through these conditions. Potential failure modes include moisture ingress at the interface and bulk plasticization of the adhesive. The adhesive should be selected to have sufficient strength based on the design of the module/device and the necessary durability for the adhesive.

OCAs are typically based on acrylate chemistry, having a refractive index of 1.47–1.48 that is comparable to glass and other important materials (e.g. PET, polarizer, etc.) in a display device. By matching the index and using an adhesive to replace an air gap, this significantly reduces the amount of light reflected at an interface (∼4%). By using an OCA, both contrast and brightness can be improved in a device.

Reliability and durability of OCA bonding in elevated temperatures and humidity can have an impact on optical properties. The adhesive can absorb moisture under these conditions, becoming supersaturated at elevated temperatures. When the adhesive returns to ambient conditions, the excess moisture phase separates in the material becoming apparent as haze in the bulk of the material. This can be avoided by using anti-whitening OCAs. Durability under UV exposure is a requirement in automotive and high performing consumer electronics applications. A common failure mode under these conditions is yellowing. During the OCA manufacturing, careful materials selection and formulation by the materials supplier can prevent yellowing by UV. Circular polarizers in display devices can reduce UV transmission to the display panel. As POL-less devices become a reality, the OCA will need to take on UV blocking functionality to protect the display device.

The electrical functionality of OCAs includes protecting sensitive touch sensors as well as enabling superior touch functionality. Industrial pressure sensitive adhesives often contain acrylic acid to achieve good adhesion. Acidic species corrode touch sensors, such as ITO. Therefore, adhesives selected for bonding in optical applications must be acid-free. OCAs can be designed with an optimal dielectric constant at the touch sensing frequency (100 kHz) from low (Dk<3) to high (Dk>9) to maximize the signal-to-noise ratio and minimize current and power to drive the touch circuit.

Whatever the form factor, OLED display modules all require additional layers for durability, user interface, and optical functionality. At least one layer each of cover window, touch sensor, and circular polarizer is generally included, and these must be laminated together with OCA or LOCA for best contrast, brightness, and mechanical durability. The cover window is generally made of glass, although in a few cases manufacturers have used plastics like polycarbonate, polyethylene terephthalate (PET), or clear polyimide (CPI).

Quoted from intechopen.com