Optical bonding is the application of a clear material within the air gap which creates a single optical layer and prevents internal light refracting. The optical layer has other benefits, explains the company. The absence of an air gap makes the unit strong with a high impact resistance. Optical bonding also prevents moisture ingress and the introduction of foreign materials within the optical stack to prevent fogging and to improve the backlight performance and contrast ratio.
Relec offers optically clear resin (OCR) bonding and optically clear adhesive (OCA) bonding. OCR bonding involves a liquid silicone based layer injected into the TFT’s air gap by a vacuum process to ensure the adhesive is evenly spread and without bubbles. The assembly is UV cured to set the silicone. There are no restrictions on the thickness of the adhesive with this process. It can be used with air gaps up to 1mm. The thicker the adhesive used, the higher impact resistance. It also offers high transmittance capabilities, high protection levels against reflection and UV.
OCA bonding uses an acrylic sheet with adhesive applied to both sides. This is laminated within the air gap to create a single optical layer. OCA bonding is a relatively simple and low cost process, but is restricted to certain sheet thicknesses. Its transmittance rate and impact resistance are less than OCR bonding and it does not protect against light reflection to the same degree as OCR bonding. The chances of yellowing due to UV exposure also increase compared with OCR bonding.
Both bonding options have advantages and the choice should be based on a number of environmental and use-case factors. Relec advises that the process needs to be performed with precision to avoid discolouration, or bubbles/ foreign material being introduced to the adhesive material. Uneven application can result in clouding in parts of the display’s active area (the Mura effect). The display specialist also warns against using an inappropriate material which does not match the composition of the TFT’s polariser as this can result in delamination of the optical bonding layer.