Relec introduces optical bonding for TFT displays

They are used in transportation systems as well as in industrial settings where they information from equipment can be relayed, viewed and/or controlled easily by the user.

TFT assemblies are typically made up of a touch panel or cover lens and the display, with a natural air gap between the two.

When an external light source (natural sunlight or ambient lighting) enters the front of the assembly (either natural sunlight or ambient lighting), the light passes through the top layer (touch panel or cover lens) and then through the natural air gap refracting and reflecting the light through the different layers, which leads to distortion and glare, making the screen’s image distorted or difficult to view.

Applying a clear material within the air gap creates a single optical layer and prevents internal light refracting. This is called optical bonding and is available as a design service from Relec Electronics.

The optical layer has other benefits, besides improving readability. 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. This is done using a vacuum process to ensure the adhesive is evenly spread and without bubbles. The assembly is UV cured to set the silicone.

One of the main advantages of OCR bonding is that there is no restriction on thickness of the adhesive; 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. 

While OCA bonding is a relatively simple and low cost process, it is restricted to certain sheet thicknesses which are available on the market. Its transmittance rate and impact resistance are also less than OCR bonding. It does not protect against light reflection to the same degree as OCR bonding and the chances of yellowing due to UV exposure increase compared[ch1]  with OCR bonding.

The best bonding option for a particular installation needs careful consideration, and should be based on a number of environmental and use-case factors. The process also has to be performed with precision and skill to avoid discolouration, or bubbles/ foreign material being introduced to the adhesive material. If the material is not applied evenly, it can result in clouding in parts of the display’s active area – known as the Mura Effect. Using an inappropriate material which does not match the composition of the TFT’s polariser can result in delamination of the optical bonding layer.

 [ch1]See earlier comment re: Compared with OCR/other processes