What is optical bonding?
Optical bonding is a process of affixing the touch panel (or just the protective glass) to the display using a liquid adhesive. In general, this process improves the parameters of the module – the optical performance, as well as durability.
The process uses an optical-grade adhesive to glue glass to the top surface of a display. It eliminates the usual air-gap between the touch panel and the display.
The air-gap is the reason for the phenomena of reflection of sunlight. So, when the display is off, the Visual Area (V.A) is never black – it is gray.
When removing the air gaps, we reduce the number of internal reflecting surfaces, which lead to degradation of optical performance. This phenomenon worsens readability (especially for outdoor environments), by decreasing the brightness and corrupting contrast.
The optical bonding process has three main stages:
- Preparation stage– first we need to take care of decontamination, and selection of the right optical clear adhesive,
- Glue dispensing – applying the optically clear adhesive on the whole display surface
- Bonding and curing –touch panel is carefully layered onto the LCD, avoiding gaps or bubbles. Then the adhesive hardens with UV light.
There are many parameters that the liquid adhesive needs to fulfill to ensure a valid process. Firstly, the liquid adhesive used for the bonding has to be transparent. It needs to have a suitable refractive index and to be made under careful control of the producer (making sure that there are no significant variations in optical properties).
On the other hand, the adhesive has to bond with enough strength. It needs to have a reasonable pot life and to cure to the proper finished bond condition, using temperatures and conditions that are friendly to optical bonding facilities.
We use an optical clear adhesive that fulfills strict parameters. Depending on the application we will adjust the best adhesive for your project to avoid undesirable effects, like for example:
- Yellowing – when the mixture is not adjusted properly, the glue may yellow under UV, chemicals or radiation,
- Mura effect – glue tensioning causing mechanical stress
- Delamination of the module – shrinkage of the adhesive at high temperatures.
We need to remember that good optical performance is not only a matter of aesthetics. Such treatment ensures the module is very rugged, thus shock, impact, and vibration resistant.
On the other hand, good readability is the crucial parameter for safety issues – for example for displays mounted in vehicles, also perfect for hot-wet and cold-dry environments.
Summarizing everything that was said above, the main advantages of optical bonding are:
- Increased ruggedness: Bonding a sheet of glass on top of the display increases the ruggedness of the display
- Condensation: The elimination of an air gap is especially important for outdoor applications. It provides better tightness of module assembly.
- Improved durability: A bonded display is better able to resist scratches, fluids, stains, and dirt.
This type of technology is getting more and more popular on the market, and so it becomes more cost-effective, also the production times curtails. Prices vary based on the size of the module, optical glass surface treatment, volume, and lead time. For optimal parameters, the price can fluctuate around a dozen USD.