Know your bonds: LOCA, OCA & Air Gap
Today, touch displays are a familiar sight in industrial applications, so making sure that they are robust and durable, while also maintaining their brilliance, is essential. This means that we pay particular attention to how each component of the display is connected – a process called ‘optical bonding’.
There are three kinds of transparent optical bonding used to connect the display, touch sensor and cover glass:
- Full Surface Bonding (which uses UV-cured liquid adhesive, known as LOCA – Liquid Optical Clear Adhesive)
- Adhesive Film (called ‘OCA’ or ‘Optical Clear Adhesive’)
- Air Gap Bonding
Learning LOCA
With this technique, the air gap between the display and touch sensor/cover glass is completely sealed using liquid adhesive. This is achieved using a dosing system which precision dispenses the adhesive according to the geometry of the display. With this technique, the touch sensor or cover glass can be applied without a single bubble. No bubbles equal no annoying reflections (because there are none between each surface), and at the same time the color brilliance and contrast are increased.
But, for industrial or outdoor applications, the layer of LOCA also intensifies impact resistance and prevents particles or moisture coming into contact with any components, which could cause permanent damage or, at the very least, compromise the image quality.
OCA, on the other hand, seals the air gap with adhesive film, which is, once again, applied with precision and then bonded to the touch sensor or cover glass. It’s a swifter process, as there is no wait for the adhesive to cure, and there is little to no thermal stress to the components. This technique too has the benefit of creating a robust, impact resistant display. Both LOCA and OCA also significantly minimize splintering of the display if it meets with a destructive force.
However, there are some less positive aspects of LOCA to consider. During the curing process, it’s possible that some areas are outside of the reach of the UV, and this can lead to air bubbles – sometimes even delamination. Because of the nature of the bonding, as soon as any contaminants or bubbles appear, the entire screen has to be replaced. For an already complex and costly process, this can be very problematic.
Air Gap Bonding
Here, we use a high-performance double-sided adhesive tape frame, which connects the display and touch unit with an air gap remaining between them, hence the name. This type of bonding is probably the easiest and simplest way to connect a display unit, which reflects in the cost. It’s a particularly strong and stable bonding technique, overall, as well as being highly versatile. It can be used to bond low-energy surfaces (such as PE and PP) or many other plastics and powder coatings.
Unlike LOCA/OCA bonding, it doesn’t have the advantages of increased readability, minimization of reflection and protection against dust and dirt particles. It also creates a comparatively thick structure, but this can be adjusted by selecting different tape sizes. There can also be issues around reduced adhesion due to an uneven surface in the cover glass. Plus, the introduction of any dirt or grease can cause components to detach.
Ultimately, it’s important to understand that there is no hierarchy when it comes to bonding – no technique is ‘better’ or ‘worse’ than the other. They simply have different qualities which are context appropriate. So, it’s always a good idea to understand the pros and cons of each process to ensure that you’re selecting the best option for your display.
Of course, our display experts are on hand to guide you in your choice of bonding and the creation of your display or touch solutions. Find out how Avnet Displays can propel your product innovation and get in touch today.
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