Optically bonding an LCD, OLED or Plasma display improves both optical performance and durability. By selecting an appropriate adhesive that matches the index of refraction of the flat panel and the overlay, General Digital is able to remove all of the air gaps, thus reducing the number of internal reflecting surfaces that can lead to degradation of optical performance. Providing a durable adhesion between the flat panel and the overlay improves the displays’ ability to resist shock, vibration and moisture.
Optical Bonding is the affixing of two optical elements to one another using a liquid adhesive. The qualifier optical implies that the adhesive is transparent, has a suitable refractive index and is made under adequate control such that there are no significant variations in the optical properties within a single bond. In this way, we differentiate bonding from lamination.
On the practical side, the adhesive must also provide adequate bond strength, have a reasonable pot life after preparing, not present any health or safety issues, be available at reasonable cost from reliable sources, and cure to the proper finished bond condition using temperatures and times that are friendly to optical bonding facilities. Fortunately, there exist a relatively large number of choices. Typical bonded overlays include contrast enhancement filters, EMI/RFI filters, vandal shields, touchscreens, and thermal heaters.
- Augmentation of Optical Performance of Flat Panel Display and Overlay
- Increased Luminance
- Increased Contrast
- Reduced Internal Reflections
- Improved Ruggedization
- Bonded display assemblies are able to withstand the high shock and vibration typically associated with MIL-STD-901 and MIL-STD-810 applications
- Vandal Prevention
- Extended Operating Temperature Range Through the Use of Optically Bonded LCD, OLED or Plasma Display Heaters
- Elimination of Condensation and Moisture Between LCD, OLED or Plasma Display and Overlay
Types of Adhesives
This adhesive has been used for more than 30 years in making bonds to displays, in both commercial and military applications. Because it is a relatively soft material, it is very feasible to rework, with minimal risk, any bonds that have a problem. The major drawback to this material is a tendency to form debris if the edge of the bond is rubbed during handling. To reduce the severity of this, we seal the edges of the bond using thin black tape or a configuration that does not expose the edge to abrasion.
Another adhesive that we have used successfully is a flexible epoxy formulation. This makes a much more rigid bond than silicone, but does not have the tendency to form particulate debris. It is not reworkable in the event any problems arise during production or use. However, for some designs in which the use of tape to seal the edge is not practical, this may be the adhesive of choice.
Polyurethane adhesives possess good environmental integrity for use in high altitudes and may be superior at low temperatures. However, they exhibit a severe yellowing over time when exposed to high ambient (solar) lighting conditions. Because of this tendency to yellow, General Digital Optical Bonding Laboratories discourages the use of polyurethane adhesives
There are numerous configurations available, including over-the-frame, in-the-frame and under-the-frame, as well as behind-the-display and frameless designs. Each have their own unique attributes.
- Over-the-frame and in-the-frame bond designs require that the flat panel display first be sealed to the OEM metal frame to eliminate the possibility that the adhesive might leak into the backlight and contaminate the films. A non-corrosive silicone formulation is used to effect this seal.
- Under-the-frame, behind-the-display and frameless bonds require that the flat panel display be disassembled before the bond can be made. After the bond is cured, either the OEM frame or a custom-designed frame must be affixed to the display.
Our first experience in optical bonding was with under-the-frame bonds for the GenStar™ monitor, a 20.1" LCD used in an FAA control tower application. This large size created some severe concerns about handling and yield. Our experiences with this were, however, very good. During 2002, General Digital Optical Bonding Laboratories bonded approximately 500 of these displays with a yield in excess of 95%. It was our early success with this product that encouraged us to offer the bonding service to other customers.
The in-the-frame design is currently in limited production, primarily for avionic applications. The adhesive is epoxy, which was selected so that the bezel could be affixed to the display’s metal frame, with the bonded antireflective glass extending into the opening, reducing any issues of viewing angle and parallax to the display.
One drawback to the under-the-frame and in-the-frame bonds is that they are not suitable for making a liquid-proof seal to the bezel of the monitor. To accomplish this, monitor builders usually prefer to form a seal to the antireflective glass. In this case, an over-the-frame bond is necessary. This bond design is also required in the case of an optically bonded touch sensor. The details of the bond may vary slightly, depending on the nature of the overlay. However, in general, an appropriate gasket is placed onto the face of the OEM frame, forming a well-defined bond thickness and protecting the edge of the seal from abrasion.
It is, of course, possible to bond more than one overlay to the display. A specific example is the use of a conductive glass to control EMI, which is bonded to a touch sensor. As of this writing, General Digital Optical Bonding Laboratories has produced over 475 displays of this design for an aerospace application.
Types of Bonds
- Gasketed Optical Bond
- This bond design is the preferred over-the-frame bond. A gasket is applied between the LCD, OLED or Plasma display and the overlay and the space between them is then filled with adhesive.
- Good thermal and mechanical performance on a wide variety of sizes and overlays.
- Shimmed Optical Bond
- This bond design controls the thickness of the bond through the use of shims during curing.
- Suitable for under-the-frame bonds. Can be used with over-the-frame, but a gasketed bond is preferred.
- Static Optical Bond
- Bond thickness is determined by the interaction of adhesive properties, overlay weight, time and temperature.
- Only suitable for under-the-frame bonds.
- Provides the thinnest bond line.
- Rework of defective or damaged assemblies may be difficult or impossible.
Optical Bond Complexity
- Single Optical Bond Layer
- Multiple Optical Bond Layers
- Triple bond is the most complex bond to date
- Optical Bonding to Both Surfaces (front and back) of LCD, OLED or Plasma Display
- Glass (overlay) to Flat Panel
- Glass to Glass (robust anti-vandal shield, similar to automotive safety glass)
- Polycarbonate to Glass (flat panel) [under development]
- Contrast Enhancement Filters
- Glass (32%, 62%, 75%, 92% [Clear], Triple Notch)
- Anti-Vandal Shields
Rugged filters are available in various thicknesses and diagonal sizes to protect the display from harm. Highly resistant to impact, scratching, and chemicals, they are available in several surface finishes to provide optimum viewing of a flat panel display based on a particular application. All finishes are designed for antiglare, maximum light transmission properties, and durability.
- Substrates used can be polycarbonate, acrylic, chemically strengthened glass, or thick (6 mm or more) glass.
- In many cases, bonding standard glass to the LCD, OLED or Plasma display is sufficient to withstand significant abuse in the field without failure.
- EMI/RFI Shields
EMI/RFI shielding is necessary to protect against spurious RF signals or co-existing electronic or electrical systems, as well as providing high electrical isolation properties. Various applications, such as the military and medical equipment markets, can benefit the use of EMI/RFI shielding.
- Enable Display Assemblies to Satisfy FCC and MIL-STD-461 Requirements
- Optically Clear ITO Coatings Down to 5 Ohms/Square
- Coating must be done to one or both sides of the glass.
- A busbar is provided all around to ensure good ground contact in the final enclosure.
- The busbar can be eliminated in some designs where the EMI glass is bonded to a frame at General Digital using a conductive adhesive.
- Busbar configuration will depend on over-all system design constraints:
- U-shaped used to bring conductivity from the bonded side of the glass to the top.
- L-shaped used to bring conductivity from the ITO surface to the edge for contact with an EMI gasket mounted along the side of the display.
- Surface busbar used when the ITO surface is on the outside of the display assembly and the EMI gasket will be between the assembly and the monitor bezel.
Heaters help to extend the operating temperatures of flat panel displays and backlights, which will improve the LCD, OLED or Plasma display performance, especially at low operating temperatures. Typical applications include avionic displays, ruggedized electronic devices, handheld terminals, and outdoor card readers.
- Optically Clear (ITO)
- Can be Combined with Other Coatings and Filters
- Extends the Expected Life of Backlights When Operated at Cold Temperatures
- Touch Sensors
Touch sensor technology simplifies and improves the human interface to the display system. Rather than using keyboards, switches or buttons, designs now include touch technology to increase design flexibility while delivering a better overall user experience.
- Types of Touch Technology Available
- Infrared (IR)
- Wave (SAW)
- General Digital supports virtually any touch technology or manufacturer
- Types of Touch Technology Available
- Privacy Filters
Privacy filters protect the security of on-screen data. Only persons directly in front of the filter will see the screen clearly. A specially-designed film blacks out the screen at viewing angles greater than 30 degrees, side to side.
- Various filter sizes available
Specialty Optical Bonds
- General Digital-Patented Bonding Integration Technique
- Facilitates integration of bonded display subassembly into sealed enclosures
- Minimizes potential for compression bi-refringence or bond de-lamination
- Reduces risk of damage during shipping and receiving
- Extended Temperature Optical Bond
- Special bonding technique tested to survive from –45° C to 70° C at a rising rate of 10° C/minute
- Cold Temperature Optical Bond
Today’s display users are finding it more common for their units to operate in extreme temperatures. A growing trend is for the display assembly to survive in extreme cold per MIL-STD-810F. Low temperatures can have adverse effects on display systems, which can temporarily or permanently impair the visibility and operation of the display. It is imperative that this requirement be taken into consideration during the design phase. Examples of problems that may result when exposed to these extreme temperatures may include:
- Change in impact strength and reduced strength
- Condensation and freezing of water
- Static fatigue of restrained glass
- Anomalies in the optical bond including delamination, crystallization of bond material, bubbles, etc., which may or may not return to normal after temperature stabilization.
General Digital’s Optical Bonding Laboratories has thermal tested displays to -55° C. Bond failure occurs after temperature cycling; delamination and other anomalies are present. Our cold temperature bond formula prevents delamination and other bonding anomalies. We understand the needs of these unique display users and have designed a bond formulation with these considerations in mind. In addition to providing the optical bonding benefits of improved brightness and contrast, cold temperature optical bonding improves the overall ruggedization of a display. We also offer in-house thermal testing to your specific requirements. Please contact Jennifer Derleth to learn more about our cold temperature optical bonding solution, and to discuss your design and the many ways in which you can benefit from this solution.
Types of Filter Coatings/Etchings
- Antireflective (AR)
- Single-Sided (recommended for bonded filters)
- Double-Sided (recommended for non-bonded filters)
- Antiglare (AG)
- Chemically Etched
- Mechanically Etched
- Easy-Clean (smudge resistant)
For further information regarding optical bonding or to request a quote for your application, please contact one of our qualified Sales Engineers.