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For decades now, direct bonding has been used to improve the viewability and durability of displays deployed in specialized military, avionics and medical applications. Recently, however, we've also started to see a considerable surge in its use in commercial applications ranging from tablet and notebook personal computers to marine electronics and touch screen devices.
At its simplest, direct bonding refers to the process of laminating a substrate directly to the front of a display. This is performed to overcome many of the issues that arise in displays with traditional coverplates. In most standard displays, for example, an air gap exists between the display and the coverplate. Traditional coverplates experience issues with parallax and scratching; additionally this air gap creates multiple surfaces with high reflectivity making the display and coverplate extremely susceptible to condensation (figure 1). While these issues may not pose problems in displays used indoors in controlled environments, they can dramatically reduce the performance of displays deployed in demanding environments where extreme temperatures, bright light and shock are present.
In the past, displays deployed in specialized applications, such as military equipment, were generally the only ones exposed to harsh environmental factors. Today, however, technology is going mobile all around us. Where physicians used to pencil detailed notes on paper and clip the sheets into charts, we now see them entering information directly into tablet computers. Architects, scientists and engineers carry their computers to record and access data on the spot. The need for direct bonding in high-volume consumer applications has never been more apparent than now. The problem is, while demand grows for the technology, adoption has not kept pace. A variety of factors contribute to the lag in adoption, but two of the most important are the fact that many of the incumbent technologies cannot accommodate larger display sizes, support a wide range of assembly options and design requirements, or cost-effectively scale to the high production levels required by consumer applications. Many manufacturers seeking to enter this market find themselves facing numerous barriers. To overcome some of these barriers, DuPont offers several solutions, including licensing Vertakô bonding technology for starting or enhancing in-house bonding process, or contracting with their high-volume direct bonding facility in Shenzhen, China.
Direct bonding solves many of the problems that arise with traditional coverplates, making it advantageous in applications requiring outdoor viewability and the durability to withstand impact, vibration, extreme temperatures, altitudes and dust. DuPont Vertakô bonding technology, for example, actually can improve sunlight readability up to 400 percent and impact and scratch resistance up to 300 percent. By optically bonding an anti-reflective glass, plastic or touch sensor directly to the front of an LCD display, Vertak bonding technology eliminates the air gap between the reflective surfaces of the cover glass and display, and allows for reductions in reflectance and the number of anti-reflective treatments needed (figure 2). The elimination of the air gap also prevents condensation and enables thinner and lighter designs.
By offering numerous glass thicknesses and optional finishes as well as a wide range of assembly options, direct bonding can meet a wide variety of application and design requirements such as in-frame, over-frame and on-frame designs. With in-frame bonding, the cover glass is bonded to the LCD area only, inside the frame of the original LCD. There is no mechanical link to the frame of the LCD and the LCD must be mounted via the mounting holes of the OEM panel (figure 3). The on-frame bonding process bonds the cover glass to all or part of the front bezel of the original LCD and to the full LCD front surface. Unlike with in-frame bonding, the glass size has very few constraints and the cover glass can be used as a mounting surface. On-frame bonding is the preferred method for bonding cover glass to LCD modules (figure 4). The over-frame bonding approach consists of bonding the cover glass, which can extend beyond the LCD bezel frame up to a maximum of 1.0 mm, to all or part of the front bezel of the original LCD. As with on-frame bonding, the cover glass can be used as a mounting surface. Vertak technology is a single-stage, glass-on-glass bonding process of any size LCD, from a 1-inch diagonal display to 65-plus inches.