Necessity is the mother of invention. It is rumored that the first version of the Apple iPhone™ had a plastic cover made from the same material used in touch panels for decades. However, after a few weeks in the jeans pocket of Steve Jobs, the touch screen was so badly scratched (possibly from other pocket items such as keys) that he ordered a change from a plastic to a glass cover.
Thus, Apple™ changed the touch device market by incorporating a seamless, protective cover glass on the top of the touch screen. This design change was both cosmetic (no ridges on the front) and functional, in that it protected the touch screen from wearing out. The phone’s “cover glass” or “cover lens” was ordinary window glass that chemically strengthened, cut with a hole for the home button, a slot for the earphone, and had a simple black decoration on the back. Today, the cover glass has become much more, and in monetary terms, has eclipsed the cost of the touch sensor.
Compared to plastic, the glass had the advantage of better optical properties, scratch resistance, and electrical (touch) performance, but the disadvantage of breaking on impact. To prevent against breakage, glass is hardened by either heat tempering or chemical tempering [insert link to TI white paper on glass]. Since heat tempering can leave tiny ripples that can distort the display image, all touch screen cover glass is chemically strengthened.
The cover glass is made from large sheets of glass, usually .55mm, .75mm or 1.1mm thick, cut to the approximate final size by an “XY” glass cutter. In high volume operations (quantities larger than 5,000 units per month), the small rectangular sheets are glued together into a brick using beeswax and then ground into needed shape by a grinding wheel. Once the glass brick is ground a diamond drill bit cuts holes in the stack. The beeswax is melted off and the glass is made ready for routing for such things as the earphone slot. At this point, the glass may be polished to remove manufacturing residue, but most likely placed into a high-temperature salt bath for 8 to 16 hours for chemical strengthening.
For cover glass volumes of less than 5,000 units per month, a numerical control (NC) machine is used to grind and seam the edge. Then, holes and slots are cut into the glass, followed by chemical tempering.
After the cover glass is strengthened, the non-touch side (back) of the glass is printed with one or more colors. It is then attached to the sensor and finally installed onto the device.
The next big change occurred with the introduction of AAS glass by Corning™, branded as Gorilla Glass™ (soon followed by Dragon Trail™, Xensation™, and others). When chemically strengthened, AAS glass has about the same break resistance as standard cover glass, but when the AAS glass was scratched (think keys again), it did not lose its “strength” in the same way conventional glass did. Thus Samsung and Apple advertised the use of this glass in their phones and it became commonly used.
Proceeding Gorilla Glass™, changes to cover glass next came in the form of coatings on the glass. The advent and popularity of the “selfie” created the need for better optical performance on a phone’s front-facing camera. Thin chromatic coatings were put behind the peephole, and anti-fingerprint coatings are now added to the surface of the cover glass to keep the image from the display clear and sharp. Historically touch panels had anti-glare coatings, and there is renewed interest in this feature as well. Anti-reflective coatings, combined with an anti-wear coating, helps sunlight readability.
Primarily for design reasons, Touch International is now bending cover glass into 2d and 3d shapes; the first 2d production phone is the Galaxy Edge™. For “blackout” looks, the cover lenses are also tinted so that the display is only seen when it is on.
Touch International believes that cover glass will either be eliminated or will be replaced by plastic. Though plastic failed in the past due to scratching, there are anti-scratch coatings for plastic that Touch International applies that has the same hardness of glass. These coatings are expensive, but as prices come down, so will the requirement for glass as the lens.
And, due to advances we have made in touch sensors, we are now incorporating the touch panel directly into the plastic housing which we also manufacture. So the cover glass and touch sensor are incorporated into the “box” and both the cover glass and touch panel, as we know them, will be gone.
Touch International has more information in the touch screen white papers on this subject.
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