Self-healing Films

Touch Screen Devices Heal Like Living Organisms?

Touch screen devices with self-healing capabilities may become the new standard in the near future.  Who wouldn’t want a phone or tablet that could heal itself?  Seems like magic.

Well, it’s not magic nor is it new technology, but it is not yet common. Self-healing films are considered “smart” materials which are able to self-repair after damages such as daily wear. This is similar to how living organisms are able to heal after being injured (although without the scars).  So, the film is like a protective layer of skin for devices, but better than skin since it is replaceable.  Film with pressure sensitive adhesive (PSA) can be applied by the touch screen user.  When the film is at the end of its life-cycle a user can simply peel the film off the smart phone or touch screen surface and reapply a new layer of film.  Some self-healing films also have an  anti-smudge (AS) effect which makes finger prints less visible and easier to wipe off the surface.  Screens can always look brand new.

For now, there are different coatings available to enhance the durability and wear of a touch screen.
AS AG AR Coatings

The Evolution of Touch Screen Cover Glass

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.

Touch Screen Cover Glass 1

Compared to plastic, 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.

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.

Red-Cover-Glass-Full-720

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 peep hole, and anti-fingerprint coatings are now added to the surface of the cover glass to keep 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, which 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 “black out” 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.

All trademarks and registered trademarks are the property of their respective owners.  Touch International is not affiliate with Apple, Corning, Gorilla Glass, Galaxy Edge, Dragon Trail, Xensation, or the iPhone.

Projected Capacitive Borders

Wise Words from a Touch Screen Engineer

By L. Gaulin – Rockstar Touchscreen Engineer


Projected Capacitive Touch Screen Technology and Borders 

With the advent and ever increasing popularity of smart phones and tablet computers, projected capacitive (PCAP) touch sensors are becoming more prevalent every day.  This prevalence is leading more and more product development teams to look at including touch features in their new designs.  In turn these designs push the boundary of what is possible for PCAP sensor, in both capability and form factor.

The size of a PCAP sensor is directly related to the size of the display active area and the borders needed to have a linearly sensitive, reliable sensor that can be manufactured efficiently.  Many different options are available for hosting the conductive traces that make up the bulk of that border, all with their own pros, cons and costs.  Ideally, the sensor and the display would have the same active and outer areas, but as display borders get narrower, the touch sensor industry is striving to keep pace.

Capacitive Touch Borders

By far the most common type of projected capacitive touch screen traces is the printed metal trace, usually Ag (silver).  There are three main methods for creating these traces: printing, laser ablation and sputter deposition.  These are listed in increasing trace density and price.

The printing option is the cheapest and fastest method, but the traces are limited by the screen or ink deposition resolution.  This typically results in traces in the above 100um width range.  At ~200um pitch, these traces take up considerable amount of room that adds up quickly and can take up more space than the display.  These traces are best used for a small number of I/O or instances where low trace resistance is paramount.  This method is also well suited for dual sided conductive ITO coated glass (DITO) as it is independent of what is on the opposite side of the substrate.

The laser ablation technique is the Touch International preferred method when designing custom projected capacitive touch panels.  This entails a large swath of Ag (silver) ink being printed where the traces need to be, followed by a laser ablation that isolates and shapes the traces.  The traces that this process yields are usually in the 50um range, which allows twice as many traces as printing alone could accommodate in the same area.  Unfortunately as the laser passes right through the glass, any material on the far side of the glass can be damaged.  This means that DITO sensors cannot have overlapping traces or conductive areas if they hope to use the laser ablation technique.  While the time needed to laser scribe each line is greater than printed traces, it is far less than the duration of a sputtering deposition.

The last common trace creation method is sputtered deposition.  Capital expenditures needed to accomplish this process are in excess of all others, thus it is rare than any company has this capability in house, Touch International included.  Due to this, the cost and lead time for the sputtered traces is by far the largest.  Inversely, the traces are usually sub 50um, which makes them the smallest option and therefor the smallest borders.  Only when all other options have been ruled out does sputtering become a viable option.

No matter what trace creation method is used, the bond between the flexible printed circuit (FPC) tails that is needed to move the sensor signal to the controlling electronics is, by comparison, a large feature.  With this limitation, the tail exit edge is always the largest side and must be planned around accordingly.  Balancing the border width, cost, time and external constraints is what Touch International does best.  Our broad industry experience in tune with our eyes on the future of the industry keep our capabilities in pace with the ever evolving touch display and multi-touch panel marketplace.

Keep calm and multi-touch on.