Thermal Management for Touch Screen Displays

It’s Hot Y’all:  A Texan’s Guide to Thermal Management for Touch Screen Displays

We lost count of how many days it was over 100 degrees here in Austin, Texas this July.  We can tell you that Accuweather lists the “Actual Temp” of this past Saturday as 107 degrees.  Yep, that’s hot.  We’re often tasked with providing touch display solutions for places even hotter than a sunny Texas July.  Grab a tall glass of sweet iced tea, and check out the various methods we have to manufacture touch screen displays that can navigate a desert combat zone.

  • Cooling Fans
    • It’s almost too obvious to list, but it can’t be left out. There’s a myriad of cooling fan options for both consumer and industrial applications.  Costs vary greatly, mostly depending on the materials used.  Higher end cooling fans can be aluminum or copper with optimized heat pipes, integrated heat sinks, and high quality ball bearing assemblies.  On the lower end of the cost spectrum, you can find plastic fans of any shape and size imaginable just to get some air moving through your assembly.
    • When designing a complete touch display that will require a fan, you must consider the additional power required, mounting options, and the additional space required before you finalize your housing design.
  • Heat Sinks
    • Heat sinks, or heat exchangers, transfer heat generated by an electronic or mechanical device into the atmosphere (or liquid cooling system) by maximizing the surface area. The fluid transfers the heat away from the device, allowing it to cool to the desired temperature. Metals with high thermal conductivity are used. Aluminum alloys are the most common heat sink material.  Aluminum costs less, weighs less, and gets the job done for most applications. For environments where higher levels of thermal conductivity are a requirement for operation, copper provides the best heat dissipation. Some heat sinks use a combination of aluminum fins with a copper base to save on cost and weight where they can.
    • Much like the cooling fan market, there are endless options available. Heat sinks can be combined with other thermal management materials such as heat exchange gaskets and cooling fans.
  • Housing Material and Design
    • Typical touch display housing materials are steel and/or aluminum. Steel costs less and is more durable than aluminum, making it an attractive option for most industrial applications.  Aluminum costs more than steel, but is lighter weight, and a good conductor of heat, allowing for better dissipation in high heat applications where you might incorporate a heat sink and fan combination.  Aluminum is the primary choice for field/mobile applications, automotive, medical, and aerospace applications.
    • There are more exotic (and expensive) materials such as magnesium, titanium, and a variety of hybrid alloys in an emerging market. These are typically found in niche applications more so the industrial space and come with increased costs.
  • Heat Exchange/Dissipation Gaskets
    • Gaskets offer a wide variety of increasingly efficient solutions for thermal management. Heat interface pads can efficient transfer thermal energy from an LCD to a heat sink.  Heat dissipation gaskets can cover flexible printed circuits to protect sensitive electronic components.  Hybrid metal gaskets can transfer heat from a touch display to a housing or heat sink.  Since gaskets are highly conformable, they can easily eliminate surface irregularities.
    • Gaskets can be used to mount displays, touch screens, optical filters, and complete assemblies. Lightweight and corrosion resistant, they can be laser cut and formed for an precise fit.
  • Hot mirrors
    • Hot mirrors are multilayer dielectric coated substrates (glass/plastic/film) that separate infrared (IR) from the visible part of the spectrum. This protects displays and optical components from solar heat and radiation.  LCD components are very susceptible to high heat and solar energy.  The “blackout” effect can happen quickly resulting in image degradation and in some cases permanent damage to sensitive components.  This can be a critical failure in outdoor applications such as public use terminals, mobile computing devices, industrial controls, and military field applications.
    • Hot mirrors offer high optical transmittance combined with high IR blocking, making it ideal for everything from surgical lasers to fish finders. They can be combined with touch screens, EMI filters, and LCD heaters to survive the most demanding environments.
  • Extended Temperature Displays
    • Designing the right touch display starts with choosing the right LCD. LCDs of every size have a wide variety of features, and every manufacturer has their strengths and weaknesses.  For industrial applications such as agricultural equipment or military field computing, heat and direct sunlight must be taken into consideration. Standard LCD operating temperatures are 0-50 degrees Celsius (0-122F).  That’s fine for the TV on your wall or the monitor on your desk, but not for walking around in the desert or in a hot factory.  Once the display goes beyond that range, you start to get the “black out” effect and lose the image.  In some cases, the damage can be permanent.  Extended temperature displays typically have an operating temperature of 0-85 degrees Celsius (185F), greatly reducing the risk of “black out” for most industrial environments.
    • For our OFM Series Open Frame Monitors we exclusively selected extended temperature LCDs. This allows us to offer sunlight readability without the cost and time of changing the entire assembly.

As you can see, when it comes to keeping things cool, we have more than a few tools in the box.  From half-inch optical filters to mobile computers on the battle field, we design and manufacture products that beat the heat.  Our extensive experience with various industrial markets that share similar environmental concerns, yet have unique application requirements and design cycles makes us a versatile partner for efficient solutions to thermal management in touch screen displays.

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.