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I’m working on an open source 17″ tablet for the DIY 3D printing community and am in desperate need of a 17″ – 17.1″ USB multi-touch screen overlay with a 16:9 or 16:10 aspect ratio. So far, I am only coming across single touch resistive and IR multi touch panels. We only really need dual touch panels with as good light transparency as possible, as these are graphics editing machines we are making. Projective capacitive panels would be the ideal technology I assume. How can I find such a beast?

So you want a 17 inch projected capacitive touch screen and are having a tough time finding one? You have chosen wisely, because for the printing business, you will want the super sharp image that you get using this optically clear technology.

Even though Touch International makes projected capacitive sensors up to 22 inches, I have to tell you, you are on the cutting edge of wanting the “big guns” for projected capacitive touch screens. Except under special request, all of our projected capacitive panels have to have a perfect image (no shadows), they must do multi-touch (two or more fingers, no ghosting), and they have to have a touch response of less than a 1/10th of a second (among other things). Because of these requirements, we are not always the fastest to market, but we’re sure to deliver some of the best touch screens around.

There are two key reasons that larger-sized projected capacitive is difficult to do. The first is that no-shadow touch screens require very precise manufacturing equipment of the type used to make semi-conductors or TFT displays. Most commonly available are machines that will process projected capacitive sensor sizes up to 15 inches, which is why that size is easy to find.  You may know that Touch International has just opened its new China factory, and can process up to 17 inch sensors, so you are in luck (pretty soon).  We can (and do) make 19 inch, and larger, projected capacitive panels using other methods, but they break the visible line requirement, so they will not work for you.

The second complication to making big projected capacitive panels is the speed at which the multiple touches are recognized, which is dictated by the touch electronics. As I am sure you know, having read my white papers on the subject, projected capacitive is a scanned system which means that every row and column must be “energized”, and then interpolated to get an overall resolution of 1024 by 1024, which is a lot of work for those little silicon buggers.  As the screens get bigger, there are more rows and columns to move into action. This requires multiplexing, or, most often, it takes multiple fancy projected capacitive ASICs working together in a master/slave relationship. A single ASIC, which will handle 17 inch screens, has just hit the market (two touch maximum), so, once again, you are just in time.

So, we may be your source, but you will need to hold your breath a little longer – maybe until Q2.

Touch Guy

I stepped on my fancy iPhone 4 and whole screen is completely cracked! I can barely see the images, but somehow it still works! How is this possible? – Astonished Jim, Little Rock, Arkansas

Dear Astonished Jim in Arkansas,

Touch guy is about to go on vacation, so thank you for an easy question! As I’m sure you know, the iPhone uses projected capacitive technology and one of the main benefits of this technology is that it will “last forever.” Touch Guy did not really mean that you could smash it, or in your case, step on it, and it would still work. But, wow, maybe you can…

So what gives? First, I want you to know that there are several ways to make projected capacitive touch sensors, but all of these methods involve putting the transparent sensing conductors, called ITO (Indium Tin Oxide), behind a protective front layer (I call this layer a lens). The lens on an iPhone is made of a thin layer of glass – you can look in the on/off hole and see how thin this glass is. Behind it is the touch sensor, which is a thicker (0.5mm) layer of glass, and is optically bonded to the thin lens. I say “optically bonded” because there is a continuous glue layer in-between that eliminates the air gap between the touch panel and the lens and is part of the reason the display looks so good.

There are a lot of ways to bond the layers together; the material used is generally known as “OCA” (you guessed it, Optically Clear Adhesive). All automotive windows are two glass layers optically bonded together, and the internal OCA, along with tempering, is what makes the glass “safety glass” with all of its protective properties. When you get a crack in your car window from “that darn rock”, only the outer layer of glass usually cracks. So Jim, that is your answer – when you klutzed the iPhone, you cracked the outer layer of thin glass, but the touch sensing layer remained intact and worked just fine through the broken lens.

But I know you want to know more! Touch International makes all plastic projected capacitive sensors which will not break (think hand-held games for children). They also use chemically strengthened glass for the lens and touch sensor to make it harder to break. There is also another type of cover glass, alumina silicate, which can increase the strength if the lens is likely to get scratched, by, say, the keys in your pocket.

Astonished-Jim-with-the-cracked screen will probably enjoy this story: the iPhone introduction was delayed for months, because the first models of the product had an unbreakable plastic projected capacitive screen. Of course, the big man himself (if you don’t know who this is then tough luck), got the first prototype. By the end of the third week, the junk in his pockets had scratched the lens, making him very, very unhappy. So the lens maker, having invested quite a tidy sum in a plastic lens making facility, had to start over with glass.

We hear that around 25,000 smart phone lenses break each month, so you are not alone. Replacement sensors are available and there you tube tutorials on how to replace them (not easy).

Whew! That was a long answer! If you still want to know more, download the Touch International white paper “Projected Capacitive Touch Screens” – it has a lot of good stuff in it!

And Touch Guy is off! (And planning to opt-out of being X-rayed at the airport)

What adhesives do you recommend for bonding the touch screen to the LCD? – Scott

Dear Scott:

Touch Guy could simply say to use any VHB adhesive (VHB stands for very high bond), most commonly 3M 4956 mm, but it is not his style to be anything less than verbose. Touch Guy is usually not negative either, but he needs to be this time. There are two big no-no’s… neoprene and common RTV Silicone. This is because both of them are acid producing (sulfuric acid from neoprene and glacial acetic acid [vinegar] from the RTV). Those acid molecules will jump out and eat away at your electronics and your touch screen thin films. You can use a water based RTV silicone, but the cure time is much longer. And, while you are at it, resist the temptation to make a watertight seal between your plastic enclosure and your resistive touch top screen. This is because plastic (usually PMMA) excursion under temperature is usually more than the touch surface, which may leave you with a puffy topsheet.

Where was I… oh, back to that VHB that you asked about. You should use 1.6 mil (mm) thick adhesive, because even the most noise resistant touch technology (resistive) benefits from being away from those busy-beaver transistors in the display and it will reduce “puddling” (as is the case with “puffy”, a very technical term) from heavy touchers. Most companies provide strips of VHB, but many of Touch International’s touch screens come with a continuous die-cut gasket which gives you an elegant dust and water resistant seal.

The VHB has a shelf-life and will begin to activate (grow stronger) over time and pressure, so don’t use old stuff. Once aligned to the LCD, you will have a few seconds to easily pull off the touch panel and reposition it if you make a mistake. Most important of all, you really need to do this in a clean-room because the smallest dust particle in the gap will be grounds for your customer to send product back.

Don’t forget, Touch Guy can optically bond your touch panel for better optics and more rugged performance.

So, Scott, stick away!!!

Until next time,
Touch Guy