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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

Can I hurt a touch screen by touching it? – Ben Harris, Cucamonga, CA

Ah Ben, the question we Touch guys have always feared. I assume your question is akin to asking, “Does it hurt a tire if you use it on the road?” However, I think what you really mean is, “Does touching the screen wear it out”?

Well in the early days of touch (1970’s), some touch sensors did wear out when finger oils, ladies’ hand creams and Hawaiian-breezes corroded the thin films. (Hmm, wonder what those creams did to their hands?)

Today, all sensors are advertised to withstand a minimum of 5 million touches. It is even rumored that the president of one unnamed touch company (Microtouch) hired a homeless guy to actually touch the screen that many times. Each of the technologies wear out differently and there is a difference between electrical wear (it does not work anymore) and cosmetic wear (the display is hard to read).

The newest entry into the touch arena is also the most vulnerable to electrical failure. When the in-cell technology fails, it is going to take your display with it, so it’s a double whammy on the checkbook. This happens because you must actually touch and depress the LCD surface itself, which puddles the liquid crystal and scratches the first surface. (Remember your mother telling you to keep your hands off the TV?)

Surface capacitive is the most vulnerable to electrical wear. Touching the same spot multiple times will actually wear through the thin film, down to the glass substrate, rendering it non-linear. Of course, a big scratch from a diamond ring will do-it-in quickly. And bye-bye to SAW technology when that same guy puts a big ‘ol scratch in it too.

In terms of electrical and cosmetic failures, resistive 4-wire touch screens are the most vulnerable. Touching in one place will rub a shiny spot in the hard coat of the top polyester (PET) layer. Touching in this same spot will also wear and crack the top conductive layer and leave it an electrical basket-case. The same number of touches on a 5-wire resistive will not damage it electrically; however the same glossy spot will appear.

In the end, the winners in the wear-me-out contest are projected capacitive (p-cap) and Infra-Red which work without needing to touch anything. Nothing to wear, no electrical surface to scratch and thus these two touch technologies can advertise infinite electrical life.

So Benny, as is the case with that tire, there are 10,000 mile touch screens and 1,000,000 mile touch screens, it just depends.

Until next time,
Touch Guy

I need to use the most dependable touch screen for our new product, which one? – Gerry, Mooselookmequintic, ME

Hey Gerry,

(BTW, is that really your town?), I like questions where I don’t have to give a wishy-washy answer. First, for the neophytes amongst you, there are five commercially viable touch technologies, from a list longer than the number of letters in your city….they are, Resistive (four types), Surface Capacitive (two types), Infrared, Surface Acoustic Wave (SAW), and Projective Capacitive or, as we affectionately call it, P-Cap, and you know as iPhone-type. (Those of you with corner-cameras, fibers on the edge, and in-cell systems, come back when you have real sales.)

Before I give you the answer, as the cognoscenti amongst you know, any touch technology will work for 80% of the applications, so those of you with the projects that need a specific type, dependability may not be the best question to ask. But the answer to which touch screen is most likely to be running in the morning, is, TA DA, P-Cap.

Why is it that Touch International has built millions of P-Cap touch screens over the last four years and not one has come back for non operation? The reason is that projective technology has nothing to wear out, the place you touch is just glass, which has a 9H scratch resistance (yeah, sapphire glass is better, but you can’t afford it), and the electronics are simple. With p-cap, the sensors are typically made the same way the safety glass windows in your car were made, so it is tough to break, which gives this technology the edge on the runner-up in the dependability category, which is SAW. The electronics are simple and minimal, which gives p-Cap a bigger edge over the third place (dependable touch) technology, which is infrared and its dozens of expensive emitters and phototransistors.

Touch International makes p-cap sensors from 1.8 inch to 22 inch, so, if the most dependable touch technology is your first priority, Gerry, my man, you know how to answer your cranky boss…just tell him P-Cap.

Until next time,
Touch Guy