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Hey Touch Guy, I’m working on a sort of digital signage / kiosk type application that will hopefully be everywhere from hotel lobbies to office buildings. The plan is to start out around 32” and eventually grow to 60”. I’m thinking I want Projected Capacitive for the touch screens for all the usual reasons , but people are telling me optical touch is the way to go due to size. So who’s right and what’s the best? PCAP or optical? -The Sign Guy

Hey Sign Guy,

The big ones, usually defined as over 32 inch LCD’s, have long been the domain of optical systems with an occasional sighting of an acoustic (external and internal noise generator) or surface capacitive sensor, as well as a smattering of the other touch technologies. Optical is defined as IR beam or corner camera type. Recent advances in optical touch systems have added the ability to do multi-touch which had been one of the drawbacks. Frankly sign guy, optical is your best choice today. But do not sign any long term commitments. The market wants p-cap with pen for large format displays. Why, you inquire? The same reasons the small display guys want it….low cost, high reliability/durability, multi-touch, gestures, and good image quality – in addition to the twelve other things people like about the technology. The holy-grail of big formats seems to be white-boards (think every school room and meeting chamber) as opposed to the slow growing digital signage market (for touch displays). But, as Touch Guy has pointed out, p-cap has changed the sleepy old days of resistive touch manufacturing to major advances every six months…..that 82 inch, $35,000 p-cap touch panel you saw at the trade show last year will be in your budget within 12 months. Already 32 inch p-cap is common and cheap, due to advances in the materials, IC’s, and manufacturing techniques. Passive pen is not quite ready for the big ’uns yet, but when it is, Touch Guy advises you buckle your seat belt.

Until next time,
-Touch Guy

What is the difference between liquid optical bonding and dry optical bonding?

Mr. Inquisitive,

At last, a question testing Touch Guy’s expertise. Touch International is an expert in all of the ways to assemble and laminate p-cap touch sensors including OCA or optically clear adhesive using pressure and heat, DFA or dry film adhesive using vacuum and heat, OCR or optically clear resin, using heat and UV radiation, and two-part epoxy, using chemical cross-linking.

Optical bonding, however, is a process which attaches the touch panel (or other filter or overlay) directly to the LCD, filling the small air gap between the front of the display and the back of the touch panel. The reason one chooses an optical bond is to make it easier to read the display through the overlay, especially in sunlight, and most importantly to make the touch screen/LCD assembly more rugged, sometimes called the Army boot-kick test. Touch guy only does liquid optical bonding, so he had to seek expertise of others to discuss the advantages and disadvantages of dry optical bonding.

There are three common ways to do optical bonding which include a two-part Epoxy using a long heat cure, OCR using UV radiation, and dry optical bonding using heat and pressure. The names for the optical bonding methods are not the same processes for same names used for the optical laminations described above; dry bonding is the same method as OCA lamination, and OCR optical bonding is unlike the OCR lamination process.

Now that you have read all of that as background, the answer is that, if you get a properly manufactured optically bonded assembly, there is almost no difference between the three methods. Almost all of the differences have to do with the manufacturer which does the optical bonding for you. The differences are in cost of equipment, manufacturing time, space requirements in the factory, skill levels (none of the methods are easy to do), cost of materials, damage to LCD, design of overlay, and the ability to rework parts with blemishes.

Here is a chart which shows the steps for liquid optical bonding (OCR) and dry optical bonding (OCA):

optical bonding process

In addition to the optical bonding line in the US, TI has recently opened its new China-based clean room line for optical bonding consumer products.

Throw some more questions at me! I’ll be happy to answer any more, until then…
Touch Guy

I’m mounting a projected capacitive touch screen to a 17” monitor and am trying to find out how much of a gap I need to leave between the touch screen and LCD glass. Is the gap dependent upon the type of touch screen used? Also, do you have any suggestions for how to mount it to the LCD or bezel?

Dear Victor:

As they say on the London Underground, “Mind the Gap”.  The separation between the back of the touch sensor and the front of the LCD is important for two reasons—optical and electrical.

In general, one wants the touch sensor to be as close to the display as practical, so that there is no pronounced parallax error.  Parallax occurs when the finger or probe comes in at an angle and is detected “too early”, and thus misses the actual target — such as touching a “z” on the on-screen keyboard, instead of the intended “a”. This is most pronounced in older style infra-red touch technology, because the LED’s are relatively high above the display, but can be a problem with any touch sensor located too far from the display.

But putting the touch panel too close can cause problems if you are able to deflect the touch screen so that it hits the LCD. Your 17” touch panel will probably not have this problem because larger touch screens have a rigid back layer that will not easily bend.  Smaller, thinner, touch panels, however, can be pushed into the LCD and a rainbow (moiré pattern) can occur, which is not good for the life of the LCD.  In a few cases, for LCD’s without a metal frame, you can actually put the touch screen on the LCD, and the moiré pattern will be annoyingly permanent.

The LCD and backlight can also create enough electrical noise to slow or stop the operation of the sensor, so there needs to be a gap between the LCD and the touch sensor.  The required gap size, however, is not a constant.  The needed separation varies with the LCD model and touch technology; those lightning-fast tiny switching transistors do emit some electrical noise, believe it or not.  For projective capacitive technology, the necessary gap also varies with the IC manufacturer.

So, to mind your gap, a separation (from the surface of the LCD) should be the thickness of the metal frame plus 10/1000’s, which is usually the thickness of the adhesive that holds the touch screen to the LCD.  Sometimes, you will be tricked by a particularly noisy LCD and need a larger gap, also handled by better tuning of the electronics, but 40/1000’s (bezel plus adhesive), as  a minimum, will usually work.

Ta ta,

Touch Guy

Do you have a question for Touch Guy? Send him an e-mail at asktouchguy@touchintl.com.