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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 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)