Types of Glass in TFT LCD Displays

Welcome at Riverdi University. In this lecture we’ll talk about different kinds of glass in TFT LCD displays and surfaces that we use to protect displays, or we can use to protect with the glass the entire devices

We will talk about different types of glass in TFT LCD displays, then the surface treatments, what we do to achieve different parameters of glass surfaces, about the hardness – important when we want to protect something, then about painting the glass, how we do it and what we can achieve, IK rate, how much mechanical impact we can place on the glass, and will it withstand this still and at the end about laminated glass, why we laminate glass and what we can achieve by doing that.

Why do we even talk about glass in TFT LCD displays?

The most important thing with the glass in TFT LCD displays is to protect the display, but not only. As you can see on the pictures above, glass is an element of the design of the devices. It makes devices look better and can be designed in a way that protects not only the display, but the entire surface of a device, like for example for the coffee machine on the picture above, where we have a display with some additional graphic that covers the whole front of the device. Glass is one of the best materials that we use in electronics to protect screens, because it is very hard and it is hard to scratch. It is mechanically strong, cheap, and exceptionally good in optics. For glass, the transparency rate is typically more than 90% or even 95% percent. It is widely available, we know a lot of techniques how to manufacture it and how to prepare it for some special advanced designs as we can change the shape of glass quite easily nowadays.

Now we will talk about types of glass that we use to protect screens and devices. Mainly we use two types of glass in TFT LCD displays, one is chemically strengthened glass, that we call CS type glass, the other is thermally tempered glass, hardened glass where we use hot temperature to make it stronger. For our standard products we typically use on the touch screens chemically strengthened glass. Our standard thickness is 1.1-millimeter thickness. This kind of glass is pretty strong, comparing to the regular glass. Chemical strengthening means that we treat the surface with ions, usually silver ions. We increase the strength of the surface of the glass because glass usually breaks when the surface breaks. We do not change the glass internally with chemical strengthening, we just change the surface hardness, and it is enough to make the glass much stronger.

As you can see in the table above, with chemical strengthening we can make glass even 6 or 8 times mechanically stronger than the regular one. This is a very long process; it can take several hours, and we need hot temperature, 400 or more degrees. Thermally tempered glass is a separate way of strengthening glass. We use hot temperature and very fast cooling to make the glass stronger. We need a higher temperature, 700 degrees in this process, but it is much faster, it takes just several minutes, and we achieve strong glass, 4 to 5 times stronger than regular float glass. Thermally tempered glass is not as strong as chemically strengthened glass. It is cheaper, but we cannot use it for thin glass. The thinnest glass that we can thermally temper is 3-4 millimeter. If the glass is thinner, with hot temperature it starts floating and the surface will not be flat again. So, if we have a thick glass, it would be cheaper to use the thermally tempered solution. That is why it is more popular. For thinner glass we use chemical strengthening, because we cannot use the thermally tempered solution.

Now we will talk about the other difference between these two methods of strengthening glass. On the left side of the picture above, you can see chemically strengthened glass broken, and on the right side there is thermally tempered glass broken. Chemically strengthened glass breaks like regular glass because we do not change the internal part of the glass. We only make the surface stronger, but inside the glass is the same as regular float glass, and it breaks just like it. Thermally tempered glass changes the internal structure of the glass and it breaks into very small pieces. In many cases it is better because it is safer for humans, that is why we normally use thermally tempered glass in cars or in places where broken glass may injure people.

Another property or type of glass that we will talk about is Optiwhite and Float. Float is the most common glass that we use in architecture designs, but also in many touchscreens. The float glass is the most common, most popular and the cheapest, but sometimes we have specific requirements. We sometimes need to have very good color reproduction, especially light colors, white color. Then we use glass called Optiwhite. To achieve that we need to remove the iron from the glass. Float glass has a little bit of iron which makes it green or greenish. If we look straight through the glass, we may not see that but if we look like from an angle, we can see the green color. If we put a white background, we will also see this greenish color a little bit. So, if there are specific requirements, we use Optiwhite, it is especially worth considering if you have a white background. Usually, the Optiwhite is a little bit more expensive, so it is worth checking with the manufacturer of the display what we can use in our case.

Now we know how glass is made, how it is being strengthened, how it breaks and what types of glass, Float and Optiwhite, we have. To continue, we will talk about surface treatments other than strengthening. The other treatments that we use are anti-glare, anti-fingerprint, anti-reflective and anti-bacterial. About anti-reflective treatment we have talked in another video about the contrast and brightness of the outdoor LCD displays. In that clip l was demonstrating how significantly the anti-reflective surface can increase contrast. Anti-glare is, like the name says, a little bit blurring the image, but the light that is reflected is not like from the mirror, it is not glare anymore.

On the picture above there are examples of glass. One of them is a little bit blurry, it is anti-glare and the other one is clear – it is anti-reflective. In the past, anti-glare glass was more popular and used in some commercial devices, but later manufacturers have found that devices with anti-glare are being sold less frequently than the glare ones. It is because as humans we think that there is something wrong with a little bit blurry image even if the reflections are lower. When we are in a shop and looking at phones, we do not see the image clearly and we think that there is something wrong and we do not want this device. That is why we do not see any more anti-glare glass in consumer products. Everything is glare in consumer products, it could be anti-reflective or could be only regular without any surface treatment. But in the professional market that we are working on, like medical devices, military devices, we have many projects where we use anti-glare and anti-reflective treatments, both solutions to reduce reflections and increase contrast.

Next, we have anti-fingerprint treatment. All of you have this on your phones and you know how it works. It works better or worse, usually worse, so we can see a lot of fingers especially when anti-fingerprint is connected with anti-reflective treatment. On the anti-glare we do not see the fingerprints because it is blurry. So, both solutions have different advantages and disadvantages. As the last treatment we have antibacterial surfaces. There are even surfaces that are anti-Covid tested. We can use them in some special cases, for medical devices or public purpose devices. This is an important and growing market, because we have more and more screens in public places, for example in McDonald’s or other places with automated cash register machines where we touch the screens and exchange bacteria with other people. In such cases it is really important to have antibacterial surface treatments, that is why we offer it, usually for bigger screens for public solutions.

Now let us talk about hardness of glass in TFT LCD displays. Of course, to talk about hardness we need to measure it. For that we have the Mohs scale where we have 11 different levels of hardness. Like you see on the picture above, the 10^th  is diamond and the 1^st  is talk. What we normally use is glass with hardness between 5 and 7. In some cases we also use Gorilla glass with hardness 9. It is used on our phones or tablets. As you can see, we can achieve hardness 7 with chemically strengthened glass and usually 6 with thermally strengthened glass. Gorilla glass is also chemically strengthened glass, patented by the Corning company and it is the strongest that we can achieve in the cover glass to protect the screen.

This scale is about surface hardness – how hard is it to scratch the surface. As you know, even glass with hardness 9 can be scratched, everybody has some scratches on their phone because this hard layer is very thin – 10 micrometers only. If we put enough force and break this barrier, then we have soft glass with hardness 6 or even lower, that is why we have the scratches.

What is TFT LCD grayscale inversion

A couple more words about Gorilla glass. Now there is the sixth generation of Gorilla Glass on the market. The goal for Corning company and Gorilla Glass is to make the glass as strong and as light as possible, because most of the cases are handheld devices, where we want the glass to be light, that is why we want to make it very thin. We have also other companies that are making equivalents of Gorilla Glass, like Dragontrail from AGC or Xensation from Shott. They are not so popular but in many mobile phones or tablets on the market you can find these types of glass.

Now let us talk about the painting. We know the types of glass that we use in TFT LCD displays, we know how to make the glass stronger, we know the surface treatments, how to make the glass less reflective or anti-fingerprint or antibacterial, but it is not enough because glass will only be transparent. If we want to cover it, we need to paint it. Typically, we paint glass with the technique called Screen Printing. It is the most popular, cheapest and fastest technique.

When we do the Screen Printing, we need a screen for each color, so to minimize cost, we try to reduce the number of colors to 2–4, like the background and the colored logo. Each color is a different process, we need to wait until the previous painting dries and then we need to put another screen and print another color. More colors mean a longer process and of course a higher cost. Of course, we can change the shape of the glass, we can make rounded corners or custom design of the glass, but it is expensive because first it is just the rectangular piece, then you need to go to the CNC machine to make the proper shape of the glass.

The other printing technique we use nowadays is Jet Printing. It is like a regular printer that you use in your office. You can print every color, even paintings. This process is long, and it is expensive, so it is reserved for special applications, but we can do it if it is necessary.

Now we will talk about mechanical impact protection. It is different than the surface hardness we talked about before. On the picture above, we have the test and scale to measure the mechanical strength of glass, that means how much energy we can put on the glass before it breaks. It is measured in IK rate. IK rate is a scale where we have different levels and different energy that will boost. For example, if we want to test IK 9, we need to take 5-kilogram mass from 200-millimeter height. The mass is kept above the tested glass using an electromagnet, then we just drop it, and we see if it breaks or not. If not, of course the test is passed.

If the glass has not passed the test, we can try to change the glass type from thermally tempered to chemically strengthened or go to a thicker glass.

The last point in this article is laminated glass. We laminate glass because of a few reasons. First, what is laminated glass. Laminated glass is like putting the film inside two glass sheets. This process is expensive, we need pressure, we need temperature, we need time, and we need an exceptionally clean environment, because when we laminate together two sheets of glass, we need to be sure that no particles get inside. This kind of process needs to be done in a Clean Room, so it is expensive, but as you see on the picture above, even if the glass is broken, it still holds up because of the laminated film inside.

We laminate glass mainly because of two reasons. One is mechanical strength and impact. We use it even in our homes. Many windows used nowadays are anti-vandal and that means they are laminated glass, and they are extraordinarily strong. The other reason to laminate glass is to put a film inside with some properties, usually to block the UV or IR light. IR means infrared so heat and UV means ultraviolet, short wavelength, extremely dangerous for electronics. When we have an outdoor application, some customers want to protect the displays, touchscreens or the e-paper displays also against UV. Then we use laminated glass and as you can see on the chart above the IR cut film and UV cut film are both transparent for visible light. We can see everything through them, but what is higher and what is lower is cut by UV and IR films. Most often we use only UV cut film because UV is more dangerous, for example it makes the film sensors for capacitive touchscreens turn yellow or it can decrease the contrast of the TFT (Thin Film Transistor) display by damaging the polarizer or color filters. The IR film is used in some applications to protect the display from heat. If we add it, we can decrease the temperature of the display surface. In another video we were talking about High-TN, so liquid crystals that can work in very high temperatures. For this kind of liquid crystals, we usually do not need to decrease the temperature of the surface because they can go up to 100 or 110 degrees, but regular displays can work up to 50- or 70-degrees maximum temperature. Using the IR cut film can solve the problem with blackening and increasing the display temperature too much.