The document discusses different types of glass used in touch panels, including soda-lime glass, chemically strengthened glass, alkali-alumina-silicate glass (Gorilla Glass), tempered glass, and sapphire glass. It provides details on the chemical composition and manufacturing processes of each type. The key point is that while alkali-alumina-silicate glass has better scratch resistance due to deeper ion exchange during strengthening, regular chemically strengthened soda-lime glass performs almost as well in break tests if the surface is not scratched.

1. Examining Glass Substrates:
An Insider’s Guide to Stronger & Scratch Resistant Glass
Gary L. Barrett, Chief Technology Officer, Touch International

2. Introduction
One of the biggest benefits to integrating projected capacitive touch technology into
interactive displays is its durability, potentially lasting beyond the use of most devices.
This is because the most common touch surface is now glass, as opposed to flexible
plastic in legacy products. However, while the touch panel may never wear out, the glass
can break, which often ends the touch function. So, making the touch panel less
susceptible to breakage makes the device a better one.
In the pursuit of making better touch panels, touch manufacturers have experimented
with many kinds of glass. Currently, the most common types of glass used in touch
panels include, basic soda-lime, chemically strengthened, and the now very popular
alkali-alumina-silicate glass, often referred to as Gorilla Glass. Tempered glass and
sapphire glass are also occasionally used.
Background
Window glass, used for almost all glass products, is commonly called soda-lime glass or
float glass. It is made of 75% silica (SiO2), soda or sodium carbonate (Na2CO3), and lime
or calcium oxide (CaO), along with some other trace elements. These trace elements
actually affect the color, which you can see from the edge, and vary by the manufacturers
- green in the case of most U.S. manufacturers and blue from the rest (some customers
care about the color of the glass). In a continuous process, these raw ingredients are
mixed and melted. Once liquid, the elements flow over the edge of the melting chamber
and become a long river of hot liquid-glass floating on molten tin where it spreads to the
right thickness and is slowly cooled into flat glass. This process is the reason common
window glass is sometimes called float glass, though the other-chemistry glass we will
discuss can also be termed “float glass”. Because of the difficulty in starting up these
machines, they run continuously for at least 5 years before they are turned off, rebuilt,
and restarted. The image below illustrates the float glass manufacturing process.
Source: Tangram Technology, LTD Fig. 1

3. Testing Glass
Common tests used to classify glass (or plastic) include measuring breakage resistance,
with a drop ball test, pencil hardness, through a scratch test, and light transmission, using
a spectrophotometer. For reference, regular soda-lime glass has a pencil hardness of 7H
and light transmission of 92%.
Breaking Glass
To test break resistance via a drop ball test, a 2” diameter steel ball weighing 0.5Kg is
dropped systematically at varying heights up to 1.3 meters onto the glass below. The
image below illustrates the standard setup for a drop ball test.
Drop Ball Test
Steel
Locking
PVC
Touch Sensor
bonded on
LCD Stand
Source: Touch International Fig. 2

4. Chemical Strengthening
Many years ago, it was discovered that glass could be strengthened by putting the
surfaces in compression. One way of doing this is to replace the sodium ions (from the
sodium carbonate) with potassium ions (usually from potassium nitrate [KnO3]); the
potassium ions will bond with the carbonate (remember valence from chemistry class)
better than sodium, and they are bigger than the sodium ions. In a molten bath of
potassium nitrate, the glass is soaked, allowing the big potassium ions to trade places
with the smaller sodium ions, and when the glass cools, the surface where the exchange
took place, is in compression, and therefore stronger.
The chart below compares the strength of chemically strengthened (CS) float glass to
ordinary float glass.
Source: http://www.xinology.com/en/eg_cpml_temper-chemicalI.HTM Fig. 3
It is important to note that this transition of ions only occurs at the surface of the glass
where the glass and potassium salts connect. This hot-soak process should take 16 hours
at 380°C to reach 90% of the theoretical maximum strength. It is believed that it takes 30
hours to get 100% of the maximum strength. Some companies cheat and only soak for 8
hours, which interrupts the ion exchange, with a corresponding reduction in strength.

5. Tempered Glass
Tempered glass is made by heating glass to around 620°C, usually in a conveyor oven,
and then quickly cooling, often called quenching, the surface, with jets of cold air. This
puts the surface into compression, and the hotter glass underneath into tension. If regular
glass will break at 10,000 PSI, tempered glass will break around 25,000 PSI.
Far more tempered glass is produced each year than chemically strengthened glass and is
found most commonly in glass doors and appliances. It is cheaper than CS glass, and
when it breaks, is usually less hazardous; however, the process often leaves optical
imperfections and will often not work with the thinner glass needed for touch screen
applications. A touch screen mounted onto the backside of a shop window is the most
common place to see a tempered glass touch screen, and occasionally in outdoor security
access or transit applications.
Sapphire Glass
Sapphire Glass, which is nearly pure Aluminum Oxide (Al2O3), provides the ultimate in
scratch resistance. Historically, it was used as watch crystals, and today is often used in
grocery store UPC scanners that have heavy cans dragged over them all day. Sapphire
glass is made by heating the aluminum oxide to melting point and then drawing into a
boule, and when cooled, cut and polished into a window. Due to its high cost and limited
size, it is only occasionally used for special touch applications.
Source: Corning Museum of Glass Fig. 4

6. Alkali-Alumina-Silicate (AAS) Glass
The hot, new “super-glass” is currently Alkali-Alumina-Silicate (AAS), also known as
aluminosilicate or high-ion-exchange glass, which is similar to soda-lime glass, except
that it has more aluminum oxide. In its raw state, it has slightly better light transmission
and slightly better break resistance than regular soda-lime (note the word slightly).
Corning has brought much interest and attention to this glass in the past few years,
spending millions of dollars promoting its break-resistant glass product, Gorilla 2. From
the iPhone 4 to emerging touch screen keyboards, Gorilla Glass is becoming associated
with a host of cutting edge products. In response, the big guns of the glass business,
Schott (Germany) and Asahi (Japan) have released Xensation and Dragon Trail,
respectively. Due to Corning’s slick marketing campaign, however, Gorilla Glass
currently has a strong edge in this market.
Products likely to be scratched, including the smart phone in your pocket, will benefit
from this glass, but at six times the cost of regular glass, expect to pay a premium.
Alkali-Alumina-Silicate glass is made either as fusion glass (Corning) (See Figure 3) or
by the float process (Schott and Asahi) described above. For fusion glass, the molten
alumina-silicate glass falls over the edge of the melting box on two sides, and meets
together below the box and “fuses” into one sheet. Corning uses the fusion process to
make Pyrex (boro-silicate glass), and it has some advantages because there is no residual
tin, which can be a problem for other chemicals, such as the ones used in LCDs. Also of
note, some believe the tin can impede the potassium/sodium ion exchange so the glass is
sometimes polished before chemically strengthening; Asahi does not believe polishing is
necessary and does not recommend it.
The chemical strengthening process is the same for alkali-alumia-silicate glass as it is for
ordinary soda-lime glass. The big difference is that soda-lime and AAS glass surfaces are
not the same. In the case of alumina-silicate, there are bigger holes in the surface, which
allows the potassium ions to go deeper into the glass and thus create a deeper
compression layer. After chemically strengthening, the alumina-silicate becomes Gorilla,
Xensation and Dragon Trail glass.
Here is the kicker: In terms of scratch resistance, and break resistance, there is very little
difference between ordinary chemically strengthened soda-lime glass and Gorilla,
Xensation and Dragon Trail.
So why pay big money for glass with high-ion-exchange? It is a term the glass guys like
to call depth-of-layer (DOL) or thickness, which refers to the ion exchange which is
much deeper in AAS glass. This deep ion exchange allows AAS glass to take a lot of
scratching and still retain its break resistance.
The original iPhone, through the iPhone 3GS, did not use Gorilla Glass. The use of
Gorilla Glass was touted as a big advance in the iPhone 4; however, there were more
reports of breakage on the iPhone 4. The breakage spike was originally attributed to the
high tension level in the ultra-thin new glass, but was later was found to be a design
problem with the phone. It should be noted that all chemically strengthened glass,

7. whether float or AAS, is very vulnerable to cracking if the edge is hit sharply in any way.
The iPhone ushered in the era of flush mount surface (no protective bezel), so greater
care must be given to the exposure of the glass edge.
Summary
Is High-Ion-Exchange Glass worth the higher price tag? Unfortunately, we cannot show
you a comparison chart on the benefits of these high-ion-exchange products, because the
manufacturers consider them proprietary and are subject to non-disclosure agreements.
By observation, and through Touch International’s experience with these products, here
are our findings:
1. High-Ion-Exchange Glass demonstrates much better break resistance after
scratching.
2. Ordinary float glass, which has been chemically strengthened, performs almost as
well in break tests if there is no scratching of the surface.
3. Among the three big brands of AAS glass, the performance is generally about the
same. Differences include depth-of-layer (over-all scratch resistance resulting in
breakage) and performance in the ball drop test (mostly due to time in the chem-
strengthening bath).