Displays, Control Board

Introduction to Cover Glass for Displays

Cover Glass (Cover Lens) is primarily used as the outermost layer of touch screens. The main raw material for these products is ultra-thin flat glass, which offers features such as impact resistance, scratch resistance, oil and fingerprint resistance, and enhanced light transmittance. It is currently widely used in various electronic consumer products with touch and display functionalities.

1. Classification of Glass

a. Soda-lime glass: Primarily composed of SiO₂, with additional content of 15% Na₂O and 16% CaO.
b. Aluminosilicate glass: Mainly composed of SiO₂ and Al₂O₃.
c. Quartz glass: Contains more than 99.5% SiO₂.
d. High-silica glass: Contains approximately 96% SiO₂.
e. Lead-silicate glass: Mainly composed of SiO₂ and PbO.
f. Borosilicate glass: Primarily made up of SiO₂ and B₂O₃.
g. Phosphate glass: Mainly composed of phosphorus pentoxide (P₂O₅).

Types c through g are rarely used in displays, so they will not be discussed here.

2. Processing Techniques for Glass Raw Materials

a. Float Glass

Float glass is produced using raw materials such as sea sand, quartz sandstone powder, soda ash, and dolomite. These materials are mixed and melted at high temperatures in a furnace. The molten glass continuously flows from the furnace and floats on the surface of a molten metal bath, forming a uniformly thick, flat glass ribbon that is flame-polished. After cooling and hardening, the glass separates from the molten metal, and it is then annealed and cut to create transparent, colorless flat glass. The forming process of float glass is completed in a tin bath with protective gas, resulting in a distinction between the tin side and the air side of the glass.

b. Overflow Process:

In the overflow process, molten glass enters the overflow channel from the feeder section and flows downward along the surface of a long overflow trough. The glass converges at the bottom tip of a wedge-shaped body under the overflow trough, forming a glass ribbon. After annealing, this process creates flat glass. This method is currently a popular technique for manufacturing ultra-thin cover glass, offering high processing yield, good quality, and overall excellent performance. Unlike float glass, overflow glass does not have a tin side or an air side.

3. Introduction to Soda-Lime Glass

a. Also known as soda glass (English: soda-lime glass), it is processed using the float method, hence also called float glass. Due to the presence of a small amount of iron ions, the glass appears green when viewed from the side, and is therefore also referred to as green glass.

b. Thickness of Soda-Lime Glass: 0.3–10.0 mm

c. Brands of Soda-Lime Glass:

  • Japanese brands: Asahi Glass Co. (AGC), Nippon Sheet Glass Co. (NSG), Central Glass (CENTRAL), etc.
  • Chinese brands: CSG Holding, Xinyi Glass, Luoyang Glass, AVIC Sanxin, Jinjing Group, etc.
  • Taiwanese brand: Taiwan Glass (TGC).

4. Introduction to High Aluminosilicate Glass (High Alumina Glass)

a. Brands of High Alumina GlassUnited States: Corning Gorilla Glass, an eco-friendly aluminosilicate glass produced by Corning Incorporated.Japan: Dragontrail Glass, produced by AGC Inc. This glass is commonly referred to as “Dragontrail Glass.”China: Panda Glass, produced by Xuhong Company, is a high alumina glass. Other manufacturers include CSG Holding and Kibing Group.

b. Cover Glass ProcessingCompanies involved in processing cover glass include Lens Technology, Boen Optics, Shenzhen Xinhao, G-Tech Optoelectronics, Jiangxi Firstar , BYD, and others.

5. Chemical Strengthening of Glass

a. Principle:

The glass is immersed in a molten salt bath (KNO₃). The high concentration of K⁺ ions penetrates the glass surface and replaces the Na⁺ ions within the glass. Since the ionic radius of K⁺ is larger than that of Na⁺, this substitution increases the surface density of the glass, generating compressive stress on the surface. This process enhances the glass’s strength through chemical reinforcement.

 

b. Test Items for Chemical Strengthening

Depth of Layer (DOL): Indicates the depth of the stress layer after the glass has been strengthened.

Compressive Stress (CS): Represents the surface compressive stress of the chemically strengthened glass.

Surface Hardness: Evaluated using a pencil hardness test.

Drop Ball Test: A destructive test to assess the glass’s impact resistance.

Note:

  1. Based on our project experience, we recommend the following:

    a. Use 1.1 mm thick glass for IK04.

    b. Use 1.8 mm thick glass for IK06.

    c. Use 3.0 mm thick glass for IK08.

    d. Use 6.0 mm thick glass for IK10.

  2. Physically tempered glass is mainly recommended when safety is a priority for the customer. This is because, when broken, physically tempered glass shatters into small granular pieces, unlike chemically tempered glass, which can break into sharp shards, posing a safety hazard.
  3. For chemically strengthened glass, to enhance safety, optical bonding or applying an anti-shatter film to the surface can prevent glass fragments from scattering upon breakage.

6. Production Process Flow for Glass Cover Lens

Cutting → CNC (shaping, drilling, edging, and chamfering) → Ultrasonic Cleaning → Chemical Strengthening → Ultrasonic Cleaning → Full Inspection of Blank Glass → Screen Printing → Baking → Full Inspection of Glass → Ultrasonic Cleaning → Surface AR Coating → AF Anti-Fingerprint Coating → Full Inspection of Glass → Film Coating and Packaging.

Key steps are explained as follows:

a. Cutting

The original glass sheet is cut with a diamond wheel cutter and then snapped into rectangular pieces that are 20-30 mm larger on each side than the final product dimensions.

b. CNC (Shaping, Drilling, Edging, and Chamfering)

Using high-hardness diamond grinding wheels rotating at high speed, the glass substrate undergoes mechanical grinding under excellent cooling and lubrication conditions to achieve the desired structural dimensions. Different tool shapes and grit sizes are designed to meet various processing requirements.

c. Chemical Strengthening

At high temperatures, an ion exchange occurs between the glass and KNO₃, where ions from KNO₃ replace the ions in the glass. Due to the larger atomic radius of the replacement ions, the surface of the glass undergoes compressive stress after tempering. When the glass is subjected to external force, this compressive layer can offset some of the tensile stress, preventing the glass from breaking. This compressive stress increases the glass’s resistance to bending and impact. Factors affecting the strength performance of chemically tempered glass (such as drop ball tests and four-point bending tests) include: 1) Tempering performance indicators of the glass (DOL, CS); 2) Internal and surface defects of the glass (micro-cracks and scratches); 3) Edge chipping and hidden damage formed during CNC processing; 4) Inherent defects in the glass raw material (impurities in the raw material, uneven areas, air bubbles, and inclusions, which are uncontrollable factors).

d. Polishing

The glass material is ground and polished using a double-sided grinder equipped with polishing pads and polishing powder. This process removes surface impurities and micro-cracks, enhancing the glass’s surface smoothness and reducing roughness. The main component of the polishing powder is cerium oxide. Cerium oxide polishing powder particles are polygonal with distinct edges, having an average diameter of about 2 microns and a hardness of Mohs 7-8. The particle size and purity of cerium oxide polishing powder directly affect the polishing outcome.

e. Ultrasonic Cleaning

When high-frequency vibrations (28–40 kHz) are transmitted to the cleaning medium, the liquid medium generates nearly vacuum-like cavitation bubbles. As these bubbles collide, merge, and dissipate, they create localized pressure bursts of several thousand atmospheres within the liquid. Such high pressure causes surrounding materials to undergo various physical and chemical changes, a process known as “cavitation.” Cavitation can break chemical bonds in material molecules, leading to physical changes (dissolution, adsorption, emulsification, dispersion) and chemical changes (oxidation, reduction, decomposition, synthesis), effectively removing contaminants and cleaning the product.

f. Printing

The principle of printing involves creating a stencil using photosensitive materials. Ink is placed in the screen frame, and a squeegee applies pressure to push the ink through the screen mesh openings onto the substrate, forming patterns and text identical to the original design.

g. Coating

Under vacuum conditions (10⁻³ Pa), an electron gun emits a high-speed electron beam to bombard and heat the coating material, causing it to evaporate and deposit onto the substrate surface, forming a thin film. Coating equipment primarily consists of a vacuum system, an evaporation system, and a film thickness monitoring system. Common coatings include functional films like AF (anti-fingerprint), AR (anti-reflective), AG (anti-glare), high-hardness films, decorative films such as NCVM (Non-Conductive Vacuum Metallization), and iridescent films.

7. IK Rating

IK ratings are an international classification that indicate the degree of protection provided by electrical enclosures against external mechanical impacts.

IK ratings are defined as IK00 to IK10. The IK rating scale identifies the ability of an enclosure to resist impact energy levels measured in joules (J) in accordance with IEC 62262 (2002).

IEC 62262 specifies how the enclosure must be mounted for testing, the atmospheric conditions required, the quantity and distribution of the test impacts and the impact hammer to be used for each level of IK rating. The test is carried out by a Charpy pendulum impact tester.

IK00 Not protected

IK01 Protected against 0.14 joules impact.
Equivalent to impact of 0.25 kg mass dropped from 56 mm above impacted surface.

IK02 Protected against 0.2 joules impact.
Equivalent to impact of 0.25 kg mass dropped from 80 mm above impacted surface.

IK03 Protected against 0.35 joules impact.
Equivalent to impact of 0.25 kg mass dropped from 140 mm above impacted surface.

IK04 Protected against 0.5 joules impact.
Equivalent to impact of 0.25 kg mass dropped from 200 mm above impacted surface.

IK05 Protected against 0.7 joules impact.
Equivalent to impact of 0.25 kg mass dropped from 280 mm above impacted surface.

IK06 Protected against 1 joules impact.
Equivalent to impact of 0.25 kg mass dropped from 400 mm above impacted surface.

IK07 Protected against 2 joules impact.
Equivalent to impact of 0.5 kg mass dropped from 400 mm above impacted surface.

IK08 Protected against 5 joules impact.
Equivalent to impact of 1.7 kg mass dropped from 300 mm above impacted surface.

IK09 Protected against 10 joules impact.
Equivalent to impact of 5 kg mass dropped from 200 mm above impacted surface.

IK10 Protected against 20 joules impact.
Equivalent to impact of 5 kg mass dropped from 400 mm above impacted surface.

 

If you have any questions about Display Cover Glass, please contact Orient Display support engineers

 

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