Durability and safety considerations typically require the glass to be strengthened (hardened), using either chemical or thermal processes. Chemical hardening is possible with all glass, while thermal hardening is typically only available for thicknesses over about three millimeters.
Of the two approaches, chemical hardening can make surfaces more scratch resistant, but it only affects the outer surfaces. Thermal hardening, on the other hand, hardens the material throughout, including below the surface. From a safety perspective, thermally strengthened (tempered) glass shatters into smaller pieces with less-sharp edges than untreated or chemically strengthened glass. The type of strengthening used on the glass also affects the types of inks that can be used to print on it.
-- Chemically hardened glass: organic inks are typical, which offer a wider choice of hues compared to ceramic color but less ultraviolet resistance.
-- Thermally hardened glass: requires ceramic color, which is superior in terms of scratch resistance, but because printing is done on the back side of the cover lens, that resistance is only relevant during manufacturing.
Note: While thermal tempering typically requires a minimum thickness of ~3mm, specialized processes enable Avnet to be one of the few suppliers that can offer ceramic colors on glass with thicknesses down to 1.8mm.
Ion-exchange strengthened glass
An option if your device needs a particularly robust cover lens, ion-exchange strengthened (alumino-silicate) glass is created using a chemical process that exchanges the native sodium ions in soda-lime glass for larger potassium ions. The result of that process is denser and more durable glass than other options, enabling lightweight, slim device form factors, especially with bezel-less designs.
This is the technology behind commercial offerings such as Gorilla Glass (made by Corning), Dragontail (made by Asahi) and Xensation (made by Schott). Relatively high cost and minimum order quantity (MOQ) associated with alumino-silicate glass, as well as a maximum thickness of 3mm, have made it less broadly used in the industry than other options for cover lenses.
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