Glass: A Group Of Familiar Materials With Varying Properties

The knowledge to manufacture glass products is already 4500 – 5000 years old, and glass in its different compounds is one of the most commonly used man-made materials. The material technical definition of glass is loose, but, typically, inorganic products manufactured by fusing and then solidifying without crystallizing can be categorized as glasses. In the electronics industry the use of glasses ranges from parts in different electromechanical modules, such as displays and lighting devices, to resistors and substrates in multi-chip modules and integrated passive components.

The main compound in glass is silica, SiO₂, in its non-crystalline form. Depending on the presence of other compounds, glass can be categorized according to several main types, roughly in the order of rising cost and usability for demanding technical applications (see Table 1). The main features, governed by the additional elements, are also mentioned briefly.

Table 1. Examples of Main Glass Compositions

Type of Glass Properties Soda Lime Contains around 15 % soda (Na2C03) and 5 – 10% lime (CaO). Only moderate tolerance for high temperatures or temperature transients. Softening point (loses its form due to its own weight) in the range of 650 – 700oC. Thermal expansion coefficient 95 x 10-7 1/K, increasing with the temperature as for all glass types. Low cost. Used for household glassware. Lead Typically contains at least 25% PbO. High refractive index. Cannot tolerate high temperatures or temperature transients. Easy to fabricate and machine due to low hardness. Softening point around 400oC. Used for crystalware and art glass, also in some thermometer tubing. High dielectric constant, and therefore used in some electrical applications. Borosilicate Contains at least 5% boron oxide. Example: PYREX®. High temperature and chemical corrosion resistance. Moderate cost. Thermal expansion coefficient around 40 x 10-7 1/K. Used for household oven-resistance dishes, lightning equipment (bulbs), and electronics packaging. Aluminosilicate Still higher temperature and corrosion resistance than borosilicate glass. Used in glass-based resistors. 96% Silica Example: VYCOR®. High temperature resistance, strain point 890oC. Softening point 1530oC. Thermal expansion coefficient 7.5 x 10-7 1/K. Used for space applications. Fused Silica Pure non-crystalline silica. Still higher temperature resistance than 96% silica. Strain point 990oC. Softening point 1585oC. Thermal expansion coefficient 5.5 x 10-7 1/K. Used for space applications, EMC filler for IC chips, and optical fibers.

Note: All of these main categories, except fused silica, also have several glass compositions, and some of them are also mixed (Potash Soda Lead, for example).

Mechanical properties are not significantly affected by the chemical composition. Mechanically glass is nearly perfectly elastic, meaning that it returns to its original state after mechanical loading. This is valid as long as the loading force does not exceed the strength of the material and the temperature is under the glass transformation temperature, where it gradually changes from its solid state into “plastic”. Glass is extremely resistant to compressive and shear stresses, but not to tensile stresses. As commonly known from daily use, any small manufacturing defects, or tiny cracks caused afterwards, can act as stress concentration points, locally increasing the stress and thus decreasing the mechanical strength of the product dramatically.

Thermal conductivity of glasses typically varies between 0.5 – 1.5 W/(m K), the highest being for fused silica and lowest for lead glass. The thermal conductivity measurement is complicated because heat is also transferred internally by radiation. The thermal conductivity is non-linear, increasing with temperature. And, as for any processed material, the exact compound and the processing parameters affect the thermal properties significantly, causing the literature values to vary widely.