Glass fiber

Glass fiber is glass pulled into extremely thin threads. A single filament for electronics is often around 5 to 9 micrometers across, roughly a tenth the width of a hair. Glass that thin is flexible and strong, and bundles of filaments can be twisted into yarn and woven like fabric.

The glass used for circuit boards is called E-glass, the E standing for electrical, because it insulates well. Its recipe:

• Silicon dioxide, about half, the main glass former.
• Calcium oxide, which makes the glass melt and flow more easily.
• Aluminum oxide, which adds strength and durability.
• Boron trioxide, which lowers the melting point further.

The mix is melted in a furnace at around 1,500 °C. The molten glass flows to a bushing, a heated platinum alloy plate with hundreds or thousands of small holes. Streams of glass flow out and are pulled away at high speed, stretching into filaments, which are cooled, coated with a protective sizing and gathered into strands. The yarn is woven into thin, even cloth.

That cloth is the backbone of FR-4 laminate board material and of the package substrate under chips. Without it, the epoxy would be weak and would expand too much when heated.