Book Excerpt: 'The Printed Circuit Designer’s Guide to… High Performance Materials,' Chapter 2

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Evolution of Glass Fabric
Materials such as cotton, paper and glass mat were used before woven glass fiber started mass production and became the dominant fabric used for PCB composites. Early PCBs were low layer count and used a heavy glass weave, such as 7628 or 7629, as the primary component. Since the mid ‘70s, glass manufacturers have introduced new and thinner glass weaves to enable ultra-thin laminate and substrate materials.

Woven glass fabric is impregnated with resin and when fully cured or dried, provides the mechanical, chemical, and electrical properties for rigid printed circuit applications. The characteristics and properties of glass fiber and fabrics are defined by IPC-4412 “Specification for Finished Fabric Woven from E-Glass for Printed Boards.” There are compositions of glass fibers that will also be discussed including the next generation of lower-Dk glass fabrics now in development.

Glass is a mix of several inorganic minerals which are melted in a furnace. Temperatures are maintained above 1000°C so the components can be melted and homogenized. The molten glass is then drawn from platinum bushings to form fibers which are collected to make a yarn which is collected onto a bobbin. These bobbins are then processed into an electronics fabric designed to be used in production of laminate and prepreg. Figure 2.1 is a review of the whole process from raw material to the fiber and process to be woven as glass cloth.

Figure 2.1: Glass fiber/glass cloth manufacturing process. (Source: Taiwan Glass)

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