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78 The PCB Magazine • March 2014 F e a t u r e by Alun Morgan isola group europe Developments in Glass Yarns and Fabric Constructions Glass fibres are nothing new; the ancient Egyptians reportedly drew coarse fibres from heat softened glass. Modern usage, however, began in the 1930s with the founding of the Owens-Corning Fiberglas Corporation and the granting in 1938 of a patent covering their commercial manufacturing process. Fibres exhibit markedly different properties to those of their bulk parent materials. Glass fi- bres can exhibit tensile strengths up to 5 GPa, which is around 100x higher than that of bulk glass, the difference being attributable to the reduction of the effect of surface defects, the control of which remains an important perfor- mance parameter. The use of woven glass fibres in PCB sub- strates dates back to the 1960s where they were used as a high performance replacement for pa- per reinforcements. Woven glass fibre provided an ideal reinforcement to complement the prop- erties of epoxy resin systems which were being rapidly deployed in electronics by adding high tensile strength and dimensional stability to the composite material forming the substrate. Glass fibres brought not only the properties of high tensile strength and dimensional stabil- ity but also high thermal resistance, good chem- ical resistance, insensitivity to moisture and of course that of being a good electrical insulator. The process of glass fibre production re- mained largely unchanged over 50 years, how- ever there have been a number of recent impor- tant developments that have enabled substrates made with woven glass fabrics to adapt to the changing requirements of circuit design. In particular changes have been necessary to ac- commodate microvia technology requirements, improving CAF (conductive anodic filamenta- tion) requirements and very importantly to extending the usable frequency range of glass reinforced substrates. To understand the developments it is use- ful to review the process by which woven glass fabrics are made. The process begins with a glass formulation which is tailored for a particular application. Glass is formulated mainly of sili-