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80 The PCB Magazine • March 2014 con dioxide (Silica, SiO 2 ), to which varying pro- portions of principally metal oxides are added (Table 1). Glass fibre reinforcements are named ac- cording to the properties imparted by these for- mulations, the letter designation being taken from a characteristic property: E-glass is electri- cal grade and has a low alkali content. It exhibits excellent electrical insulation and low moisture absorption. E-glass accounts for the majority of glass fibre production used for reinforcement. D-glass has a low dielectric constant (Dk), how- ever its mechanical properties are not so good as E- or S-glass. C-glass is chemical glass, having very high chemical resistance and S-glass is a high-strength glass having a much higher ten- sile strength than that of E-glass. This list is not exhaustive and there have been recent variants having similar properties to D-glass which will be discussed later. The glass formulation is melted in a fur- nace and the molten glass is then mechanically drawn into single filaments through small holes in a platinum/rhodium alloy bushing. The fila- ments are next gathered into bundles called strands and are then coiled onto bobbins to form a yarn (Figure 1). During the strand form- ing process a size is applied in order to protect the glass surface to avoid the formation of de- fects that would weaken the fibres. The individual filaments for PCB substrate use are commonly between 5 and 9 microns in diameter and are gathered together into stands of between 204 and 408 filaments to form a yarn, see Table 2. It is interesting to note that 1 square foot of 0.062" glass fibre reinforced PCB laminate contains over 500 miles of individual glass filaments. The yarns have individual des- ignations, shown in Figure 2. table 1: glass composition—main ingredients, by weight%. Figure 1: glass yarn production. DEVELOPMENTS IN GLASS YARNS AND FABRIC CONSTRUCTIONS continues