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82 The PCB Magazine • March 2014 Next, we must look in detail at the woven glass fabric and to consider the inherent limita- tions placed on PCB performance and design. The traditional glass cloth styles and their com- position were originally chosen with thickness yield, ease of manufacture and mechanical properties as the primary drivers. There was lit- tle concern at the time to consider the effects of inhomogeneity in the resultant PCB substrates, indeed this did not matter greatly in terms of the PCB technology of the day. Considering that the PCB substrate is a com- posite material comprising a resin system and a woven glass reinforcement it is necessary to un- derstand how the properties of the component parts affect the behaviour of the whole, bear- ing in mind that the macro and micro behav- iour may also differ. Referring to Figures 5 and 6, the reducing density of the glass fibre matrix with decreasing thickness can be clearly seen. The picture of 106 style glass fabric in Figure 6 demonstrates that for this style over 40% of the available area is occupied with gaps in the glass fibres which will be filled with resin in the resulting substrate. The issue we must explore is the interaction of a signal transmitted on the conductive layer with the micro structure of the substrate underneath. With conductive feature sizes decreasing these are now of the same order as the features arising from the weave pattern leading there- fore to opportunities of interaction. The speed of the electrical signal, which is actually the speed of the associated electromagnetic wave, can be considered by calculating the velocity factor which is the speed at which a signal pass- es through the medium relative to the speed of light. The velocity factor is expressed as the re- ciprocal of the square root of the dielectric con- stant of the transmission medium. The dielectric constant of E-glass is around 6.6, whereas the dielectric constant of a typical epoxy resin is around 3.5. From these figures we Figure 3: glass weaving process. Figure 4: plain weave fabric. DEVELOPMENTS IN GLASS YARNS AND FABRIC CONSTRUCTIONS continues