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88 The PCB Magazine • March 2014 The use of spread glass has brought a num- ber of other benefits which were alluded to ear- lier in this article. The use of lasers to form mi- crovias in PCBs is commonplace. However the laser ablation rate of glass is significantly lower than that of the resin matrix. An early use of spread glass was to minimise the effect of this differential ablation rate by presenting the laser with a more homogeneous target thus reducing the number of laser pulses required and improv- ing the quality of the hole. Another significant improvement deriving from the use of spread glass is in the ability to better wet the fibres during the application of the finish which is used as a coupling agent to form a chemical bond between the glass reinforce- ment and the resin matrix. This benefit extends also to the impregnation process whereby the resin is coated onto the glass fabric to produce a prepreg. The use of traditional tightly bound bundles of glass fibres can lead to defects in the resin/glass interface that can compromise me- chanical and chemical integrity. In some cases these defects lying along the fibres can present pathways for conductive anodic filamentation (CAF) to propagate leading to abrupt failure. The use of spread fibres greatly improves the ac- cessibility of the individual fibres thus enabling improved wetting which along with advances in finish chemistry has led to greater reliability in service. In the future PCB substrates will no doubt utilize even better fibres and resins and will incorporate entirely new materials, including those on the nano scale. We are indebted to the researchers and developers worldwide who continue to advance our knowledge and pro- duce ever more advanced and functional ma- terials to transform the designers' dreams into reality. PCB Figure 10: glass fabric development—standard weave→square weave→spread glass. alun morgan is director of oem marketing, isola group europe. DEVELOPMENTS IN GLASS YARNS AND FABRIC CONSTRUCTIONS continues