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March 2014 • The PCB Magazine 81 The first part of the weaving process is called warping, which is the operation through which the yarn on the bobbins is transferred to section beams at the required spacing to form the warp threads which run in the machine direction and are up to 3 kilometres in length. Depend- ing on the total number of threads required for a given type of glass weave (style), several sec- tional beams are produced and consolidated in a group called a set which is the input for the slashing process where additional size is added to protect the fibres from breakage through the weaving process. Once the loom beam is installed on the weaving loom, fabric is formed by interlacing filling yarns (picks, weft threads) orthogonal to warp threads (ends) (Figure 3); the most com- mon interlacing of fill and warp ends (weave pattern) is plain weave although others such as basket, twill and satin are available. The woven fabric is wound onto a roll for subsequent heat cleaning to remove the size and then onto fin- ishing operations. In order to provide maximum adhesion of the fabric to the various resins matrices, a coupling agent (finish) is applied to the fabric by impregnating in a "finish bath"; additionally proprietary fibre spreading treatments can also be applied at this stage. The resultant plain weave fabric is shown in Figure 4. Different yarns are combined in different combinations to weave the variety of "styles" that are subsequently used for PCB substrate production. Table 3 shows the composition of the traditional styles available. table 2: glass yarn common types. Figure 2: glass yarn designation. DEVELOPMENTS IN GLASS YARNS AND FABRIC CONSTRUCTIONS continues