Issue link: https://iconnect007.uberflip.com/i/271756
March 2014 • The PCB Magazine 87 perfectly aligned differential pair, Figure 9 shows the effect of only 80ps of skew to the signal, the reader can see that the eye is almost completely closed leading to a loss of signal integrity. Early efforts to mitigate this effect relied on rotating the circuitry relative to the weave pat- tern in order to provide an equal path constitu- tion for both parts of the differential pair. How- ever this proved to be an expensive solution as it resulted in large amounts of material wastage in panelisation inefficiency. There are a number of techniques directly related to the glass fabric that can be used to mitigate and even completely eliminate this effect. The first relies on altering the glass fab- ric construction in order to close up the gaps between the glass yarns. The effect is to pres- ent the conductor with a more homogeneous substrate over which to pass thus minimising the micro Dk effects described above. In recent years a number of traditional glass cloth styles have been substituted by a "square weave" al- ternative. The square weave alternative adds ad- ditional glass yarns and balances up the warp and weft directions so that they have the same, or nearly the same, thread count in the warp and weft direction. Table 4 shows the construc- tion of some popular square weave glass cloth styles. There are further techniques that effec- tively spread the glass fibres out so that they lay flatter and no longer remain in tightly twisted threads thus distributing the avail- able fibres into the gaps between the threads. A number of methods have been employed in recent years to produce this effect; some rely on a change to the yarn production and weaving process, some to an additional post weaving process and some to a combination of techniques. Figure 10 shows photomicro - graphs of standard 106 style glass, the square weave version 1067 and finally a spread glass version of 1067. The reader can clearly see how the large gaps in the standard 106 glass fabric have been all but eliminated in the spread glass version. There is now also a novel solution by which skew can be all but eliminated. Isola's newly developed GigaSync™ product offers laminates and prepregs with engineered glass weaves hav- ing the same dielectric constant as the resin sys- tem. Matching the Dk of both component parts of the composite substrate means the elimina- tion of the skew effect as the micro composi- tion of the substrate can no longer introduce variability to the signal path. This applies to all combinations of glass styles used in a stack-up as the resin content no longer has an influence on the dielectric constant thus simplifying the electrical design. This innovation is an enabler to achieve data transmission rates of 100 Gbps and beyond. table 4: selected square-weave woven glass fabrics. DEVELOPMENTS IN GLASS YARNS AND FABRIC CONSTRUCTIONS continues