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42 DESIGN007 MAGAZINE I MAY 2020 drilling into a block wall with steel rebar in- serts. The lack of consistency would give even the best mechanical drill a tough time. On the PCB front, the material suppliers in- novated by developing flat or spread glass cloth; this modified the traditional warp and woof (weft in the U.K.) of the fibre and most- ly eliminated the gaps in the cloth where the threads cross over (Figure 4). This is achieved by mechanical or water jet blasting sometimes combined with threads that are "low twist" (i.e., the glass yarn has few- er twists per metre, making it easier to flatten out). This step-function improvement in mate- rials for laser drilling has a hidden side effect. The more even lay of the glass also helps to eliminate the variations of dielectric constant that used to be experienced by signals tran- siting along traces, which ran predominantly over fibre or predominantly over resin. The win for drilling presented SI engineers with a win for signal integrity too. (There is a "but" coming, which isn't a problem, but is some- thing you should be aware of.) A third benefit of flat glass is that the cloth lies flatter and gives overall more control over fin- ished thickness and thick- ness variation (here comes the "but"), but a benefit of the "old school" glass styles with the apertures between the crossover of the warp and woof of the cloth is that the apertures allow the easy passage of resin from one side of the cloth to the oth- er—so more of the total res- in content on top and below the cloth could flow from one side or the other into the gaps between the trac- es to ensure a reliable lami- nation process and minimal risk of resin starvation. For the same resin con- tent, flat glass may prevent the resin flowing across the glass boundary, so before you change glass styles, you should consult with your material supplier and take advice on the resin content require- ments for successful lamination with flat glass. This is not a problem with the material; it sim- ply means that when using the newer mate- rial styles, you need to ensure you understand what questions you need to ask to make sure your material supplier can help you specify the optimum build materials. Having a material and stackup discussion before submitting the design or—even better— before starting to lay out the board is critical for complex designs. Moving stackup design as early in the process as possible gives both designers and fabricators time to discuss the trade-offs and benefits of different material choices. DESIGN007 Martyn Gaudion is managing director of Polar Instruments Ltd. To read past columns or contact Gaudion, click here. Figure 4: Flattened glass cloth/epoxy resin weave. (Source: Ventec International Group)