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Design007-June2019

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72 DESIGN007 MAGAZINE I JUNE 2019 Practical Evaluations of Glass Weave Effect Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION The impact of glass weave effect on PCB electrical performance is a topic that has been investigated for many years. From my perspec- tive, the initial investigators for the glass weave effect were mostly from the high-speed digital industry. However, as more RF applications are using millimeter-wave (mmWave) frequencies, the RF industry is now pursuing more studies for the potential of glass weave to impact these applications. The basic issue for the glass weave effect is that many laminates have a layer, or layers, of woven fiberglass used to improve mechanical properties. The raw glass used to make the wo- ven glass reinforcement layers is typically an E-glass, which has an approximate dielectric constant (Dk) of 6. As an example, for many laminates that have an overall Dk of 3.5, the resin system must have a much lower Dk than the glass, so the overall Dk of the laminate will be roughly 3.5. For glass-reinforced laminates, and when looking down through the laminate, in isolated areas, the Dk of the laminate will be different in an area where there are glass bundles from the woven glass layer compared to areas where there are openings in the woven glass layer. In the open areas of the glass weave pattern, the Dk will be that of the resin system, and will probably have a Dk value around 2.1 to 3.0, depending on the laminate construction. In an area of the laminate where there is a combi- nation of resin and one layer of glass bundles, the Dk will be somewhere between 2.1 and 6. In another area where the glass bundles inter- sect to form the grid of glass reinforcement, there will be two layers of glass as a knuckle and also the resin system. In these knuckle ar- eas, the Dk will be the highest for the laminate. This means that within a small area of the lam- inate, there can be three areas of distinctly dif- ferent Dk values.

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