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CAVITY PCB SOFTWARE
PCB design software applications are not set up to consider these effects simply because they can’t be addressed during the design phase. Over long distances, this skew accumulates and causes signals to desynchronize. When a trace in this arrangement is routed over a cavity in the substrate, the signal moves slightly faster, coming out of sync with the other signals. The skew between two ends of a differential pair, or between nets routed in parallel. The fiber weave effect primarily refers to skew that accumulates along an interconnect. Let’s look at each of these three effects. Although the fiber weave effect primarily refers to skew, it also refers to losses, dispersion, and cavity resonances in the substrate. These variations in signal velocity are responsible for the fiber weave effect. It is these cavities in the substrate that cause variations in the signal velocity as the signal traverses an interconnect. The dielectric constant of PCB substrate materials is normally quoted as a single measurement, although this value depends on the measurement method and is basically equivalent to a length-averaged value for the two materials.Ĭavities formed in glass weave substrates with different tightness. The epoxy filler a dielectric constant approximately 3 and varying levels of loss, depending on the exact resin used in the laminate. The glass used to form the weave structure has a dielectric constant of approximately 6 with very low loss. The dielectric constants of these two materials are very different. The panel is then filled with a resin to harden the panel and allow bonding to other laminate layers. PCB laminates are formed from a square-grid of glass strands that are woven together into a panel. Let’s look at some of the important information needed to understand the fiber weave effect and how you can avoid signal integrity problems in mmWave and higher frequency ranges. It also requires judicious material selection to prevent signal degradation at a receiver. The signal integrity problems that arise in PCB substrates can be solved in some creative ways, but it requires understanding how you signals interact with the substrate as they are routed. It is this periodic inhomogeneity in the PCB substrate and the fibrous nature of the substrate that causes the fiber weave effect. The inhomogeneity in the substrate is periodic. The fiber weave structure causes a PCB substrate to act like an inhomogeneous anisotropic material, where behavior of electromagnetic waves in the material depends on the direction waves flow in the material and the location in the material.
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The image above is an SEM image of the fiberglass weave that is used to form PCB substrates. This is an important aspect of the fiberglass weave that is less known among many designers, but it produces some odd effects at high frequencies and ultra-fast edge rates. One of the many signal integrity aspects at high GHz frequencies is the fiber weave effect. Anyone working beyond microwave frequencies will need to consider new aspects of signal integrity they may have never considered.