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44 DESIGN007 MAGAZINE I AUGUST 2018 Dielectric constant and dielectric loss are not a function of the geometry of the transmission line—they are a function of the dielectric mate- rial in which the signal propagates, their dis- tribution in the PCB stackup, and the applied frequency. These mechanisms contribute to the frequency-dependent loss and degrade the bandwidth and speed of the signal. The sig- nal quality transmitted through the medium and picked up at the receiver will be affected by any impedance discontinuities and by the losses of the dielectric materials. The glass epoxy material (FR-4) commonly used for PCBs has negligible loss for digital applications below 1 GHz. But at higher fre- quencies the loss is of greater concern. Also, the entire bandwidth of the signal needs to be considered. For instance, a 10Gbps square wave is made up of a series of odd harmonics. It will have a fundament frequency of 5GHz, a third harmonic of 15GHz, a fifth of 25GHz and possible higher odd harmonics. These high harmonics can suffer excessive losses in amplitude and a degradation of edge sharpness which results in distortion of the signal eye. Plus, when the frequency exceeds 1GHz, cop- per roughness, conductor loss, skin effect and skew, due to variations of glass weave in the dielectric, begin to come into play. Also of interest is the glass transition (Tg) temperature, which is the point at which a glassy solid changes to an amorphous resin/ epoxy. If the reflow temperature exceeds the Tg, for an extended period, the material rap- idly expands in the Z-axis. Plus, the mechanical material properties—strength and bonds in the material—degrade rapidly. A high Tg guards against barrel cracking and pad fracture during reflow. Standard FR-4 has a Tg of 135-170C, whereas the high-speed materials are generally over 200°C. Therefore, Tg is not a factor that needs to be considered for high-speed design but is certainly worth checking. With so many materials to choose from which is the best for your specific product? Low cost generally means low quality. But also, the price of poor yields drives up the final material cost. Dielectric material selection is usually driven Figure 1: Loss profile for ultralow-loss dielectric materials (source: iCD Materials Planner).