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36 DESIGN007 MAGAZINE I MAY 2024 ese properties are independent of trans- mission line geometry but depend on the dielectric material and frequency. ey are a function of the dielectric material in which the signal propagates their distribution in the PCB stackup and the applied frequency. ese mechanisms contribute to the frequency- dependent loss and degradation of the speed of the signal. e signal quality transmitted through the medium and picked up at the receiver will be affected by any impedance dis- continuities and by the losses of the dielectric materials. e glass epoxy material (FR-4) commonly used for PCBs exhibits negligible loss for digi- tal applications below 1 GHz. But at higher frequencies, the loss is of greater concern. It is crucial to consider the entire bandwidth of the signal. For instance, a 10 Gbps square wave is made up of a series of odd harmonics. It will have a fundamental frequency of 5 GHz, a third harmonic of 15 GHz, a fih of 25 GHz, and possibly higher odd harmonics. ese high harmonics can suffer excessive losses in amplitude and a degradation of edge sharp- ness, which results in distortion of the signal eye. Plus, when the frequency exceeds 1 GHz, copper 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 tem- perature (Tg), 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 rapidly expands in the Z-axis. Also, the mechanical material properties—strength and bonds in the mate- rial—degrade rapidly. A high Tg guards against barrel cracking and pad fracture during reflow. Standard FR-4 has a Tg of 135-170°C, whereas the high-speed materials are generally over 200°C. erefore, Tg is not a factor that needs Figure 1: Loss profile for ultra low-loss dielectric materials. (Source: iCD Materials Planner)