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20 DESIGN007 MAGAZINE I MAY 2023 the speed of light, or 6 inches per nanosecond. e electrical properties of a dielectric mate- rial can be described by two terms: 1. e dielectric constant (Dk) or relative permittivity (Er) is the ratio of the amount of electromagnetic energy stored in a material by an applied voltage. It describes how the material increases the capacitance and decreases the velocity of propagation in the material. 2. e dissipation factor (Df ) or dielectric loss/loss tangent (tan δ) is a parameter of a dielectric material that quantifies its inherent dissipation of electromagnetic energy. Dielectric constant and dielectric loss are not a function of the geometry of the trans- mission line. Rather, they are a function of the dielectric material in which the signal propa- gates their distribution in the PCB stackup and the applied frequency. ese mechanisms contribute to the frequency-dependent loss and to degrade the speed of the signal. e signal quality transmitted through the media and picked up at the receiver will be affected by any impedance discontinuities and losses of dielectric materials. e glass epoxy material (FR-4) commonly used for PCBs has a negli- gible loss for digital applications below 1 GHz. But at higher frequencies, the loss is of greater concern. If the signal has a fast rise/ fall time, then the electro- magnetic wave needs to propagate at a higher speed, and therefore the Dk needs to be low to enable this. If a material with a high dielectric constant is placed in an elec- tric field, the magnitude of that field will be measurably reduced within the volume of the dielectric. erefore, a lower Dk is desirable for high- speed design. Conversely, a high Dk material is very good at condensing electric fields, so having it between the planes increases planar capacitance. An efficient dielectric material supports a varying charge with minimal dissipation of energy in the form of heat. ere are two main forms of loss that may dissipate energy within a dielectric: 1. Conduction loss is the flow of charge through the material that causes energy dissipation. 2. Dielectric loss is the dissipation of energy through the movement of charges in an alternating electromagnetic field as polarization switches direction. Dielectric loss is especially high around the resonant frequencies of the polarization mech- anisms as the polarization lags behind the applied field, causing an interaction between the field and the dielectric's polarization that results in heating (Figure 1). ere are also thermal factors to consider. e most important is the glass transition tem- perature (Tg), 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 rapidly expands in the Z-axis. Plus, mechanical mate- rial properties degrade rapidly—strength and Figure 1: Dielectric constant and dielectric loss vs. resonant frequency.