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30 DESIGN007 MAGAZINE I SEPTEMBER 2020 6. Dielectric Constant The main considerations for dielectric con- stant, or Dk, (Figure 6) include: a. A dielectric constant of 4.3 is typical. b. Low loss materials tend to have lower dielectric constants (~3). c. Microstrip (outer layer) dielectric should have lower Dk to slow the velocity of propagation of the EM wave to match the stripline speed. d. A high dielectric constant creates the most planar capacitance, which is used to lower the impedance of the power distribution network. This is good for plane-to-plane coupling. Start with a value of 4, but if the frequency is 10 GHz or more, use 3 as a starter. Taking all of the above into account, one should first plan the stackup using virtual materials (Figure 7) to get in the ballpark. In other words, pick an appropriate value, and then hone in on these values with the actual materials stocked. In this case, I am designing the stackup for three different technologies (50/100-ohm digi- tal, 40/80-ohm DDR3, and 90-ohm USB). The dielectric constant and thickness, together with the trace width and clearance, need to provide near correct impedances for these three tech- nologies (Table 1). A characteristic impedance of 40–60 ohms is typically used for a digital design. However, this value becomes more critical as the edge rates become faster. Also, different technolo- gies have their specific impedance require- ments. For example, Ethernet is 100 ohms and USB 90 ohms differential, and DDR2 memory is Figure 6: Impedance vs. dielectric constant. Figure 7: Virtual materials for microstrip, stripline, and dual stripline configurations (iCD Stackup Planner).