Issue link: https://iconnect007.uberflip.com/i/1071356
80 DESIGN007 MAGAZINE I JANUARY 2019 As you can see, the currents in the strip are pretty much the same as expected in cases with the solid planes. However, the cutouts destruct the return current flow; it has to bypass the cutouts. Also, the currents flow on the opposite side of the meshed plane. As you will see, that may cause unwanted coupling to the traces shielded by the meshed planes. All of those effects must be simulated to design predictable interconnects. Characteristic Impedance Now, let's get back to the basics and take a look at the characteristic impedance of the dominant mode in the periodic structure formed by the trace over a meshed plane. Any deviation from the link target impedance can cause the increase of the reflection losses in interconnect link. With the stackup defined in Figure 1, a 65-µm wide trace over the solid reference plane gives about a 51-ohm transmission line impedance at 1 GHz (green lines in Figure 5). Relatively large cutouts in the reference plane right below the strips increase the impedance to about 62 ohms (red lines in Figure 5). This may cause the design failure due to excessive reflection losses or possible resonances in the system with non-uniform impedance links. This is the worst-case scenario for this size of cutouts. The trace may be shifted to have more metal below the trace to provide a better path for the return current. This shift reduces the impedance down to 55.5 ohms (blue lines in Figure 5). The deficiency of the reference conductor area reduces the capacitance and increases the inductance of the periodic structure. In reality, without control of the trace position over the cutouts, one should expect the impedance variations from 55.5 to 62 ohms in this case. Can you get it back down to 50 ohms? Yes, but, unfortunately, only for a particular position of the trace. For instance, if you increase the trace width to 100 µm, it is going to be about 50 ohms if it goes directly over the cutouts in the reference planes (black lines in Figure 5). The impedance will decrease substantially if the trace is shifted. In general, all possible scenarios must be simulated. Attenuation and Delay Other important interconnect design para- meters are signal attenuation and delay. Those two parameters are derived from the complex propagation constant of a transmission line or periodic structure mode. Attenuation is the energy loss to heat up dielectric (polarization losses) and conductor (conduction losses). The minimal phase delay corresponds to the signal front delay in general. Attenuation in dB/mm and phase delay in ps/mm for different configurations are compared in Figure 6. Note that attenuation, delay, and charac- teristic impedance depend on the material Figure 5: Characteristic impedance for traces over solid and meshed plane (left) and corresponding TDR of a 10-cm trace segment.