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March 2015 • The PCB Design Magazine 61 One of the keys to determining the optimal PCB stackup is to understand how and where the return signals actually flow. The schematic only shows the signal path whereas the return path is implicit. The ICD Stackup Planner al- lows the designer to determine any number of single-ended and differential impedance tech- nologies on the same substrate. In Figure 3, I have simulated 50/100 (digital), 40/80 (DDR3) and 50/90 ohm (USB) on the same substrate. It is recommended to use as many GND planes as possible in the stackup. The de- coupling capacitor and IC GNDs naturally provide stitching vias to connect the GND planes in most cases. However, any plane, not just ground, can act as the return path of a signal. Care must be taken to ensure there is a provision for enough decoupling between the power and ground planes, in this case. But still, even after putting as many supplies as possible on the power planes, without doubling up, we soon run out of planes. So what do we do with the other supplies? In the above screenshot of Figure 3, 1V8 beyond design Figure 3: impedance altered to 40/80 ohms by pours under the DDR3 devices. Figure 4: supplies routed directly under the iC on the top layer. SPLIT PLAnES In MULTILAyER PCBS continues