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38 DESIGN007 MAGAZINE I APRIL 2021 four to layer one or layer three, a ground tran- sition via is required, to connect the dielectric between the traces on layer four in the Z-axis to the dielectrics in the triplet formed by lay- ers one, two and three. is pattern must be repeated to create a solid foundation for EM field control. A six-layer board stack is extremely power- ful, providing two sets of paired dielectrics and four routing layers that form good trans- mission lines. Again, care must be taken when routing between these paired dielectrics, to connect the spaces in the Z-axis. e plumbing must be intact from the source of the energy to the load, in all three dimensions. Some signals can never be compromised. e crystal circuit and power supply are two very good examples of this. e one dielectric rule must always be enforced for these signals, with the traces routed over continuous ground copper on an adjacent layer or co-planar ground copper (a poor second choice). Analysis of the PDN is the next step on the road to signal integrity and compliance. In the case of the power supply, not only is the one dielectric rule inviolate, but the capacity of the dielectric to carry EM fields must be adequate to supply the needed energy for the devices connected to it. is is not achieved by increased copper weight, but by having an impedance that matches the need. in- ner dielectrics, wider conductors, or mul- tiple parallel transmission lines are required for good power distribution, as are properly sized and placed capacitors in the PDN. e capacity of the power supply transmission lines is critical. If you need five amps, you need the dielec- tric that has the capacity to deliver this much energy. Current flow is a measure of the amount of EM field energy passing through the dielectric. Five amps is five coulombs of energy per second, much like the idea of water flow in a pipe, measured in gallons per minute. If you need five gallons of water per second, a one-gallon-per-minute pipe will not do the job. e same is true for the PDN. If you need five amps, you must provide the spaces that can carry that much energy. It is the dielectric that is the key here, not the conductors. Add- ing more copper weight does not increase the capacity of the dielectric. Again, only reducing the dielectric thickness, increasing the copper conductor surface area (trace width) or adding more parallel transmission lines, will increase the capacity of the PDN. In conclusion, with proper EM field plan- ning, you can oen avoid having to simulate a PCB design. e first line of defense for achieving good signal integrity and compli- ance is to review the PCB design. Look for areas where the one dielectric rule was vio- lated. Start with the board stack. Are there paired dielectrics? Are they connected to the other layers properly? Are there ground tran- sition vias where needed? is is especially important in circuits such as SMPS designs, where the components are placed on both sides of the board. Properly connecting the top dielectric, adjacent to the top layer, to the bottom dielectric is critical. Each power via needs a ground transition via. Does the PDN provide adequate capacity and properly placed components? In many cases, parallel Z-axis transmission lines are needed to ensure there is enough capacity to allow the energy to move between components. As Ralph Morrison always said, "Buildings have walls and halls. People travel in the halls, not the walls. Circuits have traces and spaces. Energy and signals travel in the spaces, not the traces." Design the spaces, by using the traces. To help you all remember this, enjoy "All About the Space," a remake of the Meghan Trainor song, featuring my daughter Breezy Beeker on vocals. DESIGN007 Dan Beeker is a senior principal engineer at NXP Semiconductor.