Issue link: https://iconnect007.uberflip.com/i/1369942
26 DESIGN007 MAGAZINE I MAY 2021 simple circuit trace routing. A successful PC board design accounts for both viewpoints. In order to satisfy both the circuit and field theory viewpoints, we now see the importance of adjacent power and power return planes, as well as adjacent signal and signal return planes. Signal or power routes referenced to a single plane will always have a defined return path back to the source. Figure 3 shows how the electromagnetic field stays within the dielec- tric on both sides of the return plane. e dielectric is not shown for clarity. On the other hand, if a signal passes through two reference planes (Figure 4), things get a lot trickier. If the two planes are the same poten- tial (for example, both are return planes), then simple connecting vias may be added adjacent to the signal via. ese will form a nice defined return path back to the source. If the two planes are differing potentials (for example, power and return), then stitch- ing capacitors must be placed adjacent to the signal via. Lack of a defined return path will cause the electromagnetic wave to propagate throughout the dielectric, causing cross cou- pling to other signal vias and leakage and radia- tion out the board edges as shown. For example, let's take a look at a poor (but very typical) board stack-up that I see oen (Figure 5). Notice the power and power return Figure 3: A signal trace passing through a single reference plane. Figure 4: A signal trace passing through two reference planes. If the reference planes are the same potential (signal or power returns, for example), then stitching vias next to the signal via should be sufficient. However, if the planes are different potentials (power and return, for example), then stitching capacitors must be installed very close to the signal via. Lack of a defined return path will cause the electromagnetic field to leak around the dielectric, as shown, and couple into other signal vias or radiate out board edges.