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18 The PCB Design Magazine • April 2014 pads/lands and traces that are non-critical. Via- in-pad could be used or a just short (<200mil) fanout to layer 2 using a 12 mil trace in an at- tempt to maintain impedance. In order to change reference planes: • If there are multiple ground planes, then place a stitching via as close as possible to each layer transition (signal via) • If power planes are also used as the refer- ence plane, then place decoupling capacitors as close as possible to each layer transition When you plan your stackup, be aware of which planes—either power or ground—will be the return path for your critical signals and make sure there is an unobstructed return path. The best way to think of this is to imagine rout- ing a return trace adjacent to each signal trace on the reference plane: Where is the best place for the current to flow, and is it unobstructed? The reference plane(s) adjacent to each signal layer allows the return current to flow as closely as possible to the signal trace, reducing induc- tance and loop area. A large loop area will create higher emissions of electromagnetic radiation, so we need to keep these as tightly coupled to the trace as possible. The best way to go about this is to look at each signal layer with respect to its associated reference plane. Figure 3 shows an internal sig- nal layer and the ground plane. Make sure that there are no large cut-out areas or lines of anti- pads on connectors that completely block off the ground, forcing the return paths to deviate from their associated traces. This is best done in a CAM/Gerber viewer as what you see—is what you get. Whereas, in the CAD database you may not see the correct anti-pad or plane cut-out siz- es. You may have to reduce the size of the anti- pads if they are too close. SMT boards are not so bad, in this respect, but through-hole connec- tors generally create issues. Also, keep in mind that high speed signals MyTHBuSTING: THERE ARE NO ONE-WAy TRIPS! continues feature Figure 4: signal layer 3 with respect to ground plane layer 2.

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