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28 DESIGN007 MAGAZINE I JULY 2019 struct higher layer count boards. In gen- eral, these boards contain more planes, and therefore, the issues associated with split power planes can usually be avoided. Also, 10+ layers require very thin dielec- trics to reduce the total board thickness. This naturally provides tight coupling be- tween the adjacent signal and plane lay- ers, reducing crosstalk and electromag- netic emissions. Additional rules for high- speed design were defined. The number one question about determining the re- quired layer count was also addressed. In this month's column, I will add to my stackup planning series with this final chapter (Part 5), covering all of the latest concepts in stackup design. The speed of a digital signal does not depend intrinsically on the speed of electrons but rath- er on the speed of energy transfer between elec- tronic components. The actual velocity of elec- trons through a conductor is very slow (~10 mm per second); however, the "knock-on" ef- fect is very fast as it follows the electromag- netic field. The energy propagates between the signal trace and return path(s) as an electro- magnetic wave (Figure 1). And the speed of this wave varies depending on the layer in the multilayer substrate and the surrounding di- electric materials. For instance, the wave will travel at approximately half the speed of light in a typical FR-4 material with a dielectric con- stant (Dk) of four. The electromagnetic fields of a microstrip (outer) layer are shown in Figure 2. The fields tend to radiate outward, as there is only one solid plane beneath, which blocks the emis- sions. It is obvious that this configuration is not recommended for routing high-speed sin- gle-ended signals. The electromagnetic fields surrounding the microstrip exist partially with- in the dielectric material(s) and partially with- in the surrounding air. Since air has a dielectric constant of one, which is always lower than that of FR-4 (typically four), mixing a little air into the equation will speed up the signal prop - agation. You can see this in the kinks in the field lines as they travel into the air region from the dielectric material. Adding a solder mask (Dk of three) will put another kink in the fields. A stripline is any trace sandwiched between reference planes on both sides (Figure 3). The electric fields (blue) of a stripline are totally contained between the two solid planes where- as; the magnetic fields (red) are also limited vertically by the planes. Radiation is reduced dramatically and limited to just the edge fring- ing fields due to the shielding effects of the planes. Signals that travel in the same dielec- tric material will couple. This may be good for a differential pair, but is not desirable for non- related signals as part of the aggressor signal will couple to the victim depending on the sep- aration of traces. Figure 1: Digital signals travel as a wave of electromag- netic energy in a multilayer PCB. Figure 3: Stripline configuration, differential pair. Figure 2: Microstrip electric (blue) and magnetic (red) fields.

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