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PCBD-Mar2014

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March 2014 • The PCB Design Magazine 53 Microstrip layers are those that are fabri- cated on the outside of the substrate, top and bottom. Traces on these layers are referenced to the plane below/above, whether it be a ground or power plane. The trace has a dielectric mate- rial on either side: FR-4 below and air above. If you look closely at Figure 2, you can see where the field lines refract as they pass from the FR-4 (green) to the air dielectric. This is because the dielectric constant of FR-4 for example (~4.3) changes to air (1.0). The electric field is absorbed by the plane on one side and both the electric and magnetic fields also radiate into the air. Stripline traces however, are totally embed- ded in dielectric material between planes. This may be a combination of materials—and dielec- tric constants—if for example multiple prepregs are stacked to obtain the desired thickness. The electric field is completely contained within the dielectric and blocked by the planes. The mag- netic field still tends to radiate, from the board edges, but is limited somewhat. Since the electromagnetic fields of microstrip layers partly reside within the surrounding air, the speed of propagation of a signal traveling on the microstrip trace is therefore partly de- termined by the dielectric properties of the PCB material and partly by the surrounding air. Mi- crostrip traces are usually faster than stripline traces because the dielectric constant of air is lower than that of FR-4. There are two exceptions to this: 1. If you use a microstrip prepreg with a high dielectric constant (Isola 370HR, 3.92) and in the stripline configuration a low dielectric constant (fastRise FR-27 & TSM-26, ~2.7) as in Figure 3. The combination of air (1.0) and Isola 370HR (3.92) brings the total dielectric constant to approximately that of the Taconic fastRise materials, thus the traces exhibit a similar delay. 2. If you add a liquid photo-imageable sol- der mask, with a dielectric constant of say 3.3, to the outer layers, then the microstrip delay changes again. So it is best to compare the de- lays using a simulator otherwise you are just guessing. Contrary to what you may believe, the prop- agation delay of a serpentine trace is less than the delay through an equivalent length straight trace. The signal is sped up because a portion of the signal will propagate perpendicular to the serpentine. And, this also varies with the type of serpentine pattern used. For example, the ser- pentine pattern may have long parallel lengths spaced close together in the "U" bend coupling the signal many times through the serpentine pattern. This self-coupling (forward and reverse crosstalk) shortens the electrical path. But in theory, the forward crosstalk—far-end cross- talk—does not exist in the stripline configura- tion. Please see my previous column, Beyond Design: A New Slant on Matched-Length Rout- ing, for further details. Also, if two traces of equal length are ref- erenced to different planes, then the return paths may be considerably different and add round-trip delay. This cannot be simulated, so it is important that the return paths are deter- mined to be as short as possible. If planes are changed, then stitching vias or capacitors need beyond design Matched Length Matched Delay continues Figure 3: isola 370hr (microstrip) and fastrise Fr-27 & TsM-26 (stripline) exhibit similar delay.

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