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PCBD-Aug2015

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50 The PCB Design Magazine • August 2015 I have mentioned in previous columns that planes should not be split. This mainly refers to having one continuous GND plane for all supplies including analog. But since, in this case, the return current path has been forced into the GND planes then the path is not in- terrupted. Looking at the asymmetric stripling configuration between layers 2 and 4 of Fig- ure 4, we have a ratio of 14.8:4. This means that at least 79% of this current will flow in the GND plane. However, if you also take into account the increased inductance, of the alter- nate return path, this almost totally eliminates the power plane. It is important to place GND stitching vias close to each signal layer transi- tion to ensure there is a low inductance path between GND planes. Figure 5 incorporates ITEC IT-180A 2GHz materials. This material is very common in Asia and is suitable for low-cost, high-speed applica- tions. It has a low dielectric constant and low dielectric loss of just 0.015 with a glass transi- tion temperature of 180C. Since the "real" val- ues are quite different to the "virtual" values, I have used to initially construct the stackup, the variables must be adjusted to provide the cor- rect impedances for the design. The impedance plots of Figure 5, project the correct values of trace width (4 mils), trace thickness (2 mils), trace clearance (12 mils) and dielectric thickness (2.8 mils) given the dielec- tric constant (3.9) for the ITEC IT-180A 2GHz material to achieve the target single ended and differential impedance. This also provides the differential coupling point whereby increasing the trace spacing has little effect as the imped- ance rolls off. In this case, 12 mils is ideal for 100 ohms differential impedance. Next month, Part 4 of the Stackup Planning series will continue detailing the construction of 10+ stackup layer configurations. Once the basics are defined, the pattern for higher layer- count structures becomes evident. Points to Remember • Each signal layer should be adjacent to, and closely coupled to, an uninterrupted reference plane, which creates a clear re- turn path and eliminates broadside cross- talk. • Although power planes can be used as ref- erence planes, ground is more effective. • At high frequencies, the return current takes the path of least inductance. • In order to force the current onto the ground (GND) plane of an unbalanced stripline configuration, h2 needs to be at least twice h1; three times is even better. • An eight-layer board can be thought of as a six-layer board with optimum EMC performance and should not be utilized to improve routability. STACKUP PLANNING, PART 3 continues beyond design Figure 5: impedance plots for the microstrip layers.

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