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48 The PCB Design Magazine • August 2015 equation 1 equation 2 Equations 1 and 2 define the relationship between the ratio of the thicknesses of the di- electrics to the close and far planes and the re- sultant return current distribution. Table 1 sim- plifies the calculations presenting the percent- age of return current, in each plane, for differ- ent ratios of h2:h1. If the configuration is dual asymmetric stripline (two signal layers between the planes), then these ratios still apply and can be calculated by adding the two (or more) di- electric thicknesses to the far plane representing the h2 value. Eight-Layer Stackup An eight-layer board with six routing layers is not recommended. If six routing layers are required, then a 10-layer board should be used. For that reason, 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. Although there are many possible configurations of eight layers, I will only focus on the two that I consider to possess superior performance. Having said that, there are a total of four signal layers and four plane layers to consider in the each configuration. 1. The stackup of Figure 3 is the standard eight-layer configuration. This stackup has many good attributes including: • All signal layers are adjacent to, and close- ly coupled to, an uninterrupted reference GND plane, which creates a clear return path and eliminates broadside crosstalk. • Stripline return current is biased toward the GND planes. • There is good planar capacitance (4 mils VDD to GND spacing) to reduce AC im- pedance at high frequencies. • ECM could also be substituted for the center planes, further improving the PDN performance. • Crosstalk is minimized by closely coupling the signal to plane layers. • EMI is minimized by routing critical sig- nals between the planes. • The substrate can accommodate a number of different technologies. For example: 50/100 ohm digital and 90 ohm USB. STACKUP PLANNING, PART 3 continues beyond design Figure 2: Close plane is h1 (5 mils), far plane is h2 (15 mils). Table 1: Percentage of return current in the close and far planes of asymmetric striplines.