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July 2015 • The PCB Design Magazine 41 ohms differential. In the Figure 4 impedance plot, you can see that a ~15 mil trace width is required to achieve ~50 ohms impedance. 2. Since the stackup is symmetrical, this top section of the stackup can be mirrored to create the entire stackup of Figure 5. You need to find a center core of appropriate thickness to beef-up the total board thickness to about 62 mils. A 39 mil core is selected from the Isola FR406 range to achieve this. Now in this case, I was not able to tightly couple the ground and power planes because the center core had to be used to beef-up the total board thickness. However, to improve the EMC performance of a four-layer board, it is best to space the signal layers as close as possible to the planes, and use a large core between the power and ground plane keeping the overall thickness of the substrate to ~ 62 mils. For high-speed ap - plications, you could reduce the trace width to say 4 mils and prepreg thickness to ~3 mils as shown in the six-layer stackup of Figure 7. This is the most cost-effective and most overlooked way to improve the performance of a four-layer PCB. There are three advantages to this configura- tion: 1. The signal loop areas are smaller and therefore produce less differential mode radiation. Tight coupling between the signal and reference planes can amount to ~10 dB reduction in the trace loop ra- diation compared a stackup with equally spaced layers. 2. This also reduces the plane impedance (inductance) hence reduces the common- mode radiation from the cables connected to the board. 3. And, tight coupling will also decrease the crosstalk between traces. Six Layer Stackup Generally a six-layer board is created by add- ing two more signal layers between the planes of the standard four layer configuration. This has a huge advantage, in that embedding high- speed signals between the planes can reduce electromagnetic radiation by up to 10 dB. Em- bedding signals between the planes also reduces susceptibility to radiation, as well as providing ESD protection. So, not only do we prevent noise from being radiated, but we also reduce the possibility of being affected by an external source. Figure 6, illustrates the dramatic difference of radiated emissions from high-speed signals routed on the outer microstrip layers (left) com- pared to those routed on the embedded inner stripline layers (right). You can see a notable 10 STACkUP PLANNING, PART 2 continues beyond design Figure 5: Completed four-layer stackup with ~ 50/100 ohms impedance.