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PCB-Oct2017

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14 The PCB Magazine • October 2017 but for EMI, emission levels must be kept in the microvolt and microamp range. Ground impedance is at the root of virtually all signal integrity and EMI problems. To keep a low ground impedance is mandatory for both EMI and signal integrity. This is achieved with a solid ground plane. In fact, the main problem with ground impedance is the discontinuities that occur in the signal path, and it has a major impact on characteristic impedance control. Copper thickness is not an important fac- tor. At high bit rates, the skin effect dominates so the signal is pushed to the copper surface, which means that additional copper thickness is irrelevant. These days, with more and more HDI de- signs operating well up in the GHz frequencies, characteristic impedance control becomes more important, but also more challenging to main- tain, since distances between layers are shrunk. HDI and microvias require less distance be- tween layers, and the fact that more layers will be squeezed within a given PCB thickness. The biggest problem with maintaining im- pedance control is the signal path discontinui- ties, including the return path on ground plane. Ideally there should be a copper plane immedi- ately underneath critical signals and the signal should refer to this ground plane without in- terruptions. The worst scenario (Figure 2) can be if the signal is leaving the ground plane and continues (e.g., along a voltage plane). It is less problematic if the signal goes through the GND plane and continues on the other side of the plane (Figure 3). There is no issue with the reference GND since it is the same—just make sure that vias are kept as small and short as possible. The keep out (anti pad) diameter around the vias shall also be kept at a minimum. There can also be cuts in the plane such as to large anti-pads (Figure 4). In this example, half of a differential pair is exposed over a miss- PLANNING A PCB: SIGNAL INTEGRITY AND CONTROLLED IMPEDANCE CONSIDERATIONS Figure 2: The return path, shown in yellow, is discontinuous if switching planes. Figure 3: Return path shown in yellow. Reduced discontinuity if kept to same plane. Figure 4: Unbalanced differential pairs due to too wide openings in GND plane and/or signals routed to close to pad. Figure 5: Disrupted return path due to cutout in the GND plane. Return path will go around the cutout.

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