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44 DESIGN007 MAGAZINE I FEBRUARY 2020 inch and extremely low inductance, 3M ECM is the highest capacitance density material on the market. Table 1 lists available ECMs. ECMs have excellent stability of dielectric constant (Dk) and dissipation loss (Df) up to 15 GHz. These specialized ECMs are precision-fab- ricated laminates with extremely thin dielec- trics. However, if your budget is limited, then a planar capacitor can be constructed with two build-up layers so that they are close to the top/bottom of the stackup. Alternatively, use a thin 2-mil laminate as a plane pair on the inner layers. These are not as good as ECM but much better than a typical thicker laminate. Please check the minimum thickness with your fabri- cation shop first. The goal of designing a high-performance PDN is to reduce the impedance peaks below the target impedance level and push the peak frequency components above the bandwidth of the signals. To achieve this, one needs to reduce cavity resonance and emissions. • A thin dielectric in the plane cavity is the most effective way of reducing the peak amplitude of the modal resonance. It reduces spreading inductance and the impedance of the cavity and reduces the resonance peaks by damping the high- frequency components. Thinner plane separation implies less area of equivalent magnetic current at the plane pair edge, or equivalently less local fringing field volume, and therefore lower emissions for a given field strength • A dielectric material with a high Dk should be selected to add more planar capacitance. This is contrary to the typical choice of high-speed materials that require a low Dk. Remember, we are talking about the dielectric embedded between the planes, which have little impact on the signal properties • The parallel resonant frequencies of the cavity can be pushed up above the maxi- mum bandwidth of the signals by reduc- ing the plane size and by adding stitching vias between (similar) planes of a cavity • Where the length of a rectangular plane is a simple multiple of its width—such as 1, 1.5, or 2—the resonant frequencies of the length and width directions will coin- cide at some frequencies, causing higher-Q peaks (more intense resonances) than usual. Thus, it is best to avoid square planes and simple L:W ratios by choosing irrational numbers • When plane pairs resonate, their emis- sions come from the fringing fields at the board edges. With ground/power plane pairs, edge-fired emissions can be reduced by reducing the plane separation, as de- scribed earlier, but this technique cannot generally be used for multiple planes. Al- ternatively, make the power planes slightly smaller (~200 mils) than the GND plane. This modifies the pattern of the fringing fields, pulling them back from the edge, and may help reduce emissions to some extent Table 1: ECM (available in the iCD Dielectric Materials Library).