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32 The PCB Design Magazine • January 2016 As you can see, there are four planes in the center of the board, two power and two ground. This is where tight coupling, between adjacent planes, can be utilized to add planar capacitance at low cost and dramatically reduce the AC im- pedance at the high end. There are thin sheets of Isola 370HR 1080 prepreg (2.8 mils thick) be- tween both planes pairs. Given the effects of the capacitors equiva- lent series inductance (ESL) and mounting in- ductance, the added planar capacitance still re- duces the overall impedance to approximately the target impedance up to 1GHz as in Figure 2. Now, this is not easy to do using standard stackups. With the continuous trend to smaller feature sizes and faster signal rise times, planar capacitor laminate (PCL), also known as embedded capac- itor material (ECM), is becoming a cost-effective solution to further improved power integrity. This technology provides an effective approach for decoupling high-performance ICs whilst also reducing electromagnetic interference. Plane pair cavity resonances contribute to emissions. Smaller 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. However, the smaller the plane separation, the higher the Q of the cavity can be, implying a higher field strength at the plane pair edges. Embedded capacitance technology allows for a very thin dielectric layer (0.24–2.0 mil) that provides distributive decoupling capaci- tance and takes the place of conventional dis- crete decoupling capacitors over 1GHz. Unfor- tunately, standard decoupling capacitors have little effect over 1GHz and the only way to re- duce the AC impedance of the PDN above this frequency is to use ECM or alternatively die capacitance. These ultra-thin laminates replace the conventional power and ground planes and have excellent stability of dielectric constant and dielectric loss up to 15GHz. The thinner layers of ECM, also significantly reduces the ca- beyond design PLANE CRAzY, PART 2 Figure 2: DDr3 PDn with low impedance up to 1gHz.

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