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October 2017 • The PCB Magazine 47 pled near-end noise would be given by the fol- lowing equation:" [2] Crosstalk in HDI substrates is reduced by the shorter coupled lengths and by the lower dielec- tric constant by as much as 50%. Shorter trace lengths will radiate less, and traces with thinner dielectric will radiate less. The example in Fig- ure 5 shows that the shorter the coupled length, the less the mutual inductance (Lm), and the thinner the traces, the less the mutual capaci- tance (Cm). Moreover, the thinner the distance to the reference plane, the lower the near-end crosstalk will be, or the same cross talk for a longer coupled length. With length reductions of 2x and dielectric thickness reductions of 2x over conventional boards, the radiated field from HDI signal loops might be reduced by as much as 4x, which is 12 dB. Simultaneous Switching Noise and Voltage Rails The actual circuit performance varies with the rise-time of signals. Because most of these larger/higher-performance HDI boards deal with high-speed computer busses and telecom signals, they are very sensitive to noise and sig- nal reflections. Simultaneous switching noise (SSN) is the most difficult type of noise to con- trol. Switching noise originates from the instan- taneous demand for current as devices turn on and off. Any drop in the power supply voltage will adversely affect components, the power distribution in the board, and how devices are connected to ground. Keeping the inductance of the power and ground distribution low and the inductance to the ground connections low reduces this type of switching noise or 'ground bounce.' Conceptually, that reduction looks like the following equation: To manage simultaneous switching noise (SSN), the focus needs to be on: 1) minimizing di/dt; 2) good selection of decoupling capacitance; 3) careful induction management. Reducing L effective can be helped by HDI tech- niques. The use of area array packages instead of peripheral leaded packages is one way. Watch- ing how ground is assigned on peripheral lead- ed packages is another. Increasing the number of power/ground leads to packages and using a power/ground plane in the package (even a floating plane) helps. However, the major issue is board layout. Nearly 70% of BGA and QFP in- ductance is due to breakout routing on the PCB or the ground return path. The choice of mi- HDI'S BENEFICIAL INFLUENCE ON HIGH-FREQUENCY SIGNAL INTEGRITY Figure 4: Near-end crosstalk coefficient (Source: Eric Bogatin [3] ). Figure 5: Crosstalk vs. length (Source: Buchanan [6] ).