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October 2017 • The PCB Magazine 51 HDI'S BENEFICIAL INFLUENCE ON HIGH-FREQUENCY SIGNAL INTEGRITY ground planes drop. "When the dielectric thickness is 1 mil, there can be as low as 33 pH/square of loop in- ductance. When the current travels in conduc- tors that are shaped like a square and have the same length as width, the loop inductance is in- dependent of the size of the square. This repre- sents a rough approximation to the typical in- terconnect inductance in the power and ground planes. Between the decoupling capacitor pads and the chip attach pads, the connecting loop inductance can be about one square's worth. Thin HDI layers can keep the power and ground inductance very low [3] ". Signal Propagation and Interconnect Delay Higher via density, microvias (smaller vias), and via-in-pads allow components to be placed closer together, reducing wiring delays by a fac- tor of up to 50%. Signals propagating through a transmission line have a characteristic veloc- ity that is primarily determined by the square root of the effective dielectric constant of the surrounding medium. The reciprocal of this ve- locity is the signal propagation delay per unit of conductor length. This is for a classical mi- crostrip topology, but if the conductor is locat- ed between two reference planes, then it is pro- portional to the square root of the material's di- electric constant: Where: e = dielectric constant of substrate Interconnects with a time delay shorter than about 20% of the rise time of the signal may not need to be terminated. EMI Radiation Ground Return Path Voltage and current waves are supported by propagating electric and magnetic fields. The ideal return path is continuous and uniform. This is usually not the case in high-speed dense circuit boards. The more the return path is non- ideal (with discontinuities), the more it produc- es ground loops. The ground loop of the con- ventional through-hole with innerlayer power and ground was discussed earlier and is shown in Figures 7 and 10. The characteristic of con- tinuous and uniform is illustrated in Figures 9a, 9b, and 9c. Figure 9a: Innerlayer plane ground plane relief. Figure 9b: Primary side ground plane relief (with the ground removed for clarity). Figure 9c: Secondary side ground plane relief (with the ground removed for clarity).