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44 The PCB Magazine • October 2017 from the adage "Smaller and closer is better!" That is, HDI's main contribution is miniaturiza- tion! The signal integrity improvements for HDI come from three phenomena: noise reduction, EMI radiation reduction, and improved signal propagation and lower attenuation. Noise Reduction Four categories of noise describe the various effects (Table 1) [2] . 1. Signal quality of one net and its return path (ringing due to reflections) 2. Crosstalk between two or more nets (noise pulses due to switching on neighboring lines) 3. Switching noise (noise on power and ground lines/planes) 4. EMI Each of these four categories has specific causes (Figure 2). By identifying the cause in each category of problems, design- and technol- ogy-based solutions can be identified and im- plemented [3] . Noise can come from many sources in the board layout, such as: 4 Changes in trace width 4 Plane splits 4 Cutouts in power/ground planes 4 Via antipads (the opening in a power or ground plane for a drilled hole so it doesn't make contact) 4 Insufficient plane capabilities 4 Excessive stubs, branched or bifurcated traces 4 Component lead frames 4 Improper impedance matching and termination networks 4 Coupling between signals 4 Varying loads and logic families Signal Quality of One Net and its Return Path HDI is a fabrication technology of minia- turization that has two main benefits: a small- er substrate and improved SI. The smaller sub- strate is due to the shorter interconnect length, smaller vias, and thinner dielectrics of lower di- electric constant materials. These things also improve the signal integrity. With HDI, devices can be brought so close together (both from a surface point of view and using the secondary or backside of the intercon- nect) that the signal may not need to be termi- nated. "Interconnects with a time delay short- er than about 20% of the rise time of the sig- nal may not need to be terminated [2] ." The in- terconnect length is given by: Where: t = signal rise time in nsec, and er = dielectric constant of the material [2] . For a rise time of 1 nsec for FR-4, this is only 1.14 inches. The signal return path is just as important as the signal path, and exists whether you pro- vided for it or not. The signal return path con- tributes to the inductance, capacitance, and re- sistance experienced by the signal. The signal return current will seek the path of minimum energy, which has the least impedance. For low frequencies, this path will have the least resis- tance; for high frequencies, the path will min- imize the current loop. At higher frequencies, inductance dominates over resistance, so the re- turn path follows the signal path even though it meets higher resistance. Figure 2: Four categories of SI problems (Source: Eric Bogatin [3] ). HDI'S BENEFICIAL INFLUENCE ON HIGH-FREQUENCY SIGNAL INTEGRITY