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52 The PCB Magazine • October 2017 In Figure 9a, the ground planes of a high- speed dense multilayer are shown. For this 9.2- inch by 6.3-inch 18-layer board, 8.46 square inches of copper is etched away to make room for the through-holes. Figure 9b is the 10-lay- er HDI multilayer that replaced the original 18-layer multilayer. The surface ground plane (primary side, Figure 9b) has only 6.63 square inches removed and the secondary side (Figure 9c) has only 6.35 square inches removed. This is 21.6% and 24.9%, respectively, less discontinui- ties for the return path. In addition, you can see that at the fine-pitch BGA devices, the ground copper goes substantially all the way into the center ground pins. The return signal may encounter breaks in the ground plane, which are a known source of noise, but many do not recognize that large through-hole antipads on the PWR/GND planes under finer pitch BGAs can also cause ground loops that will create noise. When generated under the BGA, ground loops are very difficult to locate. This situation is depicted in Figure 10. Details on Improved RFI and EMI At 1.00 mm pitch BGAs, which are 0.040" center to center, the copper left between the antipads of a 0.013" dia. PTH is 0.007" or less. When you drop to 0.8 mm pitch devic- es (0.032"), a 0.013" diameter TH would wipe out all the copper. Dropping to 0.008" diame- ter drilled holes will provide only 0.004" of cop- HDI'S BENEFICIAL INFLUENCE ON HIGH-FREQUENCY SIGNAL INTEGRITY per between the antipads. This ideal situation does not take into consideration drill accuracy and the difficulties in metallizing such a small hole. On higher I/O BGAs, only one trace can be routed out between the adjacent holes. There will be a number of signal aberrations because the reference planes will be too small. At 0.65 mm pitch (0.032"), not only do you have to use HDI, but the microvias may have to be stacked. The 22% and 25 % greater ground return area on moving GRN to the SMT layers not only reduces noise, but the two grounds on the outer layers can serve as a Faraday Cage to minimize radiated emissions. Decoupling The energy storage capability of a multilayer can be significantly enhanced by HDI. Figure 11 compares the normal decoupling with an im- proved HDI structure. The normal through-hole multilayer is simulated in Figures 12a through 12d. The inductance of the TH vias contributes to the reduced effect of the capacitors as fre- quency goes up. As shown in Figures 12c and 12d, the through-holes act as excessive stubs af- fecting impedance, phasing, and signal reflec- tions. The vias represent nearly 3 nH each. Bogatin points out that "Even by using he- roic efforts to get the inductance of the pow- er and ground distribution of the substrate as Figure 10: Ground loops. (Source: Steve Bird [7] ) Figure 11: A comparison of conventional decoupling capacitor versus the HDI approach where GRN is flooded on the surface and PWR is layer two. (Source: Steve Bird [8] )