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NOVEMBER 2021 I DESIGN007 MAGAZINE 33 view of top strip. e scale in all three plots is 0 to 120 nA/m. In both the hollow and solid barrel cases, the current is clearly flowing on the near side of the via barrel. ere is also a small amount of current that flows down the inside surface of the via barrel. e return current in the ground plane can also be seen in Figures 2a and 3a. We tend to forget that microstrip is a two-conduc- tor system—the return path is just as important as the signal path. Figures 2b and 3b show a top view of the complete structure. Note the non- uniform current distribution across the width of the strip. Finally, in Figures 2c and 3c, there is a view of the bottom side of the top strip. Comparing the last two views we note that there is more current on the bottom side than the top side of the trace. At low frequencies, the current would split more or less equally between the top and bottom surfaces of the strip. But, as frequency increases, the current distribution shis toward the bottom side of the strip due to the proximity effect. e cur- rent distribution of the hollow and solid bar- rel has not changed significantly at 18 GHz, although this would be quite different at DC. e equivalent inductances for the hollow via and solid via cases are virtually identical. At 18 GHz, skin depth effects alone will force the current to the surface of the solid via. Filled vias have little impact on signal integ- rity at high frequency. However, there is a definite benefit in using filled vias if thermal relief is the goal. At DC, filled vias also provide Figure 2: Via with hollow barrel. Figure 3: Via with solid barrel.

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