Issue link: https://iconnect007.uberflip.com/i/1140547
58 DESIGN007 MAGAZINE I JULY 2019 has the shortest via height (2.65 mils)—thus the lowest interconnection inductance (a total series inductance of 0.95 nH)—and incurs the lowest impedance. At 1 GHz, a further increase of via height to 7.95 mils in model 1B worsens the impedance by 1.5ohm versus model 1A. Model 1C with the highest via height of 13.95 mils and largest interconnection inductance (a total series inductance of 1.42 nH) incurs addi- tional 1-ohm impedance at 1 GHz compared to the other two models. B. Impact of Trace Length To investigate the effect of interconnection inductance due to trace length on the imped- ance of decoupling capacitors, the modeling and de-embedding steps are repeated for mod- el 1D and 1E with trace length, l, at each end of the capacitor set as 10 mils and 0 mils, re- spectively. Models 1A, 1D, and 1E have the same stitching via height, h, of 2.65 mils. A 3DEM structure of model 1E with a 50-mil fix- ture trace is shown in Figure 7. With reference to impedance plots shown in Figure 8, the three models experience the same resonance frequency—103 MHz. Across the wideband range, model 1E with no inter- connect trace but with via-in-pad and thus the lowest interconnection inductance (total series inductance of 0.68 nH) experiences the low- Figure 6: Plots of impedance (L) and series inductance (R) for models with varying stitching via heights after de-embedding. Figure 7: A 3DEM structure of model 1E together with a 50-mil fixture trace.