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AUGUST 2021 I DESIGN007 MAGAZINE 57 via barrels is decreased to 5-mil in length. On the other hand, the TL model E is like model D, except the short bar connecting the two via barrels is widened to 9-mil width. As shown in the TDR plot depicted in Fig- ure 3, the through-hole via stub in TL model A causes a capacitive impedance discontinu- ity of 22 ohms. With stacked microvias, the capacitive impedance mismatch experienced by TL model B is reduced to 3 ohms. Mean- while, with a short bar in the staggered micro- via, the impedance mismatch encountered by TL model C becomes inductive and rises to 6 ohms. e inductive discontinuity in staggered microvias can be diminished by shortening and widening the short bar, as experienced by model D (5-ohm discontinuity) and E (4-ohm discontinuity) respectively. Hence, the stacked microvia experiences the minimal imped- ance mismatch vs. the other via structures. But if staggered micro- vias are selected for sig- nal routing, the short bar shall be kept at the minimal length to min- imize impedance mis- match to comply with DFM standards. In the Sdd21 plot depicted in Figure 4, the through-hole via stub in TL model A causes a resonant dip of -65 dB at 22 GHz. With stacked microvias, TL model B experi- ences Sdd21 of -3.6 dB at 10 GHz and -9.5 dB at 50 GHz respectively. Meanwhile, the impact of inductive impedance discontinuity due to the short bar in the staggered microvia, becomes significant for frequency range beyond 20 GHz. e Sdd21 encountered by TL model C is worsened by 0.1 dB at 20 GHz and 1.1 dB at 50 GHz respectively, versus TL model B. By shortening and widening the short bar in the staggered microvia, the Sdd21 at 50 GHz encountered by model D and E is reduced by 0.4 dB and 0.6 dB respectively, versus C. ere- fore, stacked microvias experience the minimal insertion loss. e impact of via structure is further studied by analysing an eye diagram at the receiving (Rx) end, with the injection of non-return-to- Figure 2c: Cross-sectional (left) and top (right) view of staggered microvia. Figure 3: TDR plots for TL model A, B, C, D and E.