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72 PCB007 MAGAZINE I OCTOBER 2019 ate layer transitions to avoid the point-to-point connections on the same layer. To minimize signal distortion/reflection, point-to-point con- nections are used in favour of via layer transi- tions. In this section, we analyse the SI per- formance of layer-to-layer transitions using VeCS-2 technology. Enabling the use of layer transition with little reduction in the SI performance of the trans- mission line will simplify the routing of a lot of designs. This reduces the complexity of the de- sign (e.g., layer reduction, eliminates sequen- tial build-ups, etc.). For the SI performance analysis, we used "Simbeor THz 2017.01" with thanks to Yuriy Schlepnev for translating our VeCS design in the analysis and simulation tool. From the mechanical VeCS-2 design princi- ples, the following VeCS element was created. We used one differential pair per VeCS, where the rest of the element is shield- ed in a top view representation (Figure 9). We created an eight-layer stackup targeting a 95-ohm dif- ferential impedance, as shown in the 3D view in Figure 10. When we perform the second route, we clean the bottom sec- tion, create the vertical traces, and remove the stub on the top of the VeCS traces. This can all be done in one cycle. Layer 3 and 7 are the signal layers, and the others are reference layers. The first objective in the analysis was to perform a time-domain simulation and tune the trace width and anti-pad sizes such that we create the smallest distortion possible. We started with a traditional capacitive response as expected form a layer transition (e.g., via through hole) and began to tune the VeCS ele- ment such that we got close to a flat line. More modification changed the response from a ca- pacitive (red and blue lines) to an inductive (green line) element, as is shown in Figure 11. We used the results of the TD simulation to perform the 30 Gb/s bitstream eye-diagram Figure 9: Top view of the VecS element. Figure 10: Eight-layer stackup. Figure 11: TDR response as a function of the variable vertical trace widths.