There are a lot of reasons to mix SI and PI analysis – trying to figure out a worst-case stimuli to determine PDN performance, trying to see the effect of a poor PDN on signals due to SSN issues, or just being able to properly analyze a single-ended via. In fact, to get an accurate model of a single-ended via, you have to mix SI and PI. The reason is that the power distribution network, or PDN – decoupling capacitors, stitching vias, and plane pairs – make up the signal return path for a single-ended via, thus determining the “impedance”, delay, and crosstalk characteristics of the via. This is especially crucial when modeling very fast, single-ended signals like DDR3 and DDR4 signals. In HyperLynx, you can extract via models which include the PDN; this is achieved by the PI engine running in the background during the via model extraction. The result is a very accurate wideband single-ended via model that properly captures the via’s electrical characteristics. Once extracted, this model can be used in other simulations again and again. Another option is to use the SI/PI co-simulation option, which generates these models on-the-fly.
To learn more about mixing SI and PI, take a look at this article in EDN: