In high-speed digital systems with signals at 20 Gbps, standard approaches used to study the signal integrity at PCB and package level are not valid anymore. IBIS models are no longer accurate, and H-spice transient simulations face convergence and stability problems. Another important issue is the preservation of passivity and causality related to the inaccurate material characterization of the electromagnetic models. How should these problems be addressed? In the paper answers are provided by considering a 26-layer board as a test case. An electromagnetic model is presented and validated by means of time and frequency domain measurements. A practical workflow and design rules for the analysis of differential signals are also proposed.