In this paper, we suggest a simulation flow that allows the early detection of crosstalk noise, caused by power supplies on nearby sensitive signals, in high density mixed-signals boards. This paper focuses on the PCB and power supply modelling and discusses the accuracy of ground planes modelling in S-parameters modules, as current return paths in ground planes can be a major source of coupling. Our approach is based on, and validated with, a comparative study between simulation results and measurements on a real case study board.