In this paper, we suggest a simulation method that allows the early detection of crosstalk noise, caused by power supplies on nearby sensitive signals, in high density mixedsignals 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 means of coupling. Our approach is based on, and validated with a comparative study between simulation results and measurements on a real case study board developed by Atos for HPC applications.