You would have to bear in mind that the components mounted on the PCB will certainly alter the mass and most likely alter the stiffness. The mass is usually easy to get hold of, the increased stiffness is a much longer way. Usually, when the PCB is part of a bigger ElMech assy, I will try to do a simple sine sweep test of the PCB with components, and then tune my FE model to produce the same results. The tuned model is then part of the total assy model.