wlcsp said:
You gotta have prior knowledge about your model. You should at least have some idea how the electromagnetic field would behave within your model. The best examples for this case are the CPW and rectangular waveguide.
1.CPW
We know that it consists of one signal line and two ground line on left and right side of the signal line. The CPW mode is that the electric field will equally distributed from the signal line to the ground line. The electric fields are are parallel to the symmetry plane. Therefore, perfect-H symmetry can be used. The same can be done to microstrip.
The E field in the substrate between trace and ground plane, no problem, perpendicular to trace and ground, so use pefect H, But, the exact mode of microstrip line is not TEM moed, there must be E field component along the surface of trace and ground. If you use perfect H, that means you neglect this part of field, is it?