Analyst-MP typically treats solid electrical conductors by applying an appropriate boundary condition to the conductor surface instead of solving inside the conductor material. The boundary condition replacement is an excellent approximation in most cases, and solving inside the conductor may be very costly since the fields vary rapidly close to the conductor surface.
Nevertheless, sometimes it is more appropriate to solve inside conductors, for instance if the material is thin compared to the skin depth or if corner effects are important. Driven-frequency and eigenmode simulations allow the user this choice. To do this, set the Solve Inside Conductors value for the materials's properties to Yes: