Im asking about the nature of vias (conductor or dielectric), im importing a structure build in ADS into HFWorks to simulate it, i want to know if i will apply material to vias parts or just apply PEC(perfect electric conductor) as boundary condition
If you include metal/conductor layers below and above the via into simulation - you cannot define a boundary condition at via.
Current and voltage drop on via is not known and will be established through a self-consistent solution.
If, on the other hand, via leads to "outside" (for example, via on the topmost metal layer included into simulation) - sometimes, you can define a boundary condition on it (voltage or current - and possibly with some series resistance).
If via dimensions are large (as compared to a characteristic scale of voltage or current non-uniformity) - you would need to discretize it.
If via is small in size and current non-uniformity inside it is not important - it can be treated as a vertical resistor, without discretization.
Most simulators, though, are not providing such an option - to switch (automatically or not) between the dimensionality representation of metals and vias.
Normally, vias are conductors, not dielectrics.