Creo Simulate > Reference Links > Guidelines for Surface-Surface Connections and Interfaces (FEM mode)
  
Guidelines for Surface-Surface Connections and Interfaces (FEM mode)
There are several types of connections and interfaces in FEM mode that you can create using two surfaces. These entities include:
Interfaces (FEM mode only)
Contacts
Rigid links
Perimeter welds
To mesh your model successfully, you should keep the following points in mind when you create surface-surface connections and interfaces:
In most cases, the selected surfaces must belong to different parts in your assembly. If you are creating an interface, however, you can select a quilt surface and a solid surface on the same part.
You cannot select two surfaces of different geometric types. For example, you cannot create a surface-surface perimeter weld between a cylindrical surface and a planar surface.
If you want to create a solid mesh in which the meshes on the two surfaces are compatible, you cannot use offset mate constraints when you create the assembly in Creo Parametric. If there is any offset between the mated surfaces, Creo Simulate will not generate a compatible mesh.