The Surface Roughness subnode is available from the context menu (right-click the Impedance Boundary Condition or Transition Boundary Condition parent node) or from the Physics toolbar, Attributes menu.

Select a Surface roughness model — Sawtooth (the default) or Snowball. For Sawtooth, enter a value or expression for the Surface roughness ΔRMS (SI unit: m). For Snowball, enter a Snowball hexagon cell area Ahex (SI unit: m2). Then enter values for the Snowball radius (SI unit: m) and Number of snowballs in the table, adding as many rows as you need.

Select the Skin depth type to be Physics-controlled (the default) or User defined. For User defined, enter the Skin depth δ (SI unit: m).

The Surface Roughness increases the impedance, and consequently it decreases the surface current density of Impedance Boundary Condition or Transition Boundary Condition, proportional to the ratio between the impedance of a rough surface and that of a smooth surface:

where ΔRMS is the root mean square of the surface roughness, and δ is the skip depth of the material (Ref. 1).

where Ahex is the hexagonal area of a unit cell, Ni is the number of snowballs, ai is the radius of a snowball, and δ is the skip depth of the material (Ref. 2).

For Transition Boundary Condition, select Side — Upside (the default) or Downside to define on which side the Surface Roughness is applied. The red arrow visualized on the selected boundaries always indicates the upside.