Surface Roughness
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.
Surface Roughness
Select a Surface roughness modelSawtooth (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:
Sawtooth model
where ΔRMS is the root mean square of the surface roughness, and δ is the skip depth of the material (Ref. 1).
Snowball model
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 SideUpside (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.
References
1. E. Hammerstad, O. Jensen, “Accurate Models for Microstrip Computer-Aided Design”, Microwave symposium Digest, 1980 IEEE MTT-S International, pp. 407–409, May 1980
2. P.G. Huray, The Foundation of Signal Integrity, Wiley-IEEE Press, 2010