Conductor loss settings in AXIEM AWR

Ohm's law works at low frequency when the volume current fills the conductor section. The resistance at low frequency is defined as R_DC = 1/σt, where t is the metal thickness and σ is the conductivity.

Skin effect shows that at high frequency, when the current is constrained to the area adjacent to the metal surface, the penetration depth is defined as skin depth:

The overall impedance is composed from low-frequency and high-frequency resistance values.

In AXIEM software, conductors are modeled as either thin or thick. A thin model has mesh only on the conductor bottom (where the unknown current is solved); a thick model has mesh on all metal boundaries.

Thin Metal Modeling

The AXIEM software only meshes the bottom of the conductor; therefore, the unknown current is in only one layer. This results in fewer unknowns and faster problem solving.

You can turn on thin metal modeling in the Options dialog box on the Mesh tab by selecting Model as Zero Thickness.

Thick Metal Modeling

The AXIEM software meshes all metal surfaces in thick metal modeling; so, the unknown current is on all sides of the metal surface. When the frequency is high enough, the skin depth is a fraction of the metal cross-section. At low frequency, the current is uniformly distributed inside the metal. The DC surface resistance must be doubled for the current on the top and bottom layers, such that when they are in parallel, they give the correct DC resistance.

Enhanced Thin Metal Modeling

At high frequency, due to the skin effect, surface current flows on all sides of the conductor. In a thin metal model, since only the conductor bottom is meshed, all of the current is forced to flow on the bottom surface. This reduces the area in which current flows, and the loss is overestimated. To imitate the real current flow, you reformulate the high-frequency resistance.

In the AXIEM software, you can turn on the enhanced thin metal model by selecting the secondary option Enhanced Loss Model for Thin Mesh. This option only applies to metal defined by conductivity and thickness. It does not apply to metal modeled by an impedance definition.

Enhanced Thick Metal Modeling

For thick metal modeling, at low frequency, a common practice in commercial EDA software is to redefine R_DC=1/(σt/2)=2/σt. However, this does not correctly represent the currents on the sidewalls and tends to underestimate the loss.

To solve this problem, you need to investigate the volume current density and model it correctly with unknowns on all sides of the trades. So, R_DC is no longer a constant.

In the AXIEM software, this enhanced thick metal modeling is the default.

Loss Model for Conductor Surface Roughness

In the manufacturing procedure, copper foils are treated for roughness to increase adhesion. Conductor-surface roughness has undesirable effects on the loss and dispersion (frequency dependence of ε _eff) at high frequency, when the skin depth approaches the height of the roughness grooves. The AXIEM software can correctly capture these effects.

You can define effective conductor-surface roughness (RMS) in the Element Options - ENCLOSURE Properties dialog box on the Materials tab.

Loss Modeling for Vias

Vias can be in any shape; however, most commercial software only use the metal thickness defined in the material type to get DC resistance, which is often incorrect. The AXIEM software can calculate the DC resistance for vias in arbitrary shapes. You can select the secondary Alternative Via Resistance Calculation option to better calculate loss for vias.

Team SimTech  

Cadence Design Systems