What's Good About PCB SI Metal Surface Roughness? SPB16.3 Has Some New Enhancements!

Happy New Year!

Electromagnetic Solution 2D (EMS2D) is designed for accurate transmission line parameter extraction over a full frequency range from DC up to the frequency of interest. However, this accuracy depends on an accurate and complete physical model provided to the solver by users. For example, the parameter for conductor surface roughness is missing from the input parameter list to the solver.

In multiple GBPS digital designs, the conductor surface roughness can contribute an extra 30% or more energy loss. Therefore this effect should be included in the SI analysis and the field solver should provide the lossy transmission line model with roughness effect included.

The EMS2D field solver has been enhanced in the SPB16.3 PCB SI release to include the effects of conductor surface roughness.

Roughness Model

The roughness effect is analyzed by the Hammerstad model. Due to the surface roughness, the conductor resistance per unit length is significantly increased:

 
The Hammerstad model is especially accurate for good conductors and is working very well when roughness is 1 to 3 µm (RMS). An alternative modified Hammerstad model is also provided which, based on published literature report, is more accurate than the standard Hammerstad model in certain cases.

Based on this model, effective conductivity is induced and used in the electromagnetic equations within EMS2D. As a consequence, both resistance and inductance will be modified at a low frequency range and all transmission line parameters will be impacted at high frequency when full-wave computations are done. Furthermore, the insertion loss in S parameter modeling will be larger than without the roughness effect.

Options for Defining Surface Roughness

Conductor surface roughness is defined for the EMS2D field solver by the following two parameters:

• SurfaceRoughnessModel
• RoughnessRMS

The SurfaceRoughnessModel defines the type of model that is to be used by EMS2D to compute the effects of surface roughness. The possible values for this parameter are:

• Hammerstad - Use the Hammerstad model
• ModifiedHammerstad - Use the modified Hammerstad mode
• ExponentialModel - Use a new experimental model. We don't currently have sufficient evidence for the accuracy of this model.

The RoughnessRMS parameter defines the root mean square value of the roughness surface. Typically this value is a few µm. For ½ oz. copper foil, the surface roughness is about 1.9 µm; for 1 oz. copper, it is about 2.4 µm; for 2 oz. copper, about 2.9 µm.

The above two parameters must be used simultaneously. EMS2D will not produce a roughness model if either is missing.

 
The Surface Roughness Model field has a pull-down with values of Hammerstad, Modified Hammerstad, Exponential and None. A value of None indicates that surface roughness is not to be modeled.

The Surface Roughness RMS value field allows you to specify the roughness RMS value. The default unit for this field is µm.

These two new preference values are saved in the signoise.run/casex directory. A change in either of these values causes any existing simulation data in the case to be invalid.

Two environment variables are also available to enable roughness model extraction. Similar to the options in the GUI, the two environment variables are:

• EmsRoughnessModel, takes one of the following values:
  • 1 = use the Hammerstad model
  • 2 = use the Modified Hammerstad model
  • 3 = use the Exponential model
• EmsSurfaceRoughnessRMS defines the roughness RMS value (in meters).

Please share your experiences with this new modeling capability.

Jerry "GenPart" Grzenia