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Best Practices for Efficient and Effective Planar EM Simulation 16 www.cadence.com/go/awr The thickness of lines for a project can be toggled on and off globally using the Model as Zero Thickness setting under the Options>Mesh tab. The right picture in Figure 16 shows the mesh for a thin microstrip line with the total number of facets at 57. Edge meshing is used, as discussed in Best Practice Tip #5, and the mesh consists of 57 unknowns for this simple problem. The lower right picture shows the mesh for thick metal. The top, the bottom, and the sides of the line are meshed, and the number of unknowns is 204, larger than the infinitely thin case. Note that edge meshing is still used for the thick metal case. As discussed in Best Practice Tip #5, edge meshing for thick metal is not usually set by default. The author usually runs thick metal with edge meshing, arguing that it can't hurt much, and the current still peaks at the edge of the line, although not as dramatically as the infinitely thin case. For most common PCB geometries, simulating with zero-thickness lines is preferred. However, there are cases where thickness can be important: f MMICs: The signal lines are usually set in the PDK to simulate non-zero thickness lines. The metal layers, when combined, can be several microns thick for a MMIC, which is a significant fraction of the typical 100µm substrate thickness. It is therefore important to keep the lines a non-zero thickness. This is critical for structures like spirals. f Internal signal layers in modules: The signal line can have a thickness that is a significant fraction of the distance to the neighboring ground planes. f Tightly coupled, distributed filters: The coupling between the filter elements can be sensitive to the line thicknesses. f Spiral inductors, especially on chips: The spacing between turns can be about the same distance as the line thickness. f Coplanar lines: The capacitive coupling of the signal lines to the side grounds depends on the thickness of the metal; therefore, non-zero thickness metal should be used. Unfortunately, because of the ground plane this raises the mesh count significantly. One trick that can be tried is to break the ground plane up into two shapes, where a rectangle about the width of the gap is set to non-zero thickness and the rest of the ground plane is set to zero thickness. The capacitive coupling is accurately modeled, and the mesh count is not increased as much as by making the entire ground plane non-zero thickness (Figure 17). The mesh density is set to low for the power plane and to medium for the neighboring non-zero thickness strip. Edge meshing for thick metal is turned on. f The vertical distances in the STACKUP have been set for non-zero thickness lines: When a layout is imported from an external layout tool, the STACKUP has already been set. If the lines and ground planes are then set to draw without thickness, the vertical positioning will be wrong. For example, vias will be the wrong length. Figure 17: Side-ground polygons set to thick metal (top left), grounds set to thin metal except for the region next to the signal line (top right), and the mesh showing the thick side-ground strips with edge meshing for thick metal enabled (bottom)

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