Best Practices for Efficient and Effective Planar EM Simulation
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Geometry simplification rules are used in the EM layout to invoke negative layer operations, which are used for cutting holes
in power planes. Typically, negative layers are needed for vias in multi-layer boards to cut out holes (anti-pads) in internal
power planes and to cut metal away from a planar circuit when the surface is covered in metal. This technique of "ground
flooding" the circuit side of the board is often used by designers for mechanical, thermal, and electrical reasons. Figure 12
shows a simple example of a signal line and attached via surrounded by a ground plane.
Figure 12: Ground flood on a positive layer Cu_01+ with negative layer automatically drawn on Cu_01_ (left), hole cut in the ground flood with
shape simplification rules (right)
Three draw layers are needed when using negative layers. They all have the same name with the exception that the negative
layer ends with a minus sign and the positive layer ends with a plus sign. The normal layer is listed first, followed by the
negative layer, and finishing with the positive layer. The negative layer cuts a hole in the positive layer. Therefore, the designer
draws the ground plane on the positive layer, and the anti-pad is on the negative layer. In Figure 12, an artwork cell is used for
the via and a PCell for the signal line. Both layout cells already have negative layers assigned. The shape simplification rules
are used to subtract the negative layer from the positive one, but simplification rules are not executed until the preview
geometry command is used. This shows the final layout sent to AWR AXIEM analysis. The example in Figure 12 shows a hole in
the ground plane for the signal via and around the signal line. Finally, note that when using an EM extraction shape, simplifi-
cation rules can be set up in the extraction; if no rules exist, the negative layers are still subtracted from the positive layers.
