This application note covers some valuable layout concepts for doing printed circuit board designs using Microwave Office. It demonstrates how to use positive and negative layers to ease drawing ground flood regions. By utilizing some simple setup procedures it is possible to have all lines draw with a user-specified spacing to the ground flood automatically. This technique greatly simplifies drawing the ground flood when the initial layout for a circuit is complete. This application note also discusses various ways that Gerber files can be created in Microwave Office. One way is to merge the positive and negative layers to make one Gerber file. The other way is to keep the positive and negative layers separate if using a paint-scratch-paint processing technique.
Board Layout Simplification Techniques
This Application note demonstrates:
- the capability of Microwave Office to streamline Printed Circuit Board (PCB) designs
- how to properly set up Microwave Office to do PCB layout in an efficient manner, and the advantages of this approach
- several options for exporting the layout, focusing on different options for creating Gerber files.
In a PCB design, all of the components (active and passive) are used as discrete parts. The circuit is then assembled on a microwave substrate with metal patterns etched or machined to make transmission lines. This document assumes that the reader has a strong understanding of Microwave Office layout concepts. For more information on layout, see the Microwave Office User's Guide "Layout" chapter and the layout principles application notes available on AWR's web site (www.mwoffice.com). Additionally, the Microwave office project used to make all the figures in this application note is available.
The examples included here use a process technology that has a two-sided microwave board with thin-film resistors on both sides of the board. Chip components are only allowed on the top of the board. This may not be a practical technology, but it helps demonstrate many of the concepts discussed here. These techniques and discussions can easily be extended too multi-layer boards.
Typically, when designers are doing a PCB design, they design the entire circuit to fit into a fixed area. The last step is to draw the top ground plane so that it covers any area not populated by other components. While there is nothing wrong with this approach, drawing the top ground can be difficult if the board is complex and especially if there are curved lines or circles in the layout. This technique becomes even more difficult if the designer needs to make changes. Now instead of just moving one component in the layout, the component and the drawn ground plane all must be moved. The technique presented here eliminates all of these problems and allows the software to draw the proper spacings from transmission lines, pads, components, etc. to the ground plane.
Board Layout Setup
The Microwave Office layout tool simplifies the PCB layout through the use of positive and negative layers. The Microwave Office User Guide contains an excellent explanation of positive and negative layers. The basics of this concept are that for a named layer (e.g., Metal) you can have a normal layer, a positive layer, and a negative layer. The positive layer is specified by adding a "+" after the layer name (e.g., Metal+ is the positive for Metal) and the negative layer is specified by adding a "-" after the layer name (e.g., Metal-). These three layers represent one mas layer used for processing. When combined, the shapes on the negative (Metal-) layer are subtracted from the positive (Metal+) layer and the results are added to the normal layer (Metal).
When a layout is exported, Microwave Office can merge the normal, positive, and negative layers when creating the exported file. You can also specify to export each of these layers separately. Finally, you can use te built-in scripting environment to create one file that has all three layers' information using the Gerber format, allowing positive and negative layers. These formats are used with photopolotter machines that utilize a "paint-scratch-paint" processing technique. To visualize what a layer looks like when the normal, positive, and negative layers are merged, the drawing layers in Microwave Office for these layers need to be set up in a particular manner.
Figure 1 shows a microstrip line as a basic example of how these layers are used. The entire board surface is drawn with the positive layer, then the negative layer is drawn to cut a hole in the positive layer. This layer is drawn larger than the microstrip geometry. Finally, the normal layer is drawn to the exact dimensions of the microstrip line. The offset of the negative layer ffrom the normal layer creates the spacing to the ground plane.
The key to making this work is defining the necessary layers before designing the circuit. For the technology used in this example, layers are needed for:
- top and bottom metal
- top and bottom resistor
- package leads
- board outline
The following sections cover the steps to properly create the drawing layers, the layer mapping, and the line types for the chosen technology.
Layout Process File (LPF)
Board Layout Examples
Line Type Settings
Moving Lines Between Layers (Layer Mapping)
Adding Cutout Regions
Adding Cutout Regions to Artwork Cells
Drawing Ground Spacings for Unassociated Drawing Objects
Exporting the Artwork