BoardSurfers: Specifying Layer Information for Multi-Layer Rigid and Flex Stackups

To manufacture a product that performs as you intended, it is imperative that you share accurate stackup definitions with the board fabricator or manufacturer. An accurate stackup definition is critical to getting the best possible performance from a design. The layer stackup also affects cross-talk and net impedance; and these two factors, in turn, drive the functional performance of a PCB.

Allegro^® layout editors – Allegro^® PCB Editor and Allegro^® Package Designer Plus – include a spreadsheet-based user interface, Cross-section Editor to help you define an accurate layer stackup with key layer information to avoid any electronic component design failure. Layer information includes the number of layers and the material used for the layers in a stackup, and the attributes of each layer, such as design impedance, layer thickness, and conductivity. With Cross-section Editor, defining dielectric and conductor layers and external mask layers for rigid and flex board designs is easier than ever before.

This blog post shows how you can define a multi-layer stackup for rigid and flex PCBs and specify layer information for a design using Cross-section Editor.

Cross-section Editor

You can define a layer stackup and specify attributes of each layer in Cross-section Editor. You can access Cross-section Editor from any Allegro layout editor using the Setup – Cross-section menu command. Alternatively, use the xsection command.

Cross-section Editor displays ordered layers of the active design and the attributes of each layer, such as its thickness, material, conductivity, and so on.

Single-Stackup and Multilayer-Stackup

Cross-section Editor provides support for both single-stackup and multilayer-stackup. The single-stackup mode is the default mode, where all the electrical layers, such as conductor, plane, and dielectric are displayed in the Primary tab. 

Along with the multiple-stackup definitions support for electrical layers, this mode also supports multiple-stackup definitions for non-electrical layers such as soldermask and coverlay, which can be used in rigid, flex, or rigid-flex PCBs. You can enable the multilayer-stackup mode using the View – Multi Stackups mode menu command.

What Layer Information Can I Specify in Cross-section Editor? 

To define a useful layer stackup for a PCB design, it is important to first understand what layer information can be added using Cross-section Editor. Some of the key layer information that you can specify includes, layer material, thickness, dielectric constant, loss tangent, and so on. We’ll explore more as we move forward in this post.

Layer Sequencing

Layer sequencing is one of the many factors that determine the performance of a PCB from a signal integrity point of view. It is also an effective way to reduce the radiation from the loops formed on a PCB. The number of layers in a stackup is determined by the number of signals to be routed and the operating frequency.

A design with a poor stackup may lead to failure. For example, if there is a large separation between the power and ground planes in a multi-layer design, the interplane capacitance between the adjacent power and ground planes will be insufficient and may not provide adequate decoupling. The loss of interplane capacitance can be compensated by providing tight coupling between signal and plane layers. Adding more layers between the plane layers and placing the signal layers close to the plane layers, increases the interplane capacitance and improves the EMC performance as a result.

Adding a Layer

To add layers to stackup:

1. Right-click the Objects column and choose Add Layers.
2. Select the layer type, its function, and material in the Add Layers dialog box.
3. Optionally, select the position of the new layer.
4. Add non-conductor layers, such as mask and coating layers, if required.
5. Click Add.

Layer Materials

After adding the conductor and dielectric layers, the next step is to select the materials that are used for the conductor, plane, or dielectric layers in the Material column in Cross-section Editor. The most used materials are Copper and FR4.

Cadence provides a global default materials file, materials.dat that contains typical industry fabrication materials. You can edit the materials file to add or remove materials. This file is located at: <install_dir>\share\pcb\text.

Dielectric Constant and Loss Tangent

The dielectric constant and loss tangent properties of the PCB materials also affect the electrical behavior of the design. You can specify these attributes in the Dielectric Constant and Loss Tangent columns.

Allegro layout editor displays an error message in the Command window if any incorrect values are specified in these columns.

Shielding

Shielding prevents electrical signals on adjacent layers from interacting with each other. You can select the check box in the Shield column for a plane layer so that the simulator treats the layer as a pseudo-infinite reference plane for a transmission line.

Impedance 

Impedance for the traces can be specified in the Impedance column. The impedance depends on the dielectric constant value of the material, thickness and width of traces, and the distance from the reference plane.

You can also view both single-ended and differential impedance for microstrip and stripline in Cross-section Editor. You can either select the edge-coupled or broadside-coupled differential impedance in the Diff Coupling Type column.

Etch Factor

The Etch Factor column allows you to set an etch-factor angle for the etch, on each conductor and plane layer. If you enter a value outside of valid ranges, Cross-section Editor displays a warning and ignores the new value.

Negative Artwork

For power and ground plane layers where the entire plane is solid-filled, you can select the check box in the Negative Artwork column to reduce the artwork file size and the plotting time of the film. This creates the artwork in a reverse image where the voids are solid-filled, and copper areas are shown as clear.

Conclusion

With the increase in the layer count, sharing accurate stackup definitions with the board fabrication facility has become more critical than ever before. Specifying the layer stackup information in Cross-section Editor ensures that your design is correct by construction and less prone to failure. This information can also be passed on to MCAD systems, eliminating unnecessary MCAD-ECAD iterations.

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About BoardSurfers

The BoardSurfers series provides solutions to the various tasks related to the creation and management of PCB design using the Allegro platform products. The name and logo of this series are designed to resonate with the vision of making the design and manufacturing tasks enjoyable just like surfing the waves. Regular, new blog posts by experts cover every aspect of the PCB design process, such as library management, schematic design, constraint management, stackup design, placement, routing, artwork, verification, and much more.