Multilayer PCB Design: Stackup Considerations With OrCAD X

July 8, 2024

Cadence PCB Solutions

Key Takeaways

Choose the right substrate and dielectric materials based on your PCB’s application to optimize performance.
Implement efficient routing and thermal management techniques to enhance signal integrity and heat dissipation.
Utilize advanced OrCAD X features like cross-section editors, via arrays, and constraint managers to streamline design and ensure manufacturability.

OrCAD X cross-section editor for multilayer PCB

OrCAD X cross-section editor for a multilayer PCB

Multilayer PCB Design Layer Stackup

The main difference between a double-layer and multilayer board setup is in the way the layer stackup is planned. The following are some of the points you will need to consider while planning your board layer stackup.

Multilayer PCB Design Layer Stackup Considerations

Parameter	Description
Performance	Fabrication materials are influenced by operational speed and the operating environment of the final board. Advanced materials may be better suited for specific applications, but can affect impedance calculations.
Cost	The materials used and the layer count and configuration directly affect the overall cost of building the board. It is important to work with the manufacturer to consider all options.
Density	The routing density influences the board layer stackup configuration. Adding layers later in the design process can be difficult and costly.
Circuitry	Sensitive signals may require stripline layers and additional ground planes. Analog and digital circuitry should be separated with dedicated ground planes, and onboard power supplies need isolation, affecting the layer configuration.

Material Selection

The substrate material forms the foundation of the PCB. Common materials include:

FR4: The most widely used, offers a good balance of performance and cost. It is suitable for most applications but has limitations in high-frequency designs.
Polyimide: Offers excellent thermal stability and flexibility, ideal for high-temperature and flexible circuit applications.
Rogers RO4000, R03000, and RO4830: High-frequency materials that provide low dielectric loss and are suitable for RF and microwave applications.

Furthermore, the dielectric material between copper layers affects the PCB’s electrical properties. Key parameters include dielectric constant (Dk) and dissipation factor (Df). Low Dk and Df values are preferred for high-frequency applications to minimize signal loss.

Once you’ve gathered your data and created your board layer stackup in the layout database, you can start placing and routing the board.

A Different Perspective on Placement and Routing

Aspect	Description
Internal Traces	In a multi-layer layout, think in "3D" as multiple layers interact, unlike a two-layer board which only has a top and bottom. Avoid placing noisy parts above sensitive inner-layer routes.
Component Placement	Placing components is similar to a double-sided board, but you have more routing space, as most routing will be on inner layers. This allows for more components and efficient use of surface layers for short, direct routes.
Thermal Management	Efficient thermal management is crucial. Use thermal vias to transfer heat from hot components to internal copper planes. Place power and ground planes adjacent to each other for effective heat spreading.

Routing for Multilayer PCB Design

Internal trace routing and power planes are a joy to work with, but at the same time, there are some important considerations as well:

Multi layer boards will typically have more components and, therefore, more routing than a double-sided board, so plan accordingly. Depending on the board technology, some of this routing may have specific routing widths and spaces or other requirements, such as differential pairs or impedance-controlled traces.
Some routing will require a stripline layer structure and must be routed on layers adjacent to ground planes. Additionally, sensitive routing must be crossed perpendicularly on adjacent internal signal layers to help reduce any possible broadside coupling or crosstalk.
Ground planes will have many vias for connectivity, which could affect signal return paths. This requires carefully planning your routing to avoid blocking up the planes.
Split planes need to be laid out so that sensitive signals don’t cross the splits and thereby ruin their return path. A situation like this can create a lot of noise on the board.

Once the placement and routing are done and checked, the rest of the design work will be similar to a double-sided board. Now, you are ready to have the boards built.

Older legacy PCB footprints may be inadequate for multilayer designs, adding further requirements. Depending on your CAD system, you may need to add layers or attributes to a footprint. Access to an advanced PCB design system with links to online library services can provide the latest and most accurate PCB footprint data.

Finalizing the Design With Documentation and Output Files

To get your multilayer design out for manufacturing, you will need to create the same kind of documentation that you’ve always created, but with a few exceptions.

Your manufacturing drawings might need more details. Your fabrication drawing will need a multilayer board stackup detail and notes, including the specifics of how the board will be built.
If you are using Gerber files for your manufacturing outputs, you will obviously need to generate additional files for the multiple board layers. This is where an advanced set of CAD tools can help you create and manage your manufacturing output files.

OrCAD X Features for Multilayer PCB Design

OrCAD X Feature	How it Helps Your Multilayer PCB Designs
Cross-Section Editor	Adjusts the dielectric constant for high-speed signals to calculate impedance. Visualizes the stack-up with a preview of layers and vias, and allows importing/exporting cross-section configurations for reuse.
Via Arrays	Create vias in various patterns and configurations tailored to design needs (staggered, centered, surrounding a trace or shape). Efficient via placement manages signal integrity, improves routing density, and enhances flexibility.
Constraint Manager	Sets and manages design rules for trace widths, spacing, and via usage across layers. Ensures adherence to specific electrical and physical requirements. Provides real-time feedback and error-checking. Streamlines the design process and reduces rework.
Interactive Routing	Offers real-time, constraint-driven feedback during routing. Features like hug, shove, and clearance view help manage trace placement and avoid routing violations.
Design for Manufacturing (DFM) Checks	Provides a comprehensive suite of rules for manufacturability, including constraints for fabrication, assembly, and testing.
Via Arrays	Creates vias in various patterns and configurations tailored to design needs (staggered, centered, surrounding a trace or shape). Efficient via placement manages signal integrity, improves routing density, and enhances flexibility and efficiency in complex multilayer designs.

Fortunately, there are PCB design systems available that already have the tools you need for successful multilayer PCB design. OrCAD X PCB Designer is the type of advanced system that will give you access to online CAD library services, board outline creation wizards, and manufacturing and documentation generation utilities.

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