Rigid-Flex Command Setup and Layer Stack Design in Altium Designer

 Rigid-flex PCBs integrate rigid sections and flexible circuits onto a single board, providing engineering teams with design and assembly benefits. However, these hybrid rigid-flex designs bring additional complexity when it comes to configuration and layer planning.

This guide will cover critical steps in Altium Designer to setup and use dedicated rigid-flex design rules, define rigid and flex regions, properly generate layer stacks, and produce manufacturing outputs. With the right approach, rigid-flex designs can be handled smoothly within Altium's unified PCB editing environment.

Rigid-Flex Design Benefits

Before jumping into the details, let's review the key advantages of a rigid-flex approach:

• Simplified assembly - Integrating multiple PCBs onto a single flex assembly reduces mechanical parts and connectors.
• Space/weight savings - Folding flex regions allows fitting PCBs into tight spaces while reducing overall weight.
• Increased reliability - Direct connections between rigid areas on a single assembly eliminate connectors and solder joints that could fail.
• Enhanced serviceability -Components can be placed on rigid islands for easy access, with interconnects routed through flex layers.
• Noise reduction - Careful segmentation and isolation of circuits using both rigid and flex regions lowers coupling and interference.
• Improved manufacturability - Rigid-flex allows combining incompatible or advanced fabrication steps onto specific sections of a single board.
• Cost effectiveness - A single rigid-flex product can replace complex, expensive mechanical assemblies and custom connectors.

Realizing these benefits does require added planning for the layer stack, flex-rigid transitions, and manufacturing steps compared to simple rigid boards. Proper command setup in Altium streamlines the process.

Command Manager Configuration

Altium's Commands system allows customizing the user interface with useful shortcuts and options tailored to specialized design scenarios like rigid-flex. Creating a dedicated command set for these boards simplifies accessing associated tools and layers.

To define rigid-flex commands:

1. Open the Commands editor from the Tools menu.
2. Use the New button in the Commands editor toolbar to create a new command set.
3. Rename the new set to something descriptive like RigidFlex.
4. Create needed commands like:
   • Split Rigid-Flex - Maps to the Panel button for defining rigid/flex regions
   • Validate Rigid-Flex - Validates rigid-flex constraints
   • Generate Stackup - Runs the layer stack manager
   • Fabrication Outputs - Generates manufacturing files
   • etc.
5. Organize these into a logical ribbon tab structure as needed.
6. Under System > Space Mappings, assign the RigidFlex command set to an open function key like F11.

Now when designing rigid-flex boards, simply pressing F11 maps the UI to your customized ribbon tabs, shortcuts, and layers for convenient access. The command set helps simplify rigid-flex workflows in Altium.

Layer Stack Design

A critical step when working with rigid-flex designs is properly configuring the layer stack. Separate stacks will be required for the rigid and flex sections. Key considerations include:

Rigid Sections

Rigid portions of the PCB typically use a traditional symmetrical, copper-balanced stackup:

• Multiple signal layers to accommodate required routing density
• Paired power and ground planes for power integrity
• Additional internal planes for shielding if needed
• Typically thicker dielectrics for rigidity and reduced layer counts

Flex Sections

Flexible zones require a much simpler stackup optimized for flexing:

• 2-4 layers total
• Thinner dielectrics like polyimide
• Copper on one or both sides for traces
• Avoiding buried vias under components

Forbidden Layers

Ensure the rigid and flex layer stacks define different core thicknesses and avoid sharing layers. Any shared layers can lead to manufacturing issues. The information about which layers belong to each region is stored in Altium as Forbidden Layers.

Defining Rigid-Flex Regions

With suitable layer stacks configured, the next step is delineating the extents of the rigid and flex areas of the PCB. This is done using the Split Rigid-Flex command added earlier.

To define rigid-flex regions:

1. Run the Split Rigid-Flex command. This opens the Regions mode.
2. Use the region drawing tools to outline the areas that should be rigid vs flexible.
   • Closed polylines define rigid sections
   • Open polylines define flexible sections
3. Set region meta information like side assignment, layer stacks, etc.
4. Review warnings for any constraint violations.

Properly defining the regions applies the configured mechanical layer stack to each area of the PCB. The editor dynamically switches layer visibility and DRC rules depending on the region.

Layer Stack Manager

With the rigid-flex regions in place, running the Generate Stackup command opens the Layer Stack Manager. This provides an interactive interface to tune layer settings across the entire board.

In the Layer Stack Manager when working with rigid-flex designs:

• Review warnings for any violations or improperly defined layers
• Ensure different core thicknesses in rigid vs. flex regions
• Verify minimum bend radius rules are defined for the flex layer stack
• Visualize and edit layer material types like flex dielectrics vs. rigid FR-4
• Check that critical clearance and impedance rules differ between layer stacks
• Generate a PDF report to inspect the final defined stackup

Properly configuring layer settings is crucial to avoiding DFM issues and receiving accurate quotes from manufacturers. The Layer Stack Manager provides an intuitive way to visualize and tune the hybrid rigid-flex stackup within Altium.

Rigid-Flex Design Rules

In addition to the layer stack, specialized rigid-flex design rules need to be defined. These include constraints like:

Bend constraints - Specify minimum bend radius for flex areas based on materials and stackup. This prevents overflexing.

Region clearance - Maintain a minimum clearance between rigid-flex region boundaries. This provides tolerance for misalignment.

Corner style - Configure corner shape at rigid-flex intersections as rounded or chamfered. Impact ease of flexing.

Jumper restrictions - Limit trace length allowed to span between regions. Long jumpers can break.

Pad coverage - Define a coverage area that must be free of copper around pads to allow flexing.

Testpoint requirements - Require testpoints at region intersections to validate alignment during assembly.

No-fly zones - Specify keepout zones around rigid-flex intersections where components cannot be placed.

Proper rigid-flex rules prevent designing boards that cannot be practically manufactured. The constraints dynamically check for violations during layout.

Manufacturing Outputs

Prior to generating final manufacturing files, run the Validate Rigid-Flex command to check the design against all defined rigid-flex constraints. Any errors or warnings must be addressed before fabrication.

Finally, use the Fabrication Outputs command to step through generating:

• Separate Gerber files per region to allow different fabrication steps
• Testpoint and fixture drawings
• Drill drawings with separate tooling for rigid vs flex areas
• IPC-compliant assembly drawings detailing rigid-flex layers and keepouts
• Accurate rigid-flex and impedance information in the IPC netlist report
• Layer stack PDFs per region for sharing detailed stackup info with manufacturers

These outputs provide the layered fabrication data needed to produce complex rigid-flex boards. The layer-aware outputs eliminate manual effort to generate proper rigid-flex manufacturing documentation.

Rigid-Flex Design Tips

When working with rigid-flex designs, keep these additional tips in mind:

• Minimize components in flex areas to avoid fractures
• Avoid 90 degree rigid-flex intersections - use rounded corners or chamfers instead for easier flexing
• Use hatched polygons in layout to clearly indicate flex vs. rigid zones
• Verify electrical rules differ between layer stacks (e.g. trace width/spacing)
• Consider isolating analog and digital sections onto separate rigid zones to reduce noise
• Place debug headers at rigid-flex junctions to simplify testing the finished assembly

Rigid-flex undoubtedly adds complexity to the PCB layout and manufacturing processes. However, with careful stackup planning and rule configuration in Altium, these hybrid boards can be designed smoothly. Keep layer stacks isolated, define robust constraints, and leverage the specialized rigid-flex outputs to eliminate surprises.

FAQs

What are the biggest challenges when first designing rigid-flex PCBs?

For engineers new to working with rigid-flex boards, the most common challenges are:

• Adjusting to adding rigid/flex region planning to the normal PCB workflow
• Learning techniques to cleanly route across multiple layer stacks
• Accounting for layer plan differences between regions
• Implementing and validating the specialized rigid-flex design rules
• Modifying regular design practices like thermal relief approaches
• Generating and reviewing the additional manufacturing documentation

With experience and practice, these areas quickly become second nature. The dedicated rigid-flex tools and outputs in Altium greatly help smooth the learning curve.

When should rigid-flex PCB technology be used vs. standard rigid boards?

The most common situations where a rigid-flex approach makes sense are:

• Tight height or weight constraints prevent using separate standard PCBs
• Complex assemblies with high connector counts that would benefit from consolidation
• Applications requiring serviceability or modularization within a single assembly
• Cost savings by replacing mechanical parts with flex circuits
• Circuits requiring different or mutually incompatible fabrication approaches
• High frequency or noise sensitive circuits that can be isolated on separate rigid zones

For simpler applications that do not require these benefits, standard rigid PCBs remain preferable to avoid added rigid-flex complexity.

What are the best online resources for learning more about rigid-flex design?

Some great resources for further learning include:

• Altium Academy rigid-flex design courses
• IPC rigid-flex standards like IPC-2226 and IPC-2223
• Design guides from manufacturers like Minco and Flexdude
• Altium's rigid-flex video tutorials on YouTube
• Nokia's classic "Rigid-Flex Design Guide" (Google for PDF)
• Technical rigid-flex articles on resources like EDN

Examining reference designs, manufacturer guidelines, and industry standards are great ways to master the nuances of rigid-flex PCB design.

What should I look for when choosing a rigid-flex PCB manufacturer?

Key capabilities to look for in a qualified rigid-flex supplier include:

• Experience with different rigid-flex layer counts and materials like polyimide
• In-house advanced processes like laser direct structuring
• Tools to handlerigid-flex-specific requirements like selective gold plating
• Electrical test capabilities to validate the complex assemblies
• Expertise checking and discussing rigid-flex designs for manufacturability
• Willingness to build small-batch prototype runs cost effectively
• Experience dealing with EMS partners for final assembly

Finding a manufacturer experienced with rigid-flex provides valuable guidance during the design process and ensures your finished boards meet the application requirements as planned.

Conclusion

Rigid-flex PCB technology enables solutions not feasible using standard rigid boards alone. The combination of rigid and flexible substrates opens exciting design possibilities. With Altium's dedicated tools for planning rigid-flex layer stacks, defining regions, implementing constraints, and generating outputs, these hybrid designs can be expertly created and validated. While requiring planning and practice, rigid-flex design unlocks innovation possibilities beyond traditional PCBs.