Rigid-Flex PCB Strain Relief Designs: Best Practices With OrCAD X

June 20, 2024

Cadence PCB Solutions

Key Takeaways

Strain relief at the transition points of rigid-flex PCBs is crucial for preventing damage due to bending or manipulation.
Key design aspects include maintaining a minimum height difference, ensuring sufficient flex section length, avoiding components in high-stress areas, and calculating the appropriate bend radius.
Strain relief involves applying a flexible, bead-like epoxy or silicone using precision tools at the flex-rigid junctions.

Rigid-flex PCB strain relief is shown as a tapered filet.

An important design aspect in rigid-flex PCB design is the transition line(s) between a flexible section and rigid section. To alleviate strain in rigid-flex or flex circuits, a flexible material bead is applied at the transition, known as a rigid flex PCB strain relief. Once this material sets, it forms a tapered filet (shown above).

Common Types of PCB Strain Relief

Type of PCB Strain Relief	Description
Stiffeners	Rigid materials like FR4 or polyimide are attached to the flex PCB to provide additional support and prevent bending or twisting.
Adhesive Tapes	Double-sided tapes are applied to the flex PCB to offer extra support and prevent bending or twisting.
Cable Ties	Plastic or metal ties are used to secure the flex PCB to a rigid structure or component, thus preventing bending or twisting.

About Rigid-Flex PCB Strain Relief

The primary function of rigid-flex PCB strain relief is to shield components from stress incurred during bending or manipulation at a design's transition point. Issues can arise in rigid-flex circuits or flex designs with stiffeners where the flexible circuit layers emerge from rigid areas. If a bending requirement is near the transition, it might force the flex layers to bend sharply, creating a crease that surpasses the maximum bending capacity of the flex layer's construction. Such a sharp bend can deform the copper circuits, causing them to harden and become less ductile.

Rigid-Flex PCB Strain Relief Design Considerations

Design Requirement	Description
Minimum Rigid to Flex Height	Minimum height difference of 0.010” between flex and rigid areas. Allows for strain relief application, considering the material's viscosity. Too thick of an application can protrude above a rigid area. Protrusion hinders flush placement of solder paste stencils.
Flex Section Length	Typical strain relief bead width: 1-2 mm. Minimum manufacturable length for flex sections: 3mm. Excessive strain relief can leave insufficient unencapsulated flex length.
Application Sides	Strain relief is usually applied to both sides of flex layers at transition locations. In designs with unbalanced/offset flex layers, strain relief may be feasible on one side. Feasibility influenced by height constraints relative to rigid areas.
Bend Area Design	Avoid pads, holes, and vias in bend areas due to mechanical stress. Place these features on non-bending flexible areas or rigid sections. Use anchors and teardrop connections for vias/holes on flexible sections. Ensure features are at least 15 mils from the edge of the stackup.
Conductor Spacing and Bend Radius	Maintain 5mm spacing between rigid boards and flex circuit bend areas. Calculate minimum bend radius by multiplying flex thickness by a factor (6-20 depending on layer count). For dynamic bends, use 100 times the thickness for 1-2 layer flex designs.
Conductor Width	Conductor widths should be at least five times their thickness. Use the thinnest copper trace that can safely carry the intended signal. Adjust trace width as necessary. Minimum trace width for signals of ½ oz. copper: 3 mils.
Air Gap in Rigid-Flex	In multi-layer flex designs, configure flex layers in independent pairs with air gaps. Known as a "loose leaf design." Increases flexibility of the design. Beneficial in complex assemblies.
Thermal Expansion	Most adhesive-based flex materials are unfilled, leading to z-axis thermal expansion, causing potential delamination and intermittent faults post-soldering. Mitigate by using a robust pad stack for components soldered on flex material. Especially important in miniaturized and wearable technology.

How Rigid Flex Strain Relief Is Applied

At the junction of the flexible and rigid sections of a circuit, a small bead of polymer is applied to mitigate physical stress at the interface. This strain relief material could be a flexible epoxy, acrylic, RTV silicone, or polysulfide compound. Application is typically carried out using either a manual or pneumatically assisted syringe. The horizontal dimension of the strain relief typically ranges from 1.0mm to 2.5mm.

OrCAD X for Rigid-Flex PCB Design

Feature in OrCAD X	Description	Benefits of Rigid-Flex PCB Strain Relief
3D Engine	Enables visualization of rigid and flex designs within tight enclosures, incorporating material flexure and bend radii adjustments.	Allows designers to optimize the assembly of systems, reducing potential errors related to mechanical integration. Useful for ensuring that strain reliefs are positioned accurately to manage stress transitions.
Cross Section Editor	Provides a tool to define and visualize the cross-sectional layers of the PCB, enabling the differentiation between rigid and flex areas along with the application of various stack-ups.	Aids precise design of transitions and stack-ups at strain relief points, ensuring proper thickness and material properties are maintained to prevent bending and twisting at critical areas.
Import and Export of Mechanical Data (IDX Import)	Allows the importation of mechanical design data, including defined zones and bends, from external mechanical tools into OrCAD X.	Ensures the mechanical and electrical designs are aligned, which is vital for applying strain reliefs where mechanical stress is anticipated during flexing.
Generating 3D Exports	Supports exporting 3D data of the board design in multiple formats for ECAD and MCAD analysis.	Useful for evaluating the physical assembly and integration of the design, ensuring that strain reliefs function as intended under real-world mechanical stresses and movements.
Interactive 3D Visualization	Offers tools to visualize and manipulate the design in 3D, including bending flex regions and running collision detection.	Verifies the spatial arrangement and mechanical clearance of components in flex regions, ensuring that strain reliefs are effective and do not interfere with other components.

To experience all the features geared for rigid flex PCB strain relief design, learn more about OrCAD X here. Discover how OrCAD X can streamline your PCB design process and enhance project outcomes.

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