Rigid flex PCBs are becoming the go-to solution for engineers seeking compact, lightweight, and highly reliable circuit designs. Whether you are a beginner in PCB design or a purchasing manager exploring options for your product, understanding the basics of rigid flex PCB stack up is essential in 2025. This guide walks you through everything you need to know, from structure and materials to common design mistakes and best practices.
1. Introduction to Rigid Flex PCBs
Rigid flex printed circuit boards combine the advantages of both rigid and flexible PCBs into a single circuit. They consist of multiple layers of flexible polyimide substrates and rigid FR4 sections laminated together. This hybrid structure allows for tight packaging, reduced connectors, and enhanced reliability under dynamic conditions.
You’ll commonly find rigid flex PCBs in aerospace systems, medical devices, industrial automation, consumer electronics, and modern automotive electronics—anywhere where space-saving and vibration resistance are key.
2. What Is a PCB Stack Up?
A PCB stack up refers to the configuration and arrangement of conductive copper layers and insulating dielectric materials that make up the board. In rigid flex PCBs, the stack up is especially crucial as it directly influences electrical performance, mechanical strength, and manufacturability.
Unlike standard rigid boards, rigid flex stack ups must accommodate both static and dynamic conditions, including bending during installation. The planning phase defines how many layers are rigid, how many are flexible, how signal layers are arranged, and where ground or power planes are located.
3. Basic Layer Structure of Rigid Flex PCB
In a typical rigid flex stack up, the rigid sections contain multiple copper layers laminated with prepreg and core materials, just like a standard multilayer PCB. The flexible section, on the other hand, usually includes one to three copper layers supported by polyimide films.
For example, a common configuration might involve a six-layer design where four layers are rigid and two layers extend through the flex areas. These flex layers interconnect different rigid zones, allowing for folding or dynamic movement while maintaining signal continuity.
4. Materials Used in Rigid Flex PCB Stack Ups
The performance and durability of a rigid flex PCB depend heavily on material selection. The rigid parts typically use FR4, which offers good mechanical strength and cost efficiency. The flexible parts use polyimide, known for its excellent thermal and chemical resistance.
Adhesives may be used between the layers, but many modern designs rely on adhesive-less bonding for better reliability. Coverlays made of polyimide films and applied with flexible adhesives protect the outer flex layers. Prepreg materials are used to bond layers during lamination and to maintain dielectric integrity.
5. Design Considerations for Stack Up Planning
Proper stack up design is essential to avoid performance issues or manufacturing delays. Here are some key considerations:
- Signal Integrity and Impedance Control: To avoid signal degradation, the placement of power and ground planes must be symmetrical and well-defined. The spacing between layers should support the required impedance levels for high-speed signals.
- Bend Radius and Flex Reliability: The flexible areas must meet minimum bend radius guidelines. Typically, a larger bend radius improves reliability and reduces the risk of cracking during use.
- Via Types and Placement: Designers often use blind or buried vias to connect inner layers without impacting flex zones. Vias should never be placed in areas that need to bend, as they can crack over time.
6. Manufacturing Process Overview
Rigid flex PCBs are manufactured using a combination of rigid board lamination and flexible circuit fabrication. The process starts by patterning copper layers on both rigid and flex materials, followed by lamination, drilling, plating, and surface finishing.
During lamination, precise alignment is critical to ensure the flexible layers match the rigid layers. The manufacturer must also account for z-axis expansion and dimensional tolerances, especially in multi-layer designs. Quality inspection includes electrical testing, X-ray analysis, and mechanical reliability testing.
7. Common Mistakes Beginners Should Avoid
When working on your first rigid flex PCB design, it’s easy to make a few costly mistakes. Avoid the following pitfalls:
- Over-bending the flex areas: Exceeding the minimum bend radius can cause copper fatigue and eventual failure.
- Poor layer symmetry: Imbalanced stack ups can lead to warping or delamination during reflow soldering.
- Ignoring DFM (Design for Manufacturability) guidelines: Without consulting your manufacturer early, your design might not meet process tolerances or material limitations.
8. Rigid Flex Stack Up vs. Traditional Rigid or Flex PCBs
Rigid flex stack ups offer a balance between structure and flexibility that traditional PCBs can’t match. Compared to rigid boards, they eliminate bulky connectors and reduce overall weight. Compared to pure flex circuits, they add mechanical support and ease of handling.
While the unit price of rigid flex PCBs is generally higher—ranging from $0.50 to $2.50 per square inch depending on complexity—they reduce total system cost by simplifying assembly and improving reliability.
9. Conclusion: How to Get Started with Your First Stack Up Design
If you’re ready to start your first rigid flex project, begin by defining the functional zones in your design—what needs to stay rigid and what must flex. Then, work with a trusted PCB manufacturer to finalize your stack up, choosing the right layer count, materials, and via strategy.
Software tools like Altium Designer, KiCad, or Eagle offer features for defining stack ups and validating impedance and clearance requirements. Early collaboration with your fabrication partner will ensure a smooth transition from design to manufacturing.
10. FAQs About Rigid Flex PCB Stack Up
Can I simulate the stack up before fabrication?
Yes, most advanced PCB design software allows you to simulate impedance and mechanical strain based on your stack up parameters.
How much does a basic rigid flex stack up cost in 2025?
As of 2025, a basic 4-layer rigid flex PCB (2 rigid + 2 flex layers) might cost between $1.20 and $1.80 per square inch for low-volume production. Complex HDI or multilayer designs can go beyond $3.00 per square inch.