Rigid Flex PCB Capabilities

Introduction to Rigid Flex PCBs

Rigid flex PCBs are a type of printed circuit board that combines the benefits of both rigid and flexible circuit boards. They consist of rigid PCB sections connected by flexible circuits, enabling the PCB to be bent and folded into compact or complex 3D configurations.

Rigid flex PCBs offer many advantages over traditional rigid PCBs:

• Reduced size and weight
• Improved reliability
• Eliminated connectors
• Easier assembly
• Design flexibility
• Better signal integrity

The combination of rigid and flex circuits in a single PCB provides a versatile solution for applications requiring high density interconnects, compact packaging, and dynamic flexing capabilities.

Typical Rigid Flex PCB stackup

A typical rigid flex PCB stackup consists of alternating layers of rigid and flexible substrates laminated together:

Layer	Material
Top Cover	Polyimide or Photoimageable solder mask
Top Flex Circuit	Copper on polyimide substrate
Adhesive	Acrylic or epoxy
Rigid FR4 Layer	Copper clad laminate
Rigid FR4 Core	FR4 glass epoxy
Bottom Rigid FR4 Layer	Copper clad laminate
Adhesive	Acrylic or epoxy
Bottom Flex Circuit	Copper on polyimide substrate
Bottom Cover	Polyimide or solder mask

The number of layers and exact stackup configuration can be customized based on the specific electrical and mechanical requirements of the application. Rigid flex PCBs can incorporate any combination of single sided, double sided or multilayer flex and rigid circuits.

Flex Circuit Materials

The flexible portions of rigid flex PCBs are fabricated using thin, flexible substrates like polyimide films. The most common materials are:

• Polyimide (PI) – Kapton by DuPont is the most widely used polyimide film. It provides excellent thermal, mechanical, chemical and electrical properties.

• Polyester (PET) – Lower cost than polyimide but also lower temperature resistance and dimensional stability. Suitable for some dynamic flexing applications.

The copper conductors are typically plated or laminated to the flexible substrate. Adhesives like acrylic or epoxy bond the copper to the substrate. Coverlayers of polyimide film or photoimageable solder mask are used as outer insulating layers on the flex circuits.

For the rigid portions, standard FR4 epoxy glass laminates and prepregs are commonly used. The rigid and flex materials are carefully selected to have compatible thermal expansion and bonding properties.

Rigid Flex PCB Fabrication Process

The rigid flex PCB manufacturing process combines the methods used for rigid and flexible circuit boards. The key steps are:

1. Imaging and etching the inner layer flex and rigid circuits
2. Laminating the flex layers with insulating coverlays
3. Laminating the rigid and flex layers together using specialized epoxies and high pressure/temperature
4. Drilling through-holes and vias
5. Plating the holes to connect copper layers
6. Outer layer imaging and etching
7. Applying solder mask and silkscreen
8. Cutting out flex circuit regions and rigid board outline
9. Testing and inspection

Fabricating rigid flex PCBs requires specialized equipment, experience and controls to ensure proper layer alignment, bonding, and hole plating between the rigid and flex materials. Partnering with a PCB manufacturer that has specific rigid flex capabilities is crucial to achieving a reliable design.

Rigid Flex Design Guidelines

Properly designing the rigid flex circuit board is critical to achieving a manufacturable, reliable and cost-effective solution. Some key rigid flex PCB Design Guidelines include:

Flex Circuit Routing

• Avoid abrupt changes in conductor width or direction. Use curved traces and tear-drop pads.
• Maintain uniform copper density across the flex circuit to balance stresses. Add copper thieving if needed.
• Provide stress relief at the rigid-to-flex interface using curved traces or regions void of copper.
• Follow minimum bend radius and cycle requirements for the flex material and application.

Rigid-Flex Transitions

• Locate rigid-to-flex transitions away from areas of high stress or flexing.
• Stagger the rigid-to-flex transitions layer to layer to minimize stress concentration.
• Specify Z-axis expansion values for the rigid and flex laminates to ensure aligned interconnects.
• Consider adding epoxy fillets at rigid-flex boundaries for mechanical reinforcement.

Bookbinder Construction

• Use “bookbinder” construction where the flex layers are laminated on the outer surfaces of the rigid board. This allows the flex to be bent away from the rigid board in installation.
• For dynamic flex applications, loose leaf or tailed bookbinder designs allow the greatest freedom of movement between rigid and flex sections.

Bookbinder Type	Best For
Continuous	Static flex installations requiring compact design
Loose Leaf	Flexing to 180° or more where flex movement is restricted
Tailed	Applications with complex or repetitive dynamic flexing

Following expert rigid flex design principles during PCB layout is essential for achieving the ideal balance of density, reliability, manufacturability and cost for the end application.

Rigid Flex Applications

Rigid flex PCBs are used across many industries in applications that require high reliability, high density, and 3D packaging. Some common uses include:

Aerospace and Defense

• Avionics
• Missiles
• Satellites
• Radar systems

Medical Devices

• Hearing aids
• Pacemakers
• Insulin pumps
• Endoscopes

Consumer Electronics

• Smartphones
• Wearables
• Cameras
• Gaming devices

Automotive

• Engine control units
• Sensors
• Displays
• Driver assist systems

Industrial

• Robotics
• Automation equipment
• Test and measurement devices
• Portable instruments

The ability to replace multiple rigid PCBs and connectors with a single rigid flex assembly provides significant size, weight, and reliability improvements critical for these applications. As miniaturization and electronics integration continues to increase, the adoption of rigid flex PCBs will accelerate.

Advantages of Rigid Flex PCBs

In summary, rigid flex PCBs provide many benefits compared to rigid PCBs or assembled flexible circuits:

Reduced Size and Weight

By combining circuits and eliminating connectors, rigid flex PCBs enable highly compact and lightweight electronic packaging. A single rigid flex assembly can often replace multiple rigid PCBs, connectors, and cables for significant space and weight reduction.

Improved Reliability

Rigid flex PCBs have fewer interconnections and failure points compared to multiple rigidboards and connectors. The reduction in solder joints improves reliability, especially in high vibration and shock environments. The direct connection of the flex to the rigid board also provides greater bonding strength.

Simplified Assembly

With rigid flex PCBs, the flexible circuits are already integrated into the rigid board which eliminates the need for cables and connectors. This simplifies the final assembly process and lowers the risk of defects.Rigid flex designs also reduce the total part count for easier supply chain management.

Enhanced Signal Integrity

Rigid flex PCBs can provide better signal integrity than wire and connector solutions. The planar structure of the flex circuit has well-controlled impedance for high speed signaling. Rigid flex also minimizes EMI/RFI by eliminating bulky wires that can act as antennas.

Increased Design Flexibility

By combining rigid and flexible substrates, rigid flex PCBs open up new possibilities for dense 3D packaging of electronics. The flexible circuits act like a hinge, allowing the PCB to be folded to fit in tight, conformal spaces. Rigid flex stackups can also be customized with mixed materials and thicknesses for each layer.

While rigid flex PCBs have higher initial costs than rigid boards due to their specialized manufacturing needs, they provide significant system level cost savings by reducing assembly time, part counts, and increasing product yields and reliability. As electronics advance in complexity and miniaturization, rigid flex will become an increasingly vital technology.

Frequently Asked Questions

What are the differences between rigid and Flexible PCBs?

Rigid PCBs use a solid substrate material, typically FR4 epoxy laminate. They are strong and stiff, but cannot be bent. Flexible PCBs use thin, bendable substrates like polyimide that allow the circuit to flex and fold. Rigid flex PCBs combine both rigid and flexible substrates for circuits that have both rigid and flexible portions.

How much flex capability does a rigid flex circuit have?

The flexing capability depends on the specific design and materials. In general, most flexible circuits can bend to a radius about 6x the total thickness of the flex without damaging the conductors. For dynamic flexing applications, 10-20x the thickness is a common design guideline. The number of flex layers, copper thickness, and coverlayer materials also impact the flexibility.

What is bookbinder construction?

Bookbinder construction refers to when the flexible circuit layers are laminated to the top and/or bottom outer surfaces of a rigid PCB. This allows the flex layers to be bent away from the rigid board, similar to the cover of a book. Bookbinders provide an efficient method for connecting rigid PCBs together with integrated flex “hinges” for dense 3D packaging.

Are rigid flex PCBs repairable?

Rigid flex PCBs are difficult to repair due to the complex lamination of the rigid and flex materials. Replacing a damaged component may require specialized microsoldering rework equipment and technician skills. Rigid flex PCBs are often treated as full replaceable units rather than repaired. The best strategy is to use known-good subassemblies and conduct thorough testing.

How do you specify rigid flex PCB layer stackups?

Rigid flex PCB stackups are typically specified using a combination of numerical callouts and visual diagrams. The number and order of rigid and flex layers is defined along with the material types and thicknesses for each layer. Critical design values like copper weights, nominal vs pressed thicknesses, and material CPKs are also specified. A cross-section drawing is the clearest way to communicate the layer stack design intent to the fabricator.

Conclusion

Rigid flex PCBs offer a powerful solution for applications demanding high density interconnects, reliability, and flexible 3D packaging. By combining hardboard and flexible circuits into a single PCB, rigid flex technology enables the next generation of miniaturized and high-performance electronic products.

However, rigid flex PCBs also require careful design planning and experienced manufacturing to achieve a reliable and cost-effective result. By understanding rigid flex materials, stackups, and design guidelines, PCB designers can unlock the full potential of rigid flex integration. As density and performance needs continue to evolve, rigid flex PCBs will undoubtedly remain a vital cornerstone for electronics advancement.