rigid flex pcb cross section

Common Flex & Rigid-Flex PCB Constructions

Flex and rigid-flex PCB constructions have many variations that allow for a wide range of applications and solutions. A significant difference to rigid PCB constructions is that uneven layer counts are allowed and frequently used. The primary reasons being reduced flex thickness, improved flexibility and reduced part cost.

The following are examples of some of the more commonly used constructions found in applications throughout the industry.

Flex PCB Construction

Flex circuit layer count can vary from 1-6. Designs with greater than 3-layers will have limited bend capabilities due to their thickness. All constructions can utilize both plated through holes (PTH) and surface mount technology (SMT) components for either interconnect or active designs.

Stiffeners are commonly either Polyimide or FR4, depending upon the application. Stainless steel and aluminum stiffeners can also be used for special applications.

1-Layer Dual Access Flex PCB:

• Common Application(s): ZIF to ZIF interconnect.
• Allows access to one layer of copper from both sides of the design.

1-Layer Flex PCB:

• Common Applications(s): Connector to connector flex, functional circuit.
• Flex core can be either Adhesiveless or Adhesive based depending upon finished thickness required to meet bend requirements.

2-Layer Flex PCB:

• Common application(s): Surface micro strip controlled impedance, ZIF to ZIF interconnect, PTH or SMT Connector interconnect,
• Flex core can be either Adhesiveless or Adhesive based depending upon finished thickness required to meet bend requirements.

3-Layer Multilayer Flex PCB:

• Common application(s): Stripline controlled impedance, radio frequency (RF)/electromagnetic interference (EMI) shielded.
• Can utilize either FR4 or polyimide stiffeners.
• Flex cores are typically adhesiveless to minimize flex thickness and allow for bend requirements.
• Higher layer counts available. Review of bend requirements recommended.

Rigid-Flex PCB Construction

The combination of rigid PCB technology and flex circuit technology allows for a very wide variety of rigid and flex layer counts and combinations. Rigid areas can utilize blind and buried vias, with some limitations, and via in pad technology. Rigid area layer counts can range from 2–20+ and flex area layer counts from 1–6+. Designs can have multiple rigid sections and can incorporate flex tails with ZIF contacts or connectors.

While not limited to it is preferred that the flex layers be centrally located within the rigid area so as to minimize any potential warp and twist. Many configurations allow for controlled impedance and or shielding in the flex areas.

The examples below contain only 1 or 2 rigid areas for simplicity.

2-Layer Rigid with 1 Flex Layer:

Common application(s): Simple designs with limited requirements.

3-Layer Rigid with 1 Flex Layer:

4-Layer Rigid with 2 Flex Layers:

4-Layer Rigid with 2 Flex Layers and ZIF Contacts:

5-Layer Rigid with 3 Flex Layers:

6-Layer Rigid with 4 Flex Layers:

Flex layers configured as two pairs of 2 layers with air gap between pairs for improved reliability and flexibility.

12 Layer Rigid with 4 Flex Layers:

Flex layers configured as two pairs of 2 layers with air gap between pairs for improved reliability and flexibility, which allows for controlled impedance in flex areas.

Summary

Considering all the possible combinations, we have only displayed a small sample of the available flex and rigid-flex PCB constructions. When designing your flex circuit, it is important to use the IPC specifications for specific flexible materials, circuit design, performance, and assembly of flexible circuits.

Please contact us if you have any questions or need assistance determining which construction best meets your flex and rigid-flex design requirements. During Epec's quoting process, we carefully examine specifications, materials and construction in order to minimize expenses and eliminate any technical issues, delivering the best possible product to our customers!

Key Takeaways:

• Flex & Rigid–Flex Versatility:
  • Flex and rigid–flex constructions offer wide-ranging applications with variations in layer counts and configurations to meet specific needs.
  • Unlike rigid PCB constructions, uneven layer counts are common in flex circuits, reducing thickness, improving flexibility, and lowering costs.
• Flex Circuit Constructions:
  • 1-Layer Dual Access Flex:
    • Applications: ZIF-to-ZIF interconnects.
    • Access to a single copper layer from both sides.
  • 1-Layer Flex:
    • Applications: Connector-to-connector flex or functional circuits.
    • Adhesiveless or adhesive-based cores based on bend requirements.
  • 2-Layer Flex:
    • Applications: Controlled impedance, ZIF-to-ZIF interconnects, PTH, or SMT connectors.
    • Core type (adhesiveless or adhesive) depends on thickness and bend requirements.
  • 3-Layer Multilayer Flex:
    • Applications: Stripline controlled impedance, RF/EMI shielding.
    • Typically, adhesiveless cores for thinner, more flexible designs.
    • Higher layer counts are available with review of bend requirements.
• Rigid-Flex Constructions:
  • Combines rigid PCB and flex circuit technologies for flexible, complex designs.
  • Rigid area layers: 2–20+; Flex area layers: 1–6+.
  • Capable of supporting blind/buried vias, via-in-pad technology, and controlled impedance in flex areas.
  • Preferred design locates flex layers centrally within rigid areas to minimize warp and twist.
  • Examples:
    • 2-Layer Rigid with 1 Flex Layer: Simple designs with minimal requirements.
    • 4-Layer Rigid with 2 Flex Layers & ZIF Contacts: Incorporates ZIF contacts for connectors.
    • 6-Layer Rigid with 4 Flex Layers: Flex layers configured in pairs with air gaps for flexibility and reliability.
    • 12-Layer Rigid with 4 Flex Layers: Supports controlled impedance and enhanced reliability through air-gap configurations.
  • Material Considerations:
    • Common stiffener materials include FR4 and polyimide, with specialized options like aluminum for heat dissipation and stainless steel for mechanical strength.
    • Flex cores can be adhesiveless for thinner constructions or adhesive-based for specific requirements.
  • Design Recommendations:
    • Ensure careful selection of layer counts and materials to meet flexibility, thickness, and electrical performance needs.
    • Centralize flex layers in rigid sections and review designs with high layer counts to ensure reliability.
  • Customization & Support:
    • Constructions highlighted are a small sample of available options.
    • Epec offers expertise to help tailor constructions to specific design and application requirements.

Watch Our Video:

Designing Flex and Rigid Flex PCB Stack-Ups with Advanced Software

See our in-depth tutorial on creating a stack-up using our advanced software! In this video, we walk you through the process of generating a new stack-up, starting with the default settings and customizing it to fit your specific requirements.

Topics: Flex & Rigid-Flex PCB's