4/17/2025

Advanced design and insights on flexible and rigid-flex PCBs

In modern electronics, flexible and rigid-flex PCBs have become essential for compact, lightweight, and high-performance applications. From aerospace to medical devices, their ability to replace traditional rigid boards and connectors enhances design reliability and space optimization. With the increasing complexity of high-density interconnects (HDI) and miniaturized components, engineers face new challenges in ensuring manufacturability, reliability, and cost-effectiveness.

This article explores key advancements in flexible and rigid-flex PCB design, covering new materials, stack-up configurations, HDI integration, and emerging technologies such as chip-on-flex (COF) and laser microvia formation.

Advanced rigid-flex PCB constructions

The evolution of rigid-flex PCB designs has introduced several key innovations:

  • Multi-layer flexible structures: There is a growing trend toward higher layer counts in flexible circuits to support complex interconnections.
  • Blind and buried vias: These interconnect technologies enable greater routing density while maintaining board flexibility.
  • ZIF (Zero Insertion Force) connectors: Integrated ZIF structures simplify assembly and improve connector reliability.
  • Component mounting in blind vias: Enhancing PCB density by directly integrating components within buried cavities.
  • Air gap constructions: Essential for designs with four or more flexible layers, this technique eliminates adhesive layers in rigid sections, ensuring higher via reliability and improved flex performance.
  • Rigid areas with multiple thicknesses: While technically feasible, dual-thickness rigid sections significantly increase manufacturing complexity and cost.
  • EMI/RF shielding with non-metallic films: Instead of copper layers, specialized shielding films (such as Tatsuta and A+) effectively reduce electromagnetic interference without the additional weight and cost.

The impact of HDI on flexible PCB design

As PCB density increases, the integration of HDI technology into flexible circuits is gaining traction:

  • Microvias: Laser-drilled microvias (50-100 μm) enable ultra-fine pitch BGA designs.
  • Thin-Dielectric Stacks: Dielectric layers as thin as 25 μm enhance signal integrity in high-speed applications.
  • Cross-Hatched Ground Planes: Essential for flex applications requiring high-frequency signal transmission while maintaining flexibility.
  • HDI + Rigid-Flex Integration: Combining HDI features with rigid-flex architectures allows for smaller form factors and reduced assembly costs by eliminating external connectors.

Chip-on-flex (COF): the future of high-density packaging

Chip-on-Flex (COF) technology is revolutionizing high-performance flex circuits, particularly in wearables, aerospace, and medical electronics:

  • Miniaturized packaging: CSP (Chip-Scale Packages) reduce footprint while offering high I/O density.
  • Wire bonding vs. flip-chip: Both bonding techniques are viable for COF, depending on thermal and mechanical constraints.
  • Encapsulation & thermal management: Protective coatings such as conformal coatings and potting enhance longevity and environmental resistance.
  • BGA COF applications: Fine-pitch BGAs (down to 0.4 mm pitch) require ultra-thin flex substrates and precise solder mask openings to maintain reliability.

Key design considerations for flexible & rigid-flex PCBs

Engineers designing flexible and rigid-flex PCBs must consider mechanical, thermal, and electrical performance to ensure reliability:

1. Bend radius and flexibility

  • A minimum bend radius of 10x the flex thickness is recommended to prevent stress fractures.
  • For dynamic flex applications, avoid plated through-holes in bending areas to prevent mechanical fatigue.

2. Coverlay vs. solder mask

  • Coverlay (Kapton + Adhesive) is preferred for flex circuits, as it withstands repeated bending without cracking.
  • Flexible solder masks are an alternative for fine-pitch component areas where coverlay openings are not practical.

3. Reinforcements & stiffeners

  • FR4 stiffeners provide mechanical support for connector areas and soldered components.
  • Aluminum stiffeners are used for thermal dissipation in power applications.
  • Polyimide stiffeners add structural rigidity while maintaining a lower weight profile.

4. Routing & trace design for flex reliability

  • Rounded traces instead of sharp corners minimize stress concentration.
  • Teardrop pads improve via strength and prevent annular ring breakage.
  • Avoiding vias in bend areas ensures better long-term mechanical performance.

The role of laser technology in HDI flex PCB manufacturing

Advanced laser processing is redefining microvia fabrication, enabling ultra-compact flex PCB designs:

  • UV laser drilling: Achieves microvias down to 30 μm, critical for HDI flex boards.
  • Direct laser structuring: Allows for ultra-fine traces (30/30 μm) without requiring traditional photolithography.
  • Solder mask laser ablation: Enables precise opening formation for BGA pads and ultra-fine-pitch components.

The advancement of flexible and rigid-flex PCBs is driving a new era of miniaturization, reliability, and high-density interconnects. With the integration of HDI, laser microvias, and chip-on-flex packaging, modern flex PCB designs offer unprecedented performance in compact, high-reliability applications.

Collaboration with experienced manufacturers is essential to optimize design complexity, reduce costs, and improve production efficiency. As flex and rigid-flex PCB technology continues to evolve, leveraging cutting-edge materials, advanced fabrication methods, and smart design principles will be key to pushing the boundaries of electronic innovation.

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