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How to Control Flex PCB Impedance ?

Introduction

Maintaining controlled impedances on flexible printed circuit boards (flex PCBs) is critical for high frequency applications like RF circuits, high speed networking, automated testers, and medical imaging equipment. The challenges of variable dielectric thickness, dynamic bending, and conductor adhesion require special modeling and fabrication methods to achieve consistent impedances.

This article provides an overview of techniques to design and manufacture controlled impedance flexible circuits to ensure signal integrity and maximize performance.

Impedance Control Importance

Properly controlling impedance on flex PCBs provides several benefits:

  • Minimizes signal reflections that cause noise and interference
  • Allows matching with drivers, transmission lines, and receivers
  • Enables high frequency performance beyond just physical flexibility
  • Reduces EMI generation and susceptibility
  • Avoids resonances that can impact signal quality
  • Optimizes power transfer and efficiency up to microwave frequencies

Flex PCBs without impedance control should be limited to low frequency analog or digital signals below 10-20MHz that are more tolerant to impedance mismatches and reflections.

Modeling Flexible PCB Impedance

Single-sided Flex PCB
Single-sided Flex PCB

Accurate modeling of impedance on flex PCBs considers:

  • Thin, variable dielectric thickness
  • Lack of solid reference plane
  • Impact of bends/folds on dielectric spacing
  • Deformation when bent that changes spacing
  • Varying conductor width and profile

Common modeling approaches include:

2D Field Solvers

Most PCB modeling tools rely on 2D field solvers. Requires detailed cross-section definition considering bending, spacing, dielectric properties, and adhesive thicknesses. Provides good correlation to actual flex impedance with proper inputs.

3D Electromagnetic Solvers

Full 3D EM solvers offer the highest accuracy by modeling complex effects of bending, dielectric variations, and component placement. The computational requirements limit applications to smaller flex regions.

Lumped Element Models

A lumped parameter model approximates the distributed transmission line as discrete inductive, capacitive, and resistive elements. Quicker computations but reduced accuracy. Useful for initial estimates.

Validation Prototypes

Building controlled impedance test coupons allows empirical measurement and refinement of the models. This tuning of the simulation tools improves correlation and accuracy.

Developing accurate models requires careful attention to all physical construction details of the flex laminate materials and stackup.

Flex Stackup Design

Key considerations when developing the flexible PCB stackup include:

  • Select flexible laminate materials with tight impedance tolerances and stability over bending.
  • Minimize number of laminate layers which makes modeling easier.
  • Add reference planes wherever feasible to provide low impedance AC return paths.
  • Maintain symmetry between layer dielectric materials and thicknesses.
  • Use thicker copper layers to reduce resistive losses. 1oz baseline with 2oz in high current areas.
  • Model effects of solder mask thickness on impedance.
  • Ensure good registration between layers to prevent variations.

An optimized stackup minimizes the variability of parameters impacting impedance as circuits flex during use.

Trace Geometry Planning

With the stackup defined, transmission line trace geometry can be selected:

  • Choose initial trace width based on target impedance, typically between 100-250μm for 50Ω.
  • Ensure sufficient insulation clearance around traces based on voltage.
  • Use thicker traces than rigid PCBs due to greater roughness.
  • Increase spacing between adjacent traces to control coupling.
  • Minimize number of tight bend angles which cause impedance spikes.

Simulation of actual circuit trace geometry with the defined stackup provides the route to optimizing widths and spacings to hit target impedances.

Maintaining Impedance Under Bending

flexible-circuit-board-manufacturers

Special considerations help maintain consistency when flexed:

  • Model effects of dynamic bending and folding during use to quantify impedance deviations.
  • Limit the minimum bend radius to reduce impedance variations and conductor strain.
  • Use thinner laminate materials to provide better flexibility without deforming spacing and dielectric thickness.
  • Select laminate materials with elasticity to return to uniform spacing after bending.
  • Increase spacing between conductors to compensate for thickness changes under bend stress.

Understanding impedance variability under bending through modeling, material selection, and design allows mitigating changes when circuits are flexed in actual use.

Manufacturing Processes for Controlled Impedance

Fabrication processes must be optimized for impedance tolerances:

  • Surface preparation to remove oxides and promote polymer adhesion
  • Etch processes tuned to achieve precise trace geometry and minimize undercuts
  • Registration between layers of +/- 0.05mm or better
  • Symmetrical bond and lamination pressures to maintain dielectric spacing
  • Minimize adhesive voids which allow variability in dielectric constant
  • Conductor thickness uniformity within 5% across panel
  • Cure oven with airflow control to prevent temperature gradients

Tight tolerances and process controls are critical for consistent, repeatable results compared to standard flex PCBs.

Validation Testing

To ensure accuracy, testing production boards is necessary:

  • Test coupon evaluation – measure impedance on multi-point coupons for statistical analysis
  • Microsectioning – inspect critical layers for proper geometry, spacing and thickness
  • Time domain reflectometry – verify impedance uniformity along trace length
  • Gain-phase analysis – validate performance meets RF signal response requirements

Correlating measurements with modeled predictions allows further refinement of models and improvement of processes to achieve target impedances.

Summary

  • Controlling impedance on flex PCBs requires accurate modeling considering bending and materials.
  • Stackup symmetry, smaller layer count, and reference planes aid impedance control.
  • Tight trace dimensions, controlled fabrication processes, and microsection validation enable repeatability.
  • Modeling and measuring impedance under dynamic bending improves reliability.
  • With robust design-manufacturing coordination, flexible PCBs can deliver controlled impedances.

Following comprehensive guidelines allows developing flex PCBs with the impedance control needed for mission-critical and high frequency applications.

FAQ

How much does bending decrease the impedance on flex PCBs?

Typical drop is 10-25% when flexed to moderate bend radii. Sharp, tight bends can reduce impedance by over 50% in extreme cases. The effect worsens with thinner flex materials.

Does solder mask thickness impact impedance on flex circuits?

Yes, variability in solder mask thickness and its proximity to traces impacts the capacitance to ground, affecting impedance. Keeping thickness uniform through tight process control is important.

Can flex PCBs use microstrips instead of striplines?

Yes, but a microstrip construction lacks a controlled reference plane and is more susceptible to bending variations. A stripline provides the most consistent impedance under dynamic flexing.

Are there impedance test points on flex PCBs?

Test coupons containing impedance measurement points are often included in the fabrication panel. This allows characterization and correlation to modeling predictions.

How often should controlled impedance models be updated?

Models should be refined based on measured results every 6-12 months. This compensates for any process changes over time. More frequent updates are recommended when first characterizing.