How to Select the Right Foaming Technology for Micro Coaxial Cable Insulation?

Engineering Comparison of Physical Foam, Lotus Foam, and Expanded PTFE Tape Technologies

In the design of high-frequency micro coaxial cables ranging from 38 AWG to 46 AWG, achieving low signal attenuation and precise characteristic impedance control (50 Ω or 75 Ω) within extremely limited dimensions requires advanced insulation engineering.

The fundamental engineering approach is to introduce air void structures into the insulation layer to significantly reduce the material’s relative dielectric constant.

This article compares the engineering selection logic of physical foaming, lotus foam, and expanded PTFE tape insulation technologies from three key perspectives:

- Electrical performance

- Dynamic bending reliability

- Environmental adaptability

1. Physical Foaming: The Balanced Choice for High-Speed Applications

Physical foaming technology relies on injecting high-pressure inert gas, typically nitrogen, directly into molten polymer or fluoropolymer materials during the extrusion process.

When the material exits the extrusion die, controlled pressure release creates a uniformly distributed closed-cell micro air bubble structure inside the insulation layer.

Core Technical Characteristics

Uniform Closed-Cell Geometry:

The independent cellular structure blocks continuous pathways inside the insulation layer, allowing the cable to maintain excellent structural integrity under mechanical compression.

Stable Dielectric Constant (1.4–1.7):

Provides predictable characteristic impedance control with excellent consistency between production batches.

Fine Conductor Compatibility:

Supports continuous extrusion with high concentricity over ultra-fine conductors ranging from 38 AWG to 46 AWG.

Because the closed-cell structure effectively prevents gas and moisture migration, physically foamed FEP/PFA insulation maintains excellent capacitance stability and low high-frequency attenuation during long-length continuous production.

Main Application Areas:

Medical Micro Coaxial Cable Assemblies, Industrial and Robotics Applications, RF and AR/VR Systems.

2. Lotus Foam: Extremely Low Dielectric Performance and Engineering Limitations

Lotus foam technology utilizes specially designed extrusion dies to create a parallel pore network extending longitudinally through the insulation layer, forming a structure similar to the cross-section of a lotus root.

Electrical Performance Advantages:

This structure significantly increases the internal gas volume fraction (Void Fraction) within the insulation layer, enabling an extremely low dielectric constant.

Key Engineering Trade-offs and Limitations

Moisture Sensitivity:

Unlike closed-cell structures, the semi-continuous pore network inside lotus foam can become a pathway for environmental moisture penetration under high-humidity conditions.

Mechanical Vulnerability Under Flexure:

The high void ratio reduces compressive strength. Under long-term dynamic bending or small-radius bending conditions, local cellular structures may collapse, resulting in inner conductor eccentricity, impedance dips, and signal reflection.

Engineering Recommendation:

Lotus foam should not simply be considered the next-generation replacement technology for traditional foamed fluoropolymer insulation. Its application should be limited to static or short-distance high-frequency transmission environments with controlled humidity and limited mechanical movement.

3. Expanded PTFE Tape: A High-Performance Solution for Specialized Applications

Expanded PTFE tape insulation is manufactured by mechanically stretching extruded PTFE tape films or substrates under specific thermal processing conditions.

Unlike traditional foam structures, this process creates a microporous network composed of interconnected nodes and fibrils.

Core Technical Characteristics

Excellent Thermal and Chemical Stability:

Maintains mechanical and electrical performance from -200°C to +260°C while resisting aggressive chemical environments.

Extremely Low Dielectric Loss and High Velocity of Propagation (VVP):

Due to its high microscopic air content, VVP can exceed 85%.

Excellent Radial Flexibility:

The fibril-based structure allows tight wrapping around ultra-fine conductors without radial cracking.

Main Application Areas:

Aerospace and avionics systems, defense and extreme environment RF applications, ultrasonic inspection equipment.

4. Engineering Decision Framework for Insulation Selection

Selecting the optimal foaming technology for micro coaxial cable insulation requires balancing electrical target parameters, mechanical stress conditions, environmental exposure, and cost efficiency.

Physical Foaming PFA:

Dielectric Constant: 1.4–1.7

Structure: Uniform closed-cell structure

Impedance Stability: Excellent

Flex Life: High

Moisture Resistance: High

Applications: Medical ultrasound, AR/VR, display buses, industrial robotics

Lotus Foam:

Dielectric Constant: 1.2–1.4

Structure: Open/semi-continuous pore structure

Impedance Stability: Medium and highly affected by humidity

Flex Life: Medium to low

Moisture Resistance: Low

Applications: Static high-frequency connections and low-humidity testing environments

Expanded PTFE Tape:

Dielectric Constant: 1.3–1.6

Structure: Node and fibril network structure

Impedance Stability: High

Applications: Specialized high-performance environments

Conclusion

In micro coaxial cable design, there is no universal insulation technology capable of meeting all high-frequency application requirements.

Although lotus foam provides an extremely low dielectric constant, its open pore structure introduces unavoidable mechanical and environmental sensitivity.

For the majority of high-speed micro coaxial cable applications, especially those requiring dynamic bending capability, precise impedance control, and reliable mass production, physical foaming provides the most balanced engineering solution.

It achieves the optimal combination of electrical efficiency, mechanical durability, and manufacturing consistency, making physically foamed FEP/PFA insulation the preferred choice for advanced micro coaxial cable assemblies used in medical imaging, AR/VR devices, industrial robotics, and high-frequency signal transmission systems.