IPC-1756 Explained: High-Frequency Flex PCB Dielectric Standards

Anyone who’s designed flex circuits for RF applications knows the frustration. You’ve got a 5G antenna design that needs to bend around a smartphone chassis, but the moment you start looking for material specs, you realize most flex datasheets focus on mechanical properties, not high-frequency performance. That’s exactly the gap IPC-1756 fills.

I’ve spent years working with flexible circuits in high-speed applications, and IPC-1756 has become one of those standards I reference constantly. In this guide, I’ll break down everything you need to know about IPC-1756 — from material classifications to practical selection criteria. Whether you’re designing flex antennas, high-speed flex interconnects, or rigid-flex assemblies for mmWave applications, this article will help you navigate the standard effectively.

What Is IPC-1756?

IPC-1756 is the industry standard titled “Flexible Printed Board Base Dielectrics and Cover Materials for High Speed/High Frequency Applications” published by IPC. Think of it as the flex circuit counterpart to IPC-1758 (which covers rigid high-frequency materials).

The standard addresses both the base dielectric films that form the core of flexible circuits and the coverlay/cover film materials that protect and insulate the outer layers. This dual focus is critical because in flex circuits, both layers significantly impact high-frequency performance.

Why Flex Circuits Need Their Own High-Frequency Standard

You might wonder why flex materials can’t just follow IPC-1758. The answer comes down to fundamental material differences:

Rigid boards use glass-reinforced laminates where the glass weave dominates mechanical properties. Flex circuits use unreinforced polymer films — primarily polyimide — where the base dielectric IS the structural element. This changes everything about how we characterize and specify materials.

Flex circuits also have unique requirements that rigid boards don’t face:

• Dynamic flexing capability while maintaining electrical performance
• Coverlay materials that must match base dielectric properties
• Adhesive layers that can degrade high-frequency performance
• Thinner constructions where every micron affects impedance

IPC-1756 addresses all of these flex-specific concerns within a high-frequency context.

Scope and Purpose of IPC-1756

IPC-1756 covers flexible base dielectric materials and cover materials specifically engineered for high-speed digital and high-frequency analog applications. The standard applies to:

Materials Covered by IPC-1756

Applications Addressed

What IPC-1756 Does NOT Cover

The standard excludes:

• Standard polyimide flex materials for non-high-frequency use (covered by IPC-4203/4204)
• Rigid laminate materials (covered by IPC-1758 and IPC-4101)
• Finished flex circuit requirements (covered by IPC-6013)
• Rigid-flex specific construction requirements (addressed in IPC-2223)

IPC-1756 Material Classification System

Just like IPC-1758 classifies rigid materials, IPC-1756 provides a classification framework for flexible high-frequency materials. This makes comparing materials from different suppliers much more straightforward.

Classification by Dielectric Constant (Dk)

Classification by Dissipation Factor (Df)

When I’m selecting materials for a 28 GHz phased array antenna on flex, I start with Class A or B materials in the Ultra-Low or Very Low Loss grade. For a Bluetooth wearable, Class C with Low Loss grade is typically sufficient and more cost-effective.

Key Base Dielectric Materials in IPC-1756

Understanding the actual material options helps translate IPC-1756 classifications into real-world choices.

Liquid Crystal Polymer (LCP)

LCP has become the go-to material for high-frequency flex applications. Here’s why:

LCP’s killer feature is its near-zero moisture absorption. Standard polyimide absorbs 2-3% moisture, which shifts Dk significantly. LCP stays stable regardless of humidity — critical for outdoor 5G antennas.

Modified Polyimide (MPI)

Modified polyimide offers a middle ground between standard polyimide and LCP:

MPI works well for applications where LCP’s cost premium isn’t justified but standard polyimide’s losses are too high.

PTFE-Based Flex Materials

For the lowest possible loss, PTFE flex films deliver:

PTFE flex is niche — I typically only specify it for test fixtures or ultra-low-loss interconnects where dynamic flexing isn’t required.

Read more IPC Standards:

IPC-7092: Complete Guide to Embedded Components in PCB Design

IPC-7091: Complete Guide to 3D Component Design & Assembly Standard

IPC-6903: Complete Glossary of Additive Circuitry & Flexible Hybrid Electronics Terms

IPC-6901: Printed Electronics Categories, Levels & Performance Criteria by Application

IPC-6018 Explained: Microwave PCB Requirements, PTFE Materials & Impedance Control

IPC-6017 Complete Guide: PCB Embedded Passive & Active Circuitry Requirements

IPC-6015 Explained: MCM-L Organic Substrate Specification for Multichip Modules

IPC-6013 Complete Guide: Flex & Rigid-Flex PCB Types, Classes & Acceptance Criteria

IPC-6012 Complete Guide: Rigid PCB Qualification, Class 2 vs Class 3 & Acceptance Criteria

IPC-6011 Explained: The Foundation for IPC-6012 & PCB Performance Standards

IPC-4921: Substrate Classification, Testing & Specification Sheets for Printed Electronics

IPC-4781 Guide: Silkscreen Ink Types, Testing & Qualification for PCBs

IPC-4761 Explained: Via Tenting, Plugging, Filling & Capping Guide

IPC-4591: Functional Conductive Materials Standard for Printed Electronics

IPC-4204 Explained: Complete Guide to Flexible Copper Clad Laminates for Flex PCB Fabrication

Coverlay and Cover Film Requirements

Here’s something that trips up a lot of designers: your coverlay matters just as much as your base dielectric for high-frequency performance. IPC-1756 addresses this directly.

Why Coverlay Properties Matter

In a typical microstrip flex design, the electromagnetic field extends above the conductor into the coverlay. If your base dielectric has Df of 0.002 but your coverlay adhesive has Df of 0.02, you’ve compromised your entire design.

IPC-1756 Coverlay Classifications

The Adhesive Problem

Traditional flex coverlays use acrylic or epoxy adhesives with Df values of 0.02-0.04 at 10 GHz. That’s 10x worse than LCP film. IPC-1756 recognizes this and includes specifications for:

• Low-loss adhesive systems (Df < 0.01)
• Adhesiveless constructions using thermoplastic bonding
• Selective coverlay application (adhesive-free in RF areas)

My standard practice for any flex design above 6 GHz: specify adhesiveless coverlay or at minimum low-loss adhesive systems in the RF signal path.

IPC-1756 vs. IPC-1758: Understanding the Differences

Since both standards address high-frequency materials, it’s worth understanding how they differ.

Direct Comparison

When to Use Each Standard

Design Considerations for IPC-1756 Materials

Selecting the right material is only half the battle. Here’s what I’ve learned about designing with high-frequency flex materials.

Impedance Control Challenges

Flex circuits are thin — often 50-100 µm total thickness. This makes impedance control tricky:

Bend Radius and Electrical Performance

This surprised me early in my career: bending a flex circuit changes its electrical characteristics. The outer surface stretches, the inner compresses, and trace geometry shifts. For dynamic flex applications at high frequencies:

• Design traces perpendicular to bend axis when possible
• Avoid RF traces in tight bend radius areas
• Use stiffeners to define bend locations away from critical RF sections
• Test electrical performance in the bent configuration

Hybrid Stackup Strategies

For complex designs, I often use hybrid approaches:

Practical Material Selection Guide

Here’s my workflow for selecting IPC-1756 materials:

Step 1: Define Frequency Requirements

Step 2: Assess Mechanical Requirements

• Static flex only → More material options
• Dynamic flex (< 100 cycles) → LCP or high-grade PI
• Dynamic flex (> 100 cycles) → LCP preferred, careful design review

Step 3: Consider Environmental Factors

Outdoor or high-humidity applications demand low moisture absorption materials. LCP excels here. For controlled environments, MPI can be acceptable.

Step 4: Evaluate Total Cost

Useful Resources and Downloads

Official IPC Resources

• IPC-1756 Standard Document: Available for purchase at IPC Official Store
• IPC-4204 Flexible Metal-Clad Dielectrics: Companion specification for base materials
• IPC-6013 Flexible Circuit Qualification: Overall flex PCB requirements
• IPC-2223 Sectional Design Standard for Flex: Design guidelines for flex and rigid-flex

Material Supplier Technical Resources

• Rogers Corporation: ULTRALAM 3000 LCP series datasheets and design guides
• Panasonic: FELIOS LCP material technical documentation
• DuPont: Pyralux AP Plus and HF series for high-frequency flex
• Shengyi Technology: SF305 and high-frequency flex offerings
• Murata: MetroCirc LCP material data

Design and Simulation Tools

• Polar Instruments: Speedstack Flex for impedance planning
• Ansys HFSS: Full-wave simulation with flex material libraries
• Cadence Allegro: Flex design with material-aware stackup management

Frequently Asked Questions About IPC-1756

What is the main difference between IPC-1756 and IPC-4204?

IPC-4204 is the general specification for flexible metal-clad dielectrics covering all flex materials regardless of frequency performance. IPC-1756 specifically addresses materials designed for high-speed and high-frequency applications, including detailed Dk/Df characterization at multiple frequencies, material classification systems for electrical performance, and coverlay requirements for RF applications. If your design operates above 1 GHz or has controlled impedance requirements, IPC-1756 is the more appropriate reference.

Can I use standard polyimide flex materials for 5G applications?

For Sub-6 GHz 5G applications, modified polyimide (MPI) materials meeting IPC-1756 Low Loss or Very Low Loss grades can work adequately. However, for mmWave 5G frequencies (24 GHz, 28 GHz, 39 GHz), standard polyimide’s high dissipation factor (Df > 0.015) causes excessive signal loss. You’ll need LCP or similar ultra-low-loss materials classified under IPC-1756 for acceptable antenna efficiency and signal integrity at these frequencies.

How does moisture absorption affect high-frequency flex performance?

Moisture absorption is critical for flex circuits because water has a Dk of approximately 80 — far higher than any flex dielectric. When polyimide absorbs 2-3% moisture, its effective Dk can shift by 5-10%, throwing off impedance calculations and antenna tuning. This is why IPC-1756 includes moisture absorption specifications and why LCP (< 0.04% absorption) is preferred for applications exposed to varying humidity. For outdoor 5G antennas, moisture stability often drives the material choice toward LCP regardless of other factors.

What coverlay should I use for high-frequency flex circuits?

For optimal high-frequency performance, match your coverlay to your base dielectric. LCP base should use LCP cover film or adhesiveless coverlay. If using modified polyimide, specify a low-loss coverlay system with Df < 0.01 at your operating frequency. Never use standard acrylic or epoxy adhesive coverlays in RF signal areas — their Df of 0.02-0.04 will dominate your losses. Consider selective coverlay application, leaving RF-critical areas uncovered or using conformal coating instead.

Is IPC-1756 required for rigid-flex high-frequency designs?

For rigid-flex assemblies operating at high frequencies, both IPC-1756 and IPC-1758 apply — IPC-1758 for the rigid sections and IPC-1756 for the flexible sections. The interface between rigid and flex areas requires careful attention to impedance matching since the materials have different Dk values. Many designers route critical high-frequency signals through only one material type (either all-rigid or all-flex paths) to avoid the complexity of transitioning between different dielectric systems.

Conclusion

IPC-1756 fills a critical gap in the flex circuit world. As wireless frequencies climb and flexible electronics become more sophisticated, having a standardized framework for specifying high-frequency flex materials isn’t just convenient — it’s essential.

The key points to remember: LCP dominates for mmWave applications due to its combination of low loss and moisture stability. Don’t forget about coverlay — it’s half your stackup in many flex constructions. And always consider the complete material system, including adhesives, when evaluating high-frequency performance.

Whether you’re designing 5G flex antennas, high-speed flex interconnects, or wearable RF devices, familiarity with IPC-1756 will help you specify materials that actually meet your electrical requirements rather than discovering problems after fabrication.

Meta Description:

IPC-1756 defines standards for high-frequency flex PCB materials including LCP and modified polyimide. This guide covers material classification, Dk/Df specifications, coverlay requirements, and selection criteria for 5G, mmWave, and RF flex circuit applications.

Alternative Meta Descriptions:

1. Learn about IPC-1756 — the flex circuit standard for high-frequency dielectrics. Covers LCP, MPI materials, coverlay specs, and practical selection guidance for 5G antennas and RF flex designs.
2. IPC-1756 explained by a PCB engineer: Complete guide to high-speed flex materials, Dk/Df classifications, and design considerations for mmWave and high-frequency flexible circuit applications.