IPC-4202 Explained: Complete Guide to Flexible PCB Base Dielectric Materials

Selecting the wrong dielectric material for a flex circuit is an expensive mistake. The board might crack during dynamic flexing, delaminate during reflow, or fail impedance requirements because the material properties were never specified correctly. These problems often trace back to vague material callouts like “polyimide” without any reference to an actual specification.

IPC-4202 solves this problem by establishing standardized specifications for flexible base dielectric materials. This standard defines material types, performance requirements, and a classification system that ensures designers and fabricators communicate clearly about material requirements. Whether you’re designing a simple single-layer flex or a complex multilayer rigid-flex, IPC-4202 provides the framework for specifying base dielectric materials correctly.

This guide explains what IPC-4202 covers, how the classification system works, and how to select the right flexible dielectric material for your application.

What Is IPC-4202? Understanding the Flexible Dielectric Standard

IPC-4202, officially titled “Specification for Flexible Base Dielectrics for Use in Flexible Printed Boards,” establishes the classification system, qualification requirements, and quality conformance standards for flexible base dielectric materials. The current version is IPC-4202C, released in January 2022 as a 36-page document.

This standard covers the unclad flexible films, laminates, and prepregs used as the base dielectric in flexible printed circuits. It does not cover metal-clad materials (covered by IPC-4204) or coverlay materials (covered by IPC-4203).

IPC-4202 Standard Overview

Attribute	Details
Current Version	IPC-4202C
Release Date	January 2022
Pages	36
Supersedes	IPC-4202B, IPC-FC-231C
Scope	Flexible base dielectrics (films, laminates, prepregs)
Primary Use	Flexible and rigid-flex printed boards

What IPC-4202 Provides

Content	Description
Classification system	Standardized material identification
Specification sheets	Detailed requirements for each material type
Qualification requirements	Testing for material conformance
Quality conformance	Ongoing production testing requirements
High-frequency properties	Dk and Df at 1 MHz, 1 GHz, and 10 GHz

IPC-4202 Material Types: Flexible Dielectric Options

IPC-4202 covers several flexible dielectric material types, each with distinct properties suited to different applications. Understanding these materials is essential for proper specification.

Polyimide (PI) Films

Polyimide is the dominant material in flexible circuit fabrication. DuPont Kapton is the most recognized brand, though other manufacturers produce equivalent materials. Polyimide offers an exceptional balance of thermal stability, mechanical strength, and electrical properties.

Property	Typical Value
Temperature range	-269°C to +400°C
Dielectric constant (Dk)	3.2 to 3.5 at 1 MHz
Dissipation factor (Df)	0.002 to 0.003 at 1 MHz
Tensile strength	165 to 230 MPa
Elongation	70% to 100%
Moisture absorption	2.5% to 3.0%

Kapton FPC, specifically designed for flex circuit applications, meets IPC-4202C requirements and provides enhanced dimensional stability and adhesion compared to general-purpose polyimide films.

Polyester (PET) Films

Polyethylene terephthalate (PET) offers a lower-cost alternative to polyimide for less demanding applications. PET provides good flexibility and adequate electrical properties but cannot withstand soldering temperatures.

Property	Typical Value
Maximum temperature	105°C to 150°C
Dielectric constant (Dk)	3.0 to 3.3 at 1 MHz
Dissipation factor (Df)	0.002 to 0.005 at 1 MHz
Tensile strength	170 to 200 MPa
Cost	Lower than polyimide
Lead-free reflow	Not compatible

Polyethylene Naphthalate (PEN) Films

PEN occupies a middle ground between PET and polyimide in terms of both performance and cost. It offers better thermal stability than PET while remaining less expensive than polyimide.

Property	Typical Value
Glass transition (Tg)	~120°C
Dielectric constant (Dk)	~3.0 at 1 MHz
Temperature capability	Higher than PET, lower than PI
Applications	Automotive sensors, moderate-heat environments

Liquid Crystal Polymer (LCP) Films

LCP provides exceptional high-frequency performance and very low moisture absorption. It is the preferred choice for RF and microwave flexible circuits where signal integrity at high frequencies is critical.

Property	Typical Value
Dielectric constant (Dk)	2.9 at 1 GHz
Dissipation factor (Df)	0.002 at 1 GHz
Moisture absorption	<0.1%
Dk stability	Excellent (stable vs frequency and humidity)
Cost	Higher than polyimide

Fluorocarbon (PTFE) Films

Fluorocarbon materials, including PTFE-based films, offer the lowest dielectric constant and loss among flexible materials. IPC-4202 includes specification sheets for adhesive fluorocarbon films for high-frequency applications.

Property	Typical Value
Dielectric constant (Dk)	2.1 to 2.5
Dissipation factor (Df)	<0.001
Temperature capability	Excellent
Processing	More difficult than polyimide

Reinforced Materials

In addition to unsupported films, IPC-4202 covers reinforced flexible materials using various reinforcement types.

Reinforcement Code	Type	Description
1	Non-reinforced	Pure film, maximum flexibility
2	Nonwoven	Random fiber mat reinforcement
3	Woven	Woven fabric reinforcement (glass, aramid)
4	Combination	Woven + nonwoven combination

Reinforcement letters identify the fiber type: A = glass, B = polyester, C = aramid.

IPC-4202 Flexible Dielectric Material Comparison

Selecting the right material requires understanding how different dielectrics compare across key properties.

Material Properties Comparison

Property	Polyimide	PET	PEN	LCP
Max operating temp	250°C+	105°C	155°C	280°C
Lead-free reflow	Yes	No	Limited	Yes
Dk at 1 MHz	3.2-3.5	3.0-3.3	~3.0	2.9
Df at 1 MHz	0.002	0.002	0.005	0.002
Moisture absorption	2.5-3%	0.3%	0.4%	<0.1%
Dynamic flex life	Excellent	Good	Good	Good
Relative cost	High	Low	Medium	Very high

Application Selection Guide

Application	Recommended Material	Reason
Dynamic flex (continuous bending)	Polyimide	Best flex life, durability
Static flex (bend-to-install)	PET or Polyimide	Cost vs performance tradeoff
High temperature (>150°C)	Polyimide or LCP	Thermal stability
Lead-free assembly	Polyimide or LCP	Reflow compatibility
RF/microwave (>1 GHz)	LCP	Lowest Dk and Df
High humidity environment	LCP	Lowest moisture absorption
Cost-sensitive consumer	PET	Lowest cost
Automotive under-hood	Polyimide	Temperature + reliability

IPC-4202 Slash Sheet System: How to Specify Materials

Like IPC-4101 for rigid materials, IPC-4202 uses a slash sheet system to identify specific material types. Each slash sheet defines the requirements for a particular category of flexible base dielectric.

Understanding IPC-4202 Slash Sheet Designations

The nonspecific designation format is used by designers on master drawings:

IPC-4202/X

Where X is the specification sheet number identifying the material type.

Slash Sheet	Material Type
IPC-4202/1	Unsupported polyimide flexible base dielectrics
IPC-4202/2	Polyester (PET) flexible base dielectrics
IPC-4202/3	Reinforced flexible dielectrics
Additional sheets	Fluorocarbon, PEN, other specialty materials

IPC-4202 Designation Example

A typical nonspecific designation on a master drawing:

IPC-4202/1

This indicates the design requires unsupported polyimide film per IPC-4202 specification sheet 1. The fabricator then selects a specific material (such as DuPont Kapton FPC) that conforms to this specification.

Specific Designation Format

For purchase orders and fabrication, a more detailed specific designation is used that includes additional parameters:

Parameter	Description
Base specification	IPC-4202/X
Reinforcement	Type and material code
Thickness	Film thickness designation
Additional properties	As required by design

The specific designation format provides fabricator-level detail and should not be used on master drawings, as it requires specialized knowledge to interpret correctly.

IPC-4202 Key Material Properties and Requirements

IPC-4202 establishes requirements for critical material properties that affect flex circuit performance and reliability.

Mechanical Properties

Property	Requirement	Why It Matters
Elongation	>30% minimum	Survives repeated bending
Tensile strength	Per slash sheet	Withstands handling and assembly
Dimensional stability	<0.1% shrinkage	Maintains registration during processing
Tear resistance	Per material type	Prevents damage during handling

Electrical Properties

Property	Test Frequencies	Importance
Dielectric constant (Dk)	1 MHz, 1 GHz, 10 GHz	Impedance control
Dissipation factor (Df)	1 MHz, 1 GHz, 10 GHz	Signal loss
Dielectric breakdown	Per IPC-TM-650	Voltage withstand
Insulation resistance	Per IPC-TM-650	Leakage prevention

The inclusion of high-frequency dielectric properties (1 GHz and 10 GHz) in recent IPC-4202 revisions reflects the increasing importance of signal integrity in modern flex circuit designs.

Thermal Properties

Property	Typical Requirements
Thermal stability	Withstand reflow temperatures
Flammability	UL 94 rating as specified
Coefficient of thermal expansion	Matched to copper for reliability

Peel Strength Requirements

Requirement	Typical Value
Minimum peel strength	6 lbs/inch (1.05 N/mm)
After thermal stress	Maintain specified minimum
After humidity exposure	Maintain specified minimum

IPC-4202 vs IPC-4203 vs IPC-4204: Related Flex Standards

Understanding how IPC-4202 relates to other flexible circuit material standards helps ensure complete and correct material specification.

IPC Flexible Circuit Material Standards Comparison

Standard	Covers	Primary Use
IPC-4202	Base dielectrics (films)	Unclad flexible substrates
IPC-4203	Coverlay and bonding materials	Cover films, adhesive-coated dielectrics
IPC-4204	Metal-clad dielectrics	Copper-clad flex laminates
IPC-4562	Metal foil	Copper foil for flex circuits

When to Use Each Standard

Design Element	Applicable Standard
Base film (unclad)	IPC-4202
Copper-clad laminate	IPC-4204
Coverlay film	IPC-4203
Bondply/adhesive film	IPC-4203
Copper foil specification	IPC-4562

IPC-4204 Material Designation Example

For copper-clad flex materials, IPC-4204 uses a comprehensive designation system:

IPC-4204/11 E1E2 CU W7 1S/1S

This specifies the slash sheet (/11), construction type (E1E2), copper type, weight (W7), and surface treatment on each side.

IPC-4202 Design Considerations for Flex Circuits

Proper material specification requires understanding how material properties affect design decisions.

Thickness Selection

Thickness	Bend Radius	Applications
12.5 µm (0.5 mil)	Very tight (<0.5 mm)	Ultra-thin, tight bends
25 µm (1 mil)	Tight (0.5-1 mm)	Standard dynamic flex
50 µm (2 mil)	Moderate (1-2 mm)	General purpose
75-125 µm (3-5 mil)	Larger (>2 mm)	Stiffened areas, static flex

Adhesive vs Adhesiveless Constructions

Construction	Description	Best For
Adhesive-based	Acrylic or epoxy adhesive bonds copper to dielectric	Single/double layer flex, cost-sensitive
Adhesiveless	Direct bond or cast construction	Rigid-flex, high-reliability, fine-pitch

Adhesiveless constructions provide better dimensional stability and thermal performance, making them preferred for rigid-flex and high-layer-count designs.

Dynamic vs Static Flex Considerations

Flex Type	Material Requirement	IPC-4202 Relevance
Dynamic (continuous)	High elongation, excellent flex life	Specify polyimide per /1
Static (install only)	Good flexibility, may use lower-cost options	PET or polyimide acceptable

Useful Resources for IPC-4202 Implementation

Official IPC Standards:

• IPC-4202C Specification for Flexible Base Dielectrics (shop.ipc.org)
• IPC-4203 Cover and Bonding Material for Flexible Printed Circuitry
• IPC-4204 Flexible Metal-Clad Dielectrics
• IPC-2223 Sectional Design Standard for Flexible Printed Boards
• IPC-6013 Qualification and Performance Specification for Flexible Printed Boards

Material Manufacturer Resources:

• DuPont Kapton Technical Data Sheets (dupont.com)
• DuPont Kapton FPC Data Sheet (IPC-4202C compliant)
• SKC Kolon PI Film Technical Library
• UBE Industries Polyimide Film Resources

Related Test Methods:

• IPC-TM-650 Test Methods Manual
• ASTM D149 Dielectric Breakdown Voltage
• ASTM D2520 Complex Permittivity at Microwave Frequencies

Frequently Asked Questions About IPC-4202

What is the difference between IPC-4202 and IPC-4204?

IPC-4202 covers unclad flexible base dielectric materials—the films used as the substrate before copper is applied. IPC-4204 covers metal-clad (copper-clad) flexible dielectrics—the complete laminate with copper already bonded to the film. Designers typically reference IPC-4204 for copper-clad flex materials and IPC-4202 when specifying bondply or unclad dielectric layers in multilayer constructions. Both standards work together: IPC-4204 metal-clad materials use base films that conform to IPC-4202 requirements.

Can I use PET film for circuits that will be soldered?

PET (polyester) cannot withstand standard soldering temperatures, especially lead-free reflow profiles that peak at 260°C. PET is limited to applications where components are attached using conductive adhesives, mechanical connections, or low-temperature soldering processes. For any circuit requiring conventional soldering, specify polyimide or LCP materials per IPC-4202. The temperature limitation is why PET is primarily used in low-cost consumer applications where soldering is not required.

How do I specify IPC-4202 materials on my drawing?

Use the nonspecific designation format “IPC-4202/X” where X is the appropriate slash sheet number. For polyimide films, this would be “IPC-4202/1.” Add any additional requirements in notes, such as thickness, flame retardancy class, or specific electrical properties. Do not specify materials by brand name (like “Kapton”)—use the IPC specification to ensure multiple qualified sources can supply conforming material. The fabricator then selects a specific material that meets your IPC-4202 requirements.

Why does IPC-4202 include high-frequency dielectric properties?

Modern flex circuits increasingly operate at high frequencies, particularly in 5G devices, automotive radar systems, and high-speed data applications. The inclusion of dielectric properties at 1 GHz and 10 GHz allows designers to specify materials with known high-frequency performance. This is critical for impedance-controlled designs where Dk and Df values at operating frequency directly affect signal integrity. LCP materials specified per IPC-4202 provide the best high-frequency performance among flexible dielectrics.

What determines whether to use adhesive-based or adhesiveless flex materials?

The choice depends on application requirements. Adhesive-based constructions (copper bonded to polyimide with acrylic or epoxy adhesive) are cost-effective for single and double-layer flex circuits. Adhesiveless constructions provide better dimensional stability, higher temperature capability, and improved reliability for rigid-flex, high-layer-count, and fine-pitch applications. IPC-4204 defines various adhesive and adhesiveless constructions, with the underlying dielectric meeting IPC-4202 specifications.

Applying IPC-4202 in Your Flex Circuit Designs

Proper material specification is fundamental to flex circuit reliability. IPC-4202 provides the framework for clearly communicating dielectric requirements between designers, fabricators, and material suppliers. By specifying materials using IPC-4202 slash sheets rather than brand names, you ensure designs can be sourced from multiple qualified suppliers while maintaining consistent performance.

For most flex circuit applications, polyimide per IPC-4202/1 provides the best balance of flexibility, thermal stability, and electrical performance. When cost is the primary driver and soldering is not required, PET offers a lower-cost alternative. For high-frequency applications where signal integrity is critical, LCP provides the lowest loss and best Dk stability.

Understanding IPC-4202 also means understanding its relationship to IPC-4203 (coverlays), IPC-4204 (metal-clad laminates), and IPC-2223 (design standard). Together, these standards create a complete framework for flex circuit material specification that ensures your designs perform reliably in their intended applications.