Complete guide to DuPont Kapton FN film: FEP-coated polyimide for heat sealing, moisture barriers, and flex PCB dielectrics — specs, comparisons, and FAQs.

If you’ve ever spec’d a flex circuit for a high-temperature environment and run into the moisture sensitivity wall with uncoated polyimide, you already understand why DuPont Kapton FN exists. It’s not a general-purpose upgrade to the baseline HN film — it solves a specific set of problems that come up repeatedly in flex PCB designs for aerospace wiring, harsh industrial environments, and any application where heat sealing is part of the assembly process. This guide breaks down exactly what Kapton FN is, how its FEP coating changes the material’s behavior in a flex circuit stack, and where it outperforms and underperforms its Kapton siblings.

What Is DuPont Kapton FN Film?

DuPont Kapton FN is a general-purpose HN film that is coated or laminated on one or both sides with Teflon FEP fluoropolymer. Kapton FN imparts heat sealability, provides a moisture barrier, and enhances chemical resistance.

That three-layer structure — FEP / Kapton HN / FEP — is what defines the entire product. The core polyimide is the same high-performance Kapton HN engineers have been using for over five decades, carrying all its established thermal stability, dielectric strength, and mechanical toughness. The FEP (fluorinated ethylene propylene) coating adds a new set of functional properties on top of that foundation — most importantly, the ability to thermally bond directly to conductors and adjacent layers without adhesives.

Kapton FN is recommended in applications that require a heat bondable film, or moisture and chemical resistance beyond the capabilities of uncoated Kapton films. Kapton FN meets ASTM D-5213 (type 2, item A) requirements.

For DuPont PCB fabricators and flex circuit designers, that ASTM compliance is the baseline validation that the material meets published industry standards for composite polyimide-fluoropolymer films.

Understanding the FN Product Code System

The product code on Kapton FN isn’t arbitrary — it encodes the exact construction of the film. A three-digit system is used in which the middle digit represents the nominal thickness of the base Kapton film in mils. The first and third digits represent the nominal thickness of the FEP fluoropolymer resin coating in mils. The symbol 9 is used to represent 13 µm (0.5 mil) and 6 to represent 2.5 µm (0.1 mil). For example, 120FN616 is a 120-gauge structure consisting of a 25 µm (1 mil) base film with a 2.5 µm (0.1 mil) coating on each side.

Once you understand that decoding logic, the datasheet table becomes a direct design tool.

Product Code	Total Gauge	Base Kapton HN	FEP Side 1	FEP Side 2	Construction
120FN616	120	25 µm (1 mil)	2.5 µm (0.1 mil)	2.5 µm (0.1 mil)	Double-sided
150FN019	150	25 µm (1 mil)	13 µm (0.5 mil)	None	Single-sided
250FN029	250	50 µm (2 mil)	None	13 µm (0.5 mil)	Single-sided

Single-sided constructions are used where bonding or sealing is required on only one face — for example, bonding a conductor array to a substrate on one side while the other remains uncoated polyimide.

Key Electrical Properties of Kapton FN Film

The FEP coating changes the bulk electrical properties of the composite relative to uncoated Kapton HN. Because FEP has a lower dielectric constant than polyimide, the FN composite lands between the two materials depending on the FEP coating fraction.

Property	120FN616	150FN019	250FN029	Test Method
Dielectric Strength (V/µm)	272 (6,900 V/mil)	197 (5,000 V/mil)	197 (5,000 V/mil)	ASTM D-149-91
Dielectric Constant (Dk)	3.1	2.7	3.0	ASTM D-150-92
Dissipation Factor (Df)	0.0015	0.0013	0.0013	ASTM D-150-92
Volume Resistivity @ 23°C (Ω·cm)	1.4 × 10¹⁷	2.3 × 10¹⁷	1.9 × 10¹⁷	ASTM D-257-91
Volume Resistivity @ 200°C (Ω·cm)	4.4 × 10¹⁴	3.6 × 10¹⁴	3.7 × 10¹⁴	ASTM D-257-91

A few things stand out for the working engineer. The Dk of 2.7–3.1 is slightly lower than uncoated Kapton HN (3.5 typical), which is mildly favorable for signal integrity in high-speed flex circuits. The Df of ~0.0013–0.0015 is low enough to be useful in RF interconnect designs. And the volume resistivity remains comfortably high across the operating temperature range, confirming the film doesn’t become a leakage path at elevated temperatures.

What the FEP Coating Actually Does in a Flex Circuit

Heat Sealability Without Adhesives

This is the headline feature for cable and harness applications. Kapton FN is the HN type coated on one or both sides with Teflon FEP, which enhances chemical resistance and imparts heat sealability. It is used for the covering of copper wires and cables in high temperature applications.

In spiral-wrap wire insulation designs, the FEP layer fuses to itself and to the copper conductor surface under controlled temperature and pressure — no liquid adhesive, no pot-life constraints, no outgassing concerns from an adhesive cure cycle. For cable harnesses destined for aerospace or military environments, this is a significant reliability advantage. Adhesive bond lines are potential failure points under thermal cycling; a direct FEP-to-FEP heat seal is structurally simpler.

Moisture Barrier Performance

Kapton FN imparts heat sealability, provides a moisture barrier, and enhances chemical resistance. Its heat bonding and sealing capabilities make it ideal for a range of industrial applications.

Uncoated Kapton HN absorbs moisture — the polyimide backbone is hygroscopic to a measurable degree. In circuit designs where moisture ingress can affect dielectric properties or cause delamination, the FEP surface coating acts as an effective barrier. The fluoropolymer’s intrinsic hydrophobicity means water doesn’t wet the surface or penetrate the FEP layer readily. For sealed flex assemblies in marine, outdoor, or humidity-controlled industrial environments, this makes a real performance difference.

Chemical Resistance Beyond Uncoated Polyimide

FEP is chemically inert to virtually all industrial solvents, acids, and bases that would cause concern in PCB fabrication or end-use environments. The Kapton FN composite therefore inherits FEP’s chemical shielding, protecting the polyimide core from chemical attack in process environments or through-life exposure to cleaning agents and fuels.

Kapton FN vs. Other Kapton Grades: How to Choose

Engineers frequently ask which Kapton type belongs in a specific flex design. The answer depends heavily on what the film is doing in the stack.

Kapton Grade	Base Film	FEP Coating	Primary Use Case
HN	Polyimide	None	General-purpose substrate, motor insulation, tape
FN	Polyimide (HN)	One or both sides	Cable insulation, heat-seal applications, moisture-sensitive environments
FPC	Polyimide (treated)	None	Flex circuit base dielectric, superior dimensional stability
XP	Polyimide (HN)	Proprietary fluorocarbon	Higher bond strength at elevated temperatures vs. FN
FWR	Polyimide	FEP (hydrolysis-resistant)	Enhanced hydrolytic stability vs. FN
MT	Thermally conductive PI	Optional FEP (FMT)	Heat management applications

Kapton FN is typically used for heat sealing, provides an excellent moisture barrier, and is chemically resistant. Recommended applications for Kapton FN are those that require a heat bondable film, or higher resistance to moisture and chemicals than uncoated Kapton films can provide.

The important distinction for flex PCB designers: if your application is a traditional flexible printed circuit where the polyimide is serving as the base dielectric substrate, the preferred material is usually Kapton FPC or Kapton HN — not FN. Kapton FPC is specifically optimized for superior adhesion and low shrinkage in flex circuit fabrication. Kapton FN shines where heat sealing and moisture protection are the primary requirements, rather than the fine-pitch dimensional stability that FPC delivers.

A proprietary fluorocarbon resin coating on one or both sides of Kapton HN polyimide film delivers excellent adhesive properties and mechanical strength at high temperatures, and outperforms Kapton FN polyimide films in bond retention at temperature. If you need bond strength retention above 150°C, Kapton XP is worth evaluating alongside FN.

Applications Where Kapton FN Is the Right Specification

Aerospace and Military Wire Harnesses

The aerospace wire insulation market is one of the core applications Kapton FN was designed for. Spiral-wrapped harnesses in aircraft, satellites, and military vehicles use the heat-fusible FEP layer to create sealed, lightweight wire bundles that resist fuels, hydraulic fluids, and moisture infiltration. The temperature range of the underlying Kapton HN core — reliably functional from -269°C to +400°C — covers the full environmental envelope these applications demand.

Industrial Cable Jacketing

Kapton FN is the HN type coated on one or both sides with Teflon FEP, and is used for the covering of copper wires and cables in high temperature applications. For industrial drive cables, motor lead insulation, and wiring in chemical process environments, FN offers a practical combination of dielectric integrity and chemical inertness that conventional PVC or XLPE cable jackets cannot match at elevated temperatures.

Sealed Flex Circuit Assemblies

Where a flex circuit needs to be environmentally sealed — submersible sensors, outdoor instrumentations, or implantable-adjacent medical devices — Kapton FN constructions allow the circuit assembly to be heat-sealed along its edges. The FEP layer bonds to itself or to compatible fluoropolymer surfaces without requiring separate adhesive tapes or liquid encapsulants, simplifying the assembly process and reducing the number of potential leak paths.

High-Frequency Signal Lines in Harsh Environments

The low Dk (down to 2.7 for 150FN019) and very low Df (0.0013) of the FN composite make it worth considering for flex transmission lines operating in RF or microwave frequencies where dielectric losses matter. Combined with the chemical resistance, this creates a useful niche for FN in antenna flex feeds, RF interconnects in defense electronics, and downhole instrumentation where the low-loss dielectric is needed alongside chemical protection.

Physical Properties at a Glance

Property	Value	Notes
Temperature Range	-269°C to +400°C	Inherited from Kapton HN core
Flammability	UL 94V-0	Self-extinguishing
ASTM Compliance	D-5213 Type 2, Item A	Composite polyimide-fluoropolymer
Constructions Available	Single-sided or double-sided FEP	Per product code
Standard Sheet Supply	18 × 24 in (457 × 610 mm)	Check distributor for roll availability
Density	1.53–1.67 g/cc	Depends on FEP fraction
Chemical Resistance	Excellent	Resistant to most solvents, acids, fuels

Processing Notes for Fabricators

Working with Kapton FN in a production environment requires awareness of a few handling differences compared to uncoated polyimide films. The FEP coating surface is slippery relative to HN, which affects web tension management in roll-to-roll processes. Bonding parameters (temperature, pressure, dwell time) need to be qualified for the specific FEP coating thickness — the 13 µm coating constructions require more energy input to reach the FEP melt point than the 2.5 µm coating variants.

The processing of Kapton can generate a strong static charge. Unless this charge is bled off as it forms by using ionizing radiation or tinsel, it can build to many thousands of volts and discharge to people or metal equipment. In dust- or solvent-laden air, a flash fire or explosion could result. This static management requirement applies specifically to the FN film as well — grounding and ionization bars are standard practice in high-speed processing lines.

Frequently Asked Questions About DuPont Kapton FN

Q1: Can Kapton FN be used as the base dielectric substrate in a flexible printed circuit?

Technically yes, but it’s rarely the optimal choice. Kapton FPC is the preferred base dielectric for flex circuits because it’s specifically engineered for superior adhesion and low shrinkage during lamination — properties that directly affect yield and dimensional accuracy. Kapton FN’s FEP coating doesn’t add benefit in a standard adhesive-laminated flex circuit stack. Use FN where heat sealing or moisture protection are driving requirements; use FPC or HN where dielectric substrate performance comes first.

Q2: What is the heat-sealing temperature range for the FEP coating on Kapton FN?

FEP fluoropolymer typically fuses in the range of 200°C to 270°C depending on pressure and dwell time. The exact bonding window needs to be qualified for the specific FN construction you’re using, as the coating thickness affects energy transfer. Your lamination equipment’s platen temperature, dwell, and pressure settings will all be variables in the qualification process.

Q3: Is Kapton FN available with single-sided or double-sided FEP coating?

Both constructions are standard. Single-sided and double-sided constructions are commercially available. For cable spiral-wrap applications where both surfaces need to bond, double-sided FN (such as 120FN616) is specified. Where only one bond surface is needed, single-sided variants like 150FN019 are the typical choice and offer a cost advantage.

Q4: How does Kapton FN compare to Kapton XP for high-temperature bond retention?

Kapton XP has high bonding strength at elevated temperatures with copper, itself, and other materials. It exhibits the same mechanical, chemical, and electrical properties as Kapton FN in both high and low temperatures. The key difference is that XP’s proprietary fluorocarbon coating retains bond strength more effectively above 150°C compared to FN. If your sealed assembly will see sustained elevated temperatures in service, the Kapton XP technical data sheet bond strength retention curves are worth reviewing before committing to FN.

Q5: Does the FEP coating on Kapton FN affect its radiation resistance for space applications?

The Kapton HN core has well-documented radiation resistance — it’s been used on spacecraft and satellites for decades. The FEP layer adds fluoropolymer chemistry to the composite. FEP itself is reasonably radiation resistant, though it can crosslink under high gamma or electron dose. For space applications with significant radiation exposure, always review the specific radiation test data with your material supplier rather than relying on datasheet values alone. Kapton FN has been used in space-rated cable assemblies, but the radiation environment of your specific mission needs to be matched against the material qualification data.

Useful Resources for Engineers Specifying Kapton FN

Resource	What You’ll Find
DuPont Kapton FN Official Product Page	Product family overview, selection guide for all Kapton types
Kapton FN Technical Data Sheet (American Durafilm)	Full property tables for 120FN616, 150FN019, 250FN029
Kapton Summary of Properties (Marian Inc. PDF)	Complete HN and FN property tables including chemical resistance
ASTM D-5213 Standard	Specification covering polyimide-fluoropolymer composite films
IPC-FC-231 Flexible Base Dielectrics	Industry standards for flex circuit base material specifications
American Durafilm Kapton Distributor Page	Stocking distributor with FN, HN, and FPC availability
Insulectro Kapton Product Support	Technical support for flex circuit laminate material selection

Making the Call: Is Kapton FN Right for Your Design?

From a practical engineering standpoint, Kapton FN earns its place in the spec list when at least two of these conditions apply: you need to heat-seal the film to itself or to a conductor, you need meaningful moisture barrier performance at the dielectric layer, or you’re operating in a chemical environment that would challenge uncoated polyimide. For straightforward flex PCB base dielectric applications without those requirements, Kapton FPC or HN is almost certainly the cleaner choice.

Where FN consistently wins is in cable insulation and harness applications that combine the thermal performance of polyimide with the chemical inertness and self-bonding capability of FEP — a combination that no single-component material can match in the same thickness and weight envelope.

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Complete guide to DuPont Kapton FN film: FEP-coated polyimide for heat sealing, moisture barriers, and flex PCB dielectrics — specs, comparisons, and FAQs.