DuPont Kapton FN Film: FEP-Coated Polyimide for Flex Circuit Dielectrics

When DuPont Kapton HN alone isn’t enough — when the application needs heat sealability, better moisture barrier performance, or chemical resistance that uncoated polyimide can’t deliver — that’s where DuPont Kapton FN comes in. Kapton FN is a composite film: the proven Kapton HN polyimide base with Teflon® FEP fluoropolymer coated or laminated on one or both sides. The result is a material that carries all of Kapton’s thermal range and mechanical toughness while adding the heat-bondable, low-dielectric-loss, chemically inert properties of FEP fluoropolymer.

For PCB engineers and electronics designers, understanding when to specify Kapton FN instead of Kapton HN is a real design decision — one that affects fabrication process, electrical performance, bond strength, and cost. This guide covers the complete technical picture for Kapton FN, from its construction logic to full electrical and mechanical properties, the product code system, standard constructions, and where it’s actually used in practice.

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. It is recommended in applications that require a heat bondable film, or moisture and chemical resistance beyond the capabilities of uncoated Kapton films.

The material is manufactured and certified to ASTM D-5213 (type 2, item A) requirements, manufactured according to product specifications in H-38479, Bulletin GS-96-7. Unlike bare Kapton HN (which cannot be heat sealed — it is not heat sealable in uncoated form), the FEP coating layer on Kapton FN melts at approximately 260–270°C under pressure, creating a strong, hermetic bond to another FEP surface, to copper foil, or to other heat-sealable materials without adhesives.

This is the core value proposition for flex circuit applications: Kapton FN enables adhesive-free bonding during lamination, reducing the stackup thickness, eliminating acrylic or epoxy adhesive layers that can degrade at temperature extremes, and improving the dielectric consistency of the final composite.

For a broader view of how Kapton FN fits into DuPont’s polyimide and flex circuit material ecosystem, DuPont PCB covers the full portfolio from Kapton films through Pyralux laminates and specialty PCB materials.

Decoding the Kapton FN Product Code System

The FN product code is one of the most confusing aspects of the Kapton family for engineers encountering it for the first time. A three-digit numbering system distinguishes between constructions where the same total gauge can be achieved with different polyimide-to-FEP ratios:

The code format: [FEP side 1 thickness] + [Polyimide thickness] + [FEP side 2 thickness]

Where:

• The number 9 represents 0.5 mil (13 µm) FEP
• The number 6 represents 0.1 mil (2.5 µm) FEP
• Standard mil values represent their nominal mil thickness

Example: 120FN616 = 120-gauge total, consisting of 1 mil (25 µm) polyimide base with 0.1 mil (2.5 µm) FEP coating on each side (6 + 1 + 6 = “616” construction, 0.1 + 1.0 + 0.1 = 1.2 mil = 120 gauge).

Product Code	Total Gauge (mil)	Polyimide (µm/mil)	FEP Side 1 (µm/mil)	FEP Side 2 (µm/mil)	Configuration
100FN999	2.0 mil	25 µm / 1.0 mil	13 µm / 0.5 mil	13 µm / 0.5 mil	Coated both sides
120FN616	1.2 mil	25 µm / 1.0 mil	2.5 µm / 0.1 mil	2.5 µm / 0.1 mil	Thin coat both sides
150FN019	1.5 mil	25 µm / 1.0 mil	13 µm / 0.5 mil	—	One side coated
200FN011	2.0 mil	25 µm / 1.0 mil	25 µm / 1.0 mil	—	One side coated
200FN919	2.0 mil	25 µm / 1.0 mil	13 µm / 0.5 mil	13 µm / 0.5 mil	Coated both sides
250FN029	2.5 mil	50 µm / 2.0 mil	13 µm / 0.5 mil	—	One side coated
300FN021	3.0 mil	50 µm / 2.0 mil	25 µm / 1.0 mil	—	One side coated
300FN929	3.0 mil	50 µm / 2.0 mil	13 µm / 0.5 mil	13 µm / 0.5 mil	Coated both sides
400FN022	4.0 mil	50 µm / 2.0 mil	50 µm / 2.0 mil	—	One side coated
400FN031	4.0 mil	75 µm / 3.0 mil	25 µm / 1.0 mil	—	One side coated
500FN131	5.0 mil	75 µm / 3.0 mil	25 µm / 1.0 mil	25 µm / 1.0 mil	Coated both sides
600FN051	6.0 mil	125 µm / 5.0 mil	25 µm / 1.0 mil	—	One side coated

Understanding which construction is needed for your application is the first decision in any Kapton FN specification. Double-sided FEP coating is required for heat-sealed tubing and hermetically sealed pouches; single-sided coating is common for flex circuit dielectrics where only one bonding interface is needed.

Complete Technical Properties of DuPont Kapton FN

Mechanical Properties

The FEP fluoropolymer coating modifies the mechanical properties of the composite compared to bare Kapton HN. Because FEP has significantly lower tensile strength (around 20–25 MPa) than polyimide (231 MPa), the composite properties depend heavily on the polyimide-to-FEP ratio. The three key grades from the official DuPont FN datasheet (K-15347):

Property	120FN616	150FN019	250FN029	Test Method
Polyimide content (wt%)	80%	57%	73%	—
FEP content (wt%)	20%	43%	27%	—
Density (g/cc)	1.53	1.67	1.57	ASTM D-1505
Ultimate Tensile Strength at 23°C	207 MPa (30,000 psi)	162 MPa (23,500 psi)	200 MPa (29,000 psi)	ASTM D-882
Ultimate Tensile Strength at 200°C	121 MPa (17,500 psi)	89 MPa (13,000 psi)	115 MPa (17,000 psi)	ASTM D-882
Ultimate Elongation at 23°C (%)	75%	70%	85%	ASTM D-882
Tensile Modulus at 23°C	2.48 GPa (360,000 psi)	2.28 GPa (330,000 psi)	2.62 GPa (380,000 psi)	ASTM D-882
Tensile Modulus at 200°C	1.62 GPa (235,000 psi)	1.14 GPa (165,000 psi)	1.38 GPa (200,000 psi)	ASTM D-882
Tear Strength, initial (Graves), N	11.8 N	11.5 N	17.8 N	ASTM D-882

The tensile strength and modulus values are lower than bare Kapton HN (231 MPa, 2.5 GPa) because the softer FEP layer is load-bearing in parallel. In the 150FN019 construction — where the FEP represents 43% of total weight — the effect is most pronounced. For applications where maximum tensile strength is needed alongside FEP coating, the 120FN616 construction (80% polyimide by weight, thin 0.1 mil FEP on each side) preserves the best mechanical properties while still enabling heat sealing.

Electrical Properties

The FEP fluoropolymer coating lowers the composite’s dielectric constant and dissipation factor below that of bare Kapton HN — an unexpected benefit that comes from the FEP’s inherently lower Dk (approximately 2.1) averaging into the composite structure:

Property	120FN616	150FN019	250FN029	Kapton HN 100 (for comparison)	Test Method
Dielectric Strength, V/µm (V/mil)	272 (6,900)	197 (5,000)	197 (5,000)	303 (7,700)	ASTM D-149
Dielectric Constant (Dk) at 1 kHz	3.1	2.7	3.0	3.4	ASTM D-150
Dissipation Factor (Df) at 1 kHz	0.0015	0.0013	0.0013	0.0018	ASTM D-150
Volume Resistivity at 23°C (Ω·cm)	1.4 × 10¹⁷	2.3 × 10¹⁷	1.9 × 10¹⁷	1.5 × 10¹⁷	ASTM D-257
Volume Resistivity at 200°C (Ω·cm)	4.4 × 10¹⁴	3.6 × 10¹⁴	3.7 × 10¹⁴	—	ASTM D-257

The lower Dk (2.7 for 150FN019 vs. 3.4 for 100HN) and slightly lower Df make Kapton FN attractive for high-frequency flex circuit dielectric applications where signal integrity on adjacent copper layers benefits from reduced capacitive coupling and lower dielectric losses. This is a property improvement over HN, not a compromise.

Chemical and Moisture Properties

This is where Kapton FN’s FEP coating delivers its most significant advantage over bare HN:

Property	120FN616	150FN019	400FN022	Test Method
Moisture Absorption at 50% RH (%)	1.3%	0.8%	0.4%	ASTM D-570
Moisture Absorption at 98% RH (%)	2.5%	1.7%	1.2%	ASTM D-570
Water Vapor Permeability (g/m²·24h)	17.5	9.6	2.4	ASTM E-96

Bare Kapton HN absorbs approximately 2.9% moisture at 100% RH. The FEP coating on Kapton FN reduces moisture absorption significantly — and thicker FEP coatings reduce it further. The 400FN022 construction (2 mil polyimide + 2 mil FEP) achieves 1.2% moisture absorption at 98% RH and a water vapor permeability of only 2.4 g/m²·24h, making it the construction of choice for hermetic packaging and moisture-sensitive electronic enclosures.

Heat Sealing Performance: The Defining Capability of Kapton FN

Film-to-Film Sealing

The FEP fluoropolymer layer enables thermal bonding between two Kapton FN surfaces (FEP side to FEP side) at temperatures achievable with standard heat seal tooling. The minimum peel strength for FEP-to-FEP heat seal bonds on Kapton FN (except 120FN616, which has thinner FEP) is 450 g/in (1.7 N/cm), tested per standard heat seal peel strength methods.

Film-to-Copper Bonding

When the FEP side of Kapton FN is sealed to copper foil under heat and pressure, the minimum peel strength achieved is 300 g/in (1.2 N/cm), measured with the FEP side sealed to the untreated side of 3/4 oz copper foil. This FEP-to-copper bonding capability is how Kapton FN functions as a coverlay or bonding layer in flex circuits — the FEP layer bonds directly to the copper conductor surface without a separate acrylic or epoxy adhesive.

HN-to-FEP Layer Bond Integrity

The HN-to-FEP interface within the composite itself is specified at a minimum cold peel strength of 225 g/in (0.87 N/cm) (except 120FN616, which has a thinner interface due to the minimal 0.1 mil FEP layer). This internal bond is what prevents delamination of the composite during processing and in service.

Kapton FN vs. Kapton HN: When Does the FEP Coating Justify the Upgrade?

Design Requirement	Kapton HN	Kapton FN
Heat sealability required	✗ Not heat sealable	✓ FEP enables thermal bonding
Moisture barrier performance	Moderate (2.9% at 100% RH)	Improved (0.4–2.5% depending on construction)
Chemical resistance to solvents	Good	Better — FEP outer surface is chemically inert
Minimum dielectric constant	Dk 3.4	Dk 2.7–3.1 (lower due to FEP)
Maximum tensile strength	231 MPa	162–207 MPa (reduced by FEP layer)
Dynamic flex endurance	Excellent (285,000 cycles at 1 mil)	Reduced (FEP layer affects flex life)
Adhesive-free bonding in multilayer	Not possible	✓ FEP bonds under heat and pressure
Cost	Lower	Higher

The decision rule: if you need heat sealability, moisture barrier improvement, or adhesive-free copper bonding, specify Kapton FN. If your application is primarily mechanical flex performance or maximum tensile integrity and you don’t need the FEP functionality, Kapton HN is the correct and more economical choice.

How Kapton FN Is Used in Flex Circuit Fabrication

As an Adhesiveless Flex Circuit Dielectric

In conventional adhesive-based flex circuits, an acrylic or epoxy adhesive bonds the Kapton substrate to the copper foil. These adhesives add thickness, reduce thermal performance (acrylic adhesives typically rated to 105°C UL thermal index vs. 220°C+ for polyimide), and can creep under long-term mechanical stress.

Kapton FN eliminates the adhesive layer. The FEP side of the film bonds directly to copper foil under controlled heat and pressure conditions — typically 270–300°C at 50–200 psi — creating a composite with no adhesive interface. The result is a thinner, thermally superior laminate. DuPont’s Pyralux LF and LF (Low Flow) product family uses this Kapton FN bonding mechanism as its construction basis.

As a Flex Circuit Coverlay

Kapton FN is used as a coverlay over the etched copper conductors on the outer layers of a flex circuit. The FEP side bonds to the copper and adjacent Kapton during the lamination cycle. Compared to liquid photoimageable (LPI) solder mask, Kapton FN coverlay offers superior flex endurance, temperature rating, and resistance to chemical exposure — critical for dynamic flex applications and harsh-environment deployments.

Heater Circuits and Thermal Insulation

For resistance heater circuits laminated between two dielectric layers, Kapton FN’s heat sealability makes it the standard construction: the resistive conductor traces are sandwiched between two Kapton FN sheets, sealed at their edges under heat and pressure to create a hermetic, robust, flexible heater. This construction is standard in aerospace, medical, and industrial heating elements.

Tubing and Formed Parts

Kapton FN film can be seam-sealed into tubes of any diameter by bringing two FEP surfaces together under heat. The resulting tubes are used for wire wrapping insulation, catheter insulation, high-temperature protective sleeves, and chemical-resistant tubing where Kapton’s thermal properties and FEP’s chemical resistance are both required.

Applications Summary

Application	Why Kapton FN	Recommended Construction
Adhesiveless flex circuit laminates	FEP bonds to Cu without adhesive layer	150FN019 or 200FN011
Flex circuit coverlay	Heat sealable, flexible, high-temperature	150FN019 (one-sided)
Hermetic sealing pouches	FEP-to-FEP seal, moisture barrier	100FN999 or 200FN919 (both sides)
Flexible heater circuits	Seals conductor between two FN layers	150FN019 or 200FN919
RF/microwave flex substrates	Lower Dk (2.7) vs. HN (3.4)	150FN019 (lowest Dk)
Aerospace wiring insulation	Chemical inertness, high temp rating	400FN022 (heavy moisture barrier)
Chemical-resistant insulation	FEP outer layer resists solvents/acids	300FN021 or 400FN022

Useful Resources for PCB Engineers

Resource	Link
Kapton FN Technical Data Sheet (K-15347)	American Durafilm PDF
DuPont Kapton FN Product Page (Qnity)	dupont.com — Kapton FN
Kapton General Specifications (EI-10167) — FN Constructions Table	Qnity PDF
DuPont Kapton Summary of Properties — Includes FN Data	Marian Inc PDF
Fralock — Kapton FN Data Sheet	fralock.com
CS Hyde — Kapton FN Film Products	cshyde.com
DuPont PCB Materials Overview	pcbsync.com/Dupont-pcb
IPC-4202 — Flexible Base Dielectrics for Flex PCBs	ipc.org
IPC-6013 — Qualification for Flexible PCBs (Class 1/2/3)	ipc.org

5 FAQs About DuPont Kapton FN Film

Q1: What does the “9” and “6” mean in Kapton FN product codes like 150FN019 and 120FN616? The Kapton FN product code encodes FEP layer thicknesses in a compressed notation. In the FN code, the digit “9” represents 0.5 mil (13 µm) of FEP, and “6” represents 0.1 mil (2.5 µm) of FEP. So 150FN019 means: total gauge 150, polyimide = 1 mil (the middle digit “1”), FEP on one side = 0.5 mil (“9” = 0.5 mil = 13 µm), and no coating on the other side (“” = 0). Meanwhile 120FN616 means: polyimide = 1 mil, FEP on each side = 0.1 mil (“6” = 0.1 mil). This notation is explained in the DuPont Summary of Properties document and is essential for correctly interpreting the product lineup when comparing constructions.

Q2: What are the heat sealing conditions for Kapton FN? The FEP fluoropolymer coating melts and flows at approximately 260–280°C, enabling heat sealing under pressure. Typical heat sealing parameters for Kapton FN are 270–300°C, 50–200 psi (0.35–1.4 MPa), and 5–30 seconds dwell time depending on FEP coating thickness and substrate thickness. Refer to DuPont technical bulletin H-55005-2 (Teflon FEP fabrication techniques) for detailed heat sealing parameters. The 120FN616 construction (0.1 mil FEP per side) requires more precise thermal control than heavier FEP constructions because the thin coating has less thermal mass and can over-bond or under-bond with small process variation.

Q3: Can Kapton FN be used as a flex circuit coverlay for IPC-6013 Class 3 applications? Yes, Kapton FN is a valid coverlay material for IPC-6013 Class 3 flex circuit applications. Its polyimide base provides the high-temperature rating (200°C+ UL index) and flex endurance required for Class 3 qualification, while the FEP coating provides the adhesive-free bonding to copper. For Class 3 dynamic flex qualification, the flex circuit design should use the thinnest appropriate FEP construction (typically 150FN019 or thinner) and validate bend radius, cycle life, and temperature cycling per IPC-TM-650 methods. The FEP layer does slightly reduce dynamic flex endurance compared to bare Kapton HN, so flex cycle validation is mandatory rather than assumed.

Q4: How does Kapton FN compare to Pyralux LF for adhesiveless flex circuit construction? Pyralux LF (Low Flow) and Pyralux LF adhesiveless are DuPont product families that use Kapton-based polyimide technology with FEP bonding layers to create adhesiveless copper-clad laminates — the same principle as Kapton FN, but supplied as a ready-to-use double-sided or single-sided copper-clad laminate. Kapton FN film is the raw dielectric material that fabricators use to build their own laminates or as coverlay. If you need a copper-clad laminate for imaging and etching, specify Pyralux (LF, AP, or relevant variant). If you need the raw dielectric film for a custom construction, coverlay fabrication, or sealing application, specify Kapton FN.

Q5: Does the FEP coating affect the dimensional stability of Kapton FN compared to HN? The FEP fluoropolymer coating adds a layer with higher CTE (approximately 120–150 ppm/°C for FEP vs. 20 ppm/°C for polyimide) than the underlying Kapton HN. In the composite, this creates a bimetal-effect where temperature changes produce differential thermal stresses between the polyimide and FEP layers. For most flex circuit applications, this effect is managed through design — using constructions with balanced coatings (FEP on both sides) for symmetric stackups, and following the same thermal pre-conditioning discipline as bare Kapton HN before imaging (bake at 150°C for 30 minutes to release residual stresses). For high-precision applications with tight dimensional tolerances, Kapton VN (dimensionally stabilized, no FEP) is a better choice than Kapton FN.

Engineering Perspective

DuPont Kapton FN adds a specific capability set to the Kapton family that no amount of adhesive engineering can fully replicate: truly adhesiveless heat bonding to itself and to copper, with the chemical inertness of FEP on the outer surface and Kapton’s full thermal range intact. The trade-offs are straightforward — lower tensile strength than bare HN, slightly reduced flex endurance, and higher unit cost — but for the applications where these trade-offs are acceptable, the FEP coating enables construction simplifications (thinner stackups, no adhesive layer, lower z-axis impedance variation) that pay back in both performance and long-term reliability. The 150FN019 construction is the workhorse of the lineup for single-sided flex circuit coverlay and adhesiveless laminate applications, while 100FN999 and 200FN919 (both-sides coated) handle sealed assemblies and heater circuits. Specifying the right FN construction from that table is the engineering work — the film itself is a proven, well-documented material with decades of production history.