Electrical Polyimide Film
Polyimide film for electrical insulation: dielectric strength by thickness, 25-125 micron, slit to 8 mm. Class H, 260 C. Choose the film for your voltage class.
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Product Introduction
Description Electrical polyimide film
Polyimide film is specified for electrical insulation by one relationship above all others: dielectric strength falls as thickness rises. A 25 µm film holds 7.7 kV per 0.025 mm; a 125 µm film, five times thicker, holds only about 2.5 times the voltage. That is why you choose a polyimide film by the breakdown voltage your insulation actually needs - not by picking the thickest film you can afford, and not by accepting "5,000 volts" as a specification from your supplier.
This page gives you the numbers. It covers uncoated polyimide film in the standard thickness band 0.03 – 0.125 mm (25 / 50 / 75 / 125 µm), supplied in roll or slit to width, for Class H motor and transformer insulation, magnet wire, flexible circuits and high-temperature electrical applications.
Thickness: 25 / 50 / 75 / 125 µm standard · 0.015 – 0.25 mm available Width: slit from 8 mm up to 1040 mm Colour: amber / brown, natural Thermal class: H (180 °C) · continuous service −269 °C to +260 °C Forms: roll · slit roll · sheet · die-cut
What This Film Is - and What It Is Not
It is uncoated polyimide film. A cast film of polyimide resin (PMDA–ODA chemistry), amber or brown, supplied without adhesive, without FEP coating and without any backing.
Three products are commonly confused with it - make sure you are specifying the right one:
| Product | What it is | Typical use | Where to find it |
|---|---|---|---|
| Uncoated polyimide film (this page) | Plain PI film, no coating | Slot, interphase and layer insulation; magnet wire wrap; FPC substrate | This page |
| FEP-coated polyimide film (Kapton® FN type) | PI film with a heat-sealable fluoropolymer layer on one or both faces | Bondable magnet wire insulation and self-adhering wraps - heat seals to itself | Ask us separately |
| Polyimide tape | PI film with a silicone or acrylic adhesive | Masking during wave soldering and powder coating; high-temperature holding | Polyimide film tape |
Dielectric Strength by Thickness
The core table. Values are for standard polyimide film (DuPont Kapton® HN reference data, AC dielectric strength, 60 Hz).
| Thickness | Dielectric strength | Per mil | Breakdown voltage | Typical use |
|---|---|---|---|---|
| 25 µm (1 mil) | 303 kV/mm | 7,700 V/mil | ≈ 7.6 kV | Thin-wall turn and interturn insulation; where space is critical |
| 50 µm (2 mil) | 240 kV/mm | 6,100 V/mil | ≈ 12.0 kV | General slot and layer insulation |
| 75 µm (3 mil) | 205 kV/mm | 5,200 V/mil | ≈ 15.4 kV | Higher-voltage slot insulation, transformer interlayer |
| 125 µm (5 mil) | 154 kV/mm | 3,900 V/mil | ≈ 19.3 kV | Highest-voltage sections, mec |
Thicker film gives you more total withstand voltage but lower dielectric strength per unit thickness. If your constraint is space, thin film is more efficient per millimetre of insulation. If your constraint is mechanical toughness or partial-discharge margin, thick film is the answer. Optimising for the wrong one is a common and expensive specification error.
Choosing Film Thickness for Your Working Voltage
The film's breakdown voltage is not your system's rating. Work through it in this order:
Working voltage - the rms voltage the insulation sees in service
Required withstand voltage - apply your safety factor and the insulation-coordination rule you work to (for example IEC 60664-1, or the relevant motor or transformer standard)
Read the required breakdown from the table above and select the thickness that exceeds it
Add margin for temperature, humidity, mechanical stress during winding, and ageing
Test the finished construction - not the bare film
Worked examples
| Working voltage | With 3× safety factor | Required breakdown | Suggested film |
|---|---|---|---|
| 1,000 V | 3,000 V | < 7.6 kV | 25 µm - ample margin |
| 3,000 V | 9,000 V | < 12.0 kV | 50 µm |
| 6,000 V | 18,000 V | < 19.3 kV | 125 µm, or reconsider the construction |
Other Electrical Properties
| Property | Typical value | Note |
|---|---|---|
| Dielectric constant | ≈ 3.4 at 1 kHz (range 2.7 – 3.5 by film type and frequency) | Stable across frequency and temperature - one reason PI suits high-frequency and high-temperature circuits |
| Dissipation factor | ≈ 0.003 at 60 Hz | Low loss |
| Volume resistivity | ≈ 10¹⁷ Ω·cm | High insulation resistance, retained at temperature |
| Surface resistivity | ≈ 10¹⁶ Ω | - |
| Dielectric strength at elevated temperature | Retained up to the film's thermal limit | The defining advantage of polyimide over PET and most other films |
Slit to Your Required Width
Polyimide film is bought to fit the insulation you are making, so slitting is part of the specification, not an afterthought.
| Parameter | Range |
|---|---|
| Minimum slit width | 8 mm |
| Maximum width | 1040 mm |
| Core | 3" / 6", paper or plastic |
| Edge condition | Clean, burr-free edge - essential where the film is wrapped under tension |
| Also available | Sheet and die-cut pieces to drawing |
Why the minimum width matters: narrow slitting is where most suppliers fail. A film slit to 8 mm has to come off the slitter with a clean edge and consistent tension, or it breaks on the customer's winding machine. Tell us your width and the winding tension and we will confirm what we can hold.
Temperature and Insulation Class
| Property | Value |
|---|---|
| Thermal class | H (180 °C) |
| Continuous service range | −269 °C to +260 °C |
| Flammability | UL 94 VTM-0 |
| Behaviour at temperature |
No melting point - the film does not melt and flow; it retains mechanical and dielectric performance across the range
|
APPLICATION
Class H motor slot insulation
Holds dielectric strength at 180 °C where polyester films cannot
Thickness: 75 – 125 µm
Interphase and turn-to-turn insulation
Thin enough to fit inside the winding without reducing copper fill
Thickness: 25 – 50 µm
Dry-type transformer layer insulation
Layer-to-layer dielectric separation at Class H temperature
Thickness: 50 – 125 µm
Magnet wire wrap
High dielectric strength in a very thin wall
Thickness: 25 – 50 µm (or FEP-coated for a bonded wall)
Flexible printed circuits (FPC)
Substrate, coverlay and stiffener at solder temperatures
Thickness: 25 – 50 µm
High-temperature cable insulation
Retains insulation resistance at temperature
Thickness: 50 – 125 µm
Voice coil insulation
Withstands coil temperatures in loudspeaker and transducer assemblies
Thickness: 25 – 50 µm
EV battery and busbar insulation
Dielectric barrier with thermal and mechanical durability
Thickness: 50 – 125 µm
Die-cut insulation parts
Washers, spacers, shields to drawing
Thickness: Any
How to Specify Your Film
| Line | Example |
|---|---|
| Thickness | 50 µm or other thickness |
| Required breakdown voltage | ≥ 10 kV, or "for 3 kV working voltage with 3× safety factor" |
| Slit width | 12 mm, ± tolerance |
| Form | Roll / slit roll / sheet / die-cut to drawing |
| Insulation class | Class H (180 °C) |
| Quantity | By weight or by metres, per width |
If you would rather not calculate it: send the working voltage, the insulation class and the space available for the insulation, and we will propose a thickness and confirm the slit width.
FAQ
What is the dielectric strength of your polyimide film? It depends on thickness, so it must always be quoted with one. Standard values (Kapton® HN reference data): 25 µm ≈ 303 kV/mm (7.6 kV breakdown); 50 µm ≈ 240 kV/mm (12.0 kV); 75 µm ≈ 205 kV/mm (15.4 kV); 125 µm ≈ 154 kV/mm (19.3 kV). Be cautious of any supplier quoting a single voltage figure with no thickness attached.
Why does thicker film have lower dielectric strength? Because dielectric strength is measured per unit thickness. Thicker films contain more volume in which imperfections, voids and partial-discharge sites can exist, so the field they withstand per millimetre is lower - even though the total breakdown voltage is higher.
Which thickness should I use? Start from the working voltage and apply your safety factor, then read the table. As a guide: 25 µm suits thin-wall turn and interturn insulation; 50 µm suits general slot and layer insulation; 75 and 125 µm suit higher-voltage sections and where mechanical toughness matters. Tell us the application and we will confirm.
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