PI Film Thermal Conductivity Testing: Methods, Standards, and Technical Guide

PI film thermal conductivity testing is essential for evaluating the heat dissipation performance of polyimide films used in flexible electronics, EV batteries, and high-frequency communication devices. Professional laboratories measure thermal conductivity, thermal diffusivity, and specific heat capacity using internationally recognized methods such as laser flash analysis, transient plane source, and guarded heat flow meter techniques. This guide explains the testing principles, sample requirements, applicable standards, and typical applications of thermal conductivity measurement for PI films.

Technical Overview

Polyimide (PI) film is a high-performance engineering polymer widely used in flexible printed circuits, electric vehicle battery insulation, aerospace components, and advanced electronic packaging. Its outstanding thermal stability, excellent dielectric strength, and superior mechanical toughness make it the material of choice for demanding thermal management applications. Thermal conductivity, typically expressed in W/(m·K), is one of the most critical parameters determining how effectively a PI film transfers and dissipates heat.

Standard unfilled PI film exhibits a through-thickness thermal conductivity of approximately 0.10 to 0.25 W/(m·K), while modified formulations containing thermally conductive fillers such as boron nitride, aluminum nitride, aluminum oxide, graphite, or graphene can reach values from 1 to 10 W/(m·K) or higher. Because PI films are anisotropic, in-plane thermal conductivity can differ significantly from through-thickness values, and both directions may need to be characterized depending on the end use. Accurate measurement requires careful sample preparation, appropriate method selection, and strict adherence to recognized international standards.

Testing Samples

Thermal conductivity testing can be performed on a wide range of polyimide film materials supplied by manufacturers, converters, or end-product assemblers. Common sample types include:

Standard Unfilled PI Film

Conventional yellow or amber polyimide film, such as Kapton-type and Upilex-type materials, used as base substrates and electrical insulation layers. Typical thicknesses range from 7.5 μm to 125 μm, and multiple layers are usually stacked to meet minimum thickness requirements for reliable measurement.

Thermally Conductive Filled PI Film

Composite PI films loaded with boron nitride, aluminum nitride, alumina, graphite, or graphene fillers, engineered for enhanced heat spreading in flexible circuits, battery packs, and power electronics.

Black PI Film and Graphite-Modified Film

Pigmented or carbon-filled PI films offering improved thermal spreading and light-absorbing characteristics for optical sensors and display applications.

Colorless and Transparent PI Film

Low-coloration polyimide films used in flexible OLED displays and cover windows, where thermal properties must be verified alongside optical transmittance.

FCCL Substrate Film and Laminates

Polyimide films used as dielectric layers in flexible copper-clad laminates, adhesives, and coverlay systems, which can be tested individually or as built-up multilayer structures.

PI Coatings, Microporous Films, and Aerogels

Coated PI layers, porous thermal insulation films, and PI aerogel materials requiring low-conductivity measurement using methods suitable for insulating materials.

Testing Items

A comprehensive thermal characterization program for PI film typically includes the following measurable parameters:

• Through-thickness (cross-plane) thermal conductivity, in W/(m·K)
• In-plane (lateral direction) thermal conductivity for anisotropic films
• Thermal diffusivity, in mm²/s
• Specific heat capacity (cp), in J/(g·K)
• Thermal resistance and thermal impedance of stacked film assemblies
• Temperature-dependent thermal conductivity across a defined temperature range, for example -40 °C to 300 °C
• Density verification, required for converting diffusivity into conductivity
• Thickness uniformity and contact-interface evaluation for stacked samples

Testing Methods

Because PI films are extremely thin, special procedures such as multi-layer stacking, thin-film sensors, or adapted sample holders are applied. The most widely accepted methods are described below.

Laser Flash Method (LFA)

The laser flash technique measures thermal diffusivity by applying a short energy pulse to one side of the sample and recording the temperature rise on the opposite side with an infrared detector. Combined with independently measured specific heat and density values, thermal conductivity is calculated. It is performed in accordance with ASTM E1461 and ISO 22007-4, covers a broad temperature range, and is the preferred method for filled and high-conductivity PI films. Thin PI films are typically stacked to a total thickness of 0.5 mm to 3 mm, and thin graphite coating is applied to ensure uniform energy absorption and emission.

Transient Plane Source Method (TPS / Hot Disk)

A spiral nickel sensor is sandwiched between two pieces of the film and acts as both heater and temperature monitor. From the transient temperature response, thermal conductivity and diffusivity are derived in accordance with ISO 22007-2. This method covers a wide conductivity range from insulating to highly conductive materials, can measure in-plane and through-thickness properties using anisotropic modules, and requires minimal sample preparation.

Guarded Heat Flow Meter Method

The sample is placed between two temperature-controlled plates under a calibrated transducer, and steady-state heat flux is measured in accordance with ASTM E1530. This approach is well suited for moderate to low conductivity films and stacked specimens, providing reliable through-thickness conductivity at a single controlled mean temperature.

Guarded Hot Plate Method

As an absolute steady-state reference method per ASTM C177 and ISO 8302, the guarded hot plate technique delivers the highest accuracy for thermal insulation materials and is suitable for microporous PI films and aerogel insulation used in cryogenic or high-temperature service.

ASTM D5470 Thermal Impedance Testing

For PI films functioning as thermal interface or insulation layers in power modules, ASTM D5470 measures thermal impedance versus applied pressure, separating bulk thermal resistance from contact resistance. This method is essential for evaluating thermally conductive PI films used between heat sources and heat sinks.

Thin-Film Specialized Methods

For freestanding films below 100 μm requiring single-layer measurement, the 3-omega method, time-domain thermoreflectance (TDTR), and modified transient plane source sensors can be applied to determine intrinsic in-plane and cross-plane conductivity without stacking artifacts.

Applications

Thermal conductivity data for PI film directly supports design, qualification, and quality control across many industries:

• Flexible printed circuits and rigid-flex boards, where the PI dielectric layer governs heat spreading from dense copper traces and components
• Electric vehicle lithium-ion battery packs, where PI films serve as thermal runaway barriers and electrical insulation between cells
• 5G and high-frequency antennas, where low dielectric loss PI substrates must also manage device operating temperatures
• Flexible OLED displays and organic electronics, where thermal properties influence lifetime and color stability
• Power semiconductor modules and inverters, where filled PI films act as thermally conductive electrical isolation layers
• Aerospace and satellite systems, where PI films endure extreme thermal cycling and vacuum environments
• Heat-resistant masking, barcodes, and thermal insulation liners in manufacturing processes
• Research and development of next-generation thermally conductive polymer nanocomposites and film materials

FAQ

What is the thermal conductivity of standard PI film?

Unfilled polyimide film generally shows a through-thickness thermal conductivity of about 0.10 to 0.25 W/(m·K), while boron nitride filled or graphite modified PI films can reach several W/(m·K). Exact values depend on formulation, thickness, orientation, and measurement temperature.

Why are multiple PI film layers stacked for testing?

Individual PI films are often thinner than the minimum thickness that laser flash and steady-state instruments can measure accurately. Stacking several identical layers increases total thickness, reduces measurement uncertainty, and produces an averaged through-thickness result representative of the material.

What is the difference between in-plane and through-thickness conductivity?

Through-thickness conductivity describes heat flow perpendicular to the film surface, which governs heat transfer into substrates and heat sinks. In-plane conductivity describes heat spreading along the film plane, which is significantly higher in oriented or graphite-filled films. Both directions should be characterized for anisotropic materials.

Which testing standard should be selected?

Laser flash analysis per ASTM E1461 or ISO 22007-4 is recommended for filled and high-conductivity films across wide temperature ranges. Transient plane source per ISO 22007-2 suits rapid broadband screening, while ASTM E1530, ASTM C177, and ASTM D5470 are chosen for steady-state, insulation, and interface application scenarios respectively.

What sample size and quantity are required?

Typical laser flash specimens are 10 to 12.7 mm discs or squares, while transient plane source samples are 20 to 50 mm squares; sufficient material is needed for stacking or duplicate pieces and for density and specific heat verification. Exact dimensions are confirmed with the laboratory before testing.

Can testing be performed at elevated or sub-ambient temperatures?

Yes. Laser flash and transient systems can characterize PI films from cryogenic temperatures up to approximately 300 °C or beyond, generating thermal conductivity curves versus temperature that reflect real operating conditions.