Polyimide (PI) films are among the most thermally stable polymeric materials available, supporting continuous service temperatures up to 260°C and short-term excursions beyond 400°C in demanding applications such as flexible printed circuits, EV batteries, and aerospace insulation. A rigorous high-temperature resistance evaluation verifies critical parameters including glass transition temperature, thermal decomposition behavior, dimensional stability, and long-term aging performance. This article outlines the complete testing framework for PI film heat resistance assessment, covering sample types, testing items, standardized methods, and typical acceptance criteria.
Technical Overview
Polyimide (PI) film is a high-performance aromatic polymer material produced through polycondensation of dianhydride and diamine monomers followed by imidization at elevated temperatures. Its rigid imide ring structure delivers exceptional thermal stability, chemical resistance, and dielectric performance that few engineering polymers can match. Standard PMDA-ODA type PI film, for example, exhibits a glass transition temperature (Tg) in the range of 360–410°C, a 5% weight-loss decomposition temperature above 500°C, and a recognized relative temperature index (RTI) of approximately 240–260°C for continuous long-term service.
Because PI films are routinely deployed in environments where sustained or cyclic heat exposure determines product reliability—flexible printed circuits undergoing reflow soldering, battery cells during thermal runaway mitigation, and spacecraft subjected to thermal cycling—a systematic evaluation of high-temperature resistance is essential. A complete assessment characterizes thermal transitions, decomposition kinetics, dimensional stability, retention of mechanical and electrical properties after thermal aging, and long-term thermal endurance. Testing must be performed in accordance with recognized standards such as ASTM, ISO, IEC, and IPC methods, using conditioned specimens and calibrated instrumentation.
Testing Samples
Standard Polyimide Films
Conventional PMDA-ODA based films (comparable to Kapton HN or equivalent grades), typically supplied in thicknesses from 7.5 μm to 125 μm. These samples are evaluated for baseline thermal properties, including Tg, decomposition temperature, and heat shrinkage in both machine direction (MD) and transverse direction (TD).
Low-CTE and High-Modulus PI Films
BPDA-based or molecularly oriented films designed for chip-on-film (COF) packaging, fine-line FPC, and flexible display substrates. These samples require additional focus on coefficient of thermal expansion (CTE), dimensional stability during solder reflow, and modulus retention at elevated temperatures.
Functional and Modified PI Films
- Black PI films for photovoltaic and optical light-blocking applications
- Colorless and transparent PI (CPI) films for flexible display cover substrates
- Heat-sealable or thermoplastic PI films for laminate bonding
- Thermally conductive PI films filled with boron nitride or alumina
- Corona-resistant PI films for traction motor and inverter insulation
Composite and Laminated Structures
Coverlay films, flexible copper-clad laminates (FCCL), PI adhesive tapes, and PI-based thermal barrier composites are tested as finished structures to evaluate interfacial stability, delamination resistance, and adhesive performance under thermal stress.
Testing Items
The high-temperature resistance evaluation of PI film encompasses the following core testing items, each addressing a distinct aspect of thermal performance:
- Glass transition temperature (Tg): determined by DSC, DMA, or TMA to identify the onset of segmental mobility and softening behavior
- Thermal decomposition temperature (Td): temperature at 5% and 10% weight loss measured by thermogravimetric analysis (TGA), evaluated in both nitrogen and air atmospheres
- Char yield and residual mass: residue percentage at 700–800°C, an indicator of structural aromaticity and flame resistance
- Coefficient of thermal expansion (CTE): linear dimensional change below and above Tg, critical for matching with copper and silicon in electronic assemblies
- Heat shrinkage (dimensional stability): permanent MD/TD dimensional change after exposure to defined temperatures such as 150°C, 200°C, or 260°C for specified durations
- Thermal aging performance: retention of tensile strength, elongation at break, and dielectric properties after long-term oven aging at elevated temperatures
- High-temperature elongation (HT-elongation): ductility measured directly at service temperature, a key qualification metric for FPC-grade film
- Relative temperature index (RTI): long-term thermal endurance rating established through Arrhenius-based aging studies per UL 746B
- Thermal cycling resistance: resistance to cracking, delamination, or property degradation after repeated temperature cycling, typically -55°C to 250°C or wider ranges
- Surface and appearance stability: color change (ΔE), yellowing index, blistering, and outgassing after thermal exposure
- Electrical properties at elevated temperature: dielectric constant, dissipation factor, insulation resistance, and dielectric breakdown strength measured at controlled high temperatures
Testing Methods
Thermogravimetric Analysis (TGA)
TGA measures weight loss as a function of temperature to determine decomposition onset, Td5%, Td10%, and char yield. Testing follows ASTM E1131 or ISO 11358, typically heating 5–10 mg specimens from room temperature to 800°C at 10°C/min under nitrogen purge, with complementary runs in air to assess oxidative stability. Isothermal TGA at service temperature may also be performed to quantify long-term weight loss rates.
Differential Scanning Calorimetry (DSC)
DSC per ASTM E1356 or ISO 11357 identifies the glass transition as a step change in heat flow. Because fully imidized PI exhibits a weak thermal transition, DMA is often preferred for precise Tg determination. DSC also verifies residual solvent content and degree of imidization through absence of exothermic curing peaks.
Dynamic Mechanical Analysis (DMA)
DMA per ASTM D7028 or ISO 6721 measures storage modulus and tan delta as temperature sweeps from room temperature to above 400°C. The tan delta peak temperature provides a sensitive and reproducible Tg value, while modulus retention curves quantify stiffness behavior across the operating range.
Thermomechanical Analysis (TMA)
TMA per ASTM E831 measures dimensional change under minimal load to determine CTE below and above Tg. Low-CTE grades for electronics are typically specified below 20 ppm/°C in the range of 50–250°C, and TMA data confirm compliance.
Heat Shrinkage Testing
Dimensional stability is evaluated by conditioning marked coupons (commonly 100 mm × 100 mm or 150 mm × 150 mm) at specified temperatures—such as 200°C for 2 hours or 260°C for 30 minutes to simulate reflow—then calculating shrinkage as (L0 − L1)/L0 × 100% in both MD and TD. IPC-TM-650 method 2.2.3A and ASTM D5214 provide the standard framework, with premium FPC grades typically requiring shrinkage below 0.05%.
Long-Term Thermal Aging
Oven aging per ASTM D3045 and UL 746B exposes specimens to multiple elevated temperatures (for example, 250°C, 275°C, and 300°C) for extended periods up to several thousand hours. Tensile strength and elongation are periodically measured per ASTM D882, and property retention curves are extrapolated to establish the RTI at which a critical property retains 50% of its initial value after 100,000 hours.
Thermal Cycling and Combined Environmental Testing
Cycling between extreme temperatures (for example, -196°C to 300°C for aerospace qualification, or -40°C to 150°C for automotive electronics) validates resistance to fatigue-induced cracking and delamination. Post-cycling inspection includes microscopy for microcracking, peel strength measurement for laminates, and dielectric testing to confirm insulation integrity.
Applications
The high-temperature resistance evaluation of PI film directly supports qualification and quality control across the following industries:
- Flexible printed circuits (FPC) and rigid-flex boards: film must withstand lead-free reflow at 260°C+ without excessive shrinkage, wrinkling, or delamination
- Electric vehicle and lithium-ion batteries: PI films serve as cell-level electrical and thermal barrier insulation, where performance under abusive thermal conditions is safety-critical
- Aerospace and spacecraft: multi-layer insulation blankets, wire insulation, and thermal control films must survive vacuum, radiation, and extreme thermal cycling
- High-temperature wire and cable: magnet wire and wrapping insulation for motors, transformers, and traction systems rated to 180–240°C classes
- Flexible displays and OLED substrates: colorless PI substrates require thermal dimensional stability compatible with LTPS processing temperatures approaching 350–450°C
- Photovoltaic modules: black PI films and backsheet layers must maintain insulation and appearance through 25-year outdoor thermal exposure
- Semiconductor manufacturing: high-temperature masking tapes, carrier films, and wafer processing consumables exposed to repeated bake cycles
FAQ
What is the maximum continuous service temperature of standard PI film?
Standard PMDA-ODA PI film carries a UL relative temperature index of approximately 240–260°C for long-term continuous use, and can tolerate short-term excursions up to 400°C. However, actual service limits depend on mechanical loading, atmosphere, and required property retention, which is why project-specific aging evaluation is recommended.
Why is heat shrinkage such an important test item for FPC applications?
During solder reflow, FPC substrates experience temperatures of 260°C or higher. Excessive film shrinkage causes circuit pattern distortion, pad misalignment, warpage, and coverlay blistering. Premium electronic-grade PI film is therefore specified with MD/TD shrinkage typically below 0.05% after reflow-simulating exposure.
What specimen quantities and dimensions are required for a full evaluation?
TGA requires approximately 10–20 mg per run; DSC uses 5–10 mg discs; TMA and DMA specimens follow instrument-specific geometries; shrinkage coupons are typically 100 mm × 100 mm; and tensile testing per ASTM D882 requires die-cut strips, commonly 10 mm wide × 150 mm long, with a minimum of five specimens per condition. Long-term aging programs should reserve sufficient specimens for all planned measurement intervals.
How is the glass transition temperature of PI film best measured when the DSC transition is weak?
Fully imidized PI often shows a subtle, low-energy step in DSC, making DMA the preferred technique. The tan delta peak temperature from a DMA temperature sweep provides a sharp, reproducible Tg, and TMA can further corroborate the transition through the CTE inflection point.
What is the typical turnaround time for high-temperature resistance testing?
Baseline thermal characterization (TGA, DSC, DMA, TMA, shrinkage) is generally completed within 5–10 business days. Thermal cycling programs typically require one to three weeks depending on cycle count, while full long-term RTI aging studies extend over weeks to months due to the extended oven exposure durations mandated by UL 746B.
How do test conditions in air versus nitrogen affect decomposition results?
In nitrogen, decomposition reflects pure thermal scission of the polymer backbone and typically yields Td5% values above 500°C with substantial char. In air, oxidative degradation begins at lower temperatures and char yield is reduced, so both atmospheres should be tested to bracket real-world behavior ranging from inert processing environments to open-air service conditions.