Reliability Evaluation of Thermally Conductive Materials for New Energy Vehicles

Thermally conductive materials are the silent guardians of new energy vehicles, transferring heat away from battery cells, power modules, and motor controllers to ensure safe and efficient operation over a vehicle's lifetime. Reliability evaluation of these materials combines thermal performance testing, electrical characterization, and environmental durability assessment against international standards such as ASTM D5470, ISO 22007, IEC 60068, and UL 94. This article provides a comprehensive technical overview of thermal interface material reliability evaluation for electric vehicles, covering sample types, testing items, methods, and industry applications.

Technical Overview

New energy vehicles (NEVs), including battery electric vehicles (BEVs), plug-in hybrid vehicles (PHEVs), and fuel cell vehicles (FCVs), rely heavily on power electronics and lithium-ion battery systems that generate significant heat during charge and discharge cycles. Thermally conductive materials—including thermal interface materials (TIMs), gap fillers, thermal greases, phase change materials, thermally conductive adhesives, and graphite sheets—serve as the critical thermal bridge between heat sources and heat sinks. Their performance directly impacts battery cycle life, power semiconductor junction temperatures, and overall vehicle safety.

Unlike consumer electronics, automotive thermal materials must endure 10 to 15 years of service under extreme conditions: temperature swings from -40°C to +150°C, mechanical vibration, humidity, salt mist, coolant exposure, and thousands of thermal cycles. Reliability evaluation therefore goes beyond initial thermal conductivity measurement. A rigorous assessment program verifies that thermal resistance, mechanical properties, electrical insulation, and flame retardancy remain within specification after long-term environmental aging and cyclic stress. Vehicle OEMs typically require materials to pass qualification protocols aligned with IATF 16949 quality management systems and test methods defined by ISO, IEC, ASTM, UL, and GB standards, often validated by accredited third-party laboratories.

Testing Samples

Thermal Gap Pads and Gap Fillers

Soft silicone or non-silicone polymer pads filled with thermally conductive ceramic or metal oxides, used between battery cells, modules, and cooling plates. Typical thicknesses range from 0.5 mm to 10 mm with thermal conductivity ratings from 1 to 12 W/(m·K).

Thermal Greases and Pastes

Dispensable compounds applied between power semiconductors (IGBT, SiC MOSFET) and heat sinks. Reliability concerns include pump-out, phase separation, oil bleeding, and dry-out over extended thermal cycling.

Thermal Gels and Dispensable Gap Fillers

Cure-in-place silicone or urethane gels that combine low modulus with stable thermal performance, widely used in large-area battery pack assemblies where tolerance stack-up is significant.

Thermally Conductive Adhesives and Films

Structural bonding materials providing both mechanical fixation and heat transfer, including epoxy, silicone, and acrylic chemistries used for cell-to-cell and cell-to-busbar attachment.

Phase Change Materials (PCMs)

Materials that soften at operating temperature to achieve wetting performance similar to grease while remaining solid at room temperature, used in power module assemblies.

Graphite Sheets and Thermally Conductive Insulation Films

Natural or synthetic graphite films for in-plane heat spreading, and polymer films (such as filled PI or PET laminates) providing electrical isolation between cells and cooling systems.

Testing Items

A complete reliability evaluation of automotive thermal materials typically covers the following performance dimensions:

• Thermal conductivity (W/(m·K)) and specific thermal impedance (°C·cm²/W) at defined pressures and thicknesses
• Thermal resistance stability before and after aging (thermal cycling, damp heat, high-temperature storage)
• Hardness (Shore 00, Shore A, Asker C) and compression deflection characteristics
• Compressive stress versus strain behavior at 10%, 25%, and 50% deflection
• Compression set under sustained heat and pressure
• Dielectric breakdown voltage (kV) and dielectric strength (kV/mm)
• Volume resistivity and surface resistivity for electrically insulating materials
• Flame retardancy classification per UL 94 (V-0, V-1, HTI ratings)
• Tensile strength, elongation, and tear resistance (for pads and films)
• Adhesive shear and peel strength (for adhesives), before and after environmental exposure
• Oil bleeding, volatiles content, and outgassing behavior
• Compatibility with coolants, battery electrolyte vapors, and copper substrates
• Weight loss after high-temperature aging and thermal shock resistance
• Vibration and mechanical shock endurance per automotive environmental standards

Testing Methods

Thermal Conductivity and Thermal Impedance Testing

Steady-state heat flow meter methods per ASTM D5470 measure thermal impedance as a function of pressure and thickness, allowing extraction of bulk thermal conductivity and interfacial contact resistance. Laser flash methods per ASTM E1461 and ISO 22007-4, and transient plane source methods per ISO 22007-2, provide rapid diffusivity-based conductivity determination. Chinese national standards such as GB/T 10294 and GB/T 11205 are frequently referenced for domestic qualification programs.

Environmental Durability Testing

Thermal cycling between -40°C and +125°C or +150°C is performed per IEC 60068-2-14 or GB/T 2423.22 to simulate daily and seasonal temperature swings, typically for 500 to 3000 cycles. Damp heat steady-state testing at 85°C and 85% relative humidity per IEC 60068-2-78 or GB/T 2423.3, and high-temperature storage at 125°C to 200°C per IEC 60068-2-2, reveal degradation of polymer matrices and conductive filler networks. Post-aging retesting of thermal resistance quantifies performance drift limits, commonly required to remain within 10% to 20% of initial values.

Electrical Safety Testing

Dielectric breakdown voltage is measured per ASTM D149 or IEC 60243-1 using ramp or step voltage application across the material thickness. Volume and surface resistivity are determined per ASTM D257 or IEC 60093 to confirm insulation integrity in high-voltage battery environments exceeding 400 V and reaching 800 V or 1000 V in modern platforms.

Mechanical and Physical Testing

Hardness is measured per ASTM D2240 (Shore durometer) or equivalent Asker scales. Compression and compression set behavior are evaluated per ASTM D395 and ASTM D575. Tensile strength and elongation follow ASTM D412 or ISO 37. Adhesive joint strength is assessed per ASTM D1002 (lap shear) and ASTM D1876 (T-peel), with additional testing after thermal and humidity aging.

Flammability Testing

Burning behavior is classified per UL 94 vertical and horizontal burn tests, with V-0 classification commonly mandated for battery pack materials. Glow-wire ignition tests per IEC 60695-2 series may supplement flammability assessment for components near potential ignition sources.

Reliability Test Sequencing

Automotive qualification programs often apply combined stress sequences: preconditioning, followed by thermal shock, damp heat, high-temperature aging, and vibration per IEC 60068-2-6 or GB/T 2423.10, with thermal and electrical re-measurement after each stage. Such sequences reflect real-world duty cycles and are frequently executed in accredited third-party laboratories under IATF 16949-aligned supplier qualification frameworks.

Applications

Reliability-evaluated thermal materials are deployed throughout the electric vehicle thermal management architecture:

• Battery packs: gap pads and gels between prismatic, pouch, or cylindrical cells and liquid cooling plates, maintaining cell temperature uniformity within 3°C to 5°C
• Power electronics: thermal greases and phase change films interfacing IGBT and SiC MOSFET modules with liquid-cooled cold plates in motor inverters
• On-board chargers (OBC) and DC-DC converters: gap fillers and insulating thermal pads for magnetics and power switching devices
• Battery management systems (BMS): thermal interface pads dissipating heat from control boards
• Charging infrastructure: thermally conductive potting compounds and gap pads in high-power DC charging modules and connectors
• Electric drive motors: thermally conductive epoxies for winding heat dissipation and slot insulation
• Fuel cell vehicles: thermal management materials for stack end plates and power conditioning units

FAQ

What does reliability evaluation of thermal conductive materials involve?

It is a structured testing program that verifies thermal conductivity, thermal impedance stability, mechanical integrity, electrical insulation, and flame retardancy of thermal interface materials before and after environmental stresses such as thermal cycling, damp heat, and high-temperature aging, using standardized methods from ISO, IEC, ASTM, UL, and GB frameworks.

Why is post-aging thermal resistance testing so important?

Initial thermal conductivity alone does not guarantee long-term performance. Polymer aging, filler settling, oil bleed, pump-out, and interfacial delamination can all increase thermal resistance over time. Measuring thermal resistance after accelerated aging demonstrates whether a material will maintain heat transfer performance across a 10-to-15-year vehicle service life.

Which standards are most commonly referenced?

Thermal properties are typically measured per ASTM D5470, ASTM E1461, and ISO 22007 series; environmental durability per IEC 60068 and GB/T 2423 series; dielectric strength per ASTM D149 and IEC 60243; and flammability per UL 94. OEM-specific specifications supplement these international standards.

How long does a typical qualification program take?

Baseline characterization can be completed within a few weeks, but full reliability sequences including 1000 or more thermal cycles, 1000 hours of damp heat, and post-aging retesting commonly require three to six months. High-acceleration stress testing such as HAST can shorten certain aging conditions when correlated to field data.

Should testing be performed in-house or externally?

Suppliers often conduct in-process screening, while formal qualification data for OEM submissions is generally generated by accredited third-party laboratories with ISO/IEC 17025-accredited scopes, ensuring impartiality and international recognition of test results.