Performance Evaluation of Thermal Conductive Pads for Power Batteries

Thermal conductive pads are critical thermal interface materials in electric vehicle power battery systems, transferring heat from lithium-ion cells to cooling plates while providing electrical insulation and mechanical compliance. Rigorous performance evaluation of these pads ensures battery pack safety, extends service life, and prevents thermal management failures. This article examines the key testing items, internationally recognized methods such as ASTM D5470 and ISO 22007, and qualification strategies used by accredited third-party laboratories and battery manufacturers worldwide.

Technical Overview

Thermal conductive pads, also known as thermal gap pads or thermal interface materials (TIMs), are soft elastomeric sheets installed between battery cells, module housings, and liquid-cooling plates in power battery packs. Because trapped air is an extremely poor heat conductor (approximately 0.026 W/m·K), even flat-looking machined surfaces create significant contact resistance at their interfaces. A compliant thermal pad displaces this air, conforming to surface roughness and tolerance stack-ups to establish an efficient heat transfer path from the cell to the coolant.

Typical pads consist of a silicone or non-silicone polymer matrix loaded with thermally conductive ceramic fillers such as alumina, aluminum nitride, boron nitride, or graphite. Commercial products deliver thermal conductivity in the range of 0.5 to 12 W/m·K, with 1 to 6 W/m·K being the most common specification for automotive battery applications. In addition to heat transfer, the pad frequently serves as an electrical barrier between high-voltage cells and the grounded cooling plate, making dielectric performance as important as thermal performance.

Systematic performance evaluation is essential because a thermal pad must satisfy thermal, mechanical, electrical, and long-term reliability requirements simultaneously. A material that performs well at the beginning of life but hardens, bleeds silicone oil, or loses insulation capability after years of vibration and thermal cycling can accelerate capacity fade or contribute to thermal runaway propagation. Objective testing to recognized standards enables design engineers to compare candidate materials and validate them against automotive qualification targets.

Testing Samples

Silicone-Based Thermal Pads

Silicone elastomer pads are the most widely used category in power battery packs owing to their softness, wide operating temperature range (typically -40°C to 200°C), excellent compression recovery, and stable performance over time. Evaluation focuses not only on initial properties but also on low-molecular-weight siloxane volatilization, which can contaminate electrical contacts and optical components.

Non-Silicone Thermal Pads

Acrylic, polyurethane, and polyolefin-based pads are selected for siloxane-sensitive applications. These materials generally exhibit lower oil bleed and lower volatility but may offer a narrower temperature window and higher hardness than silicone equivalents, so their compression behavior and aging resistance require careful verification.

Thermal Gap Fillers and Cured-in-Place Materials

Two-part dispensable gap fillers are increasingly used in large-format battery assemblies. For laboratory evaluation, these compounds are mixed, dispensed, and cured into standard sheet thicknesses before testing so that their cured properties can be compared directly with preformed pads.

Specimen Preparation and Conditioning

Test specimens are usually die-cut from production sheets at thicknesses between 0.5 mm and 5 mm. Before testing, samples are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for at least 24 to 40 hours in accordance with ISO 291 or ASTM D618. Thermal measurements additionally require accurate thickness recording under the applied pressure, since pad thickness changes significantly with clamping force.

Testing Items

A complete performance evaluation covers five property families. Each family targets a distinct failure mode observed in battery pack service.

Thermal Performance Characteristics

• Thermal conductivity, typically 1 to 6 W/m·K for battery-grade pads
• Thermal impedance and total thermal resistance at specified pressure and thickness
• Contact resistance at the pad-to-cell and pad-to-cold-plate interfaces
• Thermal performance consistency across production lots

Mechanical and Physical Properties

• Hardness, normally reported on the Shore 00 scale for soft gap pads
• Compression deflection characteristics, describing force versus deflection behavior
• Compression set, indicating permanent deformation after sustained clamping
• Tensile strength and elongation at break, relevant to handling and tearing resistance
• Density, typically 1.5 to 3.5 g/cm³, which influences pack weight
• Thickness tolerance and surface tack

Electrical Properties

• Dielectric strength (breakdown voltage), commonly specified as kV/mm at a defined thickness
• Volume resistivity, generally required to exceed 10^12 Ω·cm
• Surface resistivity where leakage current paths are a concern

Reliability and Aging Resistance

• Heat aging resistance and retention of thermal and mechanical properties after high-temperature exposure
• Damp heat aging tolerance, typically 85°C and 85% relative humidity for 500 to 1000 hours
• Thermal cycling endurance, commonly -40°C to 85°C or 125°C for 500 to 1000 cycles
• Vibration and mechanical shock resistance in the assembled joint configuration
• Mass loss and volatile content at elevated temperature
• Silicone oil bleed tendency for silicone-based products

Safety and Regulatory Compliance

• Flammability rating, most commonly UL 94 V-0
• Limiting oxygen index for flame resistance characterization
• Restricted substance compliance under RoHS and REACH regulations

Testing Methods

Thermal Conductivity Measurement: ASTM D5470

The steady-state heat flow method described in ASTM D5470 is the reference technique for thermal interface materials. A specimen is clamped between two metered bars at controlled pressure while heat flows axially through the stack. Measuring the temperature gradient across the bars yields the total thermal impedance, which includes both bulk resistance and contact resistances. By testing multiple thicknesses and extrapolating to zero thickness, laboratories separate the intrinsic thermal conductivity from the interfacial contributions. Because the test reproduces the pressure and surface conditions of a real battery assembly, ASTM D5470 data are the most service-relevant and form the basis of most datasheet specifications.

Rapid Screening Methods: ISO 22007 Series

The transient plane source method of ISO 22007-2 allows fast thermal conductivity measurement and is widely used for incoming material inspection and research screening. The laser flash method of ISO 22007-4 and ASTM E1461 measures thermal diffusivity, from which conductivity is calculated using independently measured density and specific heat. These transient techniques are efficient but capture contact resistance differently than steady-state methods, so results should be correlated with ASTM D5470 data before critical design decisions.

Hardness and Compression Behavior Testing

Pad hardness is measured with a durometer according to ASTM D2240, almost always using the Shore 00 scale because battery pads are too soft for Shore A measurement. Soft products range from roughly 10 to 80 Shore 00. Compression deflection behavior is characterized by compressing specimens at defined deflection percentages, often 25 to 50 percent, and recording the resulting pressure, following the general principles of ASTM D575. This force-deflection curve tells designers whether the pad can absorb tolerance gaps without imposing excessive stress on prismatic or pouch cells. Compression set is determined under constant deflection using ASTM D395 Method B or ISO 815, with well-qualified pads exhibiting values below 10 to 20 percent.

Tensile, Elongation, and Density Testing

Tensile strength and elongation at break are measured with dumbbell specimens according to ASTM D412 or ISO 37, which also corresponds to GB/T 528. These properties indicate resistance to tearing during assembly and during differential thermal expansion of the joined parts. Density is determined by the hydrostatic method of ASTM D792 or ISO 1183, referenced in Chinese practice by GB/T 533.

Electrical Testing Methods

Dielectric breakdown voltage is measured by applying a ramping AC voltage across the pad between electrodes until failure, following ASTM D149 or IEC 60243-1, with an equivalent procedure in GB/T 1695. Results are reported as breakdown voltage or dielectric strength in kV/mm for a stated thickness. Volume resistivity is measured with a guarded electrode configuration per ASTM D257 or IEC 62631-3-1, and historically under GB/T 1692. Because insulation performance can degrade after aging, electrical testing should be repeated on aged specimens, not only on virgin material.

Flammability Testing

Flame retardancy is verified with the UL 94 vertical burn test, where V-0 is the standard requirement for materials located near lithium-ion cells. The limiting oxygen index determined by ASTM D2863 provides supplementary data on the minimum oxygen concentration required to sustain combustion, useful for comparing filler formulations.

Environmental Durability and Aging Protocols

Accelerated aging is performed in environmental chambers following IEC 60068-2-78 for steady damp heat and IEC 60068-2-14 for rapid temperature change, supplemented by oven heat aging under ISO 188 or ASTM D573. After exposure, specimens are re-tested for thermal conductivity, dielectric strength, hardness, and compression set, and property retention is expressed as a percentage of initial values. Volatile content and oil bleed are assessed gravimetrically by conditioning samples at elevated temperature, such as 150°C to 200°C for 24 hours, and recording mass loss; low-volatility silicone grades should show mass loss below approximately one percent. A comprehensive qualification program conducted by accredited third-party laboratories combines these aging exposures with vibration profiles representative of the target vehicle platform.

Applications

Thermal conductive pads are deployed at multiple locations within a power battery system, and each location imposes a slightly different balance of requirements.

• Cell-to-cold plate interfaces, where the pad conducts heat from the cell bottom or side into the liquid cooling plate and must maintain dielectric isolation between the high-voltage cell and the grounded plate
• Cell-to-cell heat spreading layers, which equalize temperature differences across a module and slow propagation during a single-cell thermal runaway event
• Module cover and busbar interfaces, where pads couple hot interconnects to heat sinks or structural housings
• Energy storage system (ESS) enclosures, which use larger pads between rack-level battery blocks and cooling surfaces with demanding long-life requirements
• Adjacent power electronics such as inverters and on-board chargers, which share qualification approaches with battery thermal interface materials

In all of these positions, the pad operates under continuous clamping pressure while experiencing vibration, relative motion from thermal expansion, and wide temperature swings. Successful designs therefore treat the pad as a structural component of the thermal management system, not merely as a sheet commodity, and qualify it with the full testing scope described above.

FAQ

What thermal conductivity value should a power battery thermal pad have?

Most automotive battery applications specify pads between 1 and 6 W/m·K. Higher numbers are not automatically better, because thicker or harder pads with very high filler loading can increase contact resistance or stress cells. The selection should be based on measured thermal impedance at the actual stack pressure, thickness, and interface surfaces of the specific pack design.

Why is thermal impedance often more meaningful than thermal conductivity?

Thermal conductivity is an intrinsic material property, but the resistance that matters in service is the total thermal impedance, which includes the bulk resistance of the pad plus the contact resistances at both interfaces. A softer, lower-conductivity pad that wets surfaces well under modest pressure can outperform a harder, higher-conductivity pad in the real assembly. ASTM D5470 directly measures this service-relevant impedance.

How soft should a thermal pad be for battery applications?

Battery pads typically fall between 10 and 80 Shore 00. Softer pads conform to larger tolerance gaps and impose less pressure on pouch and prismatic cells, but extremely soft materials may have lower tensile strength and higher compression set. The optimum hardness is chosen by balancing the compression deflection curve against allowable cell pressure, usually with a target deflection of 20 to 50 percent in the installed state.

Is silicone oil bleed a concern in battery packs?

Yes. Low-molecular-weight siloxanes can migrate from silicone pads onto connector contacts, relays, or optical sensors, potentially causing contact failure. For sensitive designs, engineers specify low-volatility silicone grades verified by mass loss and oil bleed testing, or select non-silicone alternatives, accepting possible trade-offs in softness and temperature range.

Must thermal pads provide electrical insulation?

In most pack architectures, yes. Pads between cells and cooling plates often serve as part of the high-voltage insulation system, so dielectric strength and volume resistivity must be verified both initially and after aging, moisture exposure, and mechanical stress. A pad that loses dielectric capability late in life creates a serious safety hazard.

How is long-term reliability over the battery lifespan verified?

Laboratories use accelerated aging protocols, including 1000-hour damp heat exposure at 85°C and 85% relative humidity, 500 to 1000 thermal cycles between -40°C and the maximum design temperature, prolonged oven aging, and vibration testing, after which thermal, electrical, and mechanical properties are re-measured. Acceptance criteria typically require retention of at least 80 to 90 percent of initial thermal conductivity and dielectric strength with compression set kept below defined limits, and this testing is generally performed by accredited third-party laboratories to support supplier qualification.