Thermal Stability Analysis of Thermal Conductive Pads for Power Batteries

Thermal stability analysis of thermal conductive pads for power batteries is a critical qualification process that verifies whether thermal interface materials can sustain their thermal conductivity, mechanical resilience, and electrical insulation throughout the service life of an EV battery pack. This article systematically explains the testing samples, key testing items, standardized test methods (ASTM, ISO, IEC, UL, GB), and typical acceptance criteria used by accredited third-party laboratories to evaluate thermally conductive gap pads under long-term heat aging, thermal cycling, and damp-heat conditions.

Technical Overview

Thermally conductive pads, also known as thermal gap pads or thermal interface pads, are soft, highly filled polymer sheets installed between battery cells, cell modules, and liquid cooling plates in electric vehicle (EV) power battery packs. Their primary function is to displace air from rough contact interfaces, reduce contact thermal resistance, and homogenize cell temperature across the pack. Because a power battery operates continuously for eight to fifteen years under cyclic mechanical stress and fluctuating temperatures, the pad must not only transfer heat efficiently on day one, but must also remain dimensionally, mechanically, electrically, and thermally stable over the entire vehicle lifetime.

Thermal stability analysis is therefore a dedicated material qualification discipline. It quantifies how the key properties of a thermal pad drift after prolonged exposure to elevated temperature, temperature cycling, humidity, and sustained compression. Typical degradation mechanisms include polymer chain scission or additional crosslinking, volatilization or bleeding of low-molecular-weight species, filler settlement or de-bonding, progressive hardening, compression set growth, and eventual cracking or delamination. Any of these mechanisms can increase thermal impedance, create local hot spots on cells, and accelerate capacity fade or even trigger thermal events.

• Thermal stability of the heat-transfer function: retention of thermal conductivity and total thermal resistance after aging.
• Physical stability: hardness change, compression set, dimensional change, mass loss, cracking, and oil bleed behavior.
• Chemical stability: thermogravimetric decomposition behavior, volatile content, and release of contaminating siloxanes.
• Electrical safety stability: retention of dielectric strength and volume resistivity, which are essential because the pad often provides electrical isolation between cells and the grounded cooling plate.

Testing Samples

Silicone-Based Thermally Conductive Pads

The most common sample category is a silicone elastomer matrix loaded with 70 to 90 wt.% of thermally conductive ceramic fillers such as alumina, aluminum nitride, boron nitride, magnesium oxide, or graphite blends. Typical commercial grades range from 0.5 mm to 10 mm in thickness, 1 to 8 W/(m·K) in thermal conductivity, and Shore 00 30 to 70 in hardness. Silicone pads offer excellent compressibility, wide temperature capability, and strong aging resistance, but silicone-specific risks such as low-molecular-weight siloxane bleeding must be specifically evaluated.

Non-Silicone Thermally Conductive Pads

For designs sensitive to silicone contamination of electrical contacts and optical components, non-silicone pads based on acrylic, polyurethane, or EPDM chemistry are tested as an alternative sample family. These materials eliminate siloxane volatiles but often exhibit different aging kinetics, so their thermal stability window, compression set behavior, and upper service temperature limit must be independently established.

Thermally Conductive Gap Fillers and Putty-Type Materials

Dispensed or pre-formed gap fillers with very low hardness (below Shore 00 20) and putty-like materials are used to fill large, irregular gaps in module-to-plate interfaces. Their high filler content and low crosslink density make them particularly prone to oil separation and creep under sustained compression, so extended aging and vertical bleed tests are usually added to the sample evaluation plan.

Specimen Preparation and Conditioning

Before testing, specimens are die-cut to the dimensions required by each test method and conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours in accordance with ISO 291 or ASTM D618. Thickness is measured at multiple points per ASTM D374, and baseline (time-zero) values of thermal conductivity, hardness, mass, and dielectric properties are recorded so that all post-aging results can be expressed as percentage retention relative to the initial state.

Testing Items

A complete thermal stability evaluation of power battery thermal conductive pads typically covers the following testing items, each measured both in the as-received state and after defined aging exposures:

• Thermal conductivity and thermal impedance (initial and post-aging retention).
• Thermogravimetric parameters: decomposition onset temperature, 1%, 5%, and 10% mass-loss temperatures, and residual mass reflecting filler loading.
• Glass transition temperature and cure completeness by differential scanning calorimetry.
• Mass loss after long-term oven aging at 125 °C and 150 °C.
• Hardness change (Shore 00 or Shore A) after aging.
• Compression set under constant deflection.
• Volume resistivity and dielectric breakdown strength after aging.
• Oil bleed and volatile organic content, including cyclic siloxane release for silicone grades.
• Dimensional stability and surface condition, including cracking, blistering, and delamination after thermal cycling.
• Flame retardancy classification of the aged material.

Testing Methods

Thermal Conductivity and Thermal Resistance Measurement

The reference method is ASTM D5470, a steady-state guarded heat-flow method that measures apparent thermal conductivity and thermal impedance of the pad as a function of applied pressure (commonly 50 to 300 kPa) and interface temperature. This is essential because a pad works under compression in the pack, and its effective performance is dominated by contact resistance at both interfaces. Supplementary methods include the transient plane source technique per ISO 22007-2, the laser flash method per ASTM E1461, and the heat flow meter method per ASTM C518, ISO 8301, or GB/T 10294. For stability assessment, conductivity is re-measured on aged specimens and retention is calculated.

Thermogravimetric Analysis (TGA)

TGA per ASTM E1131 or ISO 11358 is performed in both nitrogen and air atmospheres from room temperature to 600 °C at 10 °C/min. The analysis reports the decomposition onset temperature, the temperatures at which 1%, 5%, and 10% mass loss occur, and the residual mass, which corresponds closely to inorganic filler content. A high-quality power battery pad generally shows a 5% mass-loss temperature well above 300 °C, confirming a wide margin above any realistic pack operating temperature, including short excursions during abuse conditions.

Differential Scanning Calorimetry (DSC)

DSC per ISO 11357 or ASTM E1356 and ASTM D3418 is used to determine the glass transition temperature, verify cure completeness through the absence of residual exotherms, and detect any post-curing or degradation reactions that occur during aging. A shift of the glass transition temperature after oven aging indicates chemical restructuring within the polymer network, which often precedes measurable mechanical changes.

Long-Term Heat Aging

Oven aging is conducted per ASTM D573 or ISO 188 at 125 °C and 150 °C for durations up to 1000 hours or longer. At scheduled intervals, specimens are removed and re-characterized for mass, hardness, thermal conductivity, and dielectric properties. Commonly applied internal acceptance targets include mass loss below 1%, hardness change within plus or minus 10 Shore units, and thermal conductivity retention of at least 90% after 1000 hours at 150 °C, although the exact limits are defined by each vehicle manufacturer's material specification.

Thermal Cycling Resistance

Thermal cycling per IEC 60068-2-14 or GB/T 2423.22 subjects specimens to repeated excursions between −40 °C and +85 °C, and in some specifications up to +105 °C, with dwell times at each extreme, for 500 to 1000 cycles. After cycling, specimens are inspected for cracks, warping, and interfacial delamination, and thermal impedance is re-measured. This test simulates the fatigue loading created by charge-discharge heat generation combined with seasonal and diurnal temperature swings experienced by the power battery.

Damp-Heat Aging

Steady-state damp-heat exposure at 85 °C and 85% relative humidity for 1000 hours is performed per IEC 60068-2-78 or GB/T 2423.3. The test evaluates moisture absorption and its effect on thermal performance, volume resistivity, and dielectric strength, which is particularly important for pads installed in packs that are not fully hermetically sealed.

Compression Set and Stress Relaxation

Compression set is measured per ASTM D395 Method B under constant deflection, typically 25% of the original thickness, for 22 hours at 70 °C or 100 °C, and additionally after 1000-hour aging sequences. Because a pad exerts contact pressure only through its elastic recovery, excessive compression set directly reduces interface pressure and raises contact thermal resistance over the vehicle life. Stress relaxation measured under constant force provides complementary information for gap designs with rigid tolerances.

Electrical Insulation Verification

Volume resistivity is measured per ASTM D257 or IEC 60093, and dielectric breakdown strength per ASTM D149 or IEC 60243-1, both before and after aging. A stable pad maintains volume resistivity above 10^9 Ω·cm and retains its declared breakdown voltage, ensuring continued electrical isolation between the high-voltage battery bus and the grounded cooling structure.

Oil Bleed and Volatile Siloxane Analysis

Bleeding tendency is evaluated by placing specimens between filter papers under a defined compressive load at elevated temperature for a fixed period and gravimetrically determining transferred oil, while total volatile content is quantified by mass loss after baking. For silicone-based pads, headspace gas chromatography–mass spectrometry identifies and quantifies volatile cyclic siloxanes, which can migrate onto connectors, relays, and control circuit boards and cause contact failure, making this a decisive acceptance item for many pack designers.

Flame Retardancy of Aged Specimens

Because polymer aging can deplete flame-retardant packages, vertical burn testing per UL 94 or GB/T 2408 is repeated on aged specimens to confirm that the material retains at least a V-0 classification, supporting the overall fire safety strategy of the battery system in line with pack-level requirements such as GB 38031.

Applications

Thermal stability analysis of thermal conductive pads directly supports the design, validation, and industrialization of battery thermal management systems. Its main applications include:

• Material qualification and supplier comparison during pack development, providing objective retention data for conductivity, mechanics, and insulation.
• Incoming inspection and production consistency control, where shortened aging protocols and TGA fingerprints are used to confirm batch-to-batch stability.
• Interface design optimization between prismatic, pouch, or cylindrical cells and liquid cooling plates, where aged compression-deflection and thermal impedance data feed thermal simulation models.
• Energy storage system (ESS) container design, where pads operate for even longer lifetimes and higher ambient temperatures than in passenger vehicles.
• Power electronics thermal management, including IGBT modules, on-board chargers, and DC fast-charging power stages that use the same pad families.
• Change management and warranty engineering, where long-term aging datasets support lifetime extrapolation and root-cause analysis of field returns.

Test reports issued by accredited third-party laboratories based on these methods are routinely used as objective evidence in design reviews, safety assessments, and customer audits across the EV supply chain.

FAQ

Why is thermal stability so important for power battery thermal conductive pads?

The pad is the thermal bottleneck between the heat source (the cell) and the heat sink (the cooling plate). If the pad hardens, bleeds oil, or takes a compression set over time, its contact pressure and thermal conductivity decline, interface temperatures rise, and cell aging accelerates. Because the pad is inaccessible once the pack is sealed, its stability must be proven by laboratory analysis before design release.

What temperature range must a power battery thermal pad withstand?

Normal pack operation spans roughly −40 °C to +85 °C at the pad interface. However, pads near cells or busbars may see sustained 105 °C to 125 °C in fast-charging and high-load scenarios, and brief higher excursions during abuse testing. TGA results confirming a 5% mass-loss temperature above 300 °C provide a comfortable safety margin over these service conditions.

How much thermal conductivity drift is acceptable after aging?

Most vehicle manufacturer specifications require retention of at least 90% of initial thermal conductivity, and in many cases 95%, after 1000 hours at 125 °C to 150 °C, with no more than a defined small increase in thermal impedance. The exact limit depends on the margin designed into the pack cooling system.

What causes silicone oil bleeding and how is it assessed?

Bleeding originates from low-molecular-weight siloxane chains and process oils that are not chemically bound in the crosslinked network. Under heat and pressure they migrate to the surface and can contaminate electrical contacts. It is assessed by gravimetric bleed tests with filter paper, by baking mass-loss measurements, and by headspace GC-MS quantification of volatile siloxanes, with tighter limits applied when the pad is located near relays or connectors.

Which standards govern the thermal stability testing of these materials?

No single standard covers everything; a composite protocol is used. Thermal conductivity follows ASTM D5470 or ISO 22007; decomposition behavior follows ASTM E1131 or ISO 11358; heat aging follows ASTM D573 or ISO 188; thermal cycling and damp heat follow IEC 60068-2-14 and IEC 60068-2-78 or the corresponding GB/T 2423 series; compression set follows ASTM D395; electrical properties follow ASTM D149, ASTM D257, and IEC 60243; and flammability follows UL 94 or GB/T 2408.

Can laboratory aging results predict real service life?

They form the technical basis for life prediction but are not a substitute for system-level validation. By comparing property retention at multiple aging temperatures and durations, engineers can apply Arrhenius-type extrapolation and add safety factors, while final confirmation comes from pack-level endurance and safety testing. This layered approach is standard practice in power battery development programs.