Long-term thermal aging testing is the core reliability experiment used to verify that thermally conductive pads installed in power battery packs can maintain their heat transfer, mechanical conformability, and electrical insulation performance over an 8-to-15-year vehicle service life. This article explains the sample requirements, test items, measurement methods, reference standards, and acceptance criteria applied by accredited third-party laboratories when qualifying thermal interface pads under prolonged elevated-temperature exposure.
Technical Overview
Thermally conductive pads (also known as gap fillers or thermal interface pads) are soft, elastomeric materials filled with thermally conductive ceramic or graphite particles, installed between lithium-ion cells, module baseplates, and liquid-cooled cold plates. Their function is to fill air gaps, reduce thermal contact resistance, and—when electrically insulating grades are used—provide dielectric separation between the cell stack and the cooling structure. During real-world operation, these pads are continuously exposed to interface temperatures of 45 to 90 degrees Celsius, with localized hot spots and fast-charging transients that can push temperatures above 100 degrees Celsius for thousands of hours.
Prolonged heat exposure drives several degradation mechanisms inside the pad, including oxidative chain scission, post-curing, changes in crosslink density, volatilization of low-molecular-weight siloxanes or plasticizers, and filler-matrix debonding. The observable consequences are hardening, shrinkage, oil bleed onto cell surfaces, cracking, increased thermal contact resistance, and reduced dielectric strength. A long-term thermal aging experiment deliberately accelerates these mechanisms by exposing the material in a forced-air oven at one or more elevated temperatures for 500 to 3000 hours or longer, then benchmarking aged properties against baseline values. The commonly referenced frameworks include ISO 188, ASTM D573, and GB/T 3512 for heat aging of elastomers, the IEC 60216 series for thermal endurance of electrical insulating materials, and UL 746B for long-term property evaluation and relative temperature index (RTI) determination of polymeric materials.
Testing Samples
Silicone-Based Thermal Pads
Filled silicone elastomer pads are the most widely used category in power battery packs, with typical thermal conductivity ratings of 1.0 to 6.0 W/(m·K), thicknesses of 0.5 to 10 mm, and Shore 00 hardness of 30 to 75. Samples are usually die-cut plaques or sheets of at least 100 mm × 100 mm, or full-size production pads, submitted in a quantity sufficient for baseline measurement plus each planned aging checkpoint and each test item.
Non-Silicone Thermal Pads
Polyurethane, acrylic, and epoxy-sponge based pads are selected where siloxane volatilization and silicone contamination must be avoided, for example near connectors, optical sensors, or in fuel-cell systems. These materials generally have lower continuous-use temperature ratings (often 105 to 125 degrees Celsius) and therefore require more conservative aging temperatures to avoid unrepresentative failure modes.
Gap Filler Gels and Putty-Like Materials
Cure-in-place dispensed gap fillers and putty materials are tested as cured plaques prepared under the customer-specified cure schedule (time, temperature, and mixing ratio). The cured specimens must be conditioned before baseline testing so that any post-curing effect is captured in the reference values rather than being mistaken for aging drift.
Sample Preparation and Conditioning
Before testing, all specimens are conditioned at 23 ± 2 degrees Celsius and 50 ± 5% relative humidity for at least 24 hours. Thickness is measured at a minimum of five positions per specimen, initial mass is recorded with a balance of 0.1 mg resolution, and each specimen is assigned a unique identifier so that property changes can be tracked on a paired basis throughout the aging campaign.
Testing Items
The long-term thermal aging experiment evaluates the retention of thermal, mechanical, and electrical properties that directly govern pack-level thermal management safety and performance:
- Thermal conductivity and thermal impedance, measured per ASTM D5470 or equivalent heat-flow methods, under defined compression pressure and interface temperature.
- Hardness (Shore 00, Shore A, or Asker C) per ASTM D2240, ISO 7619-1, or GB/T 531.1, to detect hardening or softening of the matrix.
- Compression set per ASTM D395 Method B, ISO 815-1, or GB/T 7759.1, at a specified deflection (commonly 25%), quantifying permanent loss of conformability.
- Mass change (weight loss and oil bleed), indicating volatilization of siloxanes or plasticizers that can contaminate adjacent components.
- Thickness and dimensional change after aging and recovery, reflecting shrinkage that opens interfacial gaps.
- Tensile strength and elongation at break per ASTM D412 or GB/T 528, where the pad geometry permits standard dumbbell specimens.
- Dielectric breakdown strength per IEC 60243-1 or GB/T 1408.1, confirming insulation integrity after aging.
- Volume resistivity per IEC 62631-3-1 or GB/T 1410, typically required to remain above 1.0 × 10^12 ohm-centimeters for insulating grades.
- Flammability retention per UL 94, verifying that V-0 or equivalent ratings are maintained on aged material.
- Visual and physical inspection for cracking, blistering, warping, delamination, brittleness, and surface exudation.
Testing Methods
Aging Conditions and Equipment
Samples are exposed in forced-air convection ovens with a demonstrated temperature uniformity of ± 2 degrees Celsius or better across the working volume. Specimens are placed on clean racks or hung freely, spaced at least 10 mm apart and never in direct contact with one another, following the general practice of ASTM D573. A representative aging matrix for battery thermal pads includes:
- 85 degrees Celsius for 1000 to 2000 hours, simulating sustained upper-range pack operating temperature.
- 105 degrees Celsius for 1000 hours, an intermediate accelerated condition.
- 125 degrees Celsius for 1000 to 3000 hours, the standard accelerated condition for silicone pads and the anchor point for many OEM specifications.
- 150 degrees Celsius for 500 to 1000 hours, a capability-screening condition normally limited to high-temperature silicone grades.
Measurement Schedule and Recovery
Baseline (0-hour) properties are established on at least three to five specimens per test item. Specimens are then withdrawn at defined checkpoints, for example at 250, 500, 1000, 2000, and 3000 hours. After removal from the oven, specimens are cooled and reconditioned at 23 ± 2 degrees Celsius and 50 ± 5% relative humidity for 16 to 24 hours before measurement, so that transient thermal states do not distort the results. Compression set specimens receive an additional 30-minute recovery period after release before final thickness measurement.
Data Evaluation and Acceptance Criteria
All results are expressed as percentage change relative to baseline. Typical automotive acceptance thresholds, which must always be confirmed against the specific OEM or cell-manufacturer specification, include: thermal conductivity retention of at least 80% (change within ± 20%); hardness change within ± 10 Shore units; weight loss not exceeding 1 to 2%; compression set not exceeding 20 to 30%; dielectric breakdown voltage retention of at least 80%; volume resistivity remaining above the specified insulation limit; and no visible cracking, delamination, or oil exudation. When aging is performed at three or more temperatures, an Arrhenius analysis (log of degradation rate versus reciprocal absolute temperature) can be used to derive an apparent activation energy and extrapolate the expected property retention at the actual service temperature, in line with the methodology underlying IEC 60216 and UL 746B.
Combined and Sequential Exposure
For full qualification, long-term thermal aging is frequently combined with damp heat (85 degrees Celsius and 85% relative humidity), thermal cycling between − 40 and + 85 or + 105 degrees Celsius, and vibration testing, either sequentially on the same specimens or in parallel matrices. This reflects the coupled thermal-humidity-mechanical stress state experienced by thermal pads inside a vehicle battery pack.
Applications
Long-term thermal aging data for conductive pads are used across the electric mobility and stationary storage value chain. Key applications include: thermal interface qualification between prismatic, pouch, and cylindrical cells and module cooling plates in BEV, PHEV, and HEV battery packs; dielectric barrier verification between the cell stack and liquid cold plates; material selection and second-source approval for tier-one pack integrators; aging model calibration for battery thermal management simulation; quality control release testing for incoming TIM batches; and long-duration qualification programs for grid-scale energy storage containers, where service lives of 20 years are increasingly demanded and where thermal pad reliability directly influences cell temperature uniformity, degradation rate, and propagation resistance.
FAQ
How long should a long-term thermal aging test for battery thermal pads be?
Most automotive specifications require a minimum of 1000 hours at 125 degrees Celsius for silicone pads, with extended programs of 2000 to 3000 hours for safety-critical or long-life applications. Shorter high-temperature exposures (for example 150 degrees Celsius for 500 hours) are useful screening tests but should not replace the 1000-hour baseline condition.
How is accelerated aging related to real service life?
Chemical degradation rates increase exponentially with temperature. By aging at several elevated temperatures and applying Arrhenius extrapolation per IEC 60216 and UL 746B methodology, the property retention at the actual pack operating temperature can be estimated. As a rough engineering rule, many elastomer aging processes roughly double in rate for every 10 degrees Celsius rise in temperature.
Why is oil bleed and weight loss measured so carefully?
Low-molecular-weight siloxanes that volatilize from silicone pads can condense on connectors, busbars, and cell surfaces, potentially degrading contact resistance and long-term reliability of adjacent electronics. Weight loss limits of 1 to 2% after 1000 hours of aging are therefore a common rejection criterion in battery specifications.
Do non-silicone pads need different test conditions?
Yes. Polyurethane and acrylic based pads typically have lower continuous-use ratings, so aging temperatures above 105 to 125 degrees Celsius can trigger decomposition mechanisms that would never occur in service. Test temperatures must be selected to accelerate the relevant mechanism without changing it, and this is confirmed by agreement of degradation curves across temperatures.
Which organizations publish the standards used in this test?
The experiment is built on standards from ISO, IEC, ASTM, UL, and the GB series, including ISO 188, ASTM D573, GB/T 3512, IEC 60216, UL 746B, ASTM D5470, ASTM D2240, ASTM D395, IEC 60243-1, and IEC 62631-3-1. Testing is typically performed in accredited third-party laboratories or qualified in-house facilities under these frameworks.
