Hygrothermal (damp heat) aging testing is a critical reliability evaluation for thermal conductive pads used in electric vehicle power battery packs, simulating long-term exposure to combined heat and humidity. This article explains the test principle, applicable standards such as IEC 60068-2-78 and GB/T 2423.3, sample types, measured parameters, test procedures, typical acceptance criteria, and industry applications.
Technical Overview
Thermal conductive pads (also known as gap pads or thermal gap fillers) are compliant, highly filled polymer materials installed between battery cells, module frames, and liquid cooling plates in electric vehicle (EV) power battery packs. Their function is to fill interfacial air gaps and provide a low-resistance heat transfer path while, in many designs, maintaining electrical insulation between cells and the cooling system. Because a battery pack is expected to operate reliably for 8 to 15 years, the thermal interface material must retain its thermal, mechanical, and dielectric properties throughout the vehicle lifetime.
In real-world service, battery packs encounter elevated temperatures, humidity ingress, condensation events, and thermal cycling. Sustained heat and moisture can accelerate several degradation mechanisms in thermal pads, including hydrolysis of the polymer matrix, plasticization and swelling from moisture uptake, debonding at filler–matrix interfaces, increased silicone oil bleeding, and leaching of ionic contaminants that may corrode busbars and cell terminals. Hygrothermal aging testing, typically performed as a steady-state 85°C / 85% RH exposure or as cyclic damp heat, accelerates these mechanisms in the laboratory so that material qualification can be completed in weeks or months rather than years. The test is a standard element of design validation (DV) and production validation (PV) programs and is conducted according to frameworks published by standards organizations such as IEC, ISO, ASTM, and the GB national standard system.
Testing Samples
Silicone-Based Thermal Gap Pads
The most common sample type is a silicone elastomer loaded with thermally conductive ceramic fillers such as aluminum oxide, aluminum nitride, or boron nitride, with thermal conductivities typically ranging from 1 to 6 W/m·K and thicknesses from 0.5 mm to 10 mm. Silicone pads are particularly scrutinized in damp heat aging because humidity can promote hydrolysis of siloxane bonds and increase the migration of low-molecular-weight siloxanes and ionic species.
Non-Silicone Gap Pads
Acrylic, polyurethane, or epoxy-based pads filled with thermally conductive ceramics are tested when silicone contamination (oil bleed) must be avoided, for example near optical sensors or in designs sensitive to siloxane fouling. These materials are generally more susceptible to hydrolytic degradation, making damp heat testing an essential qualification step.
Thermal Putty and Gel-Type Fillers
Form-in-place thermal putties and cured-in-place thermal gels are evaluated in their cured state. Because these materials exhibit very low hardness and high compressibility, post-aging assessments focus on oil separation, adhesion retention, and changes in thermal resistance under defined compression.
Film-Reinforced Insulating Thermal Pads
Some battery designs use thermal pads laminated with polyimide (PI) or PET films to guarantee dielectric isolation between cells and cooling plates. For these laminated structures, damp heat aging also evaluates interlayer adhesion, film delamination, and the durability of adhesive coating layers.
Specimen Preparation and Conditioning
Test specimens are cut from production-grade sheets at the thickness specified in the customer drawing. Prior to exposure, samples are conditioned in a standard atmosphere of 23°C ± 2°C and 50% ± 10% RH for at least 24 hours (or 40 hours for referee testing) in accordance with ISO 291 or ASTM D618, and baseline (unaged) properties are measured for later retention calculations.
Testing Items
A comprehensive hygrothermal aging program measures key functional properties before and after exposure and calculates the retention rate for each parameter. The core testing items include:
- Thermal conductivity and thermal impedance, measured by the transient plane source method (ISO 22007-2), laser flash method (ASTM E1461), or steady-state heat flow method (ASTM D5470)
- Hardness (Shore 00 or Shore C) per ASTM D2240 / ISO 7619-1
- Compression deflection (stress–strain) characteristics under defined deflection percentages
- Compression set per ASTM D395 / ISO 815-1, indicating permanent loss of resiliency and contact pressure
- Tensile strength and elongation at break per ASTM D412 / ISO 37
- Dielectric breakdown strength per IEC 60243-1
- Volume resistivity and surface resistivity per IEC 62631-3-1 / ASTM D257
- Dimensional and weight change (swelling or shrinkage), with moisture uptake referenced to ISO 62 / ASTM D570
- Extractable ionic contaminants (chloride, sulfate, sodium, potassium ions) using water extraction followed by ion chromatography
- Low-molecular-weight siloxane content and oil bleeding tendency for silicone materials
- Visual inspection for blistering, cracking, discoloration, surface tack change, and filler segregation
- Adhesion or tack retention for pressure-sensitive adhesive backed constructions
- Flammability rating retention per UL 94 (V-0), where applicable
Testing Methods
Test Conditions and Equipment
Steady-state damp heat exposure is most commonly performed at 85°C ± 2°C with 85% ± 5% relative humidity in accordance with IEC 60068-2-78 or its Chinese national equivalent GB/T 2423.3 (Test Cab). Cyclic damp heat testing follows IEC 60068-2-30 or GB/T 2423.4 (Test Db), applying 12-hour plus 12-hour humidity cycles at 40°C with 95% RH, optionally with a lower temperature excursion. The test chamber must provide uniform, calibrated temperature and humidity control, with continuous recording of chamber conditions and uninterrupted operation throughout the exposure.
Exposure Durations and Checkpoints
Typical exposure durations are 500 hours, 1000 hours, or 2000 hours, selected according to the automotive customer specification and the severity class of the intended application. Intermediate measurement checkpoints (for example at 250 h, 500 h, and 1000 h) allow degradation trends to be plotted as property-versus-time curves. Some development programs extrapolate the aging kinetics using Arrhenius-type modeling to estimate service life, although such extrapolations should always be validated against real-field data.
Post-Aging Evaluation Procedure
Upon completion of each exposure interval, specimens are removed from the chamber and allowed to recover in the standard conditioning atmosphere for a defined period (commonly 1 to 24 hours) before measurement. Recovery conditioning distinguishes between reversible moisture effects and permanent chemical or structural degradation. All baseline test methods are then repeated on the aged specimens using identical equipment, fixtures, and compression ratios, and property retention is calculated as the ratio of the aged value to the initial value. For compression-mounted properties such as thermal impedance, the mounting pressure and deflection must replicate the actual battery pack design condition.
Typical Acceptance Criteria
While final acceptance limits are defined by the vehicle manufacturer or cell supplier, common industry benchmarks include thermal conductivity retention of at least 80% to 90%, dielectric breakdown strength retention of at least 80% with no dielectric failure, volume resistivity remaining above 1 × 10¹² ohm·cm, weight change within approximately ±2%, compression set below 10% to 20%, no visible cracking or delamination, and extractable chloride levels maintained below the specified contamination limit (frequently in the range of a few tens of ppm) to protect copper and aluminum busbars from corrosion.
Common Failure Modes Observed
- Increase in thermal contact resistance caused by compression set and loss of interfacial pressure
- Hydrolytic embrittlement or softening of the polymer matrix, reflected in tensile property drift
- Accelerated silicone oil bleed and siloxane deposition on adjacent surfaces
- Leaching of ionic impurities leading to potential electrochemical corrosion risks
- Blistering or delamination of reinforcement films in laminated constructions
- Reduction of dielectric strength due to moisture pathways and interfacial defects
Applications
Hygrothermal aging testing of thermal conductive pads is applied throughout the EV and energy storage supply chain to qualify materials, support design decisions, and monitor production consistency. Typical application scenarios include:
- Qualification of gap pads between prismatic or pouch cells and pack-level liquid cooling plates in cell-to-pack (CTP) architectures
- Validation of cell sidewall thermal pads in prismatic module designs requiring both heat spreading and dielectric isolation
- Material selection and second-source approval for thermal interface materials in cylindrical cell module assemblies
- Reliability demonstration for pads used in battery energy storage system (BESS) enclosures operating in hot and humid climates
- Verification of pads used in battery management system (BMS) power electronics, on-board chargers, and DC-DC converters
- Periodic reliability audits and incoming material surveillance programs performed by accredited third-party laboratories
- Root cause analysis when field or end-of-line failures suggest thermal path degradation or contamination-related corrosion
FAQ
What is the most widely used damp heat aging condition for power battery thermal pads?
The steady-state 85°C / 85% RH condition per IEC 60068-2-78 (equivalently GB/T 2423.3) is the industry baseline, typically run for 1000 hours. Many automotive specifications extend the exposure to 2000 hours or supplement it with cyclic damp heat per IEC 60068-2-30.
Why are ionic contamination tests included with damp heat aging?
Humidity accelerates the leaching of chloride, sulfate, and metal cations from filled elastomers. When these ions migrate to busbars, terminals, or cell cans under humid conditions, they can initiate corrosion and leakage-current paths. Measuring extractable ion content after aging therefore directly assesses a battery-specific corrosion risk that generic material datasheets do not capture.
Can damp heat aging replace thermal cycling tests?
No. The two tests address different failure mechanisms. Damp heat aging targets hydrolysis, moisture uptake, ionic leaching, and dielectric degradation, while thermal cycling (for example, −40°C to 85°C per IEC 60068-2-14 or GB/T 2423.22) targets fatigue, delamination, and stress-driven interfacial failure. A complete validation plan combines both, often followed by combined or sequential stress profiles.
Why is thermal impedance re-measured under compression rather than only thermal conductivity?
Thermal impedance captures both the bulk conductivity of the pad and the contact resistance at the interfaces, which depends on the compression pressure maintained by the pad. Since damp heat aging can permanently reduce pad resiliency through compression set, the compressed thermal impedance is the property that most closely reflects in-vehicle heat transfer performance.
Which organizations publish the relevant standards?
The governing documents are published by IEC (IEC 60068-2 environmental testing series), ISO (ISO 291 conditioning, ISO 37 tensile, ISO 22007 thermal properties), ASTM (ASTM D5470, D2240, D395, D412, D257), UL (UL 94 flammability), and the Chinese national standards system (GB/T 2423.3, GB/T 2423.4, and related GB/T methods for rubber and insulating materials).
What should a test report include?
A compliant report should document the material and lot identification, specimen dimensions and quantity, conditioning history, chamber setpoints with calibration traceability, exposure duration and any interruptions, recovery conditions, all before-and-after property values with retention rates, photographs of post-aging appearance, ion chromatography data where applicable, and a clear conclusion against the specified acceptance limits.
