Damp heat aging testing at 85°C and 85% relative humidity is the industry-standard method for evaluating the long-term reliability of thermal conductive gels used in photovoltaic modules. This article provides a comprehensive technical overview of sample preparation, test items, measurement methods, acceptance criteria, and the governing standards—such as IEC 61215-2, IEC 61730, IEC 62788, ASTM, and GB/T 2423.3—that accredited third-party laboratories apply when qualifying thermally conductive gel materials for PV applications.
Technical Overview
Thermal conductive gels—typically two-part, addition-cured silicone systems filled with ceramic particles such as alumina (Al2O3), aluminum nitride, or boron nitride—are increasingly used in photovoltaic (PV) modules to enhance heat dissipation from solar cells to the frame, backsheet, back rail, or heat-spreading structures. Because a typical PV power plant must operate reliably for more than 25 years, these gels must retain their thermal, mechanical, and electrical properties under sustained exposure to heat and humidity.
Damp heat aging is one of the most demanding accelerated reliability tests used to qualify these materials. Specimens are exposed to a controlled environment of 85°C ± 2°C and 85% ± 5% relative humidity for 1,000 hours as a baseline, with extended durations of 2,000 hours or longer frequently specified for high-humidity climates or premium product tiers. The test conditions originate from module-level qualification requirements in IEC 61215-2 (Test MQT 13) and IEC 61730-2, while material-level characterization is supported by the IEC 62788 series, the IEC 60068-2-78 environmental testing standard, GB/T 2423.3, and relevant ASTM methods. Combined heat and moisture can hydrolyze the polymer network, weaken filler–matrix interfaces, degrade adhesion to glass, backsheets, and aluminum, and mobilize ionic impurities that compromise electrical insulation. Damp heat aging therefore serves as a critical acceptance gate for thermal interface materials in PV module designs.
Testing Samples
Bulk Gel Specimens
Cured gel plaques, pads, or sheets are cast at representative thicknesses (commonly 0.5 mm to 3 mm) and fully post-cured before testing. These specimens are used to measure thermal conductivity, hardness, tensile properties, volume resistivity, dielectric strength, and water absorption. A minimum quantity—typically at least five specimens per property per aging interval—is recommended to ensure statistical validity.
Gel-on-Substrate Adhesion Specimens
The gel is dispensed or laminated between substrates representative of the actual module construction, such as PV-grade glass, fluoropolymer backsheets (TPT or TPE structures), anodized aluminum rails, and junction-box plastics such as PBT. Lap-shear, peel, and butt-joint configurations are prepared to quantify interfacial durability after humid exposure.
Functional Module and Coupon Specimens
Mini-modules, laminate coupons, or thermal stack assemblies integrating the gel between a heat source and a heat sink are tested to measure end-of-life thermal impedance and to verify that no delamination, voiding, pump-out, or migration occurs within a realistic construction.
Testing Items
The damp heat aging program evaluates a matrix of properties before, during, and after exposure to quantify degradation:
- Visual appearance: cracking, blistering, delamination, discoloration, surface tack or reversion, filler sedimentation, and material migration
- Thermal conductivity (or thermal diffusivity) and total thermal impedance of the assembled joint
- Adhesion performance: lap-shear strength, 90° or 180° peel strength, and butt-joint tensile strength
- Shore 00 / Shore A durometer hardness
- Tensile strength and elongation at break
- Volume resistivity and dielectric strength (electric strength)
- Mass change and water uptake, indicating moisture absorption and possible extractables
- Ion contamination indicators, such as chloride and sodium content, relevant to potential corrosion of metallization
Testing Methods
Test Conditions and Duration
All specimens are conditioned prior to baseline measurement at 23°C ± 2°C and 50% ± 5% relative humidity in accordance with ISO 291 or ASTM D618. Aging is then performed in a climatic chamber maintained at 85°C ± 2°C and 85% ± 5% RH, consistent with IEC 60068-2-78 (test Cab), IEC 61215-2 MQT 13, and GB/T 2423.3. The standard duration is 1,000 hours; extended protocols of 2,000 hours or more are often applied as part of enhanced qualification sequences. Interim characterization is commonly performed at 500-hour checkpoints. After removal from the chamber, specimens are reconditioned at standard laboratory conditions for a defined recovery period (typically 2 to 24 hours depending on the property) before post-aging measurements, so that transient moisture effects are not confused with permanent degradation.
Measurement Procedures
- Thermal conductivity and diffusivity: ASTM E1461 (laser flash method), ASTM D5470 (steady-state heat flow meter / thermal impedance), or ISO 22007-2 (transient plane source)
- Volume resistivity: ASTM D257 or IEC 62631 series; dielectric strength: IEC 60243-1 or ASTM D149
- Hardness: ASTM D2240 or ISO 868
- Tensile strength and elongation: ASTM D412 or ISO 37 using die-cut dumbbells
- Lap shear: ASTM D1002 or ASTM D3163; peel adhesion: ASTM D903 or ASTM D3330
- Mass change: gravimetric measurement with a balance resolution of 0.1 mg
- Appearance: visual inspection and, where required, optical or cross-sectional microscopy of mounted specimens
Acceptance Criteria
While specific limits are agreed between manufacturer and purchaser, typical qualification requirements include: no visible cracking, blistering, or delamination; thermal conductivity retention of at least 90% of the initial value; thermal impedance increase of the assembly limited to 10% to 20%; adhesion strength retention of at least 70% to 80% of initial values with cohesive rather than adhesive failure preferred; volume resistivity remaining above the specified insulation threshold (commonly on the order of 1 × 10^12 ohm-centimeters); and successful dielectric withstand testing of the complete stack. Any evidence of gel reversion (permanent softening or tackiness), oil bleeding, or migration onto cell metallization is grounds for rejection.
Applications
Damp heat aging data for thermal conductive gels support a wide range of engineering and commercial decisions across the PV value chain:
- Qualification of new thermal interface materials before adoption in module designs, including back-rail bonding gels and junction-box thermal management compounds
- Validation of design changes such as alternative gel suppliers, cure chemistry modifications, or revised dispensing thicknesses
- Supplier quality control and incoming material lot acceptance for gel manufacturers and module assemblers
- Extended reliability demonstration for modules deployed in tropical, coastal, or high-humidity environments, where 2,000-hour or longer exposures provide additional design margin
- Supporting documentation for bankability reports, technical data sheets, and certification dossiers submitted to accredited third-party laboratories
- Comparative benchmarking of silicone gel systems against thermal pads, gap fillers, and phase-change interface materials
FAQ
What are the standard damp heat test conditions for PV thermal conductive gels?
The standard exposure is 85°C ± 2°C with 85% ± 5% relative humidity, matching the damp heat test defined for PV modules in IEC 61215-2 (MQT 13) and the general environmental procedure in IEC 60068-2-78 and GB/T 2423.3. The baseline duration is 1,000 hours.
Why is damp heat testing necessary for silicone-based gels, which are already moisture resistant?
Although silicone elastomers are inherently hydrophobic, the composite gel—comprising polymer, high loadings of ceramic filler, and adhesion interfaces—is still vulnerable. Sustained heat and humidity can hydrolyze the crosslinked network, weaken filler–matrix bonding, degrade primer layers at glass, backsheet, and aluminum interfaces, and mobilize ionic contaminants. The test verifies that the material system, not just the base polymer, survives long-term service.
What property retention levels are considered a pass?
Commonly accepted criteria include thermal conductivity retention of 90% or greater, adhesion retention of 70% to 80% or greater, no visible cracking or delamination, and electrical insulation properties remaining above specification. Exact limits are defined in the customer specification or qualification plan.
Can the damp heat test be extended beyond 1,000 hours?
Yes. Exposures of 2,000 hours or longer are frequently used for enhanced reliability programs, particularly for products targeting humid climates or premium warranty terms. Extended durations are typically specified by the purchaser or within extended-stress test protocols used for module bankability assessments.
How are specimens handled after removal from the chamber?
Specimens are transferred to standard laboratory conditions (23°C ± 2°C, 50% ± 5% RH) for a defined recovery period before measurement. This ensures that reversible moisture effects are separated from permanent chemical or physical degradation when comparing pre-aging and post-aging results.
Which organizations publish the relevant standards?
Key standards are published by IEC (IEC 61215-2, IEC 61730, IEC 62788 series, IEC 60068-2-78, IEC 60243, IEC 62631), ASTM International (ASTM D5470, E1461, D412, D2240, D257, D903, D1002), ISO (ISO 37, ISO 291, ISO 868, ISO 22007), and the Chinese national standards body (GB/T 2423.3). Testing is carried out by accredited third-party laboratories or in-house reliability facilities operating under these frameworks.