High and Low Temperature Tensile Testing of Photovoltaic Modules: Standards, Methods, and Applicatio

High and low temperature tensile testing is a critical reliability evaluation used to verify the mechanical integrity of photovoltaic module materials, interconnects, and bonded joints across the full operating temperature range. By applying controlled tensile loads inside an environmental chamber at temperatures from -40 °C to +85 °C and beyond, this test reveals how encapsulants, backsheets, ribbons, and adhesives behave under the thermal stresses encountered in real-world service. This article explains the test principle, sample types, measured parameters, standard methods, and practical applications of tensile testing for PV modules at extreme temperatures.

Technical Overview

Photovoltaic modules operate outdoors for 25 to 30 years and are continuously exposed to temperature extremes that typically span -40 °C to +85 °C, as defined in the IEC 61215 and IEC 61730 qualification frameworks. Because nearly every material inside a module is temperature-sensitive, mechanical properties measured at room temperature alone cannot predict field behavior. Ethylene-vinyl acetate (EVA), polyolefin elastomer (POE), and other encapsulants stiffen dramatically at low temperature and soften or creep at high temperature; solder alloys used for cell interconnection lose strength as temperature rises toward their melting range; and adhesive joints, such as those used for junction boxes and frame bonding, may undergo brittle-to-ductile transitions within the module service window.

High and low temperature tensile testing addresses this challenge by applying a controlled uniaxial tensile load to a specimen while it is held at a constant, precisely regulated temperature inside an environmental chamber mounted on a universal testing machine. The resulting force-displacement or stress-strain data quantify how tensile strength, yield behavior, elongation, modulus, peel resistance, and shear strength vary with temperature. Testing is performed in accordance with internationally recognized procedures from ISO, IEC, ASTM, EN, and GB standards bodies, and accredited third-party laboratories routinely use this method to support module design qualification, safety certification, material selection, and failure analysis.

Testing Samples

Encapsulant Materials

Dumbbell or rectangular strip specimens are die-cut from EVA, POE, PVB, or silicone encapsulant films and tested in accordance with IEC TS 62788-1-2, ISO 527, and GB/T 1040. These tests determine whether the polymer retains adequate strength and ductility at both temperature extremes, which is essential for maintaining lamination integrity under thermal cycling.

Backsheet and Front Sheet Films

Thin polymer films, including fluoropolymer-based backsheets and co-extruded structures, are prepared as narrow strips and tested per ISO 527-3 or ASTM D882. Temperature-dependent tensile data reveal embrittlement at low temperature and excessive softening at high temperature.

Soldered Interconnect and Ribbon Joints

Copper ribbon soldered to cell busbars, lead-free solder lap-shear coupons, and ribbon-to-ribbon splices are tested to evaluate joint strength at elevated temperatures where solder alloys weaken significantly, and at low temperatures where residual stresses from thermal mismatch are highest.

Laminated Multi-Layer Specimens

Strips cut from glass-encapsulant-backsheet laminates are tested in peel configurations, such as 90-degree, 180-degree, or T-peel modes per ASTM D903 and ASTM D1876, to measure interfacial adhesion as a function of temperature and to identify delamination risks.

Adhesive Bonded Assemblies

Junction-box-to-backsheet bonds, frame bonding sealants, structural silicones, and cable-to-connector terminations are tested in pull, shear, or peel modes at temperature extremes to validate that attachment strength remains sufficient throughout the module lifetime.

Testing Items

Depending on the specimen type and the applicable standard, the following parameters are typically measured and reported:

• Ultimate tensile strength and maximum tensile force
• Yield strength and stress at break
• Elongation at break and strain at maximum force
• Tensile modulus (Young's modulus)
• 90-degree and 180-degree peel strength
• T-peel strength for flexible laminates
• Lap shear strength for bonded and soldered joints
• Pull-off force for junction boxes, cables, and terminations
• Failure mode classification: adhesive, cohesive, or mixed failure
• Strength retention ratio relative to the room-temperature baseline, before and after thermal aging

Testing Methods

Testing is conducted using an electromechanical universal testing machine equipped with an environmental chamber capable of covering the required temperature window, commonly from -70 °C to +300 °C for general polymer and metal testing, with PV-specific test points normally set at -40 °C, +23 °C, and +85 °C, plus extended points such as +105 °C, +125 °C, or +150 °C for solder joints and high-temperature materials.

Load measurement chains are calibrated in accordance with ISO 7500-1 or ASTM E4, and chamber temperature uniformity is typically controlled within ±2 °C of the set point. Specimens are conditioned at the target temperature for a sufficient dwell time, generally 10 to 30 minutes depending on specimen mass and thermal mass, to ensure the gauge region reaches thermal equilibrium before loading begins.

Crosshead speed is selected according to the governing standard, with typical ranges of 1 mm/min to 500 mm/min: slower speeds for rigid materials and modulus determination, and faster speeds for flexible films and elastomers per ISO 527, ASTM D638, ASTM D882, ASTM D1002, ASTM D1876, and the corresponding EN and GB/T equivalent procedures. A minimum of five valid specimens per temperature condition is tested, and results are reported as mean values with standard deviations. Test results at each temperature are compared against room-temperature baselines and design requirements, and retained strength is expressed as a percentage to quantify temperature sensitivity.

Applications

High and low temperature tensile testing supports a wide range of engineering and certification activities across the photovoltaic industry:

• Design qualification and type approval in line with the IEC 61215 series and safety certification under the IEC 61730 series, including UL 61730 and EN IEC 61730 frameworks
• Material selection and comparative evaluation of encapsulants, backsheets, adhesives, and sealants before lamination process lock-in
• Validation of new interconnection technologies, including electrically conductive adhesives, multi-busbar ribbon, shingled cells, and zero-busbar designs
• Failure analysis of modules returned from the field or extracted after damp heat, thermal cycling, or humidity-freeze aging per IEC 61215 test sequences
• Incoming quality control and supplier change management for encapsulant films, ribbon, solder wire, and adhesive systems
• Reliability research, such as Arrhenius-based lifetime modeling and thermal-cycle fatigue assessment of solder joints

FAQ

Why is tensile testing performed at extreme temperatures instead of only at room temperature?

Polymeric materials and solder alloys exhibit strongly temperature-dependent mechanical behavior. A joint or film that performs well at 23 °C may embrittle at -40 °C or lose most of its strength at 85 °C. Testing at the actual operating extremes provides data that realistically reflect field conditions.

What temperature range is typically used for PV module testing?

The standard module operating envelope of -40 °C to +85 °C is used for most materials and joints. Solder interconnects and high-temperature components are often tested up to 125 °C or 150 °C to evaluate creep and softening behavior closer to practical stress conditions.

How many specimens are required for a valid test?

Most standards, including ISO 527 and related ASTM and GB/T procedures, require a minimum of five valid specimens per temperature condition to ensure statistical reliability of the reported strength and elongation values.

How is a failed or passing result determined?

Pass criteria are defined by the applicable product standard or the manufacturer's internal specification. Typical requirements include minimum strength values, minimum elongation, acceptable failure modes, and a defined strength retention percentage after accelerated aging.

Can aged samples be tested after damp heat or thermal cycling?

Yes. Tensile and peel testing after accelerated aging per IEC 61215 test sequences is a standard practice to quantify degradation of adhesion, encapsulant ductility, and solder joint strength, providing valuable input for lifetime assessment.

What equipment is required?

A calibrated universal testing machine, an environmental chamber with adequate temperature range and uniformity, appropriate grips or peel fixtures that can operate inside the chamber, and calibrated load cells and displacement or strain measurement devices are the essential components of a compliant test setup.