Internal Stress Determination for Photovoltaic Backsheets: Principles, Methods, and Reliability Asse

Internal stress determination for photovoltaic backsheets is a critical reliability test that quantifies the residual and thermally induced stresses embedded within multi-layer backsheet structures after lamination and during field aging. By measuring stress distribution, curl behavior, and dimensional stability, manufacturers can predict cracking, delamination, and module failure risks before products reach the market. This article explains the testing principles, sample requirements, measurement methods, relevant standards such as IEC 61730 and IEC 61215, and practical applications of backsheet internal stress analysis.

Technical Overview

The photovoltaic backsheet is a multi-layer functional composite—typically consisting of fluoropolymer films (PVF, PVDF, THV, or ETFE), a PET core, and adhesive tie layers—that provides electrical insulation, moisture barrier protection, and mechanical support at the rear side of a crystalline silicon or thin-film module. During module lamination, the backsheet is bonded to the encapsulant (EVA or POE) at temperatures of approximately 140–155 °C, and the assembly is subsequently cooled to room temperature. Because each layer possesses a different coefficient of thermal expansion (CTE), elastic modulus, and viscoelastic response, significant internal stresses are frozen into the structure upon cooling.

Internal stress in backsheets originates from several coupled mechanisms: thermal expansion mismatch between the backsheet, encapsulant, glass, and cells; cure-induced shrinkage of the encapsulant; solvent evaporation and orientation relaxation within the fluoropolymer and PET layers during processing; and hygroscopic swelling driven by moisture absorption. When these stresses exceed the interlaminar bond strength or the cohesive strength of individual layers, the consequences include backsheet curling, edge lifting, cracking, chalking, interlayer delamination, and eventual insulation failure. Quantifying internal stress is therefore an essential element of backsheet material qualification, lamination process optimization, and long-term field reliability prediction.

Internal stress determination is performed by accredited third-party laboratories as well as in-house quality and R&D departments, following internationally recognized frameworks from IEC, ISO, ASTM, and UL. The test data support compliance with safety standards such as IEC 61730-2 and design qualification standards such as IEC 61215-2, where backsheet integrity must be demonstrated after thermal cycling, damp heat, and humidity-freeze exposure.

Testing Samples

Free-Standing Backsheet Sheets

Unlaminated backsheet rolls or flat sheets are tested to establish the baseline residual stress introduced during film co-extrusion, coating, and lamination of the backsheet itself. Specimens are typically cut into strips (for example, 150 mm × 25 mm) in both machine direction (MD) and transverse direction (TD) to capture anisotropic stress states arising from film orientation during manufacturing.

Glass/Encapsulant/Backsheet Laminates

Mini-laminates replicating the actual module stack—glass, encapsulant, and backsheet without cells—are prepared under production or controlled laboratory lamination conditions. These specimens reveal the stress state created by the lamination cycle and encapsulant cure shrinkage. Free-standing strips cut from these laminates, with the glass removed, exhibit measurable curl that can be converted into stress values.

Finished Module Coupons and Field-Returned Samples

Sections harvested from finished modules, from modules after accelerated aging (damp heat at 85 °C/85% RH, thermal cycling from −40 °C to +85 °C per IEC 61215-2 MQT 11 and MQT 12), or from field-failed modules are tested to characterize stress evolution during service and to support failure analysis of cracking or delamination.

Sample Preparation Requirements

• Condition specimens at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours prior to testing, in accordance with ISO 291.
• Cut edges cleanly without burrs, micro-cracks, or heat-affected zones that could release or redistribute stress artificially.
• Mark the machine direction and the air-side versus cell-side (inner) surface on every specimen to preserve orientation reference during measurement.
• Avoid any bending, folding, or scraping of the specimen surfaces, as mechanical damage distorts the intrinsic curvature used for stress calculation.
• Recommended minimum sample quantity: five replicates per direction per material lot to ensure statistical significance.

Testing Items

Internal stress determination programs typically cover the following measurable quantities:

• Residual internal stress: the frozen-in biaxial stress state of each backsheet layer at room temperature, expressed in MPa.
• Curl radius and curl height: the curvature adopted by a freed strip or bilayer laminate, the primary observable from which stress is computed.
• Thermal mismatch stress: stress generated between backsheet and adjacent module materials across the operating temperature range (−40 °C to +85 °C and beyond).
• CTE (coefficient of thermal expansion) of the backsheet in MD and TD below and above the PET glass transition temperature, required as input for stress modeling.
• Dimensional shrinkage of the backsheet after thermal exposure (for example, 150 °C for 30–60 min), an indicator of orientation-related stress release.
• Encapsulant cure shrinkage contribution: the stress component attributable to crosslinking shrinkage of EVA or POE.
• Stress relaxation and creep behavior under sustained strain, evaluated by dynamic mechanical analysis (DMA).
• Interlaminar peel strength before and after stress-inducing treatments, to assess whether internal stress degrades adhesion.
• Stress redistribution after accelerated aging, comparing as-received versus damp-heat or thermal-cycled specimens.

Testing Methods

Strip Curl / Curvature Method

The most widely used approach for laminate-level internal stress is the curvature method, based on classical laminate plate theory and the bimetallic-strip analogy. A rectangular strip containing the backsheet and encapsulant (with or without glass) is released from the constraint of the surrounding structure. The sample curls into an arc, and the radius of curvature R, or the chord height h, is measured using optical profilometry, laser displacement sensors, or a calibrated dial gauge on a flat reference plane. From the curvature, layer thicknesses, and elastic moduli, the bending moment and the residual stress distribution are calculated, commonly through solutions analogous to the Stoney equation extended to multilayer stacks. Elevated-temperature versions of the test, performed on a heated stage, map how internal stress evolves as the structure passes through the PET glass transition (around 78–85 °C for the amorphous phase) and the encapsulant softening point.

Thermal Mechanical Analysis (TMA)

TMA per ASTM E831 or ISO 11359 measures the dimensional change of backsheet specimens as a function of temperature at a low applied load. The resulting CTE values in MD and TD, combined with modulus data, feed the thermal mismatch stress calculation σ = Δα × ΔT × E /(1−ν), where Δα is the CTE difference between adjoining layers, ΔT the temperature excursion, E the elastic modulus, and ν the Poisson ratio. TMA also detects shrinkage onset, which signals the temperature at which frozen-in orientation stresses begin to relax.

Dynamic Mechanical Analysis (DMA)

DMA per ASTM D4065 or ISO 6721 provides the temperature-dependent storage modulus and viscoelastic relaxation spectra of each backsheet layer. These data are essential because stress relaxation above the glass transition can significantly reduce effective internal stress, and accurate lifetime prediction requires time-temperature superposition of the relaxation behavior.

Layer Removal / Incremental Etching Method

For through-thickness stress profiling, material is removed incrementally—by controlled plasma etching, chemical stripping, or precision microtomy—from one surface while the resulting curvature change is monitored after each removal step. Solving the inverse problem of curvature evolution versus removed depth yields the depth-resolved residual stress profile, which is particularly valuable for coated and co-extruded backsheets where near-surface stresses dominate cracking initiation.

Raman Spectroscopy and Photoelasticity (Selected Cases)

For transparent or semi-transparent layers, micro-Raman peak shift analysis or photoelastic birefringence measurement can localize stress at the micrometer scale, complementing the macroscopic curvature methods when investigating stress concentration near cracks, edges, and the junction-box attachment region.

Combined Aging and Stress Monitoring

To connect internal stress with long-term reliability, laboratories combine the above measurements with sequential environmental conditioning:

• Damp heat exposure at 85 °C and 85% RH per IEC 61215-2 MQT 13, followed by re-measurement of curl, peel strength, and stress profile.
• Thermal cycling between −40 °C and +85 °C per IEC 61215-2 MQT 11 (200 to 600 cycles), with intermediate curvature measurements to track stress evolution and detect embrittlement.
• Humidity-freeze testing per IEC 61215-2 MQT 12 to evaluate stress amplification caused by ice formation in micro-voids.
• UV preconditioning per IEC 61215-2 MQT 09 when photodegradation-induced chalking and stress development are of concern.

Data Interpretation and Acceptance Considerations

Reported results normally include curl radius, computed layer stresses in MPa with uncertainty, stress direction (MD/TD, tensile/compressive), CTE values, and comparison against the interlaminar peel strength of the backsheet. A backsheet is considered at risk when computed internal tensile stress approaches the cohesive strength of the PET core or the adhesive tie layer, when curl after lamination exceeds typical module aesthetics and edge-seal tolerances, or when stress grows substantially after accelerated aging, indicating progressive embrittlement and latent cracking propensity.

Applications

• Incoming material quality control: screening backsheet lots for excessive residual stress and shrinkage before they enter module production lines.
• Lamination process optimization: adjusting lamination temperature profiles, pressing cycles, and cure parameters to minimize thermal mismatch stress and post-lamination curl.
• New backsheet qualification: comparing fluoropolymer-based (TPT, KPK), coextruded multilayer, and PET-based structures during material selection and IEC 61730 / IEC 61215 certification programs.
• Encapsulant pairing studies: evaluating EVA versus POE combinations to balance cure shrinkage stress against adhesion performance.
• Failure analysis: determining whether field cracking, yellowing-edge delamination, or insulation breakdown originates from excessive internal stress, thermal mismatch, or environmental degradation.
• Lifetime and warranty modeling: feeding measured stress and relaxation data into finite-element models and kinetic degradation frameworks for 25–30-year service life prediction.
• Support for certification and due-diligence reporting: providing objective internal stress data for bankability assessments and independent laboratory test reports.

FAQ

Why is internal stress in backsheets important for module reliability?

Internal stress acts as a permanent driving force for backsheet cracking, chalking, and interlayer delamination. When amplified by daily thermal cycling and moisture ingress over 25 years of service, even moderate initial stress can propagate micro-cracks that compromise electrical insulation and trigger safety failures addressed by IEC 61730.

What causes internal stress in a photovoltaic backsheet?

The main causes are the CTE mismatch among fluoropolymer, PET, and adhesive layers; polymer orientation frozen in during film manufacturing; encapsulant cure shrinkage during lamination; and hygrothermal swelling or drying after environmental exposure. Stress magnitude depends on lamination temperature, cooling rate, and layer thickness ratios.

How is internal stress actually measured if it is locked inside the material?

Because internal stress cannot be read directly, it is inferred from the deformation that occurs when constraints are released. The strip curl method measures the curvature of a freed specimen and converts it to stress through laminate mechanics; TMA and DMA supply the CTE and modulus inputs; layer-removal techniques map the stress profile through the thickness.

Which standards govern backsheet internal stress and related testing?

There is no single dedicated international standard for internal stress itself; testing is built on a framework combining IEC 61730-1/-2 (safety), IEC 61215-2 (design qualification, MQT sequences), IEC 62788 (measurement procedures for polymeric materials), ASTM E831 and ISO 11359 (TMA), ASTM D4065 and ISO 6721 (DMA), ASTM D882 (tensile properties), and UL 746B (polymeric materials long-term properties). Accredited third-party laboratories integrate these into a coherent stress determination protocol.

What sample size and conditioning are required for a typical test?

Standard practice uses strips of approximately 150 mm × 25 mm cut in both machine and transverse directions, conditioned at 23 ± 2 °C and 50 ± 5% RH for at least 24 hours, with a minimum of five replicates per condition. For laminate studies, mini-laminates should replicate production lamination parameters as closely as possible.

Can internal stress be reduced after a backsheet is already laminated?

Only partially. Post-lamination annealing above the PET glass transition can relax some stress, but it also risks dimensional distortion and reduced adhesion. The effective strategy is prevention: selecting low-shrinkage backsheets, matching CTE values across the stack, and optimizing the lamination cooling profile.

How do test results correlate with field failures?

Studies of field-returned modules show that backsheets with high residual stress combined with low post-aging peel strength exhibit the highest incidence of cracking and insulation faults. Correlating measured stress evolution during damp heat and thermal cycling with outdoor failure statistics is the accepted method for establishing material-specific acceptance limits.