Substrate glass strain point determination is a critical thermal characterization test that establishes the temperature below which the glass behaves as an essentially rigid elastic solid, directly defining its usable thermal budget in display, optoelectronic, and semiconductor manufacturing processes. Performed in accredited third-party laboratories in accordance with internationally recognized standards such as ASTM C598, ASTM C336, and the ISO 7884 series, this measurement quantifies the temperature at which the glass viscosity reaches 10^13.5 Pa·s. This article explains the underlying principles, sample requirements, test procedures, typical result ranges, and practical applications of strain point testing for substrate glass.
Technical Overview
The strain point (StrP) of glass is defined as the temperature at which its internal viscosity reaches 10^13.5 Pa·s (equivalent to 10^14.5 poise). It marks the lower boundary of the transformation range, sometimes called the annealing range. Below the strain point, molecular rearrangement is so slow that residual stresses in the glass are effectively "frozen in" and will not relax within practical time scales. Above the strain point, the glass begins to exhibit measurable viscoelastic behavior, and prolonged exposure can lead to warpage, compaction (thermal shrinkage), and dimensional drift.
For substrate glass, the strain point is one of the most important thermal properties because it defines the maximum temperature at which a thin, large-area sheet can be processed while maintaining dimensional stability. Modern flat-panel display substrates, particularly alkali-free aluminosilicate and borosilicate glasses, are routinely subjected to high-temperature fabrication steps such as thin-film transistor (TFT) array formation, deposition, crystallization, and annealing. Typical strain point values range from approximately 510 °C for soda-lime glass to 590–710 °C for advanced alkali-free display glass, while special high-strain-point glasses used for low-temperature polysilicon (LTPS) processing can exceed 700 °C.
Related reference points on the viscosity–temperature curve include the annealing point (10^12 Pa·s), at which residual stresses relax in minutes, and the softening point (approximately 10^6.6 Pa·s), at which the glass begins to deform under its own weight. Strain point determination is typically performed using beam-bending viscometry or fiber-elongation viscometry, following established test methods published by standards organizations such as ASTM International and ISO.
Testing Samples
Beam or Bar Specimens for Beam-Bending Viscometry
Beam-bending measurements use small rectangular bars or cylindrical rods cut or ground from representative glass panels or cullet. Typical dimensions are a length of 50–100 mm with a diameter or side width of 3–6 mm, fabricated to the tolerances specified in the applicable standard. Specimens must be free of visible defects, bubbles, stones, and striae that could bias the viscous response.
- Specimens must be fully annealed to remove residual stress before testing, verified using a polariscope or stress-optical instrument.
- Surfaces should be ground and polished smoothly, with edges chamfered to avoid stress concentration artifacts.
- Diameter or thickness is measured at multiple positions, since the viscosity calculation is highly sensitive to geometry.
Drawn Glass Fibers for Fiber-Elongation Viscometry
Fiber-elongation measurements use a thin glass fiber drawn from the melt or from a representative sample of the substrate glass, commonly with a diameter of approximately 0.6–0.75 mm and a fused ball at the upper end for gripping. The effective gauge length is typically on the order of 100 mm.
- The fiber must be straight, uniform in diameter along its length, and free of draw-induced stresses.
- The fiber composition must faithfully represent the substrate lot being evaluated.
- Careful handling is required to prevent surface damage or contamination that could cause premature failure under load.
Panel Coupons for Comparative Screening
Small coupons cut from finished substrate sheets may be used for dilatometric or thermomechanical analysis (TMA) screening of the transformation range. These tests provide rapid comparative data useful for quality trending, although they do not replace absolute viscosity-based strain point determination.
Testing Items
A typical strain point determination program for substrate glass includes the following measurable outputs:
- Strain point temperature, reported in °C, corresponding to a viscosity of 10^13.5 Pa·s.
- Annealing point temperature (10^12 Pa·s), frequently measured in the same test campaign.
- Softening point temperature (approximately 10^6.6 Pa·s), where required for complete thermal characterization.
- Discrete viscosity–temperature data across the transformation range, typically from about 10^12 to 10^14 Pa·s or wider.
- Fitted viscosity–temperature curve parameters, such as Fulcher (VFT) equation constants, for process modeling.
- Apparent activation energy of viscous flow in the transformation range, where specified.
Testing Methods
Beam-Bending Viscometry
Beam-bending viscometry, standardized in ASTM C598 and covered within the ISO 7884 viscosity series, is the most widely used method for direct strain point measurement of substrate glass. A horizontal specimen is supported on two knife edges inside a precisely controlled vertical tube furnace, and a known load is applied at the midpoint. The rate of central deflection (sag rate) is monitored continuously with a displacement transducer or optical system.
- Viscosity is calculated from the applied load, specimen geometry, support span, and measured sag rate; it is inversely proportional to the deflection rate.
- The furnace is ramped through the transformation range under a controlled schedule, with temperature measured by a calibrated thermocouple positioned adjacent to the specimen.
- Measurements are recorded at multiple temperatures, and the temperature at which the calculated viscosity equals 10^13.5 Pa·s is interpolated as the strain point.
- Within-laboratory repeatability is typically within about 1–2 °C, with between-laboratory reproducibility on the order of 4–6 °C.
Fiber-Elongation Viscometry
Fiber-elongation viscometry, standardized in ASTM C336 and also addressed in the ISO 7884 series, measures the elongation rate of a vertically suspended glass fiber subjected to a calibrated deadweight inside a tube furnace. Because the load is constant, the elongation rate is inversely proportional to viscosity.
- The annealing point is identified as the temperature at which the fiber elongation rate corresponds to a viscosity of 10^12 Pa·s.
- The strain point is then determined as the temperature, obtained from the same controlled cooling trace, at which the elongation rate falls to one-tenth of the annealing-point value, corresponding to the one-decade-higher viscosity of 10^13.5 Pa·s.
- This method is efficient and well suited to comparative qualification of melt or cullet samples.
Transformation-Range Screening by Dilatometry and TMA
Dilatometric methods, such as those consistent with ASTM E228, provide the thermal expansion curve and dilatometric transformation temperature. These results correlate with, but are not identical to, the strain point: the dilatometric transition typically lies between the strain point and the annealing point. Dilatometry and TMA are valuable for rapid incoming inspection, lot trending, and thermal shrinkage (compaction) studies of thin substrate sheets.
Calibration, Reference Materials, and Data Treatment
- Furnace temperature and displacement systems are calibrated using certified reference glasses traceable to national metrology institutes, such as NIST viscosity standards.
- Residual stress in specimens is checked and relieved before testing to avoid systematic bias.
- Viscosity data across the transformation range are fitted to a Fulcher (VFT)-type relation, and the strain point is extracted from the fitted curve for improved precision.
- Test reports state the method applied, specimen geometry, calibration traceability, measured temperatures, and estimated measurement uncertainty.
Applications
Strain point data for substrate glass directly support design, process, and quality decisions across multiple industries:
- Display substrate qualification: verifying that TFT-LCD and OLED substrate glasses withstand array fabrication, deposition, and annealing thermal budgets without warpage or compaction.
- LTPS and advanced backplane processes: confirming strain points of 650 °C or higher where elevated crystallization and activation temperatures are required.
- Chemical strengthening of cover glass: ion-exchange processes must be conducted below the strain point to preserve compressive stress without viscous relaxation.
- Forming and annealing lehr design: establishing factory annealing schedules referenced to the strain and annealing points to ensure low residual stress and clean cutting.
- Frit sealing and encapsulation: matching substrate strain points with sealing glass process windows for OLED encapsulation and hermetic package assembly.
- Glass composition development: benchmarking new alkali-free or high-strain-point formulations during research and pilot production.
- Photovoltaic and optoelectronic packaging: qualifying glass sheets and wafers exposed to repeated thermal cycling during module assembly.
FAQ
What is the strain point of substrate glass?
The strain point is the temperature at which the glass viscosity equals 10^13.5 Pa·s (10^14.5 poise). Below this temperature, the glass behaves essentially as a brittle elastic solid, and internal stresses relax negligibly over practical time scales, which is why it defines the upper limit of the dimensionally stable processing window.
How does the strain point differ from the annealing point, softening point, and Tg?
The annealing point corresponds to 10^12 Pa·s and represents the temperature at which stresses relax within minutes, making it the target for stress-relief annealing. The softening point, at roughly 10^6.6 Pa·s, is where glass deforms under its own weight. The dilatometric glass transition temperature (Tg) is an expansion-based parameter that generally falls between the strain point and the annealing point and should not be used interchangeably with the strain point.
Which test method is preferred for thin display substrate glass?
Beam-bending viscometry per ASTM C598 is generally preferred for absolute strain point determination because it accommodates machined bar specimens cut directly from production panels and delivers good precision. Fiber-elongation viscometry per ASTM C336 is a well-established alternative, particularly when representative fibers can be drawn from the melt or cullet.
How much sample material is required?
For beam bending, several machined bars approximately 50–100 mm long are typically sufficient. For fiber elongation, a small quantity of cullet or offcut material from which fibers can be drawn is needed. Screening by dilatometry requires only small coupons, making it convenient for limited sample quantities.
What precision and accuracy can be expected?
With calibrated equipment and properly annealed specimens, within-laboratory repeatability is typically 1–2 °C, while between-laboratory reproducibility is commonly on the order of 4–6 °C. Reporting the calibration traceability and measurement uncertainty is an essential element of a compliant test report.
Why must ion-exchange strengthening be performed below the strain point?
Above the strain point, viscous flow allows the compressive stress generated by ion exchange to relax, reducing surface compressive stress and strength. Keeping the strengthening salt-bath temperature safely below the strain point preserves the intended depth of layer and compressive stress profile.
What is a typical turnaround time for strain point testing?
In accredited third-party laboratories, routine strain point determination is commonly completed within five to ten working days after sample receipt, depending on specimen preparation requirements, annealing needs, and the scope of the accompanying viscosity characterization.