Acid and alkali resistance testing quantifies the chemical durability of substrate glass used in flat-panel displays, semiconductor packaging, and advanced electronics manufacturing. This technical guide explains the governing ISO, ASTM, GB, and JIS standards, typical sample preparation requirements, gravimetric and surface-analysis test procedures, and the acceptance criteria commonly used to qualify glass substrates for high-reliability production environments.
Technical Overview
Glass substrates are thin, high-flatness sheets of alkali-free boroaluminosilicate, borosilicate, or aluminosilicate glass that serve as the structural and functional platform for TFT arrays, OLED devices, glass-core semiconductor packages, and through-glass-via (TGV) interposers. Throughout fabrication, these substrates are exposed to aggressive aqueous chemistries, including acidic cleaners and etchants (hydrochloric, sulfuric, and nitric acids), buffered oxide etch solutions, alkaline developers such as TMAH, strippers, ammonia- and hydroxide-based cleaners, and electroless plating baths. Acid and alkali resistance testing measures how well the glass surface withstands such exposures without degrading.
Chemical attack on glass proceeds by two principal mechanisms. In acidic media, attack is dominated by ion-exchange leaching, in which network modifier ions (boron, aluminum, alkali, and alkaline-earth species) are selectively extracted while the silicate framework remains largely intact. In alkaline media, hydroxide ions directly break silicon–oxygen–silicon network bonds, causing uniform bulk dissolution of the glass surface. The practical consequences include mass loss, thickness reduction, increased surface roughness and haze, pitting, dimensional change, and degradation of mechanical strength.
Durability is quantified as mass loss per unit surface area (mg/cm² or mg/dm²), etch depth or etch rate, and changes in post-exposure optical and surface properties. Testing is performed to standardized methods published by ISO, ASTM, GB, and JIS, either in-house or at accredited third-party laboratories, and supports incoming inspection, supplier qualification, process window validation, and failure analysis.
Testing Samples
Alkali-Free Display Glass Substrates
Alkali-free boroaluminosilicate glass, containing less than 0.1 wt% alkali oxides, is the standard substrate for TFT-LCD and OLED panel fabrication. Chemical durability evaluation focuses on tolerance to acid cleaning steps, HF-free etch processes, and repeated wet-cleaning cycles, with thicknesses typically ranging from 0.1 mm to 0.7 mm cut from production-size sheets into test coupons.
Borosilicate and Aluminosilicate Substrates
Borosilicate and high-alumina aluminosilicate glasses are used for glass-core substrates, TGV interposers, and cover windows. These grades encounter strong alkaline chemistries during desmear, seeding, and electroless copper plating, making boiling alkali resistance a critical qualification parameter. Ion-exchanged aluminosilicate cover glasses additionally require verification that chemical durability is not compromised by surface compressive stress layers.
Soda-Lime and Specialty Glass Substrates
Soda-lime float glass and specialty glasses are tested as comparison baselines or for architectural, photovoltaic, and industrial applications. Because sodium and calcium leaching is more pronounced in these compositions, acid resistance testing is especially important, and float-side versus tin-side orientation may be recorded as part of the test plan.
Sample Preparation Requirements
- Coupon dimensions typically range from 30 mm × 30 mm to 50 mm × 50 mm, or as specified by the client, with a minimum of three and preferably five parallel specimens per test condition.
- Edges are ground and polished (or fire-polished) to remove machining damage that would otherwise dominate the attack; the total exposed surface area, including edges, is calculated from measured dimensions.
- Specimens are cleaned with a non-reactive detergent, rinsed with deionized water and a volatile solvent, dried at 105–120 °C, cooled in a desiccator, and weighed to 0.1 mg before and after exposure.
- The surface-area-to-solution-volume ratio is controlled and reported; reagent volume is typically maintained at no less than 5–10 mL per cm² of glass surface to prevent exhaustion of the attacking species.
- Coated or patterned parts can be tested by agreement, with untested faces masked using chemically inert PTFE fixtures where a one-sided exposure is required.
Testing Items
Chemical durability programs for substrate glass are built around the following measurable items, each reported with test conditions, exposure time, and statistical treatment of replicate specimens.
- Acid resistance: mass loss per unit area and etch rate after exposure to boiling hydrochloric acid or defined dilute acid solutions, the primary gravimetric metric.
- Alkali resistance: mass loss per unit area and etch rate after exposure to boiling sodium hydroxide or mixed alkali (NaOH/Na₂CO₃) solution.
- Hydrolytic resistance: supplementary grain titration classification of intrinsic water durability of the glass composition.
- Post-exposure surface quality: change in haze and total transmittance, surface roughness (Ra, Rq), and optical microscopy inspection for staining, spotting, and pitting.
- Dimensional stability: change in thickness, total thickness variation, and warp after chemical exposure.
- Extract analysis: quantification of leached species (Si, B, Al, Na, Ca) in the test solution by ICP-OES, providing mechanistic insight and early-stage sensitivity beyond gravimetry.
Testing Methods
Boiling Acid Resistance Test
The test follows acid durability methods such as ISO 1776 (resistance to attack by hydrochloric acid at 100 °C) and the acid durability procedures of JIS R 3502. Prepared coupons are suspended on chemically inert holders in freshly prepared 6 mol/L hydrochloric acid held at boiling reflux (approximately 108 °C for this concentration) for 3 to 6 hours, with a reflux condenser maintaining constant acid concentration. After exposure, specimens are rinsed with deionized water and solvent, dried, and reweighed. The result is calculated as the mass difference divided by the total exposed surface area and expressed in mg/cm² or mg/dm², then classified into the acid resistance grades defined by the applicable standard. Blank runs and reference glass specimens are included in each test series for quality control.
Mild Acid Exposure Test
To simulate production cleaning chemistry, coupons are immersed in 5–10 wt% citric acid or dilute mineral acids at 60–95 °C for 0.5 to 2 hours. Acceptance is typically based on the absence of visible staining, a haze increase below a defined limit (commonly 0.5 percentage points), and mass loss below a specified threshold agreed with the customer. Hydrofluoric-acid-based etch-rate testing is handled as a separate mixed-acid etch study, because HF attack proceeds by a distinct mechanism and is outside the scope of standard acid resistance classification.
Boiling Alkali Resistance Test
The reference method follows ISO 695, resistance to attack by a boiling aqueous solution of mixed alkali. Equal volumes of 1 mol/L sodium hydroxide and 0.5 mol/L sodium carbonate are combined, coupons are immersed at boiling reflux for 3 hours, and the mass loss per unit area is determined and classified into the alkali resistance grades specified by the standard. For high-silica, alkali-free substrate glasses, this is usually the limiting durability metric because hydroxide attack uniformly dissolves the silicate network rather than merely leaching modifiers. Application-specific variants include immersion in 5–10 wt% NaOH or in simulated developer and stripper solutions at defined temperatures and times, with results reported as etch rate in nm/min.
Hydrolytic Resistance Test (Supplementary)
Grain titration methods per ISO 719 and its GB national equivalent GB/T 6582 evaluate intrinsic water durability of the composition: glass crushed and sieved to 300–500 µm grains is boiled in water at 98 °C for one hour, and the leached alkali in the extract is titrated with 0.01 mol/L HCl, classifying the glass from HGB 1 (highest resistance) to HGB 5. The more severe ISO 720 autoclave method at 121 °C is applied where extended water exposure is relevant. These data are widely used for incoming lot screening and composition control.
Data Reduction and Surface Characterization
Gravimetric results are converted to etch depth using d = 10·Δm/(ρ·A), where d is the etch depth in µm, Δm the mass loss in mg, ρ the glass density in g/cm³ (typically 2.2–2.6 for substrate glasses), and A the exposed area in cm²; etch rate is the depth divided by exposure time. Supporting instrumentation includes:
- An analytical balance with 0.1 mg readability and antistatic shielding for repeatable gravimetry.
- Stylus profilometry or atomic force microscopy for Ra and Rq before and after exposure.
- A hazemeter, following ASTM D1003 methodology, for haze and total transmittance change.
- ICP-OES analysis of test solutions for leached element quantification, plus optical microscopy and SEM for pitting and surface texture assessment.
Quality controls include triplicate specimens, reference glasses with known durability, continuous temperature logging, reagent standardization, and control charting of reference-sample results across test campaigns.
Applications
Acid and alkali resistance data are applied across the entire lifecycle of substrate glass, from composition development through fab incoming inspection and failure analysis.
- Incoming inspection and supplier qualification of display glass substrates for TFT-LCD and OLED fabrication lines.
- Process development and material qualification for glass-core substrates and TGV interposers in semiconductor advanced packaging, including resistance to desmear, etchant, and electroless plating baths.
- Cover window and touch sensor durability qualification against household cleaners and repeated wet cleaning.
- Supporting durability programs for photovoltaic cover glass, combined with damp-heat testing in IEC 61215-type sequences.
- Failure analysis of surface defects, bath contamination incidents, and lot acceptance disputes, performed by accredited third-party laboratories when independent verification is required.
FAQ
Which standards govern acid and alkali resistance testing of substrate glass?
The principal methods are ISO 695 for boiling mixed-alkali attack, ISO 1776 for hydrochloric acid attack at 100 °C, ISO 719 and ISO 720 for hydrolytic resistance, ASTM C225-type chemical attack procedures, the GB national equivalent GB/T 6582, and the chemical durability procedures of JIS R 3502. The appropriate method is selected based on the glass composition, the target process chemistry, and customer specification.
What sample size and quantity are required?
Coupons of 30 mm × 30 mm to 50 mm × 50 mm cut from production sheets are typical, with three to five replicates per condition to provide statistical confidence. Ground and polished edges, defined cleaning, and precise dimensional measurement are prerequisites, because the reported result is normalized to total exposed surface area.
What are typical acceptance criteria?
Specifications are application-specific, but high-durability substrate grades commonly target single-digit to low-tens mg/dm² losses in boiling acid exposure, alkali losses within the top resistance grades of ISO 695, haze increase below about 0.5 percentage points, and etch rates of only a few nm/min in simulated cleaning chemistry. Exact limits are agreed between glass supplier and end user.
Why is alkali resistance usually the weaker property of high-silica substrate glass?
In acid, attack is largely limited to leaching of modifier ions while the silicate network survives, whereas hydroxide ions cleave the silicon–oxygen network directly and dissolve the glass uniformly. Consequently, alkaline exposure governs thickness loss and is the dominant limiting condition in chemical durability qualification.
How long does testing take?
Exposure times are typically 3 to 6 hours per condition, with additional time for coupon preparation, conditioning, weighing, and surface characterization. Complete test reports, including classification and replicate statistics, are commonly issued within five to ten working days by accredited third-party laboratories or in-house reliability laboratories.
Can coated or patterned substrates be tested?
Yes. Coated, metallized, or patterned parts can be evaluated with results normalized to exposed area, with masking used to isolate specific faces. Combined gravimetric, optical, and ICP-OES analysis distinguishes coating degradation from attack on the underlying substrate glass.