Substrate glass porosity measurement is a critical quality control and characterization process for glass substrates used in flat panel displays, semiconductor packaging, and electronic ceramics. Accurate determination of open, closed, and total porosity helps manufacturers evaluate density uniformity, hermeticity, mechanical strength, and long-term reliability. This article outlines the testing samples, measurable parameters, standard methods, industrial applications, and frequently asked questions related to substrate glass porosity determination.
Technical Overview
Porosity is defined as the ratio of the volume of void spaces (pores) within a material to its total bulk volume, typically expressed as a percentage. In substrate glass materials, porosity is a decisive factor governing dielectric performance, thermal conductivity, mechanical strength, moisture resistance, and optical scattering behavior. Even trace-level porosity in display or semiconductor glass substrates can lead to device degradation, delamination, or failure under thermal cycling conditions.
Porosity in glass-based substrates is generally classified into three categories:
- Total porosity: the entire void volume within the bulk material, including both open and closed pores.
- Open (apparent) porosity: interconnected pores that are accessible from the surface and can absorb liquids or gases.
- Closed porosity: isolated pores completely enclosed within the glass matrix, which can only be evaluated indirectly by combining bulk density and true (skeletal) density measurements.
Fused and float-produced display glass substrates are engineered to be essentially fully dense, with porosity approaching zero, while glass-ceramic and sintered porous glass substrates may contain intentionally controlled porosity ranging from a few percent to over 40 percent by volume. Accredited third-party laboratories routinely perform porosity determination in accordance with international standards such as ASTM C373, ISO 18754, ISO 15901, and ISO 12154.
Testing Samples
Display Glass Substrates
Alkali-free boroaluminosilicate and aluminosilicate glass sheets used in LCD, OLED, and microLED display panels. These thin, large-area substrates (typically 0.1 to 0.7 mm thick) are tested to verify densification quality and confirm the absence of process-induced microvoids.
Glass-Ceramic and LTCC Substrates
Sintered glass-ceramic substrates and low-temperature co-fired ceramic (LTCC) glass composites used in electronic packaging, RF modules, and multilayer interconnect boards. These materials contain engineered porosity that must fall within tight specification windows.
Porous Glass Substrates
High-silica porous glass, controlled-pore glass carriers, membrane supports, and glass filter substrates. For these products, pore size distribution and interconnected porosity are key functional parameters rather than defects.
Powder, Green Tape, and Sintered Specimens
Raw glass frits, binder-filled green tapes, and laboratory-sintered compacts evaluated during process development to study densification behavior and sintering kinetics.
Testing Items
Typical test items reported in a substrate glass porosity determination include:
- Total porosity (% volume)
- Open (apparent) porosity (% volume)
- Closed porosity (% volume)
- Bulk density (g/cm³)
- Apparent density and relative density (% theoretical density)
- True (skeletal) density by gas pycnometry (g/cm³)
- Water absorption (% mass)
- Pore volume and specific pore volume (cm³/g)
- Pore size distribution, median and mean pore diameter (nm to µm)
- Maximum pore size and pore throat size
Testing Methods
Archimedes Immersion Method
The most widely used approach, performed per ASTM C373, ISO 18754, and ISO 5017. The specimen is dried to constant mass (Md), saturated with distilled water under vacuum, weighed in the saturated state (Ms), and weighed while suspended in water (Mw). Apparent porosity is calculated as P(%) = (Ms − Md) / (Ms − Mw) × 100, and bulk density equals Md / (Ms − Mw) multiplied by the water density. The method is accurate, economical, and suitable for open porosity above approximately 0.5 percent.
Mercury Intrusion Porosimetry
Conducted according to ISO 15901-1 and ASTM D4404. Mercury is forced into the specimen under incrementally increasing pressure, and the intruded volume is related to pore size through the Washburn equation. This technique characterizes pore size distributions from roughly 3 nm to 360 µm and is particularly valuable for porous glass and glass-ceramic substrates.
Gas (Helium) Pycnometry
Performed per ASTM B923 and ISO 12154. Helium gas penetrates even nanometer-scale open pores to measure true skeletal density. When combined with geometrically or Archimedes-derived bulk volume, total and closed porosity can be calculated. This is the preferred approach for near-fully-dense display glass substrates, where closed porosity levels below 0.1 percent must be resolved.
Microscopic Image Analysis
Cross-sectional specimens prepared by grinding and polishing are examined by optical microscopy or SEM, and pore area fractions are quantified by image analysis. This method provides spatial information on pore morphology, location, and uniformity across the substrate thickness.
X-Ray Computed Tomography
A non-destructive technique that reconstructs the three-dimensional pore network, distinguishing open from closed porosity and revealing internal voids, bubbles, and delamination without sectioning the sample.
Applications
Substrate glass porosity determination supports quality assurance and product development across multiple industries:
- Flat panel display manufacturing: verifying densification and hermetic integrity of Gen 4.5 through Gen 10.5 mother glass and cover glass.
- Semiconductor and advanced packaging: qualification of glass interposers, through-glass via (TGV) substrates, and glass core build-up laminates.
- Electronic ceramics and RF modules: process control of LTCC and glass-ceramic substrate sintering lines.
- Photovoltaics and optoelectronics: evaluation of glass carrier substrates and diffusion sources.
- Filtration and separation: characterization of porous glass membranes and catalyst supports.
- Research and failure analysis: densification studies, sintering optimization, and root-cause investigation of field failures.
FAQ
Q: What is the difference between porosity and water absorption?
A: Porosity is a volumetric fraction of voids, whereas water absorption is a mass-based ratio of absorbed liquid to dry specimen mass. Both are obtained from the same Archimedes test data, but they describe different material attributes.
Q: Which method should be selected for closed porosity?
A: Closed porosity cannot be measured by liquid immersion or mercury intrusion alone. It is determined by combining true density from helium pycnometry with bulk density from the Archimedes method or dimensional measurement.
Q: What are typical porosity values for display substrate glass?
A: Fully densified display glass substrates normally exhibit total porosity below 0.5 percent, frequently near the detection limit. Glass-ceramic LTCC substrates typically range from 2 to 15 percent depending on the formulation and sintering schedule.
Q: What sample size and quantity are required?
A: Archimedes testing generally requires specimens of at least 1 to 10 grams or practical coupon dimensions of approximately 10 mm × 10 mm, with a minimum of three to five specimens per condition for statistical validity.
Q: How long does porosity testing take?
A: Standard Archimedes and pycnometry tests are usually completed within one to three working days, while mercury intrusion porosimetry and X-ray CT studies may require additional time for sample preparation and data analysis.
Q: Can testing be performed to recognized standards?
A: Yes. Accredited third-party laboratories routinely conduct porosity determination in conformity with ASTM, ISO, and GB standard methods, and can issue test reports suitable for supplier qualification, incoming inspection, and regulatory submission.