Water absorption is one of the most critical reliability indicators for fiberglass resin boards such as FR-4, G10, G11, and GPO-3 laminates, directly affecting dielectric performance, dimensional stability, and long-term durability. This article presents a comprehensive technical guide to water absorption evaluation of fiberglass resin boards, covering specimen preparation, test conditions based on ISO 62, ASTM D570, GB/T 1034, and related standards, calculation methods, result interpretation, and typical acceptance criteria. Engineers, quality managers, and material specifiers can use this information to select appropriate materials and validate product quality through accredited third-party laboratories.
Technical Overview
Fiberglass resin boards are composite laminates manufactured by impregnating woven or mat glass reinforcements with thermosetting resins such as epoxy, unsaturated polyester, or phenolic systems, followed by hot pressing and curing. They are widely used as electrical insulation components, PCB base materials, and structural parts because of their excellent dielectric strength, mechanical strength, heat resistance, and flame-retardant properties. However, because cured resin matrices and the fiber-resin interface are mildly hygroscopic, these laminates absorb a small but measurable amount of water when exposed to humid air or direct immersion.
Water absorption evaluation quantifies the mass of water taken up by a conditioned specimen under strictly defined environmental conditions. The absorbed water acts as a plasticizer and conductor, degrading multiple performance characteristics simultaneously. For this reason, water absorption is classified as a fundamental material property in product standards such as IEC 60893 and is routinely verified during incoming inspection, material qualification, and periodic type testing by accredited third-party laboratories.
- Reduced dielectric strength and insulation resistance, measured per IEC 60243-1 and IEC 62631-3
- Increased dissipation factor and permittivity, affecting high-frequency signal integrity
- Dimensional swelling and warping that compromise machining tolerances and assembly fit
- Lowered flexural and tensile strength due to matrix plasticization and interface attack
- Blistering or delamination during thermal shock, wave soldering, or reflow processes
A low and reproducible water absorption value therefore demonstrates a well-cured matrix, a dense fiber-resin interface, and low internal porosity, which together predict stable performance in humid operating environments.
Testing Samples
Standard Specimen Geometry
The preferred specimen for immersion testing is a flat square plaque of 50 mm × 50 mm, taken at the full thickness of the board under evaluation. ISO 62 and GB/T 1034 recommend this geometry with a preferred thickness of 3 mm for molded materials, while for laminate products the actual product thickness is retained because absorption is strongly thickness dependent. ASTM D570 commonly uses rectangular strips of 76.2 mm × 25.4 mm (3 in × 1 in) cut from the sheet at production thickness. When comparing results between suppliers or test reports, the specimen dimensions and thickness must always be reported together with the percentage value.
Specimen Conditioning and Preparation
Specimen preparation has a decisive influence on the accuracy of the result. The standard sequence is as follows:
- Cut specimens cleanly without chipping, cracking, or heat damage, and remove burrs by light polishing with fine abrasive paper
- Dry the specimens in a ventilated oven at 50 ± 3 °C (or as specified by the material standard) until constant mass is reached, typically within 24 hours for thin laminates
- Cool the dried specimens to room temperature in a desiccator over silica gel to prevent re-absorption of atmospheric moisture
- Weigh each specimen immediately on an analytical balance with a resolution of 0.1 mg and record the dry mass as the initial value
- Test at least three specimens per condition and report individual and mean values to account for material homogeneity
Special Test Configurations
For finished components or engineering evaluations, tests can also be performed on machined parts, drilled panels, or cut-to-size insulators to reflect real exposure conditions. In such cases, exposed cut edges, drilled holes, and milled slots increase the effective absorption path, and results are not directly comparable to molded plaque data. Some specifications additionally require testing after humidity storage at 93% relative humidity or after accelerated aging, in which case specimens are preconditioned in a climate chamber before weighing or before electrical retesting.
Testing Items
A complete water absorption evaluation of fiberglass resin boards usually comprises several complementary test items, each probing a different moisture exposure scenario. Common test items include:
- 24-hour cold water immersion absorption at 23 °C, the baseline quality indicator for all laminate grades
- Short-duration immersion, typically 2 hours at 23 °C, used for thin materials and rapid screening
- Boiling water immersion absorption, typically 30 minutes to 2 hours, revealing interface quality and porosity under severe conditions
- Long-term saturation immersion, 7 days or longer, to determine the equilibrium moisture uptake for service modeling
- Humidity absorption at controlled relative humidity, for example 50% RH or 93% RH at 23 °C, simulating atmospheric exposure
- Post-immersion electrical verification, including dielectric strength, insulation resistance, and dissipation factor, to quantify functional impact
The specific test items selected depend on the material specification, the applicable product standard, and the end-use environment. For electrical insulation grades governed by IEC 60893, the 24-hour immersion value at 23 °C and the boiling water value are the two most frequently quoted figures on material datasheets.
Testing Methods
Immersion Method per ISO 62 and GB/T 1034
In the standard immersion method, conditioned and weighed specimens are fully submerged in distilled or deionized water maintained at 23 ± 1.0 °C, with the water volume kept at a high ratio to specimen surface area, for example at least 8 mL per cm² of specimen surface, so that absorbed water does not measurably alter the bath composition. After 24 hours, each specimen is removed from the water, all surface droplets are immediately blotted off with a dry lint-free cloth, and the wet mass is determined within one minute of removal to prevent evaporative loss. For thin specimens below 1 mm, an abbreviated 2-hour immersion period may be applied where specified.
ASTM D570 Procedure
ASTM D570 follows the same gravimetric principle with slightly different details in specimen size, blotting technique, and timing. A widely applied variant for laminate materials is the 2-hour boiling immersion: specimens are immersed in boiling water for 120 minutes, then transferred to room-temperature water for 15 minutes to cool before blotting and weighing. This accelerated condition exposes weak fiber-resin bonding, residual porosity, and under-cured resin, and forms part of the material qualification requirements referenced in UL 746A for polymeric materials used in electrical equipment.
Humidity Exposure Method
For applications where the board is exposed to humid air rather than liquid water, specimens are placed in a conditioning chamber at 23 °C and either 50% RH or 93% RH until equilibrium mass is reached, which may take several days to weeks depending on thickness. The moisture uptake is calculated from the mass difference relative to the dry state. This method is particularly relevant for electronics applications, where humidity absorption correlates with solder-reflow resistance and long-term electrical drift.
Result Calculation and Expression
The water absorption percentage is calculated from the dry mass and the wet mass using the standard gravimetric relationship, expressed as the mass difference divided by the dry mass multiplied by 100. For example, a specimen of 12.0000 g dry mass that weighs 12.0150 g after 24-hour immersion has a water absorption of 0.125%. Some product standards, including IEC 60893, alternatively express the result in milligrams of absorbed water for a defined specimen size, because percentage values vary with thickness while the absolute uptake of a fixed specimen better reflects material quality. Typical 24-hour immersion values at 23 °C are approximately 0.05% to 0.20% for well-cured epoxy glass grades such as G10 and FR-4, and 0.15% to 0.40% for polyester mat grades such as GPO-3, while boiling water values are several times higher. Exact acceptance limits depend on the grade, thickness, and governing specification.
Influencing Factors and Quality Interpretation
Several material and procedural factors must be controlled for meaningful comparison:
- Specimen thickness: thinner specimens show higher percentage absorption in the same period because diffusion distance is shorter
- Cure degree: under-cured resin contains residual reactive groups and free monomers that raise moisture uptake
- Glass content and sizing: silane coupling agents on the fibers improve interface adhesion and lower water ingress
- Voids and porosity: entrapped air or microvoids create capillary paths that sharply increase absorption
- Cut edges: machined edges expose raw fiber and are preferential ingress routes, so edge condition must be consistent
- Water temperature and immersion time: elevated temperature accelerates diffusion, which is why boiling tests are more discriminating
Absorption values significantly above the specification limit usually indicate incomplete cure, excessive porosity, poor interface bonding, or material contamination, and warrant investigation of the lamination process and raw materials.
Applications
Water absorption evaluation supports quality control and material selection across all major application fields of fiberglass resin boards:
- Electrical insulation: slot wedges, phase barriers, spacers, and support bars in transformers, switchgear, motors, and generators operating in humid or outdoor environments
- Printed circuit boards: FR-4 type copper-clad substrates, where moisture control is essential for solder-reflow reliability and stable high-frequency dielectric behavior
- Power electronics: busbar insulation, GPO-3 structural insulation, and arc-resistant panels in low- and medium-voltage distribution equipment
- Transportation: insulation and structural components in railway traction systems, EV battery enclosures, and marine electrical installations
- Renewable energy: insulating parts in wind turbine converters, combiner boxes, and photovoltaic junction equipment exposed to temperature and humidity cycling
- General engineering: chemical-resistant spacers, templates, and machined parts where dimensional stability under humidity is required
For safety-critical and export-oriented products, manufacturers commonly validate water absorption and the associated electrical retention properties through accredited third-party laboratories, with test reports referenced against IEC, ASTM, EN, GB, UL, or JIS requirements as demanded by the target market.
FAQ
What is a good water absorption value for fiberglass resin boards?
For high-quality epoxy glass laminates such as G10 and FR-4, a 24-hour immersion value at 23 °C of 0.10% to 0.20% or lower is generally considered good, while premium grades can achieve below 0.10%. Polyester mat grades such as GPO-3 typically fall between 0.15% and 0.40%. The acceptance limit must always be checked against the specific material grade and thickness range defined in the governing specification.
What are the differences between ISO 62 and ASTM D570?
Both standards use the gravimetric immersion principle, but they differ in preferred specimen dimensions, drying conditions, water bath control, and the available exposure variants. ISO 62 and its Chinese counterpart GB/T 1034 specify a 50 mm × 50 mm specimen with methods for 23 °C immersion, boiling water, and controlled-humidity exposure, whereas ASTM D570 defines its own specimen sizes and a 2-hour boiling procedure widely used in North American qualification programs. Results from the two standards show the same trends but should not be compared numerically without noting the method.
Why is water absorption so important for electrical insulation boards?
Water is electrically conductive relative to the resin matrix, so even small absorbed amounts measurably reduce dielectric strength, lower insulation resistance, and increase dielectric loss. Absorbed moisture also swells the laminate, changing dimensions and internal stress, and can cause blistering during thermal excursions. Low water absorption is therefore a strong predictor of reliable long-term insulation performance.
Does specimen thickness affect the reported result?
Yes. Because moisture enters primarily through the two large faces, thinner specimens reach a higher absorbed percentage in the same immersion time, while thick specimens require longer to saturate. Always compare results at the same thickness, or use the milligram-per-specimen expression provided in standards such as IEC 60893 for thickness-independent assessment.
Can water absorption testing be combined with other evaluations?
Yes, and this is recommended practice. A typical qualification program pairs water absorption with post-immersion dielectric strength per IEC 60243-1, insulation resistance per IEC 62631-3, and comparative tracking index per IEC 60112, together with flexural strength and thermal resistance, giving a complete picture of how moisture affects the material in service. Such combined evaluations are routinely performed by accredited third-party laboratories.