Resin Fiberglass Board Vertical Burning Test: UL 94 and IEC 60695-11-10 Flammability Testing Explain

The vertical burning test is the industry-standard method for evaluating the flame-retardant performance of resin fiberglass boards, the glass fabric reinforced laminates widely used as electrical insulation materials and printed circuit board substrates. This guide explains how resin fiberglass sheets are classified under UL 94 and IEC 60695-11-10, covering specimen preparation, the 50 W burner procedure, V-0/V-1/V-2 rating criteria, and typical industrial applications. Whether you are qualifying FR-4 laminate for electronics or insulation panels for switchgear, understanding the vertical burning test is essential for material selection and regulatory compliance.

Technical Overview

Resin fiberglass board, also known as glass fiber reinforced laminate, FR-4, G10, or G11 sheet, is a thermosetting composite in which woven glass fabric is impregnated with resin, typically epoxy, phenolic, or silicone, and cured under heat and pressure. Because these boards are routinely installed in electrical equipment, enclosures, and electronic assemblies, their ability to resist ignition and self-extinguish after removal of a flame is a critical safety property.

The vertical burning test evaluates this property by applying a small, standardized burner flame to the lower edge of a vertically oriented specimen and measuring how long the material continues to burn (afterflame), glow (afterglow), and whether it releases flaming drips capable of igniting combustible material placed beneath it. The vertical orientation is deliberately severe: molten resin and flaming particles fall away from the specimen, simulating worst-case behavior in real installations.

The most widely applied test methods and classification schemes include:

• UL 94, the North American standard for tests for flammability of plastic materials for parts in devices and appliances, which defines the V-0, V-1, and V-2 vertical ratings.
• IEC 60695-11-10, the international fire hazard testing standard specifying 50 W horizontal and vertical flame test methods, technically aligned with the UL 94 small-scale vertical test.
• GB/T 5169.16, the Chinese national standard adopting the IEC 60695-11-10 method.
• ASTM D3801, a comparable method for measuring the comparative burning characteristics of solid plastics in a vertical position.
• ISO 9773, which covers vertical burning tests for thin, flexible specimens that require rod support.

For more demanding requirements, the 500 W flame tests of UL 94 and IEC 60695-11-20 define the higher 5VA and 5VB classifications, which may be specified for enclosure materials and fire barriers.

Testing Samples

Specimen Dimensions and Preparation

Standard vertical burning specimens for resin fiberglass board are rectangular strips measuring 125 mm in length by 13 mm in width, cut at the thickness at which the material will be marketed. Specimens must be cut cleanly with smooth edges, free of burrs, cracks, and delamination, since edge quality can influence afterflame behavior. Reference marks are made at 25 mm and 100 mm above the lower edge to assist in observing flame spread.

Because flammability ratings are thickness-dependent, the material is normally evaluated at both its minimum and maximum commercial thicknesses, and the reported rating applies only to the thicknesses actually tested. A full evaluation typically requires two sets of five specimens per thickness, with retests performed when borderline results occur.

Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 48 hours, in accordance with IEC 60695-11-10 and UL 94 conditioning requirements.

Material Types Commonly Submitted

• FR-4 epoxy glass cloth laminate, the standard substrate for rigid printed circuit boards.
• G10 and G11 epoxy glass laminates used for electrical and mechanical insulation.
• Phenolic resin fiberglass laminates for higher-temperature insulation applications.
• Silicone resin glass fabric boards used as flexible heat-resistant barriers and arc shields.
• Halogen-free flame-retardant laminates developed for environmentally sensitive applications.
• Thin or flexible sheets below the self-supporting limit, which are tested with mandrel support in accordance with ISO 9773.

Testing Items

During a vertical burning test on resin fiberglass board, the laboratory records and evaluates the following parameters for each specimen:

• Afterflame time after the first flame application, designated t1.
• Afterflame time after the second flame application, designated t2.
• Afterglow time after the second flame application, designated t3, measured after flaming ceases.
• Whether flaming drips or falling particles ignite the indicator cotton placed beneath the specimen.
• Whether flaming or glowing reaches the specimen clamp or propagates the full length of the specimen.
• Visual observations such as charring length, resin regression, delamination, and glass fiber exposure.
• The final classification achieved: V-0, V-1, V-2, or no rating (NR) if none of the criteria are met.

Testing Methods

Apparatus and Test Setup

The test is conducted in a draft-free enclosure equipped with an observation window, an exhaust system for combustion products, and a calibrated burner. A 50 W premixed methane or natural gas flame is adjusted to produce a blue flame of 20 ± 1 mm in height. The specimen is clamped at its upper end so that its long axis is vertical, and the burner tube is positioned 10 mm below the bottom edge of the specimen. A pad of dry, absorbent surgical cotton approximately 50 mm by 50 mm and no more than 6 mm thick is placed 300 mm below the point of flame application to catch flaming drips.

Test Procedure

• The burner flame is applied to the center of the specimen's lower edge for 10 ± 0.5 seconds, then withdrawn, and the first afterflame time t1 is recorded.
• Immediately after flaming ceases, the flame is reapplied for a second 10 ± 0.5 second interval, then withdrawn again.
• The second afterflame time t2 and, once flaming stops, the afterglow time t3 are recorded, together with any ignition of the cotton indicator.
• The procedure is repeated for all five specimens in the set, and the individual and cumulative afterflame times are compared against the classification criteria.
• If only one specimen fails a criterion within defined limits, a second set of five specimens may be tested and must satisfy stricter cumulative requirements for a rating to be granted.

Classification Criteria

The observed results are compared with the following criteria, where V-0 represents the most stringent vertical burning classification:

• V-0: afterflame time for each individual application does not exceed 10 seconds; the total afterflame time for the set does not exceed 50 seconds; afterflame plus afterglow after the second application does not exceed 30 seconds; no flaming drips ignite the cotton; no flaming reaches the clamp.
• V-1: individual afterflame time does not exceed 30 seconds; total afterflame time for the set does not exceed 250 seconds; afterflame plus afterglow after the second application does not exceed 60 seconds; no flaming drips ignite the cotton.
• V-2: identical timing limits to V-1, but brief flaming drips that ignite the cotton indicator are permitted.
• NR (no rating): assigned when specimens exceed the V-2 criteria, burn to the clamp, or exhibit excessive flaming.

The final test report states the classification obtained, the specimen thickness, the conditioning regime, the gas and flame parameters used, and detailed observations for every specimen. End-product safety standards such as IEC 62368-1, IEC 60335-1, IEC 60947 series, and UL 746A frequently reference these ratings when defining material requirements.

Applications

Resin fiberglass boards perform well in vertical burning tests because the glass fabric is non-combustible and provides a structural skeleton that limits flame spread, while the resin phase is formulated with reactive or additive flame retardants, or with inherently char-forming chemistries, to achieve rapid self-extinguishing behavior. This combination allows many commercial laminates to attain V-0 classification even at thin gauges. Typical applications that depend on these verified ratings include:

• Printed circuit board substrates and insulation sheets for consumer, industrial, and automotive electronics.
• Busbar supports, phase barriers, and mounting panels in low-voltage and medium-voltage switchgear.
• Transformer spacers, layer insulation, and arc-resistant barriers in power distribution equipment.
• Slot insulation, wedges, and structural insulating components in electric motors and generators.
• Thermal and electrical barriers in electric vehicle battery modules and power electronics assemblies.
• Rail transit and aerospace interior components subject to stringent fire safety requirements.
• Terminal boards, LED lighting substrates, and insulating parts within household appliances.

Manufacturers supplying these industries typically commission vertical burning tests at accredited third-party laboratories or maintain in-house facilities validated against the referenced standards, so that material certifications remain current with each formulation or thickness change.

FAQ

What does a UL 94 V-0 rating mean for a resin fiberglass board?

A V-0 rating means that when a 20 mm flame is applied twice for 10 seconds to the bottom edge of a vertical specimen, each afterflame period lasts no more than 10 seconds, the total afterflame time for the five-specimen set is no more than 50 seconds, and no flaming drips ignite the cotton placed below. It is the highest standard vertical rating and indicates excellent self-extinguishing performance.

Is IEC 60695-11-10 the same as the UL 94 vertical burning test?

The two methods are technically harmonized: both use a 50 W flame, 125 mm by 13 mm specimens, two 10-second flame applications, and identical V-0/V-1/V-2 criteria. Test reports from accredited third-party laboratories conducted to either standard are generally accepted interchangeably, although some end-product specifications explicitly name one standard.

Why must samples be tested at different thicknesses?

Burning behavior depends strongly on specimen thickness, because thicker sections store more heat and resin mass while conducting heat away more slowly. A rating obtained at one thickness is valid only for that thickness, so materials are evaluated at both the thinnest and thickest forms offered to establish the complete certified thickness range.

How many specimens are required for a full evaluation?

A standard evaluation uses two sets of five conditioned specimens per thickness. If one set produces a single marginal result, a retest set of five additional specimens may be run under the stricter cumulative criteria defined in UL 94 and IEC 60695-11-10.

Can halogen-free resin fiberglass boards achieve a V-0 rating?

Yes. Modern halogen-free laminates use phosphorus-based, nitrogen-based, or inorganic filler flame-retardant systems, often combined with the inherent char-forming tendency of certain resins and the non-combustible glass fabric, and many commercially achieve V-0 classification without brominated or chlorinated compounds.

How long does the vertical burning test take?

The physical test for a single set of five specimens is completed within an hour, but a full program including conditioning (minimum 48 hours), testing at multiple thicknesses, and report issuance typically requires one to two weeks at an accredited third-party laboratory, depending on sample throughput.