The horizontal burning test is a fundamental flammability evaluation for resin fiberglass boards — glass-fiber-reinforced thermoset laminates such as FR4, G10, and G11 that are widely used in electrical insulation and printed circuit board applications. Performed in accordance with recognized standards including UL 94, IEC 60695-11-10, ASTM D635, and GB/T 2408, the test determines the linear burning rate, ignition behavior, and flame propagation characteristics of the material. This article provides a comprehensive overview of the testing samples, evaluated parameters, test methodology, applications, and frequently asked questions related to horizontal burning tests for resin fiberglass boards.
Technical Overview
Resin fiberglass boards are composite laminates manufactured by impregnating woven glass fiber cloth with thermosetting resins — most commonly epoxy, phenolic, or polyester — and consolidating the stack under elevated temperature and pressure. The resulting material combines high mechanical strength, excellent electrical insulation, strong dimensional stability, and reliable thermal performance, which explains its widespread use in electronics, electrical equipment, and structural applications. However, because these boards are frequently installed near energized circuits, connectors, and other potential ignition sources, their flammability behavior must be quantified before the material can be qualified for end use.
The horizontal burning test is a small-scale laboratory flame test in which a horizontally supported bar-shaped specimen is exposed to a low-energy (nominally 50 W) diffusion flame. The test measures whether and how quickly the specimen ignites, how far the flame front propagates, and whether the material self-extinguishes. Results are expressed as a linear burning rate in mm/min and, depending on the standard applied, as a flammability classification such as HB (Horizontal Burning) under UL 94 or an equivalent category under IEC 60695-11-10 and GB/T 2408. Manufacturers, design engineers, and accredited third-party laboratories rely on these results for material selection, quality control, and regulatory compliance.
Testing Samples
Correct specimen selection and preparation are essential for obtaining representative and reproducible results. The following sample categories are commonly submitted for horizontal burning tests:
Epoxy Glass Fiber Laminates (FR4, G10, G11)
Specimens are cut from finished laminate sheets at production thickness, representing the standard substrate material used for printed circuit boards, insulation panels, and machined electrical components. Both flame-retardant (FR4) and non-flame-retardant (G10) grades may be evaluated to determine their respective burning classifications.
Phenolic and Polyester Resin Fiberglass Boards
Boards based on phenolic or unsaturated polyester resin systems, frequently used in higher-temperature or cost-sensitive applications, are tested to verify their burning rate and suitability for specific end-use requirements.
Finished Parts and Machined Components
Insulation barriers, terminal boards, busbar supports, gaskets, and other machined parts may be evaluated as representative specimens, provided they can be prepared to the required bar geometry.
Specimen Preparation and Conditioning Requirements
- Standard specimen dimensions of 125 mm × 13 mm, with the thickness equal to the actual end-use thickness of the material.
- Specimens are typically tested at both the minimum and maximum thickness of the declared thickness range, since burning behavior varies with thickness.
- Reference marks are applied at 25 mm and 100 mm from the end exposed to the flame to enable burning rate calculation.
- Edges are cut cleanly and smoothly, free of burrs, cracks, and delamination that could influence flame propagation.
- Prior to testing, specimens are conditioned at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a minimum of 48 hours.
- A minimum of three specimens per condition is generally tested, with additional sets evaluated if inconsistent results are observed.
Testing Items
The horizontal burning test evaluates a defined set of flammability parameters for each specimen:
- Ignition behavior: whether the specimen ignites when exposed to the applied flame and the time required for ignition.
- Afterflame time: the duration for which flaming combustion persists after the ignition source is removed.
- Afterglow time: the duration of glowing combustion without a visible flame following flame extinction.
- Linear burning rate: the speed of flame front propagation in mm/min, measured between the 25 mm and 100 mm reference marks.
- Extent of burning: the distance the flame front travels and whether it reaches or passes the 100 mm reference mark.
- Flaming drips: whether flaming particles or drops are released that could ignite adjacent materials such as cotton indicators.
- Classification compliance: verification of conformity with the applicable criteria, for example the UL 94 HB class.
Testing Methods
Horizontal burning tests for resin fiberglass boards are conducted according to internationally and nationally recognized flammability standards, including:
- UL 94, Tests for Flammability of Plastic Materials for Parts in Devices and Appliances — Horizontal Burning (HB) method.
- IEC 60695-11-10, Fire hazard testing — Part 11-10: Test flames — 50 W horizontal and vertical flame test methods, which defines the FH series of horizontal burning categories.
- ASTM D635, Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position.
- GB/T 2408, the Chinese national standard for the determination of burning characteristics of plastics in horizontal and vertical orientations, technically aligned with IEC 60695-11-10.
- ISO 1210, the international standard corresponding to the 50 W horizontal and vertical flame test methods.
Test Apparatus
The test is performed in a ventilated but draft-free enclosure using a laboratory burner producing a blue flame of 20 mm ± 1 mm in height, a specimen support that holds the bar in the specified horizontal orientation, precision timing devices, and calibrated reference marks. Ambient conditions in the test chamber are monitored and controlled to ensure reproducibility.
Test Procedure
- The specimen is clamped so that its long axis is horizontal and its transverse axis is inclined at 45° to the horizontal, exposing the free end of the bar.
- The burner is tilted at 45° and positioned so that the 20 mm blue flame contacts the end of the specimen.
- The flame is applied for a maximum of 30 seconds, or until ignition occurs, after which the burner is withdrawn.
- Timing begins when the flame front reaches the 25 mm reference mark and ends when the flame front reaches the 100 mm mark or when flaming combustion ceases, whichever occurs first.
- Afterflame time, afterglow time, extent of burning, and any flaming drips are recorded for each specimen.
- The linear burning rate is calculated as the distance traveled (75 mm between marks) divided by the measured elapsed time.
- The test is repeated for the full set of specimens, and results are evaluated against the applicable acceptance criteria.
Acceptance Criteria (UL 94 HB Class)
A resin fiberglass board is classified HB when it meets any one of the following criteria:
- The burning rate does not exceed 40 mm/min for specimens with a thickness between 3.0 mm and 5.0 mm.
- The burning rate does not exceed 75 mm/min for specimens with a thickness less than 3.0 mm.
- Burning ceases before the flame front reaches the 100 mm reference mark.
Under the IEC 60695-11-10 and GB/T 2408 frameworks, analogous horizontal burning categories are assigned based on whether the flame front reaches the reference mark, the measured afterflame and afterglow times, and the observed burning rate.
Applications
Horizontal burning test data for resin fiberglass boards support material qualification across a broad range of industries:
- Printed circuit board substrates and copper-clad laminates for consumer, industrial, and automotive electronics.
- Electrical insulation components such as terminal boards, busbar supports, phase barriers, and switchgear spacers.
- Transformer insulation structures, layer washers, and winding supports.
- Rail transit and aerospace interior components requiring documented flammability performance.
- Structural and mechanical parts, including gears, gaskets, and mounting plates, used near heat or ignition sources.
- Building and construction applications where flame propagation characteristics of composite panels must be demonstrated.
FAQ
What is the difference between the HB class and the V classes in UL 94?
The HB class is based on a horizontal specimen orientation and evaluates the linear burning rate, whereas the V classes (V-0, V-1, V-2) use a vertical orientation with repeated flame applications and impose strict afterflame time, afterglow, and flaming drip limits. The HB class represents the least stringent flammability rating in the UL 94 system and is suitable for materials in applications where lower fire exposure is anticipated or where large thick sections are involved.
Why must specimens be tested at different thicknesses?
Burning behavior is thickness-dependent: thin sections generally ignite more easily and burn faster than thick sections. Testing at both the minimum and maximum thickness of the declared range ensures that the assigned classification is valid for the entire product range offered to customers.
How many specimens are required for a horizontal burning test?
A minimum of three specimens per set is typically tested for each thickness and condition. If a single specimen exhibits behavior inconsistent with the others, an additional set is tested and evaluated in accordance with the governing standard before a final classification is assigned.
How long does testing take, including sample conditioning?
Conditioning requires at least 48 hours, and the mechanical testing itself is short, typically completed within one working day once conditioned specimens are available. Overall turnaround, including report issuance by an accredited third-party laboratory, is commonly within five to ten working days depending on the scope of the test program.
Can horizontal burning test results be compared between different laboratories?
Yes, provided the tests are performed under the same standard, specimen geometry, thickness, and conditioning regime. Accredited third-party laboratories operating under ISO/IEC 17025 quality management systems maintain calibrated equipment and controlled procedures to ensure inter-laboratory reproducibility.