Flame retardancy evaluation of insulation films for flexible printed circuits (FPC) is essential for verifying fire safety compliance in consumer electronics, automotive systems, and medical devices. This article explains the key materials involved, the flammability ratings such as UL 94 V-0, and the internationally recognized test methods—including IEC 60695, ISO 4589-2, and ASTM D2863—used by accredited third-party laboratories to assess coverlay and polyimide film performance.
Technical Overview
Flexible printed circuits (FPC) rely on thin polymeric insulation layers to provide electrical isolation, mechanical flexibility, and environmental protection. These insulation films—typically polyimide (PI), polyester (PET), or liquid crystal polymer (LCP) substrates combined with adhesive-based coverlays—are inherently thin and often operate in close proximity to current-carrying conductors. Under fault conditions such as overcurrent, short circuit, or external thermal events, the insulation film can ignite and propagate flame, making systematic flame retardancy evaluation a critical element of FPC material qualification and product safety certification.
Flame retardancy assessment for FPC insulation films follows a layered framework: material-level classification of the polymeric film and adhesive system, component-level verification of the finished coverlay or laminated structure, and end-product compliance under safety standards such as IEC 62368-1 and IEC 60695-based fire hazard testing. Flame retardant performance is typically achieved through halogenated flame retardant systems or increasingly through halogen-free alternatives based on phosphorus-nitrogen chemistry and inorganic fillers, driven by environmental requirements such as the restriction of hazardous substances regulations.
- Material safety objective: prevent ignition and flame propagation under electrical or thermal fault conditions
- Regulatory driver: compliance with UL, IEC, EN, and GB framework requirements for electronic equipment
- Verification approach: standardized laboratory burning tests on conditioned film specimens of defined thickness
Testing Samples
Base Insulation Films (Polyimide and Polyester)
Raw substrate films such as 12.5 µm, 25 µm, and 50 µm polyimide film or PET film are tested as received from the material manufacturer. Because flammability classification is thickness-dependent, each commercial thickness grade must be evaluated separately, with specimens cut to the standard dimensions of 125 mm × 13 mm for vertical and horizontal burning tests.
Coverlay Films
Coverlay—polyimide film coated with a semi-cured acrylic, epoxy, or modified adhesive layer—is the most common FPC insulation and protection material. Specimens are prepared from fully laminated constructions that reflect the actual press cycle, adhesive flow, and surface conditions used in production, since adhesive formulation and lamination parameters directly influence ignition behavior.
Adhesive and Bonding Films
Pure adhesive films, bonding sheets, and thermally cured epoxy or acrylic systems used in multilayer FPC and rigid-flex assemblies are evaluated as free-standing films. Where the adhesive is too thin or flexible to self-support, specimens may be supported according to the applicable test protocol, and this support condition must be documented in the test report.
Finished Flexible Circuits and Assemblies
Circuit-level verification is performed on finished FPC sections or assembled modules to confirm that the complete construction—including copper patterns, solder mask, stiffeners, and connectors—maintains the intended flammability classification. Specimen conditioning is typically 48 hours at 23 ± 2 °C and 50 ± 5 % relative humidity prior to testing.
Testing Items
The evaluation scope for FPC insulation film flame retardancy generally includes the following test items, selected according to the target market and applicable product safety standard:
- UL 94 vertical burning classification: V-0, V-1, or V-2 rating based on afterflame time, afterglow, and cotton ignition by flaming drips
- UL 94 horizontal burning classification: HB rating based on linear burning rate across a marked gauge length
- UL 94 advanced vertical classifications: 5VA and 5VB for enhanced requirements including specimen burn-through resistance
- Limiting oxygen index (LOI): quantitative determination of the minimum oxygen concentration supporting combustion, tested per ISO 4589-2, ASTM D2863, or GB/T 2406.2
- Glow wire flammability index (GWFI): per IEC 60695-2-12, assessing ignition and self-extinguishing behavior against a heated glow wire
- Glow wire ignition temperature (GWIT): per IEC 60695-2-13, determining the temperature at which ignition occurs
- Needle flame test: per IEC 60695-11-5, simulating small fault-source flame exposure
- Smoke density and combustion gas analysis: evaluation of smoke opacity and toxic or corrosive gas release relevant to enclosed electronic enclosures
- Halogen content verification: supporting halogen-free claims through measurement of chlorine and bromine content, typically per IEC 61249-2-21 limits
Testing Methods
UL 94 Vertical Burning Test (V-0 / V-1 / V-2)
Specimens of 125 mm × 13 mm are mounted vertically and exposed twice to a 20 mm methane flame for 10 seconds each. Afterflame times t1 (after first application) and t2 (after second application) are recorded along with afterglow time and whether flaming drips ignite surgical cotton placed below the specimen. Five specimens are tested in the first set; if results fall near classification limits, a second set of five specimens is required. For the highest V-0 classification, individual afterflame times must not exceed 10 seconds, the total afterflame time for the set must not exceed 50 seconds, and no cotton ignition is permitted. The equivalent international procedure is described in IEC 60695-11-10 and GB/T 2408.
UL 94 Horizontal Burning Test (HB)
Specimens are clamped horizontally at one end with a 45° tilt at the free end, and the flame is applied for 30 seconds. The burning rate is measured between 25 mm and 100 mm reference marks. A typical HB classification requires either no burning past the 100 mm mark or a burning rate not exceeding 40 mm/min for thin materials, subject to the thickness-dependent criteria defined in the standard.
Limiting Oxygen Index Test
Per ISO 4589-2, ASTM D2863, and GB/T 2406.2, specimens are supported vertically in a glass chimney while a controlled nitrogen-oxygen mixture flows upward. A small flame is applied to the specimen top, and the oxygen concentration is adjusted to determine the minimum level that just supports candle-like combustion for 3 minutes or a 50 mm burn length. FPC insulation materials intended for demanding applications commonly demonstrate LOI values above 30 %, and polyimide films frequently exceed 40 %.
Glow Wire Tests
Per IEC 60695-2-11, -12, and -13, a nickel-chromium glow wire heated to a specified temperature (typically 550 °C to 960 °C) is pressed against the specimen surface. GWFI determines whether the specimen ignites and extinguishes within a defined time without igniting surrounding tissue paper, while GWIT identifies the temperature 25 K above the highest temperature at which ignition occurs within the contact period. These tests are mandatory fire hazard assessments for many electrical equipment standards.
Needle Flame Test
Per IEC 60695-11-5, a small propane flame approximately 12 mm high is applied to the specimen for a defined exposure time. The assessment considers whether the specimen ignites, whether the flame self-extinguishes, and whether a pine board or tissue paper beneath the specimen is ignited by burning drops.
Applications
Flame retardancy evaluation of FPC insulation films supports material qualification and product certification across multiple industries:
- Consumer electronics: smartphones, tablets, laptops, and display modules where high-density flexible circuits operate close to batteries and power management circuits
- Automotive and electrification: battery management systems, charging connectors, lighting harnesses, and in-vehicle sensor modules requiring compliance with automotive fire safety requirements
- Medical devices: wearable monitors, imaging probes, and implantable-adjacent electronics where material biocompatibility and fire safety must coexist
- Telecommunications and data centers: high-speed flexible interconnects and antenna modules in servers and 5G infrastructure
- Aerospace and industrial equipment: flexible harnesses and control circuits exposed to demanding thermal environments
- Material development: comparison of halogen-free versus halogenated flame retardant formulations during new coverlay and adhesive development
FAQ
What is the difference between UL 94 V-0 and V-2?
Both classifications use the same vertical burning test, but V-0 requires shorter afterflame times (maximum 10 seconds per ignition) and no ignition of the cotton indicator, whereas V-2 permits flaming drips that may ignite cotton. For most FPC insulation applications, V-0 is the specified target classification.
Why must each film thickness be tested separately?
Flammability performance is thickness-dependent: thinner specimens generally ignite more easily and burn faster. Standards organizations and certification schemes therefore assign ratings only to the specific thickness range actually tested, and extrapolation to untested thicknesses is not permitted without additional evaluation.
How does the glow wire test differ from the UL 94 test?
UL 94 applies an open flame to simulate exposure to a small ignition source, while glow wire tests press a heated element against the specimen to simulate thermal stress from overloaded or glowing components. Many end-product safety standards, such as IEC 62368-1, require glow wire data for materials located near potential ignition sources.
What does halogen-free mean for insulation films?
Halogen-free FPC materials are generally defined per IEC 61249-2-21, limiting chlorine to below 900 ppm, bromine to below 900 ppm, and total halogen content to below 1500 ppm. Halogen-free flame retardancy is typically achieved with phosphorus-nitrogen or inorganic filler systems and is verified through combustion ion chromatography analysis.
How many specimens are required for a full evaluation?
A standard UL 94 vertical burning series requires five specimens per set, with two sets (ten specimens) needed when the first set shows borderline results. LOI testing typically requires fifteen to twenty specimens to bracket the critical oxygen concentration. Exact sample quantities should be confirmed with the accredited third-party laboratory performing the evaluation.
How long does flame retardancy evaluation take?
Standard burning tests such as UL 94 or LOI can often be completed within one to two weeks, including specimen conditioning. Programs involving formal component recognition, multiple thickness grades, or additional smoke and toxicity assessments require additional time for documentation and, where applicable, factory inspection steps.