I. Incipient Stage: No unified classification system (1930–1971)
Following the Industrial Revolution, plastics and foam materials became widely adopted, with EVA, rubber, and resins extensively used in household appliances, insulation, and electronics; however, there was no standardized flame retardancy evaluation system.
1930s: The American ASTM standard introduced the first generation of combustion testing, measuring only horizontal burning rates and distinguishing between "slow-burning" and "fast-burning," without provisions for vertical combustion or droplet risk assessment.
1950s–1960s: Incidents of fires involving electronic equipment surged dramatically; the presence of molten droplets that could ignite secondary fires posed a significant hazard, yet there was no unified global classification system for vertical flame retardancy.
Key industry challenges: For EVA and conventional rubber, fire resistance can only be assessed visually; test results from different laboratories for the same material are entirely inconsistent; and there is no unified safety standard for export-oriented electronics and PCB industries.
II. Birth of the First Standard: Establishment of a Basic Three-Level System (1972–1990, UL94 Versions 1–3)
In 1972, UL released the first edition of UL94-the world's inaugural classification standard for plastic flame retardancy-laying the foundation for modern flame retardant grading systems.
Two new core tests have been added: horizontal combustion (HB) and vertical combustion.
The original vertical model was divided into only three categories: V-0, V-1, and V-2. The key evaluation criteria were residual flame duration and whether the dripping liquid ignited the cotton pad.
The third edition (1980) refined the testing procedures, which were adopted by the U.S. Department of Defense and became a mandatory reference for electronics and home appliances.
Limitations: Only compatible with thick, rigid plastics; no dedicated testing exists for thin foam materials (e.g., EVA, EPDM foam, silicone foam); flame resistance ratings for thin materials are generally exaggerated and insufficient to meet PCB's high-temperature stress-relief requirements.
III. Graded Improvement: Addition of an ultra-high flame retardancy grade (5V) (1991–2012, Fourth and Fifth Editions)
The fourth edition (1991) and fifth edition (1996) introduced two major critical upgrades, establishing the flame retardancy classification framework that remains in use to this day.
Newly added 5VA and 5VB flame resistance ratings (higher than V-0), simulating scenarios with sustained high temperatures and large-scale open flames;
5VA: Non-perforated plates with no holes, compliant with high-end IC substrate standards, high-temperature press requirements, and mandatory specifications for energy storage materials.
5VB: Allows slight perforations; a universal high-temperature structural component.
Add special testing provisions for thin films and thin foam materials to address testing distortion issues with EVA and EPDM foam.
A pivotal industry milestone: Electronic components, PCBs, and new energy materials now require mandatory UL94 certification labeling, while low-grade combustible HB-grade foam materials are gradually being phased out of industrial production lines.
Early environmental concerns: Europe detected dioxins produced when brominated flame retardants burn, prompting restrictions on traditional halogen-based flame-retardant EVA foam.
IV. Refined Iteration: Unified Global Testing Standards (6th Edition, 2013)
The sixth edition of UL94 was released in 2013, significantly tightening testing tolerances to align with global foreign trade supply chains.
Standardize the flame height, calorific value, sample dimensions, and environmental temperature/humidity of the BenSheng lamp to eliminate testing discrepancies among laboratories in China, the United States, and Europe.
The impact of thickness on flame retardancy rating is clear: for the same silicone foam, a 3 mm thick sample classified as V-0 may be downgraded to V-1 if the foam layer is only 0.5 mm thick.
Compliant with international ISO and IEC flame resistance standards; PCB consumables exported to the EU and Southeast Asia are uniformly certified under UL94 classification.
Environmental regulations have been implemented concurrently: the EU's RoHS directive restricts polybrominated flame retardants, prompting the industry to shift toward halogen-free V0 foam silicone and phase-out halogen-containing flame-retardant materials such as EVA and EPDM.
V. Latest Modern Standards: Dual Requirements for Environmental Protection and High-Temperature Operation (7th Edition, 2023, currently effective)
The 7th edition released in 2023 is the globally standardized version, featuring core enhancements optimized for foam cushioning materials and PCB thermal pressing applications.
Refine the identification criteria for thin foam sponges, distinguishing their combustion characteristics among closed-cell silicone, open-cell EPDM, and soft EVA.
Enhance flame retardancy stability testing after high-temperature aging: The material's flame retardancy rating must not show significant degradation when exposed to prolonged heating at 200°C.
Addressing critical industry challenges: EVA and conventional EPDM flame retardants exhibit high-temperature leaching; repeated thermal compression causes degradation from V0 to HB, rendering them unsuitable for PCB production lines.
Flame-retardant foam silicone, renowned for its high-temperature resistance and absence of phase separation while maintaining a V0 fire rating, has become the mainstream material for high-temperature thermal insulation applications.
New criteria for low-smoke and non-toxic combustion byproducts have been introduced to align with workshop fire safety requirements and clean production standards for printed circuit boards.
Global trend: Flame retardancy ratings now encompass not only "self-extinguishing upon exposure to fire," but also three additional criteria-temperature resistance durability, halogen-free environmental friendliness, and no leaching.