UL94 Flame Rating for Potting Compounds: V-0, V-1, V-2, and HB Explained

What Do UL94 Flame Ratings V-0, V-1, V-2, and HB Mean for Potting Compounds?

Figure 1. UL94 classifies materials by how they respond to ignition - not by whether they catch fire. V-0, V-1, V-2, and HB describe measurably different behaviors under the same test condition. | Fong Yong Chemical Co., Ltd.

UL94 is a flammability standard developed by Underwriters Laboratories that classifies plastic and encapsulant materials by how they respond to an ignition source, not by whether they will burn. There are four main ratings, from most to least flame-resistant: V-0 - the material self-extinguishes within 10 seconds after the ignition source is removed, with no burning drips; V-1 - self-extinguishes within 30 seconds, no burning drips; V-2 - self-extinguishes within 30 seconds, burning drips are permitted; HB (horizontal burn) - the material slows the rate of flame spread when burned horizontally, but does not self-extinguish when held vertically. HB is the lowest classification and is not considered flame-retardant by most regulatory bodies or end-product safety standards. Most electronics safety standards - including IEC 62368-1, UL 1709, and automotive OEM material specifications - require potting and encapsulant materials to be rated V-0 or V-1 minimum. Specifying an HB-rated potting compound for electronics applications where V-0 or V-1 is required by the end-product safety standard creates a certification gap. To confirm which UL94 rating applies to a specific Fong Yong formula, request the Technical Data Sheet (TDS) and confirm the UL94 classification listed for that product.

What UL94 Measures - and What It Doesn't

UL94 measures a material's response to an ignition source applied under controlled laboratory conditions. Specifically, it tests:

• Whether the material self-extinguishes after the ignition source is removed
• How long it takes to self-extinguish
• Whether the material drips burning particles that could ignite secondary materials below it

UL94 does not measure:

• Whether the material will catch fire in real-world conditions
• How the material behaves in a sustained fire (only a brief laboratory ignition)
• Smoke generation or toxic gas emission during combustion
• Whether the encapsulated assembly as a whole meets any fire safety standard

UL94 is a material classification, not a product safety certification. A UL94 V-0 rated potting compound contributes to an electronics assembly meeting end-product safety requirements - it does not by itself certify the assembly. The end-product safety standard (IEC 62368-1, UL 1012, etc.) determines what material rating is required.

The Four Ratings: HB, V-2, V-1, and V-0

UL94 ratings are assigned based on test performance. Listed from least to most flame-resistant:

HB - Horizontal Burn
The specimen is tested horizontally. The material must slow the rate of flame spread below a defined threshold, but is not required to self-extinguish. HB is the minimum classification for any material that passes UL94 testing. It is not considered flame-retardant by most regulatory bodies. End-product safety standards that require flame resistance specify V-2 or higher - not HB.

V-2 - Vertical Burn, Drips Permitted
The specimen is tested vertically. After a 10-second flame application, the material must self-extinguish within 30 seconds. Burning drips are permitted (the cotton indicator below the specimen may ignite). V-2 indicates the material self-extinguishes, but the burning drips it produces can ignite secondary materials in the assembly or enclosure.

V-1 - Vertical Burn, No Drips
Same test as V-2, but the material must self-extinguish within 30 seconds without producing burning drips. The cotton indicator below the specimen must not ignite. V-1 is the minimum rating for many commercial electronics and building automation applications.

V-0 - Vertical Burn, Self-Extinguishes in ≤10 Seconds, No Drips
The strictest rating in the standard V series. After 10-second flame application, the material must self-extinguish within 10 seconds. No burning drips. The cotton indicator must not ignite. V-0 is required by most consumer electronics, automotive, and medical device safety standards where flame containment is critical.

How the UL94 Vertical Burn Test (V Series) Works

Figure 2. UL94 V-series vertical burn test setup. The specimen is held vertically in a clamp. A defined flame is applied to the bottom edge for 10 seconds. After flame removal, self-extinction time is measured. A cotton indicator below detects burning drips.

The test procedure for V-0, V-1, and V-2 ratings is standardized:

Specimen preparation: The material is cast into a bar of defined dimensions (typically 125 mm × 13 mm × 3.2 mm or other specified thickness). Five specimens are tested per rating claim.

Conditioning: Specimens are conditioned at 23°C / 50% RH for 48 hours before testing.

Flame application: A defined Bunsen burner flame is applied to the bottom edge of the vertically held specimen for exactly 10 seconds, then removed.

First timing: The time for the specimen to self-extinguish is recorded (t₁).

Second flame application: If the specimen self-extinguishes, the flame is applied again for 10 seconds.

Second timing: The time for the second self-extinction is recorded (t₂).

Drip evaluation: A cotton indicator is positioned below the specimen to detect burning drips. Any ignition of the cotton fails the specimen for V-0 and V-1.

V-0 requires: t₁ ≤ 10s, t₂ ≤ 10s, total of all five specimens ≤ 50s, no burning drips.

V-1 requires: t₁ ≤ 30s, t₂ ≤ 30s, total of all five specimens ≤ 250s, no burning drips.

V-2 requires: t₁ ≤ 30s, t₂ ≤ 30s, total of all five specimens ≤ 250s, burning drips permitted.

Which Rating Does Your Application Actually Require?

The required UL94 rating depends on the end-product safety standard that applies to the assembly, not on assumptions about the application environment. Common application-to-standard mappings:

• Consumer electronics (audio, video, IT equipment): IEC 62368-1 requires V-1 minimum for encapsulants and insulation materials in many configurations; V-0 is required in specific fire-enclosure contexts.
• Automotive electronics: OEM material specifications typically require V-0 for potting and encapsulant materials used in vehicle electronics. USCAR, AEC-Q200 qualified components, and OEM supplier requirements commonly cite V-0.
• Medical devices: IEC 60601-1 requires V-1 minimum for patient-contact and near-patient equipment. Class III or implantable device encapsulants may have additional requirements beyond standard UL94.
• Industrial control and power equipment: IEC 61010-1 and IEC 60950-1/62368-1 require V-1 minimum for many encapsulant applications. V-0 is required where fire containment within the equipment is a design requirement.
• Building automation and HVAC controls: UL 508A and similar standards for industrial control panels typically require V-0 or V-1 for encapsulant materials in control assemblies installed in occupied buildings.
• LED drivers and power supplies: UL 8750 (LED equipment) and UL 1012 (power units) require V-1 or V-0 depending on the output voltage, power level, and installation type.

Figure 3. Minimum UL94 rating by application category. Confirm the specific clause of the applicable end-product safety standard - the minimum required rating varies by installation environment, output voltage, and enclosure type within each category.

The practical recommendation: check the end-product safety standard requirement before specifying a potting compound. Do not assume V-0 is always required - it is over-specified for some applications. Do not assume HB is acceptable - it is insufficient for virtually all regulated electronics applications.

UL94 for Potting Compounds vs Enclosure Materials - an Important Distinction

UL94 applies to both the enclosure (housing, shell) and the internal materials (PCB, potting compound, wire insulation) of an electronics product, but the same rating requirement does not always apply to both.

In many end-product safety standards, the enclosure must meet V-0 while the internal potting compound may be permitted at V-1 if it is fully enclosed within a compliant housing. In other configurations - particularly open-frame or DIN rail-mounted equipment where the internal assembly is exposed - the potting compound must independently meet V-0.

This distinction matters for cost: V-0 flame retardant additives in epoxy formulations typically affect optical clarity, cost, and sometimes mechanical properties. Specifying V-0 for a potting compound in an application where V-1 would satisfy the standard requirement adds cost and potentially compromises other specifications unnecessarily.

Confirm the specific clause of the applicable safety standard before finalizing the UL94 requirement for the potting compound specifically - not just for the finished product generally.

Comparison Table: V-0 vs V-1 vs V-2 vs HB

Verifying a Potting Compound's UL94 Rating Before Specifying It

UL94 ratings for potting compounds and encapsulants are reported in the product's Technical Data Sheet (TDS). When verifying a supplier's UL94 claim:

• Confirm the rating is listed in the TDS, not just stated in marketing material. UL94 rating claims in promotional text without TDS documentation cannot be verified and should not be used for compliance purposes.
• Confirm the thickness at which the rating was tested. UL94 ratings are thickness-dependent. A material rated V-0 at 3.2 mm may not achieve V-0 at 1.6 mm. The TDS should specify the specimen thickness used in testing.
• Confirm the UL Yellow Card listing if required. For product certifications that require verified UL94 data, check the UL Product iQ database (iq.ul.com) for the material's listing under the Yellow Card system. This provides independently verified data beyond the supplier's self-reported TDS values.
• Confirm the cure schedule used for testing. Some epoxy formulations achieve different UL94 ratings depending on whether they were RT-cured or heat-cured. Confirm that the TDS rating applies to the cure schedule you will use in production.

To confirm the UL94 rating for a specific Fong Yong formula, request the Technical Data Sheet directly. Specify the product code, the thickness requirement for your application, and the applicable safety standard so the correct data can be provided.