V-0 vs VTM-0: Understanding UL 94 Flame Ratings for Thermal Pads

When engineers compare thermal pads for electronic applications, thermal conductivity is usually one of the first specifications they check. But for products used near power electronics, batteries, connectors, and other heat-generating components, flammability can be equally important.

This is where UL 94 ratings come into the discussion.

Two ratings that often cause confusion are UL 94 V-0 and VTM-0. They may look similar on a datasheet, but they are not the same classification and should not be treated as interchangeable.

The difference comes down to the test method, specimen configuration, and type of material being evaluated.

What Is UL 94?

UL 94 is a standard used to classify the flammability characteristics of plastic materials used in electrical and electronic equipment.

The standard includes several test methods, such as:

• HB horizontal burning
• V-0, V-1 and V-2 vertical burning
• 5VA and 5VB vertical burning
• VTM-0, VTM-1 and VTM-2 for thin materials
• HBF, HF-1 and HF-2 for certain low-density foam materials

For thermal interface materials, V-0 and VTM-0 are two ratings that engineers may encounter when reviewing thermal pads, insulating films, tapes, and related materials.

UL describes VTM as a test method for thin materials that generally cannot support themselves in a horizontal position.

V-0 vs VTM-0: What's the Difference?

The easiest way to understand the difference is to look at how the specimens are tested.

UL 94 V-0

The standard V test uses a vertically mounted bar specimen. A 20 mm flame is applied twice, for 10 seconds each time.

The material must meet the V-0 limits for afterflame, total flaming time, afterglow, dripping, and burning toward the holding clamp.

UL 94 VTM-0

The VTM test is intended for thin materials that are not suitable for the standard V test.

A sheet specimen is wrapped around a mandrel to create a cylindrical specimen. The flame is then applied twice, for 3 seconds each time.

The material is evaluated for afterflame, afterglow, burning progression, and flaming drips.

Are V-0 and VTM-0 Equivalent?

No.

This is one of the most important points to remember.

UL explicitly states that:

VTM-0 cannot be considered equivalent to V-0 because the test methods are different.

That does not mean VTM-0 is a "lower" rating.

It means that the two classifications belong to different test methods.

A VTM-0 material has met the requirements of the VTM-0 test. A V-0 material has met the requirements of the V-0 test.

Do V-0 and VTM-0 Have Similar Pass Criteria?

Yes, the main classification limits are closely aligned.

For both V-0 and VTM-0, the top classification includes requirements such as:

• Individual afterflame time of no more than 10 seconds
• Total afterflame time of no more than 50 seconds
• Limited afterflame/afterglow following the second flame application
• No flaming drips that ignite the cotton indicator

However, the flame application procedure is different.

The different test configuration is why the two ratings should not be written as if they were the same certification.

Why Are Some Thin Materials Tested Using VTM?

Imagine trying to hold a very thin flexible film vertically and expose it to a flame.

The material may distort, shrink, curl, or otherwise fail to maintain the geometry required for the standard V test.

The VTM method addresses this type of material.

The specimen is prepared from a sheet approximately 200 mm long and 50 mm wide and wrapped around a small mandrel. This creates a test configuration that is more suitable for thin, flexible materials.

This is why VTM classifications are commonly encountered with materials such as:

• Flexible films
• Thin insulating sheets
• Thermal tapes
• Thin polymer layers
• Flexible electronic materials

Does Thickness Determine Whether a Material Is V-0 or VTM-0?

Thickness matters, but there is no universal thickness cutoff.

It is tempting to use a simple rule such as:

"Below 3 mm = VTM-0."

That rule is not reliable.

The applicable method depends on the material's physical characteristics and whether it is suitable for the standard V test. UL recognition data also show that some materials can have different classifications at different thicknesses.

For this reason, engineers should always check the actual tested thickness, rather than assuming that a rating automatically applies across an entire thickness range.

What Does This Mean When Selecting a Thermal Pad?

For a thermal pad, UL 94 is only one part of the material specification.

A proper selection should consider:

Thermal performance

• Thermal conductivity
• Thermal impedance
• Bond-line thickness
• Contact resistance

Mechanical performance

• Hardness
• Compressibility
• Compression set
• Conformability

Electrical performance

• Dielectric strength
• Volume resistivity
• Insulation requirements

Compliance

• UL 94 rating
• RoHS
• REACH
• Other customer or industry requirements

A thermal pad with excellent thermal conductivity is not necessarily the best choice if it cannot meet the required mechanical or flammability specifications.

How to Check a Supplier's UL 94 Claim

If a thermal pad datasheet states "UL 94 V-0" or "UL 94 VTM-0," ask the supplier for more information.

Check the material grade

Make sure the UL recognition applies to the exact grade being quoted.

Check the thickness

Confirm that the tested thickness covers the thickness used in your design.

Check the test method

Determine whether the classification comes from the V or VTM test.

Check the certification

Ask for the applicable UL recognition or Yellow Card information so that the rating can be independently verified.

UL's Product iQ database can be used to search certified products, components, and materials.

Common Mistakes When Reading UL 94 Ratings

Mistake 1: Assuming V-0 is always better than VTM-0

It isn't.

They are different classifications for different test configurations.

Mistake 2: Assuming a rating applies to every thickness

It doesn't.

Flammability performance can change with specimen thickness.

Mistake 3: Looking only at the datasheet

A datasheet statement should be checked against the applicable certification documentation.

Mistake 4: Treating UL 94 as a complete product fire-safety qualification

UL 94 evaluates material flammability under a defined laboratory test. Final product compliance depends on the applicable end-product standard and design.

V-0 or VTM-0: Which One Should You Specify?

The answer should come from the end-product requirement and the physical characteristics of the material.

A good selection process is:

1. Identify the customer's or end-product standard.
2. Determine the actual TIM thickness and construction.
3. Confirm which UL 94 test method applies.
4. Verify the material's UL recognition at the required thickness.
5. Keep the certification documentation with the engineering qualification records.

This approach avoids one of the most common mistakes in TIM procurement: choosing a material based on a flame-rating label without checking whether that rating actually applies to the material configuration being purchased.

Final Takeaway

UL 94 V-0 and VTM-0 are not two grades on the same scale.

They are classifications obtained through different test methods.

V-0 uses the standard vertical burning test, while VTM-0 is intended for thin materials that are not suitable for that test configuration. The main classification limits are closely aligned, but the specimen geometry and flame application procedures are different.

For thermal pads and other TIMs, the safest approach is to verify three things together: material grade, tested thickness, and test method.

That is much more meaningful than simply asking whether a material is "V-0" or "VTM-0."