"UL 94V-0" printed on a flex PCB quote is easy to read as a finished-board certificate. It rarely is. UL recognizes constructions built from specific materials, and the flame rating that matters for a thin flex circuit is often a VTM rating, not V-0. This guide explains what the terms mean, why the two ratings are not interchangeable, and which documents to ask for before you accept a flammability claim from any supplier.
It is a specification and verification guide, not a statement about any particular supplier's recognitions. Confirm the current scope of any recognition in UL's database before you rely on it.
Judgment first: how much UL evidence does your flex need?
- Design perspective — decide the requirement from the end product. Whether a flame rating is required, and at what level, comes from the product's safety standard or your customer's specification, not from the flex supplier. Write the required rating and the standard that imposes it on the drawing.
- Manufacturing perspective — the rating belongs to a construction. A recognized flex construction names the base laminate, adhesive, copper and coverlay. Swapping any of them can leave the finished flex outside the recognized construction, so changes need a check against the supplier's recognition.
- Purchasing perspective — ask for the file, not the adjective. A quote line reading "UL 94V-0" is a claim. The evidence is the supplier's UL file number and the listed construction, which you can check yourself.
These are selection recommendations, not a guarantee that any given flex will pass your product's safety review.
| Claim you may see | What it can mean | What to ask for |
|---|---|---|
| "UL 94V-0 material" | The base polymer or laminate is rated V-0 at a stated thickness | Datasheet with the rating, thickness and UL file number |
| "UL 94 VTM-0" | A thin film passed the vertical thin-material test | The datasheet line and file number for each film in the stack |
| "UL recognized flex" | The manufacturer holds a recognition covering listed constructions | The manufacturer's file number and the construction that matches your build |
| "UL certified PCB" | Often imprecise wording for one of the above | A request to restate it as a specific recognition and file |
What UL actually recognizes on a flex circuit
UL does not generally test and certify each individual flex circuit. As one flex manufacturer's article puts it, UL "does not require each flex circuit to be tested and certified, but, rather the materials that make the circuit"; the recognized stack-ups combine polyimide, copper, adhesive and coverlay in defined configurations (Designs on Flex: Underwriters Laboratory and Flexible Printed Circuits).
The standard that applies to the flex constructions is UL 796F. Its scope covers flexible-material printed wiring board constructions used as components in flexible, flex-to-install, rigid and multilayer rigid-flex applications, with and without stiffeners and adhesives. It gives data on physical, electrical, flammability and thermal properties of the construction. UL states that meeting it does not make the product acceptable in an end product without further investigation; the end-use standard still decides (UL 796F summary). Rigid boards fall under UL 796.
UL 94 V-0 in plain terms
UL 94's vertical burning test holds a flame to a clamped specimen twice for 10 seconds each and records how long it keeps burning and whether it ignites cotton below. Five specimens are tested. The criteria for the three vertical ratings are:
| Criterion | V-0 | V-1 | V-2 |
|---|---|---|---|
| Burning time per specimen after a flame application | ≤ 10 s | ≤ 30 s | ≤ 30 s |
| Total burning time, five specimens | ≤ 50 s | ≤ 250 s | ≤ 250 s |
| Afterglow after the second flame application | ≤ 30 s | ≤ 60 s | ≤ 60 s |
| Cotton ignited by drips | No | No | Yes |
| Burns up to the holding clamp | No | No | No |
Source: MG Chemicals, Understanding the UL94 Vertical Burn Test. V-0 is the most demanding of the three. The test is run on a specimen of a stated thickness, and a rating applies to that material at that thickness, not to the polymer family in general.
Why flex often shows VTM-0, not V-0
Polyimide films and coverlays are thin and flexible. UL 94 provides the VTM ratings (VTM-0, VTM-1, VTM-2) for materials too thin or flexible to be tested in the standard vertical test, which can distort or shrink. In the VTM test the film is rolled around a mandrel and held vertically, and the flame is applied for a shorter time; the pass criteria mirror the V ratings (CAPLINQ, UL 94V vs UL 94 VTM). The same source stresses that VTM-0 is not equivalent to V-0, because the test methods differ.
That is why a coverlay data sheet may show VTM-0: for example, DuPont lists its Pyralux FR coverlay products with a UL 94 VTM-0 rating and UL file E124294 (DuPont Pyralux FR). That is one vendor's published datasheet claim, not a statement about your board. Read each film in your stack for its own rating and thickness.
Build the stack so the rating survives
A flame rating is not a property you add at the end; it follows the stack-up. Check these points on the drawing and with the supplier:
- Every material in the build. List base laminate, adhesive, copper, coverlay, stiffener and bond-ply by manufacturer and product name, so each can be matched to a datasheet and a recognition.
- Thickness. A rating applies at a tested thickness. If your build is thicker or thinner than the recognized construction, ask the supplier how that is covered.
- Stiffeners and adhesives. UL 796F covers constructions with and without stiffeners and adhesives, so a stiffener choice can matter. Review it with the supplier; do not assume a generic stiffener is covered. See the flex stiffener service and the coverlay versus solder mask guide.
- Substitutions. If the supplier proposes an alternate laminate or coverlay for availability, require written confirmation that the alternate is inside the recognized construction before approval.
- Operating conditions. Ask for any maximum operating temperature or other limit attached to the recognition, since a recognition may carry conditions.
Our flex material guide compares polyimide, PET and LCP, and the fabrication drawing guide shows where to put material and rating notes.
What to send and what to ask for
| Drawing or RFQ item | Required definition |
|---|---|
| Required flammability rating | The rating and the standard that requires it (end-product or customer specification) |
| Stack-up | Manufacturer and product for each material, thickness of each layer |
| Rating basis | V-0 or VTM-0 per material, with the datasheet and UL file number |
| Supplier recognition | The supplier's UL file number and the listed construction that matches your build |
| Changes | A rule that any material change is reviewed against the recognition before use |
| Evidence at delivery | The document set you expect with the first article and with each lot, as agreed in the quote |
| Verification | You check the file number and listed construction in UL's database yourself |
Send this package for a capability review through the flex PCB service or, for mixed assemblies, the rigid-flex service. A standard quotation does not establish that a particular construction is recognized.
FAQ
Is my flex PCB UL certified?
Ask a more precise question: which recognition covers it, under whose file number, and for which listed construction. UL generally recognizes constructions made from specific materials, so a finished flex is covered only if its build matches a recognized construction. The supplier should give you the file number to verify.
What is the difference between UL 94 V-0 and VTM-0?
Both describe how a material burns after a flame is applied, with similar pass criteria. V-0 is run on a specimen in the standard vertical test; VTM-0 is a variant for thin or flexible films. They are different test methods and are not equivalent, so do not treat a VTM-0 film as V-0 or the reverse.
Does polyimide have a flame rating?
Polyimide films and flex coverlays commonly carry a published UL 94 rating, often VTM-0 for thin films, but the value depends on the product and thickness. Use the datasheet and file number for the actual film in your stack, not a generic statement about polyimide.
Do I need UL 796F for my flex circuit?
Only if your end-product standard or customer requires a recognized flex construction. UL 796F is the standard that covers flexible-material printed wiring board constructions, and UL says meeting it does not by itself make a part acceptable in an end product. Your product's safety standard decides.
Can I change the coverlay or adhesive after approval?
Treat any change as a re-approval. A different adhesive or coverlay can leave the build outside the recognized construction. Require the supplier to confirm in writing that the alternate is covered before you accept it.
Does a UL flame rating mean the flex is safe in my product?
No. A flame rating describes how a material burns in a specific laboratory test. It is one input to your product's safety assessment, which also covers temperature, electrical load, enclosure and the applicable end-use standard.