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All About Flex: FAQs on UL Listings for Flexible Circuits

Most electrical consumer products in the U.S. have an Underwriters Laboratory (UL) listing, which means that the product meets certain safety standards. In some cases, it may be a requirement for a product to be UL listed; in other cases a UL certification is chosen as part of a sales and marketing decision. UL requirements are driven from the end product down to the component level. The requirement for flexible circuitry recognition is driven by the end product specification as flexible circuits themselves are not sold directly to the consumer. The applicable UL spec for flexible circuits is UL796. The following are some common questions and answers relating to UL recognition.

What is UL?
 UL stands for Underwriters Laboratory and is a global organization that provides safety standards for consumer products.

What is the difference between UL Listed, UL Classification and UL Recognized^[1]?
UL Listing means that UL has tested representative samples of a product and determined that it meets UL’s requirements. These requirements are often based on UL’s published and nationally recognized Standards for Safety. A UL listing can only be applied to the final product as sold to the consumer.

UL Classification typically means that UL has tested and evaluated representative samples with respect to certain properties of the product. Product that passes flammability testing would be granted a Flammability classification mark.

UL Recognition means that UL has evaluated components or materials intended for use in a complete product or system. These components are intended only for end-use products that may be eligible for UL certification. Since most flexible circuits are components, it would be technically incorrect to state that a flexible circuit is UL listed; it can only be UL recognized.

How do you get a product UL Recognized?
There are two basic approaches: 

1. Submit the generic product construction for UL listing. This requires extensive planning, product/sample build and testing. The process can take over 6 months, but once completed, the supplier has limited latitude in material and process changes. To receive qualification for a generic flexible circuit product listing, the fabricator chooses many possible variations that would be common (i.e. thickness, number of layers and trace configuration) and submits samples for test with construction ranges to cover each of the common variations.
2. A second option is to submit a specific part number for testing. This is the fastest way to get a part listed as the qualification process can be completed in as little as a week. The main drawback with this method is there is little room for any design or process variation. The marking only applies to the exact construction and layout of the submitted part. If a customer wanted to change the copper thickness or polyimide thickness, those variations would need to be submitted for testing again.

What are the flammability ratings?
UL 94 covers several variations of flammability ratings. These ratings include:^[2]

• HB: slow burning on a horizontal specimen; burning rate <76 mm/min for thickness <3 mm or burning stops before 100 mm
• V-2: burning stops within 30 seconds on a vertical specimen; drips of flaming particles are allowed
• V-1: burning stops within 30 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed
• V-0: burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed
• 5VB: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may develop a hole
• 5VA: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may not develop a hole

Does UL Recognition also relate to product performance?
UL only tests for safety issues. Aa UL listing in general does not apply to product performance, unless it relates to safety.

Are process parameters included in the listing?
Any processing parameter that could affect aging or degradation of the material must be submitted as part of the qualification process. Consequently, the UL listing will be restricted to those processing conditions. Temperature, pressure, chemicals and humidity are process parameters that are considered to potentially age or degrade materials.

How does UL monitor for compliance?
UL auditors will perform a quarterly (unannounced) visit. During this visit the auditor will inspect the applicable materials, processing records, and work in process to assure compliance.

What would be an example of non-compliance?
Any material used in a product build that was not specified in the listing would be an example of non-compliance.  Any recorded processing condition falling outside of the submitted range would be considered an incident of non-compliance. Failure to adequately track or record materials and conditions would also be incidents of non-compliance.

What happens if UL finds non-compliance?
When an incident of non-compliance is found, UL allows the company to submit a written response. Non-compliance can result in significant fines and restrictions on applying UL markings on products.

References

1. UL’s Specific Guidelines and Rules
2.  Wikipedia on UL_94

Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.