Flex-rigid printed circuit boards with UL Classification

6th October 2014

Jacqueline Regnier

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Receiving a UL Mark for flex-rigid printed circuit boards is not easy, as there are many different combinations of materials and configurations. After a time-intensive and close coordination with the UL engineers, an enormous amount of test patterns and high costs, Würth Elektronik was able to record a big achievement: four flex-rigid printed circuit boards received the UL Listing. Currently, approximately 50% of all of Würth Elektronik’s flex-rigid printed circuit boards carry the UL Mark.

“Historically it has always been difficult to receive a UL Mark for flex-rigid printed circuit boards, as the then exclusively-used acrylic adhesives were not flame-resistant and did not withstand the flammability tests”, explains Andreas Schilpp, Product Manager for Flex-Rigid Printed Circuit Boards at Würth Elektronik. “Today we have flame-resistant acrylic adhesives as well as epoxy adhesives and epoxy prepregs. In spite of the improved material, it is not easy to receive a UL Mark for flex-rigid printed circuit boards.”

The UL-standards are very comprehensive and consider various risks based on the scope of use, the materials used and the procedures in production and processing. For electrical appliances the purpose is mainly to ensure fire prevention and electrical safety.

PCBs are not independent products, rather important components in electronic devices. Therefore, they are not UL-certified, but marked with the UL-Recognized Component test mark. Such test mark proves that ULcertified components can be used in UL-listed products or systems. After Würth Elektronik’s standard printed circuit boards already had the UL Mark for quite some time, Würth Elektronik embraced the challenge a while back and the very complex project with the UL engineers led to success: four UL-types could be listed for flex-rigid and that for the USA as well as for Canada!

“The effort really was worth it”, says Andreas Schilpp and presents the solutions:

- Flammability according to the UL94 standard: Würth Elektronik flex-rigid printed circuit boards meet the highest requirement of UL standards: “UL94 V-1 or better” with precisely V-1 or even V-0.
- Maximum Operating Temperature (MOT): Würth Elektronik flexrigid printed circuit boards passed the tests in accordance with UL796F for MOT120.
- Direct Support of Current-carrying Parts: The UL types of the flexrigid printed circuit boards meet the Direct Support Requirements (DSR) in accordance with UL746A.
- Comparative Tracking Index (CTI; ASTM D3638): For requirements regarding resistance to tracking, Würth Elektronik offer flex rigid solutions up to CTI Range-tracking Index 250 – 400V (PLC 2).

Should new customer-requirements need to be fulfilled, further UL Marks for flex-rigid printed circuit boards from Würth Elektronik are bound to follow.

“We will gladly determine which of these types suits the application of our customers based on their requirements”, says Andreas Schilpp. “We are prepared for any eventuality.”