Designs on Flex: Underwriters Laboratory and Flexible Printed Circuits

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Underwriters Laboratory is a worldwide safety consulting and certification company headquartered in Northbrook, Illinois. UL certifies, validates, tests, inspects, audits, and advises on safety of electrical and electronic products.

I am sure you have seen the UL logo on many of your electrical and electronic products at home. Electronic equipment, in many cases, requires a UL certification.  In turn, the components that make up the electronic or electrical system must be certified. 

What Does UL test?

The test process for UL varies based on what is being certified, but in essence, the tests consist of determining how flammable a product is and the functionality of the circuit. In most cases, UL will perform flammability on the consumer product to insure its safety.

For UL 796F standard,

There are two categories of the safety test: flammability and function.

  • Adhesion
  • Bending / fatigue
  • Current / voltage limit

UL 94 Standard 

These requirements cover tests for flammability of plastic materials used for parts in devices and appliances.  There are three grades of flammability rating:

1)Vertical Burning Test: V-0, V-1, or V-2

2) Thin Material Vertical Burning Test: VTM-0, VTM-1, or VTM-2

3) Horizontal Burning Test: HB

UL Testing of Flex Materials

To obtain UL certification for a flex circuit is a little different.  UL does not require each flex circuit to be tested and certified, but, rather the materials that make the circuit- (specific stack up of polyimide, copper, FR-4 and adhesive).

To get approval from UL, a flex circuit manufacturer provides samples of a specific construction for flammability and flexibility testing. UL will test and approve that specific construction. Once the construction is approved, the flex manufacturer can use that specific construction on other circuits as a UL-approved construction.


From a high level, here are the procedures to get specific flex materials certified for flammability from UL.

  • Flex vendor submits request to UL have a flammability test
  • UL evaluates and creates a test plan,, procedures and processes. Test plan includes flammability test
  • Flex vendor manufactures test samples (coupons) and provides all required paperwork

Testing and certification takes 8-10 weeks.



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