October 8, 2015

Underwriters Laboratory (UL) 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 functionality of the circuit. In most cases UL will perform flammability on the consumer product to insure its safety.


UL Standards for Flexible Circuits
Standard for Printed Wiring Board  – UL 796
Standard for Printed Wiring Board includes special classification for flexible printed wiring –  UL 796F
Tests for Flammability of Plastic Materials for Parts in Devices and Appliances – UL 94

For UL 796F standard, there are 2 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 3 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


Flex circuits and UL
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 , FR4 and adhesive). Flexible circuit customers only ever care about flammability.

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


Testing Process
Here is an example test procedure for flammability from UL.
• PFC submits a material stack to UL
• UL evaluates and creates a test plan
• Test plan includes flammability test only
• PFC manufactures test samples.
• PFC ships to UL for testing.
Qualification takes 8-10 weeks

vertical burning test


UL Approval
Once the material has been approved by UL it is posted on the UL web site under each particular vendor. Below is the post you will find for PFC. You can go to the PFC website to find the link if you ever need the information: https://pfcflex.com/design-center/flex-circuit-materials/ul-constructions/

Here is what is provided on the UL web site for PFC.



Designing a UL approved flex circuit
As part of the design process with your flex vendor, there should be a discussion that incorporates circuit thickness requirements, bendability/flexibility, plating, RoHS, and assembly. Your flex vendor should assist/recommend a material stack that fits the circuit’s requirements. UL should be part of that discussion. All these requirements should be incorporated/included into a drawing supplied to the circuit vendor.

For UL approved materials, your flex vendor will go to their list of approved UL materials (such as the one above) and choose a material that fits your flex requirements. If the material is not on the list, then there may be a need to qualify a new material through UL.

Approved Stack example:
Vendor supplied material stack and associated information.



Marking a flex circuit for UL
The standard UL logo is not placed on the circuit itself since the circuit it self has not gone through the UL certification testing.  But, since the material stack and adhesives have gone through the testing, the part is stamped with the UL approved material.

Marking: Company name or trademark  or file number and type designation. May be followed by a suffix to denote factory identification or burning test classification.

PFC is recognized for a full range of DuPont materials (including AP and FR laminates and coverlays) and also a full range of Panasonic and Espanex materials. Additionally, PFC just had the full line of Taiyo “BN” solder masks recognized for any color.

example of flex markings

Example of Flex Markings


In the end…
So now you have been educated, relatively speaking, on the UL process for flex circuits.  Again, if there are UL requirements for your flex, or flex assembly, make sure you talk to your flex vendor about their UL approved materials. In some instances they may not have exactly what you need and may have to go through a UL test and qualification to supply the stack up required.